Page 219 - Materials Science and Engineering An Introduction

6.10 Hardness • 191

6.9 COMPRESSIVE, SHEAR, AND TORSIONAL
DEFORMATIONS
Of course, metals may experience plastic deformation under the influence of ap-
plied compressive, shear, and torsional loads. The resulting stress–strain behavior
into the plastic region is similar to the tensile counterpart (Figure 6.10a: yielding
and the associated curvature). However, for compression, there is no maximum
because necking does not occur; furthermore, the mode of fracture is different from
that for tension.

Concept Check 6.3 Make a schematic plot showing the tensile engineering stress–strain
behavior for a typical metal alloy to the point of fracture. Now superimpose on this plot a sche-
matic compressive engineering stress–strain curve for the same alloy. Explain any differences
between the two curves.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]

6.10 HARDNESS
hardness Another mechanical property that may be important to consider is hardness, which is a
measure of a material’s resistance to localized plastic deformation (e.g., a small dent or
a scratch). Early hardness tests were based on natural minerals with a scale constructed
solely on the ability of one material to scratch another that was softer. A qualitative
and somewhat arbitrary hardness indexing scheme was devised, termed the Mohs scale,
which ranged from 1 on the soft end for talc to 10 for diamond. Quantitative hardness
techniques have been developed over the years in which a small indenter is forced into
the surface of a material to be tested under controlled conditions of load and rate of
application. The depth or size of the resulting indentation is measured and related to
a hardness number; the softer the material, the larger and deeper the indentation, and
the lower the hardness index number. Measured hardnesses are only relative (rather
than absolute), and care should be exercised when comparing values determined by
different techniques.
Hardness tests are performed more frequently than any other mechanical test for
several reasons:
1. They are simple and inexpensive—typically, no special specimen need be pre-
pared, and the testing apparatus is relatively inexpensive.
2. The test is nondestructive—the specimen is neither fractured nor excessively de-
formed; a small indentation is the only deformation.
3. Other mechanical properties often may be estimated from hardness data, such as
tensile strength (see Figure 6.19).
Rockwell Hardness Tests 13
The Rockwell tests constitute the most common method used to measure hardness
because they are so simple to perform and require no special skills. Several different
scales may be used from possible combinations of various indenters and different loads
a process that permits the testing of virtually all metal alloys (as well as some polymers).

13 ASTM Standard E18, “Standard Test Methods for Rockwell Hardness of Metallic Materials.”