All things *WILL* fatigue over time being flexed/bent/etc
All things *WILL* fatigue FASTER at the stress raiser point(s)
Designers were taught that in engineering classes from high school on up
It is 'when' it will fatigue...starts out with micro stress cracks...both
surface and internal
Propagation to become ever larger cracks...till it cracks clean through
Alloy, hardness (whether alloy, working, temp, etc), cross section,
design attributes (designed stress raiser that are then mitigated by
design) factored by the type of force, frequency, amplitude...etc
ALL truck frames and even unibody/Monocoque flex under *ANY* kind of loading
No matter static or dynamic or both
The higher the natural frequency (harmonic) of any given structural
member, the longer it will take to fatigue....factored by amplitude,
which will shorten that time to failure
For a pickup, I'd spec it to have a minimum of TENS OF MILLIONS of
cycles, at max design spec loading for MTBF...during the warranty
period...plus a few for good measure (margin)
Pickups with this kind of load has very low frequency at any load
Increasing the box section and thickness will increase the harmonic
(frequency) vs this very low frequency load
BUT...if the load is over the spec MAXIMUM load...that will toss the
designed MTBF out the window.
Reason is that during the upper end of each load cycle (up and then down)
the material (metal in this case) will go beyond elasticity and into
plasticity
Once will leave it bent and not spring back
Too many times will start to introduce micro stress fractures that
will propagate into ever larger cracks till it cracks clean through
From the image...that trucks 'top' flange of the frame rail was fatigued
more than the bottom. So the bottom was still connected
If both cracked and the web cracked, then separation would occur
It's not anyone OEM is better than the other. Most engineers go through
the same basic courses in college
The main differentiator is the design philosophy of the OEM, eng
department and the biggie here...their bean counter management's
allowance of per-centage design margin or lack of any...
