So, you’ve got a tricky durability problem to solve, a budget, and a deadline. Let’s look at a helpful framework for sorting which Endurica workflows you need. In the grid below, each row represents a potential approach you can take. The approaches are, in order of increasing complexity and cost, the Infinite Life approach, the Safe Life approach, the Damage Tolerant approach, and the Fail Safe approach.
The Infinite Life approach is by far the simplest approach. Here, we say that damage will not be allowed at all. All locations in the part must operate, at all times, below the fatigue limit (ie intrinsic strength) of the rubber. The required material testing is minimal: we need only know the fatigue limit T0 and the crack precursor size c0. We avoid the question of how long the part may last, and we focus on whether or not we can expect indefinite life. We report a safety factor S indicating the relative margin (ie S = T0 / T) by which each potential failure location avoids crack development. When S>1, we predict infinite life. For S<=1, failure occurs in finite time and we must then go on to the next approach…
In the Safe Life approach, the chief concern is whether or not the part’s estimated finite life is adequate relative to the target life. The material characterization now becomes more sophisticated. We must quantify the various “special effects” that govern the crack growth rate law (strain crystallization, temperature, frequency, etc.). We consider the specific load case(s), then compute and report the number of repeats that the part can endure. If the estimated worst-case life is greater than the target life then we may say that the design is safe under the assumptions considered. If not, then we may need to increase the part’s load capacity, or alternatively to decrease the applied loading to a safe level. In critical situations, we may also consider implementing the next level…
The Damage Tolerant approach acknowledges that, whatever the reasons for damage, the risk of failure always exists and therefore should be actively monitored. This approach monitors damage development via inspection and via tracking of accrued damage under actual loading history. A standard nominal load case may be assumed for the purpose of computing a remaining residual life, given the actual loading history to date. Changes in material properties due to cyclic softening or ageing may also be tracked and considered in computing forecasts of remaining life.
The Fail Safe approach takes for granted that failure is going to occur, and obliges the designer to implement measures that allow for this to happen safely. This can take the form of a secondary / redundant load path that carries the load once the primary load path has failed. It can take the form of a sacrificial weak link / “mechanical fuse” that prevents operation beyond safe limits. It can take the form of a Digital Twin that monitors structural health, senses damage, and requests maintenance when critical damage occurs.
The last three columns of the grid show which Endurica fatigue solver workflows align with each design approach. The Endurica solvers give you complete coverage of all approaches. Whether you need a quick Infinite Life analysis of safety factors for a simple part, or deep analysis of Damage Tolerance or Fail Safety, or anything in-between, our solvers have just what you need to get durability right.
Road load signals are notoriously difficult to work with. The signals feature so many different time increments that it becomes too much to directly model efficiently in FEA. It is difficult to tell which portions of the loading do the most damage. Experimental fatigue testing would be too time-consuming and costly to run on the full complex road load signal. For these reasons simplifying road loads into block cycle schedules has become the gold standard for working with road load signals. Experimental testing and FEA modeling are more manageable when using a block cycle schedule instead of the full road load signal. Traditional methods of converting a road load signal to block cycle schedule can often fall short. Endurica recently added a built-in method in the Endurica CL software that uses the power of critical plane analysis and rain-flow counting to automate block cycle creation.
