Fatigue and Fracture Mechanics - Frequently Asked Questions
What Savings Can Be Achieved By Undertaking A Fatigue/Fracture Mechanics Analysis?
Fracture Mechanics, used in conjunction with appropriate non destructive testing (NDT), facilitates quantitative assessment of the potential for catastrophic failure. With rational and appropriate inspection procedures this quantitative knowledge allows the component to be kept in service safely, at least until a scheduled inspection or maintenance period, when it can be repaired or replaced without significant loss of production costs and without running the risks and costs associated with catastrophic failure
A Fracture Mechanics sensitivity assessment of loading cases, often allows particularly detrimental loading cases to be identified, and enables appropriate action to be taken to avoid premature failure. By knowing which aspect of the loading/environment/manufacture is most detrimental attention can be focused on these areas and thus allow the problem to be dealt with in the most cost effective manner.
What Materials Are Affected By Fatigue Failure?
Both non crystalline and crystalline materials are affected by fatigue with metals and polymers being of principal concern. Contrary to popular belief brittle materials such as cast irons and cements often fail by fatigue.
What Loading Conditions Lead To Fatigue?
All loading conditions that cause a cyclic tensile stress above the threshold for a particular material will cause fatigue. These loading conditions include direct loading and those due to thermal cycling. Fatigue failure generally does not occur in compression.
Static stresses such as residual stresses do not contribute to fatigue directly, but may contribute to failure by reducing the threshold for fatigue initiation. Furthermore, when they are tensile in nature (as in welds) static stress may result in compressive cyclic stresses, that would not normally influence fatigue, introducing a tensile cyclic component.
What Affects Fatigue Failure?
Amongst others fatigue crack growth rate/failure is affected by:
- The magnitude and mean value of the cyclic stress applied to the component.
- The frequency and shape of the cyclic stress component.
- The load history - overload of the right magnitude and frequency of application can decrease fatigue crack growth rate.
- Material microstructure and properties.
- The operating environment effects both the crack growth rate and the threshold for fatigue crack initiation.
Is Increasing Toughness Or Reducing Initial Flaw Size More Beneficial In Preventing Fatigue Failure ?
Both flaw size and toughness effect the propensity for both brittle fracture and fatigue. In fatigue the number of cycles to failure is an exponential function of the difference between the critical and initial flaw sizes. For any given initial flaw size increasing the toughness increases the number of cycles to failure, however, since the function is exponential decreasing the initial flaw size by a similar percentage has a significantly more marked effect. As such in a fatigue scenario, time and money spent increasing the toughness of the material can be more profitably used to decrease the initial flaw size.
How Is Fatigue Failure Characterised?
The fracture surface of a fatigue crack is normally flat and often characterised by so called 'clamshell' and 'ratchet' marks. Clamshell marks are semi circular rings radiating from a common point, that are caused by overloads. Ratchet marks are small steps at the edge of the fracture surface that are caused when cracks that initiate on multiple levels join up to form a single crack. Both these features are macroscopic and when doing a failure analysis are often the first pointers to fatigue. The most definite evidence of fatigue failure in ductile materials are striations, microscopic areas of deformation caused during extension of the crack during each load cycle.