Stress, a familiar feeling to all of us, it makes people exceed their normal performance limits, or incapacitates until depression.  How can it be explained that the same phenomena can cause individuals to experience things in different or even in opposite ways? People are different from one other, and variety of circumstances is infinite.

Let’s think about two stressful situations!

Case 1: One morning the supervisor of shot peening is in his office after having quarrelled at home over breakfast. His wife dented his car when coming home from an expensive beauty parlor, and an arrogant teenager ignored his parental concern about coming home too late the night before. He reads a complaint from his boss that a previous shot peening lot was faulty and has caused significant loss of money and if the new lot is not properly peened by the end of the day, our supervisor would be free to look for a new employer.

Case 2: A young gentlemen has been chatting through Facebook with a girl. Both share similar senses of humor and areas of interest. Finally they meet and realize that both are keen young shot peeners. The young lady wants to introduce the young man to her new shot peening machine, equipped with the state of the art dust collector, and maybe they will test the system together that same evening. Quite a stressful proposal indeed, this makes our young friend eager to do his best when the time comes.

Two stressful situations, prediction of the first case is unpleasant, detrimental, while latter is pleasant and possibly beneficial.
Now we want to measure directly the actual stress levels of both individuals, regardless of whether the stresses are pleasant or not. Can we make such a measurement? Short and simple answer is:  No, we cannot! Even though everyone is absolutely aware that such stresses exist without doubt; a short and simple explanation is that there is no such measurable quantity as stress. Something else must be measured!
Let’s leave our stressed gentlemen for a while, and give a thought to some fundamental principles of mechanics, such as;

• Conservation of mass the mass of the body is constant (definition of the body)
• Equilibrium of linear momentum
• Equilibrium of the angular momentum
• Conservation of energy, i.e. the 1st law of thermodynamics, the perpetual motion machine, "perpetuum mobile", is impossible. In other words, energy can change its appearance, but it will not disappear or appear on any macroscopic scale
• Increase of entropy. Every system will spontaneously try to minimize its energy, which is why things break, i.e. the 2nd law of thermodynamics. No one has ever seen a macroscopic structure spontaneously repair without any energy input.

These principles of mechanics must be kept in mind when considering possible methods for measuring any mechanical phenomena.
If we think about energy, there are several different forms of energy; kinetic, thermal, chemical, electric, etc., even mass can be considered a form of energy. There is no absolute measure of energy; it is the work that one system does on another system. All of the above mentioned energies can be expressed with exactly the same unit, even though their obvious appearance is totally different. So, energy must be an abstract quantity and it is.
Mechanical stress is also a quantity of energy; it is the elastic energy in a body. The importance of stress is that crack propagation is release of elastic energy, the elastic energy transforms in to heat and new free surface formation.  If we want to measure this abstract quantity, there is no other method but to measure another value, as we learned from the stressed people mentioned earlier, that is somehow related to the stress. As an example, we can measure the change of the electrical resistance of the strain gage, which in turn is proportional to the strain of the body or we can measure interatomic displacements by measuring the angle of diffracted X-ray or neutron radiation, etc. From these measurable quantities it is possible to derive the stress and those measurable quantities must be proportional to stress.
When applying an external load to a body, all these measurements are relatively easy to make because there is an apparent change to the stresses that the body must bear. When considering residual stresses the situation is totally different even though the body experiences exactly the same kind of stresses as an external loading. Residual stresses are locked into the body, being in equilibrium (according to fundamental principles of mechanics) and in that sense they are both “invisible and stored” energy. Without making any major misjudgement, residual stresses can be defined as an uneven distribution of dislocations.  By definition, if the residual stresses are not in equilibrium then some sort of motion, or strains, would exist. Strains are actually redistribution of dislocations and certain measurement techniques are based on their motion. For making residual stresses "visible", there are both destructive and nondestructive methods available.
Destructive methods are based on the fact that when making new a free surface, by drilling or cutting, stresses must be reorganised in order to minimize their elastic energy. When this takes place, strains can be observed.
By using non-destructive methods, it is possible to measure; spacing of crystallographic planes of the material, velocity of sound, magnetic properties of the material, and so on, when such properties are proportional to stress.
So it can be said that those interesting but unmeasurable stresses can be calculated or derived from some other measurable quantity, and this is done every day everywhere in the world. The motivation to measure stresses is that an enormous amount of money, as well as human lives are lost every year due to excessive stresses.
Let’s not forget our poor supervisor and excited young shot peener. Both are experiencing quite high stresses, but how could we measure the stress levels? How about blood pressure, hormonal balance, and analysis of their blushing, sweating, or some other value that is proportional to their residual stress?
This is how nature works.

For questions contact juha@mfn.li