16-03-2018, 09:10 PM
(This post was last modified: 16-03-2018, 09:21 PM by Bob Culver.)
This is not the ideal heading for a discourse on fatigue but....I have read that in constant load lab tests life is only about 10% remaining after first (microscopic) crack. The real world is very different and from the sheer number of cracked cranks and steering arms these must be in this state for a very large chunk of total life.
As evident from any basic text, a slight increase on load or decrease in the fatigue limit has a huge effect on the cycles to failure. Hence, along with more sustained revs, so many failures since hotting up with raised c.r since the 1960s.
If heating reduces the strength from say 50 tsi to 40 that represents a huge reduction in life cycles at the same loadings.
When testing cranks I start at the back and work forward. Apart from one in the car, I never got beyond number 3! Many intact cranks are brazed so have obviously done a fair mileage. Apart from heating, welds usually show a crack at the edge of the weld
As evident from any basic text, a slight increase on load or decrease in the fatigue limit has a huge effect on the cycles to failure. Hence, along with more sustained revs, so many failures since hotting up with raised c.r since the 1960s.
If heating reduces the strength from say 50 tsi to 40 that represents a huge reduction in life cycles at the same loadings.
When testing cranks I start at the back and work forward. Apart from one in the car, I never got beyond number 3! Many intact cranks are brazed so have obviously done a fair mileage. Apart from heating, welds usually show a crack at the edge of the weld