Finite element (FE) calculations are used to develop a comprehensive understanding of the dynamic response of sandwich beams subjected to underwater blast loading, including the effects of fluid–structure interaction. Design maps are constructed to show the regimes of behaviour over a broad range of loading intensity, sandwich panel geometry and material strength. Over the entire range of parameters investigated, the time-scale associated with the initial fluid–structure interaction phase up to the instant of first cavitation in the fluid is much smaller than the time-scales associated with the core compression and the bending/stretching responses of the sandwich beam. Consequently, this initial fluid–structure interaction phase decouples from the subsequent phases of response. Four regimes of behaviour exist: the period of sandwich core compression either couples or decouples with the period of the beam bending, and the core either densifies partially or fully. These regimes of behaviour are charted on maps using axes of blast impulse and core strength. The simulations indicate that continued loading by the fluid during the core compression phase and the beam bending/stretching phase cannot be neglected. Consequently, analyses that neglect full fluid–structure interaction during the structural responses provide only estimates of performance metrics such as back face deflection and reaction forces at the supports. The calculations here also indicate that appropriately designed sandwich beams undergo significantly smaller back face deflections and exert smaller support forces than monolithic beams of equal mass. The optimum transverse core strength is determined for minimizing the back face deflection or support reactions at a given blast impulse. Typically, the transverse core strength that minimizes back face deflection is 40% below the value that minimizes the support reaction. Moreover, the optimal core strength depends upon the level of blast impulse, with higher strength cores required for higher intensity blasts.

A diamondlike carbon (DLC) thin film was deposited onto a stainless steel substrate using a plasma-enhanced chemical vapor deposition (PECVD) process. Nanoindentation, coupled with focused-ion-beam (FIB) milling, was used to investigate contact-induced deformation and fracture in this coating system. Following initial elastic contact between the coating and the indenter and apparent plastic yield of the substrate, pop-ins were observed in the load–displacement curve, indicative of coating fracture. However, FIB cross-sectional images of indentations revealed the presence of ring, radial, and lateral cracks at loads much lower than the critical load for the first observed pop-ins. Finite element modeling was used, and the properties of the substrate and the film were calibrated by fitting the simulated load–displacement curves to experimental data. Then, based upon the experimental observations of damage evolution in this coating system, the stress distributions relevant to initiate ring, radial, and lateral cracks in the coating were ascertained. Furthermore, the effects of substrate yield stress and coating residual stress on the formation of these cracks were investigated.

The dynamic out-of-plane compressive response of stainless steel corrugated and Y-frame sandwich cores have been investigated for impact velocities ranging from quasi-static to 200ms−1. Laboratory-scale sandwich cores of relative density 2.5% were manufactured and the stresses on the front and rear faces of the dynamically compressed sandwich cores were measured using a direct impact Kolsky bar. Direct observational evidence is provided for micro-inertial stabilisation of both topologies against elastic buckling at impact velocities below 30ms−1. At higher impact velocities, plastic waves within the core members result in the front face stresses increasing with increasing velocity while the rear face stresses remain approximately constant. While the finite element calculations predict the rear face stresses and dynamic deformation modes to reasonable accuracy, the relatively slow response time of the measurement apparatus results in poor agreement between the measured and predicted front face stresses. The finite element calculations also demonstrate that material strain-rate effects have a negligible effect upon the dynamic compressive response of laboratory-scale and full-scale sandwich cores.

Fatigue crack propagation was investigated in homogeneous and graded alumina–epoxy composite specimens, produced via a multi-step infiltration technique. Crack-extension toughening was observed in homogeneous composite specimens under monotonic loading and attributed to the development of a bridging zone behind the crack-tip. A similar increase in crack propagation resistance was observed under cyclic loading. This was quantified, in an approximate manner, using an adjustment based on the Paris-law relation. Cracks in graded composite specimens underwent deflection and significant variations in crack-propagation resistance were observed as the crack traversed the graded region. The adjustment procedure provided a useful approximate method for characterising these variations, which resulted from crack-extension effects and the spatial variation of both intrinsic and extrinsic crack-growth resistance.

An analytical model is developed to classify the impulsive response of sandwich beams based on the relative time-scales of core compression and the bending/stretching response of the sandwich beam. It is shown that an overlap in time scales leads to a coupled response and to the possibility of an enhanced shock resistance. Four regimes of behaviour are defined: decoupled responses with the sandwich core densifying partially or completely, and coupled responses with partial or full core densification. These regimes are marked on maps with axes chosen from the sandwich beam transverse core strength, the sandwich beam aspect ratio and the level of blast impulse. In addition to predicting the time-scales involved in the response of the sandwich beam, the analytical model is used to estimate the back face deflection, the degree of core compression and the magnitude of the support reactions. The predictions of the analytical model are compared with finite element (FE) simulations of impulsively loaded sandwich beams comprising an anisotropic foam core and elastic, ideally plastic face-sheets. The analytical and numerical predictions are in good agreement up to the end of core compression. However, the analytical model under-predicts the peak back face deflection and over-predicts the support reactions, especially for sandwich beams with high strength cores. The FE calculations are employed to construct design charts to select the optimum transverse core strength that either minimises the back face deflections or support reactions for a given sandwich beam aspect ratio or blast impulse. Typically, the value of the transverse core strength that minimises the back face deflection also minimises the support reactions. However, the optimal core strength depends on the level of blast impulse, with higher strength cores required for greater blasts.

Layered, graded material structures exhibit anisotropies in elastic properties and failure resistance that can strongly influence the propagation of cracks, and hence overall structural integrity. This study examines the influences on propagation trajectory for cracks initially oriented parallel to the layers in layered, graded alumina-epoxy composites, produced experimentally by infiltration of layered porous alumina bodies. Notched specimens were tested under monotonic and cyclic bending loading. Finite element (FE) modelling was used to predict crack-tip stress fields and crack propagation paths. Measured initial crack deflection angles agreed with predictions from FE results, and observations from phase-shifted Moire interferometry. Crack propagation paths showed good agreement with FE predictions, except when cracks were influenced by interfaces between layers. Influences of elastic property gradient, microstructural heterogeneity and toughness anisotropy at interfaces are addressed, and the implications for structural reliability of layered structures are discussed. (C) 2006 Elsevier Ltd. All rights reserved.

ABSTRACTDeflection and deviation of cracks commonly occurs because of asymmetry in crack‐tip stresses in both homogeneous materials and functionally graded materials (FGMs); yet the analysis of curved cracks has been limited to simple crack shapes, otherwise the analysis would involve extensive levels of computation. The present study investigates the approximation of curved cracks with simplified shapes. A simple analytical model justifying the use of crack‐shape approximations, developed in an earlier study on stationary curved cracks in homogeneous materials, is outlined. Then, the approach is applied to propagating cracks in both homogeneous and graded material structures. Results are presented from finite element (FE) simulations of crack propagation using exact and simplified crack shapes. The use of an approximated crack shape can provide basic estimates for crack propagation path and critical load. However, systematic divergence can occur between predictions for exact and approximated crack shapes, particularly in inhomogeneous material configurations, and so the development of solutions for non‐straight cracks in FGMs would be expedient.

Cracks in stepped and continuously graded material specimens under flexural loading were investigated via finite element analysis. Calculation of mechanical energy release rates and propagation angles with crack-opening displacement correlation and the local symmetry (KII = 0) criterion, respectively, provided results most efficiently and accurately, as compared with compliance and J-integral approaches and other deflection criteria. A routine was developed for automatic crack extension and remeshing, enabling simulation of incremental crack propagation. Effects of gradient profile and crack geometry on crack-tip stresses and crack propagation path are examined, and implications of these for optimal design of graded components against failure by fast fracture are discussed.

Plastic yielding at or near the crack tip can significantly influence the behaviour of cracks, and accordingly should be included in simulations of crack growth; however, this can introduce a number of computational challenges. In this study, a finite-element model, for simulating mixed-mode crack propagation in linear elastic materials, was modified to incorporate yielding. A routine for automatic crack extension and re-meshing enabled simulation of incremental crack propagation. Particular issues, including calculation of fracture parameters, crack propagation direction under mixed-mode loading and retention of plastic strain history, are addressed. Crack propagation was simulated in homogeneous and layered Cu/W composites, employing thermal and mechanical properties previously obtained from experiments. Two effects of plasticity on crack-tip stresses are predicted: (i) compliance mismatch leads to stress intensity factor amplification or ‘anti-shielding’, and (ii) accumulation of plastic strains leads to increases in effective toughness. Competition between these determines the structural reliability of the interface region.

Cracks situated parallel to, and very near, the interface in layered, ductile‐brittle composite specimens were investigated with finite‐element analysis. Elastic, plastic and thermal properties previously obtained from experiments were utilized in the model. A routine was employed for automatic crack extension and remeshing, enabling simulation of incremental crack propagation. The elastic, thermal and plastic contributions to crack propagation behavior were investigated, along with the variation of these with crack length and crack‐tip position. Thermal residual stresses are shown to have a large influence on crack path, although this is mitigated to some extent by plasticity. The implications on the inherent reliability of joints and layered materials containing brittle constituents are discussed.

Recent experimental and theoretical studies of the fracture and fatigue behaviour of composites with graded material composition are reviewed. The effects of compositional and microstructural gradation on crack-tip stress fields, stress intensity factors and crack extension direction are summarised for cracks initially oriented both parallel and perpendicular to the gradient direction. Spatial variation of intrinsic fracture toughness, residual stresses, crack-wake effects, and the scale of the material gradient have all been observed to strongly influence crack propagation behaviour and failure resistance. Pertinent issues such as composite property prediction, crack deflection criteria, stress intensity factors for curved cracks and crack bridging are also discussed.

Methods of predicting effective mechanical properties of composites with an interpenetrating network structure are currently not well understood, particularly for cases in which the constituent materials have widely differing properties. Alumina–epoxy composites with an interpenetrating composite structure have been produced via an infiltration process and the elastic properties were measured via the impulse excitation technique. A strong dependence of properties on composition and processing was observed. Properties were compared with several mixing law predictions made using compositional data obtained from microstructural analysis. The effective medium approximation (EMA) was shown to predict properties adequately, whilst others models proved inappropriate.