rel="nofollow" href="#ulink_4907c1d3-2800-580f-b182-46377658941d">Figures 4.7, 4.8, and 4.9 illustrate the sequence of events.
Figure 4.7 How road surfaces get damaged.
Figure 4.8 Tires “drop” into defect and climb out.
The time when we notice the initial defect is the start of the incipient failure, denoted by point x at time ti in Figure 4.10. The droop of the curve shows the rate of growth of the pothole. At some point in time, this condition becomes unacceptable, as the road is no longer safe to use. The norm used to determine its acceptability is dependent on the operating context. The higher the speed of the vehicles and the greater their loading, the stricter are the acceptance standards. The dotted lines show the relative levels of acceptability, which are dependent on road speeds and loading. At the point of intersection with the curve, indicated by the point y at time tf, it is not safe to drive on the road any longer. In other words, it has failed. The time taken for the condition to deteriorate from x to y, that is, tf – ti, is the incipiency interval.
Figure 4.9 The ‘drop’ energydamages the road further.
Figure 4.10 Incipiency interval (tf – ti).
The second example is of a welded structure, such as a pressure vessel or steel frame of a building. When originally fabricated, some minor cracks would have remained in the welds. At the time of construction, these cracks either escaped detection or were not serious enough to trace and repair. After commissioning the structure,these welds experience loads, which can fluctuate in magnitude, direction, or both. When there are cracks in the welds, the effective cross-sectional area is smaller, resulting in higher stresses. At the tip of the crack (refer to Figure 4.11), the material can become plastic due to stress concentration. The most stressed part of the weld will yield, resulting in the crack propagating further. This raises the stress just beyond this point, ensuring the continuous propagation of the crack. In due course, the crack can grow to such an extent that the weld as a whole is no longer able to perform its function.
Figure 4.11 Crack propagation in a weld.
The incipiency interval may be very short, as in the case of light bulbs, or very long, as in the case of weld crack propagation. A large number of failures have incipiency intervals ranging from weeks to several months or years. Bearing failures, general corrosion, and weld crack propagation are all examples of such failures. Nowlan and Heap2 refer to the point x in Figure 4.10 as the point of potential failure, and the point y as the point of functional failure. Moubray7 refers to it as the P-F curve, where points P and F correspond to points x and y in Figure 4.10. Therange of variance in incipiency is shown in Figure 4.12.
Figure 4.12 Examples of incipiency intervals.
Even in the case of a single failure mode in a given operating context, the droop of the incipiency curve may vary. Thus, there is a range of incipiency intervals, as illustrated in Figure 4.13. This range introduces uncertainty in determining the incipiency interval.
Figure 4.13 Variations in incipiency intervals.
4.7 LIMITS TO THE APPLICATION OF CONDITION MONITORING
When the incipiency is very short, the time available to plan or execute maintenance action is also very small. In such cases, it is difficult to plan replacement before failure by monitoring the component’s condition. When incipiency intervals are in weeks, months, or years, condition monitoring is often an effective way to plan component replacement. Condition monitoring is feasible when it is possible to measure the change in performance, using human senses or instruments. It follows that we cannot monitor hidden or unrevealed failures.
Proponents of condition-based maintenance are correct when they highlight their ability to predict failures. Any predictive capability enhances the decision making process. However they sometimes give the impression that condition monitoring systems will solve all our problems. We know that all failures do not lend themselves to condition monitoring. The failure must exhibit incipiency, it must be feasible to measure it, and the interval must be of reasonable duration. We must always ask the providers of condition monitoring services to demonstrate how they meet these requirements.
4.8 AGE RELATED FAILURE DISTRIBUTION
A system consists of many pieces of equipment, each of which has several components. Each component can fail in one or more ways. In Chapter 3, we looked at the six failure patterns identified by the Nowlan and Heap2 team. You will recall that these failure patterns are plots of the hazard rates against time. Other studies such as Broberg and MSP reported similar results—see Reference 3 in Chapter 3.
Prior to the Nowlan and Heap study, the belief was that all failures followed the so-called bath-tub curve. Their results showed that this pattern was only applicable to 4% of all the failure modes.
Fourteen percent showed a constant failure pattern, and if we ignore the failures that took place early in life, a further 75% also followed this pattern. The remaining 11% (including 4% of the bath tub) of the failure modes exhibited a distinct relationship to age. Should we concern ourselves with this relatively small proportion of failures that exhibit an age-relationship?
To answer this question, we need to know whether any of these failure modes could result in serious consequences. If so, they acquire a new level of respect. With a skewed distribution, a strategy based on an assumed constant failure pattern will not be satisfactory. Therefore, we cannot assume that all failures exhibit a constant hazard rate pattern, as long as any of the remaining 11% matter.
When we assemble components to build equipment, each component failure-mode affects the overall failure rate. These individual component failure-modes may have exhibited a distinct age-related failure pattern. When any failure takes
