The presence of an operator is not required to identify an evident failure (tyre
threads, oil leaks), but a hidden failure needs a second event to be identified. For
example, to confirm whether the brake or reverse lights work, the car driver needs as
assistant standing behind the car, when he carries out the required manoeuvre.
20.1.5 Incipient Failures
When deterioration occurs gradually over a time period, the start of deterioration will
be noticed at some time. Incipiency indicates the point at which failure starts. As the
deterioration evolves, the moment of functional failure is obtained when the performance is no longer tolerable. The time from the beginning of incipiency to functional
failure is called the incipiency interval. With evident failures exhibiting incipiency, it
is possible to forecast when functional failures are likely to occur.
20.1.6 Incipiency
Incipiency can occur at several levels. There are small components, such as light
bulbs, ball bearings, where the failure mechanism may progress rapidly. Failure is
usually initiated by deformation or fatigue arising from mechanical or thermal
stresses, or some other reason. The deterioration rate may also be much slower,
varying from weeks to years.
Consider a road comprising a small surface defect or unevenness arising from
poor finishing. As vehicles repeatedly drive over this defect, the tyres go into the
depression and then climb out to the original level. This creates an impact load on the
road surface as well as on the vehicle suspension. This road impact causes it to be
further damaged, creating a deeper depression. The next vehicle then meets a bigger
depression, and creates still further damage to the road. If repairs are not carried out,
the depression ultimately turns into a pothole, rendering it unsafe to drive on this
portion of the road. Figures 20.2a–c below illustrate this sequence of events.
The time when the initial defect is noticed indicates the beginning of the incipient
failure, denoted by point x at time t i , in Fig. 20.3 below. The slouching of the curve
shows the pothole growth rate. Eventually, the road becomes unsafe for use. The
norm used to decide the road’s suitability depends on the operating context. If the
vehicles drive at high speed with heavy loads, the acceptance standards will be more
stringent. The dotted lines indicate the relative acceptability levels, which depend on
road speeds and loading.
At the point y at time t f , where the acceptable level (dotted) line meets the road
surface condition curve, it is no longer safe to drive on the road. In other words, the
road has failed. The time during which the road condition has deteriorated from x to
y, i.e., t f – t i , is the incipiency interval.
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20 Human Resources Management for Infrastructure
threads, oil leaks), but a hidden failure needs a second event to be identified. For
example, to confirm whether the brake or reverse lights work, the car driver needs as
assistant standing behind the car, when he carries out the required manoeuvre.
20.1.5 Incipient Failures
When deterioration occurs gradually over a time period, the start of deterioration will
be noticed at some time. Incipiency indicates the point at which failure starts. As the
deterioration evolves, the moment of functional failure is obtained when the performance is no longer tolerable. The time from the beginning of incipiency to functional
failure is called the incipiency interval. With evident failures exhibiting incipiency, it
is possible to forecast when functional failures are likely to occur.
20.1.6 Incipiency
Incipiency can occur at several levels. There are small components, such as light
bulbs, ball bearings, where the failure mechanism may progress rapidly. Failure is
usually initiated by deformation or fatigue arising from mechanical or thermal
stresses, or some other reason. The deterioration rate may also be much slower,
varying from weeks to years.
Consider a road comprising a small surface defect or unevenness arising from
poor finishing. As vehicles repeatedly drive over this defect, the tyres go into the
depression and then climb out to the original level. This creates an impact load on the
road surface as well as on the vehicle suspension. This road impact causes it to be
further damaged, creating a deeper depression. The next vehicle then meets a bigger
depression, and creates still further damage to the road. If repairs are not carried out,
the depression ultimately turns into a pothole, rendering it unsafe to drive on this
portion of the road. Figures 20.2a–c below illustrate this sequence of events.
The time when the initial defect is noticed indicates the beginning of the incipient
failure, denoted by point x at time t i , in Fig. 20.3 below. The slouching of the curve
shows the pothole growth rate. Eventually, the road becomes unsafe for use. The
norm used to decide the road’s suitability depends on the operating context. If the
vehicles drive at high speed with heavy loads, the acceptance standards will be more
stringent. The dotted lines indicate the relative acceptability levels, which depend on
road speeds and loading.
At the point y at time t f , where the acceptable level (dotted) line meets the road
surface condition curve, it is no longer safe to drive on the road. In other words, the
road has failed. The time during which the road condition has deteriorated from x to
y, i.e., t f – t i , is the incipiency interval.
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20 Human Resources Management for Infrastructure