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Description
The use of composite materials in aerospace vehicles has allowed for creation of lighter structures that provide excellent stiffness. Although they allow for easy disassembly and inspection of critical assemblies, bolted joints are common damage initiation sites in composite structures. In order for aging aircraft to be considered for life extension programs, the damage at these locations must be characterized and well-understood. Failure studies require that data be provided through experimental tests. The drawback of the present test standard for bolted composite joints is that it was initially designed for testing polymers. This thesis discusses deficiencies in the measurement of bearing strength as a result of this shortcoming, although the battery of bearing tests is modeled after this established test standard. The goal of the present work is to characterize the bearing load induced damage in laminates with pin bearing and countersunk fasteners through a series of bearing tests on laminated composites using a new modified ASTM D5961 fixture designed for testing countersunk bolted joints. This thesis examines variations in measurement of bearing strength that comes with using different test fixtures. The reliability of each test fixture to represent real conditions is evaluated using test fixtures that represent real joint configurations. Results for damage conditions and joint responses are compared. This thesis presents a study of the damage initiation and accumulation in composite specimens loaded in bearing using three different test fixtures: Procedure A, Procedure C, and Procedure A Modified. Procedure A, which is used to load the test specimen in double-shear via a straight pin, is a well known test fixture that comes from ASTM D5961. Procedure C has a countersunk fastener that loads the test specimen in single-shear. These types of fasteners are preferred in the aerospace industry over traditional bolts in order to maintain a smooth aerodynamic profile on exterior aircraft surfaces. Because of pin bending that is observed in previous experiments and in literature during single-shear tests, a new test fixture called Procedure A Modified was developed that allows for a countersunk pin to be used in double shear in order to provide more stiffness. This fixture seeks to replicate bolted joints found in many aircraft, including the first generation of military aircraft built with composite primary structures. The effect of clamp-up, which provides improved bearing strength, is studied with each test fixture because the presence of a bolt preload is a common practice in the aerospace industry. Characterizing the damage accumulated in each of the three joints both with and without bolt preload is essential to evaluating effectivity of each test fixture in simulating actual bolted joints. Although the pattern of damage accumulation in the three test fixtures used appear visually different, each is characterized by shear cracking that leads to fiber buckling. The damage initiation site in Procedure A is the hole surface; in Procedure C it is the junction between the thru-hole and the countersink surface. Because of reduced pin bending when compared to Procedure C, Procedure A Modified causes damage in the thru-hole region of the hole. This test fixture also provides greater ultimate bearing strength, load, and stiffness response than the other test fixtures. Procedure A Modified can be adopted as a new fixture for testing composite countersunk bolted joints due to the reduction of pin bending but attention must be given to proper hole and specimen sizing if scaling to larger test coupons is desired.
