hertz stress numbers w/ relationship to tensile & yield of metal

[duk748]

hello - please forgive my elementary explaination of this question but here goes - - what is the relationship between the numbers in hertz stress (i.e. 81k as shown in an fea problem)to the yield & tensile limits of a particular grade of steel or ductile iron - we have a design we are working on & after the fea was done the analysis shows that at a number of points along a lug the hertz stress is 81k - i have a fundamental understanding of hertz stress but would like to know at which point along the flowing of the metal as the stress start to go down does it become acceptable & safe in the design - or at what limit is it safe based on the material properties so that one or both of the pieces will not develop a crack or fail - please forgive me if i am not stating this right - just need to know for my personal knoweldge - any help would be appreciated - thank you

 

[Metaljon]

If you are concerned about excessive contact stress and the risk of cracking then you need to think wider than Hertz stress. Contact problems usually involved a whole lot more than a single quasi-static loading of a simple geometry. If you're reviewing how critical it is in respect to yield, using typically yield conditions such as Von Mises stress is one way to resolve this problem. In a specific application, a lot can be inferred from the value of the contact pressure itself. Just make sure if you're actually trying to get to the failure of the surface you don't take too simplistic an approach. Have a look at some tribology books and the number of mechanisms for surface failure and what different issues affect the failure mode. There are lots of different criteria and models utilizing different measures of stress, deformation and material properties.

 

[salmon2]

Hertz theory applys only to elastic cylinder or sphere shape contact, it is a theoretical solution. Basically it is only good for light contact, meaning everything is below yield strength.

If the max stress from Hertz theory is larger than material yield strength, then you can't use Herrz any more. You can use fea, elastic-plastic fea to model your problem. Then I will get the average bearing or contact stress and check it against either yield or tensile strength depending on your design code and application which will bring in design factors to consideration. As Metaljon suggested, I will also check von Mises stress from fea for structural stability, again dependent on your design code. These are two things I will check for this contact problem.