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Magnesium alloy (AZ31) based metal matrix composite reinforced with carbon nanotubes (CNTs) was fabricated using the technique of disintegrated melt deposition followed by hot extrusion. In this research paper, the microstructure, hardness, tensile properties, tensile fracture, high cycle fatigue characteristics, and final fracture behavior of CNTs-reinforced magnesium alloy composite (denoted as AZ31/1.0 vol.% CNT or AZ31/CNT) is presented, discussed, and compared with the unreinforced counterpart (AZ31). The elastic modulus, yield strength, tensile strength of the reinforced magnesium alloy was noticeably higher compared to the unreinforced counterpart. The ductility, quantified both by elongation-to-failure and reduction in cross-section area of the composite was higher than the monolithic counterpart. A comparison of the CNT-reinforced magnesium alloy with the unreinforced counterpart revealed a noticeable improvement in cyclic fatigue life at the load ratios tested. At all values of maximum stress, both the reinforced and unreinforced magnesium alloy was found to degrade the cyclic fatigue life at a lower ratio, i.e., under conditions of fully reversed loading. The viable mechanisms responsible for the enhanced cyclic fatigue life and tensile behavior of the composite are rationalized in light of macroscopic fracture mode and intrinsic microscopic mechanisms governing fracture. [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]

Composite materials demonstrate a considerable extent of heterogeneity. A non-uniform spatial distribution of reinforcement results in variations of local properties of fibrous laminates. This non-uniformity not only affects effective properties of composite materials but is also a crucial factor in initiation and development of damage and fracture processes that are also spatially non-uniform. Such randomness in microstructure and in failure evolution is responsible for non-uniform distributions of stresses in composite specimens even under externally uniform loading, resulting, for instance, in a random distribution of matrix cracks in cross-ply laminates. The paper deals with statistical features of a distribution of carbon fibres in a transversal cross-sectional area in a unidirectional composite with epoxy matrix, based on various approaches used to quantify its microscopic randomness. A random character of the fibres’ distribution results in fluctuations of local elastic moduli in composites, the bounds of which depend on the characteristic length scale. A lattice model to study damage and fracture evolution in laminates, linking randomness of microstructure with macroscopic properties, is discussed. An example of simulations of matrix cracking in a carbon fibre/epoxy cross-ply laminate is given. [ABSTRACT FROM AUTHOR]

This paper presents the findings of work carried out to describe the micromechanisms of radial crack growth in wood. TR and TL cracks are both radial cracks but TR grows radially and TL longitudinally. TR cracks are known to show higher fracture toughness than TL cracks. The TR fracture surfaces also indicate a more tortuous crack path. Since the reason for this is unclear, details of the TR crack growth mechanisms in green Pinus sylvestris L were studied. This was done by in-situ optical microscopy as the crack was cutting through alternating layers of soft earlywood and stiff latewood. At the scale of individual cells, the crack tip advanced by separating cell walls at the middle lamella in a splitting or peeling mode. At the scale of growth rings, stick-slip type of crack growth was observed and new crack planes were often formed. The stress distribution in a material with alternating stiff and soft layers is causing this. This stress distribution also contributes to the tendency for inclined cracks to deviate in the radial direction. For interpretation of fracture mechanisms, the importance of scale interaction and the combined influences of microstructure and stress state are emphasized. [ABSTRACT FROM AUTHOR]

In this paper the influence of notch acuity and test temperature on the impact behavior of aluminum alloy 6061 is presented and discussed. Notch angles of 45°, 60°, 75° and 90° were chosen for a standard charpy impact test specimen containing two such notches positioned at right angles to the applied load. For a given angle of the notch the dynamic fracture toughness increased with an increase in test temperature. At a given test temperature, the impact toughness of a ductile microstructure decreased with an increase in notch severity. For the least severe notch dynamic fracture surfaces revealed the occurrence of localized mixed-mode deformation at the elevated temperature. An increase in notch severity resulted in essentially Mode-I dominated fracture at all test temperatures. The results are discussed in light of alloy microstructure, fracture mechanisms and deformation field ahead of the advancing crack tip. [ABSTRACT FROM AUTHOR]

We developed a facile and low-cost approach to prepare lightweight and high-strength magnesium-matrix composites with a nacre-inspired laminated structure. First, lamellar Mg2B2O5 whisker (Mg2B2O5w) scaffolds with initial solid loadings of 10, 15 and 20 vol% were prepared by ice templating. The wettability between a molten AZ91D alloy and the Mg2B2O5w scaffold was greatly improved by the incorporation of nano-SiO2 sol in the aqueous slurry, making the preparation of nacre-mimetic AZ91D/Mg2B2O5w composite by way of pressureless infiltration feasible. The SiO2 content in the Mg2B2O5w scaffold has a significant effect on the processing and the microstructure and properties of the composites. The optimum SiO2 content was about 6-8 wt% of the total ceramic loading. A lower SiO2 content resulted in incomplete infiltration, while a higher content led to the formation of a large quantity of Mg2Si in the composite. The flexural strength of the composites seemed independent of the initial ceramic loading (10-20 vol%), whereas the compressive strength and elastic modulus increased considerably and the crack-growth fracture toughness decreased with increasing ceramic content. The mechanism for such variations was addressed. [ABSTRACT FROM AUTHOR]

We investigate the influence of the microstructure on the fracture properties of calcium aluminate cement/polymer composites. We carry out microscopic scratch tests during which a Rockwell C diamond probe pushes across the surface of a polished specimen under a linearly increasing vertical force. We extend the scratch fracture method to heterogeneous materials. The scratch test induces a ductile-to-brittle transition as the penetration depth increases. Scanning electron microscopy imaging shows that the low porosity and the strong cement-binder interphase favor toughening mechanisms such as crack trapping and bridging. Nonlinear fracture mechanics theory yields the fracture toughness in the fracture-driven regime. The fracture toughness of macro-defect-free (MDF) cement is found to decrease as the polymer-to-cement ratio increases. This decrease in the fracture resistance can be explained by the decrease in anhydrous cement content and the increase in the inter-particle distance between cement grains. By evaluating the fracture toughness of the micro-constituents of MDF cement, we show that the high value of the fracture toughness at the composite level stems from tough calcium aluminate phases and a highly packed non-porous granular microstructure. [ABSTRACT FROM AUTHOR]

In situ observations of crack propagation in sulfur-doped coarse-grained nickel were performed for the specimens with grain boundary microstructure pre-determined by SEM/EBSD analysis. The role of grain boundary microstructure was studied in the crack propagation in nickel embrittled by grain boundary segregation of sulfur. It was found that the main crack tends to predominantly propagate along random boundaries, and the crack propagation rate can be locally accelerated at the grain boundary network with a high connectivity of random boundaries. On the other hand, the cracks can propagated along fracture-resistant low-Σ coincidence site lattice (CSL) boundary only when the trace of the grain boundary is arranged being almost parallel to slip bands in the adjacent grains. The local crack propagation rate was found to become lower when a crack propagated along low-Σ CSL boundaries. Moreover, when the crack propagation is inhibited by low-Σ CSL boundaries, the branching of propagating crack occurs at partially cracked triple junctions. The crack propagation can locally slow down due to the occurrence of crack branching. The optimum grain boundary microstructure for the control of sulfur segregation-induced brittle fracture is discussed on the basis of new findings obtained from the in situ observations on crack propagation and fracture processes in polycrystalline nickel. [ABSTRACT FROM AUTHOR]

The effect of reheating by following passes on the hydrogen embrittlement of MAG weld metal for HT780 class steels has been investigated by using specimens subjected to simulated thermal cycles. The hydrogen-charged specimens exhibited transgranular quasi-cleavage fracture and intergranular fracture along prior austenite grain boundaries on slow strain rate tensile (SSRT) tests, depending on the reheated temperature and charged hydrogen content. The reduction in elongation of hydrogen-charged specimens became more significant when intergranular fracture occurred. When specimens in as-welded state and precedently reheated at coarse grained HAZ temperature of 1,623 K were reheated at a tempering temperature of 873 K, significant amount of intergranular fracture occurred at charged hydrogen contents above 3 ppm in spite of the decrease in hardness. The specimen reheated at 1,173 K showed no intergranular fracture even after receiving the reheating at 873 K at a hydrogen content of 6 ppm, suggesting the strong influence of the prior austenite grain size on the hydrogen-induced intergranular embrittlement. The measurement of hydrogen content desorbed from the hydrogen-charged specimen at room temperature suggested that the intergranular fracture caused by the reheating at 873 K was associated with an increase in susceptibility to hydrogen embrittlement of the prior austenite grain boundary itself rather than a decrease in the amounts of trapping sites such as dislocation and retained austenite. [ABSTRACT FROM AUTHOR]

Microstructure and fracture behavior of brazed joint between commercially pure titanium and low carbon steel using silver (Ag–34Cu–2Ti) and copper (Cu–12Mn–2Ni ) based alloys have been characterized to determine the effect of brazing parameters and chemical composition on the strength of brazed joints. It is found that the shear strength of brazed joints strongly depends on the lap width. Furthermore, the fracture path and the value of shear strength significantly changed with the type of filler alloy. The two filler metals showed metallurgical interaction with steel and titanium forming different kinds of intermetallic compounds such as CuTi, Cu2Ti, and FeTi with silver based filler and Ti2Cu, FeTi and TiCuFe with copper based filler. [ABSTRACT FROM AUTHOR]

Fracture behavior of a two-phase TiAl alloy was investigated using notched specimens. Fracture surfaces and metallographic sections of surviving notch in double notched specimens are observed. The fracture process of notched specimens of TiAl alloys was described as that several inter-lamellar cracks initiate and extend directly from the notch root and propagate preferentially along the interfaces between lamellae and stop at various obstacles. With increasing applied load, cracks connect with each other and propagate further by translamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, could be reducing the driving force for crack propagation. The higher toughness of near fully lamellar microstructure than that of finer duplex microstructure is attributed to the path of crack propagation. On the fracture surfaces of the finer duplex microstructure, more low-energy-spending interlamellar fracture facets are observed, which means that it is easier for crack to bypass a fine duplex lamellar grain with lamellae perpendicular to the main crack and to take a interlamellar path. [ABSTRACT FROM AUTHOR]

The surface scratch resistance, damage evolution and damage detection during microscratching of pure AlN and AlN with an Al2O3 oxide layer (∼0.1 μm) were investigated. The microscratch experiments were performed in a progressive load-ramped mode where the load was linearly increased from 0 to 3 N as the scratch progressed with a loading rate of 0.2 N/min. The experiments resulted to two different failure mechanisms; Ductile scratching and fracture dominated scratching. In the case of pure AlN, inconsistent different kinds of damages were found to initiate at low critical load measured as 0.30 N. This critical load was also found to decrease with increasing the surface roughness, material density and grain size of AlN. In the case of pure AlN with an Al2O3 oxide layer, the critical load for crack initiation was measured as 0.45 N, higher than pure AlN. This critical load was found to generate uniform discontinuous chipping. The distance between these defects decreased and the failure mode gradually transformed to continuous chipping as the scratch progressed. The surface modification of AlN with the use of an Al2O3 oxide layer was found to improve the tribological properties of AlN. [ABSTRACT FROM AUTHOR]

Crack propagation paths in the low temperature brittle fracture of 18Cr-18Mn-0.7N austenitic steel were investigated by means of scanning electron microscopy. Corresponding relationships of the fracture facets with microstructures were established by the simultaneous observation of the fracture surface and the microstructure of the adjacent side surface. It was shown that the annealing twin boundary cracking occurred during fracture. A great deal of twins formed during solution treatment of the steel, with steps several microns high on the twin boundaries, and a considerable amount of planar deformation structures developed on {111} planes in the fracture process. The fracture facets of the annealing twin boundary are fairly flat and smooth, with bent steps of micron-scale height and a pattern of three sets of parallel straight-lines intersecting at 60°. But there is no river pattern on the facets. The bent steps result from partial propagation of crack along steps that are developed on annealing twin boundaries during solution treatment while the line-pattern is the intersection traces of the planar deformation structures with the fracture facet. It is believed that the annealing twin boundary cracking is attributed to the stress concentration arising at the intersection of planar deformation structures and the annealing twin boundaries. [ABSTRACT FROM AUTHOR]

The damage process in composite laminates subjected to cryogenic cooling was monitored employing a thermo-acoustic emission (AE) technique. The thermo-AE signals processed with a short-time Fourier transform could be classified into three different types which were correlated with individual microfracture processes. In the initial stage of cryogenic cooling, very strong AE signals with low and high frequency bands were dominantly detected showing that large cracks accompanying fiber breakages were developed mainly. With an increase in the cooling time, weak emissions with low frequency bands became prevalent indicating the propagation of microfractures in the matrix and/or fiber-matrix interface. Similar types of AE signals, however, having weak amplitudes, were also observed for the cryogenically-treated specimens during thermal heating and cooling load cycles. Thus, analysis of thermo-AE behavior through the thermal load cycle led to the nondestructive evaluation for the cryogenic damage of composites. [ABSTRACT FROM AUTHOR]

The interface microstructure and mechanical strength of joints obtained by diffusion bonding of alloy Ti-6A1-4V to AISI 316L stainless steel are presented. Bonding took place at 850, 900 and 950°C, maintained for different lengths of time (60 to 180 min). The highest recorded shear strength was equal to 382 MPa and was displayed by the specimen bonded at 950°C/180 min. Three different fracture paths were identified, each correlated to a characteristic strength range. From microprobe measurements and x-ray diffraction studies the following interface reaction products were identified: σ phase, α-Ti, Fe2Ti, β-Ti and Fe-Ti. The interface growth kinetics obeyed a quadratic law, indicating diffusion controlled growth of the β-Ti layer. Finally, from correlations identified between β-Ti layer thickness and shear strength variations, some hypotheses on the role of both pores and interface reaction product on mechanical strength were presented. [ABSTRACT FROM AUTHOR]

The toughening behavior of short glass fiber reinforced toughened polymers was studied using fracture mechanics and microscopic techniques. The essential work of fracture (EWF) analysis shows that the inclusion of short glass fibers not only provided a stiffening effect but also a toughening influence. It was observed that rubber-related toughening and fiber-related toughening were competitive in nature for the reinforced, toughened nylon 6,6. When the matrix stress was substantially reduced by the presence of short fibers via the load-shedding mechanism, rubber toughening was severely curtailed. At higher fiber volume fractions, fiber pull-out work contributed significantly to the enhancement of the specific essential fracture work. Fiber-end plasticity was evident under microscopic examination. [ABSTRACT FROM AUTHOR]

The specific essential work of fracture, we, has been measured for a relatively thick walled uPVC pipe as a function of position through the wall of the pipe. we was highest at the surface of the pipe and decreased significantly at the centre of the pipe wall. The variation in we through the wall of the pipe correlated with the processing level of the uPVC material as measured by the critical temperature, Tc. The variability in the measured values of we was substantially higher in the centre of the pipe where the processing levels were lower. This was likely to be a result of the variability in the microstructure of the material where poor processing had introduced regions of poor fusion of primary PVC particles. [ABSTRACT FROM AUTHOR]

Existing models indicate that the mechanical properties of brittle foams in shear depend on relative density, cell size, cell geometry and the modulus of rupture of solid cell walls. A series of mechanical testing on alumina cement foams with various relative densities were conducted to measure their shear modulus, shear strength and fracture toughness. Experimental data are compared to the existing theoretical models. Results suggest that the existing models for closed-cell foams are applicable to describe the mechanical properties of alumina cement foams in shear. Also, the microstructure coefficients included in the theoretical expressions for mechanical properties of alumina cement foams have been determined. [ABSTRACT FROM AUTHOR]

Mechanical testing of an advanced thermoplastic polyimide (LaRCTM-SI) with known variations in molecular weight was performed over a range of temperatures below the glass transition temperature. The physical characterization, elastic properties and notched tensile strength were all determined as a function of molecular weight and test temperature. It was shown that notched tensile strength is a strong function of both temperature and molecular weight, whereas stiffness is only a strong function of temperature. A critical molecular weight ( Mc) was observed to occur at a weight-average molecular weight (― Mw) of ∼22000 g/mol below which, the notched tensile strength decreases rapidly. This critical molecular weight transition is temperature-independent. Furthermore, inelastic analysis showed that low molecular weight materials tended to fail in a brittle manner, whereas high molecular weight materials exhibited ductile failure. The microstructural images supported these findings. [ABSTRACT FROM AUTHOR]

The addition of both elements (Cr and Ni) and carbides (SiC and WC) during laser surface alloying under different processing speeds produced surfaces with both enhanced hardness wear resistance and corrosion properties compared to the base AISI 4340 steel material. These effects were due to the evolution of unique microstructures within the laser-processed region, which includes austenite, ferrite, martensite, Fe- and Si-based carbides and the retention of the original carbides (SiC and WC) in various combinations. The chromium and nickel stabilized the austenite and ferrite but reduced the formation of martensite that is useful to increase the hardness and prevent cracking. Also, the substantial dissociation of the original carbides (SIC and WC) into elemental silicon and tungsten supplemented the stabilization of ferrite and reduction in the hardness. The presence of the undissociated carbides and some martensite formation provided substantial increases in the microhardness. The improvement of both the mechanical properties and corrosion resistance might be self-exclusive due to the reduction of the carbides and the subsequent inability of the matrix to prevent cracking. [ABSTRACT FROM AUTHOR]

Two kinds of α-Si3N4 powders with different properties (high purity and low purity) were pressureless sintered using MgAl2O4-ZrO2 as sintering additives. The grain growth was controlled by sintering time and temperature. The fracture toughness was determined using indentation microfracture (IM) and single-edge-precracked-beam (SEPB) methods. A discrepancy of fracture toughness was found between the values obtained by these two methods, and the SEPB method provided higher fracture toughness than the IM method. Materials with more large elongated Si3N4 grains gave higher fracture toughness, R-curve behavior and larger discrepancy. This is contributed to the effects of grain bridging, pull-out and deflection by the large elongated Si3N4 grains. Comparing the specimens from two kinds of α-Si3N4 powders with different purity level, both gave high fracture toughness of over 9 MPa · m1/2 by the SEPB method, while the E10 samples have a little higher flexural strength. [ABSTRACT FROM AUTHOR]

The failure of a craze ahead of a crack growing under steady state conditions in a glassy polymer is investigated by modeling the craze microstructure using a highly anisotropic network of springs. A rate dependent drawing law is used to determine the shape of the craze-bulk interface. Approximate analytical results are developed to link the normal stress on the craze-bulk interface, the thickness of the craze and the far field stress intensity factor to the crack propagating velocity, through the craze failure criterion and the craze microstructural parameters. The accuracy of the analytical results is examined using a detailed numerical simulation. Our analysis shows that the rate independent craze failure criterion, which assumes the failure stress for fibrils ahead of the crack tip to be a material constant independent of the crack growth rate, leads to predictions of the dependence of the craze thickness and the fracture toughness on crack growth rate that are contrary to what is found experimentally. Rate dependent craze failure criteria are then proposed. Specifically, we study a case where the crack tip fibril breaks down by rate dependent chain scission and a case where the crack tip fibril fails by rate dependent chain disentanglement. For the rate dependent chain scission criterion, the results given by the rate independent constant failure stress criterion are retrieved in the limit of low crack propagation velocity. Also, there exists a critical stress intensity factor below which steady state crack propagation is impossible, i.e., crack growth becomes unstable. [ABSTRACT FROM AUTHOR]