Chapter 2

Properties of Metals

Stress-Strain CurveFigure 2.2 A typical stress- strain curve obtained from a tension test, showing various features.

Strain-

• The unit deformation of a material when stress is applied.

Stress-

• The load per unit of area on a stress-strain diagram.

Tensile-Test Specimen and Machine

(b)

Figure 2.1 (a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths. (b) A typical tensile-testing machine.

Tensile stress-

• Refers to an object loaded in tension, denoting the longitudinal force that causes the fibers of a material to elongate.

Temperature Effects on Stress-Strain Curves

Figure 2.7 Typical effects of temperature on stress-strain curves. Note that temperature affects the modulus of elasticity, the yield stress, the ultimate tensile strength, and the toughness (area under the curve) of materials.

Proportional limit-

• On the stress-strain diagram, the point where the ratio of stress to strain (i.e., the modulus of elasticity) is no longer a constant, or where that line is no longer straight.

Stress-Strain CurveFigure 2.2 A typical stress- strain curve obtained from a tension test, showing various features.

Yield strength-

• The stress at which a material deviates by a specified amount of strain from the region where stress and strain are proportional.

Ultimate tensile strength or UTS-

• The stress equal to the maximum load achieved in a tensile test divided by the original area of the specimen.

Modulus of elasticity-

• The ratio of the unit stress to the unit deformation (strain) of a structural material; a constant as long as the unit stress is below the elastic limit.

• Also known as Young’s Modulus

• Shearing modulus of elasticity is often called the modulus of rigidity.

Creep-

• Slow plastic deformation in steel and most structural metals caused by prolonged stress under the yield point at elevated temperatures.

Compressive stress-

• Refers to a member loaded in compression, which gives rise to either a given reduction in volume

Shear load-

• A load that tends to force materials apart by application of side-slip action.

Tensile load-

• A load applied to a part or parts that attempts to pull apart by a stretching action.

Ductility-

• The property of a material that allows it to fail permanently, or to exhibit plasticity without rupture while under tension.

Impact test-• A test that applies an impact load (by a swinging

hammer) to a small notched specimen. • The data from the test are in energy units, joules

or inch- pounds. • The test is performed with the specimens at

different temperatures; the results determine the notch toughness of the metals at the temperature tested.

• There are two types of specimens-– Izod – Charpy, with Charpy the one primarily used in the

United States.

Impact Test SpecimensFigure 2.18 Impact test specimens: (a) Charpy; (b) Izod.

Malleability-

• The ability of a metal to deform permanently without rupture when loaded in compression.

Fatigue in metals-

• The tendency of a metal to fail by breaking or cracking under conditions of repeated cyclical stressing that takes place well below the ultimate tensile strength.

Hardening-

• The process of increasing the hardness of a ferrous alloy by austenitizing and quenching; also, the process of increasing the hardness of some stainless steels and nonferrous alloys by solution heat treatment and precipitation.

Hardness-

• The property of a metal that allows it to resist being permanently deformed.

• This property is divided into three categories: – the resistance to penetration– the resistance to abrasion– elastic hardness.

Hardness TestsFigure 2.12 General characteristics of hardness-testing methods and formulas for calculating hardness. The quantity P is the load applied. Source: H. W. Hayden, et al., The Structure and Properties of Materials, Vol. III (John Wiley & Sons, 1965).

Plasticity-

• The quality of material such that it can be deformed without breaking.

• Clay is a completely plastic material.

• Metals exhibit plasticity in varying amounts.