What Is The Stress-Strain Curve Of Brittle Materials?
Many of the characteristics of a material can be deduced from the tensile test. It is more convenient to compare materials in terms of stresses and strains, rather than loads and extensions of a particular specimen of a material.
The stress at any stage is the ratio of the load of the original cross-sectional area of the test specimen; the strain is the elongation of a unit length of the test specimen.
For stresses up to about 750 MN/m2 the stress-strain curve is linear, showing that the material obeys Hooke’s law in this range; the material is also elastic in this range, and no permanent extensions remain after removal of the stresses.
The ratio of stress to strain for this linear region is usually about 200 GN/m2 for steels; this ratio is known as Young’s modulus and is denoted by E. The strain at the limit of proportionality is of the order 0.003, and is small compared with strains of the order 0.100 at fracture.
We note that Young’s modulus has the units of a stress; the value of E defines the constant in the linear relation between stress and strain in the elastic range of the material. We have
for the linear-elastic range. If P is the total tensile load in a bar, A its cross-sectional area, and Lo its length, then
where e is the extension of the length Lo. Thus the expansion is given by
If the material is stressed beyond the linear-elastic range the limit of proportionality is exceeded, and the strains increase non-linearly with the stresses. Moreover, removal of the stress leaves the material with some permanent extension; h range is then bothnon-linear and inelastic.
The maximum stress attained may be of the order of 1500 MNlm’, and the total extension, or elongation, at this stage may be of the order of 10%. The curve of Figure 1.5 is typical of the behaviour of brittle materials as, for example, area characterized by small permanent elongation at the breaking point; in the case of metals this is usually lo%, or less.
When a material is stressed beyond the limit of proportionality and is then unloaded, permanent deformations of the material take place. Suppose the tensile test-specimen of Figure 1.5 is stressed beyond the limit of proportionality, to a point b on the stress-strain diagram. If the stress is now removed, the stress-strain relation follows the curve bc; when the stress is completely removed there is a residual strain given by the intercept Oc on the &-axis.
If the stress is applied again, the stress-strain relation follows the curve cd initially, and finally the curve df to the breaking point. Both the unloading curve bc and the reloading curve cd are approximately parallel to the elastic line Oa; they are curved slightly in opposite directions.
The process of unloading and reloading, bcd, had little or no effect on the stress at the breaking point, the stress-strain curve being interrupted by only a small amount bd, Figure 1.6. The stress-strain curves of brittle materials for tension and compression are usually similar in form, although the stresses at the limit of proportionality and at fracture may be very different for the two loading conditions.
Typical tensile and compressive stress-strain curves for concrete are shown in Figure 1.7; the maximum stress attainable in tension is only about one-tenth of that in compression, although the slopes of the stress strain curves in the region of zero stress are nearly equal.