Select equipment conservatively and prevent unexpected failure
In our line of business, we are exposed to a wide range of very interesting failures. We also encounter certain types of failures repeatedly, as end users are unaware of the effect of certain load conditions on the performance of their component or equipment. These types of failures are particularly concerning since the fundamental understanding is well known, and is well taught at universities and colleges, but design mistakes and/or misinformed plant selection still occurs. A typical example of this concerns the understanding - or lack of it - of repeated loading and fatigue conditions. It is well known that fatigue failure accounts for between 75 to 80% of all failures, and technically the effect of cyclic stress level, stress concentrations (notches, holes, cracks etc.) is well understood, yet failures still occur surprisingly often.
Environmentally Induced Fatigue Failure
Although the effect of flaws and stresses is often considered by designers the effect of environment is sometimes overlooked. In adverse environments such as those experienced in the chemical, marine and biomedical fields the operating environment may cause failure from flaws which would otherwise not propagate.
Can high strength bolts be galvanised?
Bolts can be galvanised to good effect to prevent corrosion but one should be aware of the effects of hydrogen embrittlement when galvanising high strength bolts.
Understand thermal fatigue or risk the potential for failure
Repetitive thermal loading on a localised basis can lead to thermal fatigue and extensive surface cracking. Although relatively rare, this phenomenon can have catastrophic consequences because it is generally not well understood.
Ignore sources of atomic hydrogen that can promote hydrogen embrittlement at your peril!
Understanding the common sources of hydrogen can help assist in preventing the premature failure caused by the insidious effects of hydrogen embrittlement.
Beware of the effects of hydrogen embrittlement!
The ductility of components particularly those manufactured from high strength material can be severely compromised by hydrogen embrittlement mechanisms causing them to fail with a short period in service.
Changes in toughness associated with the operating temperature or material condition can lead to unexpected performance
Lowering the toughness of a material by changing the material composition/grade or unintentional heat treatment can lower the toughness of the material at the operating condition to promote brittle behaviour - causing unexpected failure
Beware of the insidious efforts of residual stress which can have significant effect on fatigue performance!
In cyclic loading conditions, tensile residual stresses superimpose on the applied cyclic stresses and depending on the magnitude of the residual stress can promote failure at design stress levels.
Benefits of surface finish and shot peening
In cyclic loading conditions failure by fatigue can be prevented / delayed by employing surface finish techniques like shot/laser penning.
Beware of surface damage to electroless nickel plated components
Choose the right coating for the job - flaws in electroless plated nickel coatings can cause failure of the underlying component.
Understanding the relationship between sheave size and rope life
Inappropriate selection of the sheaves can lead to premature failure of wire rope by fatigue mechanisms.
Don't underestimate the complexities of bolting - Reap the benefits of reduced bolt stiffness
Gaskets, dirt, irregular/distorted member contact faces reduce member stiffness and can help promote failure by fatigue mechanisms.