## 81 Definition of fatigue terms

First, we need to define some standard fatigue terms. Consider a cylindrical specimen loaded uniaxially by a stress, a, which varies from a minimum absolute value amin to a maximum absolute value amax, as shown in Figure 8.1. For example, for a fatigue cycle ranging from — 1MPa to — 10MPa, we take

Compression-Compression

Cycles to failure, Nf

Figure 8.1 (a) Definition of fatigue loading terms; (b) Typical S-N curve for aluminum alloys, in the form of stress range Aa versus number of cycles to failure, Nf. The endurance limit, Aae, is defined, by convention, for a fatigue life of 107 cycles

Cycles to failure, Nf

Figure 8.1 (a) Definition of fatigue loading terms; (b) Typical S-N curve for aluminum alloys, in the form of stress range Aa versus number of cycles to failure, Nf. The endurance limit, Aae, is defined, by convention, for a fatigue life of 107 cycles amin = 1 MPa and amax = 10MPa. The load ratio R is defined by

amax

It is well known that the fatigue life of structural metals such as steels and aluminum alloys is insensitive to the loading frequency, under ambient conditions. This simplification does not hold in the presence of a corrosive medium, such as a hot alkaline solution, or salt water for aluminum alloys. These broad conclusions are expected to hold also for metallic foams.

In low-cycle fatigue testing, the usual strategy is to measure the number of cycles to failure, Nf, for a given constant stress range Aa = amax — amin, and then to plot the resulting pairs of values (Nf, Aa) on log-linear axes. The resulting S-N curve is used in design for finite life (Figure 8.1(b)). Many steels have an S-N curve with a sharp knee below which life is infinite; the corresponding stress range is designated the fatigue limit. This knee is less pronounced for aluminum alloys, and it is usual to assume that 'infinite life' corresponds to a fatigue life of 107 cycles, and to refer to the associated stress range Act as the endurance limit, Aae. For conventional structural metals, a superimposed tensile mean stress lowers the fatigue strength, and the knockdown in properties can be estimated conservatively by a Goodman construction: at any fixed life, the reduction in cyclic strength is taken to be proportional to the mean stress of the fatigue cycle normalized by the ultimate tensile strength of the alloy (see any modern text on metal fatigue such as Suresh, 1991, Fuchs and Stephens, 1980 or a general reference such as Ashby and Jones, 1997). This chapter addresses the following questions: