THE SEARCH FOR ZERO IN AN UNBALANCED WORLD
We often talk about zero and balance as if they are the same thing (in the same way that stress and strain are often incorrectly transposed), but for measurements this can lead to incorrect decisions regarding structural integrity. Let’s explore what we mean by these terms, and how we can use them to better understand our measurements.
Setting an instrument to zero is merely a convenience for the measurement engineer to know how a component responds to the service loads, and to evaluate if the structure returns to the same position (“zero return”). In reality the instrument is zeroed to remove the initial state of the structure.
All structures are already under some sort of load – residual stresses from manufacturing such as welding, shot peening or cold-expansion of holes, assembly stresses from bolting and riveting, or simply the effect of gravity. Ignoring this existing state can be the difference between long-term structural integrity or catastrophic failure! Further testing can be done to investigate the assembly stresses (dismantle/reassembly) or residual stresses (for example, the hole drilling method to ASTM E837). Combining these methods gives the engineer a true understanding of the structure.
A good example of the difference between zero and balance can be found in a traditional kitchen balance scale. If we put a 1 kg weight on one side and put 1 kg of produce on the other the pointer indicates zero, in other words the scale is balanced. Yet is it obvious to a casual observer that there is 2 kg on the device (ignoring the already-significant weight of the mechanism). Taken to extreme, we could add 50 kg to each side, it would still read “zero” but most likely the device would break!
In real-world testing we most often measure the effects due to service and environmental loading, but ignoring the initial state could result in disaster.
Zero – a true zero state, no stress or load from any source. Not possible in the real world!
Balance – an arbitrary structural state where the instrument will be set to read zero despite existing loads and stresses.
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