A “Toolbox” for Forensic Engineers

Fluid Transport 189
Figure 6.13 Close-up of side of the fracture surface with flaps due to crack
branching, from which crack growth direction could be inferred.
Figure 6.14 Close-up of the crack tip showing weld lines ahead of the crack.
lines are indicative of cool tools, so that flow of molten polymer in the mold
tool is inhibited. The fact that the fitting had been screwed tightly to produce
a closure stress on the threads was indisputable as Figure 6.15 shows. So what
effect would such weld lines have when the thread was screwed tightly by the
plumber?
6.4.3 Water Pressure
Tiny weld lines could concentrate the tightening load if any were situated at
the root of a thread. One common observation of failures from screw threads
is that failure is often situated at the next-to-last thread, a feature confirmed
by theoretical analysis of the stress-concentrating effects of thread forms.
Thread failures are themselves rather common simply because threaded joints
and connections are some of the most common ways of joining components
together. However, there was no evidence of an excessive closure stress made
by the plumber when finally tightening the joint, and the cause of the failure