Thermoelastic stress analysis for detecting and characterizing static damage initiation in composite lap shear joints

Abstract

Experimental techniques using thermoelastic stress analysis (TSA) are presented to characterize static damage initiation and progression in fiber reinforced polymer (FRP) single lap shear joints. Two carbon fiber composite lap joint systems are tested: woven (fabric) laminated and uni-tape laminated orthotropic plates. The resin layer used for the bonded lap-joints is the FM300K adhesive with an approximate thickness of 10mils. Two testing methods are studied with TSA to detect damage and characterize damage severity. Several samples are removed from loading after initial cracking is suspected from observing the TSA data, and micrographic inspection is conducted to characterize and confirm the nature of their failure initiation. The proposed TSA testing schemes along with a damage index demonstrate a good ability to detect damage near the bonded edge of the joint and characterize damage severity. The damage index presented provides a distinct advantage over traditional methods such as contact gauges and acoustic emission which are unable to link detected events to the damage extent. The demonstrated TSA ability to characterize damage in lap joints is well suited for long-term fatigue tests or comparing the mechanical performance of various FRP joints.