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This paper describes a method for characterizing composite materials subjected to mode III delamination fracture using a custom-designed testing device and test equipment which allows loads or displacements to be applied to the test specimen in two directions, one axial and the other torsional. To verify the method’s functionality experimentally, a composite material made up of an epoxy matrix and unidirectional carbon-fiber reinforcement was used in conjunction with an image analysis device for the purpose of determining the displacement field in the crack front of a double cantilever beam test specimen. According to the results, this test method permits almost pure mode III fracture tests to be carried out, as the mode II component is practically negligible. Another feature of the method is the improvement in the quality and ease of inserting the specimen in the device, thus permitting more repetitive results to be obtained with less dispersion. [ABSTRACT FROM AUTHOR]

This paper presents researches in the field of processing of composite materials manufactured by water jet cutting. In the last years, the field of composite materials reinforced by carbon fiber / epoxy matrix has developed increasingly more because of the advantages they offers. One of the challenges in this area is machining of the contour of complex parts. Therefore water jet cutting is proving to be a viable solution for this problem. But, also, to manufacture complex composite parts is required to optimize the water jet cutting process. [ABSTRACT FROM AUTHOR]

Mechanically, composite laminates perform exceptionally well in-plane but poorly out-of-plane. Interlaminar damage, known as "delamination," is a major issue for composite laminates. Results from Mode-I and Mode-II experimental testing on twill-woven carbon fiber reinforced (CFRP) laminates are analyzed in this paper. Composite Mode-I fracture toughness was determined using three different methods in accordance with ASTM D5528: modified beam theory, compliance calibration, and a codified compliance calibration. Two methods, the Compliance Calibration Method and the Compliance-Based Beam Method, were used to determine the Mode-II fracture toughness in accordance with ASTM D7905. Stick-slip behavior is quite evident in the composite's Mode-I fracture toughness test findings. The MBT technique's GIc values for initiation and propagation are 0.533 and 0.679 KJ/m², respectively. When comparing the MBT approach to the industry-standard ASTM procedure for determining fracture toughness Mode-I, the MBT method was shown to be highly compatible. Furthermore, the -IIc values for the CBBM technique are 1.65 KJ/m² for non-pre cracked and 1.4 KJ/m² for pre-cracked materials. The CBBM method shows a good method to evaluate fracture toughness Mode-II, due to not needing to monitor the length of the crack during delamination growth to get the value of the fracture toughness. [ABSTRACT FROM AUTHOR]

Delamination between layers occurs during the creasing and subsequent folding of paperboard. Delamination is necessary to provide some stiffness properties, but excessive or uncontrolled delamination can weaken the fold, and therefore needs to be controlled. An understanding of the mechanics of delamination is predicated upon the availability of reliable and properly calibrated simulation tools to predict experimental observations. This paper describes a finite element simulation of paper mechanics applied to the scoring and folding of multi-ply carton board. Our goal was to provide an understanding of the mechanics of these operations and the proper models of elastic and plastic behavior of the material that enable us to simulate the deformation and delamination behavior. Our material model accounted for plasticity and sheet anisotropy in the in-plane and z-direction (ZD) dimensions. We used different ZD stress-strain curves during loading and unloading. Material parameters for in-plane deformation were obtained by fitting uniaxial stress-strain data to Ramberg-Osgood plasticity models and the ZD deformation was modeled using a modified power law. Two-dimensional strain fields resulting from loading board typical of a scoring operation were calculated. The strain field was symmetric in the initial stages, but increasing deformation led to asymmetry and heterogeneity. These regions were precursors to delamination and failure. Delamination of the layers occurred in regions of significant shear strain and resulted primarily from the development of large plastic strains. The model predictions were confirmed by experimental observation of the local strain fields using visual microscopy and linear image strain analysis. The finite element model predicted sheet delamination matching the patterns and effects that were observed in experiments. [ABSTRACT FROM AUTHOR]

Methodologies for prognostication and health monitoring (HM) can significantly impact electronic reliability for applications in which even minimal risk of failure may be unbearable. Presently, HM approaches such as the built-in self-test are based on reactive failure diagnostics and unable to determine residual-life (RL) or estimate residual-reliability. Prognostics health-monitoring (PHM) approach presented in this paper is different from state-of-art diagnostics and resides in the prefailure-space of the electronic system, in which no macro-indicators such as cracks or delamination exist. Applications for the presented PHM framework include, consumer and defense applications such as automotive safety systems including front and rear impact protection systems, chassis-control systems, x-by-wire systems, and defense applications such as avionics systems, naval electronic warfare systems. The presented PHM methodologies enable the estimation of prior damage in deployed electronics by the interrogation of the system state for systems in which the prior stress-history may be unknown or unavailable. The primary focus is on thermo-mechanical stresses. The presented methodologies will trigger repair or replacement, significantly prior to failure. The approach involves the use of condition monitoring devices which can be interrogated for damage proxies at finite time-intervals. The system's residual life is computed based on residual-life computation algorithms. Previously, we have developed several leading indicators of failure. In this paper, a mathematical approach has been presented to calculate the prior damage in electronics subjected to cyclic and isothermal thermo-mechanical loads. Electronic components operating in a harsh environment may be subjected to both temperature variations in addition to thermal aging during use-life. Data have been collected for leading indicators of failure for 95.5Sn4Ag0.5Cu first-level interconnects under both single and sequential applications of cyclic and isothermal thermo-mechanical loads. Methodology for the determination of prior damage history has been presented using non-linear least-squares method based on interrogation techniques. The methodology presented used the Levenberg–Marquardt Algorithm. The test vehicle includes various area-array packaging architectures soldered on immersion Ag finish, subjected to thermal cycling in the range of -40^\circC to 125^\circC and isothermal aging at 125^\circC. [ABSTRACT FROM PUBLISHER]

Conoidal shells are very popular roofing units owing to their aesthetic elegance and stiffness. Many parts of the globe, which were earlier assumed to be seismologically stable, are now being considered as earthquake prone. Hence the necessity to build light structures using composites has become very important. In this paper an eight-noded isoparametric shell element is applied for analyzing the bending behavior of delaminated composite conoidal shells under a uniformly distributed load with different practical boundary conditions. To ensure compatibility of deformation and equilibrium of forces and moments at the delamination crack front, a multipoint constraint algorithm is incorporated, which leads to an unsymmetrical stiffness matrix. This formulation is validated through the solution of benchmark problems. Lamination, curvature, and extent of delamination area are varied to compare the performances of delaminated conoidal shells against those with no damage. The results are carefully observed, and a set of conclusions is presented at the end of the paper. [ABSTRACT FROM AUTHOR]

In this paper, the compressive bucking and postbuckling behavior of composite laminates with through-the-width delamination are investigated. The analytical method is based on the first-order shear deformation theory, and its formulation is developed on the basis of the Rayleigh-Ritz approximation technique by the implementation of the polynomial series, which has been used for the first time in the case of the mixed mode of buckling. Both local buckling of the delaminated sublaminate and global buckling of the whole plate are investigated. Also, the contact among sublaminates is taken into account. The three-dimensional finite-element analysis is performed by using ANSYS5.4 general-purpose commercial software just to compare the finite-element method results with those obtained by the analytical model. It is noted that the significance and contribution of the current paper lies in the fact that for a rather complicated problem, very good results are obtained by using a fairly small number of degrees of freedom through the application of complete polynomial series. [ABSTRACT FROM AUTHOR]

Though many methods exist for detecting damage in a structure, few of them can be used to identify the debonding defect between the surface and inner layer of a lattice truss sandwich plate. In this paper, a vibration signal based nondestructive testing method using Uniform Load Surface (ULS) curvature is applied to detect the delamination defects of the lattice sandwich plate. The feasibility of the approach is investigated through numerical simulation by using commercial software for different damage configurations. The numerical results show that there is a good possibility to detect delamination defects in the lattice sandwich plate from vibration signals by using the ULS curvature strategy. [ABSTRACT FROM AUTHOR]

Composite structures are more susceptible to manufacturing defects than conventional materials. Fiber misalignment, cracks, and voids are typical types of manufacturing defects in laminated composites. Defects can greatly reduce structural integrity and load carrying capacity, so their effects on material and structural strengths must be understood. In this paper, the effect of manufacturing defects on the progressive delamination behavior was studied for carbon/epoxy composite laminated L-beam under four-point bending test conditions. The defects of wavy plies and pure resin that had flowed out were characterized from surface photography and then transformed into finite element modeling using semi-automatic approach to which pre-delamination and void were included. Next, progressive failure analyses were performed with the interface delamination and matrix direction failure accounted for by cohesive zone modeling. Numerical results were examined focusing on the effects of defects on the peak load reduction and the variation of delamination pattern. The stacking sequence effect was also investigated. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to identify the ideal process parameters to be set for the drilling of hybrid fibre-reinforced polymer (FRP) (kenaf and banana) composite using High-Speed Steel drill bits (5, 10, 15 mm) coated with tungsten carbide by means of statistical reproduction of the delamination factor and machining force using Taguchi–Grey Relational Analysis. Design/methodology/approach: The contemplated process parameters are Feed, Speed and Drill Diameter. The trials were carried out by taking advantage of the L-27 factorial design by Taguchi. Three factors, the three level Taguchi Orthogonal Array design in Grey Relational Analysis was used to carry out the trial study. Video Measuring System was used to identify the damage around the drill region. "Minitab 18" was used to examine the data collected by taking advantage of the various statistical and graphical tools available. Examination of variance is used to legitimize the model in identifying the most notable parameter. Findings: The optimised set of input parameters were found out successfully which are as follows: Feed Rate: 450 mm/min, Cutting Speed: 3,000 rpm and Drill Diameter of 5 mm. When these values are fed in as input the optimised output is being obtained. From ANOVA analysis, it is apparent that the Speed (contribution of 92.6%) is the most influencing parameter on the delamination factor and machining force of the FRP material. Originality/value: Optimization of process parameters on drilling of natural fibres reinforced in epoxy resin matrices using Taguchi–Grey Relational Analysis has not been previously explored. [ABSTRACT FROM AUTHOR]

Various defects produced during the manufacture of a wind turbine blade can affect the damage evolution under fatigue loads. In this paper, a calculation model based on energy dissipation was used to predict damage evolution. Based on the structure of the unidirectional laminates in the main spar of a 1.5-MW blade, specimens with voids or delamination defects of different dimensions were fabricated. Fatigue tests of the specimens were monitored with an infrared thermography system. The energy dissipation density was calculated from the surface temperature. A damage parameter D is proposed to represent the degree of damage evolution. By analyzing the energy dissipated during cyclic loading, it is possible to evaluate the critical moment when a defect evolves into initial critical damage. The calculation model agrees well with the observed damage evolution. This innovative method could be used to detect the initial damage in a main spar with defects during a fatigue test. [ABSTRACT FROM AUTHOR]

Drilling performance of carbon fiber reinforced polymer (CFRP) aggravates significantly with serious tool wear caused by highly abrasive fibers. Because of the anisotropy of CFRP, the angle between fiber orientation and cutting direction, namely, fiber cutting angle, also plays a crucial part in the drilling performance. To enhance drilling performance, this paper initially conducts a comprehensive experimental study on the effects of tool wear on thrust force, exit temperature, and hole wall defect in different fiber cutting angles in the drilling of unidirectional CFRP. The results demonstrate that the profile of worn cutting edge is an approximate elliptical arc and the half axis lengths of the elliptical arc are utilized to evaluate cutting edge wear. Tool wear on the cutting edge, flank face, and rake face all deteriorates with the number of drilled holes. Tool wear has a more significant influence on thrust force in the fiber cutting angle region of 0~90° than in the fiber cutting angle region of 90~180°. With increasing tool wear, the exit temperature distribution can still be approximate to an ellipse. The variation of exit temperature caused by tool wear in the major axis direction is still no less than that in other directions. In addition, on hole wall, tool wear causes severe crack defects in the fiber cutting angle region of 0~90° and cavity defects in the fiber cutting angle region of 90~180°. [ABSTRACT FROM AUTHOR]

An engineering approach for fatigue life prediction of fibre‐reinforced polymer composite materials is highly desirable for industries due to the complexity in damage mechanisms and their interactions. This paper presents a fatigue‐driven residual strength model considering the effect of initial delamination size and stress ratio. Static and constant amplitude fatigue tests of woven composite specimens with delamination diameters of 0, 4 and 6 mm were carried out to determine the model parameters. Good agreement with experimental results has been achieved when the modified residual strength model has been applied for fatigue life prediction of the woven composite laminate with an initial delamination diameter of 8 mm under constant amplitude load and block fatigue load. It has been demonstrated that the residual strength degradation‐based model can effectively reflect the load sequence effect on fatigue damage and hence provide more accurate fatigue life prediction than the traditional linear damage accumulation models. [ABSTRACT FROM AUTHOR]

Fiber-reinforced composite laminates are being increasingly used in various engineering components in the sectors of aerospace and green energy. Due to impacts throughout the service life of the structure, matrix breakage and delaminations significantly altering the structural integrity of the laminate can occur. Hence, robust guided wave structural health monitoring systems are required to ensure continuous safety of engineering structures. In this paper, the ultrasonic method based on the analysis of A0 mode reflecting within the defected area has been proposed to extract the length and the depth of the delamination-type defect. The technique proposed in this study extracts the depth of the damage by analyzing the magnitude variations of direct A0 mode which are caused by the difference of wave velocities in the upper and lower sub-laminates. This results in an altering and frequency-dependent forward-scattered amplitude of direct A0 mode. Furthermore, the proposed approach uses previously obtained information about the depth of the defect, which allows for the determination of the phase velocities of A0 and S0 modes in the upper and lower sub-laminates. As a result, the accuracy of the damage length estimation is increased. The performance of the proposed method was proven with 2D and 3D numerical simulations and experiments on samples with artificial defects. The method validation results showed that the proposed method with some limitations is capable of extracting the length of delamination with an approximate error below 6%. [ABSTRACT FROM AUTHOR]

To examine the crack propagation in quasi-brittle materials, cohesive models have been suggested by the works of researcher. They treat the existence of a process zone at the crack tip, with a suitable constitutive law, linking the tensile normal stress T and the relative displacement among the two crack edges. In this paper, we employed the cohesive model which examines the mode I interlaminar fracture into stitched reinforced composite laminates. The test configuration that we used in this study was a stitched reinforced double cantilever beam samples. A bilinear damage-rate-dependent cohesive traction–separation law is implemented to design the woven composite fracture and discrete nonlinear spring elements to characterize the stitches influence, its advantages and disadvantages. A novel macroscopic law adopted from a 1D micromechanical-stitching model is invented to model the stitches effect along the interface. The numerical simulations of double cantilever beam test with the present model present a significant agreement compared to the experimental results. [ABSTRACT FROM AUTHOR]

This paper reports the mode I interlaminar fracture toughness and fracture mechanisms of two-dimensional (2D) plain woven composite and three-dimensional (3D) angle-interlock woven composite. The fracture toughness behaviors were tested with double cantilever beam method at the different loading rates from 0.5 to 100 mm/min. Critical strain energy release rate was calculated to compare the difference between the 2D and the 3D woven composites. The fractographs were photographed with scanned electronic microscopy and optical microscopy to show the fracture morphologies. We found that the 3D angle-interlock woven composite has high fracture toughness than that of 2D woven composite. The binder yarns resist the crack initiation and propagation to increase the fracture toughness. While the lower in-plane stiffness of the 3D woven composites should be considered fully for designing the 3D woven composites. [ABSTRACT FROM AUTHOR]

Ceramic coatings are ideal materials for thermal protection systems such as the thermal shield of the space shuttle. In a high-temperature environment, material properties of ceramics strongly depend on the temperature. The severe mismatch of material properties between the ceramic coating and the substrate can result in progressive mechanical failure of thermal protection system. This paper investigates delamination and buckling behaviors between a temperature-dependent ceramic coating and a porous substrate. The shear stress intensity factor at the tips of the delamination crack and buckling region are derived. Based on the stress intensity factor, the critical temperature of the coating buckling from the substrate is obtained. A fitting formula of the critical buckling temperature with respect to the length-to-thickness ratio of the coating, and the buckling region is obtained. It is found that the effect of the temperature dependence of material properties on delamination and buckling is more significant for higher temperatures than for lower temperatures. The critical temperatures of delamination and buckling are overestimated if the temperature dependence of material properties is neglected. The critical temperatures of delamination and buckling increase with the porosity of the substrate. [ABSTRACT FROM AUTHOR]

Delamination, an inter-ply debonding failure phenomenon, is considered as the most undesirable and challenging failure mode in hole making of carbon fiber-reinforced plastic (CFRP) composite laminates. The existence of delamination, even small ones, noticeably damages the strength and stability of the assembled products. It is reported that delamination is responsible for up to 60% of composite components' rejection during assembling. In order to reduce delamination, rotary ultrasonic machining (RUM) has been studied and utilized in hole making of CFRP composites. Existing investigations on delamination of CFRP composites in RUM hole making are experimental studies, in which several methods (such as adjusting input variables and the use of supportive plate) to reduce delamination are reported. To understand delamination generation mechanisms and predict delamination initiation, theoretical investigations are needed. Cutting force models in RUM of CFRP composites have been developed, and it is well accepted that delamination is correlated to thrust force. However, there are no reported investigations to predict delamination initiation and study the impact of thrust force on the delamination of CFRP composites in the RUM hole-making process. Finite element analysis (FEA) could be an effective way to study the delamination in the RUM process. In this paper, FEA was conducted to predict delamination initiation and establish the relationship between the thrust force and delamination thickness in RUM, for the first time. In addition, experiments were conducted to validate the FEA model. The proposed model was proved to be effective in predicting delamination initiation, and the trends of the FEA results agreed well with those of the experimental results. [ABSTRACT FROM AUTHOR]

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This paper presents an experimental study on abrasive waterjet (AWJ) drilling of Inconel 718 and AISI 1040 steel, with an aim to understand the AWJ hole drilling performance and to investigate the process monitoring strategy by using acoustic emission (AE). In the experiments, pressure levels were set from 200 to 350 MPa at intervals of 50 MPa, and drilling time between 5 and 30 s at intervals of 5 s. It is found that the hole depth increases along with the drilling time at a slowing down penetration rate. A higher pressure results in a higher penetration rate. While there exists a direct influence of the material property on the drilling efficiency, it has limited significance with regard to the diameter of the hole produced. AE-RMS signal is demonstrated to be able to reflect the decreasing trend of the penetration rate and the influence of the pressure. A strong correlation between the penetration rate and AE-RMS values in AWJ hole drilling is shown through correlation analysis. The AE-RMS is also substantially affected by the material properties. A discrepancy of the AE-RMS under similar condition may suggest an anomaly in this cutting run. [ABSTRACT FROM AUTHOR]

Fiber metal laminate (FML) is a kind of lightweight material with excellent mechanical properties combining advantages of metal laminates and fiber reinforced composites. It has been widely used in the aerospace and transportation fields and is especially used as structural material such as aircraft skins, wings, and tails. However, under complex service conditions, interlaminar failure in FMLs greatly reduced mechanical properties of the material, even leading to serious economic and safety disasters. The failure and destruction of important structural parts of aircraft and other manned transportation vehicles are extremely unsafe for people. Therefore, it is of great significance to summarize the interlaminar failure behavior of FMLs and find ways to avoid these defects. This review paper is a collection of various researches done by many groups, which systematically discuss the interlaminar failure behaviors and their control methods of FMLs. The application status of several common FMLs in aircraft structures was given. The common interlaminar failure modes of FMLs and the testing and evaluation methods of interlaminar properties were stated. The failure mechanisms and the corresponding control methods were analyzed. Finally, the future developments of FMLs were also discussed by the authors. Through this review article, readers can obtain new research progress about the control method, the mechanism and future development on the failure behavior of FMLs in a more efficient way. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the ability of ultrasonic and electronic speckle pattern interferometry to analyse the low-velocity impact internal damage mechanisms on basalt composite laminates and to provide information on the shape and the extent of the delamination in non-destructive way. Basalt/epoxy composites with different thicknesses have been realised and characterised by mechanical tests to investigate both fibre-dominated (tensile and flexural behaviour) and matrix-dominated properties (interlaminar shear strength). Specimens were impacted at penetration and at increasing energy values, to explore the damage onset and propagation. The results showed that the damage was concentrated under the impactor–material contact point and that the composite with intermediate thickness had the best balance between the different kinds of impact damages: delamination and indentation. Further, a good agreement was found between the overall data obtained by the two non-destructive techniques, confirming the capability of both techniques to examine the composite impact damage. [ABSTRACT FROM AUTHOR]

Interfacial failure by delamination in the unidirectional AS4/PEEK laminated composite was simulated through this paper with a finite element method (FEM) based model developed with Comsol Multiphysics software. The mixed - mode bending (MMB) test was investigated numerically here after a successful validation of the model by comparing with experimental measurements of the load -displacement curve provided in the literature. The maximum delamination length of the specimens modeled, the corresponding von Mises stress distributions and the damage evolution function have been evaluated for different ratios between mode II strain energy rate and total strain energy rate GII/GT = 20, 50 and 80%. [ABSTRACT FROM AUTHOR]

Bilinear delamination failure criteria proposed by Reeder for mixed-mode (I–II) fracture of composite laminates has four limitations: require mixed-mode (I–II) fracture data by the more complex MMB test; determination of bilinear constants by two independent curve fits; failure of bilinear equations to intersect at G I m / G II m = 1 ; and the data representation by G I m versus G II m instead of the conventional G c m versus G II m / G T . Some of these limitations were overcome by limited input bilinear criteria (LIBC) proposed by Davidson and Zhao by replacing the MMB test data by DCB, ENF, and SLB (single leg beam) test data. But the LIBC retained the two discontinuous forms of the equations for no specific reason in a continuous material system. This paper eliminates the two equations by a single power law equation that covers the whole domain of fracture as in Benzeggagh and Kenane's empirical equation. This simplified mixed-mode delamination fracture criterion we call "Limited Input B–K (LIBK) criterion in recognition of the works of Davidson and Zhao, and Benzeggagh and Kenane. The criterion was validated by a large number of test data in the literature. [ABSTRACT FROM AUTHOR]

The compressive strength of composite laminates decreases seriously after being subjected to impact loading, which is an important item to be considered in the usage of composite material. In this paper, a micromechanics-based damage model is proposed to study the compressive behavior of impacted composite laminates. The micro stresses of fiber and matrix are calculated by stress amplification factors and then used to judge the failure mechanisms according to corresponding physical failure criteria. A progressive damage model based on different failure statuses of constituents is established to study the degradation of material properties. The bi-linear cohesive model is used in the research of delamination onset and propagation. The compressive behaviors of quasi-isotropic composite laminates subjected to different impact energies are investigated by this proposed method. Good agreements in terms of structure responses, failure mechanisms, and residual compressive strengths are obtained between numerical results and experimental data. The matrix cracking and delamination caused by impact loading are responsible for the initiation and propagation of buckling, which leads to the final collapse of entire laminates. Based on the numerical investigations of material parameters, the increment of mode II interlaminar fracture toughness is capable of improving the residual compressive strength significantly. [ABSTRACT FROM AUTHOR]

Process-induced damages, such as burrs and delamination, frequently occur and are the critical hole quality issues for CFRPs. These damages are strongly influenced by the axial cutting force. By employing the novel drill-milling process, the influence of the axial cutting force may be completely eliminated, and the process-induced damages can be remarkably removed. The drill-milling tools with a better performance are very critical to the drill-milling process. In this paper, according to the drill structure with V-shaped cutting edges and the mill structure with double-crossed helix flutes, two different new drill-milling tools are designed, and a series of tests are conducted on CFRPs. The result indicates that the influences of the cutting forces on the damages can be remarkably reduced during the helical milling phase by employing the drill-milling process. Under the same cutting conditions, the cutting forces of T2 are significant less than that of T1. The re-trimming effect of T2 is superior significant than that of T1 during the helical milling phase. The damages can be significantly reduced by using these two tools, especially by using T2 tool. [ABSTRACT FROM AUTHOR]

This paper experimentally analyses the delamination behaviour of a composite material under mode III fracture as a function of the loading mode applied during the fatigue tests was carried out in two different ways. First, the test specimens were loaded cyclically in a single direction of delamination until reaching a certain fracture energy and subsequently returning to the starting point. Second, cyclic loading was applied both in one direction of delamination and in the opposite direction until reaching the same fracture energies, though in two opposite directions. The experimental results revealed clear differences in the fatigue behaviour of the material when the way of applying the load varied. In both cases, a statistical analysis of the results was carried out in order to evaluate the existing differences more suitably. Furthermore, a fractographic analysis was conducted for both types of loading in order to distinguish possible differences at the microstructural level. [ABSTRACT FROM AUTHOR]

This paper studies the detection of hidden polymer matrix composite delaminations with a pitch–catch ultrasonic testing system and an agglomerative clustering algorithm. Existing ultrasonic testing methods characterize damage through normal-incidence pulse-echo measurements. Yet, these pulse-echo methods are ineffective at detecting delaminations underneath other delaminations. The ultrasonic waves only reflect off the top delamination. As a result, no information about the lower delamination is transmitted to the receiver. To address this problem, we investigate an oblique-angle pitch–catch ultrasonic testing method to transmit ultrasonic waves underneath the delaminations. The ultrasonic waves interact with the lower delaminations and carry that information to the receiver. We describe and discuss our experiments, which use two polytetrafluoro-ethylene (PTFE) inserts to simulate delaminations. We show that applying agglomerative clustering to the experimental data can successfully distinguish three regions: regions with two PTFE inserts, regions with only an upper PTFE insert, and regions with only the lower PTFE insert. Normal-incidence measurements only observe two regions. [ABSTRACT FROM AUTHOR]

Drilling holes on carbon fiber reinforced plastic/polymer (CFRP) and Ti stack structures is significantly important assembling process in aerospace manufacturing. However, it is quite a challenge that the excessive and completely different drilling loads of two materials with distinguish physical and mechanical properties will lead to frequent CFRP drilling damages and Ti burrs, as well as rapid tool failures. Therefore, understanding and controlling the drilling loads are crucial to ensure low damage and high efficiency drilling of CFRP and Ti stacks. In this paper, a novel mechanistic model was established to predict the thrust force and the torque in the drilling of CFRP and Ti stacks. The cutting loads of a twist drill were discretized along chisel edge, primary cutting edge, and minor cutting edge. An orthogonal cutting model and an oblique cutting model were proposed to represent the discretized cutting load for CFRP and Ti with the consideration of fiber cutting angles, flank face friction, and drilling parameters. Integrating the discretized cutting load, a mechanistic model for predicting thrust force and torque was developed for drilling of CFRP/Ti stack. After the coefficient calibrations, verification experiments showed that the developed model has accurately predicted the thrust force and torque under various spindle speeds/feed rates as well as the changing fiber cutting angles. [ABSTRACT FROM AUTHOR]

One of the most severe, costly, and time-consuming problems in the drilling operation is the loss of circulation. The drilling fluid accounts for 25–40% of the total cost of the drilling operation. Loss of the drilling fluid will increase the total cost of the drilling operation. Uncontrolled lost circulation of the drilling fluid may result in dangerous well control problem and in some cases the loss of the well. Fluid losses can occur in different formations such as natural fracture, induced fracture, unconsolidated, and cavernous and vugular. The objective of this paper is to deeply understand and illustrate different types of loss of circulation, techniques used to determine the loss rate, and detection of loss zones. In addition, losses mitigation techniques, different lost circulation materials (LCM), and the apparatus used to evaluate the available LCM will be explained in detail. The deep literature review illustrated that lost circulation costs 10–20% of the cost of drilling high-pressure high-temperature wells and 90% of these losses occur in fractured formations. The loss rate depends on the drilling fluid types, and it is generally higher in the case of water-based drilling fluid than in the case of oil-based drilling fluid. It is important to utilize an advanced measurement for losses with high frequency to detect the occurrences of lost circulation quickly and correctly. The measurements of flow rate (flow in and flow out) should be supported by additional data such as logging data, lost circulation information (loss rate and depth), mud properties, and surface drilling parameters. The preventive approach is the most efficient to mitigate the loss of circulation since the use of conventional LCM is not successful in most of the cases because of their limitations. It is vital to consider the size of different fractures encountered while drilling and modify the laboratory equipment to simulate this problem. Loss of circulation is affected by many parameters that are related to formation characterization, drilling parameters, fluid properties, and a lot of other known and unknown factors. Therefore, it is a challenge to predict the loss of circulation. To overcome such challenge, it is recommended to develop a new technique such as artificial intelligence to predict the thief zones and the loss rate by capturing the changes in the drilling mechanical parameters and the fluid properties. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this paper is to optimize the machining parameters to obtain the smallest average surface roughness values during drilling of the carbon fiber-reinforced polymer (CFRP) composite material with abrasive water jet (AWJ) and analyze the damage of the delamination. Design/methodology/approach: CFRP composite material had been fabricated having fiber orientations frequently used in the aerospace industry (0°/45°/90°/−45°). Three different stand-off distances (1, 2 and 3 mm), three different water pressures (1,800, 2,800 and 3,800 bar) and three different hole diameters (4, 8 and 12 mm) were selected as processing parameters. The average surface roughness values were obtained, and delamination damage was then analyzed using Taguchi optimization. Drilling experiments were performed using the Taguchi L27 orthogonal array via Minitab 17 software. The signal/noise ratio was taken into account in the evaluation of the test results. Using the Taguchi method, the control factors giving the mean surface roughness values were determined. Analysis of variance was performed using the experimental results, and the effect levels of the control factors on the average surface roughness were found. Findings: It was found that water pressure and hole diameter had a higher effect on average surface roughness, while water pressure and stand-off distance were effective on delamination. Practical implications: Owing to their excellent thermal and mechanical properties, the CFRP composite materials show greater potential for their applications in aircraft and aerospace industry. Originality/value: The novel approach is to reduce cost and spent time using Taguchi optimization as a result of AWJ drilling the material in this fiber orientation ([0°/45°/90°/−45°]s, which is often used in the aerospace industry). [ABSTRACT FROM AUTHOR]

Composite material parts are typically laid out in near-net-shape, i.e., very close to the finished product configuration. However, further machining processes are often required to meet dimensional and tolerance requirements. Drilling, edge trimming and slotting are the main cutting processes employed for carbon fiber-reinforced plastic (CFRP) composite materials. In particular, drilling stands out as the most widespread machining process of CFRP composite parts, chiefly in the aerospace industrial sector, due to the extensive use of mechanical joints, such as rivets, rather than welded or bonded joints. However, CFRP drilling is markedly challenging: due to CFRP abrasiveness, inhomogeneity and anisotropic properties, tool wear rates are inherently high leading to superior cutting forces and detrimental effects on workpiece surface quality and material integrity. Damage such as delamination, cracks or matrix thermal degradation is often observed as the result of uncontrolled tool wear or improper machining conditions. Sensor monitoring of drilling operations is, therefore, highly desirable for process conditions' optimization and tool life maximization. The development of this kind of automated control technologies for process and tool state evaluation can notably contribute to the reduction of scraps and tool costs as well as to the improvement of process productivity in the drilling of CFRP composite material parts. In this paper, multi-sensor process monitoring based on thrust force and torque signal detection and analysis was applied during drilling of CFRP/CFRP laminate stacks for the assembly of aircraft fuselage panels with the scope to evaluate the tool wear state. Different signal-processing methods were utilised to extract diverse types of features from the detected sensor signals. A machine-learning approach based on an artificial neural network (ANN) was implemented to make smart decisions on the timely execution of tool change, which is highly functional for CFRP drilling process automation. [ABSTRACT FROM AUTHOR]

A multidirectional damage model based on continuum damage mechanics for fiber-reinforced composite laminates is proposed in this paper. The influence of three main damage mechanisms, including transverse matrix cracking, local delamination, and fiber breakage, on the multidirectional stiffness properties of composite laminates is analyzed by introducing macro phenomenological damage variables. Then the mechanical behavior of elementary ply in laminates is modeled based on these damage variables. Besides, relations between micro-level damage variables and macro-level damage variables are established. Damage evolution laws of the three damage mechanisms are proposed to predict the degradation of multidirectional stiffness and failure strength of composite laminates under quasi-static loading. The experiment of cross-ply glass fiber-reinforced plastic laminates is carried out, and the prediction results show good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a new methodology for detecting various types of composite damage, such as delamination and impact damage, through the application of multimode guided waves. The basic idea is that various wave modes have different interactions with various types of composite damage. Using this method, selective excitations of pure-mode guided waves were achieved using adjustable angle beam transducers (ABTs). The tuning angles of various wave modes were calculated using Snell's law applied to the theoretical dispersion curves of composite plates. Pitch-catch experiments were conducted on a 2-mm quasi-isotropic carbon fiber-reinforced polymer (CFRP) composite plate to validate the excitations of pure fundamental symmetric mode (S0) and shear horizontal mode (SH0). The generated pure S0 mode and SH0 mode were used to detect and separate the simulated delamination and actual impact damage. It was observed that S0 mode was only sensitive to the impact damage, while SH0 mode was sensitive to both simulated delamination and impact damage. The use of pure S0 and SH0 modes allowed for damage separation. In addition, the proposed method was applied to a 3-mm-thick quasi-isotropic CFRP composite plate using multimode guided wave detection to distinguish between delamination and impact damage. The experimental results demonstrated that the proposed method has a good capability to detect and separate various damage types in composite structures. [ABSTRACT FROM AUTHOR]

The paper discusses a method for measuring changes in the electric resistance of a unidirectional carbon-fiber-reinforced plastic (CFRP) to detect an internal delamination. A numerical solution was obtained by the four-probe method for measuring the electric resistance. The influence of distance between electrodes on the efficiency of delamination detection was investigated. The problem was solved by the method of planning of experiments and constructing a surface response by using a 2D finite-element model. The distance between electrodes and the length of delamination were selected as parameters of the study. Distances between the internal and external electrodes for an effective detection of delaminations are determined. [ABSTRACT FROM AUTHOR]

One of the machining failures in composite materials is delamination damage. In this paper, machining parameters and delamination damage caused by the drilling process on ramie woven reinforced composite material with an unsaturated polyester matrix were investigated. The ramie woven used is ramie yarn type 12S/3. The machining process used 1.5 kW pillar drills, with variations in the diameter of the brad and spur drill of 4 mm, 6 mm, 8 mm and 10 mm. This work focused on the influence of machining parameters like feeds rate and spindle speed. Holes quality was analysed in terms of thrust force and delamination failure. From the results of this study, the thrust force value obtained at the time of drilling is very closely related to the delamination damage that happens. Delamination damage occurs on both sides of the holes drill. [ABSTRACT FROM AUTHOR]

This paper first examines the commonly-used thermal-moisture analogy approach in moisture diffusion analysis. We conclude that such an analogy using a normalized concentration approach does not exist in the case of soldering reflow, when the solubility of each diffusing material varies with temperature or the saturated moisture concentration is not a constant over an entire range of reflow temperatures. The whole field vapor pressure distribution of a flip chip BGA package at reflow is obtained based on a multiscale vapor pressure model. Results reveal that moisture diffusion and vapor pressure have different distributions and are not proportional. The vapor pressure in the package saturates much faster than the moisture diffusion during reflow. This implies that the vapor pressure reaches the saturated pressure level in an early stage of moisture absorption, even the package is far from moisture saturated. However, the interfacial adhesion degrades continuously with moisture absorption. Therefore, the package moisture sensitivity performance will largely reply on the adhesion strength at elevated temperature with moisture. A specially designed experiment with a selection of six different underfills for flip chip packages was conducted. Results confirm that there is no correlation between moisture absorption and the subsequent interface delamination at reflow. The adhesion at high temperature with moisture is the only key modulator that correlates well with test data. Such a parameter is a comprehensive indicator, which includes the effects of thermal mismatch, vapor pressure, temperature and moisture. In this paper, a micromechanics based mechanism analysis on interfacial delamination is also presented. With the implementation of interface properties into the model study, it shows that the critical stress, which results in the unstable void growth and delamination at interface, is significantly reduced when the effect of moisture on debonding is considered. [ABSTRACT FROM AUTHOR]

The presence of a fiber-reinforced polymer (FRP) strengthening material bonded to the tension face of a reinforced concrete beam will restrict but not prevent the opening of intermediate flexural cracks due to applied loading. Test results indicate that displacements at the toe of flexural cracks create stress concentrations at the interface of the FRP laminate and the beam, leading to the development of localized interface cracks that, typically, propagate, under the effect of the load, to join the original flexural cracks and cause delamination of the FRP system. This type of FRP delamination is commonly termed intermediate crack (IC) debonding. In this paper the analytical models published in the literature are reviewed and it was found that these models do not correlate well with measured experimental results. This paper proposes an analytical model that characterizes the interface shear stress based on two distinct sources: (1) the change in the applied moments along the length of the member and (2) stress concentrations at the intermediate cracks. The proposed model is compared to an experimental database and shown to predict extremely well most of the test results reported by other researchers. A parametric study, performed using the proposed model, indicates that the model varies with several important variables that are not captured by most of the existing models. [ABSTRACT FROM AUTHOR]

Various researchers have developed techniques to control delamination in laminated structures. One of these techniques is "interleaving," adding high toughness material to key interfaces in a laminate. This paper studies using polyester veil as a low-cost interleaf alternative to other materials and focuses on a nonwoven, polyester spunbond material. Two different interleaf thicknesses, 0.18 mm and 0.74 mm, are primarily used. In addition, fine 4 g/m2 polyester was also compared. Carbon/epoxy composites are manufactured using 2 × 2 Twill 24″–12k carbon fibers embedded in an epoxy resin, with polyester interleaves at key interfaces. Specimens are fabricated using wet hand layup and cured at room temperature in a vacuum bag. Mode I fracture toughness is measured using the double cantilever beam test and Mode II fracture toughness is examined using the end-notched flexure test. Further evaluation is made using static indentation and full penetration impact testing. Toughness is compared, and the resulting fracture surfaces are investigated. Significant improvement is seen in Mode I testing. Up to a factor of 4 increase in propagation energy per unit area resulted from the inclusion of the interleaf material. Smaller improvements were observed in Mode II, with the best cases showing an increase in propagation energy to maximum load by about a factor of 2 compared with control cases. The polyester interleaf significantly influences the fracture morphology observed in static indentation and full penetration tests. [ABSTRACT FROM AUTHOR]

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Millisecond (ms) pulsed laser drilling is currently state-of-the-art in producing acute angle film cooling holes over aero-engine and gas-turbine components made from uncoated nickel superalloys. After laser drilling, most of these components are coated with a high-temperature thermal barrier coating (TBC) to maintain the temperature of the component at a level appropriate for its application. It is desirable to produce holes over the TBC coated components; however, the current state-of-the-art ms laser drilling of acute angle holes over TBC coated materials results in a high level of coating delamination and hence is not used in manufacturing industries. The recent introduction of ms quasi-continuous wave (QCW) fiber lasers has had a significant impact on industrial laser drilling. This paper reports the results of a fundamental investigation carried out on ms QCW laser drilling of angular holes over TBC coated superalloys and provides a pragmatic solution to the issue of TBC delamination. In addition to the investigation on traditional percussion and trepanning laser drilling processes, a new method of drilling called "laser drilling post-laser TBC decoating" is evaluated with the aim of achieving delamination-free laser drilling of TBC coated aerospace alloy. [ABSTRACT FROM AUTHOR]

It is possible that the large oil and gas fields have already been discovered. The new discoveries are smaller, remote, deep water, or buried deep underground. Drilling deep wells often have to face the challenges from high temperature. It is especially challenging for drilling fluids to maintain desirable properties under a thermally-demanding environment over extended drilling time. This paper surveys the worldwide field cases of drilling fluids for high-temperature wells. According to field experiences, oil based mud and emulsion mud provides the best wellbore stability, thermal stability and rate of penetration (ROP). Water based formate mud is an alternative to oil based mud at a lower cost and less environmental impact. Formate mud also produced good wellbore integrity and good ROP. Foam mud is most suitable for drilling low-pressure formations with wellbore instability issues. The field case demonstrated foam mud was successfully implemented in a high-temperature and low-pressure well. [ABSTRACT FROM AUTHOR]

Over-melting and vaporization of resin matrix are usually found along a cut in the laser machining of carbon fiber reinforced plastic (CFRP). This is considered as the heat-affected zone (HAZ) and it has to be minimized to avoid the delamination of CFRP laminates. This paper presents the use of water flow to cool down the CFRP during the laser machining process. A thin flowing water layer induced by the impingement of low-pressure waterjet was formed on the workpiece surface, where a laser beam performed the ablation underneath the water layer. With this technique, the excessive heat and cut debris can be carried away from the workpiece by water. In this study, the effects of laser traverse speed, orientation of carbon fiber, water flow rate, and flow direction on cut dimensions and HAZ size were experimentally investigated. Using high water flow rate can limit the expansion of HAZ and also assist the material removal. In addition, the water flow directed along the laser traverse direction can increase the cut depth. The groove aspect ratio produced by the presented technique was found to be the same level as the laser ablation in air but the HAZ size was 20% smaller than the dry ablation. A predictive model for cut depth based on energy balance was also developed and analyzed in this study. The experimental findings and theoretical model presented in this work could enable a better understanding of the laser ablation in the flowing water layer and highlight the potential use of this technique for processing CFRP and other similar materials. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) composites are widely being used for structural applications due to their attractive properties. Particularly, to fabricate complicated structures by FRP composites, joining composite structure elements through fasteners require drilled holes. More than 60% of the structural FRP components are rejected owing to poor quality of holes. The FRP composites are heterogeneous due to multi-layer architecture, multi-phase material combinations, and hybridization. Hence, while drilling the behavior of FRP composites depends on various machining and tool parameters in addition to workpiece properties. As a result of drilling, the quality of drilled holes deteriorates due to delamination, matrix crack, matrix burning, fiber pull-out, buckling, spalling, and fuzzing in the drilled hole zone. To minimize all these defects researchers tried to design material, tool and machining parameters and reported. In this paper, the authors have made an attempt to summarize up-to-date progress in structure properties and drilling study of unidirectional and bidirectional multi-layered composites, three-dimensional fabric-reinforced composite and hybrid composite and similarly, emphasize the advanced characterization techniques to investigate the drilled hole quality. In addition, cryogenic assisted drilling, lubricated drilling, and micro-drilling literatures have been discussed. Based on the literature, conclusions and future scope have been reported. [ABSTRACT FROM AUTHOR]

We examined L-shaped structures prepared from ultrathin chopped carbon fiber tape reinforced thermoplastics (UT-CTT), attempting to mitigate their delamination and minimize interlaminar out-of-plane tensile stress using four-point bending tests and finite element modeling. The reasonable agreement between experimental and simulated results was achieved, and interlaminar tensile strength was obtained by finite element modeling and analytical methods. Additionally, free-shape optimization was utilized. The structural optimization problem was formulated using interlaminar tensile stress as the objective function (with constraints placed on structural stiffness and weight), and thickness variations of the inner parts of the L-shaped structure were used as design variables. Optimization reduced the interlaminar tensile stress of the L-shaped structure from 53.77 to 37.29 MPa while maintaining its original stiffness and weight, suggesting that the described approach is generally applicable to determining the interlaminar tensile strength of UT-CTT and can be used to increase the delamination initiation load. [ABSTRACT FROM AUTHOR]

Practical experience and laboratory observations indicate that corrosion affected reinforced concrete structures are prone more to cracking and delamination of the concrete than to the loss of structural strength. A review of research literature suggests that insufficient theoretical work on corrosion induced concrete delamination has been undertaken. This paper proposes a theoretical method to predict the time to concrete delamination caused by steel reinforcement corrosion. The method is based on fracture mechanics and uses crack opening in concrete as the criterion for its delamination. An analytical model is derived to determine the crack width in concrete. A numerical example is given to illustrate the proposed method and a comparison with both test results and an empirical model indicates a good agreement. It is found in the paper that the corrosion rate is the most significant single factor that affects the time to concrete delamination. The proposed method can be used for the prediction of time to concrete delamination for reinforced concrete structures located in chloride-laden environments. In principle, this provides a means for judging timely intervention and as such has the potential to prolong the service life of corrosion affected concrete structures. [ABSTRACT FROM AUTHOR]

The paper investigates thermal compatibility between wearing surface (WS) materials and glass fiber reinforced polymer (GFRP) bridge decks, and proposes a more durable hybrid WS system for GFRP decks. Wearing surface delamination problems observed on many existing GFRP bridge decks motivated the investigation and the search for a durable WS material that could alleviate the problems. Several WS materials were bonded to GFRP panels, with and without surface preparation, and tested under various environmental conditions. In addition to the standard ASTM C884 method, the testing program included two new methods for thermal compatibility testing to reflect the in-service conditions of WSs on GFRP bridge decks. The proposed methods were developed to account for the influence of freeze–thaw–heat and submerge–freeze cycles on thermal compatibility and durability. The investigation concluded that a hybrid WS system, consisting of two-layered WS materials, has the best bond quality. Applied directly on top of a GFRP deck, the top layer of the hybrid WS system had the best tire resistance, forming a nonskid riding surface. [ABSTRACT FROM AUTHOR]

This paper investigates the tensile and fatigue properties of a novel fiber-metal laminate (FML) based on a titanium alloy and carbon fiber-reinforced poly-ether-ether-ketone (PEEK). Tensile tests on unidirectional unnotched laminates have shown that their mechanical properties follow the predictions offered by a simple law of mixtures approach. Tension-tension fatigue tests on notched unidirectional FMLs have shown that these laminates offer fatigue lives up to fifty times greater than those offered by a notched monolithic titanium alloy. An examination of the failed FMLs highlighted the presence of delamination between the titanium alloy and the fiber-reinforced composite. The experimental evidence suggests that this form of damage is harmful during fatigue loading conditions. It has also been shown that delamination is more widespread in FMLs based on thick composite layers than in laminates containing thin composite layers. [ABSTRACT FROM AUTHOR]

This paper deals with a basic methodology for the application of the finite element (FE) simulation and linear elastic fracture mechanics (LEFM) to predict the onset of delamination growth. The first step of the proposed methodology is the experimental characterisation of material interlaminar fracture in pure mode I (DCB test), pure mode II (ENF test) and mixed mode I+II (MMB test). In the second step, the results of the interlaminar fracture tests are translated from load/displacement data to strain energy release rates by means of test FE simulations; thus, the critical strain energy release rate (Gc), or fracture toughness are obtained as a function of mode ratio. The third step of the methodology consists of applying FE simulations to determine the strain energy release rate (G) and mode ratio for the delamination and then comparing the result to the values of the Gc curve. The main innovative aspect of the paper is that FE simulation is used as a part of the material characterisation process; in fact, data reduction of interlaminar fracture tests is carried out by means of FE simulations. In addition, regarding the practical details of FE application to the proposed methodology, some innovative aspects are introduced, such as the analysis is carried out as a non-linear geometric problem (taken into account large displacements) and considering inside each simulation different elastic properties for the material depending on whether it is under tension or compression. These aspects and the rest of the proposed methodology are validated by the experimental results presented in the paper. [ABSTRACT FROM AUTHOR]