Downloaded from https://www.everyspec.com Chapter 2 PROPERTIES OF MATERtALS 2-1. MECHANICAL SEHAV*_ Data on the properties of some of the more In the analysis and design of any structure, pertinent engineering materials used in mis- the physical properties of available ms e design are collected and presented in are of primary importance. With relatively taii.-lar and graphical form at the end of this few exceptions, structural engineers in the past were concerned only with those proper- 2-1.1. Stress-Strain Curves. Probably the ties which might be classed as purely me- mechanical data most familiar to engineers chanical. 'ertain elastic constants, yield and are stress-straifn curves.
This information is breaking strengths, and fatigue characteris- obtained experimentally by testing a pre- tics might be cited as examples. The rela- scribed standard specimen of the material in tively recent developments in jet and rocket tension or compression and simultaneously propulsive systems, high-performance air- recording the load and elongation, in th- craft, and missiles have greatly increased the simplest of thene tests, a cylindrical *peci- complexity of structural analysis and desig~n men is loaded in tension ;r :a testing machine, problems,.
Designers of 'today's high-speed the observed load is divided by the original - in the material properties already mentioned, s'.ess, and the elongation is measured over but also in the thermal, creep and plastic a unit length to obtain the strain. When the characteristics. Indeed, it is certain that this data are presented graphically in the form of interest will extend to those physical charac- stress as a function of strain, the result is a teristics which were heretofore in the domain s ress-strain curve. Typical curves are shown of solid-scate physics. ip Figure 2-1 for steel vnd aluminum alloys.
A major reason for this extension of in- Before the information derivable from terest ir materiat properties is that ballistic these curves is discusseul, a brief review of missiles are subject to high rates of heating. the concepts of stress and strain will be pre- This heating occurs upon re-entry into the sented. Stress is the intensity of the force per atmosphere and (under certain trajectory unit area acting on a plane. This property is conditions) durinr the ascending phase of a vector quantity; the normal component of the Bight. Unless the heat is dissipated the the vector is the normal or direct stress, and temperature will rise. The loas!
carrying ca- the component in the plane, the sfr ar .stress. pacity of the strecture is affected by the In a cylindrical test specimen, the quantitj temperatures attained, and also by the length obtained by dividing the total force by the !!of time at an elevated temperature.
cross-sectional area .is the normal stre.szs act- As a genera) rule, it may be said that ele- ing on a plar.e perpendicular to the axis of vated temperatures have a deleterious effect the cylinder With a material that can be on the mechanical properties of materials, considered homogeneous and isutropic, and For example, the modulus of elasticity, yield with a cen'.rally applied load, this definition stress, and the ultimate stress usually de- is reasonzbly consistent and accurate. Inas- crease with increasing temperature. It will much as this normal stress is the only stress also be se, n that the rupture stress in acting in this simplified case, the stress con- fatigue.
or the number of cycles to failure dition is said to be uni-axial. generally decreases with an increase in The stress picture is accompanivd by a temperature. strain picture with normal and shear strains 7