Crack propagation in ceramics under cyclic loads

Stable crack growth is observed in notched plates of polycrystalline alumina subject to fully compressive far-field cyclic loads at room temperature in a moist air environment andin vacuo. The fatigue cracks propagate at a progressively decreasing velocity along the plane of the notch and in a direction macroscopically normal to the compression axis. The principal failure events leading to this effect are analysed in terms of notch-tip damage under the far-field compressive stress, microcracking, frictional sliding and opening of microcracks, and crack closure. An important contribution to such Mode I crack growth arises from the residualtensile stresses induced locally at the notch-tip when the deformation within the notch-tip process zone leaves permanent strains upon unloading from the maximum nominal compressive stress. It is shown that the phenomenon of crack growth under cyclic compressive stresses exhibits a macroscopically similar behaviour in a wide range of materials spanning the very ductile metals to extremely brittle solids, although the micromechanics of this effect are very different among the various classes of materials. The mechanisms of fatigue in ceramics are compared and contrasted with the more familiar examples of crack propagation under far-field cyclic compression in metallic systems and the implications for fracture in ceramic-metal composites and transformation toughened ceramic composites are highlighted. Strategies for some important applications of this phenomenon are recommended for the study of fracture mechanisms and for the measurement of fracture toughness in brittle solids.     

Effect of substrate roughness on the contact damage of thin brittle films on brittle substrates

The effect of substrate and surface roughness on the contact fracture of diamond-like carbon coatings on brittle soda-lime glass substrates has been investigated. The average surface roughness (R_a) of the examined samples ranged from 15 nm to 571 nm. Contact damage was simulated by means of spherical nanoindentation, and fracture was subsequently assessed by focused ion beam microscopy. It was found that, in the absence of sub-surface damage in the substrate, fracture occurs in the coating in the form of radial, and ring/cone cracks during loading, and lateral cracks during unloading. Increasing the surface roughness results in a decrease in the critical load for crack initiation during loading, and in the suppression of fracture modes during unloading from high loads. When sub-surface damage (lateral cracks) is present in the substrate, severe spalling takes place during loading, causing a large discontinuity in the load-displacement curve. The results have implications concerning the design of damage-tolerant coated systems consisting of a brittle film on a brittle substrate.     

Effect of cyclic deformation on the resistance of a material to brittle failure

A physicomechanical model is suggested for determining the critical brittle failure stress S_c in statically and cyclically deformed material. The model is based on ideas about retardation of microcracks in a deformed substructure which changes during material loading. The effect is determined by experiment of prior cyclic material deformation on resistance to cleavage. The calculated dependence of S_c on the Odquist parameter is compared with the experimental results obtained. Model ideas about the effect of material substructure on brittle failure are confirmed by fractographic studies. Unstable fatigue crack growth is studied on the basis of models for brittle and fatigue failure of BCC-metals.Translated from Problemy Prochnosti, No. 1, pp. 14–21, January, 1991.     

Experimental Analysis of the Influence of Structural Parameters on the Behavior of Glass-Fiber Reinforced Polypropylene Composites

In a composite material reinforced by short random fibers, damage results from different elementary failure mechanisms such as matrix microcracking, fiber pull out, failure of the fiber/matrix interface, failure of fibers, etc. These damages influence greatly the macroscopic behavior of composite materials. To obtain good mechanical performance of a composite material, it is important to optimize the fiber ratio and the quality of the fiber/matrix interface, which have a direct influence on the damage mentioned above. The main objective of this study is to determine the influence of structural parameters on the evolution of damage for two types of polypropylene glass-fiber reinforced composites. In parallel with the classical approach of the mechanical theory of damage, which consists in load–unload tensile tests, the use of acoustic emission allows one to follow in real time the character and the importance of damage mechanisms in the course of loading. In addition, fractographic analysis makes it possible to confirm different assumptions and conclusions from this study.     

Simulation of brittle and ductile fracture in an impact loaded prenotched plate

We analyze the initiation and propagation of a crack from a point on the surface of a circular notch-tip in an impact loaded prenotched plate. The material of the plate is assumed to exhibit strain hardening, strain-rate hardening, and softening due to the rise in temperature and porosity. The degradation of material parameters due to the evolution of damage in the form of porosity is considered. Brittle failure is assumed to initiate when the maximum tensile principal stress at a point reaches a critical level. Ductile failure is assumed to ensue when the effective plastic strain reaches a critical value. A crack initiating from the node where a failure first occurs is taken to propagate to the adjacent node that has the highest value of the failure parameter (the maximum tensile principal stress or the effective plastic strain). The opening and propagation of a crack are modeled by the node release technique. Surface tractions and the normal component of the heat flux are taken to be null on the newly created crack surfaces. For the brittle failure, the stress field around the crack tip resembles that in mode-I deformations of a prenotched plate loaded in tension. The distribution of the effective plastic strain in a small region around the surface of the notch-tip is not affected much by the initiation of a ductile fracture there except for a shift in the location of the point where the effective plastic strain is maximum. The initiation of the ductile failure is delayed when a crack is opened at the point where the brittle failure ensues.     

A critical shear displacement criterion for ductile–brittle transition under mixed mode I and II loading

Abstract

Micromechanisms producing ductile and brittle damage operate in parallel at a crack tip. The dominant mode of failure depends upon which of the two (ductile or brittle) damage parameters first reaches its critical value. This has been shown by a study of ductile–brittle transition behaviour in HY100 steel under mixed mode I and II loading. The transition from ductile to brittle behaviour in HY100 steel was found to be affected by mixed mode I and II ratio (ratio of imposed tensile and shear loading) in a manner such that with increasing shear the transition temperature decreased. In the present paper, a criterion is proposed based on the shear strain ahead of a notch tip, to predict the fracture behaviour at any given temperature and mixed mode ratio.     

Calculation of inelastic notch-tip strain-stress histories under cyclic loading

An application of the equivalent strain energy density method for calculation of elastic-plastic notch-tip strains under cyclic loading is presented. It is shown that the theoretical notch-tip strain calculations can be improved if the stress redistribution due to the plastic yielding around the notch-tip is taken into account. The energy density method, corrected for plastic yielding gave good results almost up to the general plastic yielding, i.e. $\text{S = σ}{}_{\mathrm{ys}}$. It was also found that a universal function for the elastic stress distribution ahead of a notch tip can be derived for both tension and bending loads. Several different notches and materials were analyzed. The equivalent strain energy density concept can easily be used for a simulation of the notch-tip cyclic stress-strain history if the stress concentration factor$\text{K}{}_{\mathrm{t}}$, the cyclic stress-strain curve σ−ϵ and nominal stress/load history are known. Good notch-tip strain predictions were achieved for both the monotonie and cyclic load.     

Machinability of Metal Matrix Composites Reinforced by 3-D Network Structure

Metal matrix composites reinforced by three-dimensional (3-D) continuous network structure reinforcement (3DCNRMMC) are difficult to machine due to serious tool wear and poor surface roughness caused by the brittle and hard reinforcement which interpenetrate into ductile matrix. In order to achieve the approach of low cost of 3DCNRMMC, the machinability of it needs to be understood. The influences of three cutting parameters and volume fraction of reinforcement on cutting force were analyzed in detail. The results indicate that: (1) Due to the brittle phase(s) introduced into ductile matrix of composites, there is a large fluctuation of cutting force causing deterioration of machinability. The fluctuation ranges of cutting forces, initially increase rapidly with the increase of volume fraction of reinforcement and then decrease finally, are largest at the range of the volume fraction of 55–65%; (2) The influence of cutting parameters on cutting force is obvious. With the increases of cutting speed, cutting force decreases gradually unless cutting speed exceeds the value of 209 m/min. Cutting forces increase with increasing feed rate and depth of cut; (3) Owing to the large fluctuation of cutting force, there were some cratered surfaces caused by Si₃N₄ reinforcement pulling-out and flaking-off. Some brittle phase protruding from the machined surface caused the deterioration of machined surface.     

Fatigue crack growth in polymers subjected to fully compressive cyclic loads

It is experimentally demonstrated in this work that the application of cyclic compression loads to polymeric materials, specifically high-density polyethylene and polystyrene, results in the nucleation and propagation of stable fatigue cracks. The cracks grow at a progressively slower rate along the plane of the notch in a direction perpendicular to the far-field cyclic compression axis. The overall characteristics of this compression fatigue fracture are macroscopically similar to those seen in metals, ceramics, as well as discontinuously reinforced inorganic composites. It is reasoned that the origin of this Mode I compression fatigue effect is the generation of a zone of residual tensile stress locally in the vicinity of the notch-tip upon unloading from the maximum far-field compressive stress. The residual tensile field is generated by permanent damage arising from crazing and/or shear deformation ahead of the notch-tip. Evidence for the inducement of residual tensile stresses on the crack plane is provided with the aid of micrographs of near-tip region where crazes are observed along the plane of the crack, i.e. normal to the compression loading axis. Compression fatigue crack growth in polystyrene is also highly discontinuous in the sense that the crack remains dormant during thousands of fatigue cycles following which there is a burst of crack extension, possibly in association with fracture within the craze. This intermittent growth process in cyclic compression is analogous to the formation of discontinuous growth bands during the tension fatigue of many crazeable polymers. The exhaustion of the near-tip residual tensile field and the increase in the level of crack closure with increasing crack length cause the fatigue crack to arrest. The universal features of this phenomenon are discussed in the context of ductile and brittle, non-crystalline and crystalline, as well as monolithic and composite materials.     

A stochastic model of damage accumulation in complex microstructures

A statistical approach for modeling fracture in brittle materials is presented. In particular, a microstructural-based finite element code called OOF is used in conjunction with a stochastic representation of failure that relies on the Weibull law. The OOF code, which maps materials microstructures onto finite element meshes, enables to calculate the local stress states; these stresses are used along with the statistical criterion for brittle fracture in order to determine microcrack formation and propagation. Computer simulations are performed on several microstructures of different materials types, e.g., laminates, particulate composites and polycrystals. The damage accumulation due to microcracking is characterized by the stereological measure of failed material and is investigated in order to assess the effect of microstructural features on the failure mechanism. Moreover, the approach allows to analyze the influence of the characteristic parameters for brittle materials on damage evolution.     

陶瓷基复合材料（SiCw／Y—TZP）在循环压应力下的疲劳裂纹扩展

研究了碳化硅晶须（ＳｉＣｗ）增强，Ｙ２Ｏ３稳定的ＺｒＯ２四方多晶体（Ｙ－ＴＺＰ）复合材料（ＳｉＣｗ／Ｙ－ＴＺＰ）在循环压应力作用下的疲劳特性，单边缺口弯曲试样在纵向循环压应力作用下缺口根部产生垂直于压应力的Ｉ型裂纹，类似于金属材料，在室温下循环应力导致Ｉ型裂纹的稳定扩展。压应力在缺口根部产生的不可逆损伤区在循环卸载过程中形成较大的残余拉伸应力场，使裂纹萌生并长大，同时，裂纹面产生的碎粒及晶须拔出导     

Static and dynamic flexural strength of 99.5% alumina: Relation to surface roughness

The dynamic flexural strength of ceramics is an important property for all applications involving impact loading conditions. Therefore, this work reports a systematic comparison of static and dynamic flexural strength results for 99.5% commercial alumina, obtained using a recently reported adaptation of the 1-point impact experimental technique. Specimens of the same size and systematically varying surface roughness conditions were used in this study to assess the influence of the latter on the static and dynamic strength of this material. The investigated roughness levels ranged from 0.8 μm (coarse) to 0.05 μm (fine, polished). Under static loading, reducing the surface roughness causes a 10% increase in flexural strength for polished specimens. By contrast, the dynamic flexural strength is apparently not influenced by the surface roughness. A thorough microstructural and fractographic examination reveals the presence of bulk (surface) pore-like flaws that are not obliterated by the polishing process and therefore govern the failure process. It is suggested that strength improvements can be reached by suitable surface preparation, provided no bulk pores are native in the material, some of which are present on its surface. The identification of the role of surface flaws is expected to clarify the discrepancy found in the literature as to the influence of surface roughness on the mechanical properties of brittle materials.     

A critical evaluation of theories for predicting microcracking in composite laminates

We present experimental results on 21 different layups of Hercules AS4 carbon fibre/3501-6 epoxy laminates. All laminates had 90 ° plies; some had them in the middle ([(S)/90 _n ] _s ) while some had them on a free surface ([90 _n /(S)] _s ). The supporting sublaminates, (S), where [0 _n ], [±15], or [±30]. During tensile loading, the first form of damage in all laminates was microcracking of the 90 ° plies. For each laminate we recorded both the crack density and the complete distribution of crack spacings as a function of the applied load. By rearranging various microcracking theories we developed a master-curve approach that permitted plotting the results from all laminates on a single plot. By comparing master-curve plots for different theories it was possible to critically evaluate the quality of those theories. We found that a critical-energy-release-rate criterion calculated using a two-dimensional variational stress analysis gave the best results. All microcracking theories based on a strength-failure criteria gave poor results. All microcracking theories using one-dimensional stress analyses, regardless of the failure criterion, also gave poor results.     

Effect of thermal mismatch induced residual stresses on grain boundary microcracking of titanium diboride ceramics

The effect of thermal mismatch induced residual stresses on grain boundary microcracking in titanium diboride (TiB₂) ceramics has been studied by finite element method. A cohesive zone model was used to simulate the microcracking initiation in four-point bending specimens. In particular, the microcracking was assumed to occur at a grain boundary which is located in the center of the specimen, surrounded by a thermally anisotropic area. The predicted failure strength appears to be significantly reduced by the presence of residual stresses when the cohesive energy of the microstructure is small. The failure load from experiments has been used to determine the critical damage parameters for microcracking initiation in both pristine and aluminum-infiltrated TiB₂. A viscous regularization technique is employed in the simulations to improve the rate of convergence of the solution and the effect of the value of the viscosity parameter on the simulation results, has been investigated. The effect of grain size, grain orientation, and number of employed thermally anisotropic grains, on the microcracking is also discussed.     

Damage mechanics of composite materials: I— Measurements of damage and strength

A new approach for modelling the strength of notched composites has been developed. The approach is based on the assumption that subcritical damage modifies the notch-tip stress field and that the state of subcritical damage just before failure, referred to as the terminal damage state (TDS), must have a significant influence on notched strength. The TDS was monitored for a wide range of cross-ply graphite reinforced epoxy specimens using real-time radiography. A finite element model incorporating the TDS was used to determine the modified notch-tip stress field. A simple tensile stress failure criterion has been found to predict failure very well provided that the effect of subcritical damage is considered in this way. The effect of both layup and notch size on strength can be entirely accounted for by the effect these parameters have on the terminal damage state. In the first paper of a four-part series, radiographs of c. 60 specimens have been used to characterize the notch-tip damage zone and to establish a qualitative relationship between terminal damage and notched strength.     

Sensitivity analysis of the surface integrity of monocrystalline silicon to grinding speed with same grain depth-of-cut

Mechanisms for removal of materials during the grinding process of monocrystalline silicon have been extensively studied in the past several decades. However, debates over whether the cutting speed significantly affects the surface integrity are ongoing. To address this debate, this study comprehensively investigates the effects of cutting speed on surface roughness, subsurface damage, residual stress, and grinding force for a constant grain depth-of-cut. The results illustrate that the changes in the surface roughness and subsurface damage relative to the grinding speed are less obvious when the material is removed in ductile-mode as opposed to in the brittle-ductile mixed mode. A notable finding is that there is no positive correlation between grinding force and surface integrity. The results of this study could be useful for further investigations on fundamental and technical analysis of the precision grinding of brittle materials.The full text can be downloaded at https://link.springer.com/content/pdf/10.1007%2Fs40436-020-00291-5.pdf     

Experimental study on the meso-scale milling of tungsten carbide WC-17.5Co with PCD end mills

Tungsten carbide is a material that is very difficult to cut, mainly owing to its extreme wear resistance. Its high value of yield strength, accompanied by extreme brittleness, renders its machinability extremely poor, with most tools failing. Even when cutting with tool materials of the highest quality, its mode of cutting is mainly brittle and marred by material cracking. The ductile mode of cutting is possible only at micro levels of depth of cut and feed rate. This study aims to investigate the possibility of milling the carbide material at a meso-scale using polycrystalline diamond (PCD) end mills. A series of end milling experiments were performed to study the effects of cutting speed, feed per tooth, and axial depth of cut on performance measures such as cutting forces, surface roughness, and tool wear. To characterize the wear of PCD tools, a new approach to measuring the level of damage sustained by the faces of the cutter's teeth is presented. Analyses of the experimental data show that the effects of all the cutting parameters on the three performance measures are significant. The major damage mode of the PCD end mills is found to be the intermittent micro-chipping. The progress of tool damage saw a long, stable, and steady period sandwiched between two short, abrupt, and intermittent periods. Cutting forces and surface roughness are found to rise with increments in the three cutting parameters, although the latter shows signs of reduction during the initial increase in cutting speed only. The results of this study find that an acceptable surface quality (average roughness R_a<0.2 μm) and tool life (cutting length L>600 mm) can be obtained under the conditions of the given cutting parameters. It indicates that milling with PCD tools at a meso-scale is a suitable machining method for tungsten carbides.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00298-y     

A numerical study of fracture initiation in a ductile material containing a dual population of second-phase particles—II. Dynamic loading

Some recent experimental studies with pre-notched bend specimens of 4340 steel under both static loading [A. T. Zehnder and A. J. Rosakis, J. appl. Mech. 57, 618–626 (1990)] and impact loading [A. T. Zehnder et al., Int. J. Fracture 42, 209–230 (1990)] have shown that considerable crack-tunneling occurs in the interior of the specimens prior to gross fracture initiation on the free surfaces. The final fracture of the side ligaments happens because of shear-lip formation. The tunneled region is characterized by a flat fibrous fracture surface. In Part I of this work, the static experiments of A. T. Zehnder and A. J. Rosakis [J. appl. Mech. 57, 618–626 (1990)] were analyzed using a 2D plane-strain finite-element procedure. The constitutive model that was employed in this analysis accounted for the ductile failure mechanisms of microvoid nucleation, growth and coalescence. The simulation also modeled void initiation at two populations of particles of different sizes. In this part, the same constitutive model as in Part I is used, along with a plane-strain transient finite-element procedure to analyze the impact experiments reported by A. T. Zehnder et al., [Int. J. Fracture 42, 209–230 (1990)] corresponding to an impact speed of 5 m/sec. A direct comparison is made between the static and dynamic results regarding the development of ductile failure in the ligament connecting the notch-tip and a simulated inclusion ahead of it. It is found that, to attain the same level of microvoid damage in this ligament, a larger value of J is required under dynamic loading. The strain rate and adiabatic temperature rise near the notch-tip are also examined.     

MACHINABILITY OF SOME FIBER-REINFORCED THERMOSET AND THERMOPLASTICS IN DRILLING

Polymer-based composite materials are the major candidates for substitution for conventional materials in industry. Drilling is most frequently employed among machining processes for composite materials due to the need for structural integration. In this paper, some aspects of both experimental observation and machinability are presented for thermoset-based and thermoplastic-based composites with high and low fiber loading. The experimental observation discusses chip characteristics and specific cutting energy to reveal the mechanism of material removal. These materials fracture due to the brittle reinforcement, hence the sensitivity of defects in bulk volume is demonstrated. The level of fiber loading and the deformation behavior of matrix polymer determine the extent of plasticity in chip formation and the chip length. The discussions of machinability include drilling force, surface roughness and edge integrity affected by cutting conditions (feed rate and cutting speed), drill geometry and lay-up system. An optimal domain of cutting parameters is suggested for secured machinability.     

Application of X-ray refraction topography to fibre reinforced plastics

The effect of X-ray refraction employs an unconventional small angle X-ray scattering (SAXS) technique which has been developed and applied to meet actual problems for improved non-destructive characterisation of advanced materials. The X-ray refraction technique makes use of X-ray optical effects at micro interfaces of composite materials. This method reveals the inner surface and interface concentrations in nanometer dimensions due to the short X-ray wavelength near 10⁻⁴ μm. Sub-micron crack and pore sizes are easily determined by “X-ray refractometry” without destroying the structure by cutting or polishing for microscopic techniques. The non-destructive characterisation of microfailure e.g. voids, fibre debonding, fibre cracks and microcracks of a short glass fibre reinforced polyoximethylene (POM-GF) after mechanical loading and accelerated ageing is investigated. X-ray refraction topographs are illustrated, showing the damage accumulation of POM-GF specimens after the fatigue test.