Page 538 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Rheology and Physical Tests 501

TABLE 14.3
Comparative Hardness Scales
Hardness Scale
Rockwell
Type of
Mohs Brindell M R Shore Barcol Material
2 (Gypsum) 25 100 55 Hard
16 80 40 Plastics
12 70 100 90 30
10 65 97 86 20
8 60 93 80
6 54 88 74
1 (Talc) 5 50 85 70
2 32 ca50 89 Soft Plastics
1 23 42
0.8 20 38 Rubbery
0.5 15 30

14.4.6 SHEAR STRENGTH

The measure of shear strength typically utilizes a punch-type shear fixture (Figure 14.19). The shear
strength is equal to the load divided by the area. Thus, the sample is mounted in a punch-type shear

fixture and punch—pushed down at a specified rate until shear occurs. This test is important for

sheets and films but is not typically employed for extruded or molded products.

14.4.7 ABRASION RESISTANCE
Abrasion is the wearing away of a materials surface by friction. The most widely used tests to mea-
sure abrasion resistance employs Williams, Lamborn, and Tabor abraders (ASTM D-1044). In each
test the abrader is rubbed on the material’s surface and the material loss noted.

14.4.8 FAILURE
The failure of materials can be associated with a number of parameters. Two major causes of fail-
ure are creep and fracture. The tensile strength is the nominal stress at the failure of a material.
Toughness is related to ductility. For a material to be tough, it often takes a material having a good
balance of stiffness and give.
Calculations have been made to determine the theoretical upper limits with respect to the strength
of polymers. Real materials show behaviors near to those predicted by the theoretical calculations
during the initial stress–strain determination, but vary greatly near the failure of the material. It is
believed that the major reasons for the actual tensile strength at failure being smaller than calculated
are related to imperfections, the nonhomogeneity of the polymeric structure. These fl aws, molecular
irregularities, act as the weak-link in the polymer’s behavior. These irregularities can be disloca-
tions, voids, physical crack, and energy concentrations. Even with these imperfections, they have
high strength-to-mass ratios (Table 14.2) with the highest tensile strength/density value for UHMW
polyethylene.
The fracture strengths of polymers are generally lower than those of metals and ceramics. The
mode of failure for thermosets is generally referred to as the materials being brittle. Cracks, related
to bond breakage, occur at points of excess stress. These create weak spots and may lead to fracture
if the applied stress, appropriate to create bond breakage, continues.

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