A Method for Predicting Fastener Hole Elongation in Composite Joints due to Cyclic Loading

Abstract

Based on the current operational and sustainment trends in the United States Air Force (USAF), modern airframes with high composite content in primary structures will likely undergo a service life extension program (SLEP) in the future. However, unlike for metallic airframes, the USAF currently has no experience in SLEP of composites, with few enabling technical tools. One such tool is a lifing method for assessing fatigue life used and remaining for bolted joints. Virtually all of the composite airframes currently in service in USAF are assembled with fasteners. Structural integrity requirements dictate that these joints remain functional—i.e., capable of transferring load–over the design service life. One approach to meeting this requirement is to limit the hole elongation to a maximum value—e.g., 4%-relative to the nominal hole diameter. The certification process for such joints must demonstrate that detrimental hole wear does not occur during the design service life. However, if the aircraft mission changes or service life must be increased or both, the USAF must assure that these joints will remain structurally functional. In order to recertify the composite joints for use beyond the original design service life without resorting to extensive testing, an analytical method for predicting fastener hole elongation is necessary. This paper details the development and initial verification of an engineering approach for predicting bolt hole elongation that was performed under “Assess Technology for Performing a Structural Life Extension Program for Advanced Composite Airframe Structures” program funded by the Air Force Research Laboratory (AFRL). A total of 63 bearing specimens were excised from several 36-ply, 0.187 inch thick, [44/44/11] laminates made from IM7/9773 unidirectional tape. These specimens were tested in a double lap shear configuration with 1⁄4-inch high-strength (220 ksi Ftu) stainless steel bolts. Twenty two (22) constant amplitude (CA) bearing fatigue tests were conducted at a frequency of 2.0 Hz with a stress ratio of R = 1 at six peak stress levels (Pmax), ranging from 63 ksi to 132 ksi. Percent hole elongation vs. cyles (%e-N) curves and normalized local residual stiffness vs hole elongation (k/ko-%e) per Pmax were extracted from these CA fatigue tests and used to develop a lifing method for this composite bolted joint design. The proposed lifing method estimates the cycle-by-cycle hole elongation by incrementally moving along the appropriate CA %e-N curve from a damageequivalent initial elongation state determined from residual joint stiffness data, k/ko-%e. Nineteen (19) specimens were tested under various CA block and spectrum fatigue loading to verify the lifing method. Comparison of predictions and test data showed sufficient performance by the proposed enigeering lifing approach relative to trend and consistency.