Fracture Crack Propagation Research Articles

Peridynamic Study on Fracture Mode and Crack Propagation Path of a Plate with Multiple Cracks Subjected to Uniaxial Tension

Peridynamic Study on Fracture Mode and Crack Propagation Path of a Plate with Multiple Cracks Subjected to Uniaxial Tension

Analysis of ultimate fracture bearing capacity of CHS XK-joints welds

The current study presents advanced finite element (FE) models that combine the stress weighted damage model and the crack propagation using explicit dynamic approach of commercial FE software ABAQUS. The applicability of the subroutine for ductile fracture prediction and crack propagation modeling of structural steel is confirmed by comparing the results of fracture tests performed on circumferential notch specimens. FE analysis is performed on 17 circular hollow section (CHS) XK-joint models with various sized partial penetration welds to simulate the fracture process of the joint models, and the obtained results are used to analyze the fracture ultimate bearing capacity of the joint weld. The results indicate that the crack first appears on the inner side of the weld at the crown point of the intersecting line of the tensile web member. The findings also demonstrate that the joint weld does not lose the bearing capacity completely after the initial cracking. Instead, the joint weld's bearing capacity increased with the displacement at the early stages of fracture propagation to reach the maximum value prior to gradual decrease in bearing capacity. A design formula of weld bearing capacity suitable for partial penetration weld has been proposed herein to incorporate the effects of uneven distribution of joint weld stress in the considered XK-joints.

Occurrence condition for steady crack propagation in quasi-brittle fracture and its application in determining initial fracture toughness

Crack propagation is commonly assumed to be a parallel motion called the steady state in quasi-brittle materials. The steady crack propagation can be used to determine softening traction-separation relation because one-to-one correspondence exists between the crack growth resistance and the softening traction-separation relation. However, the steady-state crack propagation that all quantities like crack opening displacement and cohesive stress keep invariable may not occur due to the limitation of the fracture process zone (FPZ). For quasi-brittle fracture, it is essential to define when the steady crack propagation occurs for ensuring the applicability to determine the softening traction-separation relation based on steady crack propagation. In this study, numerical test results of three specimen geometries, seven specimen sizes and three softening traction-separation relations are employed to analyze the steady crack propagation. Through comparison between crack growth resistance based on steady-state assumption and equivalent LEFM R-curve, a pseudo-steady state regime that is very similar to the steady crack propagation is found. At the pseudo-steady state, the relationship between the softening traction-separation relation and the crack growth resistance under steady crack growth is still applicable. The occurrence condition for the pseudo-steady crack propagation is deduced. It is found that for most laboratory specimens, the critical situation (load reaches its maximum value) can be seen as a pseudo-steady state with an error of <20%. By choosing the softening traction-separation relation based on the pseudo-steady assumption, the initial fracture toughness can be determined. The errors of the initial fracture toughness determined by this method can be approximately estimated based on the range of lFPZ/(D-ac).

Fracture prediction of NM450TP wear-resistant steel plates during air bending using a mesoscopic heterogeneous model

A mesoscopic finite element model considering material heterogeneity based on microstructure features was constructed to predict the fracture of NM450TP plates during air bending. The fracture morphology and crack initiation and propagation were specifically addressed from two typical planes of the viewing direction, namely, the section plane orthogonal to the bending axis and the bending (outer) convex surface under tension during bending. The results demonstrated that the model could accurately reproduce the initiation, propagation and final fracture morphology. In the section plane, strain shear bands appeared and then developed into the onset of the initial crack with an inclined angle of 45° to the convex surface. As bending developed, the crack randomly expanded towards the through-thickness direction, ascribed to the interaction between the stress mode and material heterogeneity. In the bending convex surface, the fracture morphology dramatically changed with the relative position between the bending axis and rolling directions. When the bending axis was perpendicular to the rolling direction, a serrated fracture line was formed. When the bending axis was parallel to the rolling direction, a nearly linear fracture line was finally formed along the bending axis.

Fracture modeling by the eigenfracture approach for the implicit material point method framework

AbstractThe material point method (MPM) is efficiently applied for the simulation of structures undergoing large deformations where fracture and crack initiation are expected. The eigenfracture approach is introduced in the paper at hand for the implicit MPM to model crack development and propagation in static and dynamic fracture of brittle elastic materials. Eigenfracture is an energetic fracture formulation applied in the postprocessing step of the implicit MPM, making its implementation relatively straightforward. Furthermore, the driving energy used to check crack propagation is evaluated using the representative crack elements (RCE), by which the crack is modeled as a discrete phenomenon. The RCE approach shows more realistic results compared to other split models. Additionally, the fracture description of reinforced materials within the MPM is also presented in this article by coupling truss finite elements to the MPM, considering the bond stress‐slip constitutive model. Two‐ and three‐dimensional problems in static and dynamic applications are presented to assess the efficacy of the approach.

A robust Moore–Penrose pseudoinverse-based static finite-element solver for simulating non-local fracture in solids

In this work, we develop a pseudoinverse-based static finite-element solver to model the elastic deformation and non-local brittle fracture of solids. The pseudoinverse of the finite-element stiffness matrix is calculated using a QR decomposition-based method which is faster than using the traditional approach based on the singular value decomposition method. This new finite-element framework has two advantages: (1) the finite-element equations can still be solved even when the finite-element stiffness matrix is singular, and (2) there is no need for introducing an artificial elastic rest energy or viscous regularization for the sole purpose of keeping the finite-element stiffness matrix non-singular in order to solve the finite-element equations. We also show that the proposed method is robust in solving chosen boundary value problems involving the elastic deformation and abrupt non-local mode I and mixed-mode brittle fracture of solids when compared to the traditional element kill method where a small but finite residual stiffness is maintained at a material in order to prevent the finite-element stiffness matrix from being singular. Hence, this proposed method allows the modeling of separation/fragmentation of solids when fracture occurs.Finally, we use the new computational framework to model the fracture of PMMA beam samples at room temperature. By calibrating the material parameters in the constitutive theory using analytical methods and fitting to a Mode I fracture experiment force–displacement response, we show that our newly-proposed computational method is able to predict the experimental fracture loci and crack propagation characteristics in PMMA beam samples undergoing mixed-mode fracture conditions to good accord.

The fracture resistance and crack propagation path of L415/N08825 bimetallic composite pipe welded joint

The microstructure, fracture resistance and crack propagation path in the L415/N08825 bimetallic composite pipe welded joint has been carried out using the single-edge notched bend (SENB) specimens. The microstructure in the welded seam is dendritic austenite. There are high-density dislocations in the weld, and ellipsoidal NbC and TiC precipitates are distributed on the dendritic austenite matrix. There is element diffusion between the weld and base metals of the interface, and the concentration distribution of main elements at the interface presents a steep concentration gradient. The J-resistance (J-R) curves of the heat-affected zone (HAZ) and welded seam of the experimental welded joint were obtained, reflecting the HAZ of the L415/N08825 bimetallic composite pipe welded joint has a better fracture resistance than the welded seam. The crack propagation path is approximately straight in all specimens, and both regions extend along grain boundaries. The fracture mode in the HAZ is a ductile fracture. The crack is close to deflect to the interface and is greatly affected by the strength and toughness mismatch of the weld metal (WM) and L415. While, the main crack growth direction and the weld grain orientation have always led to the low fracture resistance of the WM.

Multiscale modelling of fracture in graphene sheets

Most of the continuum scale processes, such as fracture, plasticity, etc., trace their origin to atomistic scale phenomena. To gain deeper insights into these processes, one needs to understand the behaviour of materials through the lens of multiscale methods. In this manuscript, we study the problem of fracture crack propagation in graphene sheets through a sequential multiscaling technique. The continuum-mechanical smoothed particle hydrodynamics (SPH) is coupled with the atomistic scale molecular dynamics (MD) simulations through proper constitutive modelling — the non-linear material properties and the mechanical equation of state which serve as the inputs to the SPH model are evaluated directly from the MD simulations. Such handshaking ensures that the continuum-scale SPH model is able to faithfully reproduce the atomistic scale stress–strain behaviour until failure. Using a pre-notched continuum scale graphene sheet, we show that the mode-I stress intensity factor obtained from our SPH model agrees well with the published literature. We subsequently study crack propagation in pre-notched graphene sheets, where the influence of the orientation of the notch is evaluated. Lastly, we take the case of a continuum-scale graphene sheet having randomly oriented cracks and identify the changes in the stress–strain behaviour vis-à-vis a pristine graphene sheet.

Microstructure evolution and strength-toughness matching mechanism of Fe-Cr-Ni gradient alloy steel prepared by direct laser deposition

A novel 70Cr8Ni2Y alloy steel powder was designed and prepared to meet the requirement for composition design and preparation of gradient alloys with strength-toughness matching the preparation of nuclear power camshafts by direct laser deposition (DLD). A three-layer gradient structure alloy sample with strength-toughness matching was successfully prepared by DLD using 70Cr8Ni2Y, 50Cr6Ni2Y and 12CrNi2Y alloy steel powder. The microstructure evolution and strength-toughness matching mechanism of the gradient sample were studied. The interfaces between the gradient layers were metallurgical bonding and had no defects such as cracks and pores. With the change of the alloy composition of the three gradient layers, the main microstructure of the sample changed from granular bainite to lath bainite, and to martensite in turn. The mechanism of strength-toughness matching was that the changes of the three alloy components made the strength of microstructure increase, and a metallurgical bond formed at the interfaces of the gradient layers. The gradient sample demonstrated strength-toughness matching of high-strength surface and good toughness core through adjusting of the gradient layers and change of microstructure. The high-strength 70Cr8Ni2Y surface layer ensured the high hardness and wear resistance of the gradient sample, and the 50Cr6Ni2Y gradient layer acted as an intermediate layer to support the surface and protect the bottom layer. The 12CrNi2Y gradient layer with good toughness played a role in delaying fracture and hindering crack propagation. The microhardness of the gradient sample showed a gradient distribution of 739-534-321 HV. The average tensile strength was 1076 MPa, the elongation was 1.6%, and the specific wear rate (SWR) of the sample surface was 9.5 × 10−5 mm3/(N·m). This paper can provide a useful reference for the preparation of nuclear power camshaft parts with gradient properties of good toughness of the core and high hardness and wear resistance of the surface by DLD.

Effect of the manufacturing parameters on the tensile and fracture properties of FDM 3D-printed PLA specimens

Fused Deposition Modeling (FDM), also known as material extrusion, is currently one of the most popular Additive Manufacturing (AM) technology on the market. In this study, two commercial FDM printers (Prusa i3 MK3 and WN400 3D Platform) were used for layer-by-layer manufacturing of polylactic acid (PLA) Dog Bone (DB) and Single Edge Notched Bend (SENB) specimens, aimed at investigating the influence of the manufacturing parameters on the tensile and fracture properties of PLA elements obtained by FDM technology. The effects of growing direction (horizontal and vertical), building orientation (0°, 45° and 90°), printer type, layer thickness (0.15 and 0.40 mm), specimen thickness (4 and 10 mm) and filament color (purple, white, black, gray, red, orange) are discussed in detail. Tensile tests were performed on DB specimens, while fracture mechanics tests on SENB specimens. Both the tensile and fracture properties of FDM 3D-printed PLA specimens have been found to be dependent on the investigated manufacturing parameters. From the microstructural analyses of the SENB fracture surfaces, it has been observed that the fracture mechanisms and crack propagation is a step-wise process. Finally, the material properties charts (Young’s modulus and mode I fracture toughness versus tensile strength) are plotted.

Effects of axial pre-force and loading rate on Mode I fracture behavior of granite

Coupled static-dynamic tests were carried out on notched semi-circular bend (NSCB) granitic specimens using a modified split Hopkinson pressure bar (SHPB) device, to investigate the effects of axial pre-force and loading rate on dynamic fracture behavior of rocks. Digital image correlation (DIC) method was adopted to obtain the deformation field and fracturing process. The results indicate that the pre-force (up to 50% static peak force) enhances the fracture parameters overall, including fracture initiation toughness, fracture energy , crack propagation velocity (CPV) and fracture propagation toughness, but the effect of loading rate is more significant than that of the pre-force. Digital images show that the increase in pre-force would cause the dominant crack to symmetrically bifurcate since the pre-force strengthened a fan-shaped area for the specimen near the incident bar. The loading rate larger than 124 GPa·m 1/2 /s could lead to the secondly fracture of the specimen and thereby more fragments. Compared with previous studies, the present investigation deepens the understanding of fracture characteristics of rocks under coupled static and dynamic loads by further reducing the pre-force interval (0.25 kN). In addition, a new method based on the DIC technique is proposed to calculate crack opening distance (COD) and CPV, which can improve the efficiency and accuracy compared to the strain gauge method. Moreover, fracture propagation toughness is also emphasized apart from the initiation toughness. The findings can provide some enlightenments for rock fracture dynamics and crack propagation of highly stressed rocks.

Simulation of crack growth for metal plates with holes based on cohesive zone model

Hole defects are very common in metal plates. The propagation of cracks in engineering structural members will be affected by the holes of the members, which will not only change the propagation direction of cracks, but also have the ability to limit the growth of cracks. Therefore, the study of material fracture crack propagation caused by the distribution of holes and other factors under loading conditions has become a problem that needs to be considered in structural design. The cohesive zone model (CZM) can effectively avoid the singularity problem of crack tip stress when simulating crack growth, and at the same time can clearly show the crack growth path. Based on exponential CZM, the finite element analysis of tensile test for metallic materials with hole defects was carried out. The crack propagation law under constant load was obtained. It was proved that CZM could simulate the crack propagation in metallic materials with holes more accurately. The influence of the pore size, shape, distribution on the crack propagation is discussed. The results show that the smaller the distance difference between holes, the greater the supporting reaction force. The smaller the radius of the hole, the higher the fracture toughness and the maximum supporting reaction force of the metal plate. And it also can be seen that as the number of holes increases, the maximum supporting reaction force and fracture toughness of the metal plate decrease. The use of staggered holes distribution has greater supporting force and better toughness. The research results of this paper provide a theoretical basis and reference for investigating the propagation and propagation of cracks in the process of damage and failure of materials and for structural design and performance evaluation of engineering materials.

Test method for fracture toughness GIC based on ubiquitiform theory

A new fracture toughness test method for obtaining the crack initiation fracture toughness GIC and crack propagation fracture toughness GC was proposed. This method was based on the basic idea of ubiquitiform geometry and experimental results showing that material fracture surfaces possess ubiquitiform characteristics. A uniaxial tensile test was performed on plate specimens with cracks made by ductilecast iron to generate Mode I fracture failure. Loading–displacement curves were recorded in accordance with the experimental process, and the crack-initiation work W1 and the crack propagation work W2 were recorded. The morphology of the fracture surface was observed through electron microscopy, and the ubiquitiformal complexity D of the fracture surface was calculated on the basis of ubiquitiform theory. The ubiquitiform fracture area of the fracture surface was further obtained. The formula for ubiquitiform fracture energy was provided and used to calculate the ubiquitiform fracture energies Guf1 and Guf2 of the tested material. By analyzing the relationship between the above ubiquitiform fracture energy and fracture toughness GIC, a test method for determining fracture toughness based on ubiquitiform fracture energy was proposed. The experimental fracture toughness test results for several cracks of different sizes were consistent within a certain range of crack lengths.

Cyclic thermal shock resistance for MgAlON–MgO composites obtained with additions of spent MgO–C brick: Microstructure characteristics, thermal shock parameter and thermal shock mechanism

Thermal shock parameters (R, R''', R'''' and Rst) of MgAlON–MgO composites obtained with additions of spent MgO–C brick were calculated using measured mechanical properties and thermal expansion coefficient, determining their resistance to fracture initiation and crack propagation. The cyclic thermal shock experiments of MgAlON–MgO composites performed from 1398 K to ambient temperature indicate that as number of thermal shock cycle increases, retained strength ratio of MgAlON and MgAlON–4.2 wt%MgO sharply decrease and then keep constant, while that of MgAlON–10.5 wt%MgO and MgAlON–15.7 wt%MgO slowly decrease. The reason for the difference is that MgAlON and MgAlON–4.2 wt%MgO show low value of R''' and R'''', and high value of R and Rst. Moreover, precipitation of impurity containing Fe may play a positive role in improvement of thermal shock resistance of MgAlON–MgO composites. MgAlON‒4.2 wt%MgO has the maximum retained strength (55 MPa) even after 5 thermal shock cycles, which is expected to be used in the metallurgical industry.

Effectiveness of prophylactic double-looped wiring in cementless Hip arthroplasty: A biomechanical study in osteoporotic bone model using impaction simulation system

BackgroundPeriprosthetic femoral fracture (PFF) is well-known complication in cementless hip arthroplasty (HA), especially in the elderly with osteoporosis. To prevent intraoperative PFFs during HA, prophylactic cerclage wiring on the calcar area during femoral broaching and implant insertion is recommended. However, biomechanical data on the benefits of cerclage wiring in osteoporotic bone during impaction are limited and controversial. This study aimed to assess the efficacy of prophylactic double-looped cerclage wiring (PDLCW) during femoral broaching in the osteoporotic bone model with an impaction simulation system. MethodsFifteen osteoporotic femur models were prepared and allocated to three groups; control group (no cerclage), PW group (received PDLCW), and CC+PW group (prepared as having calcar crack and received PDLCW). All femurs were broached under impaction force that sequentially increased until the visible fracture or calcar crack propagation was visible. The primary outcomes were mallet impaction force (MIF-CF) and mallet velocity (MV-CF) at the time of calcar fracture. The secondary outcomes were calcar fracture pattern and subsidence during calcar fracture (S-CF). ResultsPW group showed a significant increase in average MIF-CF, by 40.4% and 120.2% (p<0.001) compared to the control and CC+PW groups, respectively. The average MV-CF in PW group was also significantly greater, 13.1% and 64.6% (p<0.001), compared to the control and CC+PW groups, respectively. Control group revealed significantly greater incidence of complete calcar fracture (60% vs. 0% vs. 0%, p = 0.024), and the highest S-CF compared to those in the PW and CC+PW groups (10.6 ± 6.0 mm, 6.7 ± 4.4 mm, and 1.3 ± 2.0 mm, p = 0.020). ConclusionsThis study showed that PDLCW significantly improved hoop stress resistance by increasing the calcar fracture threshold related to mallet impaction, decreasing the risk of complete calcar fracture.

Fracture toughness and crack propagation in LX-17 and PBX 9502 insensitive high explosives

High explosive materials, and in particular insensitive high explosive (IHE) materials, are of significant interest. TATB-based LX-17 and PBX 9502 are two IHE’s and mechanical properties are an important metric of performance. In this work we present, for the first time, a study of fracture as a function of temperature and displacement rate and starter crack length on billet pressed IHE materials using a standard three point bend technique. We demonstrated that the technique can yield consistent results over time by measuring samples from the same lots 4 years later and observed that the fracture toughness of LX-17 and PBX 9502 are similar. We also determined the temperature dependence for different starter crack lengths and found that the fracture toughness generally decreases with larger starter crack sizes and temperatures. For the LX-17, where a wide temperature range of −60 C to 50 C was investigated, the impact of temperature on fracture toughness between −20 C and 23 C was less than expected. For the PBX 9502 we observed the difference in fracture toughness values between −20 C and 23 C did depend on the particular lot, and also there was a tendency of the fracture toughness to scale with the tensile strength. We also investigated the effect of a lower displacement rate and observed that while the impact on the fracture toughness was relatively small, the samples surprisingly did not break (fail) after exceeding the peak load. We took advantage of this to perform x-ray computed tomography (CT) of the samples to image crack propagation in the intact samples. We observed that cracks were generally straight and unaffected by the microstructure in the LX-17, in contrast to the PBX 9502 where the direction of the cracks would frequently change and were likely influenced by the underlying microstructure.

Microstructural Impact on Fatigue Crack Growth Behavior of Alloy 718

Alloy 718 for forged parts can form a wide range of microstructures through a variety of thermo-mechanical processes, depending on the number of remelting processes, temperature and holding time of homogenization annealing, cogging and the number of forging steps depending on the forming characteristics. In industrial practice, these processing steps are tailored to achieve specific mechanical and microstructural properties in the final product. In the present work, we investigate the dependence of the threshold of stress intensity factor range ΔKth on associated microstructural elements, namely grain size and distribution. For this purpose, a series of tests with different starting microstructures were performed at the falling stress intensity factor range, ΔK, and a load ratio of R = 0.1 to evaluate the different threshold values. Fracture initiation and crack propagation were analyzed afterward using scanning electron microscopy of the resulting fracture surfaces. In order to obtain comparable initial conditions, all specimens were brought to the same strength level by means of a two-stage aging heat treatment. In the future, this knowledge shall be used in the context of simulation-aided product development for estimating local fatigue crack propagation properties of simulated microstructures obtained from forging and heat treatment modeling.

An improved smoothed particle hydrodynamics method for simulating crack propagation and coalescence in brittle fracture of rock materials

A new meshless numerical method, namely the improved smoothed particle hydrodynamics (ISPH) method, is proposed to simulate the fracture process of rock samples with pre-existing flaws under uniaxial compression. By improving the kernel function of SPH method, the brittle fracture characteristics of materials are presented. In ISPH, a prefabricated node segment (PNS) method is used to generate arbitrarily complex flaws, which avoids the division and distortion of traditional particles. In order to further improve the numerical accuracy, a new solid boundary treatment method called coupled dynamic buffer SBT algorithm is applied to ISPH, which is verified to be effective and robust via two benchmark samples. The effect of the length of a pre-existing flaw on the fracture mode and compressive strength of the rock sample is studied by using the elastic-brittle constitutive model. The numerical prediction results are in good agreement with the experimental observations. In addition, the effect of the unconnected rate on the crack propagation and coalescence mode of a rock sample with two pre-existing flaws under uniaxial compression is also studied. The numerical simulation results are in good agreement with the experimental results, which further verify the accuracy and superiority of the ISPH method.

Study on fracture failure mechanism and crack propagation law of granite-shotcrete composite structure

Study on fracture failure mechanism and crack propagation law of granite-shotcrete composite structure

Understanding the ductility versus toughness and bendability decoupling of large elongated and fine grained Al 7475 - T6 alloy

7XXX aluminium alloys are widely used in aerospace components due to their high strength to weight ratio. However, first generation 7XXX alloys has a limited fracture toughness and crack propagation resistance. New generation of 7XXX alloys succeeds increasing fracture toughness, by playing on both alloying elements composition and purity level. One of the toughest alloy is the 7475 Al alloy, with a fracture toughness (KIc) that reaches up to 60 MPa√m in T6 conditions. Friction stir processing (FSP) is applied to 7475 Al rolled plates in order to generate a fine-grained microstructure. Heat treatments (HT) are performed in order to set rolled and FSPed microstructures into T6 peak-aged condition. While the T6 FSPed microstructure shows a significant fracture strain improvement in uniaxial tensile loading, this microstructure does not improve the crack propagation resistance compared to the T6 rolled material, neither on compact tension nor on bending specimens. Which mechanisms are responsible for this effect? A decoupling between ductility and toughness in 7XXX aluminium alloys is unexpected and the understanding of this phenomenon is of prime importance for mechanical design. This work reveals the competing effects of the FSPed microstructure on crack initiation and crack propagation. FSPed material delays the crack initiation under loading but the FSPed fine grains favour fully intergranular crack propagation. In comparison, the energy needed for the crack to cut through or circle around the rolled elongated grains is much higher in the rolled material. Indeed, elongated rolled grains act as crack arresters when the crack direction is perpendicular to the grain's main axis. In this case, failure of the rolled material leads to extensive transgranular propagation, increasing the energy absorption and rising the KIc values to higher levels. This work illustrates the formation and propagation of cracks in the rolled and FSPed 7XXX alloys microstructures in order to explain the damage sequence and the competition between transgranular and intergranular failure.