IFMASS10

to bear load and can operate safely for extended periods of time. Developments from the

1960s and before have produced the third design philosophy, damage tolerant. It is

intended expressly to address the issue of "cracked" components.

When a crack is present, an alternative controlling quantity is involved, as mode I

stress intensity factor range, (Δ

), a function of crack location, orientation, and size

within the geometry of the part. This fracture mechanics parameter is then related to the

potential for crack extension under the imposed cyclic loads for subcritical growth or the

initiation of unstable fracture. It is quite different from the other two approaches. Property

descriptions for the crack extension under cyclic loading are typically

- Δ

curves

(log crack growth rate vs. log stress-intensity range), Table 1.

The advantage of the damage tolerant design philosophy is the ability to treat cracked

objects in an appropriate fashion. The previous methods only allow for the immediate

removal of cracked structure, and here the number of cycles of crack growth over a range

of crack sizes can be estimated and fracture to be predicted. The clear tie of crack size,

orientation, and geometry to non-destructive testing (NDT) is also a benefit. Disadvan-

tages are connected with complexity in development and modelling of property data, and

required numerical integration for crack growth. The predicted lives are influenced by the

initial crack size, requiring quantitative determination of detection probability for each

type of NDT method used.

3. INFINITE-LIFE CRITERION (

CURVE)

Safe-life design based on the infinite-life criterion reflects the classic approach to

fatigue. It was developed through the 1800s and early 1900s because complex machinery

of that time increasingly produced dynamic loads, followed by increasing number of

failures. The safe-life, infinite-life design philosophy was the first to address this need.

The stress-life or

approach is one of a safe-life, infinite-life regime. It is categori-

zed as a "high cycle fatigue", with most considerations based on maintaining elastic

behaviour in the examined object. The "no cracks" is accepted, although all test results

inherently include the influence of the discontinuity population present in the object.

In this period steel was dominant metallic structural material (land transportation,

power generation, constructions). The "infinite-life" aspect of this approach is related to

the asymptotic behaviour of steels, many of which exhibited a fatigue limit or "enduran-

ce" limit at a high number of cycles (>10

). Most other materials do not exhibit this

response, displaying a continuously decreasing stress-life response, even at a great num-

ber of cycles (10

to 10

), more correctly described by fatigue strength at a given number

of cycles. Figure 1 shows a schematic comparison of these two characteristic results.

Many machine design texts cover this method to varying degrees /4-8/.

What about the

data presentation? Stress is the controlling quantity in this method.

The most typical formats for the data are to plot the log number of cycles to failure

(sample separation) versus either stress amplitude (

), maximum stress (

max

), or stress

range (Δ

) /9, 10/. Figure 2 provide plots for three constant-

value tests (

is the ration

of minimum and maximum stress in a cycle, the second dynamic variable). Note the

apparent reversal of the effect of

, although the data are identical. Clearly, while the

analytical result must be identical regardless of which graphic means is employed, the

visual influence in interpretation varies with the method of presentation.

Mean stress influence is important, and design approach must consider it. According

to Bannantine et al. /7/, the archetypal mean stress (S

) versus stress amplitude (

)