Initial Microcracks Research Articles

The cracking behavior of anodic films on cast aluminum alloy after heating in the temperature range up to 300 °C

The cracking behavior of sealed anodic films on cast aluminum alloy after heating in the temperature range up to 300 °C was studied and the effects of anodizing temperature, heating temperature and heating rate on cracking behavior were investigated. The results showed that before heating some micro-cracks were present in sealed anodic films on the aluminum alloy tested. After heating between 100 °C and 300 °C, the initial micro-cracks became wider and deeper, and new cracks also may be initiated in the film. As anodizing temperature increased, both the crack density and the crack width increased after heating, which was attributed to increase of the porosity of the anodic films formed at higher temperatures. At higher heating temperature, the cracks obviously got wider, but the crack density remained almost unchanged. Increased heating rate resulted in more cracks in the anodic film, indicating that higher strain rate may promote initiation of the micro-cracks, while cooling rate had little influence on cracking behavior.

Early failure of a zirconia femoral head prosthesis: Fracture or fatigue?

Zirconium ceramics are one of the most controversial prosthetic biomaterials with good mechanical properties on the one hand, but early failure incidents on the other hand. A case of an early failure of zirconia femoral head in a 73-year-old clergy man, who underwent a total hip arthroplasty 28 months before, is reported. The retrieved femoral head fragments, during revision surgery, were studied using scanning electron microscopy. Although a randomly occurred fracture without biomechanical significance can be a possible explanation, the collected data give evidence that initial non-critical microcracks or crazes due to contact stresses or strain mismatches or both between the ceramic head and the metallic tapered neck of the prosthesis led to the fatigue of the material. The final outcome was a catastrophic fracture of the head. Fixation of the neck into the zirconia head, with no mismatching or stress differences, would solve the fatigue-induced head destruction.

A fracture mechanics model for the analysis of micro-pitting in regard to lubricated rolling–sliding contact problems

A computational model is presented for the analysis of micro-pitting in regard to lubricated rolling–sliding contact problems. This model assumes the appearance of an initial microcrack on the contact surface due to the mechanical or thermal treatment of the material, and as a consequence of an on-going process in early the stage of exploitation. The discretised model of the contacting mechanical elements is subjected to normal loading (Hertzian contact pressure), tangential loading (friction between contacting surfaces) and internal pressure to the crack surfaces. Crack propagation is predicted as follows: (1) using modified maximum tangential stress criterion, which takes into account the influence of stress intensity factors K I and K II, T-stress, stress on the crack’s surface caused by lubricant pressure inside the crack, and the critical distance ahead of the crack tip and (2) the classical maximum tangential stress criterion, which only takes into account the influence of the stress intensity factors K I and K II. The stress intensity factor based on these two criteria is then used in a short crack growth theory to determine the fatigue life of an initial crack to extent up to micro-pit. The developed model is applied to a real spur gear pair.

Evaluation of microstructural parameters for micro/macro-line crack damage model

Considered is a line crack model that can account for multiple unsymmetrical microcracking patterns even the applied macrostress is symmetrical across the plane of the initial microcrack. The multiple unsymmetric micro-branchings arise from the three essential parameters that are implicit in the micro/macro-model. They account for the inhomogeneity and/or anisotropy of the material. Matching of the analytical predictions from the assumed fracture criteria and the physical observations are problem specific and subjective. The work prepares the ground for the development of the general scheme known as the physical–analytical matching approach. The main objective is to link the results found from several size and time scales that are referred to as multiscaling. Demonstrated is the linking of macro- and micro-effects. In particular, the model will show that micro-unsymmetry need not be explicitly invoked in the macro-geometry and/or loading. The double singularity line crack model shows that a variety of single, double and triple branching patterns can be predicted from the volume energy density criterion in contrast to the maximum stress criterion that is limited in predictive capability. There is no loss in generality by using a linear analysis as long as the scale range is segmented to exclude non-linearity. Moreover, the emphasis on the energy level and characteristic length of the micro- and macro-defects rather than precise geometric shape of the defect which can be a line, notch or other configurations. Such an approach is justified by the existence of a hierarchy of spatial and temporal combination of the material damage process. The ones that are commonly encountered are the macromechanical, microstructural and nanochemical. This study is concerned with the microstructural where macro-loading applied to a structural body can introduce microcracking because of the interaction of inherent microscopic defect with the material internal structure.

A combined fracture‐micromechanics model for tensile strain‐softening in brittle materials, based on propagation of interacting microcracks

AbstractStrain‐softening is the decline in stress at increasing strain. Although microcracking is a commonly accepted reason for strain‐softening, the majority of theoretical developments involve macroscopic damage evolution laws. To improve this situation, we propose a micromechanics‐based damage evolution law by combining (i) the propagation criterion for a single penny‐shaped crack embedded in an infinite matrix subjected to remote stresses (taken from linear‐elastic fracture mechanics) and (ii) stiffness estimates for representative material volumes comprising interacting microcracks (taken from continuum micromechanics). This combination allows for modelling tensile strain‐softening as a result of propagation of interacting microcracks, i.e. as a microstructural effect. The initial degree of damage, i.e. the initial microcrack size and the number of microcracks per unit volume, implies two different types of model‐predicted tensile strain‐softening behaviour under strain control: (i) continuous strain‐softening, which occurs in case of initial damage above a critical value, and (ii) an instantaneous stress drop at the peak load (‘snap‐back’), which occurs in case of initial damage below a critical value. Copyright © 2006 John Wiley & Sons, Ltd.

Experimental study on a mortar. Temperature effects on porosity and permeability. Residual properties or direct measurements under temperature

This study is a part of a large experimental program intended to characterize the effects of increasing temperature upon the hydraulic properties of cement based materials. Initial gas permeability and porosity values of the used material, a normalized mortar with a water / cement ratio of 0.5, clearly show the material homogeneity after a drying phase at 60 °C. The initial permeability is moreover insensitive to confining pressure variation which is evidence of the absence of significant initial micro-cracking in which the flow could occur. Residual properties (at room temperature) were measured after thermal treatments at 150 or 250 °C. These heating phases lead to a clear increase in porosity and permeability being sevenfold its initial value after a 250 °C treatment. This increase in permeability comes from two distinct effects that can be experimentally highlighted: a pore widening observed with the Klinkenberg effect being lower and a micro-crack closure occurring with the increase in confining pressure. Permeability was also measured with gas injection in samples submitted to thermal loading and slightly confined (at 4 MPa). Three levels of temperatures were used: 25, 105 and 200 °C; in a first stage the permeability remained almost constant to finally significantly increased at 200 °C. At 105 °C there is a widening of pores observed with the Klinkenberg effect but not sufficient to vary the permeability. At 200 °C micro-cracks have occurred but their influence upon permeability is lower than for treated material. This is an effect of the confining pressure applied on the material during heating.

Dynamic Torsion Testing of Nanocrystalline Coatings Using High-Speed Photography and Digital Image Correlation

The strength and ductility of microcrystalline and nanocrystalline tungstsen carbide-cobalt (WC-Co) cermets have been evaluated by employing a stored energy Kolsky bar apparatus, high-speed photography and digital image correlation. The test specimens were thin-walled tubular AI7075-T6 substrates 250 μm thick, coated with a 300 μm thick microcrystalline or nanocrystalline WC-Co layer with an average grain size of about 3 μm and 100 nm, respectively. Dynamic torsion experiments reported in this paper reveal a shear modulus of 50 GPa and a shear strength of about 50 MPa for both microcrystalline and nanocrystalline WC-Co coatings.The use of high-speed photography along with digital image correlation has shown that damage to the coating coincides with a significant softening on the stress-strain curve. The coating failed in mode III, and strong interactions between the crack faces were probably responsible for the increase in load after failure of the coating. The overall failure of the coating-substrate system was not brittle but rather progressive and controlled by the ductility of the aluminum substrate.A methodology for investigating damage kinetics and failure has been established. This methodology can be applied to examine the behavior of other advanced materials that can be manufactured as coatings on ductile substrates. Manufacturing coatings free of initial microcracks remains a significant challenge. Research on optimization of the spray deposition parameters should be pursued to produce high-quality nanostructured coatings that can fully exploit the benefits of nano-size grains.

Dynamic Torsion Testing of Nanocrystalline Coatings Using High-Speed Photography and Digital Image Correlation

The strength and ductility of microcrystalline and nanocrystalline tungstsen carbide-cobalt (WC-Co) cermets have been evaluated by employing a stored energy Kolsky bar apparatus, high-speed photography and digital image correlation. The test specimens were thin-walled tubular AI7075-T6 substrates 250 μm thick, coated with a 300 μm thick microcrystalline or nanocrystalline WC-Co layer with an average grain size of about 3 μm and 100 nm, respectively. Dynamic torsion experiments reported in this paper reveal a shear modulus of 50 GPa and a shear strength of about 50 MPa for both microcrystalline and nanocrystalline WC-Co coatings. The use of high-speed photography along with digital image correlation has shown that damage to the coating coincides with a significant softening on the stress-strain curve. The coating failed in mode III, and strong interactions between the crack faces were probably responsible for the increase in load after failure of the coating. The overall failure of the coating-substrate system was not brittle but rather progressive and controlled by the ductility of the aluminum substrate. A methodology for investigating damage kinetics and failure has been established. This methodology can be applied to examine the behavior of other advanced materials that can be manufactured as coatings on ductile substrates. Manufacturing coatings free of initial microcracks remains a significant challenge. Research on optimization of the spray deposition parameters should be pursued to produce high-quality nanostructured coatings that can fully exploit the benefits of nano-size grains.

Stochastic microcrack nucleation in lamellar solids

Growth of a crack across an interface between two grains of an elastic lamellar material having different lamellar orientations is investigated for materials having a heterogeneous spectrum of individual lamellar toughnesses. Numerical analyses carried out using a cohesive zone model and the finite element method show that microcracking in the adjacent lamellae can preferentially occur at low-toughness lamellae spatially remote from the crack tip rather than at higher-toughness lamellae close to the crack tip. An analytic model based on linear elastic fracture mechanics and an initial microcrack is shown to predict the location and macroscopic toughness at which microcrack nucleation and growth occur in good agreement with the numerical analyses, using only the initial microcrack size as a single parameter. These results demonstrate that microcrack nucleation requires a sufficiently high stress over a sufficiently large region and thus that microcrack nucleation ahead of a main crack can be the dominant small-scale damage mechanism in such heterogeneous systems.

Micromechanisms and modelling of crack initiation and growth in tool steels: role of primary carbides

Abstract Micromechanisms of damage initiation and crack growth in high speed and cold work steels are investigated using scanning electron microscopy in situ experiments. The role of primary carbides in initiation and growth of cracks in tool steels is clarified. It is shown that initial microcracks in the steels are formed in primary carbides and then join together. A hierarchical finite element model of damage initiation, which included a macroscopic model of the deformation of the specimen under real experimental conditions and a mesomechanical model of damage in real microstructures of steels, was developed. Using the hierarchical model, the conditions of local failure in the steels have been obtained. In order to study the effect of carbide arrangement on fracture, numerical simulations of fracture in steels with different ideal carbide arrangements were carried out and compared with each other. It is found that the heterogeneous arrangements of primary carbides can lead to strong deviations of a cra...

콘크리트의 파괴저항에 대한 균열속도의 영향

Tests of concrete CLWL-DCB specimens had been conducted with displacement-controlled dynamic loading. The crack velocities for 381mm crack extension were 0.80 mm/sec ~ 215m/sec. The external work and the kinetic and strain energies were derived from the measured external load and load-point displacement. The fracture resistance of a running crack was calculated from the fitted curves of the fracture energy required for the tests. The standard error of the fracture energy was less than 3.2%. The increasing rate of the fracture resistance for 28 mm initial crack extension or micro-cracking was relatively small, and then the slope of the fracture resistance increased to the maximum value at 90∼145 mm crack extension depending on crack velocity. The maximum fracture resistance remained for 185 mm crack extension, and then the faster crack velocity showed the faster decreasing rate of the maximum fracture resistance. The maximum fracture resistance increased proportionally to the logarithm of the crack velocity from 142 N/m to 217 N/m when the crack velocity was faster than 0.273 m/sec. The maximum fracture resistance of the fastest tests was similar to the average fracture energy density of 215 N/m. To measure the fracture resistance of concrete, the stable crack extension should be larger than 90∼145 mm depending on crack velocity.

微視的不均質をもつ複合組織鋼の材料特性制御 （第１報） 微視的不均質をもつフェライト‐マルテンサイト二相組織鋼における破壊じん性試験でのき裂の発生および伝ぱ挙動

It is important to study effects of microstructure on the characteristics of fracture in steels with microscopic heterogeneity. Crack initiation and propagation behaviors are of course dominant to the characteristics of fracture, and they are studied in two kind of ferrite/martensite dual-phase steels with the same micro-hardness in each microscopic phase. Three-point bending tests are performed to two kind of dual-phase steels, one has 30% volume fraction of ferrite and 70% of martensite (Ferrite 30% steel), and the other is Ferrite 70% steel. Brittle cracks are mainly initiated in the ferrite phase near the fatigue precrack tip in the center of specimen thickness by bending load, regardless of the difference of the volume fraction of microstructure. It is considered that not only the volume fraction of microstructure but also the stress-strain field due to microscopic heterogeneity affects crack generation. Crack mainly propagates along ferritic phase with connection to initial microcracks, and it is little influence on microscopic heterogeneity. Propagated crack width expands in proportion with the increase of CTOD value.

Tribological behavior of boundary lubricated sliding surfaces using three different spacing of surface profiles

The ball-on-disk type sliding tests with boundary lubricated steels were carried out to verity the effect of initial spacing in surface profiles on wear and scuffing. Three kinds of surface spacing, which are closely related with initial surface micro-cracks on sliding surfaces, were produced onAISI 1045 steel surfaces using different grinding and polishing processes. Frictional forces and time to scuffing were measured, and the shape and amount of wear particles were analyzed to compare the with original surface profiles. From the tests, it was confirmed that the size of wear particles are related closely to the original spacing of the surface profile. The time to failure and amount of wear were sensitive to the surface spacing. The wider surface spacing shows much longer sliding life and smaller amount of wear than the others. Time to scuffing was increased with increasing surface profile spacing. The size of wear particles increased while the wear and wear rateK were decreased with an increase in surface spacing. After the sliding tests, surface cracks of inner parts of the wear track formed due to scuffing were observed and compared among the specimens having the different surface spacing.

Crack Initiation Model from Asphalt Slab Tests

The Strategic Highway Research Program (SHRP) fatigue crack initiation life prediction model was based on a sophisticated analysis of the results of intensive bending beam fatigue tests under controlled stress and controlled strain. The definition of crack initiation was defined as the period during which initial microcracks developed into a macrocrack 7.5-mm deep. Due to the lack of the measured crack data, the numbers of loading cycles corresponding to the 7.5-mm deep crack were not observed from the tests but predicted based on a microfracture mechanics model. The objective of this study was to evaluate SHRP fatigue crack initiation life prediction model with the crack measurements in laboratory slab fatigue tests. The study showed that the SHRP crack initiation life model significantly underestimated the fatigue crack initiation life for the slabs. However, the ratio of the observed fatigue life from the slab tests to the crack initiation life predicted from SHRP equation was shown to be approximately constant.

A Micromechanical failure model for unidirectional fiber reinforced composites

Results from a newly developed model for failure in unidirectional fiber reinforced composites based on fracture mechanics are presented. The model is based on (i) analysis of branch crack initiation from the tip of an initial microcrack embedded in an anisotropic material, which serves as a model for the fiber reinforced composite and (ii) evaluation of anisotropy in homogenized elastic material properties and fracture toughness. Based on this model, failure envelope, dominant initial flaw orientation and failure mode for unidirectional fiber reinforced composites under a wide range of stress states are predicted. Parametric study provides quantitative evaluation of the effect of various mechanical and physical properties on the failure behavior, and their influence on strength is identified. Results from the current model are compared with those of existing phenomenological models, which show reasonable agreement.

Fatigue and Fracture Behavior of a Steel Cord/Rubber Composite

The objective of this study was to investigate the fatigue and fracture behavior of steel cord/rubber composites used in tire belts under constant cyclic strain loading. The material was a specially made tire belt layer in the form of rubber sheets reinforced with unidirectional cords consisting of two pairs of twisted steel wires. Failure mechanisms, damage development, and fatigue life were determined for single belt layers with different cord orientations. Tests were conducted at cord angles of 22°, 72°, and 90° degrees with a cyclic strain amplitude of 8.3% at a frequency of 10 Hz. Five different stages of damage development were observed: microcrack initiation, microcrack multiplication, macrocrack formation, slow macrocrack propagation, and fast macrocrack propagation leading to final failure. In the case of the 22° cord specimens, where the in-plane shear component was dominant, damage development consisted of microcrack initiation at the cord/rubber interface, the formation of more microcracks and macrocracks, and finally the formation of a major fatal macrocrack along the cord direction. In the case of 90° cord specimens, dominated by transverse tension, initial microcracks occurred within the cord, they propagated across the thickness of the specimen, and finally a major macrocrack propagated across the entire width of the specimen. The final crack propagated in part along the cord/rubber interface and in part within the cord. In the case of the 72° cord specimens, where both in-plane shear and transverse tension are critical, the initial microcracks occurred within the cord and the final macrocrack along the interface. For the same cyclic strain amplitude, the 90° specimens had the shortest fatigue life, and the 72° specimens had the longest. Additional tests were conducted at different strain amplitudes. The normalized modulus decreases slowly and nearly linearly with normalized fatigue lifetime up to a certain value of the latter, approximately 80% of the normalized logarithmic lifetime, and then it drops sharply. Cyclic strain amplitude also affects the failure mechanisms. High amplitudes produce localized damage, whereas low amplitudes produce dispersed damage. A residual life model was proposed based on stiffness degradation.

Formation of Surface Microcrack for Separation of Nonmetallic Wafers Into Chips

In recent years a technology for a high quality separation of nonmetallic materials into chips using a surface (“blind”) microcrack attracted considerable attention in the electronic industry. In this method a wafer is positioned on the translated X-Y table and is heated by a laser beam up to a temperature of the order of 300–400°C. The wafer is then cooled by an air-water spray, and a surface microcrack is formed due to relaxation of the thermal stresses. The initial microcrack with a depth of the order of several hundred microns then propagates in a subsurface region of a wafer and follows the path of the laser beam. Theoretical modeling based on the solution of the equations of thermal elasticity was performed to determine the distributions of temperature and thermal stresses that cause formation of an “edge” microcrack (at the edge of a wafer) followed by its transformation into a surface microcrack. The results of thermal stresses analysis are in an agreement with experimental observations. [S1043-7398(00)00804-5]

Impact-induced delamination of composites: a 2D simulation

The delamination process in thin composite plates subjected to low-velocity impact is simulated using a specially developed 2D cohesive/volumetric finite element scheme. Cohesive elements are introduced along the boundaries of the inner layers and inside the transverse plies to simulate the spontaneous initiation and propagation of transverse matrix cracks and delamination fronts. The analysis is performed within the framework of the finite deformation theory of elasticity to account for the nonlinear stiffening of the thin composite plate and the large rotations which accompany the fracture process. The simulation is dynamic and uses an explicit time stepping scheme. Comparison with existing experiments performed on graphite/epoxy laminates indicates that the cohesive/volumetric finite element scheme is able to capture the complex mechanisms leading to the delamination, including the initial micro-cracking of the matrix, the appearance of critical transverse matrix cracks and the rapid propagation of delamination cracks initiated at the intersections between the critical matrix cracks and the adjacent plies.

Brittle solid under compression. Part I: Gradient mechanisms of microcracking

The venerable problem of the origins of cracking and failure of a brittle heterogeneous solid (heterogen) under compression is analyzed here from a new point of view, with the cause of its microcracking and atrophy (degeneration) under load, found, first of all, in the differences in Poisson's ratio of its components.Four fundamental phenomena, found in experiments, are the basis of the new approach to the behavior and failure of a heterogen under compression. (1) The non-linear part of the ascending branch of SSc is due to formation and accumulation of stable microcracks. (2) The intrinsic elastic modulus remains constant up to the peak point. (3) Concrete and rock fail in splitting. (4) The pattern of SSc is the same for different types of concrete and rock under different types of loading.It is found that some kinds of gradient mechanisms can induce local transverse strains of tension and cause microcracking in a heterogen under compression. The first creates local strain gradients among the components due to difference in their Poisson extension, when the components with lower Poisson's ratio are tensioned in the lateral direction. There is also a mechanism which creates an internal thrust due to gradient in the elastic moduli of the component.It is shown that, in a brittle solid built from randomly oriented crystals, a population of laterally tensioned crystals, called “acrons”, are created due to gradients in Poisson's ratio of a single crystal along its three axes. The models of gradient strain in the acrons are given, including the equation of critical strains. The problem of crystals acting as “pistons” due to a process of sliding is also discussed.The gradient models explain the appearance of microcracks and their stochasticity and why, instead of growing into macrocracks they are stable, in good accordance with a vast number of experiments. Gradient mechanisms, especially that of Poisson, are universal, descriptive and based on measurable parameters. They suffice to exhaust the bearing capacity of a heterogen under increasing compression without recourse to shear stresses. They affect every heterogen under compression: rock materials, concrete, ceramics and do not need initial microcracking to initiate and realize the process of atrophy (degradation) of the brittle solid.

Fractal dimension evolution of microcrack net in disordered materials

Fractal geometry is used to evaluate the degree of disorder of crack size distribution in brittle damaging materials. The fractal dimension of the 2D microcrack net turns out to increase from one to two during the loading process and microcrack propagation. This means that the material becomes more disordered with the damage evolution. The longer cracks, in fact, propagate more rapidly than the shorter and, at the same time, the crack size distribution increases its statistical dispersion. Some numerical examples, related to different initial microcrack densities and size distributions, are illustrated with the computer simulation of the system evolution.