Papers by Keyword: Crack Growth

Abstract: The purpose of this paper is to estimate the number of cycles for fatigue crack initiation in notched plate under constant amplitude loading through tensile residual stress field of Aluminum alloy plate. Residual stress field was generated by plastic deformation using finite element method. Simulation of fatigue behavior was made on AFGROW code. It was shown that the fatigue crack initiation and propagation were affected by level of residual stress filed. In this investigation, the presence of tensile residual stresses reduces considerably the total fatigue life. Loading parameter namely stress ratio was studied. The decreasing in this parameter reduces the fatigue crack growth rate (FCGRs).

Abstract: The fatigue limit of a plain specimen of a tool steel JIS SK85 was investigated. The initial crack length in the fatigue process is related to the size of a single crystal in the case of low carbon steel. However, that is independent of the crystal size in the case of pure aluminum, a lamellar pearlitic steel, etc. In this study, the relationship between the initial crack length and the fatigue limit was investigated. The original microstructure of this material includes a spherical microstructure. However, after heat treatment under some conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of heat-treated SK85 could be evaluated by an equation in which the parameters are the hardness and initial crack length. Finally, this result was discussed based on the observations of crack growth behavior and fracture surface.

Abstract: In this study, we investigated the crack nucleation and growth and propagation on the surface of Y-TZP during isothermal phase transformation by low temperature ageing. Crack initiation and growth on the surface of Y-TZP specimen was dependent on the sintered microstructure, i.e, sintered density, grain size, pore structure, residual stress etc.. In the case of Y-TZP with 2mol % yttria content, phase transformation of tetragonal to monoclinic began on the surface and induced a crack nucleation of specimen at the initial stage of low temperature ageing. Most of cracks in 2Y-TZP by low temperature ageing were firstly formed on the surface of specimens (free surface, weak bonding grains, etc.) where the change of strain free energy for a tetragonal to monoclinic transformation was small, and surface cracks grew into the bulk interior through the grain boundaries.

Abstract: A numerical method RFPA-T (Thermal Induced Rock Failure Process Analysis) code is used to study the thermal cracking processes of quasi-brittle materials subjected to high or low temperature. The numerical results indicate that thermal stress concentrating along the interface between the matrix and the embedded grains due to their different coefficient of thermal expansion (CTE). The modeling results indicate that θ-crack is generated during temperature increment as the CTE of the embedded grain is smaller than that of the matrix. However, radial-cracks emerged when the temperature decrease. The results obtained from RFPA-T code show a good agreement with experimental evidence of crack patterns caused by thermal expansion mismatch.

Abstract: The basic principle and technique of a new and effective extended finite element method is introduced first in the paper. Compared with classical finite element method, the crack growth can be simulated by XFEM without making the crack surface associated to the mesh and without remeshing after crack growth. On the basis it is applied to analyze difficult problem of fracture and crack growth mechanism of composite pavement structural. By three-point bending test of composite structural beam, the whole process of initial crack and the crack growth of composite pavement structure is analyzed by numerical simulation. It can be divided into four key stages from cracking to crack reflection and each stage is presented by tensile stress and SATUSXFEM. The research results can provide important reference for composite structure design and proposed control countermeasure.

Abstract: The relationship between stress and crack growth was studied. Applying the software ANSYS, the method of calculating stress intensity factor of composite mode cracks was built. At the same time, the stress intensity factor of mode I crack was calculated and the influence of crack size and loading on the stress intensity factor was studied. The results show that the results of ANSYS are almost the same to analytic results, which proves that the method is feasible. When the crack is quite big or quite small, the result of ANSYS’ error is comparatively obvious, which is due to the problem of the refinement of meshing. When the crack is too big, the dense of meshing is inadequate.

Abstract: Concrete experiences thermal and hygral deformations at early ages due to it intrinsic properties and the environmental effects. Micro-cracking results on the top surface of pavement if deformations are restrained. These micro-cracks propagate transversely and downwards over time under traffic loadings, especially during early ages. This situation can be severe if upward curling conditions exist in pavements. Estimation of the remaining fatigue life of pavements with such cracks is of significance for scheduling prompt maintenance. Conventional fatigue models established based on uncracked beam tests are no longer applicable for such cases. It is necessary to develop a fracture mechanics-based fatigue model for pavement with cracks. This study provides a new fatigue life prediction methodology for pavement with cracks. Both model prediction and experimental test results suggest that fatigue life is significantly reduced if concrete develops a partial depth crack at early ages. These results can explain the observed premature transverse cracking failure in jointed concrete pavement. Crack growth behavior can be characterized as three stages, in which the steady stage is the most important one when prompt maintenance is needed to avoid structural failure.

Abstract: An extended finite element method (XFEM) for multiple crack growth in asphalt pavement is described. A discontinuous function and the two-dimensional asymptotic crack-tip displacement fields are added to the finite element approximation to account for the crack using the notion of partition of unity. This enables the domain to be modeled by finite element with no explicit meshing of the crack surfaces. Computational geometry issues associated with the representation of the crack and the enrichment of the finite element approximation are discussed. Finally, the propagation path of the cracks in asphalt pavement under different load conditions is presented.

Abstract: The improvement of acceleration behavior of crack growth was investigated with constant stress amplitude under negative stress ratio R=-1. Then a technical method to detect the fatigue crack growth was discussed. For example, when the stress amplitude exceeds a critical value, crack growth rate of overloaded specimen became higher than that of baseline which was obtained by crack growth test without applying overload. In some experimental cases, the acceleration of crack growth was observed and that could be happened on practical cases. Stop-holes were drilled at crack tips or in the vicinity of crack tips to remove the plastic zone and the effect of that on crack growth behavior were investigated. Also, steel pins were inserted into the stop-holes and its effect was discussed. Finite element method (FEM) was used to analyze the stress concentration at the edge of stop-holes. Positions of center of the stop-holes were varied at different distance from the crack tips to investigate the effect of stop-holes on fatigue crack growth. Also, stress intensity of base and stop-holed specimen was calculated. Then, the effect of stop-hole was discussed by both the experimental and analytical results. Specially, it was discussed whether the crack growth behavior was improved or not in the case of relatively smaller width specimen.

Abstract: The wedge-splitting test (WST) is a frequently used test configuration for performing stable crack fracture experiments on concrete specimens, thus allowing to determine the fracture process and crack propagation in the heterogeneous material. However, there are no standard rules regarding the wedge-splitting specimens geometry, groove dimensions or notch length. This paper concentrates on the influence of the initial notch length in geometrically identical, cubical specimens, cast from vibrated concrete. The experimental results of nine WSTs under monotonic loading, including F_sp-CMOD curves - splitting force versus crack mouth opening displacement - and fracture energy G_f, are presented. An important effect of the starting notch length on the fracture properties is observed.