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The “essential work of fracture” (EWF) method is applied to various machine-made papers. The deforming and fracturing processes of the paper samples during testing is analyzed by means of the thermographic observation. Plastic deformation zone appears in three ways when deep double edge notched tension specimens are strained under in-plane stress: i.e. 1. type (i)--appearing through whole the ligament in a vague manner and developing into a circular (or oval) zone even before or at the maximum load point; 2. type (ii)--appearing from notch tip and amalgamating into a circular (or oval) zone after the maximum load point; and 3. type (iii)--appearing from notch tip and not amalgamating into a circular (or oval) zone until the sheet failure. Specimens with small ligament length (L) are likely to belong to type (i), while those with large L to type (ii) & (iii). Among these three types, type (i) fulfills the original assumption of the EWF method best in terms of the complete ligament yielding before crack initiation. Thus the specific essential work of fracture determined using the linear relation of type (i) should be correct, although the estimated work is a little smaller than that from the linear relation of type (ii) & (iii). [ABSTRACT FROM AUTHOR]

The first part of a comprehensive review of the literature on wood published in the Journal of Materials Science since its inception in 1966 is presented. Papers are reviewed by subject ranging from the determination of the microfibril angle in the wood cell wall through to the evaluation of fatigue life. The role of moisture content in determining mechanical properties of wood is explored and mechanical properties are reported including creep, fatigue and fracture. It is concluded that JMS has played a key role in disseminating state of the art literature on new developments in the understanding of the structure-related properties of wood. [ABSTRACT FROM AUTHOR]

Using a pressure-pulsed chemical vapor infiltration technique, SiC was infiltrated from a SiCl4 (4%)–CH4 (4%)–H2 gas phase into carbonized paper preforms at 1100°C. SiC-based cellular substrates with cell wall thicknesses of 25, 50 and 100 μm were obtained by using honeycomb-shaped paper preforms as the templates. The reduction of both wall thickness t and cell pitch d of SiC-based honeycomb substrate successfully led to an increase in geometric surface area per unit volume S, keeping pressure drop Δ P at constant, besides; Δ P decreased without lowering S by the reduction of t and the increase in d. The pressure drop in the prepared honeycomb substrates depended on S2α−3 value, where α was open frontal area fraction. The compressive strength of the honeycomb substrates with t of 25 μm was about 7 MPa. The strength increased in proportion with 1 − α, which corresponded to the volume fraction of the cell wall in the honeycomb substrate. [ABSTRACT FROM AUTHOR]

Lists research papers in the volumes 39 and 40 of the "Journal of Materials Science". "Adhesively-bonded joints and repairs in metallic alloys, polymers and composite materials," by A. Baldan; "Thermo-mechanical fatigue of shape memory alloys," by E. Hornbogen; "Role of entropy of fusion in phase transformation and self-diffusion," by G.P. Tiwari, J.M. Juneja and Y. Illjima.

As a growing structural material in the engineering field, bamboo materials are mainly subjected to tension, bending, and a combination of the two during the service process when used as a beam structure. However, the asymmetric bending behavior of bamboo materials under combined loads has not been thoroughly investigated. Therefore, this paper aims to systematically study the asymmetric deformation and fracture of bamboo materials under tension, bending, and combined tension-bending loads. The tests were conducted on a novel self-made in situ apparatus. The periodic stresses fluctuations under three-point bending when loading on the lower fiber density side were analyzed, and the correlation between the shape, size and buckling performance of parenchymal cells was revealed from a mechanical point of view. Microscopy-based in situ three-point bending tests revealed a rich energy dissipation mechanism in the fracture process. It was found that the bending resistance and bending bearing capacity of the bamboo materials under combined tension-bending were significantly improved attributed to the coupling effect of tension and bending. The specimens under combined tension-bending exhibited a composite fracture mode with the combination of tensile fracture and three-point bending fracture mode. Additionally, an expression for the finite stress intensity factor was derived, which allows to characterize the fracture toughness of bamboo materials under tension, bending and combined tension-bending. This work is expected to provide new insights into the deformation and damage of bamboo materials, as well as guide the engineering applications of raw bamboo materials and the development of bionic bamboo structural materials. [ABSTRACT FROM AUTHOR]

Semi-crystalline polymers are now extensively used in various applications such as aircrafts, civilian structures, oil and gas platforms and electronics. They are, however, inherently damage prone, and over time, the formation of damages such as cavitation and crazing causes degradation in mechanical properties, which eventually results in the permanent failure of the materials. This paper provides an overview of the principal approaches to assess and characterize damage in semi-crystalline polymers. The damage characterization methods are divided into two categories: mechanical characterization methods, aiming at describing damage growth based on the measurement of mechanical or physical properties and structural characterization methods, focusing on the assessment of microdamage initiation and evolution. Special attention has been paid to in situ non-destructive damage characterization methods. For better understanding deformation/damage behavior in semi-crystalline polymers, two major damage theories currently used to describe damage development are also described. The paper concludes with a discussion of limitations and critical issues for characterizing damage in semi-crystalline polymers. [ABSTRACT FROM AUTHOR]

Fatigue damage of rail steel is an important issue affecting economic and human safety. In this paper, the effect of different quenching rates on the fatigue crack growth of hypereutectoid rail is analyzed. The material parameters C and m of the Paris formula are measured. The results show that an increase in cooling rate of rail decreases the value Paris law parameter C, increases the value m and decreases fatigue crack growth rate. This can be attributed to the decreased in the lamellar spacing and cluster diameter of the pearlite with an increase in cooling rate. More specifically, the smaller lamellar spacing and the cluster diameter make the crack pass easily through the cluster boundary of pearlite, which increases the number of zigzag crack growth paths and branching cracks that can help restrain fatigue crack growth. The fatigue fracture characteristic of hypereutectoid rail is that with an increase in cooling rate, the fatigue striation spacing and river pattern of the fracture surface become smaller, the crack growth path tends to zigzag, the tearing edges become deeper and the surface is relatively rougher. [ABSTRACT FROM AUTHOR]

Stress corrosion cracking in light water reactor is one of the most important factors threatening the safe operation of nuclear power plants. Due to the severity, generality and various safety and economic problems caused by this phenomenon, it is necessary to establish a model for predicting the stress corrosion cracking growth rates. This paper provides an overview of three main methods for predicting stress corrosion cracking growth rates in recent decades, i.e., empirical, deterministic and calculation methods, which are introduced in detail. Empirical models describe classical statistical analysis and emerging artificial neural network method, both of which are based on a large number of experimental test data mining. They are convenient and relatively accurate in predicting, but require extensive, time-consuming and expensive tests for different service environments. Deterministic models aim to establish a theoretical relationship between crack growth rate and various influencing parameters by studying the stress corrosion cracking mechanism. Many scholars have proposed different mechanisms to scientifically explain the stress corrosion cracking phenomenon and propose corresponding crack growth rate models. Calculation models reveal the mechanism of crack initiation and propagation in different layers of materials by means of finite element method based on fracture mechanics and multiscale method based on quantum mechanics. They provide new idea for future research on stress corrosion cracking and bridge the quantitative mechanism or model, but no specific stress corrosion cracking growth rate model is formed. The article concludes with the prospect, aim and direction for stress corrosion cracking mechanism and prediction model. [ABSTRACT FROM AUTHOR]

The author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. The approach is based on the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture. An earlier paper has derived a blunt flaw fracture initiation relation which gives the critical elastic flaw-tip peak stress σpcr (a “signifier” of a critical condition in the process zone) in terms of the process zone material parameters, subject to the proviso that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius ρ. The relation has been derived using a “two-extremes” procedure, whereby the separate σpcr solutions for small and large s/ρ are blended together to give an all-embracing relation that is valid for all s/ρ. A key feature of the relation is that σpcr essentially depends on only one geometrical parameter: the flaw root radius ρ. Though the relation has evolved from a consideration of the characteristics of one model, i.e. that of an elliptical flaw in an infinite solid that is subjected to an applied tensile stress, it is anticipated that the relation can be applied equally well for a wide range of geometrical configurations involving different flaw shapes. It is against this background that the present paper demonstrates that the relation also applies to the behaviour of an intrusion type flaw in the surface of a semi-infinite solid subjected to an applied tensile stress. [ABSTRACT FROM AUTHOR]

This paper is the final part of a three paper series describing the fracture and ageing behaviour of poly(hydroxybutyrate). In the first two parts conventional fracture mechanics methods were used to monitor changes both during the detrimental room temperature ageing process that occurs and after a subsequent annealing process that had been reported to reverse the ageing process. This paper reports on our studies of the morphology of fracture surfaces and how fracture proceeds in different ways in the original, ductile, “fresh”, material, the more brittle, aged material and the ductile, annealed material. We have used optical and electron microscopic techniques to examine fracture surfaces of samples which had already been well characterised by mechanical testing. The effect of ageing and high temperature annealing on the resultant fracture morphology is detailed for both thin films and bulk samples. We have found that PHB undergoes crazing before failure regardless of annealing history. We have studied the craze morphology using optical microscopy and scanning electron microscopy. Both aged and un-aged samples are found to deform in approximately the same manner, the primary difference on ageing being the volume of material that is plastically deformed. After high temperature annealing a different craze morphology has been observed. In thin films this is characterised by the formation of a dense zone of micro-crazes over a relatively large area. In bulk samples there is a distinct change in the resultant fracture surface. In both thin films and bulk systems there is an increased occurrence of fracture initiation in the spherulite cores after high temperature annealing which helps to extend the craze zone. [ABSTRACT FROM AUTHOR]

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]

This paper aims to propose a simulation procedure to predict the interlaminar fracture toughness of stitched flax fiber composites through a virtual double cantilever beam test. The proposed procedure is constituted of two steps. First, the interlaminar failure of unstitched flax fiber laminate, as the parent laminate, is modeled using cohesive elements with a nonlinear softening law in order to model the large-scale fiber bridging occurred during delamination. The experimental results are used to calibrate the parameters of the cohesive law. Second, two-node beam elements are superposed onto the cohesive interface of the parent laminate at a prescribed stitch density and distribution to model the bridging stitches present in the validation samples. The stitch material behavior and properties are obtained from the tensile test of impregnated stitch fibers. The out-of-plane flax yarn stitching was found to generate a twofold increase in the delamination resistance of the composite laminate at a medium stitch density. The FE analysis results agreed well with the experimental results, where a good fit between the predicted and experimental R-curves was achieved. [ABSTRACT FROM AUTHOR]

This paper is devoted to the study of the fracture behaviour of two industrial refractory materials thanks to the development of a new technique of digital image correlation (DIC). DIC, already known as a helpful and effective tool for the measurement of displacement and deformation fields in materials, has been adapted to take into account displacement discontinuities as cracks. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex, while each subset can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening) during the loading with a higher spatial resolution than the one obtained by standard DIC. After having presented the principle of the new technique, its metrological performances are assessed from synthetic images and the choice of crack detection criterion is discussed. The capacity of this new technique is shown through a comparative study with standard DIC. Its application is led on magnesia-spinel refractory materials, specifically to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. We show that refractories with aggregates of iron aluminate spinel present a fracture mechanism with crack branching and can dissipate more energy thanks to a longer crack network. [ABSTRACT FROM AUTHOR]

In this paper, an accurate formula for calculating the thermal residual stress field in a particle-reinforced composite are presented. Numerical examples are given to show r-variations of the thermal residual stresses. The increase in fracture toughness of matrix predicted by the thermal residual stress field is compared well with the experimentally measured increase. [ABSTRACT FROM AUTHOR]

This work studies the effect of some production parameters on the pore connectivity grade (i.e., the open-cell content associated cracks and missing cell walls) of aluminium foams produced via powder metallurgy route. Two types of precursors, extruded and hot uniaxially compressed, were used to create a varied group of Al-Si and Al-Si-Mg alloy-based foams in a wide porosity range. The cellular structure and defects were characterized by gas pycnometry and X-ray tomography. The analysis performed points to a high pore connectivity in all foam specimens, despite these materials are classified as closed celled due to their appearance, and a significant dependence on all the parameters varied. These dependences and the related mechanisms are discussed in the paper in terms of (i) the dissimilar foam evolution at initial stages (effect of precursor processing technique), (ii) the solidification shrinkage of each alloy (effect of composition) and (iii) the cell wall thinning (effect of foam porosity and local drainage). In addition, it has been observed that the interconnections are preferably located in the central parts of the samples, thus suggesting the possible effect of the cooling conditions on defect generation. [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]

High-multiaxial stresses are created in front of a crack which leads to various fracture processes in a region close to the crack tip. These processes contribute to the energy dissipation of the moving crack increasing the crack resistance of the material. One of these processes is matrix yielding around particles which may happen before debonding of the particles and the calculation of that mechanism is the subject of this paper. At first, the mechanical problem of a spherical particle within a spherical elastic-plastic rubber filled matrix under hydrostatic tensile stress was solved. With the knowledge of the stresses and displacements of such a composite element, the yielding energy around one particle was calculated. Finally, an analytical equation for the composite fracture toughness for this mechanism was obtained by integration of the yielding energy density over the normalized hydrostatic stress within the yielding zone, which provides an increase with increasing particle fraction towards a maximum. [ABSTRACT FROM AUTHOR]

This paper highlights the observation that the Wilshire equations for failure times and times to various strains, as reported in the original literature, may not be the most appropriate ones for all materials—including the one selected in this study. Further, such appropriateness can be determined by looking at the consistencies between the parameter estimates obtained using minimum creep rates in comparison to using failure times. It is shown, using 1CrMoV steel as an illustration, that the parameter consistency can be achieved by generalising the Monkman–Grant relation so that it contains a temperature correction. Indeed, the ability of the Wilshire equations to produce meaningful physical parameters, such as the activation energy, is shown to be highly dependent upon a valid specification for the Monkman–Grant relation. It is shown that variations in the measured values for some of the Wilshire parameters ( w and k3) with strain indicate that the causes of deformation are different at different strains and different stresses. Finally, the measured variations in the parameters of the Monkman–Grant relation with strain enable accurate interpolated and extrapolated creep curves to be calculated for any test condition. [ABSTRACT FROM AUTHOR]

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]

This paper aims to statistically estimate the dynamic fatigue strength in brittle materials under a wide range of stress rates. First, two probabilistic models were derived on the basis of the slow crack growth (SCG) concept in conjunction with two-parameter Weibull distribution. The first model, Model I, is a conventional probabilistic delayed-fracture model based on a concept wherein the length of the critical crack growth due to SCG is enough larger than the initial crack length. For the second model, Model II, a new probabilistic model is derived on the basis of a concept wherein the critical cracks have widely ranging lengths. Next, a four-point bending test using a wide range of stress rates was performed for soda glass and alumina ceramics. We constructed fracture probability-strength-time diagrams (F-S-T diagrams) with the experimental results of both materials using both models. The F-S-T diagrams described using Model II were in good agreement with plots of the fracture strength and the fracture time of both materials more so than Model I. [ABSTRACT FROM AUTHOR]

Monotonic tensile properties and fracture behaviour of carbon fibre filament materials, namely single/mono- and multi-filaments (two and four filaments) as well as virgin carbon tows have been evaluated and discussed. Micro composite or single fibre approach is used in this study, which facilitated the evaluation of tensile properties and nature of fracture of carbon filament materials in a relatively short time with a large number of inexpensive trials. Tensile tests have been conducted on these filament materials at ambient temperature and laboratory air atmosphere. Load–elongation and the corresponding stress–strain plots thus obtained have been analysed to understand the tensile behaviour. The peak tensile strength of single carbon filament is found to be 3.8 GPa, and the value of the resilience obtained is 19 MJ/m3. The peak tensile strength was found to increase moderately with further increase in number of filaments. However, the value of resilience was found to increase with increase in the number of fibres, which is attributed to the controlled failure of multi-filaments. On the other hand, the tensile strength of virgin carbon tow without matrix was found to be 1.13 GPa, and the value of the fracture energy was determined to be 9.9 MJ/m3, which is nearly one fourth or even less than the corresponding values of the mono- and multi-filaments. The data obtained in the case of the virgin carbon tows were further analysed to evaluate the Weibull statistical parameters. [ABSTRACT FROM AUTHOR]

In this paper, the mixed-mode interlaminar fracture behaviour of woven carbon-epoxy composite was investigated based on experimental and numerical analyses. A modified version of Arcan specimen was employed to conduct a mixed mode fracture test using a special loading device. A full range of mixed-mode loading conditions including pure mode-I and pure mode-II loading were created and tested. This test method has a simple procedure, clamping/unclamping the specimens are easy to achieve and only one type of specimen is required to generate all loading conditions. Also, finite element analysis was carried out for different loading conditions in order to determine correction factors needed for fracture toughness calculations. Interlaminar fracture toughness was determined experimentally with the modified version of the Arcan specimen under different mixed-mode loading conditions. Results indicated that the interlaminar cracked specimen is tougher in shear loading condition and weaker in tensile loading condition. Response of woven carbon-epoxy composite was also investigated through several criteria and the best criterion was selected. The interlaminar fracture surfaces of the carbon-epoxy composite under different mixed-mode loading conditions are examined by scanning electron microscopy (SEM). [ABSTRACT FROM AUTHOR]

Due to the remarkable physical and mechanical properties of individual, perfect carbon nanotubes (CNTs), they are considered to be one of the most promising new reinforcements for structural composites. Their impressive electrical and thermal properties also suggest opportunities for multifunctional applications. In the context of inorganic matrix composites, researchers have particularly focussed on CNTs as toughening elements to overcome the intrinsic brittleness of the ceramic or glass material. Although there are now a number of studies published in the literature, these inorganic systems have received much less attention than CNT/polymer matrix composites. This paper reviews the current status of the research and development of CNT-loaded ceramic matrix composite (CMC) materials. It includes a summary of the key issues related to the optimisation of CNT-based composites, with particular reference to brittle matrices and provides an overview of the processing techniques developed to optimise dispersion quality, interfaces, and density. The properties of the various composite systems are discussed, with an emphasis on toughness; a comprehensive comparative summary is provided, together with a discussion of the possible toughening mechanism that may operate. Last, a range of potential applications are discussed, concluding with a discussion of the scope for future developments in the field. [ABSTRACT FROM AUTHOR]

Knowledge of the fracture and fatigue behaviour of metallic alloys at extreme environmental temperature conditions is required to assess the safety of structural components operating in particular fields: aero-spatial, off-shore structures, power plants superconductors, polar Antarctic facilities, etc. Among the structural metallic alloys for civil, mechanical engineering and plant applications, steel S275 J2 is widely used, whereas aluminium alloys such as Al7075 T6 are significant especially for aero-spatial and polar Antarctic applications. In this paper, the main experimental mechanical characteristics of such metallic materials at room temperature as well as at low temperatures are examined. Three temperatures are considered: 293 K (+20 °C, room temperature RT), 243 K (−30 °C) and 193 K (−80 °C). The corresponding values of fracture toughness and endurance limit available in the literature are reported herein. Further, experimental tests have been performed to determine the unavailable mechanical properties. Then, the values of such fracture and fatigue parameters at various temperatures are critically discussed. [ABSTRACT FROM AUTHOR]

Composite laminates on the nanoscale have unique properties, such as high strength, high wear resistance, and biocompatibility. In this paper we report on the nanoindentation behavior of a model metal–ceramic nanolaminate consisting of alternating layers of aluminum and silicon carbide (Al/SiC) processed by PVD on a Si substrate. Composites with different layer thicknesses were fabricated and the effect of layer thickness on Young’s modulus and hardness was quantified. The effect of indentation depth on modulus and hardness was studied. The damage that took place during nanoindentation was examined by cross-sectioning the samples by focused ion beam (FIB) technique and imaging the surface using scanning electron microscopy (SEM). Finite element modeling (FEM) of nanoindentation of nanolaminates was conducted. The damage patterns observed in experiments were qualitatively supported by the numerical simulations. [ABSTRACT FROM AUTHOR]

The notched Izod impact properties of polypropylene (PP) filled with hollow glass beads (HGB) have been measured at room temperature to identify the effects of the particle contents, size and its distribution on them in the present paper. The mean diameters of the particles were 11, 35, and 70 μm, and named TK10, TK35 and TK70 respectively. The surface of the particles was pretreated with silane coupling agent. The results showed that the notched Izod impact strength ( σ I) of the filled systems increased gently with increasing the volume fraction ( φ f) of the fillers when φ f was less than 15%, and then it decreased. When φ f was 10%, σ I decreased with an increase of the mean diameter of the particles. Furthermore, the impact fracture surface of the specimens was observed by using a scanning electron microscope (SEM). The improvement of the impact toughness of the composites might be mainly attributed to the shear yielding first of the matrix around the beads to absorb relevant deformation energy under impact load. [ABSTRACT FROM AUTHOR]

In this paper we discuss various contact damage modes in thin TiN films on steel substrate with increasing load. To understand the displacement at different points along the depth of the film, we have used a TiN–AlTiN multilayered film in which each layer acts as a strain marker and we have also calculated the stresses theoretically using an elastic model of spherical indentation of a bi-layer. The study has helped to understand the physics behind different fracture phenomena, such as confinement of columnar sliding to the middle of the film, the genesis of lateral cracks during unloading, etc. We also emphasize the co-existence and competition of different modes of fracture in the film, rather than a single mode, at a particular combination of film thickness, substrate hardness and load and describe the way different modes interact in the spatial domains when they do coexist. [ABSTRACT FROM AUTHOR]

This paper examines the effects of mix compressive strength (30, 35 and 40 MPa) on the fracture initiation toughness, resistance-curve behavior and fatigue crack growth behavior of concrete mortar. The fracture initiation toughness and the resistance-curve behavior are shown to increase with increasing mix strength. The observed resistance-curve behavior is then attributed largely to the effects of ligament bridging, which are predicted using small- and large-scale bridging models. In contrast, the fatigue crack growth resistance is shown to decrease with increasing mix strength. An extended multiparameter framework was used for the modeling of fatigue crack growth. Finally, the implications of the results are discussed for the design of concrete mixtures with attractive combinations of strength, fracture toughness and fatigue crack growth resistance. [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]

The fracture process of composite laminates subjected to static or fatigue tensile loading involves sequential accumulation of intra- and interlaminar damage, in the form of transverse cracking, splitting and delamination, prior to catastrophic failure. Matrix cracking parallel to the fibres in the off-axis plies is the first damage mode observed. Since a damaged lamina within the laminate retains certain amount of its load-carrying capacity, it is important to predict accurately the stiffness properties of the laminate as a function of damage as well as progression of damage with the strain state. In this paper, theoretical modelling of matrix cracking in the off-axis plies of unbalanced symmetric composite laminates subjected to in-plane tensile loading is presented and discussed. A 2-D shear-lag analysis is used to determine ply stresses in a representative segment and the equivalent laminate concept is applied to derive expressions for Mode I, Mode II and the total strain energy release rate associated with off-axis ply cracking. Dependence of the degraded stiffness properties and strain energy release rates on the crack density and ply orientation angle is examined for glass/epoxy laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is also discussed. [ABSTRACT FROM AUTHOR]

In this paper, the effect of prestrain on the 14 behaviour as well as fracture behaviour of AA6111 has been investigated with a view to understanding its strengthening characteristics under this condition. The evaluation methods include tensile testing, differential scanning calorimetry (DSC) and fractography using scanning electron microscopy (SEM). It was found that AA6111 develops increased strength with increasing levels of prestrain during natural aging. The application of plastic prestrain prior to natural aging increases the speed with which the various phases form during natural aging, while decreasing the volume fraction of GPI zones. Prior prestrain and subsequent natural aging show improvement in the dimpled tensile fracture. However, the role of intermetallic particles is significant in that they serve as nucleation sites for fracture. [ABSTRACT FROM AUTHOR]

Ultra-fine grained steel bars were recently developed by thermo-mechanical controlled rolling with rapid cooling for increasing the strength of low carbon and low alloy steels. The developed steels are characterized by fine ferrite grains of less than 1 µm and high strength as a result of grain refinement. However, their correlations between tensile properties and impact behavior are not well understood. In this paper, impact absorbed energy (Ep) and dynamic fracture toughness (Jld) were used to evaluate the dynamic fracture behavior of the ultra-fine grained steels, and the fracture mechanisms were also investigated. For the ultra-fine grained steels, tensile stress-strain curve was shown to be correlated with the impact curve of load vs. time, and to be related to the dynamic fracture toughness. The steel with large ferrite grains, small ferrite grain colony and martensite was found to have a good combination of strength and toughness. [ABSTRACT FROM AUTHOR]

The study of bonding hard materials such as aluminium oxide and cubic boron nitride (cBN) and the nature of interfacial cohesion between these materials and glass is very important from the perspective of high precision grinding. Vitrified grinding wheels are typically used to remove large volumes of metal and to produce components with very high tolerances. It is expected that the same grinding wheel be used for both rough and finish machining operations. Therefore, the grinding wheel, and in particular its bonding system, is expected to react differently to a variety of machining operations. In order to maintain the integrity of the grinding wheel, the bonding system that is used to hold abrasive grains in place will react differently to forces that are placed on individual bonding bridges. This paper examines the role of vitrification heat treatment on the development of strength between abrasive grains and bonding bridges, and the nature of fracture and wear in vitrified grinding wheels that are used for precision grinding applications. [ABSTRACT FROM AUTHOR]

In a bending load test for brittle materials, such as ceramics for spacecraft and aircraft, decreasing the number of test specimens required is a crucial problem. This paper discusses the effectiveness of using the information of both fracture stress and fracture location to decrease the number of specimens required to obtain the same precision as the Weibull estimator. The following results were obtained: It was found that by adding the fracture location information, the precision of the Weibull parameter estimation under the optimal design became 1.5–1.9 times better compared with the case of using only the fracture stresses. This means the number of samples necessary to attain the same precision becomes 1/1.5–1/1.9. Tables and figures which give information on the number of samples necessary to attain the required precision are given. [ABSTRACT FROM AUTHOR]

A new approach to study the fracture of quasi-brittle materials is introduced: the design and testing of model materials. By model material is understood a material with enlarged microstructure and which material parameters, such as stacking and mechanical properties of particles and cohesion force, can be fully controlled. In this paper a first example to the model materials approach is presented, consisting in 5 mm steel particles bonded in a precise stacking with an epoxy-based glue. It is shown how it is possible to correlate the different fracture mechanisms and ultimate peak load of the model material to the particle pair force and to the fracture process zone size. It is also seen how a quasi-brittle behaviour is produced in the presence of mechanisms that induced the crack to shift fracture planes, that is, in presence of energy dissipative mechanisms. [ABSTRACT FROM AUTHOR]

Rock bolts have failed by Stress Corrosion Cracking (SCC). This paper presents a detailed examination of the fracture surfaces in an attempt to understand the SCC fracture mechanism. The SCC fracture surfaces, studied using Scanning Electron Microscopy (SEM), contained the following different surfaces: Tearing Topography Surface (TTS), Corrugated Irregular Surface (CIS) and Micro Void Coalescence (MVC). TTS was characterised by a ridge pattern independent of the pearlite microstructure, but having a spacing only slightly coarser than the pearlite spacing. CIS was characterised as porous irregular corrugated surfaces joined by rough slopes. MVC found in the studied rock bolts was different to that in samples failed in a pure ductile manner. The MVC observed in rock bolts was more flat and regular than the pure MVC, being attributed to hydrogen embrittling the ductile material near the crack tip. The interface between the different fracture surfaces revealed no evidence of a third mechanism involved in the transition between fracture mechanisms. The microstructure had no effect on the diffusion of hydrogen nor on the fracture mechanisms. The following SCC mechanism is consistent with the fracture surfaces. Hydrogen diffused into the material, reaching a critical concentration level. The thus embrittled material allowed a crack to propagate through the brittle region. The crack was arrested once it propagated outside the brittle region. Once the new crack was formed, corrosion reactions started producing hydrogen that diffused into the material once again. [ABSTRACT FROM AUTHOR]

In this paper, the results of the examination of the effect of boron, in the amounts of 0.26, 0.39 and 0.59 wt%, on the stereological characteristics of the structural phases present in the structure of 13%Cr–2.3%C white iron, are reviewed. The examined stereological characteristics were: volume fraction of phases, dendrite arm spacing-DAS and size of eutectic carbides. It has been concluded that with the increasing of boron content, the volume fraction of the matrix of austenite partially transformed into martensite decreases, whereas the volume fraction of the eutectic carbides increases. The increasing of the boron content also decreases the value of DAS, but at the same time the eutectic carbides become coarser. [ABSTRACT FROM AUTHOR]

This paper presents the results of tensile tests on viscoelastic films of two materials: a 100 pen bitumen and a commercial corn syrup. Experiments were performed using double-cantilever beam specimens, and butt-joint specimens, for a wide range of testing conditions. Failure by brittle fracture, voiding, and various viscous flow mechanisms was observed. The results are presented in the form of maps and are discussed in conjunction with appropriate mathematical models. [ABSTRACT FROM AUTHOR]

This paper studies the influence of Ni3AI intermetallic particles on the fracture behaviour of aluminium matrix (2014) composite materials. Intermetallics were obtained by mechanical alloying and by atomisation. The composite materials were manufactured by mixing, uniaxial compacting of a preform, and subsequent extrusion without canning or degassing. The study considered materials in extruded state and after T6 heat treatment. Assessments were made from the viewpoint of microstructure (by means of optical and scanning electron microscopy), and studying the reactions between the matrix and the reinforcement. These reactions produce a highly copper-enriched interphase. The influence of the reinforcement and state of the alloy on the fracture behaviour of the composite materials was studied through scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Measurement or prediction of the mechanical and fracture properties of foods is very important in the design, operation and optimization of processes, as well as for the control of quality of food products. This paper describes the measurement of yield stress of frozen sucrose solutions under indentation tests using a spherical indenter. Effects of composition, temperature and strain rate on yield stress of frozen sucrose solutions have also been investigated. [ABSTRACT FROM AUTHOR]

The moisture content is known to influence the properties of wood. While mechanical parameters like stiffness and strength decrease with increasing moisture content, investigations on the influence on fracture properties have been concentrated on linear elastic fracture mechanics. In this paper, the wedge splitting method is used to investigate also the non-linear fracture behaviour of sprucewood at different moisture contents. Complete load-displacement curves at four different moisture levels are evaluated. While the initial slope and the maximum stress state are decreasing with increasing moisture content the specific fracture energy is increasing. It is shown that this behaviour can be attributed to an increase in ductility which increases both the dissipated energy during crack initiation and the one in the crack propagation phase. [ABSTRACT FROM AUTHOR]

Hybrid hydroxyapatite/polymer composites were prepared by the infiltration of nylon into porous hydroxyapatite. Porous hydroxyapatite (HAp) bodies were prepared from a whisker-like powder with high aspect ratio by pressureless-sintering at various temperatures. Pore characteristics, such as the fraction of open porosity and the pore size distribution, were designed and evaluated by mercury porosimeter. Through the in situ polymerization of ε-caprolactam, infiltrated into the porous HAp body, a polymeric secondary phase network interpenetrated with the HAp phase was obtained. The obtained hybrid HAp/nylon composites were evaluated with respect to their fracture behavior, i.e., fracture energy, and in vitro bioactivity in simulated body fluid (SBF) in the present paper. These HAp/nylon hybrid composite have a KIC of 1.65 MPam1/2 and also a good bioactivity according to the results of SBF immersion tests. [ABSTRACT FROM AUTHOR]

The radial direction of wood is reinforced by an additional tissue called rays. These rays are one of the reasons for the anisotropy of wood in the transverse plane. In this paper the influence of rays on the mechanical properties like tensile strength as well as on fracture mechanical parameters like critical stress intensity factor and specific fracture energy is shown. By investigating two deciduous tree species with a similar wood structure in general but a different ray characteristic in particular the importance of this radial reinforcement of the wood structure could be demonstrated. The relevance for the living tree is discussed. [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]

The newly derived relationship between the closure traction and the crack opening displacement by the modified shear-lag model is used to investigate the tensile failure behaviors of unidirectional fiber reinforced ceramics. The critical stress for matrix cracking and the critical stress to fracture the fiber are calculated for various crack configurations. Then, the failure of composite initiates as the applied stress exceeds the smaller of the matrix cracking stress and the fiber fracture stress. The differences of results between the present analysis and Marshall and Cox are discussed. Finally, the possible tensile failure modes and the transition conditions between different failure modes are summarized in this paper. [ABSTRACT FROM AUTHOR]

The reinforcement in glass fibre reinforced cement (grc) is not present as discrete fibres, but as ‘strands’ of about 200 filaments each. This configuration greatly complicates determination of the ‘perimeter’ of the reinforcing elements, a crucial parameter in bond strength determination. Previous investigators have attempted to quantify strand perimeters but their methods have always involved at least one subjective step and are prone to operator bias. This paper describes an objective method of determining strand perimeters using digital analysis of images captured from petrological thin sections. The measured perimeters were found to be sensitive to the “threshold value” chosen for the analysis, and consideration of perimeter vs. threshold value curves eliminated subjectivity involved in the analysis. The perimeter value obtained, together with microscopic analysis of the crack patterns produced during tensile testing, were used for calculating bond parameters for different cement matrices. The development of bond with both ageing time and temperature were also studied. The method uses fundamental image analysis concepts and as such is readily adaptable to solve conceptually similar problems in a wide range of materials. [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]