抄録
This paper presents the dynamic stress-strain properties of styrofoams under impact loading acting on a limited area. The specimens with the dimension of 100_??_mm×30tmm were bonded on the end of surface of the pendulum being impacted. The impact loading was applied to the center portion of the specimens by another impact pendulum with the diameter of 50φmm. The acceleration-time curves induced in the impacted pendulum were measured. The dynamic stress and strain were calculated by integrating these acceleration-time curves numerically. The effects of the strain rate and the number of impact loadings on the dynamic properties of styrofoams were examined in comparison with the results of the uniform impact loading on the whole_ surface area described in the previous report. The cushion factor-stress properties were also discussed. The results obtained are as follows:
(1) Under localized impact loading, the strength and stiffness of styrofoams increase with increasing strain rate up to about 20/sec, beyond which they start to decrease.
(2) A remarkable difference exists between the stress-strain properties under localized impact loading and those under uniform impact loading in the strain range beyond 20%.
(3) The stiffness of styrofoams is remarkably reduced in the low strain range by the first application of localized impact loading, but the reduction becomes comparatively small in the higher strain range. The further reduction in stiffness is induced by the successive application of impact loadings.
(4) The styrofoams seems to have the critical deformation rate at the strain rate of 35/sec under localized impact loading. In this strain rate, the dynamic stress-strain relationship becomes complicated because of the fracture in the vicinity of loading area.
(5) The depth of the disintegration produced in styrofoams by localized impact loading can be indirectly estimated from the knee appeared on the cushion factor-stress curve.
