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Appendix 8Propertiesof structural materialsOur at tention here is centred mainly on ferrous andnon-ferrous metals. However the principles apply toother solid materials.A8.1 Simple tensile testIn principle the tensile test a pplies an axial strain to astandard specimen an d measurem ents are taken of thechang e in length b etwee n two specified ma rks, definedas the gauge length, and also of the resulting tensileload. Alternatively, the test could be carried out byapplying a dead load and recording the subsequentstrain.

The point a is the limit of proportionality, i . e . up tothis point th e material obey s Hookes law. Point b is theelastic limit, this means tha t any load ing up to this poinis reversible and the unloading curve retrace s theloading curve. In practice the elastic limit occurs justafter the limit of proportionality. After this point anyunloading curve is usually a straight line parallel to theelastic line. Point c is known as the yield pointsom etimes called the upper yield point. Point d is calledthe lower yield point. If the test is carried out byapplying a load rather than an extension then theextension will increase from point c without anyincrease in load to the point c . Further straining wilcause plastic deformation to take place until themaximum load is reached at point e. This is known asthe ultimate tensile load. Af ter t his a neck will form inthe sp ecime n resulting in a large reduction in thecross-section area until failure occurs at point f.

Figure A8.1Figure A 8.1 shows a typical specimen where A is theoriginal cross-section area. Figure A8.2 shows theload-extension plot for a mild steel specim en. No te thatloadorig inal-cross -section area is the nominal stres sand ex tensiodg auge length is the strain so the shape ofthe stress-strain curve is the sa me . The extension axis isshown broken since the extensions at e and f are verymuch greater than that at points a t o d .

Figure A8.2

Figure A8.3Figure A8.3 shows a similar plot for a non-ferrousmetal where it is noticed that no well-defined yieldpoint appears. At the point c the stress is known as aproof stress. For exam ple a 0.2% proof stress is onewhich when removed leaves a permanent strain of0.002.A strain of 0.002 can also be referred to as 2milli-strain (m.) or as 2000 micro-strain LE).Both the above cases are for ductile materials and thedegree of ductility is measured either by quoting the

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A8.1 Sim ple tensile test 26

final strain in the form of a percenta ge elong atio n, or inthe form of the percentage reduction of area at theneck.

For brittle materials failure occurs just after thelastic limit th ere be ing little or no plastic deform ation