Analysis of through-thickness stresses in composite beams

Includes bibliographical references (pages [48]-49)Interlaminar shear failure, or delamination, is a prominent failure mode for brittle, polymer-matrix composite materials. Interlaminar shear stresses are routinely generated in composite structures as a result of free edges, ply drop-offs, three-dimensional stress fields, etc. The accurate characterization of a composite material?s resistance to interlaminar shear is therefore prerequisite to maximum utilization of the benefits of these important engineering materials. The four-point (quarters) bending of a SCS (steel/composite/steel) test sample, a sandwich beam obtained by adhesively bonding unidirectional HERCULES ? AS4/3501- 6 graphite/epoxy material to steel strips, was a test proposed for characterizing the interlaminar shear strength of the said composite in a previous research. It was known that the only potential failure inducing stresses are the interlaminar shear stress, t xz, and the through-thickness normal stress, ctz. However, due to stress analysis complications the role of the through-thickness normal stress, a z, was addressed in the previous research in a qualitative manner only. A new finite element model, the lumped parameter model is generated for the purpose of verifying the effect of the through-thickness normal stress, ctz. The shear stress/deformation characteristics of the interfacial layers between the plies of graphite/epoxy material are included by means of lumped epoxy (resin-rich regions existing between the plies of the composite). The effects of varying the distribution of the lumped epoxy is studied. The lumped parameter model is incorporated into a nonlinear analysis involving contact elements to study the state of stress in the failure zone of the specimen at the time of failure. Results show that the failures are initiated due to a predominant interlaminar shear stress with negligible through-thickness normal stress.M.S. (Master of Science