Fracture Initiation Sites Research Articles

Effect of Nanosecond Laser Dicing on the Mechanical Strength and Fracture Mechanism of Ultrathin Si Dies With Cu Stabilization Layer

Ultrathin dies require a Cu stabilization layer, which is essentially a backside Cu layer, to prevent warpage and cracks during solder die attach and wire bonding. The dicing of Si wafers with a backside Cu layer is challenging. Mechanical blade dicing through the Cu layer causes blade clogging and damage, which eventually results in severe die chipping and cracks. Plasma dicing is costly as it requires additional photolithography and etching steps. Laser dicing is promising and is currently used to singulate thin Si wafers. However, there is no reported work on its application for dicing ultrathin wafers with a backside Cu layer. In this paper, nanosecond laser dicing of $20~\mu \text{m}$ Si dies with 0– $30~\mu \text{m}$ backside Cu was found to be feasible. The effect of nanosecond laser dicing on the die sidewall strength was evaluated with the three-point bend (3PB) test. Analytical and experimental results have shown that the Cu and AlCu layers have gone into plastic condition during the 3PB test. Comparison of the 3PB fracture loads indicates that the Si backside strength is higher than the Si frontside strength. Fractographic analysis has confirmed that the fracture initiation sites during the 3PB tests are at the die sidewall. The die sidewall defect morphologies, structures, and elemental compositions have been characterized in detail by transmission electron microscopy, and their effect on mechanical strength is discussed.

The effects of defects on the uniaxial compressive strength and failure of an advanced ceramic

This study investigates the effects of the processing-induced defect population on the dynamic compressive strength and failure of a hot-pressed boron carbide. Quantitative microscopic analysis was used to determine the distributions of three types of processing-induced inhomogeneities: aluminum nitride, small graphitic particles and pores, and larger graphitic disks. Scanning electron microscopy of fracture surfaces identifies the graphitic disks as fracture initiation sites. The size, orientation and number density of the graphitic disks are then quantified using image processing techniques. We use these defect statistics, in conjunction with recent scaling models, to explore our experimentally measured dynamic compressive strength results.

Fractographic Signs of the In-Service Degradation of Welded Joints of Oil Mains

According to the results of inspection of the damaged elements of an oil pipeline with axial welded joints, we analyze the most probable sites of fracture initiation and the mechanisms of crack propagation in the course of long-term operation. As a result of hydrostatic-pressure tests of the pipe, it becomes possible to visualize the fractographic signs of the in-service degradation of the metal in the form of dispersed damages located along the grain boundaries and observe a decrease in the resistance of grains to brittle failure. We associate the dispersed damage to pipe wall with the hydrogenation of the metal from its inner surface in the course of operation.

Crack Initiation Mechanism of a Cast Hybrid MMC in Low Cycle Fatigue

The low-cycle fatigue (LCF) behavior specially the fracture initiation mechanism in a cast hybrid metal matrix composite (MMC) was investigated in this research. Conventional three point bending fatigue test was carried out on a rectangular bar smooth specimen. Factographic analysis was conducted to observe the fracture initiation site. Experimental results showed that microcracks in LCF initiated at the particle–matrix interface which was located in the hybrid clustering region. Due to continued fatigue cycling, the interface debonding occurred, created additional secondary microcracks and the microcrack coalesced with other nearby microcracks caused the final fracture.

STUDY ON LOW-CYCLE FATIGUE BEHAVIOR OF CAST HYBRID METAL MATRIX COMPOSITES

The low-cycle fatigue (LCF) behavior, especially the fracture initiation mechanism in a cast hybrid metal matrix composite (MMC), was studied experimentally and numerically. The conventional three-point bending fatigue test was carried out and fractographic analysis was conducted to observe the fracture initiation site. Experimental results showed that microcracks in the LCF initiated at the particle–matrix interface which was located in the hybrid clustering region. Due to continued fatigue cycling, interface debonding occurred, created additional secondary microcracks and the microcrack coalesced with other nearby microcracks. As far as the numerical study is concerned, three-dimensional (3-D) unit-cell models of the hybrid MMC consisting of reinforcement clustering and non-clustering regions were developed by using the finite element method. The stress–strain distributions in both the reinforcement clustering and non-clustering regions were analyzed. The numerical results confirmed that the stress concentration occurred on the reinforcement–matrix interface located in the clustering region and provided reasonable agreement with the experimental observations.

Prediction model on cleavage fracture initiation in steels having ferrite–cementite microstructures – Part II: Model validation and discussions

The proposed prediction model on a cleavage fracture initiation in ferrite–cementite steels is applied to the fracture toughness tests using notched specimens of steels with various sizes of ferrite grain and cementite particle. The prediction results of fracture toughness and fracture initiation site showed good agreement with the experimental results for all the steels and temperatures tested. The results of parametric studies using hypothetical steels successfully show the influence of the sizes of microstructures on fracture toughness. The results also show that the transition of bottleneck process in the fracture initiation with increasing temperature.

Structure–Property–Fracture Mechanism Correlation in Heat-Affected Zone of X100 Ferrite−Bainite Pipeline Steel

Structural performance of a weld joint primarily depends on the microstructural characteristics of heat-affected zone (HAZ). In this regard, the HAZ in X100 ferrite−bainite pipeline steel was studied by separating the HAZ into intercritically reheated coarse-grained (ICCG) HAZ containing and non-containing regions. These two regions were individually evaluated for Charpy impact toughness and characterized by electron back-scattered diffraction (EBSD). Low toughness of ~50 J was obtained when the notch of impact specimen encountered ICCGHAZ and high toughness of ~180 J when the notch did not contain ICCGHAZ. Fracture surface was ~60 pct brittle in the absence of ICCGHAZ, and 95 pct brittle (excluding shear lip) in the presence of ICCGHAZ in the impact tested samples. The underlying reason is the microstructure of ICCGHAZ consisted of granular bainite and upper bainite with necklace-type martensite−austenite (M−A) constituent along grain boundaries. The presence of necklace-type M-A constituent notably increases the susceptibility of cleavage microcrack nucleation. ICCGHAZ was found to be both the initiation site of the whole fracture and cleavage facet initiation site during brittle fracture propagation stage. Furthermore, the study of secondary microcracks beneath CGHAZ and ICCGHAZ through EBSD suggested that the fracture mechanism changes from nucleation-controlled in CGHAZ to propagation-controlled in ICCGHAZ because of the presence of necklace-type M-A constituent in ICCGHAZ. Both fracture mechanisms contribute to the poor toughness of the sample contained ICCGHAZ.

Fatigue microcracks that initiate fracture are located near elevated intracortical porosity but not elevated mineralization

In vivo microcracks in cortical bone are typically observed within more highly mineralized interstitial tissue, but postmortem investigations are inherently limited to cracks that did not lead to fracture which may be misleading with respect to understanding fracture mechanisms. We hypothesized that the one fatigue microcrack which initiates fracture is located spatially adjacent to elevated intracortical porosity but not elevated mineralization. Therefore, the spatial correlation between intracortical porosity, elevated mineralization, and fatigue microdamage was investigated by combining, for the first time, sequential, nondestructive, three-dimensional micro-computed tomography (micro-CT) measurements of each in cortical bone specimens subjected to compressive fatigue loading followed by a tensile overload to fracture. Fatigue loading resulted in significant microdamage accumulation and compromised mechanical properties upon tensile overload compared to control specimens. The microdamage that initiated fracture upon tensile overload was able to be identified in all fatigue-loaded specimens using contrast-enhanced micro-CT and registered images. Two-point (or pair) correlation functions revealed a spatial correlation between microdamage at the fracture initiation site and intracortical porosity, but not highly mineralized tissue, confirming the hypothesis. This difference was unique to the fracture initiation site. Intracortical porosity and highly mineralized tissue exhibited a significantly lower and higher probability, respectively, of being located spatially adjacent to all sites of microdamage compared to the fracture initiation site. Therefore, the results of this study suggest that human cortical bone is tolerant of most microcracks, which are generally compartmentalized within the more highly mineralized interstitial tissue, but a single microcrack of sufficient size located in spatial proximity to intracortical porosity can compromise fracture resistance.

Applying fractography and fracture mechanics to the energy and mass of crack growth for glass in the mirror region

Abstract The mirror region following the fracture initiation site is considered by applying fracture mechanics principles along with experimental observations of the crack velocity at the formation of the mirror mist boundary. Several issues regarding this crack growth in glass are addressed after considering the terminal velocity of crack growth and the mirror mist boundary information on silicate glasses. A strain energy release rate criterion is applied to estimate the kinetic energy of an advancing crack in glass at the mirror/mist boundary. This energy is then utilized to estimate the effective mass of the crack at the mirror/mist boundary. It is compared with material volumes in the vicinity of the crack.

Micromechanical Testing of Fracture Initiation Sites in Welded High-Strength Low-Alloy Steel

Micropillar compression tests were performed on martensite (M) and retained austenite (A) containing constituents formed in high-strength low-alloy steel after simulated thermal weld heat treatment. Due to the complicated fine microstructure of the constituent, the resulted stress–strain curves were further analyzed by considering the post-mortem scanning electron micrographs and the morphology of the deformation of the pillars. It was possible to obtain relevant data for the uniaxial stress–strain behavior of the M–A constituent in the studied steel. Microvickers measurements, which were converted to macroscopic yield stress by empirical relations, were compared with the micropillar compression test results. Comparison showed that these empirical relations are overestimating the obtained mechanical properties of the M–A constituents.

Are asymmetric metal markings on the cone surface of ceramic femoral heads an indication of entrapped debris?

BackgroundThe probability of in vivo failure of ceramic hip joint implants is very low (0.004-0.05%). In addition to material flaws and overloading, improper handling during implantation can induce fractures of the ceramic ball head in the long term. Identifying the causes of an in vivo fracture contributes to improved understanding and potentially to further reduction of the fracture probability for patients. Asymmetric metal markings on the cone surface of in vivo ball head fractures have been reported. The question, therefore, is whether asymmetric loading is the sole cause or whether additional factors, specifically contamination entrapped in the taper fit, also contribute or are even the main cause.MethodsThe influence of the asymmetric physiological load configuration on resulting metal markings in the cone surface of an alumina femoral ball head with and without biological contaminants was investigated. Static and cyclic tests on ball heads were carried out in a load configuration of 0° (axisymmetric) and 40° in a physiological environment. The analysis of the metal marking was carried out to gain a better understanding of the processes that contribute to the generation of metal marking. Fractography was carried out to determine the fracture initiation of failed ball heads.ResultsDifferent types and sizes of residuals entrapped in the conical surface are shown to yield strongly asymmetric metal marking patterns. All heads tested without contaminants exhibited an almost homogenous distribution of residual metal markings around the circumference of the ceramic cone surface at the proximal end of the bore hole. The failure of ball heads that contained entrapped contaminants revealed a common fracture pattern. The site of fracture initiation on two of the failed heads was in the entrance region of the bore hole on the superior half of the head.ConclusionAsymmetric metal markings observed on the ball heads tested in this investigation are most probably caused by the presence of contaminants entrapped in the taper fit. Homogenous metal mark distributions around the circumference indicate proper assembly of the ball head without entrapped contaminants. It should, however, be noted that different taper designs may possibly result in different marking patterns.

Strengthening Mechanisms in MLCCs: Residual Stress Versus Crack Tip Shielding

Failure by fracture is a serious problem with multilayer ceramic capacitors (MLCCs), and the interior electrodes are known to strengthen MLCCs. Historically, it has been assumed that the dominant strengthening mechanism is crack tip shielding via direct crack tip‐electrode interactions. However, we have found that residual stresses arising from differential thermal contraction after device sintering are actually responsible for the observed increase in strength. In addition, the fracture initiation sites in MLCCs are located outside of the electrode array, so the established idea that the electrical and mechanical failure controlling flaw populations are one and the same cannot be true. Weibull distributions were compared from the bending fracture of two populations of MLCCs with barium titanate (X7R) dielectric, nickel electrodes, and the same exterior geometries (but different electrode array configurations). MLCCs had characteristic strengths of 236 MPa versus a strength of 190 MPa for 19‐ and 3‐electrode MLCCs, respectively. Fractography, a critical flaw size computation, an analytical residual stress approximation, and in situ electrical measurements taken during bending were also used to examine the fracture process and demonstrate that residual stress and not crack tip shielding is an important strengthening mechanism in MLCCs.

An efficient method for the identification of the modified Cockroft–Latham fracture criterion at elevated temperature

The paper presents the theoretical part of a method for the identification of the modified Cockroft–Latham ductile fracture criterion at elevated temperature. Quite a general viscoplastic model is adopted to describe material behavior. The original criterion is path-dependent and involves stresses. Therefore, the identification of constitutive parameters of this criterion, as well as many other ductile fracture criteria, is rather a difficult task that usually includes experimental research and numerical simulation. The latter is impossible without a precisely specified material model and boundary conditions. It is shown in the present paper that for a wide class of material models usually used to describe the behavior of materials at elevated temperatures, the criterion is significantly simplified when the site of fracture initiation is located on traction-free surfaces. In particular, this reduced criterion solely depends on two in-surface logarithmic strains. Since there are well-established experimental procedures to measure surface strains, the result obtained can be considered as a theoretical basis for the efficient method for the identification of the modified Cockroft–Latham ductile fracture criterion at elevated temperature.

CHARACTERISTICS OF FRACTURE ORIGINS IN SiC BASED CERAMICS

The paper deals with the flexural strength measurement and characterization of fracture origins of the silicon carbide materials prepared by liquid-phase-sintering. The characteristic flexural strength and Weibull´s modulus were computed using two-parameter Weibull´s distribution. Fractographic analysis of broken specimens was used to characterize the processing flaws. The strength of the investigated materials was degraded by the presence of processing flaws mainly in the form of pores, cluster of pores, silicon carbide agglomerates and sharp cracks. The fracture initiation sites are located predominantly in the volume of the specimens, sometimes close to the tensile surface. The size of fracture origins was lying in the interval from 15 m m to 400 m m.

Flexural strength and defect behaviour of polygranular graphite under different states of stress

The effect of stress state on the fracture behaviour of Gilsocarbon, an isotropic nuclear grade polygranular graphite, has been studied by employing four-point bend and ring-on-ring loading configurations to achieve uniaxial and equi-biaxial flexural stress states, respectively. Optical images of the specimens’ tensile surface were analysed by digital image correlation to measure the full-field displacements: these were used to identify the fracture initiation sites, analyse crack geometry (surface length and opening displacements) and also to calculate the J-integral strain energy release rate associated with surface crack propagation. Surface cracks that did not propagate to failure were identified and subsequently examined by X-ray computed tomography combined with digital volume correlation: measurements were made of their three-dimensional displacement fields when subjected to an opening tensile stress using a modified (flat) Brazilian Disk test geometry. The crack opening behaviour is explained by an effect of stress state on the development of the crack tip fracture process zone, which is in agreement with the effect of stress state on the measured strain energy release rates of sub-critical crack propagation. Both are attributed to the plastic constraint effect, which varies with the stress state in materials that can undergo inelastic deformation.

On the Plasticity of Interstitial-Free Steel Subjected to Cryogenic Rolling Followed by Annealing

Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near-cryogenic (−150°C to −100°C) rolling followed by annealing have been investigated. Twining has been found to be the dominant mode of deformation during rolling. Both the rolled and annealed sample exhibited limited work hardening due to high matrix resistance. In the rolled sample cleavage, nuclei acted as potential fracture initiation site. Annealing has resulted in effective blunting of cleavage nuclei giving rise to large total elongation due to prolonged damage growth process.

An Efficient Approach for Identifying Constitutive Parameters of the Modified Oyane Ductile Fracture Criterion at High Temperature

The paper presents the theoretical part of a method for identifying constitutive parameters involved in the modified Oyane ductile fracture criterion at high temperature. Quite a general rigid viscoplastic model is adopted to describe material behavior. The ductile fracture criterion is in general path-dependent and involves stresses. Therefore, the identification of constitutive parameters of this criterion is a difficult task which usually includes experimental research and numerical simulation. The latter requires a precisely specified material model and boundary conditions. It is shown in the present paper that for a wide class of material models usually used to describe the behavior of materials at high temperatures, the criterion is significantly simplified when the site of fracture initiation is located on traction free surfaces. In particular, this reduced criterion does not involve stresses. Since there are well established experimental procedures to determine the input data for the reduced criterion, the result obtained can be considered as a theoretical basis for the efficient method for identifying constitutive parameters of the modified Oyane ductile fracture criterion at high temperature. The final expression can also be used in computational models to increase the accuracy of predictions.

Diminished fracture initiation sites in ceramic layers bonded to glow-discharge treated substructure

A mechanically retentive structure and meticulous surface cleanliness are critical factors in providing fracture resistance and clinical success of metal ceramic restorations. This investigation compared the porcelain/metal interfaces of deliberate compressive fractures of ceramic crowns between conventional preparation and application of the Radio Frequency Glow Discharge Treatment (RFGDT) before each bonding step. It evaluated RFGDT’s capacity to improve wetting effectiveness and minimize porosity. Twelve metal ceramic crowns were fabricated identically. RFGDT was applied to the metal substructures of half the specimens before the ceramic layering process. All specimens were fractured in the same manner by an applied compressive force to simulate dental occlusive failure. Fracture surfaces were inspected by light and scanning electron microscopy. Quantitative analyses of images were performed to identify numbers, locations of cracks, porosity patterns, and other morphological correlates of the fracture zones. There were significantly fewer voids per millimeter at the interfaces in the RFGDT group than in the non-RFGDT group (t = 2.377, df = 9, p = 0.021). There was a significant difference in the number of horizontal cracks per millimeter between the groups (t = 2.132, df = 7, p = 0.035), with more cracks occurring in the non-RFGDT group. RFGDT can improve the integrity of metal ceramic crowns by increasing the substratum surface energy, improving porcelain wetting and spreading and thereby diminishing the numbers of interfacial voids available for initiation of fracture. Routine application of RFGDT should result in fewer cracks along metal/ceramic interfaces in all restorative preparations.

Cleavage initiation in steel: Competition between large grains and large particles

TiN particles in steel can limit austenite grain growth during reheating or welding. However, cuboidal TiN particles of large size (>1μm) can act as cleavage fracture initiation sites and affect the low temperature toughness. Based on the assumption that the local principle stress acts uniformly everywhere within a grain, for a fixed particle size, a critical value of ferrite grain size can be obtained below which the grain boundary effectively retards the propagation of micro-cracks, which can be nucleated from TiN particles. In the present study, the relative effect of ferrite grain size and TiN particles on cleavage crack propagation has been assessed. Ti-containing high strength low alloy (HSLA) steels were processed using different thermo-mechanical schedules and their microstructures and Charpy impact properties were investigated. It has been found that in spite of the presence of large TiN particles, refinement in ‘effective ferrite grain size’ can significantly improve the impact toughness, provided the matrix strength and meso-texture (i.e. the intensity of low-angle boundaries) are restricted to low values.

Effect of Intensive Plastic Deformation Near Frictional Interfaces on Ductile Fracture

A layer of intensive plastic deformation often appears in the vicinity of frictional interfaces in metal forming processes. The paper presents a study to reveal a possible effect of intensive plastic deformation in such a layer on ductile fracture. To this end, an upsetting test of special design is used to move the site of ductile fracture initiation to the friction surface independently of the effect of intensive plastic deformation on the occurrence of ductile fracture. Experimental results obtained are compared to the theoretical prediction based on a conventional empirical ductile fracture criterion. It is shown that there is some deviation of the fracture conditions predicted theoretically from the experimental results.