The importance of understanding the response of structural composites to impact cannot be overstated. This understanding includes both the impact phenomena themselves and the influence of materials properties on the impact response. This paper presents the need for instrumented testing to optimize our understanding of the impact event, especially the response of the impacted material. The conclusion is drawn that the impact force history is a more relevant measure of a materials characteristics than is the total kinetic energy of the impactor. Static and dynamic impact phenomena are assessed to lay the foundation for the eventual development of a standardized test method focused on the lower velocity range impact response behavior of composites. In addition, a relatively inexpensive but very versatile low-velocity, instrumented pendulum impact tester is described and actual test data for both graphite fiber/thermoset matrix and graphite fiber/thermoplastic matrix are compared. Actual energy absorption curves are shown. A simple method is described to allow direct measurement of the total energy exchanged during the impact event, and the use of these data to permit the vital dynamic calibration of the load cell for every different impact event is illustrated. The different stages in the damage process are characterized, for the first time, for the two materials systems studied.
Graphite, boron, S-glass, and DuPont's PRD-49-III fiber reinforced composites, as well as castings of current epoxy resin systems, were evaluated to determine the effects of moisture and high humidity on their physical properties and their room- and elevated-temperature mechanical properties. All of the neat resin castings were found to absorb moisture and swell. Associated with moisture absorption is a loss in elevated-temperature tensile strength. All of the composite systems showed weight gains and thickness increases when subjected to a high-humidity environment. However, the effect of absorbed moisture on the elevated-temperature mechanical properties of composites is determined principally by fiber orientation and test method employed. Unidirectional composites may show a significant reduction of 350°F (177°C) flexural strength due to absorbed moisture, whereas a multidirectional lay-up may show only a minor loss of 350°F tensile strength after equivalent moisture absorption. Fiber-controlled composite properties are relatively unaffected by absorbed moisture whereas matrix-controlled properties are adversely affected.
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