Part of the investigation is to determine where the crack initiated. That will give a clue to the load that caused it.
Remember, crack propagation is a function of steady and unsteady stress.
Steady stress contributors are: gravity, fastener loads, engine torque and impulse loads.
Unsteady stress contributors are: clutching, torque cycling, and engine vibration.
There are 2 types of fatigue: LCF (low cycle fatigue) and HCF (high cycle fatigue)
LCF failure is up to 100,000 cycles give or take a decade and is related to engine loading cycles.
HCF failure is way above 100,000 cycles because the unsteady stress is usually harmonic (sinusoidal), caused by vibration. Since vibration is high frequency, the cycles to failure are orders of magnitude greater.
In both cases, the more steady stress, the LOWER the ability to handle unsteady stress.
Many times, cracks originate at casting defects such as dross or voids. However, a sharp notch on the surface can also initiate a crack (that is why fillets are essential).
Call some T56 vendors.
My guess is that you had a LCF failure initiated at a defect.
I saw a fuel hemi block that was almost split in half, the front and rear faces of the block were all that were holding it together. The 3 internal webs/bulkheads were cracked in half. The crack originated at the oil hole on the main bearing bores, spread upward, into the came bore, then out the lifter gallery.
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Where'd you get yer body shims? or did you make them?
