There's a book for Engineers, entitled: 'Roark's Formulas for Stress and Strain', some describe it as 'High level Mathematics'. So what?

  • $\begingroup$ Stess = tensión. Strain = deformación. But this is not exactly an aviation question. $\endgroup$ Jan 23, 2020 at 12:05
  • $\begingroup$ This is a pure physics question and should be asked on physics.stackexchange instead. $\endgroup$
    – Bianfable
    Jan 23, 2020 at 15:51
  • $\begingroup$ No, the book cited is for aeronautical engineers. Thanks. Salut + $\endgroup$
    – Urquiola
    Jan 24, 2020 at 12:23
  • $\begingroup$ @Urquiola what makes you think that structural mechanics in a plane are different from those in a bridge? Or algebra? Or general physics? Roark, Timoshenko, Fung, etc are part of the recommended bibliography for many engineering programmes, be they Mechanical, Civil or Aeronautical. $\endgroup$ Jan 24, 2020 at 12:57
  • $\begingroup$ Despite the fact that the book is for aeronautical engineers, this is a physics question, not an aviation question. After all, aeronautical engineering involves physics. $\endgroup$ Jan 24, 2020 at 16:33

2 Answers 2


Stress is the load divided by the cross section of the material bearing it, and is expressed in units of pressure.

Strain is deformation divided by the original dimension in the direction of the deformation, and therefore is dimensionless.


Underneath the formal definitions, the intuition is

  • stress is what you do to a material;
  • strain is what it does as a result.

To a first approximation they are connected by Young's modulus, which obviously varies a great deal by the material.

You can put two materials under the same stress (one industry standard elephant, say) and the the strain will be very different depending on the material (steel might exhibit less strain than rubber, for example).

Alternatively, the same strain (eg 10% lengthening) is achieved with different stresses depending on the material (eg stress of one mouse per square inch for rubber, stress of one elephant per square inch for steel).

As you can tell from the intuition above, stress is measured in pressure (force per area) and strain is dimensionless (just a proportion or percentage, or whatever).

  • 2
    $\begingroup$ Stress is the cause and strain is the effect... And stress/strain is Young's modulus, that is a measure of the impedance of the material. In the same way as, in electricity, impedance, as a general term for resistance, is tension/current, i.e., cause/effect... $\endgroup$
    – xxavier
    Jan 23, 2020 at 13:37
  • $\begingroup$ I don't think I can agree with this answer. Sometimes what I do to a material is stretch it to a particular length, and what it does as a result is exert a particular amount of tension. Tuning a guitar is a good example of this, I think: you adjust the strain in order to produce the desired stress. $\endgroup$ Jan 24, 2020 at 16:30
  • 1
    $\begingroup$ @TerranSwett You are confusing cause and effect; you adjust the strain by regulating the load, and therefore stress, applied. What you are saying is that you adjust the force of a spring by stretching it to a set length... well, in that case I'd ask how did you make it stretch in the first place. $\endgroup$ Jan 24, 2020 at 21:03
  • $\begingroup$ @AEheresupportsMonica The causation goes both ways, doesn't it? For simplicity, imagine two balls that have opposite electric charges and which are stuck to each other. If I grab the two balls and apply an outward force to them, they will separate—stress causes strain. At the same time, however, the fact that the balls are separated causes them to exert a net force on each other—strain causes stress. It isn't an infinite regress of causation, of course, but it is a feedback loop. I don't think it's correct to label one as "the cause" and one as "the effect." $\endgroup$ Jan 25, 2020 at 0:21
  • $\begingroup$ @TerranSwett that was the case which I addressed in the penultimate paragraph. $\endgroup$
    – Dannie
    Jan 27, 2020 at 14:50

Not the answer you're looking for? Browse other questions tagged .