Damage tolerance of CFRP airframe bolted joints in bearing, following bolt pull-through failure

Abstract

The experimental study presented herein, investigated the residual strength of bolted joints on Carbon Fiber Reinforced Polymer (CFRP) airframe structures within the context of structural damage tolerance and airworthiness regulations. The damage scenario assumed, subjected a series of bolted joint CFRP laminate specimens to quasistatic bearing loading following bolt pull-through failure events of different magnitude. Representative CFRP laminate specimens manufactured from AS7/8552 carbon fiber/epoxy matrix system were artificially damaged under bolt pull-through loading, following the herein proposed modifications to the current pull-through ASTM testing procedure. The specimens were subsequently tested in static bearing loading for examining the specimen residual bearing strength. The residual joint bearing strength was related to the displacement travelled passed the initial failure stage in pull-through mode and was measured up to a maximum of a 13% decrease for the tested samples and the maximum damage imposed. The study explored of safe utilization of bolted joints at higher operating loading levels within the context of the current airworthiness regulations. The inherent damage arrest features of the joints were highlighted. The study concluded with comments and suggestions on the expansion of the current utilization spectrum of damaged bolted joints from pull-through loading in airframe design, bound by the current airworthiness certification requirements.

Study on Tensile Performance of Double-Bolted Joints between Carbon Fiber Reinforced Polymer Plate and Aluminum Plate

“…The joints with larger clearances were subjected to a delay in bearing the load, which had an effect on the load distribution in the multi-bolt joints.Park [7] investigated experimentally the effect of clamping force on the final load bearing strength of bolted joints and tested different working conditions with clamping pressures of 0, 5.2 MPa, 29.3 MPa and 58.6 MPa, respectively.The results showed that the clamping pressure inhibited the occurrence of delamination in composite plates, while Chang [8] proposed a damage model for open-cell composite plates subjected to tensile conditions, and also proposed a stiffness reduction scheme based on the damage theory, which can calculate the stress and damage to each layer of the composite plate. Giannopoulos [9] studied the static strength of carbon fiber composite panels, which was tested after the composite panels were subjected to damage in the presence of already existing damage. Several typical initial damage models are given in the paper, not only for its axial tensile case, but also for the analysis of the composite plate being crushed by applying pressure along the bolt axis, which can be used in the case of composite plates subjected to loads in different directions.…”