PREDICTION OF COMPRESSIVE STRENGTH AND KINK BANDS IN COMPOSITES USINGA WORK POTENTIAL

A recently developed method of characterizing nonlinear, inelastic beh avior of composites is described and then used to provide constitutive equations for use in the compressive strength problem of unidirection al fiber composites. This constitutive theory, which is based on a wor k potential, appears to be valid for the strain state and levels of st rain needed to predict kink band angles and compressive strength. A on e-dimensional deformation model of fiber waviness growth is then descr ibed and used to make a case that a band of wavy fibers initiates a ki nk band when local matrix cracking occurs and the axial stress equals or exceeds the predicted critical stress for local buckling. This requ irement of matrix cracking serves to define the kink band angle. For m ultiaxial stresses this angle and the compressive strength depend on a ll components of the overall or average stresses; the multiaxial state of stress may arise from external loading or from ply-to-ply interact ions in a multi-directional laminate. Equations are developed for pred icting the behavior for general in-plane loading and arbitrarily large geometric nonlinearities when the failure mechanism is microbuckling. A geometrically approximate, analytical solution is also developed. R esults for several cases are given in order to illustrate the predicte d behavior and to show that the predictions are consistent with experi mental observations.