Fracture characteristics of metal/intermetallic laminar composites produced by reaction sintering and hot pressing

Metal/intermetallic layered composites were formed by a process recently developed in which a self-propagating, high-temperature synthesis reaction was initiated at the interface between dissimilar metal foils. After the reaction, one of the metal foils was entirely consumed, resulting in a metal/intermetallic laminar composite. This study details the tensile fracture characteristics of these unique composites. Fracture mechanism and failure energy were controlled by varying the intermetallic-to-metal volume ratio. Failure initiated with the formation of cracks in the intermetallic layer. For high intermetallic-to-metal ratios, the intermetallic crack release energy was too great to prevent cracks from propagating through the metal layer and propagating the crack into the adjacent intermetallic layers, leading to a fast, low energy fracture. For lower intermetallic-to-metal ratios, the metal layers adsorbed the intermetallic crack release energy and blunted the propagating crack. Final failure resulted by ductile fracture of the metal layer after extensive intermetallic cracking.     

A Discrete Model for Simulation of Composites Plate Impact Including Coupled Intra- and Inter-ply Failure

Laminated composites can undergo complex damage mechanisms when subjected to transverse impact. For unidirectional laminates it is well recognized that delamination failure usually initiates via intra-ply shear cracks that run parallel to the fibres. These cracks extend to the interface of adjacent orthogonal plies, where they are either stopped, or propagate further as inter-ply delamination cracks. These mechanisms largely determine impact energy absorption and post-delamination bending stiffness of the laminate. Important load transfer mechanisms will occur that may lead to fibre failure and ultimate rupture of the laminate. In recent years most Finite Element (FE) models to predict delamination usually stack layers of ply elements with interface elements to represent inter-ply stiffness and treat possible delamination. The approach is computationally efficient and does give some estimate of delamination zones and damaged laminate bending stiffness. However, these models do not properly account for coupled intra-ply shear failure and delamination crack growth, and therefore cannot provide accurate results on crack initiation and propagation. An alternative discrete meso-scale FE model is presented that accounts for this coupling, which is validated against common delamination tests and impact delamination from the Compression After Impact (CAI) test. Ongoing research is using damage prediction from the CAI simulation as a basis for residual strength analysis, which will be the published in future work.     

Deformation behaviour of coextruded multilayer composites with polycarbonate and poly(styrene-acrylonitrile)

The tensile properties of coextruded multilayer composites comprised of predominantly 49 alternating layers of polycarbonate (PC) and polystyrene- acrylonitrile (SAN) were investigated in the bulk and microscopically. The bulk was characterized by three types of behaviour: brittle fracture at low strains, ductile yielding with fracture during neck formation, and formation of a stable neck followed by drawing to high extension. Optical microscopy was utilized to correlate deformation mechanisms within each phase to the observed modes of deformation in the bulk. Optical microscopy showed that in all cases the initial irreversible deformation event was the formation of cracks or crazes in the SAN layers. Good adhesion between the layers resulted in the subsequent initiation of shear bands in the polycarbonate layers at the craze tips. Interaction of crazes and shear bands produced an expanded damage zone ahead of the propagating crack which delocalized the stress and delayed fracture. The ultimate mode of fracture depended on the relative thickness of the SAN and PC layers, as determined by the composition, and the strain rate.     

SiC／Al合金层状复合材料的机械性能及损伤行为

在室温条件下测定了Ａｌ合金以连续层状形式存在于ＳｉＣ陶瓷层间并渗透入ＳｉＣ陶瓷层内、Ａｌ合金浓度呈层状变化高低相间，以及Ａｌ合金和ＳｉＣ陶瓷均匀分布相互渗透三种ＳｉＣ含量相同而结构形式不同的ＳｉＣ／Ａｌ合金复合材料的机械性能；用ＳＥＭ和光学显微镜观察分析了复合材料的断口形貌及裂纹扩展过程。结果表明，在ＳｉＣ陶瓷层间以连续层状形式存在的Ａｌ合金在应力作用下发生较大程度的塑性变形，在裂纹尾部被拉伸和形成桥接，引起能量耗散，减缓裂纹扩展速度，防止裂纹张开，使复合材料的韧性得到明显改善；ＳｉＣ／Ａｌ合金陶瓷─金属层状复合材料的损伤形式主要是ＳｉＣ陶瓷层开裂、金属层桥接和裂纹偏转。     

Channel cracking during thermal cycling of thin film multi-layers

This paper describes mechanisms that can lead to film channel cracking, even in scenarios when layer thickness is small. Computational models are used to illustrate conditions that lead to the introduction of channel cracks and their subsequent cycle- or time-dependent propagation. Results for elastic structures with periodic features are briefly reviewed to illustrate that small low-modulus sections promote cracking in adjacent layers because they allow for the release of strain energy in adjacent sections with high residual stress. Inelastic deformation in layers adjacent to the cracked layer may also act to increase the channel crack driving force, by allowing for increasing displacements that serve to release strain energy. Two inelastic mechanisms form the primary focus in this effort: rate-independent plasticity and creep. Analyses of a cracked film on an elastic-plastic layer reveal pronounced cyclic displacements (as known as ratcheting) when the misfit thermal strain amplitude in the ductile layer exceeds twice its yield strain. Similar behavior occurs in cracked films on layers susceptible to creep. Simulations are presented for both isolated cracks and periodic arrays of cracks, and illustrate that the likelihood of cracking grows dramatically with time. In both inelastic regimes, the upper limit on deformation is dictated by the residual stress in the elastic layer and the substrate dimensions. These results are discussed in the context of analytical models developed elsewhere and potential experiments.     

A [90/0]s composite laminate with part-through matrix cracks in adjacent plies

A [90/0]_s orthotropic composite laminate with part-through matrix cracks is considered. Stress intensity factors are determined for the cracks using a linear-elastic analysis. These matrix cracks run along the fiber direction of the individual plies. The crack-geometry considered here is one where the matrix cracks in adjacent plies form a cross-like pattern in the plan view of the laminate. The plies are assumed bonded by thin resin-rich adhesive layers. These adhesive layers are modeled as distributed shear springs. Each ply of the laminate is modeled as a thin elastic orthotropic layer under plane stress. The laminate is subject to both tensile and shear loading. The mathematical model for the stresses and displacements in the layers reduces to a pair of Fredholm integral equations which are solved numerically. The stress intensity factors show a strong dependence on crack-sizes and nature of loading. In particular, the magnitudes of the stress intensity factors for the matrix crack in the 0° layer are increased significantly by the crack in the adjacent 90° layer.     

A numerical analysis of failure characteristics of ductile layers in laminated composites

In this study, the failure of the ductile layers from collinear, multiple and delaminating cracks that occur in laminated composite systems was studied using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. The results indicate that, in laminated composites, void nucleation and growth ahead of the cracks occur at a much faster rate because of evolution of much higher stress values in the interface region. Except for short crack extensions, collinear and multiple cracks develop crack resistance curves similar to that seen for a crack in the ductile layer material as a homogenous isotropic cases. For delaminating crack cases, the fracture behaviour is strongly influenced by the delamination length. The resistance of the ductile layers to crack extension can be significantly reduced by short delamination lengths; however, for large delamination lengths the resistance to crack extension becomes greater than that seen for the ductile material. The results also show that, if the crack tip is at the interface, similar maximum stress values develop in the ductile layers as in the fracture test of the same ductile material, suggesting that ductile–brittle fracture transition behaviour of the ductile layers is dependent upon the extent of the cracks in the brittle layers and fracture characteristics of the brittle layers.     

The ductility of metal matrix composites - Relation to local deformation behavior and damage evolution

Powder-metallurgy metal matrix composites (MMCs) with Al-6061 matrix and SiC-particles are analyzed by means of in situ tensile tests in the scanning electron microscope and automatic local deformation analyses. The particle size (10 and 100 μm) and the particle volume fraction (10% and 20%) are varied. For comparison the pure matrix material is also tested. The formation of the shear bands with increasing load is studied, and the differences in the shear band evolution in the different materials are worked out. Cracks are initiated by particle fracture and by matrix/particle decohesion. The crack tip opening displacements (COD) at the particle failure sites are determined, as well as the critical COD-values for particle cracks propagating into the matrix material. Qualitative relations between the global ductility of the MMCs and the deformation and damage behavior, both observed at the micro-scale, are worked out.     

On-machine interlocking of 3D laminate structures for composites

By using an adjacent-layer interlocking method on a weaving machine, multi-layer preform structures are developed. The on-loom interlocking method eliminates the yarn breakage resulting from needle penetration which is the case for off-loom interlocking of fabric layers. The concept of this three-dimensional (3D) fabric design is to bind each pair of adjacent layers at one connecting point in every other plain-weave repeat within each layer. The mechanical properties of the resulting composites are investigated by means of impact, short-beam shear and the long-beam flexural testing. The failure mechanisms found in 3D on-loom interlocked composites include fiber breakage, fiber debonding and fiber pull-out.     

Fracture Behavior of Particle Reinforced Metal Matrix Composites

The contributions of the reinforcement volume fraction and annealing temperatures to crack opening force and propagation energy are systematically studied by three point bending tests and by SEM investigations. The bending test data show that for the same reinforcement volume fraction, 2618 and 7075 Al composites require much higher force to open the cracks than 6061 matrix. This relates to the much higher levels of solute elements which causes matrix hardening. Studies reveal that the energy absorption level of the materials during crack propagation depends on both matrix strength and ductility which relates to the reinforcement volume fraction, composition and heat treatment conditions. Large deformation zones are found in front of the crack tip before crack propagation which indicate a ductile failure mode for the composites. Studies also reveal that cracks initiate generally at the particle/matrix interfaces for the low volume fraction reinforced composites. However, for the high volume fraction reinforced composites, crack initiation has been found from both reinforcement/matrix interfaces and broken particles. This indicates that increasing reinforcement volume fraction and matrix strengthening tend to change the fracture mode from interface debonding to particle cleavage cracking.     

The failure processes analysis of rock slope using numerical modelling techniques

The slope failure process includes crack initiation, propagation and coalescence during the formation of a slip surface (small deformation stage) and block movement, rotation and fragmentation during the sliding process (large deformation stage). Neither the finite element method (FEM) nor the discontinuous deformation analysis method (DDA) can solve such problems satisfactorily due to the complex mechanical behaviour of slope failure. To study the entire process of slope failure, we develop here a model that combines the FEM and DDA approaches. The main concept of this approach is to first apply FEM to model crack growth behaviour and then automatically switch to the DDA module to model the post-failure process when the slip surface forms. The efficiency and simplicity of this approach lies in keeping the FEM and DDA algorithms separate and solving each equation individually. The heterogeneous nature of the slope material at the mesoscopic level is considered by assuming that the mechanical properties of individual elements follow a Weibull statistical distribution. The slope models are progressively destabilized by the critical gravity approach, and both the failure onset and the slope collapse process are analysed. Our modelling reveals that shear cracks first initiate at the toe of slope and subsequently promote the propagation of tensile fractures due to the stress accumulation at the shear crack tips. Throughout the entire failure process, failure in tension occurs at a higher rate than shear failure and plays a dominant role in the formation of the slip surface. The effects of slope angle and pre-formed cracks on the post-failure process are studied using the proposed method. This study demonstrates that the modelling approach outlined herein is able to tackle the fundamental problems of rock slope failure and offers a better understanding of the slope failure mechanisms at both the macroscopic and microscopic levels.     

Analysis of composite skin/stiffener debounding and failure under uniaxial loading

In this paper, damage mechanisms in the composite bounded skin/stiffener constructions under monotonic tension loading are investigated. The approach uses experiments to detect the failure mechanisms, two and three-dimensional stress analysis to determine the location of first matrix cracking and computational fracture mechanics to investigate the potential for cracks and delamination growth. The laminates strength and damage mechanisms obtained from both experimental and finite elements analysis are presented for several laminates lay-up configurations. Observations on the performed experiments show matrix crack initiation and propagation in the skin and near the flange tip, causing the flange to almost fully debounded from the skin in some cases, interlaminar debounding and fiber breakage up to the failure of the components. The finite elements analysis is also show that the matrix cracks are initiated in the first skin layer for most of the cases. With increasing the applied load the matrix cracks are propagated through the thickness to reach the next layer and causes delamination between the two layers. With increasing the applied load this delamination is propagated up to the occurrence of unstable delamination growth or the first fiber breakage known as the final failure of the component. The obtained experimental failure loads are compared with those calculated by the finite elements analysis.     

Experimental investigation into flexural retrofitting of reinforced concrete bridge beams using FRP composites

End cover separation and shear crack debond are the two most critical debonding modes in beams retrofitted with fibre reinforced polymer composites due the brittle nature of the failures. However, these failures are still not fully understood. A testing program including 18 rectangular reinforced concrete beams is carried out to investigate the failure mechanisms and the influence of several parameters on these debond modes. Testing shows that end cover separation starts from FRP ends and fails in the form of shear failure at steel reinforcement level at the root of the concrete teeth between shear cracks. Shear crack debond failure is due to the opening of one of those inclined cracks. Several debond prediction models are then verified with the experiment proving to work relatively well.     

PET和PAN纤维的拉伸形变和断裂过程

在扫描电子显微镜样品室中拉伸PET和PAN纤维,动态观察了纤维的形变和断裂行为,用照片记录了裂缝发生、发展和纤维最终断裂的过程。剪切带的出现和由此引起微裂缝的发生是PET纤维拉伸形变的主要特征,而个别大分子束的断裂形成裂缝的广泛出现则是PAN纤维的主要特征。研究得出纤维的结晶状况是拉伸下纤维形变和断裂行为的决定性因素。     

Numerical analysis of ductile failure of undermatched interleaf in tension

Ductile failure of an interleaf tension specimen consisting of a metal interleaf bonded between two elastic substrates, with a crack located in the centre of the metal, is studied by means of detailed finite element (FE) analyses. The rate-independent version of the Gurson model is used. This accounts for ductile failure mechanisms of micro-void nucleation, growth and coalescence within the framework of a finite deformation plasticity theory. Also, the rapid evolution of void density due to coalescence, which leads to ultimate failure, is considered. The effect of the interleaf thickness on failure (crack initiation and limited amount of crack growth) is investigated. The results show that the interleaf thickness affects crack initiation only slightly. For all specimens considered, crack initiation takes place at the crack tip. However, after crack initiation, the interleaf thickness affects stress and strain distributions significantly. Reducing the interleaf thickness significantly increases the load-carrying capacity. Moreover, reducing the interleaf thickness increases the maximum hydrostatic stress in the interleaf, which is no longer developed at the crack tip but at a distance far away from the crack tip. The resulting fracture toughness thus decreases as the interleaf thickness decreases. The shielding of the crack tip due to constrained plasticity is observed at higher load levels for interleaf specimens. This revised version was published online in August 2006 with corrections to the Cover Date.     

Failure of Laminated Composites at Thickness Discontinuities – an Experimental and Analytical Study

Failure initiation of laminated composites at a thickness discontinuity is studied experimentally with the aid of an optical microscope under combined loading of tension, transverse shear and bending, making use of three- and four-point bending arrangements. Because transverse shear produced relatively small effects in failure initiation, results are presented as tension-bending interactions. Two loading frames for three- and four-point bending were designed to apply moment and tension simultaneously to produce failure by generating a ply crack; this initiation was evaluated by finite element analysis using ABAQUS. For cross-plies bounding the interface at the base of the step it is found that a maximum strain criterion applied to the continuing (long) ply describes the failure initiation. Ultimate failure resulted at loads on the order of 25 to 35% than those at failure initiation.     

GROWTH OF FATIGUE CRACKS EMANATING FROM NOTCHES IN SiC PARTICULATE ALUMINUM COMPOSITE

Abstract— The initiation and growth of cracks emanating from blunt notches in 6061-Al alloy reinforced with 25% particulate Sic metal matrix composite were investigated. To elucidate the role of aging condition of the matrix on the fatigue behavior, the studies were carried out at T6 and overaged conditions. The results show that the number of cycles required for initiation of fatigue cracks are not influenced with the notch severity and the aging condition of the matrix. The overaging heat treatment resulted in slower fatigue crack growth rates. The failure of the Sic particles during the fatigue process is given as the reason for the both observed initiation and crack growth characteristics. It is also shown that the growth rate of cracks emanating from blunt notches can be accurately described by an equivalent stress intensity factor range Δ K _eq. This could provide an adequate engineering method for design against fatigue failure from various stress concentrations for this composite system.     

Flexural behavior of plain concrete beams strengthened with ultra high toughness cementitious composites layer

Ultra high toughness cementitious composites (UHTCC), which has metal-like deformation and crack width restricting ability, is expected to be utilized as retrofit materials. For this application, much attention needs to be paid to the working performance of structure members composed of UHTCC and existing concrete. This paper presents an investigation on the flexural behavior of plain concrete beams strengthened with UHTCC layer in tension face. The effect of UHTCC layer thicknesses on first crack load, ultimate flexural load, crack width, and load–deflection relationship is examined. The experimental results indicate that the use of UHTCC layer significantly increases the first crack load and ultimate flexural load. The first crack load and ultimate flexural load of composites beams increased with the increase of the UHTCC layer thickness. Considerable reduction in crack width was observed for composite specimens, as UHTCC layer restricted the cracks in upper concrete and dispersed them into multiple fine cracks effectively. Moreover, in comparison to plain concrete beam, composite beams could sustain the loading at a larger deflection without failure. Based on the plane section assumption, etc., a calculation method to predict the flexural capacity of composite beam was proposed. Good agreement between predictions and experiments had been obtained.     

Mode II delamination failure mechanisms of polymer matrix composites

The failure process of mode II delamination fracture is studied on the basis of the microscopic matrix failure modes (microcracks and hackles) as well as fracture mechanics principles. The crack tip matrix stresses leading to delamination is analysed by examining an adhesive bond with a crack analogous to a delamination crack in the resin layer of a composite. Such crack tip stresses induce matrix microcracks involving two major events: (a) single microcrack initiation and (b) development of multiple microcracks with regular spacing. The microcrack initiation shear stress τ* is found by the use of fracture mechanics to be related to certain resin properties (shear modulus G and mode I fracture toughness GIC) and microcrack length of the order of the resin layer thickness t (related to resin content). The more or less regular microcrack spacing S deduced from shear lag considerations can be related to resin properties GIC, G, τy (resin yield strength) and t. The multiple microcracks reduce the effective resin modulus and strongly affect the subsequent microcrack coalescence process. As a result of the detailed analysis of the failure process, mode II laminate fracture toughness GIIC can be quantitatively expressed as a function of resin GIC and (τ2y/G). The failure process modelled is used to interpret the mode II delamination behaviour of several carbon/epoxy systems studied here and that reported in the literature. This study reveals the critical importance of resin fracture (GIC related) and deformation (yielding) mechanisms in controlling mode II delamination resistance of laminated composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

INITIATION AND PROPAGATION OF SHORT FATIGUE CRACKS IN A WELD METAL

Abstract— Fatigue tests were performed using a purpose designed triangular shaped specimen to investigate the initiation and propagation of short fatigue cracks in a weld metal. It was observed that short fatigue cracks evolved from slip bands and were predominantly within ferrite grains. As the test progressed, the short crack density increased with minor changes in crack length. The growth of short cracks, in the early stage resulted mainly from coalescence with other existing cracks. The mechanism of short crack behaviour is discussed.