UC San Diego

Primary aerospace composite structures are commonly assembled with bolted joints due to their ability to transfer high loads and ease of assembly. However, when bolted joints are used beyond their originally intended design life, joint strength can be significantly reduced due to the accumulation of internal damage, necessitating frequent inspections. Furthermore, internal damage in composites (delamination, matrix cracks) can continue to propagate without visual indications, thus nondestructive testing is required. As a result, maintenance can become very costly, particularly for aircraft that are in-service beyond their designed life expectancy. By establishing a comprehensive understanding of damage propagation behavior, engineers can determine which damage modes to inspect for and reduce inspection frequency. This research aims to support and improve maintenance operations, fleet management, and aircraft design practice by investigating delamination initiations and propagations in the vicinity of fastener holes within fiber-reinforced composite materials.

Static and fatigue bearing were performed using novel test methods developed as part of this research for countersunk fastener joints: the modified countersunk double lap shear (DLS), single lap shear (SLS), and semi-circular notch (SCN) test configurations. DLS and SLS static and fatigue experimental test results were compared to study joint configuration, laminate stacking sequence, and loading condition effect on bearing damage initiation and growth under both static and fatigue loading. From static and fatigue tests, it was observed that major bearing damage accumulates in the straight shank region of the countersunk hole indicating most of the bearing load is carried by the straight shank region. Fatigue bearing test data showed that when the bolted hole elongates, stiffness decreases and internal delamination damage area growth becomes detectable through C-scan. Stated in reverse, if no measurable hole elongation is found, significant delamination is not expected. Complex damage morphology forms in this region, emanating from the loaded bearing face, and creating large wedge-shaped regions that drive delamination propagation with additional loading cycles. Additionally, optical microscopy observations indicated that pin bending might have affected bearing damage growth. In order to understand the effect of pin bending, a custom designed semi-circular notched experiment was performed on the countersunk hole geometry and compared to the DLS static experiments. Results indicated that the pin bending had no strong effect on the bearing failure morphology for the selected diameter to thickness ratio.

Finite element analysis using Virtual Crack Closure Technique (VCCT) and Hashin failure criteria in Abaqus was used to further understand the internal stress state of the specimen configurations and to investigate the rate of delamination growth and arrest in the SCN and DLS configurations. Results from FEA were used to more comprehensively understand the observations from static and fatigue experiments and to verify hypotheses formulated to explain these observations.