Papers by Keyword: Crack Propagation

Abstract: Structural loading of cellular metals is strongly affected by brittle fracture of cell struts and walls that exhibit tensile loads, e.g., during fatigue loading. The present paper summarizes results of compression, tension and cyclic loading experiments on various closed-cell metal foams and metal foam sandwiches (Alulight, Alporas, Foamtech, AFS) using various mechanical testing systems. The results were correlated with a thorough analysis of the cellular mesostructure and the cell strut/wall microstructure by means of scanning electron microscopy revealing defects, such as casting porosity and large Si precipitates in the Al-Si eutectic of aluminum cast alloy. The results of the work served for the definition of testing standards for compression testing (ISO 13314) and tensile testing (DIN 50099), which are outlined in the paper. Such standards and design guidelines are crucial for a successful implementation of cellular metals in innovative products in mechanical, automotive and energy engineering as well as in bioengineering.

Abstract: This work presents some novel results obtained by using the strain injection techniques for modeling crack propagation in challenging 3D benchmark tests. The techniques were already tested and validated by static and dynamic simulations in 2D ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_1" \o "Dias, 2012 #1526" 1-4], so the main goal of this paper is to verify if the most important advantages of the method, low computational cost and independence of the results on the finite element mesh, are kept in 3D. The methodology, implemented in the finite element framework, consists essentially in injecting those elements that are going to capture the cracks with some enhanced strain modes for improving the performance of the elements for modeling propagating material failure.

Abstract: A recent cohesive zone model is applied to the simulation of crack extension in austenitic stainless steel under large scale yielding conditions. The shape of the corresponding exponential traction-separation-relation can be modified in a wide range. In order to investigate the sensitivity regarding the cohesive zone parameters, a systematic parametric study is performed. The shape of the traction-separation envelope has a minor effect on the results compared to the cohesive strength and the work of separation. The aim is to fit experimental data by an appropriate choice of these parameters. Therefore, not only force-displacement curves should be used, but also crack growth resistance curves should be employed. A promising strategy for parameter identification is derived.

Abstract: The numerical analysis of stress-strain state of low-alloyed welded steel samples test has been considered. The mechanical heterogeneity has been estimated by the micro hardness test. The stress-strain state analysis is based on the models of linear elasticity, which are described by Lame equations for displacement. In this case the samples are considered as perfect welded samples without any welding defects. The discretization of the system of equations is done through the finite element method, and the numerical realization of the method is performed on collection of free software FEniCS. The defects influence has been estimated by stochastic modelling of viscous crack growth. The data for crack size in weld and heat affected zone was obtain from microscopic observation, and for mechanical properties from microhardness testing. The result obtained shows that, the distribution of displacement in all samples are almost the same. Between the welded zone, the heat affected zone and the external elliptic zone, the Von Mizes stress is almost the same in all three samples. Concerning the crack growth, the velocity of it propagation in welded zone is higher as much again than that in the heat affected zone. This research is beneficial to welders, modellers of structures, researches as a whole.

Abstract: In a standard body force method analysis, a mesh division is required to define the boundary of a problem and to solve a governing equation using discretization procedure. However, in the present study, a moving least square strategy is introduced to define a weight function for the density of body force doublet and therefore a crack analysis is carried out without providing a standard mesh-division. Hence, the standard crack face elements are not required at all. A variety of 3D crack problems can be analyzed simply by providing a data that only de nes a crack front. Besides the nodal points for crack front, several internal nodes are generated on the crack face to represent a distribution of unknown function. At the internal nodes, an unknown variable is assigned which uniquely de ne a distribution of the relative crack face displacement. In the present approach, a crack problem is formulated as a singular integral equation whose unknown is a value of the weight function at the internal nodal points. A crack growth can be simulated directly by changing the shape of crack front, by means of adding a new nodal point in the vicinity of the current crack front. In the present paper, the proposed method is used to simulate a coalescence of interacting planar cracks.

Abstract: The fracture mechanics assessment of materials exposed to harmful environments requires the understanding of the interaction between the soluted species and the affected mechanical behaviour. With the introduction of a mass transport mechanism the entire problem is subjected to a time frame that dictates the time-dependent action of soluted species on mechanical properties. A numerical framework within the phase field approach is presented with an embrittlement-based coupling mechanism showing the influence on crack patterns and fracture toughness. Within the phase field approach the modeling of sharp crack discontinuities is replaced by a diffusive crack model facilitating crack initiation and complex crack topologies such as curvilinear crack patterns, without the requirement of a predefined crack path. The isotropic hardening of the elasto-plastic deformation model and the local fracture criterion are affected by the species concentration. This allows for embrittlement and leads to accelerated crack propagation. An extended mass transport equation for hydrogen embrittlement, accounting for mechanical stresses and deformations, is implemented. For stabilisation purposes a staggered scheme is applied to solve the system of partial differential equations. The simulation of showcases demonstrates crack initiation and crack propagation aiming for the determination of stress-intensity factors and crack-resistance curves.

Abstract: Crack propagation around the stress concentration area causes fatigue failure. Non-destructive method is necessary for monitoring structure fatigue before destruction. We focused on a magnetic non-destructive evaluation method for crack growth. In order to understand the relation between crack propagation and changes in magnetic flux density, we observed the position of the positive and negative magnetic flux density distributions around the crack of tool steel (SCM440) plate using a scanning Hall probe microscope (SHPM). We found that the vertical component of the three-dimensional magnetic flux density moved as the crack growth. Furthermore, the magnetic component which is parallel to the tensile stress appeared just before destruction of the specimen.

Abstract: Mechanical breakage systems are generally employed to demolish a portion of a concrete building, however it is time consuming and costly. And the mechanical demolition work involves various risks such as those associated with occupational safety and presents a noise hazard to the general public living in the vicinity. Therefore, alternative methods for such work have been sought. For this purpose, a dynamic breakage system utilizing diamond-shaped charge holders was proposed to rapidly remove the desired portion of the concrete foundation. The charge holders which initiate crack growth were placed inside a concrete mass along the desired fracture plane. In this study, full-scale blast experiments utilizing the charge holders were introduced and the roughness of fracture planes was observed using a 3-dimensional photography system. In order to verify the effect of the charge holders on fracture controlling in full-scale blast experiments, the fracture processes of the concrete blocks were analyzed using the dynamic fracture process analysis (DFPA) code. The mechanism required to achieve controlled breakage was discussed after taking into account the influence of various loading conditions and crack tip velocity. It was found that the DFPA tool is a useful instrument in the analysis of full scale blast experiments.

Abstract: In order to obtain a real-time interlaminar crack fracture behavior of the laminate composites, in this paper we propose a method of extracting and measuring of interlaminar crack of laminated composite materials based on 2D image analysis via the Matlab software. Extracting the main crack image were conducted using the Matlab script including four different algorithms: the binarization, region growing, morphological, and skeleton thinning, and then a main interlaminar crack image with 1 pixel width were obtained. The length of the main crack was calculated through the sum of pixels of the skeletonized object. The calculated result was closed to the measured result, and the difference between the calculated value and measured value was 0.2%, which can prove the accuracy of the method in present work. The proposed method is of high precision, with strong anti-inference ability and experimental data is stable and reliable, which is helpful to study the crack propagation behavior of laminated composite materials.

Abstract: Experiments have been conducted on an oxide dispersion strengthened (ODS) copper Glidcop Al-15 under a range of cyclic stress-amplitudes at room temperature. Special specimens containing an artificial small hole of various diameters, i.e. 150, 200 and 300 μm, were used. Propagation process of surface cracks in the various holed specimens was recorded to investigate the fatigue crack growth rate. Scanning electron microscope (SEM) was used for the observation of the fracture surface after failure specimens. The well-known Coffin-Manson low-cycle fatigue relationship was substantially shown through the results. An equation between the growth rate of the surface crack and the stress amplitude for Gidcop Al-15 was carried out. The experiment proved that prior fatigue history makes little influence on the subsequent crack propagation property under low-cycle condition.