Fatigue testing is a well-established process, but accurate results depend on strict attention to detail.
Random selection of materials, test bar unit cells, and cutting methodology could skew results and put your application at risk of failure. In this second of our two-part fatigue testing series we explore how these key aspects of fatigue testing influence outcomes — and why it’s important to partner with an experienced supplier to leverage advanced testing and engineering plastics.
Often, fatigue testing involves stressing a dog bone-shaped test bar for a number of cycles, until the test bar breaks. To cover a range of stresses below breaking stress, four levels of testing are administered at 30%, 50%, 70%, and 90%.
The challenge lies in the assumption that the mechanical properties of the tested material are isotropic, meaning the same in all directions. In the case of glass filled engineering plastics, the mechanical properties are anisotropic and stronger in the direction of fiber orientation. Further, fiber orientation is affected by the forming process so there is no universal anisotropism.
Ideally, there is a unit cell where the microstructure of the plastic is shown to have all fibers oriented in the same direction – indicating that that area of fiber orientation is repeatable for testing purposes. Engineers must seek out and test this unit cell in order to accurately model fatigue behavior — a deceptively simple exercise.
To test properties at the point of the unit cell, rectangular plaques are injection molded and dog bone-shaped test bars are cut from the unit cell location. Fatigue bars are cut in three directions: 0 degrees in the direction of flow, 90 degrees perpendicular to flow, and 30 degrees offset from the flow direction.
Testing sample location largely drives stress testing results, which only underscores the importance of test bar location.
Moldflow simulations and prior testing confirm the maximum fiber orientation of glass filled engineering plastics is located in the center gate direction, about two-thirds down the flow length:
Knowing this gives engineers a target, and it also contributes to other critical aspects of fatigue testing:
Nylon testing is typically performed dry as molded. As such, several cutting options are available, each with its benefits and drawbacks:
This broad overview of key aspects of fatigue testing reveals how dynamic the process is, and how results inform selection of engineering plastics for applications. It also demonstrates the need to partner with a supplier experienced in materials characteristics, behaviors, and fatigue testing optimization to align plastics with your application.
Reach out to the Teknor Apex team to learn more about fatigue testing, our wide range of engineering plastics, and our expert guidance in combining the two for successful outcomes.