Narrow Results

In order to clarify the cracking and failure behavior of gray cast iron brake blocks that are used for the railway applications, macro- and micro observations regarding the cracks and the micro-structure of the used brake blocks were examined. Three brake blocks, which have different degrees of hot spots and cracking during the actual application, were selected for testing. In addition, a thermal-mechanical coupled finite element analysis (FEA) was applied to calculate the temperature and the stress field in the brake blocks during braking. As a result, it was observed that surface cracks were initiated at the hot spots and propagated into the matrix. From the observation of dispersed graphites close to the crack path, it can be said that the deterioration of materials due to the frictional heat of braking made it easy to initiate cracks at the hot spot. The hardness of the brake block was recommended to be under 85 by the Rockwell B scale in order to prevent hot spots and crack initiation. From the FEA, the procedure for the occurrence of hot spots and cracks was successfully simulated by assuming the surface roughness on the slid surface of the brake block.

The fatigue and fracture behavior of C/C composites fabricated using fine-woven carbon fiber laminates with α = 0/90° direction were investigated. Also, the phenomenon of crack growth behavior and the shear damage in the fiber bundle was discussed. Slits of several sizes were cut on both sides of a test section and different sizes of slit length were chosen. The effect of the slit configuration on crack initiation and growth behavior was observed. Specimens with blunt-notches and center-holes were also used to compare the fatigue strength and crack growth behavior. Non-propagating cracks were observed and fatigue limit was defined as the maximum stress at which specimen did not break for N = 10⁷ cycles stress application. The longest fatigue life was obtained in the case of specimens with shorter slits. The relationships between fatigue strengths and specimen shapes were analyzed by stress concentration, K_t, and stress intensity factor, K_I. The effect of slit configuration on fatigue strength was then discussed regarding both the experimental and calculated consequences.

The fatigue-induced damage and crack growth behavior were studied on the ultrafine grained copper processed by equal channel angular pressing (ECAP). At high stresses, fatigue cracks were initiated at the shear bands (SBs) formed along the shear plane of the final ECAP. At low stresses, the grain coarsening occurred due to dynamic recrystallization. The slip bands were then formed inside these grains and subsequently served as an initiation sites for cracks. The direction of crack growth, either 45° or perpendicular to the loading axis, varied depending on the stress. The formation and growth mechanisms of fatigue crack are discussed based on the micrographic observation of surface damage.

This paper presents a data-driven, similarity-based approach for prognostics of industrial and structural components. The potentiality of the approach is demonstrated on a problem of crack propagation, taken from literature. The crack growth process is described by a nonlinear model affected by nonadditive noises. A comparison is provided with an existing Monte Carlo-based estimation method, known as particle filtering.

Push–pull tests of rolled magnesium alloy AZ31 were performed with a side-notch plate specimen to examine the crack growth behavior under negative mean stress conditions in conditions of high humidity. The effects of repetition of compression on crack growth were discussed. The geometry of the fracture surface and the changes in microstructure were related to the crack growth behavior. The crack growth rate varied even when the experimental conditions were the same. When the oxidized powder appeared from the cracked surface, the crack growth rate became lower. That was related to the changes in microstructure while the crack grew with local shear plastic deformation. The variation of crack closure point was related to the local shear plastic deformation.

A single overload was applied during the crack growth process under constant stress amplitude, and retardation of crack growth was observed in the case of magnesium alloys as well as carbon steel, aluminum alloys, etc. The retardation of crack growth was related to crack closure, the fracture surface roughness, and crack tip deformation. In addition, the effects of supplying oil into the crack on crack growth behavior of an overloaded specimen were investigated in this study. The crack growth rate in the case of supplying oil became lower than in the case without supplying oil. In the case of the magnesium alloy AZ31, powder of oxide magnesium appeared from the crack after overloading. It is one of the typical behaviors of AZ31. In the case of AZ31 and AZX912, the crack growth behavior after overloading was slightly different due to the deformation of the crack tip.

The fatigue behavior of welded thin-walled T-joints made up of both circular hollow section (CHS) braces and chords, subjected to cyclic in-plane bending, is described in this paper. CHS chords and braces are of thicknesses less than 4 mm. Current fatigue design guidelines show that the design of welded tubular nodal joints is restricted to thicknesses greater than or equal to 4 mm. The increased availability and use of thin-walled (t<4 mm) tubes of high-strength steels in recent years, in structures subjected to cyclic loading, means that it is important to study the fatigue behavior of welded thin-walled tubular nodal joints. In this paper, welded thin-walled CHS-CHS T-joints subjected to constant-stress-amplitude cyclic in-plane bending range are studied. The stress concentration factors (SCFs) determined experimentally at the brace and chord crown positions are shown to be about 30% and 40% respectively of the SCFs determined using parametric equations in existing fatigue design guidelines. The fatigue tests showed that in welded thin-walled CHS-CHS T-joints, a through-thickness crack occurs when the surface crack length along the weld toes in the chord has grown to a length equal to about 40% of the circumference of the brace member. An end of test failure criterion was proposed as an alternative to the through-thickness failure criterion, in obtaining data that is suitable for determining fatigue design S-N curves.

The finite-cover element-free method FCEFM is applied to simulate the fracture and damage evolution process of geo-materials. This method is mathematically based on the finite-cover technique of manifold method and the multiple weighted moving least-square method to solve the continuous and discontinuous problems without meshing or re-meshing. The damage heterogeneity and evolutionary processes of rock mass with initial cracks are analyzed and numerically simulated by FCEFM. Using the method of probability to generate the parameters of materials randomly, the physical and mechanical properties of materials are randomly distributed in nodes or Gaussian points. And an alternating damage model together with numerical implementation which is adapted to microscopic elasto-brittle fracture analysis is proposed. Through analysis of several numerical examples, the validity and efficiency of progressive fracture analysis with use of the proposed FCEFM is demonstrated.

In this work, the natural neighbor radial point interpolation method (NNRPIM) is extended to the numeric analysis of crack propagation problems. Here, the advanced discretization meshless technique is combined with a linear elastic crack growth algorithm. The algorithm simulates the crack propagation by displacing iteratively the crack tip, which consequently requires a local remeshing. In each iteration, it is estimated the stress state in the crack tip and afterwards the direction of the crack propagation is obtained considering the maximum circumferential stress criterion.

The required local remeshing does not represent a numeric difficulty for the NNRPIM. The main advantage of the NNRPIM is its capability to fully discretize the problem domain using only an unstructured nodal distribution. Being a truly meshless method, the NNRPIM is able to define autonomously the nodal connectivity and the background integration mesh.

The classic NNRPIM formulation permits to enforce the nodal connectivity by means of two kind of influence-cells: first degree influence-cells or second degree influence-cells. This work investigates the influence of the nodal connectivity on the simulated crack propagation path. Thus, demanding benchmark crack propagation examples are studied and the obtained results are compared with reference solutions available in the literature.

The effects of cyclic loading conditions (either thermal or mechanical) on the reliability of electronic assemblies are strongly dependent on the performance of solder joints. Most solder joint fatigue models, and the supporting experimental data, do not treat the crack propagation processes that lead to failure. The benefits from a physically based description of crack propagation in solder joints include an accurate representation of the damage produced by cyclic loading and, therefore, a superior basis to evaluate attachment designs and materials. The development of crack growth rate models has been hampered by the lack of experimental capabilities to observe and characterize crack propagation in solder joints and a computational capability to describe the propagation of cracks in solder joints. This paper describes several approaches that have been used to measure crack propagation rates in solders and a new approach to quantify the crack growth rate and its driving force in solder joints by a quantitative analysis of the fracture surface topography.

Different time-dependent mechanisms such as creep, environmental surface oxidation or internal material degradation due to aging and irradiation will subject structures to the possibility of premature failures. In this paper a micro-scale finite element mesh consisting of multiple elements encased in ~50–150μm sized grains with designated grain boundaries is used to replicate shapes and dimensions to simulate an isotropic metallic microstructure. The grains are encased in pseudo-grain boundary element sets which can have different material and damage parameters compared to the grains. In this type of mesh random crack paths for intergranular and transgranular cracking conditions are allowed. It is shown that creep cracking using a uniaxial ductility constraint-based model coupled with a functionally distributed time-dependent environmentally assisted corrosion/oxidation/material degradation damage model acting on surface or internally can be realistically predicted using this model. It is also evident material properties input data have scatter especially at the sub-grain level where the measurement methods are new and not always standardised. This is dealt with in the model by employing a normal distributive probabilistic method to allow for statistically varied random damage and crack growth development. In this way it is possible to take into account the inherent variability in material input properties at the analysis stage without the need to change material properties following each run. The method could negate the need for knowing the exact material properties, which in any case is impossible to derive at the microstructural level, as results of each run can be varied using a statistically distributed critical damage criterion specified for each element.

This paper presents a proposed methodology to account for cyclic plastic response of the thin shell ANDES and line-spring finite elements. A through thickness integration scheme is employed for the shell element and stress resultant plasticity is used for the line-spring element. A simplified contact formulation to account for crack closure in the line-spring element is also presented. Numerical comparisons between the proposed models and detailed 3D analyses (pipes) are carried out and presented herein. A comparison between the present implementation and large scale experiment of a surface cracked pipe subjected to large cyclic plastic strains is also presented. The purpose of the presented implementation is to account for cyclic loading in pipeline technology where significant amount of plasticity in the loading cycles occurs.

In the present work, the Natural Neighbor Radial Point Interpolation Method (NNRPIM) is used to simulate the crack growth phenomenon in brittle materials. In order to discretize the problem domain, the NNRPIM only requires an unstructured nodal distribution. With the spatial information of the computational nodes, the NNRPIM is capable to automatically establish the nodal connectivity and to construct the interpolation functions. Additionally, using the natural neighbor geometrical concept, the NNRPIM is able to obtain, from the unstructured nodal distribution, the integration background mesh required to numerically integrate the integro-differential equations ruling the studied physical phenomenon. In this work, a crack opening path algorithm is adapted and combined with the NNRPIM. The developed algorithm is able to predict the crack growth by relocating iteratively the crack tip. The stress field in the vicinity of the crack tip is determined in each iteration and then, using the maximum circumferential stress criterion, the direction of the crack propagation is calculated. Here, the repositioning of the crack tip requires a local re-meshing. However, due to the flexibility of the natural neighbor concept, the local re-meshing do not represent a numerical difficulty. Additionally, in order to show the efficiency of the proposed approach, several demanding crack growth benchmark examples are solved.

Piezoelectric materials possess special characteristics of electromechanical coupling behavior and thus have found numerous applications such as transducers, sensors, actuators. Fracture of piezoelectric materials has drawn substantial attention of the research community and is being widely investigated for predicting their failure. Most of the research on piezoelectric materials is based on impermeable crack conditions. In the present study semi-permeable crack boundary conditions has been analyzed using the extended finite element method (XFEM). Combined Mechanical and Electrical loading with quasi-static crack growth has been considered on a pre-cracked rectangular plate with crack at its edge and center. Stress intensity factors have been evaluated by interaction integral approach using the asymptotic crack tip fields. Effect of presence of minor cracks and holes have been analyzed on the intensity factors of semi-permeable major crack.

Small crack growth behavior in poled lead zirconate titanate was examined under cyclic electric loading. A crack located at the edge of a partial electrode grew along the electrode boundary during the loading. The crack growth rate decreased with increasing crack length until a non-propagating crack was reached. The growth rate and crack length of the non-propagating crack were affected by the amplitude and bias levels of the electric loading. In the case of high-amplitude loading or negative-biased loading, the crack growth rate varied considerably because of domain switching. Finite element analysis of a three-dimensional permeable crack showed that the mode III stress intensity factor range is independent of crack length, but it decreases as a result of the frictional force of a positive electric field. Fracture surface observations showed that intergranular cracking is dominant near the tip of the non-propagating crack.

The effects of an overload on fatigue crack growth behavior have been investigated by using carbon steel. Delayed retardation and acceleration of crack growth were both observed. These phenomena depended not only on overload conditions but also on the baseline stress conditions. Moreover, the mechanical properties of the materials affected the crack growth rate after overload. It was found that crack growth accelerated when tensile residual stress was distributed in front of the crack tip. The residual stress distribution is related to the crack opening geometry at the overload stage.

In our previous study, we examined the influence of the fatigue properties of the stainless steel coated with TiN film and clarified the influence of TiN coating and the surface roughness on the fatigue property. In this study, the four point bending fatigue crack growth tests were carried out for martensitic stainless steel coated with TiN film deposited by arc ion plating method in order to investigate the effect of surface finishing on the fatigue crack behavior for film coated material. The fatigue crack growth behavior was evaluated using the replica method. As a result, the crack propagation rate of mirror polished specimens were lower than that of rough surface specimens. The crack propagation rate was especially decreased for TiN coatings deposited on the mirror polished substrate. The surface roughness near the crack initiation site increased after fatigue test. It concludes that the surface roughness of substrate influences crack propagation rate and the deposition of TiN film affected influenced crack propagation rate and fatigue strength when the surface roughness of substrate is small enough.

Detailed investigations on the stress intensity factors (SIFs) for corner cracks emanated from interference fitted dimpled rivet holes are conducted using three-dimensional finite element method. The influences of the crack length a, elliptical shape factor t, far-end stress S and interference magnitude δ on the stress intensity factors are systematically studied. The SIFs for corner cracks emanated from open holes are also investigated for comparisons. An empirical formula of the normalized SIF is proposed by use of the least square method for convenience of the engineering application, which is a function of the crack length a, elliptical shape factor t, far-end stress S, interference magnitude δ and the normalized elliptical centrifugal angle φ_n. Based on the empirical formula, a crack growth simulation for a rivet filled hole is conducted, which shows a good agreement with the test data.

Computational model based on extended isogeometric analysis is implemented for the fracture analysis of functionally graded material (FGM) reinforced with multi-walled carbon nanotube (MWCNT), i.e., FGM-MWCNT structure under mechanical and thermo-mechanical loading conditions. The FGM body taken for the study is composed of metal (Ti–6Al–4V) on the left side and ceramic (ZiO₂) on right side. For the gradation of FGM properties that change throughout the domain length, exponential law is assumed. Moreover, the FGM is reinforced with 0–5% of MWCNT to obtain FGM-MWCNT composite. In order to perform this fracture analysis, first the equivalent mechanical and thermal properties of the FGM-MWCNT composite are evaluated using some of the micromechanics models cited in the literature. Using the interaction integral approach, SIFs are extracted. The results of the test show that when the volume percentage of MWCNTs increases, it enhances the failure resistance of the composite.

In this work, the crack growth behaviour in rock joints under shear load is investigated using the localised gradient damage model (LGDM). The LGDM is shown to capture the mixed-mode fracture phenomena with mesh-independent structural response and sharp localising crack profiles using a simple isotropic scalar damage variable. A simplified staggered scheme based on an operator-split approach is adopted for numerical analyses. The shear box experiments are considered to demonstrate the predictive capabilities of the proposed scheme. Two different crack joint configurations i.e., non-overlapping and overlapping are numerically simulated and the results in terms of crack profiles and structural responses are compared with the experimental results. It is observed that the non-overlapping crack joints fail in mixed-mode whereas overlapping crack joints fail in tensile mode. Further, the non-overlapping crack joints show higher shear load capacity as compared to overlapping crack joints.