11-14-2010, 06:04 AM
BS ISO 12107:2003 Metallic materials. Fatigue testing. Statistical planning and analysis of data
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Introduction
It is known that the results of fatigue tests display significant variations even when the test is controlled very
accurately. In part, these variations are attributable to non-uniformity of test specimens. Examples of such
non-uniformity include slight differences in chemical composition, heat treatment, surface finish, etc. The
remaining part is related to the stochastic process of fatigue failure itself that is intrinsic to metallic engineering
materials.
Adequate quantification of this inherent variation is necessary to evaluate the fatigue property of a material for
the design of machines and structures. It is also necessary for test laboratories to compare materials in fatigue
behaviour, including its variation. Statistical methods are necessary to perform these tasks. They include both
the experimental planning and procedure to develop fatigue data and the analysis of the results.
Scope
1.1 Objectives
This International Standard presents methods for the experimental planning of fatigue testing and the
statistical analysis of the resulting data. The purpose is to determine the fatigue properties of metallic
materials with both a high degree of confidence and a practical number of specimens.
1.2 Fatigue properties to be analysed
This International Standard provides a method for the analysis of fatigue life properties at a variety of stress
levels using a relationship that can linearly approximate the material's response in appropriate coordinates.
Specifically, it addresses:
a) the fatigue life for a given stress, and
b) the fatigue strength for a given fatigue life.
The term “stress” in this International Standard can be replaced by “strain”, as the methods described are also
valid for the analysis of life properties as a function of strain. Fatigue strength in the case of strain-controlled
tests is considered in terms of strain, as it is ordinarily understood in terms of stress in stress-controlled tests.
1.3 Limit of application
This International Standard is limited to the analysis of fatigue data for materials exhibiting homogeneous
behaviour due to a single mechanism of fatigue failure. This refers to the statistical properties of test results
that are closely related to material behaviour under the test conditions.
In fact, specimens of a given material tested under different conditions may reveal variations in failure
mechanisms. For ordinary cases, the statistical property of resulting data represents one failure mechanism
and may permit direct analysis. Conversely, situations are encountered where the statistical behaviour is not
homogeneous. It is necessary for all such cases to be modelled by two or more individual distributions.
An example of such behaviour is often observed when failure can initiate from either a surface or internal site
at the same level of stress. Under these conditions, the data will have mixed statistical characteristics
corresponding to the different mechanisms of failure. These types of results are not considered in this
International Standard because a much higher complexity of analysis is required.
:JC_cheers:
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