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Fatigue Crack Paths. Special Issue of the International Journal Fatigue and Fracture of Engineering Materials and Structures
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Specimen for Fracture Characterization of Material under Biaxial Stress Fields
No full textThe simple circular notched specimen was originally proposed by Arcan to characterize the elastic properties of fiber-reinforced composites. Unfortunately, its optimized geometry does not allow to measure with reasonable accuracy both the material shear strength and the conditions of failure under a generic biaxial stress state, since the effects of stress concentration on the fillets of the two V-grooves and on the inner circular edges are responsible of premature fractures due to the uniaxial stress states of the notches edges.
To improve the performance of this kind of specimen, an extensive FEM analysis has been carried out by a two-dimensional parameterized model, in order to identify those geometries that make possible the fracture to occur in the minimum strength section
under the action of a uniform pure shear stress state. Furthermore, the parametric analysis has been extended to the 3D field to define alternative geometries, having variable thickness, that are able to produce with an higher probability material fracture
in the minimum strength section
Application of the generalized Kitagawa diagram on cast steel
No full textThis paper deals with the assessment of cast steel defects in terms of macroscopic shrinkage porosity. Thereby, a generalized Kitagawa diagram is developed by numerical analyses of V-notched specimens with varying opening angles, which is verified by the application of the NSIF concept on experimental fatigue tests under rotating bending and axial loading. At first, casting simulations are performed to establish representative specimen geometries. The design study focusses on macroscopic shrinkage pores with different spatial shapes. Thereby, several sample geometries are obtained and subsequently steel cast to obtain varying defect shapes and sizes. At second, fatigue tests under axial loading are conducted. Subsequent fracture surface analysis by light optical and scanning electron microscopy provide fracture mechanical based geometry parameters. Finally, the results of the experiments related to the failure relevant defect sizes are assessed by the generalized Kitagawa diagram. In addition, a numerical analysis of a representative defect is performed to illustrate the crack growth behaviour of a steel cast defect and demonstrate crack coalescence path tendencies. Summing up, the application of the NSIF approach on cast steel containing macroscopic imperfections shows a sound accordance to experiments and provides an engineering-feasible fatigue assessment method of cast steel components
The heat energy density averaged in the material structural volume to correlate the fatigue crack growth rate in stainless steel specimens
No full textThe heat energy averaged in a structural volume, Q*, was recently proposed by the authors as a fatigue damage index to correlate the fatigue behaviour of cracked and severely notched specimens, made of AISI 304L stainless steel. In this paper, the recent developments proposed by the authors to analytically link the Q* parameter to the Rice’s J-integral are presented and discussed
Calculation of 3D Residual Notch Stress Intensity Factors by means of the Peak Stress Method: a preliminary investigation
No full text. Residual stress influences the crack initiation and propagation in mechanical components. In the recent literature new approaches were developed to include the residual stress effects on the fatigue strength of welded joints. Among these, the local approach based on the calculation of the Residual Notch Stress Intensity Factors (R-NSIFs) seems very promising. Unfortunately, its main drawback is linked to the difficulty to assess such parameters by means of numerical simulation of the welding process because of the high mesh density required to capture the residual stress filed singularity. In this scenario, new numerical methodologies are needed which use coarse meshes such as the Peak Stress Method (PSM). In this work, a preliminary investigation aimed to analyze the PSM applicability for the estimation of R-NSIF values in a 3D butt-welded joint is described
Crack initiation life of titanium notched bars under uniaxial and multiaxial fatigue: calibration analysis of the potential drop method and experimental validation
No full textDuring experimental fatigue tests it may be necessary to monitor the crack initiation instant, that is often referred to a short crack depth. Several methods exist to assess the initiated crack size, the direct current potential drop (DCPD) technique being one of them. In the present contribution, the calibration curves representing the potential drop change as a function of the crack depth are derived from 3D electrical FE analyses. Based on previous experimental observations, two different crack shapes are considered: (i) circumferential and (ii) semi-elliptical surface. In the case of semi-elliptical cracks, the effects of the crack aspect ratio and the location of the potential probes are investigated in detail. In conclusion, the numerically-derived calibration curves are experimentally validated on the basis of measurements carried out on notched specimens fatigue tested under different loadings
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