Crack Orientation Research Articles

Toughening Mechanism of the Bone — Enlightenment from the Microstructure of Goat Tibia

Abstract With the continuous advancement of space exploration missions, the mechanical environment for planetary detectors is becoming increasingly severe. As a result, fatigue, fracture, large deformation and other forms of failures are more likely to occur at the load-bearing structures. As a critical part of the load-bearing structure of a goat, goat tibia has remarkable toughness because of its unique microstructures. In this investigation, firstly, the cortical bone of goat tibia was observed by SEM, and the characteristic microstructures in different regions were identified. Secondly, the cross section of cortical bone was loaded by long-term inplane stress, then the toughness of cortical bone in different regions are obtained and compared based on the orientation and distribution of cracks after the load. Thirdly, a simplified FEM model mimicking typical microstructure of the cortical bone is proposed using cohesive modeling, and then the toughening mechanism of the typical microstructure is validated with numerical simulation. Finally, the toughening mechanisms of cortical bone were discussed according to the SEM observation as well as the numerical simulation. This study of the toughening mechanism of cortical bone can be helpful for the biomimetic design of high-toughness structures.

Ductile and brittle crack-tip response in equimolar refractory high-entropy alloys

Understanding the strengthening and deformation mechanisms in refractory high-entropy alloys (HEAs), proposed as new high-temperature material, is required for improving their typically insufficient room-temperature ductility. Here, density-functional theory simulations and a continuum mechanics analysis were conducted to systematically investigate the competition between cleavage decohesion and dislocation emission from a crack tip in the body-centered cubic refractory HEAs HfNbTiZr, MoNbTaVW, MoNbTaW, MoNbTiV, and NbTiVZr. This crack-tip competition is evaluated for tensile loading and a totality of 15 crack configurations and slip systems. Our results predict that dislocation plasticity at the crack tip is generally unfavorable – although the competition is close for some crack orientations, suggesting intrinsic brittleness and low crack-tip fracture toughness in these five HEAs at zero temperature. Fluctuations in local alloy composition, investigated for HfNbTiZr, can locally reduce the resistance to dislocation emission for a slip system relative to the configuration average of that slip system, but do not change the dominant crack-tip response. In the case of single-crystal MoNbTaW, where an experimental, room-temperature fracture-toughness value is available for a crack on a {100} plane, theoretical and experimental results agree favorably. Factors that may limit the agreement are discussed. We survey the effect of material anisotropy on preferred crack tip orientations, which are found to be alloy specific. Mixed-mode loadings are found to shift the competition in favor of cleavage or dislocation nucleation, depending on crack configuration and amplified by the effect of material anisotropy on crack tip stresses.

Fracturing behavior around a blasthole in a brittle material under blasting loading

Dynamic fracturing behavior in brittle materials under blasting loading occurs in a very short time. It is usually challenging to observe the behavior experimentally. Therefore, visual observation of the fracturing behavior can be useful information for understanding dynamic fracturing and crack propagation process under blasting loading. In this study, dynamic fracturing behavior under blasting loading was investigated through experimental and numerical methods. Transparent and homogeneous PMMA was used for specimen, while DDNP(diazodinitrophenol) detonators were used to apply blasting loadings. During the blasting experiment, fracturing and crack propagation process were observed using a high-speed camera, then the crack propagation behavior was analyzed in both quantitative and qualitative manners. The results showed that gas-driven fracture in an ear-shape was more predominant than shock-wave-driven fractures. Further, the extent of crack propagation significantly depended on the amount of explosives charge. The experimental results matched well with the analytical solution based on the fracture mechanics theories. A numerical simulation was carried out using a commercial FEM software, which is capable of solving non-linear dynamic problems. The numerical simulation reasonably reproduced the cracking behavior in terms of crack length and orientation which were observed in the experiment.

Plane electroelastic problem for a cracked piezoelectric half-space subject to remote electric field action

Plane electroelastic problem is considered for a longitudinal crack in a transversely-isotropic piezoelectric half-space subject to remote electric field perpendicular to the flat surface of the half-space. The flat surface is fully covered by thin electrode and free of mechanical load. The Lekhnitskii’ generalized complex potentials are used for the solution representation. For a case of an internal crack, boundary conditions on the flat surface are satisfied analytically by use of the Cauchy integration technique. The least-square method is applied for satisfying boundary conditions on the crack surface. An edge crack is considered. Numerical investigations are carried out for a internal or an edge crack in dependency on crack orientation and location. The results showed that one of the most dangerous crack is parallel to the flat surface. A crack perpendicular to the flat surface does not cause any significant electroelastic field in a piezoelectric half-space. Location of a crack relative the flat surface influences on electromechanical coupling near its tips.

Fracture evolution in ferrite/martensite dual-phase flange steel

ABSTRACT Ferrite/martensite dual-phase steel is indispensable for manufacturing different automobile parts. In the present study, uniaxial tension tests and metallographic analyses are performed to reveal the fracture evolution in ferrite/martensite dual-phase flange steel with a high hole-expansion ratio. The results indicate that in the ferrite/martensite dual-phase steel, cracks originate at the interface between ferrite grains and martensite islands, and at the Al2O3 and sulfide inclusions. Further, dislocations obviously aggregate at the interface between the ferrite grains and martensite islands, providing many preferential crack nucleation sites. Stress concentrations tend to occur at the edge of Al2O3 and sulfide particles. In addition, the fraction of low-angle grain boundaries increases with prolonged deformation, and the size of ferrite grains gradually decreases with deformation, weakening their effect of inhibition of crack propagation. Moreover, the existence of the γ fiber texture and the {001} texture in the tensile deformation area facilitates crack propagation.

Effect of Crack Inclination and Residual Stress on the Stress Intensity Factor of Semi-elliptical Crack in a Pressure Vessel

Pressure vessels are widely used in products ranging from consumer artifacts to engineering components. Ensuring safety and reliability of pressure vessel is therefore imperative. In cases where conducting experiments is not viable, analytical or numerical methods are used. In this work, a horizontal pressure vessel is analyzed using finite element method for estimating the failure pressure and stress intensity factor in the presence of residual stress. A three-dimensional finite element model was created with a commercially available software, and a semi-elliptical crack was introduced in the pressure vessel on the outer and inner surfaces. In addition, the crack orientation was also changed, and the stress intensity factor was determined both analytically and numerically. The results showed a reasonably good agreement.

Scale bridging damage model for quasi-brittle metals informed with crack evolution statistics

Computationally efficient methods for bridging length scales, from highly resolved micro/meso-scale models that can explicitly model crack growth, to macro-scale continuum models that are more suitable for modeling large parts, have been of interest to researchers for decades. In this work, an improved brittle damage model is presented for the simulation of dynamic fracture in continuum scale quasi-brittle metal components. Crack evolution statistics, including the number, length, and orientation of individual cracks, are extracted from high-fidelity, finite discrete element method (FDEM) simulations and used to generate effective material moduli that reflect the material’s damaged state over time. This strategy allows for the retention of small-scale physical behaviors such as crack growth and coalescence in continuum scale hydrodynamic simulations. However, the high-fidelity simulations required to generate the crack statistics are computationally expensive. Thus, steps were taken to produce a flexible constitutive model to reduce the number of costly high-fidelity simulations needed to produce accurate results. A new stress based degradation criterion is introduced for the degradation of individual material zones. This allows for the development of a heterogeneous damage distribution within the bulk material. Then a flow stress model is added to the hydrodynamic simulation to account for plasticity in quasi-brittle materials. As a result, the effective moduli model can be applied to a larger range of materials. The effective moduli constitutive model is used to simulate beryllium flyer plate experiments. The results from the continuum scale simulations using statistics from a single high-fidelity simulation are found to be in excellent agreement with numerical and experimental velocity interferometer data. The same set of crack statistics are used to extrapolate the results of a higher rate flyer plate case using the effective moduli model. The extension of this model to higher rate cases shows promise for further reducing the number of costly high-fidelity simulations needed to generate crack statistics.

Deformation mechanisms and evolution of mechanical properties in damaged advanced ceramics

We investigated the uniaxial compressive behavior of damaged and intact alumina using quantitative X-ray computed tomography (XCT) analysis coupled with digital image correlation (DIC) for mechanical characterization. Internal three-dimensional crack characteristics such as crack surface area and orientation were quantified using XCT to assess the level of damage. From the quasi-static and dynamic stress–strain results, the primary effects of crack damage are to reduce the initial stiffness and rate of lateral expansion in damaged alumina. With increasing axial strain, crack closure was found to lead to a recovery of elastic properties, in some cases to intact levels, in the damaged specimens. Localized deformation mechanisms related to the crack structure, including lateral crack closure, axial crack opening and closing, and inclined crack sliding, were visualized in-situ and connected to XCT reconstructions. High-speed imaging also revealed a mixed fracture mode for damaged alumina that included axial splitting and failure along pre-existing cracks.

Evaluation of Critical Crack Orientations and Crack Growth Analysis in Compressor Blade Root Subjected to Fatigue

Abstract Aero-engine compressor blade and disk assemblies are subjected to radially outward centrifugal force due to blade rotation and the bending of the blade due to gas pressure. Static analysis is done initially to find the critical crack orientation at the root of the blade. With simulated initial cracks, the cracks are allowed to grow in low cycle fatigue (LCF) and isochromatics are captured at selected intervals. Using digital photoelasticity, isochromatic images are used along with an over-deterministic least-squares approach to find the fracture parameters. As the stress fields are very complex, a multiparameter solution with a higher number of parameters was needed to model the stress field. Crack growth angles are evaluated using maximum tangential stress (MTS), strain energy density (SED), and generalized MTS (GMTS) criteria. It is observed that the GMTS criterion predicted closer results to that of experimental values, which implied the significance of T-stress in predicting the crack growth angle.

Influence of the PZT Sensor Array Configuration on Lamb Wave Tomography Imaging with the RAPID Algorithm for Hole and Crack Detection.

The tomography technique is an effective way to quantitatively evaluate damage from reconstruction imaging in structure health monitoring (SHM). The reconstruction algorithm for the probabilistic inspection of damage (RAPID) algorithm based on the signal difference coefficient (SDC) feature is a promising approach due to its superior performance. This paper focuses on the influence of different patterns of PZT (Lead Zirconate Titanate) sensor array configurations, i.e., the circular, square, and parallel array, on reconstruction image qualities for evaluating hole and crack damage. Variable shape parameters are applied to account for the unequal damage distances of different actuator-sensor pairs. Considering the directionality scattering fields of cracks, the angular scattering pattern of the SDC values are studied by simulation. The SDC variations for different groups of sensing paths at the same actuator are applied to predict the crack orientation. An improved RAPID algorithm is proposed by defining an additional SDC value of 1 in the path along the predicted crack orientation, which is determined by the point of the actuator causing the minimal SDC variation and the center point of the initial reconstruction image of the crack. The results show that the improved RAPID algorithm is effective for the evaluation of crack damage. Reconstruction image qualities with three PZT sensor array configurations for both holes and cracks are compared. The research is significant for selecting the PZT sensor array configuration in SHM.

Fatigue‐life prediction of additively manufactured material: Effects of heat treatment and build orientation

AbstractIn this study, the crack‐growth approach is used to predict the fatigue life of 17‐4 precipitation hardening (PH) stainless steel (SS) fabricated via an additive manufacturing (AM) system in different orientations (ie, vertical and horizontal) before and after heat treatment. To perform fatigue‐life calculations, the effective stress intensity factor as a function of crack‐growth rate was obtained from testing modified compact specimens with different crack orientations in as‐built and heat‐treated conditions. The plasticity‐induced crack closure model, FASTRAN, was used to calculate fatigue lives based on the size of process‐induced defects. Results indicated that in the presence of large voids (ie, lack‐of‐fusion defects), the total fatigue life of AM 17‐4 PH SS in as‐built and heat‐treated conditions is dominated by crack growth. Effect of build orientation on fatigue life of AM 17‐4 PH SS was also captured based on the size of defects projected on a plane perpendicular to the loading direction.

A comparison between some fracture modelling approaches in 2D LEFM using finite elements

The finite element method has been widely used to solve different problems in the field of fracture mechanics. In the last two decades, new methods have been developed to improve the accuracy of the solution in 2D linear elastic fracture mechanics problems, such as the extended finite element method (XFEM) or the phantom node method (PNM). The goal of this work is to quantify the differences between some numerical approaches: standard finite element method (FEM), mechanical property degradation, interelemental crack method with multi-point constraints, XFEM and PNM. We explain the different techniques analysed together with their advantages and disadvantages. We compare these numerical techniques to model fracture using problems of reference with known solutions, evaluating their behaviour in terms of convergence with respect to the element size and accuracy of the stress intensity factor (SIF), stresses ahead the crack tip and crack propagation prediction. Some of the new techniques have shown a better accuracy in SIF calculation or stress fields ahead the crack tip and other lead to high errors in local results estimations. However, all methods reviewed here can predict crack propagation for the problems of reference of this work, showing good accuracy in crack orientation prediction.

The fracture resistance of bi-axially orientated PMMA

The mechanical performance of the transparent polymer PMMA (polymethyl methacrylate) can be significantly enhanced by the process of bi-axial stretching during manufacture. However, few studies have reported on the fracture properties of bi-axially oriented PMMA. In this work, the fracture toughness and resulting surface morphology of bi-axially orientated PMMA has been investigated by testing single edge notch bend fracture specimens under quasi-static test conditions. The orientation of the specimens with respect to the stretching direction was systematically varied such that the effect of in-plane orientation and out-of-plane orientation was determined. The out-of-plane fracture resistance was shown to be enhanced compared to the in-plane resistance and compared to amorphous PMMA. Both out-of-plane and in-plane crack orientations had little effect on fracture resistance. Complex crack growth behavior and surface morphologies were observed in the out-of-plane direction. The results have important implications for the study of damage tolerance of transparent canopies in the aircraft industry.

Multi-frequency fusion algorithm for detection of defects under fasteners with EC-GMR probe data

Rotating eddy current (EC) excitation with giant magnetoresistive (GMR) sensor pick up (EC-GMR) has proved to be a promising technique for detection of defects under fasteners in multi-layer riveted structures. Challenges in this application include arbitrary orientation of cracks, influence of dominant signal from steel fasteners and need for rapid inspection of parts. This paper presents a novel data fusion algorithm for suppressing signals from steel fastener and enhancing defect indications in multi-frequency EC-GMR measurements. The algorithm fuses transformed versions of EC-GMR probe data using a multi-dimensional constrained optimization procedure based on Sequential Least Square Programming (SLSQP). The optimal transformation parameters are determined by minimizing the field residue at a reference fastener site with no defects. Performance of the proposed algorithm is evaluated on experimental measurements acquired using EC-GMR sensor on a riveted lap-joint with steel fasteners and second layer EDM notches. Initial results demonstrate that bottom layer defects of different sizes and orientations can be successfully detected with this approach.

Effects of crack orientation and heat treatment on fatigue-crack-growth behavior of AM 17-4 PH stainless steel

The effects of heat treatment and crack orientation on fatigue-crack-growth (FCG) behavior of 17-4 precipitation hardening (PH) stainless steel (SS) fabricated via a laser powder bed fusion (LPBF) additive manufacturing (AM) system were investigated. Accurate representation of FCG thresholds, the region defining crack growth as either very slow or nonexistent, is extremely important in adoption of AM for structural applications. In this study, new methods called compression pre-cracking constant-amplitude (CPCA) and load-reduction (CPLR) were used to generate FCG rate data in the near-threshold (low-rate) regime using non-standard or modified compact specimens. For comparison, the current ASTM load-reduction (LR) method was also used. Results indicate that FCG behavior of AM 17-4 PH SS in the Paris regime was comparable to that of wrought 17-4 PH SS in the peak-age condition. Although the post-manufacturing heat treatment was found to be necessary in order to improve tensile strength of this alloy, the results of this study indicate that this particular heat treatment process had no influence on FCG behavior of LPBF 17-4 PH SS. The effect of crack orientation with respect to the build direction however was evident. FCG rates were slightly lower for specimens with a crack parallel to the build direction (i.e. longitudinal crack) in the Paris regime as compared to that of specimens with a perpendicular crack (i.e. transverse crack). The near-threshold FCG behavior for specimens with a longitudinal crack did not show the usual stress ratio (R)-shift from low to high R. The low-R data was vastly different than the data generated using the specimens with a transverse crack. Our results also imply that both the current ASTM standard LR method and the new CPLR method induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold.

Water evaporation from cracked soil under moist conditions as related to crack properties and near‐surface wind speed

AbstractThe impacts of five factors (wind speed, crack width, crack distance, crack depth and crack orientation (compared to wind direction)), on evaporation from cracked soil under moist soil conditions were investigated experimentally. Experiments were carried out under laboratory conditions in a wind tunnel with a 1 m × 1 m × 2.5 m test section using large‐scale (50 cm diameter, 25 cm depth) samples of natural soils wetted to a soil water content of 0.2 g g−1. To reduce the number of necessary experiments, a 2k factorial experimental design was used. Crack widths, crack distances, crack depths and crack orientations of 5–20 mm, 40–100 mm, 50–200 mm and 0–90°, respectively, were used. Effects of each factor on evaporation rate, individually and by interaction between multiple factors, were subsequently calculated. Results showed that individual effects of all five factors on evaporation rate were significant at the 95% level. Wind speed had the largest impact on evaporation rate, followed by crack width, crack distance, crack depth and crack orientation in that order. Overall, the four crack properties together explained 47% of the variations in evaporation rate.Highlights Evaporation from cracked, moist soil under windy conditions was investigated. Crack orientation significantly affects wind conditions close to the surface. Both wind speed and crack properties significantly affected evaporation rate. Crack properties together explained 47% of variations in evaporation rate.

Automatic Crack Detection and Measurement of Concrete Structure Using Convolutional Encoder-Decoder Network

The detection and measurement of crack at pixel level is a challenge to existing methods. To overcome this challenge, this paper proposes a convolutional encoder-decoder network (CedNet) to detect crack from image, and the maximum widths and orientations of cracks are measured using image post-processing techniques. To realize this, a database including 1800 crack images (with 761×569 pixel resolution) taken from concrete structures is built. Then the CedNet is designed, trained and validated using the built database. The validating results show 98.90% accuracy, 93.58% precision, 94.73% recall, 93.18% F-measure, 87.23% intersection over union (IoU) of crack and 98.82% IoU of background. Subsequently, the robustness and adaptability of the trained model is tested. To measure true maximum widths and orientations of cracks, a laboratory experiment is carried out to calibrate a relation between ratio (pixel distance / real distance) and field of view (camera's view range on concrete surface included in image) and distance from the smartphone to concrete surface. In the post-processing techniques, the perspective transformation is employed to correct distorted images caused by the existence of the oblique angles between the smartphone and concrete surfaces. Then the maximum widths and orientations of cracks in predicted results are measured respectively using the Euclidean distance transformation and least squares principle. As comparison, two existing deep learning-based crack detection and measurement method are used to examine the performance of the proposed approach. The comparison results show that the proposed method substantiates quite good performance to detect cracks and measure maximum widths and orientations of cracks in our database.

Numerical simulation of ultrasonic testing reliability of civil aircraft considering the influence of the angle between the sound beam axis and the crack orientation

In order to quantify the influence of the angle between the axis of the sound beam and the crack orientation on the detection reliability during the ultrasonic testing, this paper uses COMSOL Multiphysics to simulate the numerical simulation of ultrasonic testing and explores the variation of the ultrasonic detection amplitude with the angle. The function of fitting the function is given, and then the numerical simulation method of ultrasonic detection reliability considering the influence of the angle between the beam axis and the crack orientation is proposed. According to the numerical simulation case of ultrasonic detection reliability, the following conclusions are obtained. When the detecting device detects a crack of less than 2.23 mm and the detection probability is greater than 90%, the angle between the axis of the sound beam emitted by the detecting device and the direction of the crack must be greater than 70°; If the detecting device detects a crack greater than 0.7 mm and the angle is greater than 86°, the probability of detecting the crack is greater than 61.35%.

Buckling of cracked cones subjected to axial compression

Abstract The influence of crack geometry on the buckling load of axially compressed mild steel cones was presented in this paper. The following geometrical parameter was used: bottom radius-to-top radius ratio, r2/r1=2.0; top radius-to-thickness ratio, r1/t=25; axial length-to-bottom radius ratio, L/r2=2.24; nominal shell thickness, t=1mm; cone angle, β=12.6°. The local buckling phenomenon was investigated through a series of numerical computations (50 ≤ r1/t ≤ 2000). Numerical results show that crack geometry (i.e., length and orientation) influences the buckling strength of the cones differently. For instance, as the crack length increases, the loading capacity of cones drops; cones with a circumferential crack (0°) display the most severe drop. As the crack orientation increases (from 0° to 90°) the buckling strength of the cracked cones with crack length greater than 1 increases. Whereas, for cracked cones with crack length less than 1, increasing the crack orientation has little or no effect on the buckling strength. Hence, it can be said that crack orientation has a secondary effect on the buckling of cracked conical shells.

Acoustic Emission Characteristics and Failure Prediction of the Granite with Orthogonal Cracks under Compressive Loading

Natural joints existing in rock significantly affect the stability of long‐term served subsurface engineering. In this paper, granite specimens with two orthogonal cracks were made for uniaxial compressive tests. The acoustic emission monitoring (AE) and digital image correlation techniques were employed to record the acoustic events and cracking of rock. The stress, ring‐down count, and cumulative ring‐down count of AE during the tests were obtained. The b‐value of AE was calculated based on the magnitude and number of AE events. The relationship between the b‐value and rock cracking for the specimens with orthogonal cracks was discussed. Further, the effects of orthogonal cracks distribution on the b‐value and rock cracking were investigated. Experimental results show that the specimens with orthogonal cracks would undergo multiple cycles of energy accumulation‐release‐reaccumulation‐rerelease under the uniaxial compression. For the specimens with orthogonal cracks, the b‐value of AE was volatile but generally decreased until complete failure. Every cracking event during the loading made the b‐value drop and then the reaccumulation of energy made the b‐value increase or stable. The specimen with orthogonal cracks was more prone to initial cracking than the intact rock. The orientation of cracks had effects on the b‐value evolution and crack patterns. The b‐value reaching about 1.5 can be used as the failure precursor of specimens with orthogonal cracks.