Fatigue Crack Paths 2003

This paper starts with a description of the fundamentals of the thermoelastic

technique and the different approaches used for the experimental evaluation of stress

intensity factors from thermoelastic data. A description of a new methodology for the

mathematical analysis of thermoelastic images for the location of the crack tip is also

given. The technique was used to study the growth of a fatigue crack in a ferritic steel

weld. The equipment used for testing and the experimental work conducted is

described. Finally, some results showing that the crack path could be located and the

effective stress intensity factor of the growing crack could be determined.

O V E R V I OEFWE X P E R I M E N TAAPLP R O A C HTEOSF R A C T U R E

M E C H A N I C S

The accurate determination of the stress intensity factor of a crack in an engineering

component is not a straightforward process. In fatigue tests on specimens of simple

geometry it is not uncommonto use the compliance method, in which a remote

measurement of strain is used to monitor the change in specimen compliance from

which the stress intensity factor is deduced. This approach is not readily applicable to a

more complex geometry, and optical techniques based on a field of data from around

the crack provide a valuable alternative.

Many methods of evaluating the stress intensity factor from measurements of the

strain field around the crack tip have developed over the years. There is essentially at

least one method to accompany each technique of experimental stress analysis. Each

technique has its champions whohave developed, refined and validated the methods by

comparison to analytical and, or numerical results. The choice available to the engineer

or scientist who requires a means of measuring the stress intensity factor is large and

there are few works in the open literature to help make the decision on which technique

to use in a particular situation. Twothat do exist are by Sanford [1], who provided a

useful overview of the full-field optical techniques, and by Patterson and Olden [2],

who conducted a study to make direct comparisons between competing optical strain

analysis techniques.

Principles of Determining Fracture Mechanics Parameters from Experimental Data

The stress field around a crack tip can be described by the set of linear elastic field

equations [3] in which the stress intensity factors KI, KII, and KIII are parameters that

characterise the field. The evaluation of the stress intensity factors from experimental

data involves solving these equations using measurements to provide boundary

conditions. The most commonly used form of the stress field equations can be derived

using Westergaard’s [4] approach and by making a first order approximation gives: