| Description |
Demand for lighter thin-walled tubes have encouraged engineers to use composite materials. However, using composites makes it more difficult to predict material characteristics. Available test equipment is unable to evaluate thin-walled composite tube characteristics. Testing methods and fixtures capable of evaluating tube properties, including flexural strength, flexural fatigue, and flexural damage tolerance, provide a means of comparing product durability. Durability charts (charts with multiple fatigue curves) were produced using test methods from the categories mentioned. Available testing equipment is unable to evaluate flexural characteristics of thin-walled composite tubes. Available test fixtures are intended primarily for testing flat specimens. Thin-walled composite tubes require unique load applicators to more uniformly distribute loads, thus preventing localized failures. A modified four-point flexural test fixture with rubber load applicators provided satisfactory results. Flexural fatigue curves were developed by applying various loads, and rotating the tube in a four-point bend configuration. Advantages of this method include having relatively low concentrated loads and a region with a constant bending moment, which is useful when evaluating areas of interest such as joints or impacted regions. This fixture employed the same rubber load applicator concept used on the flexural strength test fixture. Evaluation of flexural damage tolerance can be done in different ways. Tubes used for this project are visually inspected between each cycle. Thus, short cracks running parallel to the fibers on the outer layer are difficult to detect. An anvil attached to the end of a pendulum seemed to produce damage parallel to tube fibers, similar to that produced during tube operation. The pendulum arc was oriented parallel to the fibers of the outer layer. Damage amounts were determined by tube properties and the amount of tube deformation caused by the impacting anvil. Composite tube durability charts were produced by developing a fatigue curve using thin-walled composite tubes and two fatigue curves using impacted tubes of different levels of deformation, illustrating the tube's damage tolerance with a vertical shift in different curves. By using the three unique test fixtures developed for this project, it is possible to develop a better understanding of the characteristics exhibited by thin-walled composite tubes. |
