異材接合体におけるき裂近傍の赤外線画像応力場と破壊強度

This paper reports the study of stress intensity factors (SIF) of three edge cracks in a finite plate under bending moments. The goal of this paper was to analyze the three edge crack interactions under such loading. Several studies can be found in literature discussing on mode I SIF. However, most of these studies obtained the SIFs using tensile force. Lack of SIF reported discussing on the SIFs obtained under bending moments. ANSYS finite element program was used to develop the finite element model where singular elements were used to model the cracks. Different crack geometries and parameters were utilized in order to characterize the SIFs. According to the present results, crack geometries played a significant role in determining the SIFs and consequently induced the crack interaction mechanisms.

In order to investigate the effects of crack size on the crack propagation behavior of a sintered silicon nitride, crack propagation tests were carried out under cyclic loads and static load. Two kinds of small semielliptical surface crack were introduced by Knoop indentor. By polishing the surface layer, the residual stress around the Knoop indentation was completely removed. The crack propagation rate of small cracks (350µm, 200µm) was lower than that of long cracks obtained with a CT specimen. On the other hand, by measuring and analyzing the crack opening displacement (COD) of 350µm crack, we were able to determine the quantity of the true stress intensity factor K Itip. K Itip was defined by the following equation K Itip K Imax- K S in which K Imax is the stress intensity factor calculated by the applied stress and the crack length. K s is the stress intensity factor shielded by bridging effects. Furthermore, we found that COD after cyclic loading was larger than that before cyclic loading. It was evident that the increase in COD after cyclic loading was attributed to the reduction of bridging force with cyclic fretting at the bridging sites. Under the cyclic loads, the reduction of bridging force resulted in an increase in K Itip which, in turn, accelerated the crack propagation rate.

This work deals with the interaction of P-waves between a moving central crack and a pair of outer cracks situated at the interface of an orthotropic layer and an elastic half-space. Initially, we considered a two-dimensional elastic wave equation in orthotropic medium. The Fourier transform has been applied to convert the basic problem to solve the set of four integral equations. These set of integral equations have been solved to to get the analytical expressions for the stress intensity factor (SIF) and crack opening displacements (COD) by using the finite Hilbert transform technique and Cooke’s result. The main objective of this work is to investigate the dynamic stress intensity factors and crack opening displacement at the tips of the cracks. The aims of the study of these physical quantities (SIF, COD) is the prediction of possible arrest of the cracks within a certain range of crack velocity by monitoring applied load. SIF and COD have been depicted graphically for various types of orthotropic materials. We presented a parametric study to explore the influence of crack growing and propagation. This result is very much applicable in bridges, roads, and buildings fractures.

This paper analyzed the interaction between two parallel vertical cracks in a transversely isotropic half space by using the boundary element method (DBEM). The crack surface is perpendicular to the free surface and isotropic plane subjected to normal and tangential uniform distributed load respectively. The stress intensity factor (SIF) values of the crack are calculated from the crack opening displacements. And the interaction effect factor of SIF values is employed to quantitatively describe the interaction between two cracks. The variations of interaction effect factor are investigated with the distances between cracks, the side ratios and free surface. Results show that the existence of the free surface exerts more obvious influence on the SIF values of the crack close to free surface in half space than those in infinite medium. But the free surface has almost no influence on the interaction between two cracks.

Crack closure mechanisms in residual stress fields generated by laser shock peening: A combined experimental-numerical approach

Closed-form solutions for two collinear dielectric cracks in a magnetoelectroelastic solid

Engineering components that are subjected to cyclic loads frequently contain imperfections which can rationally be assessed using linear elastic fracture mechanics. Fatigue crack growth models make use of the stress intensity factor (SIF) range and the maximum SIF or R-ratio to describe the driving force for crack propagation. Determining the stress intensity factor in welded components is difficult because the stress field is complex due to the structural geometry of the component, the local geometry of the weld and the residual stresses caused by the welding process. In many cases, the crack geometry will also be complex. Weight function (WF) solutions for numerous crack geometries have been published, e.g. for through cracks, edge cracks, surface cracks and corner cracks. However, the use of the WF for cracks of arbitrary shape is less developed. The point-load WF method for arbitrary planar cracks gives sufficiently accurate estimates of stress intensity factors for embedded cracks, but the results for the surface and edge cracks are less accurate. A new form of the general point-load WF more suitable for surface breaking cracks is proposed and is combined with a crack tip plasticity-based crack growth model. During numerical simulations, cracks are normally modelled using a discrete number of linear segments. A new method for shifting these segments as a crack advances is also proposed. The method is verified based on comparison with published experimental results for welded rectangular hollow section joint subjected to bending and torsion.

Mixed mode BEM analysis of multiple curvilinear cracks in the general anisotropic solids by the crack tip singular element

The modified complex variable function method with the continuity conditions of the resultant force and displacement function are used to formulate the hypersingular integral equations (HSIE) for an inclined crack and a circular arc crack lies in the upper part of bonded dissimilar materials subjected to various remote stresses.The curve length coordinate method and appropriate quadrature formulas are used to solve numerically the unknown crack opening displacement (COD) function and the traction along the crack as the right hand term of HSIE.The obtained COD is then used to compute the stress intensity factors (SIF), which control the stability behavior of bodies or materials containing cracks or flaws.Numerical results showed the behavior of the nondimensional SIF at the crack tips.It is observed that the nondimensional SIF at the crack tips depend on the various remote stresses, the elastic constants ratio, the crack geometries and the distance between the crack and the boundary.

The boundary element method is a numerical method that can be advantageously used for a wide range of engineering problems, including the stress concentration problems encountered in fracture mechanics. In linear elastic fracture mechanics (LEFM), the stress intensity factor is an important parameter. Cracks, if present in the region experiencing the modes of deformation, increase the stress amplitude significantly and this high stress may lead to premature failure of the engineering components. If the value of the SIF is known, it is possible to predict whether the crack will propagate or not. As the conventional boundary integral equation (CBIE) degenerates when a mathematical crack is modelled, a previously developed dual boundary integral equation approach has been adopted in the current work. It utilizes the hypersingular boundary integral equation (HBIE) along with the CBIE. A weakly singular form of HBIE is utilized in the current work to eliminate the hypersingularity analytically. Stress intensity factors are evaluated using the crack opening displacement (COD), displacement extrapolation (DE), and the J-integral approaches. A stand-alone code has been developed for calculating the stress intensity factors of general two-dimensional domains with cracks. The code has been validated by evaluating the stress intensity factors for the standard components, for which the stress intensity factor values are available in the literature. Accurate and well-converged results are obtained proving the robustness of the code. A linear combination of the CBIE and HBIE was applied at the crack and a significant (87–97 per cent) reduction in the condition numbers for the system of equations was observed for the examples studied. Again, the results obtained are accurate and well converged.

For an actual crack growth in structures subjected to the applied stress from the various directions, it is important to study about the fatigue crack propagation behavior under mixed-mode condition. In particular under the condition, crack surfaces tend to contact when the load is applied because of the compressive residual stress distributed near the crack and the zigzag crack surface morphology. In this study, using slant cracks with compressive residual stress induced in mode I fatigue crack propagation, discontinuous displacements (Mode I : crack opening displacement, Mode II : crack sliding displacement) were measured along the slant crack. The stress distributions along the crack were calculated from the discontinuous displacements and the stress intensity factors (KI)est and (KII)est were estimated from the calculated stress distributions. As a result, the residual stress distribution along the crack was evaluated from the calculated stress distribution. Using the estimated stress intensity factors, (KI)est and (KII)est, an interaction between the compressive residual stress and the crack surface contact behavior was discussed.

The effect of sweat on the performance of the interface between skin and flexible membrane

Analysis of uniformly-spaced coplanar embedded elliptical cracks

The influence of the residual stress field in a welded structure on crack growth and fracture is commonly assessed through its contribution to the stress intensity factor (SIF) for the crack of interest. This contribution is most often calculated by assuming a bounding through-thickness residual stress profile for the specific type of weldment with an appropriate SIF solution for the crack location, shape and structure of concern. Although more realistic residual stress profiles for stainless steel pipe girth welds have been developed recently their use, in some cases, leads to an underestimate of the SIF. A new approach is developed for determining bounding SIF values for cracks in residual stress fields of stainless steel pipe girth welds. The forms of the proposed SIF profiles are based on recently published SIF solutions for cracks in periodic residual stress fields [1]. It is shown that the SIF profiles bound those based on a large database of residual stress measurements without being excessively conservative. The outcome is a simple new method for defining more realistic SIF profiles for use in structural integrity assessments of stainless steel pipe girth welds.

Most of the engineering materials are faced with fracture phenomenon that needs to be controlled. There are several methods for controlling crack growth such as hole technique, using locks etc. In order to predict crack growth, stress intensity factor can be conducted as a useful tool. In this paper, the photoelasticity method is conducted as an experimental stress analysis tool in order to calculate the stress intensity factor (SIF). The investigations were conducted to find the effects of holes and locks on the fracture resistance and the SIF. This research is specially focused on the crack stitch as a unique technique to control crack propagation. In this technique, holes of locks are drilled close to each other in rows perpendicular to the crack then keys are inserted into the holes. Three batches including six different edge crack samples of polymethyl methacrylate rectangular plates were used for fracture tests. In the present study, the effects of locks in various geometries were investigated on the SIF. The results were obtained using a polariscope and the tensile load was applied to the specimens on the mode fracture (I) strength; then, the fracture forces were measured. The results showed that the samples with lock and hole have more fracture strength and thus their SIF decreases in comparison with the lock-less or hole-less samples.