Planar Crack Propagation Research Articles

Experimental and analytical study on non-tip initiation behavior of three-dimensional non-planar cracks in rock-like materials

The presence of fractures, joints, bedding planes, and faults within rock masses results in their inherent heterogeneity and discontinuity. These structural defects alter the mechanical properties of rock masses, reducing their structural strength and stiffness, and contributing to anisotropy. In addition to planar cracks, non-planar cracks are frequently found within rock masses due to geological evolution and local stress variations. While the mechanisms of planar crack propagation and fracture in both two-dimensional and three-dimensional spaces have been extensively studied and understood, research on non-planar cracks has largely been confined to two-dimensional aspects. This study addresses the limitations by successfully creating brittle solid specimens with three-dimensional non-planar internal cracks. Uniaxial compression tests and numerical simulations were conducted to investigate the propagations and fracture behaviors of various shapes of three-dimensional non-planar internal cracks. The experiments identified two primary macroscopic failure modes: symmetric non-planar internal cracks exhibiting non-tip initiation failure, and asymmetric non-planar internal cracks displaying tip initiation on the upper side and non-tip initiation on the lower side. The failure strengths of non-planar internal cracks were significantly higher than those of planar cracks, and the size of the cracks had minimal effect on their failure strengths. Notably, from initiation to failure, symmetric non-planar internal cracks did not generate any wing cracks, whereas asymmetric non-planar internal cracks were accompanied by petal-shaped cracks, wing cracks, and lance-shaped cracks. Under uniaxial compression, non-planar internal cracks propagated at extremely high speeds, resulting in rough and uneven fracture surfaces. In addition to Type III lance-shaped cracks, dynamic fracture characteristic areas and Wallner lines were also observed on the fracture surfaces. This study provides valuable insights into the fracture behavior of three-dimensional non-planar cracks and a reference basis for understanding their propagation and failure mechanisms.

Advancements in fracture toughness testing of ultra-high-strength steel sheets: Unraveling the crack-closure effect and unanticipated thickness independence

A novel testing method was developed to evaluate the fracture toughness of ultra-high-strength (UHS) steel sheets. The study also examined the influence of specimen thickness, utilizing specimens of varying thicknesses (1.6 ∼ 10 mm). While the toughness of all specimens was determined by the threshold for planar crack propagation originating from the fatigue pre-crack-front near the mid-thickness zone, the apparent fracture toughness was dependent on specimen thickness. This phenomenon was attributed to the crack-closure effects experienced during the pre-cracking process. In-depth analyses revealed that, in the absence of crack-closure effects, the intrinsic toughness remained unaffected by changes in specimen thickness.

Evolution of interphase stress over a crack propagation plane as a function of stress relief heat treatments in a PBF‐LB/M AlSi10Mg alloy

AbstractIn this study, we compare the residual stress state in a laser powder bed fusion (PBF‐LB/M) AlSi10Mg alloy in the as‐built (AB) condition with that after two different heat treatments (265 °C for 1 h, HT1; and 300 °C for 2 h, HT2). The bulk residual stress (RS) is determined using synchrotron X‐ray diffraction (SXRD), and near‐surface profiles are determined using laboratory energy‐dispersive X‐ray diffraction (EDXRD). The EDXRD results do not reveal any notable difference between the conditions at a depth of 350 μm, suggesting that the machining process yields a comparable residual stress state in the near‐surface regions. On the other hand, the SXRD results show that HT1 is more effective in relieving the bulk RS. It is observed that HT1 reduces the RS state in both the aluminium matrix and the silicon network. In addtion, HT2 does not have a significant impact on relaxing the RS as‐built state of the matrix, although it does induce a reduction in the RS magnitudes of the Si phase. It is concluded that the heat treatment stress relieving is effective as long as the Si‐network is not disaggregated.

Influence of creep forming temperature on the mechanical properties and high cycle fatigue strength of Al-Mg-Sc-Zr alloy

Creep forming experiments were conducted on 1.6 mm thick sheets of 5024 alloy at an initial stress of 210 MPa in the temperature range 120 to 320°C for 2 hrs. The effect of forming temperature on the spring back behaviour and the mechanical properties of the material was studied. Results showed that spring back is high in the lower temperature range. Exposure to creep forming temperature had a marginal effect on the ultimate tensile strength and yield strength. At the same time, there was significant reduction in the ductility as a result of exposure to creep forming temperature in the range of 200-280°C. The fatigue strength of the alloy for 106 cycles was found to have increased from 250 to 290 MPa after exposure of the material to creep forming temperature of 200°C. Exposure to 300°C, however, did not exhibit a similar trend. Fractography studies on fatigue tested specimens showed delamination along the planes perpendicular to the fatigue crack propagation plane within the fatigue crack initiation zone. Analysis suggests that precipitation of β (Al3Mg2) due to exposure to creep forming temperature regime appears to have been responsible for the observed change in the mechanical properties. Detailed analysis suggests the optimum creep forming temperature for 5024 alloy is 300°C for 2 hours, wherein the alloy shows the least spring back with desirable mechanical properties.

Study on Elastic Mixed Mode Fracture Behavior and II-III Coupling Effect.

Mixed mode fracture is a widely studied topic, while the coupling effects of mixed mode cracking are unclear. In this paper, elastic fracture behaviors and the coupling effects of the mixed mode cracks are studied in detail based on the finite element method, experimental study and linear elastic fracture mechanics. Results show that there always exist II-III coupling effects at the crack tips of mixed mode cracks, which have many effects on the crack tip field and crack propagation behavior. It is found that a mode II component at the tip of a mixed mode crack is the main reason for crack deflection, while the mode III components show no effect. For any mixed mode crack, mode II components at the crack tip can be divided as that by mode II loading which causes plane crack propagation, and by the coupling effect which causes spatial crack propagation. On this basis, a new fracture criterion suitable for any mixed mode crack is proposed, combined with the coupling effect and the linear elastic superposition principle. The research in this paper provides a solution to the problem of an II-III coupling effect in mixed mode fracture research and further promotes the development of fracture mechanics.

A novel ultrasonic non-destructive testing methodology to monitor fatigue crack growth in compact tension specimens.

Detection and evaluation of fatigue cracks in structural materials are extremely important for life prediction analysis as part of the structural integrity procedure. In this article, we present a novel ultrasonic measurement methodology, based on the diffraction of elastic waves at the crack tips, to monitor the fatigue crack growth near the threshold regime in compact tension specimens at different load ratios. The diffraction phenomenon of ultrasonic waves from the crack tip is demonstrated using a finite element 2D wave propagation simulation. The applicability of this methodology has also been contrasted with that of the conventional direct current potential drop method. In addition, the crack morphology obtained in the ultrasonic c-scan imaging showed a variation in the crack propagation plane as a function of the cyclic loading parameters. The results suggest that this novel methodology is sensitive to fatigue cracks and can form the basis of in situ ultrasonic-based crack measurements in metallic and non-metallic materials.

Dislocation-based crack initiation and propagation in single-crystal SrTiO3

Understanding the irreversible deformation (dislocation activation and crack formation) of functional oxides, based on which various advanced electronic devices are fabricated, is critical for the optimal structural design and mechanical reliability. Here, we demonstrate the dislocation-based crack initiation and propagation in a model perovskite, single-crystal SrTiO3, at small scale. Using nanoindentation tests with spherical tips and etch pit study, we identify the sequence of the irreversible deformation events occurred in single-crystal (001) SrTiO3. For a locally stressed volume that is free of pre-existing cracks, the material undergoes the following processes: (1) purely elastic deformation; (2) dislocation activation on primary slip planes that are 45° inclined to the surface; (3) dislocation activation on secondary slip planes perpendicular to the surface; (4) crack initiation by dislocation pile-up; followed by (5) concurrent crack propagation and dislocation multiplication and motion at higher loads. Specifically, we focus on the crack formation caused by the dislocation pile-up beneath the spherical indenter. We also identify the favorable crack propagation planes to be {110} when the (001) surface is indented. These findings in cubic SrTiO3 are believed to be applicable for other ceramic materials with cubic structure.

Fatigue Fracture Mechanism of a Nickel-Based Single Crystal Superalloy with Partially Recrystallized Grains at 550 °C by In Situ SEM Studies

The fatigue fracture mechanism of a nickel-based single crystal (NBSC) superalloy with recrystallized grains was studied at 550 °C by in situ observation with a scanning electron microscope (SEM) for the first time. Multiple crack initiations associated with recrystallized grain boundaries and carbides were observed. By analysis of the slip traces and crack propagation planes, the operated slip systems were identified to be octahedral for both single crystal substrate and recrystallized grains. Distinct crystallographic fractures dominated, accompanied by recrystallized grain boundary associated crack initiations. This is different from the widely reported solely intergranular cracking at high temperature. Fatigue crack growth rate curves showed evident fluctuation, due to the interaction of fatigue cracks with local microstructures and the crack coalescence mechanism. Both the recrystallized grains and the competition between different slip systems were responsible for the deceleration and acceleration of fatigue microstructurally small crack behavior.

Rising R-curves in particulate/fiber-reinforced resin composite layered systems

BackgroundAn anisotropic fracture behavior has already been successfully induced in a condensable resin composite by reinforcement with discontinuous glass fibers, developing a steep rise in the fracture resistance curve. There is still the need for developing such concepts further, by evaluating the behavior of layered structures that attempt to replicate the natural gradient structure of enamel and dentin. MethodsMonolithic and bilayer specimens of a flowable short-fiber particulate-reinforced composite (SFPRC) material (GC Corp.) and a commercial flowable particulate-reinforced composite (PRC) (Essentia HiFlo, GC Corp.) were produced with the fibers aligned perpendicular or randomly oriented to the plane of crack propagation. Pre-cracked specimens were tested in 3-point bending using the single-edge-V-notch-beam method with accurate displacement tracking recordrecord. Controlled crack growth experiments were conducted at progressive crack extension intervals to build R-curves using linear-elastic and elastic-plastic fracture mechanics. ResultsWe demonstrate firsthand the development of R-curves in resin composite bilayers, with crack arrest at the interface and a steep rise in crack resistance with subsequent drop within the SFPRC layer. Crack deflection and fiber bridging were found to contribute to a higher rise in the R-curves for specimens with aligned fibers; still, randomly oriented fibers showed significant R-curve development, especially at short range intervals. An increase in the vol% of fibers and a decrease in the Young's modulus contributed to a higher overall performance of the flowable vs. the condensable SFPRF materials. ConclusionsSFPRC materials show an up to 5-fold improvement in the fracture resistance over conventional PRCs, and show the potential to be used to build gradient structures by varying the fiber alignment and fibers parameters (length and packing).

Effect of build orientation on fracture and tensile behavior of A357 Al alloy processed by Selective Laser Melting

Selective Laser Melting (SLM) is an additive manufacturing technology that allows producing components through local melting of a metallic powder bed using high-power laser beam in accordance with 3D digital CAD models. Cast Al-Si-Mg alloys are broadly used due to their good processability by SLM and mechanical properties. This work investigates the mechanical behavior of A357 aluminum alloy produced by SLM in the as built and artificially aged conditions. In particular, the tensile properties and fracture toughness of the alloy were investigated according to different specimen orientations on the build plate in the first case and different crack propagation planes in the second. In addition, microstructural and fractographic analyses were performed in order to investigate crack propagation paths and fracture mechanisms. The results showed that building orientation has a strong influence on material response, mainly with regards to the fracture toughness. The melt pool boundaries act as preferential paths for crack propagation and lead to a marked drop in fracture toughness of samples with cracks perpendicular to the building direction.

Effect of process deformation history on mechanical performance of AM60B to AA6082 self-pierce riveted joints

The role of the process deformation history on the mechanical performance of self-pierce riveted (SPR) joints is examined in detail. The SPR process was simulated using a non-linear finite element model coupled with a damage criterion sensitive to the stress state. Results of the FE simulation compared closely to cross-sectional images of experimental SPR joints. The FE simulation results suggest that the strain hardening due to the piercing process is the main mechanism for joint strength under quasi-static condition. Aspects of fatigue damage were modeled including crack initiation, propagation, fretting and number of cycle to failure, and compared to experimental results. Results from the study indicate that residual stresses and plastic strain may alter the crack initiation location and crack propagation plane, leading to more accurate results when included in finite element analysis of SPR joints. Including these residual stresses and plastic strains also changed the predicted fretting locations leading to better agreement with experimental results. Lastly, linear elastic fracture mechanics were performed to estimate fatigue lives based on initial crack location and angle of crack plane. The estimation of the number of cycles to failure that included residual stresses and plastic strains resulted in a better correlation to experimental data when compared to the calculations that excluded residual stresses and plastic strains.

In-situ SEM study of slip-controlled short-crack growth in single-crystal nickel superalloy

Initiation and growth of short cracks in a nickel-based single crystal were studied by carrying out in-situ fatigue experiments within a scanning electron microscope (SEM). Specimens with two different crystallographic orientations, i.e., [001] and [111], were tested under load-controlled tension fatigue in vacuum. Slip-caused crack initiation was identified at room temperature while initiation of a mode-I crack was observed at 650 °C. Slip traces continuously developed ahead of the crack tip once initiated and acted as nuclei for early-stage crack growth at both room and high temperature (650 °C). These slip traces were caused by accumulated shear deformation of activated octahedral slip systems, which were specifically identified by analysing the surface slip traces and crack-propagation planes. The crack-growth rates were evaluated against stress intensity factor range, revealing the anomaly of slip-controlled short-crack growth. The effects of crystallographic orientations and temperature on fatigue crack growth were subsequently analysed and discussed, including the influence of microstructural features such as carbides and pores.

The relationship between low-temperature toughness and secondary crack in low-carbon bainitic weld metals

This paper aims to reveal the relationship among low-temperature toughness, microstructure and secondary crack from both microstructural and crystallographic perspectives. Instrumented Charpy tests were carried out in the upper platform, transition temperature range, as well as at lower temperatures. Quantitative fractographic analysis, microstructural characterization and secondary crack observation were conducted on half of the impact specimen. Crystallographic features were identified in the secondary crack proximity, e.g. variant selection, orientation relationship, cleavage plane of the corresponding crack matrix. Some results in literature are confirmed. Nickel addition is beneficial to hindering crack propagation as a result of microstructure improvement. The most effective barrier for crack propagation is high-angle misorientation boundaries between Bain packets and the boundaries of ferrite matrix with the microstructure of granular bainite. Additionally, high-angle misorientation boundaries between closed-packed plane packets arrest cracks in the mixed microstructure of fine lath bainite and intricate acicular ferrite. Most preferred crack propagation planes are {110} and {100} in our low carbon bainitic weld metals, with the decreasing possibility of {112} and {123}.

An Out-of-plane Loaded Double Cantilever Beam (ODCB) test to measure the critical energy release rate in mode III of adhesive joints

A novel fracture mechanical test for adhesive joints subjected to pure mode III loading is presented. The experiment uses a modified double cantilever beam specimen, which is loaded out of the plane of crack propagation. The paper presents and discusses first experimental results, applying the new test method on adhesively bonded joints of an elastic-plastic structural adhesive. The tests have been performed under quasi-static load conditions. Beside the critical energy release rate, the test gives information about the total shape of the corresponding Traction-Separation-Law at the location of the initial crack tip. The calculation of the critical energy release rate is done by two different analytical approaches. Firstly, an evaluation method based on the Irwin-Kies equation is proposed, which is strictly valid for Linear-Elastic Fracture Mechanics conditions only and which is commonly used in case of classical Double Cantilever Beam tests. The second approach deals with a calculation of the J-integral, which holds for non-linear fracture behavior as well. The two evaluation methods are applied to experimental results of the new test method; the differences between both methods are pointed out and discussed. Additionally, the obtained test results in mode III are compared to equivalent results from mode II tests, which are taken from literature to proof a possible correlation between the shear crack opening modes II and III.

Failure analysis of fretting fatigue initiation and growth on railway axle press-fits

Abstract Fretting fatigue failure of press fitted railway axle-wheel assembly was presented. Size, distribution and propagation profiles of the circumferential fretting cracks on the full-scale axles were determined by magnetic particle and metallographic slicing methods. The distribution of multiaxial stress cycle along the press-fit seat was obtained by finite element analysis (FEA). The obtained stress path was used for interpretation of fractographic evidence collected at crack initiation sites and the crack propagation plane. Metallurgical and mechanical characterization of the axle material (34CrMo4) was made on the specimens sampled from the broken axles. The threshold conditions for propagation of small fretting cracks were determined by Kitagawa analysis and El-Haddad correction method. The results of the experimental study were compared with EA1N grade steel which is given as the reference axle material in the EN standards. The metallurgical factors affecting the fretting fatigue crack initiation and propagation were investigated. The causes of the examined axle failures were associated with the deteriorated mechanical properties of the axle material.

Fatigue and Fracture Behavior of a Cold-Drawn Commercially Pure Aluminum Wire.

Fatigue properties and cracking behavior of cold-drawn commercially pure aluminum wires (CPAWs) widely used as the overhead transmission conductors were investigated. It was found that the fracture surface of the CPAWs shows an obvious four-stage fracture characteristic, i.e., crack initiation, planar crack propagation, 45°-inclined crack propagation and final rapid fracture. The crack growth mechanisms for the CPAWs were found quite different from those for the conventional coarse-grained materials. The cracks in the CPAWs firstly grow along the grain boundaries (Stage I crack growth), and then grow along the plane of maximum shear stress during the last stage of cycling (Stage II crack growth), leading to the distinctive fracture surfaces, i.e., the granular surface in the planar crack propagation region and the coarse fatigue striations in the 45°-inclined crack propagation region. The grain boundary migration was observed in the fatigued CPAWs. The increase in fatigue load enhances the dislocation recovery, increases the grain boundary migration rate, and thus promotes the occurrence of softening and damage localization up to the final failure.

Bridging micro to macroscale fracture properties in highly heterogeneous brittle solids: weak pinning versus fingering

The effect of strong toughness heterogeneities on the macroscopic failure properties of brittle solids is investigated in the context of planar crack propagation. The basic mechanism at play is that the crack is locally slowed down or even trapped when encountering tougher material. The induced front deformation results in a selection of local toughness values that reflect at larger scale on the material resistance. To unravel this complexity and bridge micro to macroscale in failure of strongly heterogeneous media, we propose a homogenization procedure based on the introduction of two complementary macroscopic properties: An apparent toughness defined from the loading required to make the crack propagate and an effective fracture energy defined from the rate of energy released by unit area of crack advance. The relationship between these homogenized properties and the features of the local toughness map is computed using an iterative perturbation method. This approach is applied to a circular crack pinned by a periodic array of obstacles invariant in the radial direction, which gives rise to two distinct propagation regimes: A weak pinning regime where the crack maintains a stationary shape after reaching an equilibrium position and a fingering regime characterized by the continuous growth of localized regions of the fronts while the other parts remain trapped. Our approach successfully bridges micro to macroscopic failure properties in both cases and illustrates how small scale heterogeneities can drastically affect the overall failure response of brittle solids. On a broader perspective, we believe that our approach can be used as a powerful tool for the rational design of heterogeneous brittle solids and interfaces with tailored failure properties.

Crack initiation and early propagation plane orientation of 2A12-T4 aluminum alloy under tension-torsion fatigue loading including mean tensile stress

The specimen fractures appearances are analyzed to investigate the effects of mean tensile stress on the fatigue crack initiation and early propagation plane orientation under axial-torsion fatigue loading for 2A12T4 aluminum alloy. The fatigue crack initiation and early propagation plane orientations are measured by optical microscope, the results of macro-analysis show that both the maximum shear stress amplitude and normal mean stress have effects on the orientations of crack initiation and propagation plane orientation, which are close to the plane of the maximum shear stress amplitude plane. With increasing the mean tensile stress, more cracks are inclined to initial and propagate on or near the maximum shear stress amplitude plane with larger normal mean stress, and the angle of deviation from the plane of maximum shear stress amplitude increases. The predicted plane orientations based on critical plane methods are compared with experimental measured results.

Irwin׳s conjecture: Crack shape adaptability in transversely isotropic solids

The planar crack propagation problem of a flat elliptical crack embedded in a brittle elastic anisotropic solid is investigated. We introduce the concept of crack shape adaptability: the ability of three-dimensional planar cracks to shape with the mechanical properties of a cracked body. A criterion based on the principle of maximum dissipation is suggested in order to determine the most stable elliptical shape. This criterion is applied to the specific case of vertical cracks in transversely isotropic solids. It is shown that contrary to the isotropic case, the circular shape (i.e. penny-shaped cracks) is not the most stable one. Upon propagation, the crack first grows non-self-similarly before it reaches a stable shape. This stable shape can be approximated by an ellipse of an aspect ratio that varies with the degree of elastic anisotropy. By way of example, we apply the so-derived crack shape adaptability criterion to shale materials. For this class of materials it is shown that once the stable shape is reached, the crack propagates at a higher rate in the horizontal direction than in the vertical direction. We also comment on the possible implications of these findings for hydraulic fracturing operations.

Crack Front Fingering During Planar Crack Propagation in Highly Heterogeneous Toughness Field

Crack pinning by tougher heterogeneities is in principle an interesting way to toughen brittle materials. To study the impact of highly heterogeneous toughness field, we investigate numerically the propagation of a tensile penny-shape planar crack within an axisymmetric heterogeneous toughness field. In particular, we take into account the large crack front deformations induced by high toughness contrasts. To compute the variations of stress intensity factor along the crack front arising from its progressive deformation, a perturbation approach based on Bueckner-Rice weight function theory is used iteratively. For low enough toughness contrasts, the crack front deforms until reaching an equilibrium shape for which the local stress intensity factor equals the local toughness value at each point of the front. For larger contrasts, however, this equilibrium shape is never reached. Instead, some points of the crack front remained pinned by strong impurities, while some other part of the front advances continuously. The mechanism at the origin of this fingering instability is finally discussed.