On the behavior of bonded joints in composite material structures

Abstract

The application of advanced reinforced composite materials in aerospace structures during the remainder of this century is widely predicted. The joining of structural components by adhesive bonding is extremely desirable, because both bolting and riveting result in the cutting of fibers as well as the introduction of stress concentrations, both of which reduce the structural efficiency. R.E. Watson states that, “The next two decades will surely see dramatic advances in structures as compared to those experienced over the last 30 years. Improved titanium alloys and the advanced high strength composites, with more strength per pound than aluminum, will be the principal materials used.” Further he writes, “New bonding techniques will gradually replace riveting in many applications, permitting greater design stresses and more efficient distribution of the materials.” Because in aerospace structures dynamic loads are always present, it is absolutely essential that the fatigue behavior of bonded joints between composite material components be better understood, in order to have available design principles and rationale to take advantage of the desirable characteristics of composite materials. To date the few isolated experimental studies of composite-composite or composite-metal adherend bonded joints have been conducted under static and/or constant amplitude cyclic loading, and no generally accepted cumulative damage theories have evolved. The present research is a systematic, analytical and experimental program of study concentrating on those parameters considered to be the most influential on the static and fatigue life of an adhesive bonded single lap joint. The objectives of the program are to better understand the reasons why certain parameters have such a large influence on the structural integrity of the joint. As a result it is hoped that considerable insight will be gained as to static and fatigue life of more complicated joints such as the doable lap, the scarf, and the stepped lap joints. The analytical as well as the experimental static and fatigue test portions of the program are reported on herein. The following parameters, deemed to be the most important, were selected for study: overlap length, adhesive thickness, orientation of the laminae of the laminated adherends (particularly the lamina immediately adjacent to the adhesive), and the effect on the fatigue life of whether or not the mean value of the fatigue load causes maximum stresses above or below the shear proportional limit of the adhesive material. The determination of stresses in the test specimens is made by an analysis method developed in this program.