Where I come from the ISUZU FSR 33H is the preferred truck for transporting heavy construction materials. The truck is capable of hauling loads in the order of 6 to 7 tonnes. However I cannot help but notice the high frequency of rear axle shaft breakages on the trucks. The most commonly reported cause of the breakages is steep uphill gear shifts. Some dread uphill shifts so much that they stop completely and start from dead stops with the assistance of the handbrake or a wheel chock. The other alternative used on very steep hills is to anticipate the loss of torque and shift into first right at the bottom of the hill. What might be causing these breakages? How can it be avoided?
Stress causes the failures. Stress may also cause fatigue cracks prior to fracture. Use a stronger material and/or a larger diameter axel. Also any notches / diameter changes are critical as they are stress concentrators. A simple one time over stress will produce a flat fracture, 90 degrees to the axis. A fatigue failure will mostly make a spiral path along the axis. Fatigue tends to indicate a stress concentrating notch. As noted , any shock loads will increase the stresses. Also ; torque stress cause spiral fatigue path in a shaft while bending stresses cause a flat fatigue path ( 90 degrees to the diameter.)
Based on my experience this is down to poor clutch control and allowing the re-engagement to be « snatched » causing a high shock loading through the drivetrain which causes the weakest point (half-shaft) to break.
For me, getting in the right gear, and possibly staying in it, is the best solution.