Received May 31, 1990; revision received June 25, 1990; accepted July 5,1990. Copyright © 1990 by S. C. Tan. Published by American Institute of Aeronautics and Astronautics, Inc., with permission. *Senior Research Scientist (on-site contractor), Air Force Materials Laboratory, WRDC/MLBM, Wright Patterson AFB; currently at Northwestern University, Evanston, IL 60298. Member AIAA. methods in the first three categories have been utilized extensively. Unfortunately, the compressive strengths of unidirectional composites measured by different test methods vary significantly.' The diversity could be attributed to the fact that the stress concentrations in the specimens are significantly different using different test methods. In addition, stability could be a problem for some of the test methods. Two of the most commonly used compression test methods are the Test Method for Compressive Properties of Unidirectional or Crossply Fiber-Resin Composites (D 3410-87), which are standard test methods utilized by the American Society fpr Testing arid Materials (ASTM). The standard ASTM D 3410 includes the Celanese and the Illinois Institute of Technology Research Institute (IITRI) test methods. The main difference between these two methods is the grip design of the fixtures. The grip construction restricts the overall tabbed specimen thickness of the Celanese test to around 0.157 in. (3.99 mm), whereas the overall tabbed specimen thickness of the IITRI test can be anywhere from 0.059 in. (1.5 mm) to 0.5 in (12.7 mm). To the author's knowledge, there is no analysis available for the Celanese and the IITRI test methods in the open literature. The small unsupported gauge length of the Celanese and IITRI test specimens [both are 0.5 in. (12.7 mm)] has created a concern that the stress distribution in the specimen may not be uniform. Therefore, the objective of this study is to analyze these compression test specimens.