Keigo Sakakura

A laser-caustic method for the measurement of mixed-mode dynamic stress-intensity factors in fast-curving fracture tests

Abstract High-speed photographs of the caustic patterns in fast-curving fracture tests were taken by a laser-caustic method that can be quickly synchronized to the initiation of a brittle-fracture event. By using these high-speed photographs, the instantaneous location of the crack tip and the characteristic dimensions of the caustic pattern were evaluated. From these data, the mixed-mode dynamic stress-intensity factors were evaluated on the basis of a theory of caustics for mixed-mode fast-running cracks, which was developed by some of the present authors. It was found that the magnitude of the mode II stress-intensity factor is closely related to the dynamic crack-curving.

Moiré interferometry measurements of neartip deformation in inhomogeneous elasticplastic fracture specimens

Abstract Moire interferometry was used for direct measurements of the crack tip behavior in the homogeneous compact tension (CT) specimens of A533B and HT80 steels, and in electron-beam welded inhomogeneous CT specimens of the two materials. These two materials have considerably different yield stresses, although their elastic properties are the same. Five types of homogeneous and inhomogeneous specimens were used in the experiment. Moire fringe patterns of the five specimen types were directly compared for elastic, elastic-plastic and unloaded stages. Although the global deformations in terms of the load versus COD relations exhibit little inhomogeneity effects, large inhomogeneity effects were observed in the moire fringe patterns in the elastic-plastic and unloaded stages. The near-tip deformations were also compared with the corresponding HRR singular fields. In a horizontal weld specimen, due to strong hardening in the weld region, the slope of the displacement field does not shift the characteristic HRR field for the A533B or the HT80 steel for increasing load.

MIXED-MODE IMPACT FRACTURE TESTS AND THEIR NUMERICAL SIMULATION

ABSTRACT In the present study, using three-point bend specimens of PMMA subjected to the impact load by a dropping rod at off-center line, a series of mixed-mode impact fracture tests were carried out. The eccentricity of loading point was systematically changed. The relations among the initiation angle, global propagation angle and the loading eccentricity were obtained. Generation phase simulation was performed for the mixed-mode impact fracture tests, using mixed-mode singular elements that can directly compute K I and K II . It was found from the simulation results that, for the cases of larger loading eccentricity, the crack surfaces in the vicinity of the crack-tip make contact each other, and subject to only mode II deformation. The fracture initiation angles in these cases agree with the maximum energy release rate criterion. This kind of phenomenon is extremely difficult to find by the experiment alone.

Experimental and Numerical Studies on Fast Curving Fracture

This paper summarizes recent experimental and numerical studies on fast curving fracture. The following topics are included; (1) Experimental techniques for high-speed photography of caustic pattern during fast crack curving, (2) a higher-order theory of caustics for mixed-mode fast running cracks, (3) a component separation method for determining mixed-mode stress intensity factors by using the path independent dynamic J integral, and (4) finite element simulation techniques for fast curving crack propagation. Pertinent experimental and numerical results are also presented.

A method of caustics for the direct experimental measurement of the T∗-integral in nonlinear and dynamic fracture mechanics problems

Abstract This paper presents a methodology for the direct experimental measurement of the T ∗ - integral . The method of caustics (the shadow pattern method) has proven to be a powerful optical method with which to measure stress intensity factors in static and dynamic fracture mechanics problems. An attempt has been made to extend the method of caustics to problems in nonlinear and dynamic fracture mechanics. The formation process of the caustic pattern for an elastoplastic crack tip in a compact-tension specimen was simulated by a finite-element analysis. The stimulated and actual caustic patterns agreed with each other very well. The relations between the T ∗ - integral and the size of the caustic pattern were obtained for various optical set-ups. The experimental measurement of the caustic pattern in the dynamic tear test specimen of a nuclear pressure vessel steel was also carried out. High-speed photographs of the caustic patterns were taken using a laser caustic method, which can be quickly synchronized to the initiation of impact loading by a hammer that was dropped on the specimen. The time variation of the T ∗ - integral during impact loading was evaluated by using the relation between the T ∗ - integral and the size of the caustic pattern that was obtained by the numerical simulation. The hybrid numerical-experimental method developed here, in conjunction with laser caustic high-speed photography, has made it possible to measure the T ∗ - integral for an elastoplastic material under impact loading conditions.

Determination of mixed-mode dynamic stress intensity factors in a fast curving fracture test using a laser caustic method.

In order to investigate the mechanism governing elastodynamic crack curving, a series of fast curving fracture tests were carried out. In the present experiments, high-speed photographs of the caustic patterns for dynamically curving cracks were taken by a laser caustic method which can be quickly synchronized to the initiation of brittle fracture event. The mixed-mode dynamic stress intensity factors were evaluated on the basis of a theory of tics for mixed-mode fast running cracks, which was developed by some of the authors. It found that the magnitude of the mode II stress. intensity factor is closely related to the dynamic crack curving.