write:compression stress and shear stress. Figure 34-9 shows examples of yield and ultimate stress for three materi-als commonly used in aircraft. The tension stress in a material is equ

compression stress and shear stress. Figure 34-9 shows examples of yield and ultimate stress for three materi-als commonly used in aircraft. The tension stress in a material is equal to the tension load divided by the area carrying the load. Compression stress is equal to the compression load divided by the area available to carry the load. Shear stress is calculated the same way. From this you may have deduced that bending and torsion loads may cause tension stress
compression stress or shear stress. You are right
to a degree. Both bending and torsion loads cause the creation of all three types of stress and tension loads create shear stress in addition to the tension stress you might expect.Let’s discuss this some more. Although you can’t normally see it
an under-standing of stress is very important to the aircraft accident investigator. When dis-cussing the strength of a material
it’s the stress the material can withstand before failing that’s important. Load will be used to calculate the level of stress within the material
but it’s the stress level in the material that determines when and where the material will fail. The maximum stress that standard aircraft structural materi-als can carry without failure is well-established and available to engineers in metals handbooks. You should also remember that the maximum tension stress a material can handle without failure may not be the same as its maximum compression stress which (as you may have guessed) need not be the same as the maximum shear stress. In fact it would be very unusual if all three were equal.Two levels of stress are important to the aircraft accident investigator: the material’s yield stress and its ultimate stress. The yield stress is the stress level at which the material experiences permanent objectionable deformation. Structural engineers design aircraft structures so that expected load-caused stresses are all below this level. The ultimate stress is the stress level at which the material will fail catastrophically. In common terms that means the material will break in two. A structure which has been exposed to loads which caused stresses to exceed yield levels will bend and deform
but it should not collapse. A structure which has been exposed to loads which caused stresses to exceed ultimate levels will collapse.B. APPEARAnCE oF mEtAL FRACtuRE SuRFACES. When load-gen-erated stresses in metal components exceed their ultimate stress level the com-ponent will fracture
separating into two or more parts. The surface texture of the fracture area and plastic deformation surrounding the fracture can provide the aircraft accident investigator with clues concerning the nature of the loads which caused the fracture and the relative strength of the material. For instance
if a fractured bolt which is supposed to be made of high strength steel
exhibits fracture evidence normally associated with a lower strength steel
the reasons for this apparent discrepancy will have to be uncovered and explained. The process by which a fracture occurs and the appearance of the fracture surface and sur-rounding material is classified as either “ductile” or “brittle.”Figure 34-9. Examples of Limit (Yield) and Maximum (Ultimate) Stress for Three Common Aircraft Structural Materials.