Papers by Author: Sang Yeob Oh

Abstract: In this study, a bar impact test of low velocity was carried out to gain an insight into the damage mechanism and sequence induced in alumina plates during quasi-static impact conditions. An experimental setup which could measure directly the impact force applied to the specimen and supply a compressive pre-stress to the specimen by utilizing an long bar impact was devised. During the bar impact testing, the influence of the pre-stress applied to the specimen along the impact direction on the fracture behavior was investigated. The measured impact force profiles explained well the damage behavior induced in alumina plates. The application of higher pre-stress to the specimen led to less damage due to the suppression of radial cracks which was caused by the increase in the apparent stiffness of the plate. The observed results showed the following sequence in damage development: The development of cone crack at the impact region, the formation of radial cracks from the rear surface of plate depending on the plate thickness, and the occurrence of crushing or fragmentation within the cone envelope.

Abstract: The damage behaviors induced in a SiC by a spherical particle impact having a different material and size were investigated. Especially, the influence of the impact velocity of a particle on the cone crack shape developed was mainly discussed. The damage induced by a particle impact was different depending on the material and the size of a particle. The ring cracks on the surface of the specimen were multiplied by increasing the impact velocity of a particle. The steel particle impact produced the larger ring cracks than that of the SiC particle. In the case of the high velocity impact of the SiC particle, the radial cracks were generated due to the inelastic deformation at the impact site. In the case of the larger particle impact, the morphology of the damages developed were similar to the case of the smaller particle one, but a percussion cone was formed from the back surface of the specimen when the impact velocity exceeded a critical value. The zenithal angle of the cone cracks developed into the SiC decreased monotonically as the particle impact velocity increased. The size and material of a particle influenced more or less on the extent of the cone crack shape. An empirical equation was obtained as a function of impact velocity of the particle, based on the quasi-static zenithal angle of the cone crack. This equation will be helpful to the computational simulation of the residual strength in ceramic components damaged by the particle impact.

Abstract: In order to investigate the effect of a confinement condition on the damage induced by a spherical impact, an experimental setup that can impact contact pressure to the specimen through a pressing die was composed. The steel and the WC balls in 3mm diameter impacted to the soda-lime glass specimen with dimension of 33×33×8m in the impact velocity range of 30m/s to 200m/s. Three different conditions are given for the impact damage investigation, which are the case without a pressing die and the cases of p=0MPa and p=200MPa with a pressing die. The stress distribution in the glass specimen by impacting the particle was also evaluated using MARC s/w system. The particle impact produced various kinds of the damage such as the ring and the cone cracks, the radial cracks and the craters. The contact pressure applied to the specimen changed stress fields in the specimen. The damage zones of the specimen without a pressing die increased as the impact velocity increased. The damage extents in the specimen with the contact pressure of 200MPa were reduced as compared with the case of those without a pressing die.