Fatigue Cracking Effects

Why does fatigue cracking get so much attention?
Basic physics tells us that for every action, there is a reaction.  When parts are manufactured and placed under a service load, the reaction is that the products will absorb the load (stress) and deform accordingly (strain).  In many common, everyday applications, the amount of deformation that can be seen or observed is often imperceptible to sight or feel.  For example, consider the floor in you house: the space and span of the floor joists installed per building codes make the deformation almost imperceptible as you walk around.  While nearly unrecognizable, the floors do deform under the load but are operating at an extremely safe level.  Even so all materials are capable of fatigue cracking, even wood and other polymers.
In many engineering applications, and particularly in critical load bearing elements, metal alloys are most commonly used.  The design of these components is maintained within safe material margins of stress and strain.  The process of fatigue cracking becomes troubling in that it may occur within this perceived safe margin.  Fatigue cracking occurs due to cycling from thousands to millions of times at stress levels below what would be considered unsafe for the material.  During cycling, the material develops microscopic material flaws leading to the formation of microscopic cracks.  Once formed, these cracks will continue to grow until they are either identified and repaired or propagate to cause catastrophic failure.  It is also worth noting that while designs can be made to reduce the probability of fatigue failure, other events may occur that dramatically increase the likelihood.  These may occur due to corrosion, incidental damage, poor manufacturing and more.  In a subsequent article, we will discuss some of the methodologies behind planning for fatigue cracking (damage tolerance, safe-life, law tolerance, etc…).
Laser peening increases fatigue life by minimizing the stress state causing the initiation and growth of the fatigue cracks.  With proper application, the observed stress can be pushed to a level low enough to prevent cracking or extend the service life up to ten times the normal.  The benefits of laser peening for fatigue applications can be developed in simulated service environments using equipment at LSP Technologies.  Using our MTS 810 servo-hydraulic test unit capable of operation at up to 35 Hz (35 cycles per second), the fatigue tolerance of materials and geometrics can be tested and evaluated with and without laser peening.  Take a look at our video for an MTS fatigue testing demonstration.