What does the author mean when he says "by the direct action of the applied load" I essentially want to know what is direct and indirect stress. I think these terms are also used in conjunction with normal stresses apart from shear.
$\begingroup$ to my experience the term direct and indirect stresses are not commonly used, and some times they depend very much on the context. E.g. in the following url direct stresses are synonymous to normal stress (which is not common). So it would be helpful if you presented a bit more on your example. $\endgroup$
Commented Dec 19, 2021 at 18:14$\begingroup$ Agree with you, these terms have ambiguity. I'm a newb so I don't really know what terms engineers use in real life, so at present I just try to understand every word that is written in the book 😅. It's nice though that I'm able to learn that not every word will mean the same to everyone in the world and engineers have to keep that in mind. $\endgroup$
Commented Dec 20, 2021 at 14:18We can use some examples.
$\begingroup$ Thank you kamran, I do get some intuition about the words direct and indirect now. I just want to think of a definition which can define direct and indirect stresses. I came up with this, please let me know what you think about it and if its generalized enough - If the internal resistive forces are all directed along the same direction, then the stress corresponding to those internal resistive forces is called a direct stress. If the internal resistive forces are all not directed along the same direction then the corresponding stress is called an indirect stress. $\endgroup$
Commented Dec 19, 2021 at 15:27$\begingroup$ Hence, if we have an area over which internal resistive forces are developed parallel to the area and all are directed in the same direction then the shear stress is a direct one. if the internal resistive forces are all parallel to the area but are not all directed in the same direction (as in torsion) then the shear stress will be an indirect one. $\endgroup$
Commented Dec 19, 2021 at 15:28 $\begingroup$A direct application of applied load/moment are the reason why we experience anything that happens within the structure (like deformations, strains, stresses etc). If not direct load/moment are applied, then nothing will happen and the body will just remain as it is. So anything that happens (for instance, warping in a rectangular shaft due to torsion, which results in tension and compression stresses as well) are a result of a direct application of load/moment. So nothing can be indirect.
Maybe the author tried to use simple words to not to confuse the audience. When you apply a transverse/lateral load to a cross section, it infers that it is a shear load since it is parallel to the cross section (and by definition, loads parallel to the cross sections result in shear stresses). I guess this is what he meant by direct load. According to this analogy, the tension and compression stresses due to warping, and even bending stresses due to transverse/lateral loads are all due to indirect loads, i.e. arising due to torsion and due to transverse/lateral loads respectively. Since it was initially taught that only an axial force could result in axial stresses (like tension and compression).
Moreover, you might also exprience shear stresses when you fix a beam on one side and apply axial force on the other. This can be a case of indirect shear stress. Think it like this, the extreme end is fixed meaning that this face cannot deform, while just its adjacent cross section is expanding/squeezing due to poisson's ratio effect in an axial loading condition. This relative deformation means that shear stress must exist.