Understanding the Effects of Environmental Conditions on the Failure of Bonded Joints

Abstract

Composites are widely utilised in aerospace structures, especially for the newer generation designs, due to their advanced properties such as high strength to weight ratios and durability. Understanding the failure of bonded joints with composite adherends is key for designers, particularly within the aerospace industry, for both a safe design and meeting the aircraft regulation standards. This thesis investigates both experimentally and numerically the effects of environmental conditions on the strength of Double Lap Joints (DLJ) with composite adherends, for different geometrical and loading conditions. The environmental conditions tested are at Room Temperature Dry (RTD), Hot Temperature Dry (HTD) and Hot Temperature Wet (HTW). The mechanical properties of both the adhesive and composite are characterised at these environmental conditions. DLJ tests for pristine, impacted and specimens with artificial defects are tested. Analytical models using simple calculations are used to determine the failure modes. Numerical model using Cohesive Zone Models (CZM) are used to predict and further understand the DLJ behaviour through sensitivity analyses.

During the characterisation tests, it is found that Mode I fracture energy increases whereas Mode II decreases with increasing temperature and moisture for both adhesive and composite. A distinct change in failure mechanism is also found with the influence of temperature and moisture. The effects of overlap length, adherend and adhesive thickness on the pristine DLJ strength at different environmental conditions are analysed and discussed. Following the pristine tests, the residual tensile strength after through-thickness impact and the effects of artificial defects (disbond) on the strength of DLJ are also studied. The impact is applied at RTD and followed by tensile tests at different environmental conditions. Disbond cases are represented by inserting an artificial defect in the bondline during the manufacturing process and is followed by tests at different environmental conditions. For all DLJ, increasing temperature and moisture had a significant effect on the strength, failure mechanisms and fracture surface.

Date of Award	23 Jan 2020
Original language	English
Awarding Institution	The University of Bristol
Sponsors	Mitsubishi Heavy Industries Ltd
Supervisor	Michael R Wisnom (Supervisor) & Xiaodong Xu (Supervisor)