Use This One Simple Trick to Ensure Rubber Part Durability

We’ve just added a new output to the Endurica fatigue solver: Safety Factor.  This feature makes it simple to focus your analysis on whether cracks have the minimum energy required to grow. Safety Factor is a quick and inexpensive way to identity potential failure locations.  It minimizes the number of assumptions you need to defend, and it is backed by hard science.  You don’t need to measure or explain the many influences that together determine how fast cracks grow.  You don’t need lengthy materials characterization experiments that take days or weeks.  You do need to know your material’s Intrinsic Strength T₀ (ie Fatigue Threshold) and its crack precursor size c₀. The test takes about an hour using the Coesfeld Intrinsic Strength Analyser.

The Safety Factor S is computed as the ratio of T₀ to the driving force T on a potential crack precursor.  If the value of the Safety Factor S = T₀/T is greater than 1, it indicates the margin by which crack growth is avoided.  If S is less than 1, it indicates that crack growth is inevitable. The calculation of the Safety Factor includes a search for the most critical plane, as we do for our full fatigue life computations.

Although the Safety Factor can’t tell you how long a part will endure, it nevertheless does offer great utility.  You can make a contour plot showing the locations in your part where the Safety Factor is the lowest.  This is a quick and inexpensive way to identity potential failure locations.  You can make statements about the reserve capacity of your design that are easy to communicate and understand with a wide audience.

The images above show a vibration isolation grommet operating under small (Safety Factor 2.6) and large displacements (Safety Factor 0.83).  Color contours indicate the Endurica-computed Safety Factor, and use the same scale for both images.  Large Safety Factors are shown in blue.  Safety Factors approaching 1 are shown in red.  Safety Factors smaller than 1 are indicated in black.  These results show that the grommet can be expected to operate indefinitely under the small displacements, but that large displacements will produce cracks at some point, in the regions colored black.