Fracture Behavior of Epoxy Resins in Fabric‐Supported Form

Abstract

Summary Elastic‐plastic fracture behavior of a structural adhesive in the bulk and bonded forms is discussed. The model adhesive chosen with and without scrim carrier cloth (neat resin) solid film adhesives exhibit a relatively brittle material behavior to justify the use of Linear Elastic Fracture Mechanics (LEFM) methods. The solid film adhesives are first cast in the form of tensile coupons to determine the bulk fracture properties with the use of single‐edge‐cracked specimen geometry. KIC evaluation is done using the procedure suggested by the ASTM standard. A K‐calibration method based on application of boundary collocation procedure to the William's stress function is utilized to relate the measured critical loads to the KIC values. The yield stresses (σY) and elastic moduli (E) values in the bulk tensile mode are also evaluated. The availability of KIC, σY, E and ν (Poisson's ratio) values makes the calculation of crack tip plastic zone radii (rYC) and fracture energy (GIC) values possible on the basis of Irwin's theory. The bulk casting procedure is done under different cure (temperature, time and cool‐down) conditions to determine optimum properties. We found that the most suitable model that would fit the stress‐strain response of the model adhesives with prediction of the stress whitening stress level, σ*, under material damage surrounding flaws and crack tips was a modified version of Ramberg‐Osgood equation. A bilinear behavior is obtained when log(ϵp) is plotted against log(σ – Θ). The plastic strain ϵp is assumed to be a function of the over‐stress above the elastic limit stress, Θ, and the stress levels defining the intersection point for the bilinear behavior are found to occur slightly below the stress whitening stress values.