Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53

2nd Revised Pages

Summary • 333

experience a ductile-to-brittle transition with a lowering of temperature, an increase

in strain rate, and/or an alteration of specimen thickness or geometry. In some glassy

thermoplastics, the crack formation process may be preceded by crazing; crazing can

lead to an increase in ductility and toughness of the material.

Impact Fracture Testing

Qualitatively, the fracture behavior of materials may be determined using Charpy

and Izod impact testing techniques. On the basis of the temperature dependence

of measured impact energy (or appearance of the fracture surface), it is possible to

ascertain whether or not a material experiences a ductile-to-brittle transition and the

temperature range over which such a transition occurs.

Cyclic Stresses (Fatigue)

TheS–NCurve

Fatigue is a common type of catastrophic failure wherein the applied stress level

fluctuates with time. Test data are plotted as stress versus the logarithm of the num-

ber of cycles to failure. For many metals and alloys, stress diminishes continuously

with increasing number of cycles at failure; fatigue strength and fatigue life are the

parameters used to characterize the fatigue behavior of these materials. On the other

hand, for other metals/alloys, at some point, stress ceases to decrease with, and be-

comes independent of, the number of cycles; the fatigue behavior of these materials

is expressed in terms of fatigue limit.

Crack Initiation and Propagation

The processes of fatigue crack initiation and propagation were discussed. Cracks

normally nucleate on the surface of a component at some point of stress concen-

tration. Two characteristic fatigue surface features are beachmarks and striations.

Beachmarks form on components that experience applied stress interruptions; they

normally may be observed with the naked eye. Fatigue striations are of microscopic

dimensions, and each is thought to represent the crack tip advance distance over a

single load cycle.

Factors That Affect Fatigue Life

Measures that may be taken to extend fatigue life include: (1) reducing the mean

stress level; (2) eliminating sharp surface discontinuities; (3) improving the surface

finish by polishing; (4) imposing surface residual compressive stresses by shot peen-

ing; and (5) case hardening by using a carburizing or nitriding process.

Environmental Effects

The fatigue behavior of materials may also be affected by the environment. Thermal

stresses may be induced in components that are exposed to elevated temperature

fluctuations and when thermal expansion and/or contraction is restrained; fatigue

in these conditions is termed thermal fatigue. The presence of a chemically active

environment may lead to a reduction in fatigue life for corrosion fatigue.

Generalized Creep Behavior

The time-dependent plastic deformation of materials subjected to a constant load

(or stress) and temperatures greater than about 0.4Tmis termed creep. A typical