ASTM F1801-20

4.1.1 This practice provides a procedure for fatigue testing in a simulated physiological environment. Axial tension-tension fatigue tests in an environmental test chamber are recommended as a standard procedure. The axial fatigue loading shall comply with Practices E466 and E467.

4.1.2 This practice is intended to assess the fatigue and corrosion fatigue properties of materials that are employed or projected to be employed for implants. This practice is suitable for studying the effects of different material treatments and surface conditions on the fatigue behavior of implant materials. The loading mode of the actual implants may be different from that of this practice. Determining the fatigue behavior of implants and implant components may require separate tests that consider the specific design and loading mode.

4.1.3 As a substitute for body fluid, 0.9 % saline solution is recommended as a standard environment. One of the various Ringer's solutions or another substitute for body fluid may also be suitable for particular tests. However, these various solutions may not give equal fatigue endurance results. The chloride ions are the most critical constituent in these solutions for initiating corrosion fatigue.

4.1.4 Because implants are manufactured from highly corrosion-resistant materials, no visible corrosion may be detectable when inspected by means of optical microscopy or scanning electron microscopy. Only a decrease of fatigue strength in the high cycle range may be noticeable. Therefore, S-N curves covering a broad fatigue loading range should be generated in the test solution and in air. Comparison of fatigue curves generated in air and saline solution may be the only way to assess the effect of the saline environment.

4.1.5 Where the fatigue behavior of a material system is already established, it may suffice to test modifications of the material properties or surface condition in only a selected stress range.

4.1.6 The recommended loading frequency of one hertz corresponds to the frequency of weight bearing during walking. For screening tests, higher test frequencies may be used; but it must be realized that higher frequencies may affect the results.

4.1.7 Summary of Standard Conditions—For inter-laboratory comparisons the following conditions are considered as the standard test. Axial tension-tension tests with cylindrical specimens in 0.9 % saline solution at 37°C and air at room temperature under a loading frequency of 1 Hz.