Hill Engineering Blog

Fatigue tests

Fatigue tests are an important part of our work at Hill Engineering. There are many reasons to perform fatigue tests including: 1) qualifying the performance of parts for production, 2) ranking and optimizing different production processes, 3) validating fatigue analysis results and associated models, 4) understanding the performance of parts already in service, and more. The following discussion covers some basic details of typical fatigue tests.

Fatigue is a material failure mechanism whereby cracks develop in a material and grow due to repeated sub-critical loading. In other words, a single loading and unloading cycle is insufficient to cause failure, but if the loading is repeated many times, typically tens of thousands to tens of millions of cycles, failure will eventually occur. Fatigue tests are designed to evaluate the fatigue behavior of a part under specific conditions.

Fatigue tests typically include the following elements: 1) a machine or device to impart a mechanical load (e.g., an actuator mounted to a rigid frame), 2) a control system to drive the actuator and monitor output, 3) a test fixture to transfer the load from the actuator to the test specimen at the intended location, and 4) a test specimen.

In many applications for fatigue tests the machine to impart the mechanical load is a single axis servo-hydraulic load frame. More complicated structural fatigue tests, fatigue tests requiring multi-axial loading, or fatigue tests executed at very high frequency require specialized equipment.

Standard test fixtures exist for common coupon geometries (bend specimens, dog bone specimens, and others). Custom test fixtures should be designed for non-standard fatigue tests that are intended to replicate specific loading conditions.

The specimen used to execute the fatigue tests is the most critical element of the system. Care should be used to design the specimen to replicate the important features of the intended application. Details to be considered when designing specimens for fatigue tests include: stress level, stress distribution (i.e., gradient and direction), material type, material microstructure and grain orientation, surface finish, among others.

Please contact us for more information about fatigue tests or related work that we have performed regarding fatigue analysis. In addition, there is more information on our website about fatigue tests.

A fatigue crack that grew from a countersunk fastener hole representative of an aircraft wing skin.