Page 97 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials

Scale

Pendulum End of S
Specimen P Starting position

(1o><1o><55mm)
Q
pecimen
Hammer
-
Swing Anvil Specimen (1O><1O><75mm)

r
Notch gc Penduium
Izod kk H 1;

(H) (bl
FIGURE 2.l9 Impact test specimens. (a) Izod; (bl Charpy.

In the Charpy test, the specimen is supported at both ends (Fig. 2.19); in the
Izod test, it is supported at one end like a cantilever beam (Fig. 2.19a). From the
amount of swing of the pendulum, the energy dissipated in breaking the specimen
can be obtained; this energy is the impact toughness of the material. Unlike hardness-
test conversions (Fig. 2.15 ), no quantitative relationships have yet been established
between Charpy and the Izod tests. Impact tests are particularly useful in determin-
ing the ductile-brittle transition temperature of materials (Section 2.10.1). Materials
that have high impact resistance generally have high strength, high ductility, and,
hence, high toughness. Sensitivity to surface defects (notch sensitivity) is important,
as it significantly lowers impact toughness, particularly in heat-treated metals and in
ceramics and glasses.

2.l0 Failure and Fracture of Materials in
Manufacturing and in Service

Failure is one of the most important aspects of material behavior, because it directly
influences the selection of a material for a particular application, the methods of
manufacturing, and the service life of the component. Because of the many factors
involved, failure and fracture of materials is a com-
plex area of study; this section focuses only on those
aspects of failure that are of particular significance
to selecting and processing materials. There are two
op 0 9 Barreiing
general types of failure:
Cracks
I. Fracture, through either internal or external
cracking; fracture is further subclassified into
two general categories: ductile and brittle
(3) (bl (C) (dl
(Figs. 2.20 and 2.21).
FIGURE 2.20 Schematic illustration of types of failures in 2. Buckling, as shown in Fig. 2.20b.
materials: (a) necking and fracture of ductile materials;
(b) buckling of ductile materials under a compressive load; Although failure of materials is generally re-
(c) fracture of brittle materials in compression; (d) cracking on garded as undesirable, some products are designed in
the barreled surface of ductile materials in compression. such a way that failure is essential for their function.