Downloaded from https://www.everyspec.com PROPERTIES OF MATERIALS 4"failure. Curves of this type are commonly noted that the mnaximnum ntress is equal to the q;aued S-N diagrams. The value of the great- stress amplitude. The maximnumr and mini- est. stress which the material can withstand mum stresses are equal in value, -)pposite in for an- infinite number of cycles without fail- sign, and correspond to) a case of completely urt is called the endurance limnit. This is a reversexd loading. When the mean stress is simple and admittedly crude measure of the other than zero, the stress amplitude core-e- fatigue streftgth of the material.
If the en- sponding to fatigue decreauses with inereas- durance limit so defined cannot be detected ing mean stress. Trypical curves at room and - -with a reasonable amount of testing, a prac- elevated temperatures are presented in the tical endurance limit is often introduced as literature.'4 the stress at which 107 load cycles do not cause failure. *Superscript numbers refer to the bibliography For the case of zero mear, stress, it will be at the end of thi chgpter. K b OTrEAN A U) ENDURANCE LI MIT TIME NUMBER OF CYCLES TO FAILURE (LOG SCALE) Figure 2-3. Fatigue Loading and Typical Strevs. Cycles to Failure (S-N) Curve 2-1.9. Other Mechanical Properties.
Of measures of the ductility of a material are great importance to the missile designer is the percent elongation in 2 inches of a test the plastic behavior of materials. Plasticity specimen and the reduction of area at frac- is related to the ability of a material to sus- ture in a tensi or test. tamn a permanent deformation without frac- The hardne,, of a material is a measure of ture, A perfectly plastic material, for exam- its ability to i-esist permanent surface de- Fle, would require no increase in stress to formation under applied concentrated lowds.
produce a continuing plastic deformation There are several standard methods of incas- oree [z 7ipld stress was reached; it would be uring tis quantity. Values of hardness for repr;s-aiid by a segment of the stress axis several materials are given in the tabies at and a horizontal straight line on the stress,.- the end of this chapter. Approximate corr-- strain diagrini.
The importance of plasticity lation between the various hardness mcales Ican be illu.,iialrd by copsidering a structure are given in t2~e literature." It can be shown which is su~ije, lowaly to a high stress level, that for many steels thie tensile stftength is If the material is ductile, plastic flow occurs, proportioial to the hardness. * and the load is ridistributed more uniformly. Neighboring points in the material absorb 22 RPBHVO the load which is shed at the point of stress 22.CE BAVO concentration, thus lowering the intensity orA Creep may be defined as the contirwing the maximum stress and lessening the possi- deformation tWat occur's in a material sub- bility of early failure.
A brittle material does jected t(; a constant stress. At room tempera- not readily flow plastically and hence there tures, the cre 4 op of engineering mat!rals is is the danger of brittle fracture. Quantitative unimportant, but at eievated temperatuires