Explain how 'Beach Marks' on a fractured wave spring cross-section indicate a fatigue failure mechanism.

'Beach marks' (also known as clamshell marks) are macroscopic evidence of progressive fatigue crack growth. Each mark represents a period of crack extension followed by a period of rest or change in load. In a wave spring, these marks typically originate at the surface of the ID or OD at the peak of a wave, where the tensile bending stress is maximum. The presence of these marks, followed by a rough 'final fracture' zone, confirms that the spring failed due to cyclic loading rather than a single over-stress event. Analysis of the spacing between marks can sometimes help estimate the number of cycles to failure and the magnitude of the stress cycles.