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.
Since fatigue cracks frequently originate from the surface of a component as a result of applied cyclic stresses, it is important that the combination of such cyclic stresses and stress raising features are well below the fatigue limit for the component, or else fatigue cracks will indeed initiate and subsequently propagate. In order to prevent fatigue cracking the cyclic stress intensity, ΔK, (combined effect of both cyclic stress and flaw parameters) associated with the stress concentration features needs to be below this fatigue threshold limit, ΔK Thres.
Such fatigue threshold values are available for specific material/environment combinations or can be measured experimentally to facilitate appropriate fatigue design, but can tend toward zero for a number of environments. Furthermore, the environment can also increase the rate of crack growth significantly leading to premature cracking and possibly failure where cracking in more benign conditions would either not occur or would only fail after an acceptable period in service.
A case in point involves an engineer who had had an older type chrome nickel hip prosthesis installed. These prostheses required fixture to the femur by means of what are effectively ‘self-tapping screws’. In locating drilled pilot holes (through the prosthesis) into the femur the surgeon inadvertently created a small drill score mark on the surface of the metal prosthesis. This scratched prosthesis was duly surgically installed and the patient happily walked with/on it for several months. However the stress concentration caused by the drilled score mark on the prosthesis surface was sufficient, together with the normal cyclic bending loads of walking and the harsh environment in the body, to initiate and propagate a fatigue crack through the component at the score mark, leading to complete component fracture and the need for a replacement of the prosthesis. This matter only came to light as the patient requested an analysis of the broken item, which highlighted crack initiation from the score mark. In this case the failure was directly related to the compounding effects of the score and the harsh environment in the human body, removal of either one of these factors would have prevented failure.
Incidentally the medical installation guidelines clearly recognise that if there is any damage to the prosthesis during the operation, the prosthesis should be discarded. In this instance the case was settled amicably.
Published in Technical Tips by Origen Engineering Solutions on 1 March 2016