Saturday, August 11, 2012

Endurance Strength -- Strength Reduction due to several surface conditions

The chart below shows the reduction in strength due to several surface conditions when the part is exposed to cyclic loading. 

What is "endurance strength"When we think about strength it's how much force is required to fail the part -- either break or bend. Our part must be strong enough to  resist the static load. However, many parts have repeated (cyclic) loads applied to them.; propellers and connecting rods are two examples).

Given enough of these cyclic loads the part can fail at a very different stress level than a one-time applied load. This point of failure when stressed repeadly is the endurance strength. We should be concerned about endurance strength for parts subject to large number of cyclic loads. The chart below shows the great reduction in endurance strength under various surface conditions.

Loss of Strength due to corrosion pitting:

The chart below is from Timkin Practical Data for Metallurgist.and shows endurance (fatigue) strength for various surface conditions. If we use the polished Lycoming connecting rod in the picture above and assume that the strength is 120 psi for a polished surface. With corrosion pitting the endurance strength drops from 70 psi to below 20 psi.

Lycoming Connecting Rod Corrosion

Read the note at the bottom!

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