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Fatigue & Fracture Toughness: Durability Analysis

          In order to prevent customer disappointment from a lack of durability, it is critical to have an understanding of a product's fatigue and fracture toughness life. Durability can be simplified to be the ability of a product to be used many times.


         Every use of a product is a load cycle. The fatigue life of your component is how many cycles it will survive without developing a flaw or crack. Fracture toughness is how many cycles your product will survive with a flaw or crack. Products that do not involve safety must meet a certain fatigue life to gain a positive reputation for durability. Products whose failure can cause injury must meet the higher standard of having a substantial fracture toughness. The fracture toughness life gives users an opportunity to discover the approaching failure before it happens.

Meet Durability Requirements

        We have the tools and knowledge to analyze the life of structural parts and can recommend design improvements to economically ensure the durability requirements of your product are met. All structural bodies including buildings, vehicles, turbines, hand tools, and others experience fatigue from cyclic loads. Fatigue is often recognized as the most common reason for the failure of parts and structures. Fatigue occurs during regular use, well within the components load ratings, and builds up over time as with the load cycles After some number of load cycles cracks initiate.


        This is usually not failure but leads to the final step in the life of the part or structure:  Crack Propagation. The rate of crack propagation is determined by the fracture toughness of the design. Fracture toughness is controlled by a combination of stress distribution and material properties. Fatigue is major reason structures, vehicles and products need maintenance or must be replaced. Understanding fatigue allows you to predict the life expectancy and maintenance requirements of a structure or product.

High Cycle Fatigue

         A subset of durability analyses is High Cycle Fatigue.  High Cycle Fatigue or HCF refers to cyclic loading that accumulates quickly.  Often a million cycles will accumulate with just a few hours of use.  The cause is usually the excitation of a resonance frequency within a part or overall structure.

         If your structure has a resonance frequency of 100hz it only takes 3 hours of exposure to a 100hz cyclic load to accumulate a million load cycles.  If only inherent structural damping is designed into the product, then the resonance amplification can be 25X.  So a small cyclic excitation load can cause big cyclic stress in the structure.   At a million cycles per 3 hours, it is obvious, that the structure or part will fail in a short period of time.  Avoiding HCF failures is performed with a combination of dynamic and fatigue analyses.   The resonance can be moved out of the operating range of damping can be added.  The good news is that FEA is good at predicting and solving HCF problems.

Contact Fatigue

           Another subset of durability analyses is Contact Fatigue.  This applies to all components that are loaded cyclically by contact with a pin or ball.  Ball bearings are a classic example.  Under a steady shaft load, the balls and races of a ball bearing will experience a very rapid accumulation of contact cycles as the shaft rotates.  The good news is that modern bearing materials have phenomenal contact strengths.

          As a result, modern bearings, if sized correctly, have good durability, but one still needs to pay attention to this detail to meet durability requirements.  We are well versed in this design/analysis activity.We use finite element analysis (FEA) along with engineering calculations in order to understand the rate at which products and structures fatigue and how to best implement design improvements to economically improve their durability.  Using FEA we can locate stresses where cracks are likely to initiate.  

         The same FEA results can then be used to determine how many load cycles it will take for the crack to propagate to failure.  The cool thing is that all of this durability and safety information is provided while your design is still just on the computer.  You will have every opportunity to make adjustments and have the analysis verify the durability improvement.

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