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Fatigue testing

Fatigue as a specific failure mechanism has been recognised since the early part of the nineteenth century but it was the development of rail travel that resulted in a major increase of interest in this type of fracture.
The premature failure of wagon axles led to Wohler in Germany investigating fatigue failure under rotating loading. This led to the design of the first standardised test - a reversing stress rotating specimen, illustrated in Fig.1.

Fig.1. Wohler rotating fatigue test

There are many mechanisms that can lead to failure but fatigue is perhaps one of the most insidious since it can lead to a catastrophic failure with little or no warning - one well known example being the failure of the Comet aircraft in the 1950s.
Failure can occur at a fluctuating load well below the yield point of the metal and below the allowable static design stress. The number of cycles at which failure occurs may vary from a couple of hundreds to millions. There will be little or no deformation at failure and the fracture has a characteristic surface, as shown in Fig.2.

Fig.2. Typical fatigue crack fracture surface

The surface is smooth and shows concentric rings, known as beach marks, that radiate from the origin; these beach marks becoming coarser as the crack propagation rate increases. Viewing the surface on a scanning electron microscope at high magnification shows each cycle of stress causes a single ripple. The component finally fails by a ductile or brittle overload.
Fatigue cracks generally start at changes in section or notches where the stress is raised locally and, as a general rule, the sharper the notch the shorter the fatigue life - one reason why cracks are so damaging.
There are two stages in the process of fatigue cracking - a period of time during which a fatigue crack is nucleated and a second stage where the crack grows incrementally leaving the ripples described above. In an unwelded component the bulk of the life is spent in initiating a fatigue crack with a shorter period spent in crack propagation.
An unwelded ferritic steel component exhibits an endurance limit - a stress below which fatigue cracking will not initiate and failure will therefore not occur. This is not the case with most non-ferrous metals or with welded joints - these have no clearly defined endurance limit.
The reason for this is that in arc welded joints there is an 'intrusion' - a small defect at the toe of the weld, perhaps only some 0.1mm deep. Provided that the applied stress is sufficiently large a crack will begin to propagate within an extremely short period of time. The endurance limit for a welded joint is therefore dependent on the intrusion size that does not result in crack propagation at the applied stress range. In the case of a welded joint, therefore, a fatigue limit - a 'safe life' is specified, often the stress to cause failure at 2x10⁶ or 10⁷ cycles.
During fatigue the stress may alternate about zero, may vary from zero to a maximum or may vary about some value above - or below - zero.
To quantify the effect of these varying stresses fatigue testing is carried out by applying a particular stress range and this is continued until the test piece fails. The number of cycles to failure is recorded and the test then repeated at a variety of different stress ranges.
This enables an S/N curve, a graph of the applied stress range, S, against N, the number of cycles to failure, to be plotted as illustrated in Fig.3. This graph shows the results of testing a plain specimen and a welded component. The endurance limit of the plain specimen is shown as the horizontal line - if the stress is below this line the test piece will last for an infinite number of cycles. The curve for the welded sample, however, continues to trend down to a point where the stress range is insufficient to cause a crack to propagate from the intrusion.

Fig.3. S/N curves for welded and unwelded specimens

By testing a series of identical specimens it is possible to develop S/N curves. In service however, there will be variations in stress range and frequency. The direction of the load may vary, the environment and the shape of the component will all affect the fatigue life, as explained later in this article.
When designing a test to determine service performance it is therefore necessary to simulate as closely as possible these conditions if an accurate life is to be determined. In order to enable the fatigue life to be calculated when the stress range varies in this random manner, the Palmgren-Miners cumulative damage rule is used.
This rule states that, if the life at a given stress is N and the number of cycles that the component has experienced is a smaller number, n, then the fatigue life that has been used up is n/N.
If the number of cycles at the various stress ranges are then added together - n₁/N₁ + n₂/N₂ + n₃/N₃ + n₄/N ₄etc - the fatigue life is used up when the sum is of all these ratios is 1. Although this does not give a precise estimate of fatigue life, Miners rule was generally regarded as being safe. This method, however, has now been superseded with the far more accurate approach detailed in the British Standard BS 7608.
The design of a welded joint has a dominant effect on fatigue life. It is therefore necessary to ensure that a structure that will experience fatigue loading in the individual joints has adequate strength. The commonest method for determining fatigue life is to refer to S/N curves that have been produced for the relevant weld designs.
The design rules for this range of joint designs were first developed by TWI and incorporated with the bridge code BS 5400 in 1980 and then into the industry design rules for offshore structures. Further refinements and improvements finally resulted in the publication of BS 7608 Code of practice for fatigue design and assessment of steel structures. This standard will be looked at in more detail in a future article.
This article was written by Gene Mathers.
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