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Abstract This paper analyzes the competition between kink nucleation and interface fracture for an interface crack (without a putative kink flaw) subject to mixed-mode loading. The simulations utilize a distributed cohesive zone approach that embeds cohesive elements throughout the entire mesh; dynamic crack path evolution occurs through a loss of cohesive traction associated with a critical separation between elements. The simulations identify mesh densities that lead to mesh-independent results for randomly oriented triangular meshes, and provide clear guidelines regarding parameters that recover toughness-controlled cracking (i.e. linear elastic fracture mechanics). The results demonstrate that, when the when the normalized bulk toughness is far from the transition between kinking and delamination, crack direction and critical loads are identical to those predicted by He and Hutchinson, who analyzed cracking from a putative flaw associated with the maximum energy release rate. Near the transition between fracture modes, kink nucleation depends on the relative size of the interface and bulk process zones, such that additional criteria are needed (beyond those postulated by He and Hutchinson). Regime maps are presented which indicate regions of kink nucleation versus delamination as a function of controlling cohesive parameters. [ABSTRACT FROM AUTHOR]

Hydrogels are becoming increasingly attractive for various practical applications owing to significant improvements in their fracture resistance over the past two decades. Notably, almost all existing methods for enhancing the fracture resistance of hydrogels aim to increase the fracture toughness Γ through multiscale material design. However, engineering the toughness of hydrogels at the material level often requires material-specific, complicated, or expensive synthesis processes. According to the fracture condition G = Γ , reducing the energy release rate G under given mechanical loads is an alternative way to enhance the fracture resistance. Little efforts, however, have been made to use this approach to enhance the fracture resistance of soft materials such as hydrogels so far, largely due to their highly nonlinear and deformable behavior. This paper formulates a theory for studying the fracture of monolayer and bilayer hydrogel thin films and proposes a structure-based strategy for enhancing the fracture resistance of hydrogels by constructing hydrogel films adhered to underlying stretchable substrates. Through both theoretical analysis and experimental validations, we demonstrate that the strategy exhibits several unique advantages: even a thin and compliant substrate can enhance the fracture resistance of hydrogel films by several folds, and make the fracture resistance of hydrogel films independent of the crack length and the hydrogel size – which dramatically affects the fracture behavior of monolayer hydrogels. The underlying mechanism lies in that the stretchable substrates can effectively constrain the crack opening displacements and reduce the energy release rate G for crack propagation in the hydrogel film. Moreover, the influence of experimentally-observed interfacial delamination on the fracture resistance of substrate-supported hydrogels is also investigated. The present work unveils a first-of-its-kind "structural-toughening" strategy for improving the fracture resistance of hydrogels by structurally regulating the energy release rate. [ABSTRACT FROM AUTHOR]

Fiber reinforced polymer (FRP) retrofitted concrete is a composite material system that is commonly used in construction for its improved strength properties or for rehabilitation of concrete structures. This material system is composed of a concrete substrate on which FRP is adhered with an epoxy matrix. When the system is subject to adverse mechanical or environmental effects, delamination of the FRP or damage at the FRPconcrete interface may occur, yet remain externally undetected because the damage is obscured by the FRP. In this paper, a robust standoff technique for finding discontinuities in FRP-retrofitted concrete at the FRP-concrete interface using a combined acoustic-laser vibrometry technique is presented. Experiments were conducted on a test specimen with a cubic discontinuity at the FRPconcrete interface. The frequency signature of the discontinuity, which provides a technique for determining the approximate size of the discontinuity, was found to be in agreement with theory. An image was constructed with a series of measurements that show the boundary and spatial vibrational behavior of the discontinuity. From that series of measurements a probability of detection and receiver operating characteristic curve was determined for the methodology. Estimates of the area rate of coverage of measurement for an operational system and suggestions for future studies are given. [ABSTRACT FROM AUTHOR]

Abstract: Copper–rubber interfaces show extensive rubber fibrillation at the micro-scale during delamination. This constitutes a major problem for identifying unambiguous interface properties, as the micromechanics of rubber fibrillation is not taken into account in conventional (macroscopic) interface characterization methods. As a result, a significant mismatch exists between micromechanical experimental observations, such as fibril length, and the obtained macroscopic cohesive zone parameters. In this paper, the fibrillation micromechanics is explicitly taken into account in the macroscopic interface description through the use of a multi-scale interface model by means of computational homogenization. In the micro-model physical small scale phenomena such as fibril deformation and debonding are taken into account. The resulting macroscopic cohesive zone parameters are quantitatively coupled to the micromechanical quantities. The micro-model results show the growth of a fibril, including the drawing of material from the bulk into the fibril, which is accompanied by large strains that are well beyond typical bulk strain values. The micromechanical contributions to the macroscopic work-of-separation are identified. It is concluded that the intrinsic adhesion energy only constitutes a small amount of the total dissipated energy. The main fraction of the work-of-separation consists of elastically stored energy in the fibril, that is dissipated through dynamical release upon instantaneous fibril debonding. Up to the moment of debonding, the fibril micromechanics are virtually unaffected by the intrinsic adhesion properties. The influence of the intrinsic adhesion parameters on the moment of debonding is shown. [Copyright &y& Elsevier]

This paper discusses the application of air-coupled ultrasonic testing for the inspection of commercial carbon-carbon (C/C) composite aircraft brake disks. Several tests were performed using air-coupled ultrasonic transducers in a throughtransmission mode by varying scan increments and resolutions. The test samples were fitted into polystyrene foam cutouts to reduce the amount of signal bypassing from the edge of the test specimens. A calibration was performed at 80% of the through-transmitted ultrasonic amplitude signals. This was carried out on a discontinuity-free area for each C/C disk tested to ensure accuracy in the results. It was found that a testing frequency of 125 kHz at a 2 mm step size provided the best results for C/C disks. The relative throughtransmitted signal drop was approximately - 1 8 dB in the discontinuous areas. This allowed for easy distinction between the discontinuous and nondiscontinuous regions in the C/C disks. After the inspection, the disks were cut open around these regions for visual inspection. It was revealed that regions in the C/C disks with significant attenuation of the through-transmitted ultrasonic signals had many delaminations and cracks. [ABSTRACT FROM AUTHOR]

Abstract: This work examines the performance of composite panels when subjected to underwater impulsive loads. The scaled fluid–structure experimental methodology developed by Espinosa and co-workers was employed. Failure modes, damage mechanisms and their distributions were identified and quantified for composite monolithic and sandwich panels subjected to typical blast loadings. The temporal evolutions of panel deflection and center deflection histories were obtained from shadow Moiré fringes acquired in real time by means of high speed photography. A linear relationship of zero intercept between peak center deflections versus applied impulse per areal mass was obtained for composite monolithic panels. For composite sandwich panels, the relationship between maximum center deflection versus applied impulse per areal mass was found to be approximately bilinear but with a higher slope. Performance improvement of sandwich versus monolithic composite panels was, therefore, established specially at sufficiently high impulses per areal mass (I 0/M¯>170ms−1). Severe failure was observed in solid panels subjected to impulses per areal mass larger than 300ms−1. Extensive fiber fracture occurred in the center of the panels, where cracks formed a cross pattern through the plate thickness and delamination was very extensive on the sample edges due to bending effects. Similar levels of damage were observed in sandwich panels but at much higher impulses per areal mass. The experimental work reported in this paper encompasses not only characterization of the dynamic performance of monolithic and sandwich panels but also post-mortem characterization by means of both non-destructive and microscopy techniques. The spatial distribution of delamination and matrix cracking were quantified, as a function of applied impulse, in both monolithic and sandwich panels. The extent of core crushing was also quantified in the case of sandwich panels. The quantified variables represent ideal metrics against which model predictive capabilities can be assessed. [Copyright &y& Elsevier]

Nondestructive testing of thick multilayered structures is challenging because of the number of layers and the increased plate thicknesses. In this paper, ultrasonic guided waves are applied to detect and characterize delaminations inside a 23-layer hybrid structural laminate that consists of metatlic, glass fiber composite and adhesive materials. A semianalytical finite element (SAFE) technique is used to generate dispersion curves and wave structures for the purpose of selecting appropriate wave structures and hence test points for discontinuity detection and characterization. One guided wave mode and frequency is chosen as an example to achieve Large in-plane particle displacements at regions of interest. The high sensitivity of the selected guided wave mode and frequency is first validated in a finite element model that simulates the interactions of the selected mode with a discontinuity. Theoretically driven experiments are then conducted and compared with bulk wave measurements. It is shown that guided waves can well detect and characterize deeply embedded damages inside thick multilayer fiber-metal laminates with suitable mode and frequency selection. [ABSTRACT FROM AUTHOR]

Abstract: The massive use of composites in primary structures of modern commercial aeroplanes is leading to the need for profound knowledge of the mechanisms of damage and failure of heterogeneous materials of this type. This knowledge requires a very fine evaluation of the stress state, which in turn is needed to calculate the parameters of fracture mechanics used to evaluate the integrity or damage tolerance of a particular component. In this paper, the initiation and growth of the damage in a typical [0/90] S laminate is investigated. Using a BEM code allowing contact between the lips of interfacial cracks appearing in the damaged configuration and based on a former generalized plane strain formulation of the problem developed by the authors, the energy release rate (ERR) is calculated for different configurations corresponding to the different intermediate states that may occur in the process of damage generation. The numerical results obtained clarify some aspects of the progression of the damage and open up the possibility, using comparison with experimental results, of understanding, and consequently improving, the performance of composite materials. [Copyright &y& Elsevier]

In this paper, a mixed numerical–experimental approach is adopted to quantitatively investigate and characterize delamination in polymer coated steel. The integral bi-material system is analysed and special attention is given to the constitutive modelling of the polymer coating, the interface and the determination of all involved parameters. An extended cohesive zone model for large displacements is proposed, allowing for a mode-dependent behaviour in large deformations. Finally, peel tests are used to characterize the interface, whereby the interfacial properties are determined through an inverse parameter identification procedure. [Copyright &y& Elsevier]

The effect of transverse shear on delamination in layered, isotropic, linear-elastic materials has been determined. In contrast to the effects of an axial load or a bending moment on the energy-release rate for delamination, the effects of shear depend on the details of the deformation in the crack-tip region. It therefore does not appear to be possible to deduce rigorous expressions for the shear component of the energy-release rate based on steady-state energy arguments or on any type of modified beam theory. The expressions for the shear component of the energy-release rate presented in this work have been obtained using finite-element approaches. By combining these results with earlier expressions for the bending-moment and axial-force components of the energy-release rates, the framework for analyzing delamination in this type of geometry has been extended to the completely general case of any arbitrary loading. The relationship between the effects of shear and other fracture phenomena such as crack-tip rotations, elastic foundations and cohesive zones are discussed in the final sections of this paper. [Copyright &y& Elsevier]

Delamination, a typical mode of interfacial damage in laminated composites, has been considered in the context of continuum damage mechanics in this paper. Interfaces where delaminations could occur are introduced between the constituent layers. A simple but appropriate continuum damage representation is proposed. A single scalar damage parameter is employed and the degradation of the interface stiffness is established. Use has been made of the concept of a damage surface to derive the damage evolution law. The damage surface is constructed so that it combines the conventional stress-based and fracture-mechanics-based failure criteria which take account of mode interaction in mixed-mode delamination problems. The damage surface shrinks as damage develops and leads to a softening interfacial constitutive law. By adjusting the shrinkage rate of the damage surface, various interfacial constitutive laws found in the literature can be reproduced. An incremental interfacial constitutive law is also derived for use in damage analysis of laminated composites, which is a non-linear problem in nature. Numerical predictions for problems involving a DCB specimen under pure mode I delamination and mixed-mode delamination in a split beam are in good agreement with available experimental data or analytical solutions. The model has also been applied to the prediction of the failure strength of overlap ply-blocking specimens. The results have been compared with available experimental and alternative theoretical ones and discussed fully. [Copyright &y& Elsevier]

• The Paris-Erdogan equation is obtained from engineering S-N curves via a linear cohesive law. • The exponent of the Paris-Erdogan equation is half of the inverse of that in the S-N curve. • The length of the process zone in fatigue is always shorter than under static loading. • The length of the process zone in fatigue is controlled by the S-N curve slope. This paper presents a unified cohesive zone formulation for describing the fatigue-driven crack onset and propagation in quasi-brittle materials. This approach is based on formulating a modified traction-displacement law involving linear softening, which accounts for the onset of fatigue failure described by materials S-N curves. The fatigue crack propagation in a steady-state regime is described as a sequence of initiation stages that progressively take place within the process zone ahead of the "zero-stress" fracture tip. It is demonstrated that the damage distribution within the process zone under cyclic load is governed by a non-linear Fredholm integral equation of the second kind, which is solved numerically using an iterative scheme. It is also shown that the Paris-Erdogan law for fatigue crack growth can be directly obtained from engineering S-N curves via the linear traction displacement model introduced here. The Paris-Erdogan propagation regime is attained when the crack propagation rate is much smaller that the length of the process zone. The exponent of the Paris-Erdogan law is equal to half the inverse of that characterising the material S-N curve. The main advantage of the cohesive zone model introduced here is that it does require neither ad-hoc additional parameters nor calibration constant to obtain the Paris-Erdogan law from material S-N data. It is also proved that the cohesive zone length in fatigue depends on the exponent of the S-N curve and it always shorter than its static counterpart. The unified cohesive zone model is validated against experimental data for the specific cases of mode I and mode II fatigue delamination growth in the carbon/epoxy composite material IM7/8552. Finally, an analysis of the role played by size effects is presented, with emphasis on the influence of the laminate thickness on the fatigue damage accumulation within the process zone and the ensuing propagation rates of cohesive cracks. [ABSTRACT FROM AUTHOR]