Fatigue Life Assessment:

MVC can offer fatigue life assessment of piping and structures at critical joints.

The problem with vibrating structures is the possibility of a fatigue failure or damage to plant, machinery or piping attached to the structure. This is made worse if a resonance condition exists, i.e., where the loading frequencies are close to one or more of the structure’s natural frequencies and dynamic magnification occurs. If the plant has run for some time - even if it is only days in some instances - it may be possible to determine if the structure is past its 'endurance' limit from the frequency of vibration. If this is the case, the structure will be in the 'indefinite fatigue life region' on the Stress vs Number of Cycles to Failure curve for the material (S-N curve). This will indicate that the structure will not fail by fatigue unless stress levels increase or additional stress concentrations are introduced to the structure. This simplistic approach applies to joints as fatigue failures do not generally occur in the parent material remote from joints.

In bolted or riveted joints, fatigue cracks will normally initiate from the bolt or rivet hole. In welded joints, cracks can initiate from the parent metal directly adjacent to the weld, or in the weld metal such as the root, the weld surface or at an internal flaw. However, as previously stated, this is a simplistic approach and the safest method of evaluation is to determine stress levels by using strain gauges.

If the structure has not exhausted the required number of stress reversals corresponding to the non-propagating stress level, then strain gauging may be necessary. If required, modal analysis can be carried out to determine mode shapes and hence the areas of maximum stress where the strain gauges will be installed. From strain readings, the number of cycles for each individual stress range can be determined. The objective is to evaluate fatigue life by determining the long term distribution of stress ranges by taking into consideration all stress variations which can reasonably be expected during the life of the structure which have magnitudes large enough to cause fatigue effects.

Strain gauges give surface stresses which is satisfactory for two dimensional or axisymmetric stress fields but if the stress fields are more complex, the standard analytically based solutions may not be sufficient. Three-dimensional methods can then be used such as Finite Elements for a full understanding of the stress patterns. See Finite Element (FE) Analysis.