Takuo Hayashi

Experimental observation of stress waves propagating in laminated composites

The theoretical research on stress waves propagating in laminated composites has been reported by many authors. However, there has been little work on experimental studies of stress waves in those materials. This paper presents an experimental investigation on stress waves propagating parallel to the layers of a laminated composite. A sandwich laminated composite consisting of two aluminum facings and an epoxy core is used as a specimen. The stress wave in the specimen is observed by use of high-speed holographic interferometry with a pulsed laser. In order to obtain the relative fringe orders, the interference fringe pattern in the reconstructed image is treated as an image-processing system with a personal computer. For the calculation of the in-plane displacement, an approximate relative-fringe-order method is used. The in-plane displacements obtained at some sampling points on the surface are smoothed by using a spline function. Distributions of the in-plane displacement and the shear stress are then obtained quantitatively over the whole analyzed field.

Photoelastic stress analysis of orthtropic materials by using an isotropic plate

Based on the two-dimensional theory of elasticity for an orthotropic plate, relations between stress components produced in two different orthotropic plates are considered and the conditions to realize the similar stress fields in different orthotropic plates are studied. On the basis of the similarity law, a convenient photoelastic method to analyze stress fields in an orthotropic plate, using an isotropic plate, is presented. Two examples are treated. One deals with the stress-concentration problem around a circular hole in a strip with edges parallel to the symmetric axis of elasticity. In the second example, the edges of the strip are assumed to be inclined by 30 deg to the elastically symmetric axes. The experimental results are compared with theoretical calculations.

Scanning-Moire Method

In the conventional moire method, a moire pattern appears as interference between a model grating and a master grating. However, a moire pattern also appears when the model grating lines are sampled by a TV camera. If the scanning lines of the TV camera are regarded as the master grating lines, the geometric relations among the moire fringe lines, the model grating lines and the scanning lines are obtained in the same way as the conventional moire method. When the TV camera scanning lines are thinned out, this method corresponds to a mismatch method in the conventional moire method. We call this the “Scanning-moire Method” (Morimoto 1984).

Combined dynamic behavior in a thin-walled tube. Study on strain rate dependency.

Numerical analysis is carried out based on characteristics under initial and boundary conditions of static pre tension and impulsive torsion. A constitutive equation of the Perzyna type is used and isotropic work-hardening is taken into account. Consequently, unloading behavior of the axial component is produced. Stress and strain distributions are changed considerably with the value of the material constant. The results of the calculation are also compared with that of the strain rate-independent theory. Both theories are compared with the experimental results.

Transient wave propagation in an elastic layered medium.

Transient wave propagation parallel to elastic layers is studied on the basis of the approximate continuum theory. The wave front expansion method is introduced to analyze the wave behavior in the near-field where the wave changes its shape as it travels. The transient solution is derived and compared with an experimental result. It is found that the theory can predict all the important qualitative features of the transient wave, and also the quantitative agreement is sufficiently good.

Bond Stress in a Laminated Composite Subjected to an Axial Impact

It is well known that when a laminated composite is deformed, delamination occures sometimes as the result of the interference of deformations between adjacent layers of different materials. In the present paper, shearing stresses caused at the interface between layers of a laminated composite under impulsive forces are treated theoretically and a similarity law is presented. Experimental studies are also performed and the results are compared with the theoretical one.

Photoelastic Studies on the Plane Stresses Produced by Moving Loads

A study is made on the plane stress fields produced in a plate by a moving load, and the general expressions for steady stress fields are obtained. Using those expressions, the stress fields due to a concentrated force moving in a plate or along its edge are calculated. The results are compared with those of the experiments by dynamic photoelasticity method with polyurethan rubber. Comparisions are made for three speed ranges : subsonic, transonic and supersonic. The ptotoelastic fringe patterns coincide fairly well with the calculated isochromatic lines if the load speed does not exceed the propagation velocity of the longitudinal stress waves.