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Fatigue fracture of fiber-reinforced polymer composites (FRP) occurs when microcracks are induced by debonding, pull-out and delamination at the interface between the matrix and fiber. This microcrack area increases with increase in fatigue cycles and a damage region is formed. In our previous paper, fatigue life of a short fiber-reinforced polymer composite consisting of glass fibers and polycarbonate matrix was found to be related not to the main crack growth behavior but to the progression behavior of the damage region. In this paper, using our proposed real time observational system, we performed detailed observations on the behavior of fatigue damage and clarified the mechanism of damage progression. Furthermore, mechanical considerations were performed by finite-element elastic-plastic stress analysis. The results mentioned above indicate that control of short fiber alignment makes it possible to release the stress concentration caused in the matrix, and disperse fatigue damage. This results in an enormous improvement in fracture toughness. [ABSTRACT FROM AUTHOR]

In the field of cultural objects conservation efficient stabilisation of fragile and failing, multilayered decorative coatings is a complex and challenging task. This paper introduces a new application of the standardised double-cantilever beam (DCB) test method to improve the understanding of the mechanical properties of failing material and to determine the effect of polymeric agents (consolidants) added for their stabilisation. The adapted DCB method was used to measure the fracture energy, G, and the fracture behaviour of brittle, protein-bound (gesso-type) foundation layers on wooden substrates that typically suffer from delamination and flaking. Wooden DCB specimens containing a brittle layer of protein glue mixed with finely ground clay powder were prepared, fractured, then consolidated with a range of commonly used polymer formulations and finally re-fractured to provide measurements for direct comparison. Consolidants tested included gelatine-based glues (bovine hide glue, isinglass; both pre-stained with Fast Green dye), acrylics (Lascaux Medium for Consolidation, Paraloid B-72/B-48N), poly(vinyl acetates) (Mowilith 50, Mowilith DMC2) and poly(vinyl alcohol) (Mowiol 3-83). Before second-phase fracture cross-sections were taken from the DCB specimens for determining penetration depth and gap-filling ability. For better visibility, the specimens containing acrylics were stained with Solvent blue G dye; iodine-potassium iodide was used for staining the other synthetic consolidants. The resulting data showed that the test method could determine measurable differences between initial G (47 ± 22 J/m) and post-consolidation G values. Also, penetration behaviour could be well characterised and valuable, and detailed information on the type and location of crack path propagation was gained. [ABSTRACT FROM AUTHOR]

The paper examines the roughening along a metal-polymer interface, to find out whether the relevant length scale is on a sub-grain level or on the grain-size level. This is relevant for understanding the possible delamination of a polymer coating on a metallic substrate. Therefore we have investigated the local lattice orientation in heavily strained ferritic steel using electron back-scatter diffraction. From that data we have calculated the components of the local orientation gradient tensor as well as the local Schmid factor for deformation along [100] and [001] on {101} and {112} slip systems. The curvature of the draw-and-redraw steel- polyethylene terephthalate (PET) laminate interface as well as the curvature of the underlying steel lattice was examined in detail. It is concluded that roughening at a sub-grain length scale along the interface is due to plasticity in the interior of the grains. [ABSTRACT FROM AUTHOR]

The effect of z-pin surface treatment on the delamination fracture properties of z-pinned unidirectional carbon fibre/epoxy prepreg laminate is presented in this paper. Cryogenic and plasma treatments were used to increase the pin/composite interface properties. Z-pin pullout tests were carried out to study the relations between the bridging force and the displacement. Mode-I double-cantilever beam tests were used to characterize the improvements in delamination toughness. It was pointed out that appropriate treatments could effectively increase the delamination fracture properties. Oxygen-containing functional groups could be induced on the pin surface through cold plasma treatment. An increasing surface energy is improving the wettability so that more chemical reactions can be generated between the epoxy group and z-pin surface. Furthermore, the surface roughness of z-pins can be extended with a plasma or cryogenic treatment. The pins obtained a larger surface area, which could wet by the epoxy matrix during the z-pin-insertion and curing process. [ABSTRACT FROM AUTHOR]

multipurpose acid treatment, which comprises purification, delamination, particle size reduction, and surface modification on milled talc samples, is proposed. Talcs of different origins and composition were studied in order to assess and compare the effects of different acid treatments. Associated minerals were dissolved away by both inorganic and monocarboxylic organic acids, rendering more pure and lower particle size talcs. The main results obtained from hydrochloric acid treatment were purified talcs, having silanol groups onto their surfaces. This treatment allowed the complete removal of carbonates and chlorite from initial talcs, allowing for a length and thickness reduction of talc particles and the change to hydrophilic character by breakage of siloxane bonds. The so-treated talc achieved a purity level enough to be used in plastic and paper industry, paints among other upgraded talc applications. On the other hand, organic acid attacks were able to graft carboxylic groups onto the talc surface. The occurrence of this reaction was verified by independent characterization techniques both direct (Fourier Transform Infrared Spectroscopy, Energy Dispersive X-ray) and indirect (X-ray Diffraction, Scanning Electronic Microscopy, hydrophobicity/hydrophilicity test). [ABSTRACT FROM AUTHOR]

A high fidelity assessment of accumulative damage of woven fabric composite structures subjected to aggressive loadings is strongly reliant on the accurate characterization of the inherent multi-scale microstructures and the underlying deformation phenomena. Damage in composite sandwich and joint structures is characterized by the coexistence of discrete (delamination) and continuum damage (matrix cracking and intralaminar damage). A purely fracture mechanics-based or a purely continuum damage mechanics-based tool alone cannot effectively characterize the interaction between the discrete and continuum damage and their compounding effect that leads to the final rupture. In this paper, a hybrid discrete and continuum damage model is developed and numerically implemented within the LS-DYNA environment via a user-defined material model. The continuum damage progression and its associated stiffness degradation are predicted based on the constituent stress/strain and their associated failure criteria while the discrete delamination damage is captured via a cohesive interface model. A multi-scale computational framework is established to bridge the response and failure predictions at constituent, ply, and laminated composite level. The calculated constituent stress and strain are used in a mechanism-driven failure criterion to predict the failure mode, failure sequence, and the synergistic interaction that leads to global stiffness degradation and the final rupture. The use of the cohesive interface model can capture the complicated delamination zone without posing the self-similar crack growth condition. The unified depiction of the continuum and discrete damage via the damage mechanics theory provides a rational way to study the coupling effects between the in-plane and the out-of-plane failure modes. The applicability and accuracy of the damage models used in the hybrid dynamic failure prediction tool are demonstrated via its application to a circular plate and a composite hat stiffener subjected to shock and low velocity impact loading. The synergistic interaction between the continuum and discrete damage is explored via its application to a sandwich beam subjected to a low velocity impact. [ABSTRACT FROM AUTHOR]

A defect-free, two-material component can be obtained via co-sintering by suitably altering the powder characteristics or compositions, as demonstrated in Part I. In this paper, a model to ascertain the suitability of material systems to be co-sintered without defects such as delamination or interface pores is presented. The model is based on the management of the stress induced due to the difference in shrinkage and an analysis of the in situ strength of the weaker material during sintering. Tool steel in combination with stainless steel admixed with boron and Fe-2Ni admixed with boron are two systems used to validate the model. The predictions of the model are in good agreement with the observations. [ABSTRACT FROM AUTHOR]

Two material powder injection molding (PIM) is a recently developed method to manufacture functionally graded components. This paper describes an experimental technique to determine the suitability of two materials to be combined via PIM. This is accomplished by comparing the individual shrinkage versus temperature behavior of the candidate systems. The concepts are validated by two material PIM, sintering, and subsequent microstructural observation. Two materials are compatible for two material powder injection molding provided they form a metallurgical bond and the sintering response of one material mimics the other. An extensive difference in sintering shrinkage, especially during the initial stage of sintering, results in defects such as cracks and delamination. Success of these concepts is elucidated by two material PIM of tool steel and boron doped austenitic stainless steel. [ABSTRACT FROM AUTHOR]

This paper describes an investigation into the low velocity impact response on complex geometry sections taken from a pultruded glass/polyester product. The empirical impact behaviour of the system was evaluated using instrumented falling weight impact testing (IFWI) in conjunction with ultrasonic C-Scan, optical microscopy and thermal deply techniques to detect delamination, matrix cracking, and fibre breakage. [ABSTRACT FROM AUTHOR]

This paper describes an investigation into the low velocity impact response of relatively simple geometry coupons taken from a pultruded glass/polyester product. The pultruded lay-up is a typical three ply design – a unidirectional fibre ply sandwiched between two continuous filament mat plies. By employing a simple geometry the impact response under transverse and longitudinal bending, and shear loading are considered. Impact tests from low energy through to final failure were conducted using the instrumented falling weight impact test technique. Through a detailed analysis involving ultrasonic C-scan, optical microscopy, and thermal deply all the major damage modes (delamination, fibre breakage and matrix cracking) were related to the force-deflection responses. [ABSTRACT FROM AUTHOR]

The intent of this paper is to investigate the mechanics of delamination growth in an isotropic, ideally-brittle layered material under an impact load and to present guidelines for the design of more energy absorbent structures. Using a finite element model, both two- and three-layered configurations are considered. Prompted by the results from this model, closed-form solutions for the interlaminar shear strength and the interfacial fracture toughness corresponding to maximum energy absorption are derived for the two-layered laminate. These solutions were used to develop guidelines for the optimal design of two- and three-layered laminates. [ABSTRACT FROM AUTHOR]

In this work the microstructure of interfaces present in heavily bi-axially deformed polymer-coated metal is studied. Cross sections of deformed polymer-coated steel are prepared using several polishing strategies, including the use of focused ion beam, and are imaged using optical microscopy and scanning electron microscopy. We find that the interfaces show significant details right down to the smallest scale observable with the preparation techniques used of about ~10 nm. Local delamination events at these deformed interfaces are observed and are found to be preferentially associated with overhanging parts on the interface. Overhanging parts are frequently observed, but only below a certain length-scale on the interfaces that are otherwise found to be self-affine up to a certain correlation length. The smallest detail includes the tail of the size distribution of the overhanging features. Together this suggests that the physical mechanisms determining the formation of critical features for adhesion operate at sub-grain level as well as at grain level. [ABSTRACT FROM AUTHOR]

Polymeric foams are extensively used as the core materials in sandwich structures and the core material is typically bonded between relatively thin fibre-composite skins. Such sandwich structures are widely used in the aerospace, marine and wind-energy industries. In the present work, various sandwich structures have been manufactured using glass-fibre-reinforced polymer (GFRP) skins with three layers of poly(vinyl chloride) foam to form the core, with the densities of the foam layers ranging from 60 to 100 kg/m3. This study has investigated the effects on the quasi-static flexural and high-velocity impact properties of the sandwich structures of: (a) the density of the polymeric-foam core used and (b) grading the density of the foam core through its thickness. The digital image correlation technique has been employed to quantitatively measure the values of the deformation, strain and onset of damage. Under quasi-static three-point and four-point bend flexural loading, the use of a low-density layer in a graded-density configuration reduced the likelihood of failure of the sandwich structure by a sudden force drop, when compared with the core configuration using a uniform (i.e. homogenous) density layer. The high-velocity impact tests were performed on the sandwich structures using a gas-gun facility with a compliant, high-density polyethylene projectile. From these impact experiments, the graded-density foam core with the relatively low-density layer located immediately behind the front (i.e. impacted) GFRP skin was found to absorb more impact energy and possess an increased penetration resistance than a homogeneous core structure. [ABSTRACT FROM AUTHOR]

Fibrous filters were produced using a novel melt-based co-extrusion and two-dimensional multiplication technology combined with a delamination technique using high-pressure water jets. The fibrous filters produced comprising continuous and rectangular polypropylene (PP)/polyamide 6 (PA6) micro/nano-fibers have structural integrity and uniform fiber distribution. The orientation procedure greatly improves the PP/PA6 crystal orientation, decreases the fiber sizes, and enhances their mechanical performance as filters. These filters have large surface area, micron-sized pores, and high porosity (~90 %), which are desirable for microfiltration applications. Structural and property analysis was performed on the PP/PA6 fibrous filters produced from varying number of plies of the composite tapes with different draw ratios and fiber width-to-thickness ratios. It was found that increasing the tape draw ratio improves the surface area and porosity of the filters, and decreases its pore size. Using more plies generates decreased filter pore size and unaffected porosity. Filters comprising fibers with higher width-to-thickness ratio have higher surface area, smaller pore size, and unchanged porosity. This melt-based, versatile technology is applicable to any melt-processable polymers to produce fibrous filters having tunable properties for various filtration applications. [ABSTRACT FROM AUTHOR]

The effect of size of silicon carbide particles on the dry sliding wear properties of composites with three different sized SiC particles (19, 93, and 146 μm) has been studied. Wear behavior of Al6061/10 vol% SiC and Al6061/10 vol% SiC/5 vol% graphite composites processed by in situ powder metallurgy technique has been investigated using a pin-on-disk wear tester. The debris and wear surfaces of samples were identified using SEM. It was found that the porosity content and hardness of Al/10SiC composites decreased by 5 vol% graphite addition. The increased SiC particle size reduced the porosity, hardness, volume loss, and coefficient of friction of both types of composites. Moreover, the hybrid composites exhibited lower coefficient of friction and wear rates. The wear mechanism changed from mostly adhesive and micro-cutting in the Al/10SiC composite containing fine SiC particles to the prominently abrasive and delamination wear by increasing of SiC particle size. While the main wear mechanism for the unreinforced alloy was adhesive wear, all the hybrid composites were worn mainly by abrasion and delamination mechanisms. [ABSTRACT FROM AUTHOR]

Time-dependent intercellular delamination behavior of human stratum corneum (SC) is explored. We demonstrate that SC is susceptible to such time-dependent subcritical delamination at loads below the critical delamination energy. Hydration of the SC together with chemical treatments using selected surfactants were shown to affect both critical and subcritical delamination behavior. Increased tissue hydration resulted in accelerated delamination growth rates consistent with behavior anticipated from critical delamination testing. Chemical treatments including 10% wt/wt sodium dodecyl sulfate (SDS) and chloroform-methanol (2:1 v:v) soaks that remove lipid were shown to suppress growth rates compared to untreated controls. Possible mechanisms for subcritical delamination involving extrinsic mechanisms of moisture-assisted reaction process at the intercellular delamination tip or intrinsic mechanisms involving kinetic relaxation processes in the SC are discussed. [ABSTRACT FROM AUTHOR]

Recently, very thick section laminates, up to 20 mm in thickness, have been proposed for the wing skins of large aircraft. Composite components in all aircraft have concerns relating to the presence of accidental damage, but there has been little work to investigate the mechanisms and effects of damage in such thick sections. In this work, carbon fibre composite laminates of up to 12 mm thickness have been subjected to dropped-weight impacts of at most 375 J. Two types of impacts were considered. The first is a central impact where the laminate is completely supported and the second a near edge impact where the laminate is partially supported so that one of its edges is free. The geometry of the damage has been studied using C-scan and deply techniques. The residual strengths of the impact-damaged laminates have been measured in tension and compression. The geometry of damage and level of strength reduction is different for central and edge impacts. Generally, an edge impact causes a greater reduction in compressive strength while a central impact causes more tensile strength reduction. [ABSTRACT FROM AUTHOR]

In this work, the interfacial response of a glass-based coating on Ti6Al4V to monotonic and cyclic Hertzian (spherical) indentation is investigated. This coating belongs to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system and it is specifically designed to be used as the inner layer of a bioactive bilayer coating with an outer layer of lower SiO2 content to ensure bioactivity. During Hertzian monotonic loading, delamination of the coating occurs, which is revealed in the microscope by the presence of a colour pattern of interference fringes at the interface. Hertzian cyclic loading at maximum loads, P max, lower than the monotonic delamination load, P Del, also generates delamination damage. A plot of P max versus the number of cycles for delamination shows a two-slope curve with a “knee” for P max close to the critical load to induce a radial crack from the interface, P Rc. The analysis of the delamination morphology and the results for different load ratios, R = P min/ P max, confirmed the existence of two different delamination mechanisms with a common feature of plastic deformation of the substrate but with a different dependence with the maximum applied load: for P max > P Rc the process is mostly controlled by the presence of the radial cracks ( P max dependent), meanwhile, for P max < P Rc radial cracks are not observed and delamination is attributed to the residual stress at the interface induced by the cyclic plastic deformation of the substrate and the elastic recovery of the coating during unloading part. [ABSTRACT FROM AUTHOR]

This article reports that in ceramic multilayered composites, consisting of layers of different composition, residual stresses are generated by different sintering strain rate between layers and thermal expansion mismatch between the layers during cooling down from sintering temperature. In Al2O3/Al2O3+3Y-TZP multilayered composites, layers with the lower thermal expansion coefficient (CTE), that is Al2O3, undergo compressive residual stress while tensile stress appears in Al2O3+3Y-TZP (AZ) layers with higher CTE. This stress field directly affects the mechanical properties of the composites and it is responsible for cracking and/or delamination. In addition, spontaneous cracking upon cooling is sometime observed as a result of different sintering strain rate and excessive mismatch in CTE between layers.

A simplified method for determining the individual mode components of the strain energy release rate of free-edge delaminations in composite laminates is proposed. Interlaminar stresses are evaluated as an interface moment and interface shear forces obtained from equilibrium equations of stress resultants at the interface between the adjacent layers. The deformation of edge-delaminated laminate is calculated by using a generalized quasi-three dimensional classical laminated plate theory developed by the authors. The analysis provides closed-form expressions for the Mode-I, Mode-II and Mode-III component of the strain energy release rate by combining the deformation of the edge-delaminated laminate with the interface moment and the interface shear forces. The presented method is compared with existing method suggested by Li for the asymmetry laminate. Comparison of the results with a finite element analysis using the virtual crack closure technique shows good agreement. [ABSTRACT FROM AUTHOR]

Vapor pressure and thermal stress induced growth and coalsecence of microvoids near interfaces of die/die-attach and die-pad/moulding compound is recognised as the precursor to interface delamination and popcorn cracking of plastic electronic packages. When exposed to humid ambient conditions prior to reflow soldering, plastic electronic packages absorb moisture, which condenses within the numerous micropores in the substrate, die attach, moulding compounds and in the vicinity of interfaces joining the different material layers. During reflow soldering, moisture in these micropores is vaporized. A representative material cell containing a single microvoid is used to investigate void growth under combined vapor pressure and thermal stress. A critical surface traction, the sum of the internal vapor pressure and the externally applied stress, defines the onset of unstable void growth. Existence of “large” microvoids lowers significantly the critical stress levels for unstable void growth. These stress levels are consistent with generally accepted estimates of stress levels for popcorn cracking given in the literature. Vapor pressure effects can be incorporated into a continuum description of stresses and strains and the Gurson-Tvergaard void growth model providing a new capability for full-field analysis of popcorn cracking in electronic packages. [ABSTRACT FROM AUTHOR]

Nondestructive evaluation of thermal stress-induced failure in the free edge region of thin composite laminates (1 mm thick) has been carried out. The damage zone which arose under a cryogenic cooling was examined using the ultrasonic C-scan and optical microscopy. Characteristic behavior of thermo-acoustic emission (AE) detected from the composite specimen during the heating and cooling cycle indicated the degree of damage which had occurred differently for as-moulded and cryogenically-treated specimens with various lay-up angles. Analysis of the vibration spectrum data obtained with short beam specimens exhibited that the failure occurrence caused an obvious decrease in resonant frequency and some considerable increase in flexural damping ratio. Total AE energy showed a feature similar to the flexural damping ratio which increased with increasing crack density. Consequently, it is thought that thermo-acoustic emission in association with flexural damping might be employed as a tool for nondestructive evaluation of thin composite laminates. [ABSTRACT FROM AUTHOR]

The friction and wear characteristics of low-pressure plasma-sprayed (LPPS) ZrO2-Cr2O3-CaF2 from room temperature to 800°C were studied by using a high-temperature reciprocating wear tester. At room temperature, friction of ZrO2-Cr2O3-CaF2 composite coating against Al2O3 sphere was quite high, and exhibited a decrease trend with the increase of load from 30 N to 80 N. At 700°C, the composite exhibited the lowest friction and wear among all selected temperature conditions. Brittle fracture and delamination with large wear sheets were considered as the dominant wear mechanism at room temperature. At 400°C, microfracture dropping become more dominated. However, plastic deformation, formation of CaF2 lubrication films and particle removal of Cr2O3 appeared as the main wear mechanisms at elevated temperatures. CaF2 acting as an effective lubricant at 600°C and 700°C reduced the friction and wear of the composite. Cr2O3 particles played a very important role on controlling the size and type of microcracks and preventing or deflecting the microcrack propagation during 700°C test. Cr2O3 particles also acted as hard barriers to resist the scratching and high-temperature deformation of ZrO2-CaF2 matrix without increasing the friction. To a great extent, wear mainly depended on the degree of debonding and removal of Cr2O3 particles at 700°C. [ABSTRACT FROM AUTHOR]

The dry sliding wear behavior of spray-deposited Al-Cu-Mn alloy and its composite reinforced with 13 vol.% SiC particles have been studied in the applied load of 5–400 N (corresponding normal stress is 0.1–8 MPa). It showed that SiC particle-reinforced AlCuMn composite produced by spray deposition process exhibited an improved wear resistance at the entire applied load range in comparison to the monolithic alloy. However, this improvement was not significant in the overall load range. With increasing the applied load, the wear rate of the composite and the monolithic alloy exhibited four different regions, therefore the wear was dominated by different wear mechanism. The former three regions all belonged to mild wear. The transition from mild to severe wear occurred at the similar critical load for both the composite and the monolithic alloy. For both the composite and the monolithic alloy, with increasing applied load, the dominant wear mechanism exhibited successively: oxidative mechanism, delamination mechanism, subsurface-cracking-assisted adhesive mechanism and adhesive mechanism. [ABSTRACT FROM AUTHOR]

Wear resistance of Titanium Nitride (TiN) coating on steel is partly influenced by delamination located beneath the surface. Acoustic microscopy is particularly suited to analyze the subsurface defects. To examine the delamination defects introduced by sliding contacts in TiN coating on steel the Lamb wave velocities have been measured by V(z) curves and compared with the theoretically calculated velocities from dispersion curves. [ABSTRACT FROM AUTHOR]

Resin Transfer Molded Composites exhibit impact induced damage mechanisms and sequences different from those shown by laminated composites due to differences in layering and compaction of reinforcement. In addition to the classical modes of damage such as matrix cracking, delamination and fiber breakage, mechanisms such as inter-bundle, intra-bundle and void pocket cracking are also seen. The presence of damage types relative to the impact regime is discussed with reference to regions of the Inelastic Energy Curve for Impact. Fiber tow level based damage evolution is also investigated, and damage mechanisms and sequences are elucidated for E-glass/vinylester plain weave based composites. [ABSTRACT FROM AUTHOR]

Mode II delamination phenomena of woven fabric carbon/epoxy composites were investigated by scanning electron microscopy. End notch flexural (ENF) test was used to examine the mode II delamination. Woven fabric composites showed two peculiar crack propagation patterns due to the complexity of woven geometry. In warp yarn region, crack propagated with forming a shear band and breaking the fiber/matrix interface. In fill yarn region, however, no shear band was observed. Considering these crack patterns, matrix shear property and fiber/matrix interfacial strength played an important role in enhancing the delamination properties of woven fabric carbon/epoxy composites. Due to the woven geometry, matrix rich positions, which are interstitial and undulated region, were formed in woven carbon/epoxy composite. In these regions, matrix fracture and complex crack path were mainly observed. [ABSTRACT FROM AUTHOR]

A two layer electron beam-physical vapor deposited (EV-PVD) thermal barrier coating (TBC) on a single crystal superalloy (René N5) substrate was characterized prior to and after thermal cycling at 2, 18, 25, 44, 50, 75, 100, 110, 150, and 175 cycles in between 200 C-1177 C. Optical microscopy, scanning electron microscopy, and thermal wave imaging techniques were used to characterize the interfacial damage. Pt-Al was used as bond coat and 8 wt % YSZ was used as outer top layer. Interfacial cracking was observed even at two thermal cycles. Thermally grown oxide (TGO) layer increased with the number of thermal cycles. After numerous cycles over 100, interfacial separation was observed to be higher at the middle than at the edges of the sample. This observation is consistent with buckling induced delamination—a possible mechanism for spallation. [ABSTRACT FROM AUTHOR]