A Progressive Fatigue Model on the Bearing Failure of Composite Bolted Joints

Abstract

Bolted composite joints have distinctive failure mechanisms due to the bolt clamping force, which could induce a hydrostatic pressure in the bearing region. Compressive failure near a bolt hole under bearing, clamp-up pressure, and constraint from non-zero degree plies takes a variety of forms at the microscales including fiber micro-buckling, fiber kinking, fiber compressive failure, matrix cracking, delamination, and out-of-plane shear cracking. The hydrostatic pressure would cause the increase of compressive strength of the composite material and hence grant a large load-carrying capability to the bolted joint from damage initiation to ultimate failure. A physics-informed modeling approach is developed for the static bearing failure analysis by considering the residual stress along the longitudinal and transverse direction in the bearing region. However, the mechanisms become more complicated for the fatigue load when hole elongation is involved. The permanent hole elongation could be caused by the releasing of the formed debris in the bearing region during the unloading stage under fatigue load. To consider this permanent deformation, two permanent strain components in the longitudinal direction and transverse direction are introduced into the developed fatigue model and their evolution with the increase of fatigue cycles. With the evolution of permanent strain implemented, the fatigue model can be utilized in the failure prediction for the composite bolted joints of various configurations such as single shear bearing (SSB), double shear bearing (DSB), and bearing and bypass (BB) coupons. Validation studies are performed using test data from National Institute for Aviation Research (NIAR) and damage characterization results to correlate the damage progression and final failure mechanisms. The validated bearing damage and failure prediction module for CB2ATA will be used as a virtual testing tool to generate additional points on the bearing and bypass interaction curve without testing.