A “Toolbox” for Forensic Engineers

74 Forensic Materials Engineering: Case Studies
crane weigher. The weigher registers 2 t. The intention is to show how good
the glue is. But let us estimate the stresses in the various links comprising
this load transfer path, first to discover which is likely to fail first if the load
were increased up to the point where something would break and then to
investigate differences if a defect were present in one or another of the
component members of the linked assembly. For the purpose of this exercise
we shall ignore the weight of the straps, the wooden blocks and the chains,
as these would be relatively insignificant compared with the 2 t being lifted.
We take the order of mechanical strength of each of the three components
to be:
Component
Nylon straps
Four, each 100 mm ¥ 2 mm
Total cross section = 800 mm 2
Steel chain
Two limbs each 50-mm 2 area
Total = 100 mm 2
Adhesive spread over blocks
50 mm 2 = 2500 mm 2
Stress (N mm –2 )
under 2000-kg
Load
Mechanical Strength of
Material (N mm –2 )
25 Tensile failure 75
200 Yields at 400
Tensile fracture 650
8 Fails at 15
While the 2000-kg mass is off the ground it applies a force of 20 kN (near
enough) acting on all components in the load transfer path (including those
inside the weigher) which is reacted to by the crane or whatever is supporting
the weighing machine. On the 2500-mm 2 area of adhesive this generates a
stress of 8 N mm –2 . The total cross-sectional area of the four 100-mm ¥ 2mm
nylon straps is 800 mm 2 , so the mean stress when supporting the load
of 20 kN is 25 N mm –2 . Every chain link comprises two limbs totaling 100
mm 2 cross-sectional area, so the mean stress in each limb is 200 N mm –2 .
Joints made with this adhesive are known to fail at stresses in the region of
15 N mm –2 , so the joint is, in fact, by far the weakest link in this load transfer
system. It does not fail (in the short time, at least) because the stress acting
is only just over one half this level. Its weakness relative to the other members
in the system is not immediately apparent due to the much greater crosssectional
area of the wooden blocks. However, if the applied load were
increased this is where the failure should occur because the chain links and
the nylon straps are working well within their limiting stresses and neither
would be expected to show any kind of mechanical distress, although they
would stretch elastically under the stress. Elastic deformation is recovered
when the force is removed.