Page 101 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

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

more than 1000><), a series of striations can be seen on fracture
surfaces, each beach mark consisting of several striations.

Improving Fatigue Strength. Fatigue life is greatly influenced
by the method of preparation of the surfaces of the part or speci-
men (Fig. 2.29). The fatigue strength of manufactured products
can be improved overall by the following methods:
a. lnducing compressive residual stresses on surfaces-for
example, by shot peening or by roller burnishing (Sec-
tion 34.2);
b. Case hardening (surface hardening) by various means
(Section 4.10);
FIGURE 2.28 Typical fatigue-fracture surface
on metals, showing beach marks. Magnification: c. Providing a fine surface finish and thereby reducing the
left, 500><; right, 1000><. Source: Courtesy of effects of notches and other surface imperfections; and
B.]. Schulze and S.L. Meiley and Packer d. Selecting appropriate materials and ensuring that they are

Engineering Associates, Inc. free from significant amounts of inclusions, voids, and
impurities.
\ Fine polishing Conversely, the following factors and processes can
O A Polishing A reduce fatigue strength: tensile residual stresses on the
Qrinding
pits (due to corrosion) that act as stress raisers; hydro-
g 10 - Fine turning surface (see Section 2.11), decarburization; surface
gen embrittlement; galvanizing; and electroplating.
.C
‘“ 20 - 'io UQ/7 IU,-ning
E3
Stress-corrosion Cracking. An otherwise ductile
Z,” 30 - metal can fail in a brittle manner by stress-corrosion
3 called stress cracking or season
_Qi cracking (also
§ 40 - cracking). Parts free from defects may develop cracks,
.E 46.0 either over time or soon after being manufactured into
asf a product. Crack propagation may be either intergran-
_Q
5 50 - ular or transgranular. The susceptibility of metals to
3
'O
CD stress-corrosion cracking depends mainly on the mate-
'I 60 - rial, on the presence and magnitude of tensile residual
stresses, and on the environment. Brass and austenitic
70 stainless steels are among metals that are highly sus-
500 800 1000 1300
f_l ceptible to stress cracking; the environment includes
Ultimate tensile strength (MPa) corrosive media such as salt water or other chemicals.
The usual procedure to avoid stress-corrosion crack-
ing is to stress relieve the part just after it is formed.
Full annealing (Section 4.11) may also be done, but
this treatment reduces the strength of cold-worked
FIGURE 2.29 Reductions in the fatigue strength of cast
steels subjected to various surface-finishing operations. Note parts.
that the reduction becomes greater as the surface roughness
and the strength of the steel increase. Source: M.R. Mitchell. Hydrogen Embrittlement. The presence of hydro-
gen can reduce ductility and can cause severe embrit-
tlement and premature failure in many metals, alloys, and nonmetallic materials.
Known as hydrogen embrittleinent, this phenomenon is especially severe in high-
strength steels. Possible sources of hydrogen arise during melting of the metal, pick-
ling (removing of surface oxides by chemical or electrochemical reaction), and
electrolysis in electroplating; other sources are water vapor in the atmosphere and
moist electrodes and fluxes used during welding. Oxygen can also cause embrittle-
ment, particularly in copper alloys.