Strain-Relief Inserts for Composite Fasteners -An Experimental Study

Abstract

The stress and strain distributions in the vicinity of single-pin lap joints in a glass-epoxy laminate were studied in a combined experimental-theoretical program. One objective was to evaluate the possibility of obtaining significant stress concentration relief through the use of isotropic material inserts. This paper concentrates on some experimen tal results. High sensitivity Moire interferometry was employed to measure surface strains in three directions within the insert ring and in the adjacent composite laminate. Strain rosette equations were then used to obtain maximum shear strains. Measurements were performed for no insert, a hard insert (aluminum), and a soft insert (epoxy) in an isotropic material and in a composite. Stress reductions were observed at bearing and ligament areas for both types of inserts, with the stiffer insert giving the greatest advantage.