Composites compression testing can be a minefield. With almost innumerable standards based on three distinct methods of load introduction, there are several fixture variations and a seemingly unlimited number of standards. However, what is the best compression testing approach, and what traps should you avoid? Despite decades of research and application, the composites community has not standardized on a single compression test method.
In the early days of composite technology, it was believed that composites should display the same properties under stress and compression as metals.
However, when experimental findings did not support this hypothesis, rather than questioning the science, technologists determined that the compression test method was flawed.
Researchers would then create an alternative method. Numerous anti-buckling solutions were developed to prevent Euler column buckling after it was observed that samples buckled upon what was thought to be a premature failure.
Despite the introduction of these anti-buckling fasteners, compression strengths remained considerably lower than tensile strengths. In reality, the reason for the lower compression strength is a distinction in failure mechanisms. In tension, the failure is caused by an overload of the fibers, whereas in compression, the individual fibers undergo micro buckling.
During curing, the differential in thermal expansion characteristics between the fiber and matrix contributes to this micro buckling. In tension, these fibers straighten to experience their full load capacity, whereas, in compression, their waviness promotes micro buckling.
It is important to note that while a tapered tab termination zone is ideal for tensile samples, it is not recommended for compression samples. While the benefits of a tapered tab in lowering stress concentrations at the tab termination area are evident and well-researched, it is of more importance to prevent micro buckling failures outside the gauge length. Therefore, it is desirable that samples not be tapered in order to provide full structural support outside of the gauge area. The bulk of compressive failures occurs at or very close to the tab termination zone as a result.
In conclusion, the optimal compression test method is the one that works for you, provided that sample preparation and test setup allow you to obtain consistently valid test results from the fixture you are utilizing. Falcon Geomatics LLC Materials is accredited to do compression testing to a variety of standards and would be pleased to assist you with any questions regarding compression testing.
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