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Thermal forces can be even higher than tensile forces on the piston.
Forces are compressive, caused by the difference in expansion rates from the piston powl to outside:
1, Bowl rim is hot, while outside is cooler, cause material to yield in compression.
2, The yielding in compression is OK, until the piston cools and tries to return to original shape. This cause cracks to form the tension in the thrust axis, but does not break the piston apart.
3, The crack are typically small, but continue radiating around the bowl rim to relieve the tension from the ongoing hot/cold cycles.
4, Thermal cracks do not propagated because they are just mechanisms to relieve tension. Once the tension is relieved by the crack, the forces are balanced.
5, Eventually, as the cracks move around the bowl rim, they will get to the pin axis. Once there they can crack the piston apart. |
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