Page 124 - Mechanical Behavior of Materials

Section 4.2 Introduction to Tension Test 125

Figure 4.6 Tensile specimens of polymers (left to right): untested specimen with a 7.6 mm
diameter test section, a partially tested specimen of high-density polyethylene (HDPE), and
broken specimens of nylon 101 and Teflon (PTFE). (Photo by R. A. Simonds.)

any time during the test:
P
σ = (4.1)
A i
Displacements in the specimen are measured within a straight central portion of constant cross
section over a gage length L i , as indicated in Fig. 4.7. Strain ε may be computed from the change
in this length, L:
L
ε = (4.2)
L i
Stress and strain, based on the initial (undeformed) dimensions, A i and L i , as just presented, are
called engineering stress and strain.