Finite element analysis of the microdroplet test method using cohesive zone model of the fiber/matrix interface

This paper presents a finite element (FE) modeling methodology to simulate and assess the importance of various factors affecting the failure mechanisms in the microdroplet test used to measure the interface shear strength (IFSS) of S-glass fiber epoxy matrix. These factors include the effect of processing-induced residual thermal stresses, progressive debonding with interfacial friction, unstable crack propagation and frictional fiber sliding on mixed-mode loading at the interface. Cohesive zone modeling approach is employed to simulate the crack propagation and interfacial failure. Mode II dominated traction separation laws are determined through numerical simulations of microdroplet test results. The importance of including the effect of residual stresses and interfacial friction in determining the cohesive traction–separation laws is illustrated. The sensitivity of the results for mode I traction–separation parameters is studied. The developed holistic modeling methodology allows for accurate determination of traction–separation behavior of the interface. This modeling effort also attempts to identify improvements to the microdroplet test method which is widely used to assess the degree of fiber/matrix adhesion.