Impact Bending Behavior of Sandwich Composite Beam

Abstract

This paper reports an investigation on the relationship between the stress propagation and the span ratio of a sandwich beam of a hybrid composite under impact load. The stress distribution in the impact bending process is very complicated when the hybrid construction has a different mechanical impedance.
Experimentally, the impact test was carried out with a three points bending method by using a falling elastic bar. The transient strain in the loading elastic bar and the dynamic flexural strain of the specimen at the backface against the impact point were measured.
The results obtained show that the initial strain of the bar depends on the material charcteristics of the beam member of the impact side, and is not affected by those of the backface.
In the case of a large span ratio, the value of the initial stress does not depend on the ratio and is always maximum during initial process. On the contrary, the maximum value of stress wave does not coincide with the initial one in the case of a small span ratio. The value of maximum stress depends on the modules of elasticity of the surface member laminated at the impact side. Therefore, the value of impact stress depends largely on the characteristics of the loading surface member, and it increases with increasing modules of elasticity.
The deformation mode of the sandwich beam after a long time period has no connection with high frequency stress waves at the impact loading point. The impact load acts as a distributed load against the lower lamina. As the span length becomes smaller and the characteristics of the face member laminated at the impact side becomes harder, the strain produced at the backface against the impact point becomes smaller. In an extreme case an inverse bending strain is produced.