In order to introduce a new structural component into service, it is imperative to reasonably pinpoint its expected structural performance and failure loads in order to determine required real world usage characteristics. The component must, as a single independent unit or as a part of a bigger whole, perform its intended function throughout its lifetime. To determine its structural integrity, a proof load test is done on a sample in a controlled environment.
Proof Load Test – A Brief Description
The underlying philosophy of a proof load test is that if the actual load conditions on the structure can be simulated in a controlled environment on the sample being tested, the sample can be pushed to its limits and can be studied meticulously to predict its performance in the real world. Once found, the failure-inducing load can be compared to the estimated actual load, and a decision can then be made as to whether or not to employ additional structural components to meet the expected load.
Performing the Test
In the case of structural wires and ropes, a sample of wire is laid in a test bed. This test bed basically uses hydraulic machinery to apply loads to the sample. Thus, the sample wire is held fixed at one end and pulled at the other end. Another instrument measures the elongation of the wire while it is being pulled. The measured quantities – the pulling force and the corresponding elongation – are then used to calculate the stress and its corresponding strain. Many such pairs of values are obtained. They are then used to plot a graph which gives the behavior of the sample.
The strain – a measure of elongation – must be below a certain limit at the design load, which is the actual load that the wire will have to withstand. If this limit is reached or exceeded, the sample is rejected. The sample will, in that case, yield while in service. Yielding refers to the phenomenon when the component will keep elongating under load and deform the entire structure as a whole. Since such a structure will never be fit for use, the sample must be rejected.
As a practical method of determining the safety and serviceability of the structure as a whole, and the component in particular, performing a proof load test is an indispensable tool in the engineer’s hand.
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