Measurement of the true in service stresses

Strain Gauging
This Tech Tip follows closely from the previous one in which the importance of accurate evaluation of the stresses in engineering design was highlighted. The sentiments expressed in this previous Tech Tip, focussed on the importance of the reliable estimation of loads and theoretical analysis of stresses during design and comparison of these estimates, to actual stresses measured experimentally during operation. The arguments presented showed that the results of theoretical stress analysis (Finite Element Analysis (FEA)) and experimental measurement (strain gauging) are complementary and are both required.

These contentions are well illustrated by a number of cases in which Origen has been involved.

Drill pipe on offshore drilling rig
Although a number of models had been developed to evaluate the stresses induced in offshore drill pipe, and these models highlighted high bending stresses were likely to be developed in the pipe adjacent to the bit, the effect of dynamic loading was difficult to determine. Measurement of the strain induced at various depths below the surface highlighted the effect of engagement with the seabed and the nature of the seabed topography had a significant effect and stress spikes of up to 40% of yield were being induced in the material. Surprisingly, the strain gauging exercise highlighted that although these stresses were largest near the interface with the seabed, stress waves moved along the length of the drill string causing dynamic interaction. Furthermore, despite a nominally constant torque being applied to drive the system, the torsional stresses in the drill pipe were highly cyclic and ‘stick- slip’ torque reversals were common, suggesting the drill string was being wound up and was subsequently unwinding as frictional forces on the bit changed.

None of these effects had been evaluated in the original models and only became fully understood after post measurement FEA modelling allowed the strain gauging data to be extrapolated through the length of the drill string.

Wind Loading.
Wind loading on structures is often not properly assessed/analysed, and in cases can induce high stresses with many repetitions – the consequences of which can be both expensive and catastrophic. One unique case involving a sports stadium comprising an annular structure supported by, and bolted to, a pre-stressed steel cantilevered framework. This annular section of the roofing together with its framework fluctuated in the wind causing significant stresses, particularly in the bolts interconnecting the framework. Unfortunately these were not designed as ‘flexible joints’ and under windy conditions the bolts were subject to substantial fluctuating (i.e. fatigue) stresses, sufficient to cause fatigue cracking and failure in certain regions of the roof structure. The high stress fluctuations in these roof bolts were confirmed by replacing them with nominally identical strain gauged bolts and monitoring the strain as a function of wind direction and speed. This strain gauging exercise showed clear correlation with wind speed and direction. Once understood, the problem could be rectified by relatively simple modifications to the support structure.

Lateral loading inducing torsion.
Often unanticipated sideways loading can occur in in engineering structures such as excavators, cranes and dredgers, where dynamic lateral loads can occur, particularly if operators are trying to increase productivity by slewing the boom at high speed to effect the next loading cycle. Although the boom and stick of excavators are designed to accommodate considerable bending loads, torsion induced by these loads can cause premature cracking at the hinge joints between both the boom and the stick as well as the boom and the body of the excavator.

In one particular case an excavator had been fitted with an extended boom and stick to extend its reach and facilitate river cleaning/dredging operations. Unfortunately, the purpose built boom and stick suffered severe cracking of the welds and side plates of the joint between the stick and the boom soon after the excavator had been purchased. Strain gauging of the stick, boom and hinge side plates showed that large magnitude cyclic stresses were induced when slewing with the stick at acute angle to the boom after a full bucket of weed (mud and garbage) had been recovered from the water-way.

These stresses were induced by forces associated with the rapid deceleration of the mass of the weed (bucket and stick) at the end of the slew cycle that were transmitted through the stick into the boom hinge joint as a torque. Although such forces are not atypical for an excavator, they were exacerbated in this case by the lever arm of the extended stick. Once the problem had been identified, limiting the maximum slew speed allowed the stresses to be reduced to below fatigue threshold limits.

Valve Stem Failure:
In an attempt to understand ongoing failures in the stem of remotely actuated valves in the cooling system of a power plant at surprisingly low stresses, the valve stems were strain gauged to measure the axial, bending and torsional stresses induced during closure. The results of the strain measurement exercise highlighted, as per the design calculations, the axial and torsional stresses induced in the stem were well below those required to cause fatigue failure. However, the results also highlighted that although the stresses were nominally due to axial and torsion loads as expected, large bending stresses were also being introduced. More detailed inspection aimed at ascertaining the cause of the bending loads showed that the original lock nut only engaged one and a half threads so, with normal tolerances. As such, the lock nut could ‘rattle’ on the shaft, and under high speed closure (in milliseconds), as was the case, the Belleville washers actuating closure could ‘lock up’ or ‘seat’ slightly eccentrically. The net effect was a slight but significant bending load being applied through the Belleville washers and the actuator housing. On the valve on which the stress measurement was undertaken this bending loading was in the vicinity of 150MPa, which was more than sufficient to cause fatigue crack initiation and propagation into the actuator shaft. Having identified the source of the bending stress, it was clear that the loading could be negated by extending the length of the lock-nut. Testing after modification, showed the bending stress had been reduced to approximately 25MPa which was well below the fatigue threshold levels (stress required to cause fatigue cracking).

These selected examples (of many) illustrate how certain loading cases introduced in components/structures are sometimes not be considered in design and may have negative outcomes for the component or structure. Good stress analysis is essential to ensure that the design can meet the engineering requirements and it is vital that the analysis not be over simplistic and that key load cases are not omitted. However, even with hind sight many loading scenarios may be difficult to model. Furthermore, the designer is often limited as to which load cases can be modelled within the constraints of time, budget and capability/capacity. However, the examples illustrate that measurement of actual stresses using strain gauging techniques can be readily applied to determine loading, qualify a design/FEA, and when assessing failure – a scenario which every designer, operator and owner hopes can be avoided!