IFMASS10

It has been shown that specimen thickness has no significant effect on the FCGR

behaviour /20/, although that is not always the case. The ability of

ΔK

to account for so

many variables has tremendous significance in the application of the data. Thus, the

FCGR behaviour expressed as

versus

ΔK

can be regarded as a fundamental mate-

rial property analogous to the yield and ultimate tensile strength, or plane strain fracture

toughness,

, /21/. Knowing this property, prediction of the crack length vs. cycles

behaviour of any component using that material and containing a preexisting crack or

crack-like defect can be obtained, as long as the fatigue stresses in the component are

known and a

expression for the crack/load configuration is available.

This is a simple exponential relation that can readily be curve fit to the desired data

portion, Fig. 9. Stress or load ratio affects crack propagation data as well. The influence is

that increasing

(here

min

max

) decreases both the threshold value at the low end

and the instability at the up end of the da/

vs.

ΔK

curve ( Fig. 10 /22/, Fig. 11 /8/).

Like other mechanical properties, microstructure affects also fatigue crack growth

characteristics. An example, given in Fig. 12, shows the combined influence of both

gamma prime and grain sizes on fatigue crack propagation in Waspaloy.

Plastics also can be analysed using this technique. Figure 13 shows a variety of

materials that are displayed in the conventional form. Many polymeric materials exhibit

substantial frequency effects, and this should be considered in the generation of data.

In application, use of

versus Δ

is completely different than either the

ε-

continuum method. Instead of providing an immediate life estimate in association

with a given stress or strain combination and a test coupon's modelled failure criterion, a

more complicated determination is required. Using the

versus Δ

relation in the part,

the applied loads are employed to assess crack extension over incremental changes in

length, and they are continuously summed to reflect the total increase. In essence, this is

the regeneration of the

curve for the specific part. Crack growth can be assessed until

fracture (achieving a critical crack size) or some other intermediate point.

It is common in several industries to use the above technique to determine intervals

between inspections to ensure structural integrity. So formulated inspection schedules

maximises the probability to detect discontinuity only in an extended, critical stage, enab-

ling to use advantages of the damage tolerant approach. Even on this technique, a crack,

once discovered, can’t necessarily be left. Removal or structural modification may be the

only acceptable alternative (e.g. in airframes). The predictive aspects of the technique can

justify continued operation under full or derated conditions till requested replacement

parts or with a stated finite-life limit for the unit (extra inspections may be required). By

probabilistic methods a quantified risk assessment can be assessed.

4.2. Test procedure

American Society for Testing and Materials (ASTM) Standard E647 /25/ is the accep-

ted guideline for fatigue crack growth rate (FCGR) testing and is applicable to a wide

variety of materials and growth rates. The testing consists of several steps, starting with

selecting the specimen size, geometry, and crack length measurement technique. When

planning the tests, the investigator must respect of the application of FCGR data. Testing

is often performed in laboratory air at room temperature; however, any gaseous or liquid

environment and temperature may be used to determine the effect of temperature and cor-

rosion on cyclic loading. Cyclic loading also may involve various waveforms for cons-

tant-amplitude loading, spectrum loading, or random loading.