^{1}

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^{2}

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To improve the accuracy for prediction of cyclic life of pieces with macrocracks we propose to use a new thermographic method. Traditionally this question is solved on the basis Paris formula which connects the speed of crack growth (SCG) with Stress intensity factor
*K*. However parameter
*K* is not identical to the SCG because
* K* doesn’t consider non-linear processes at the top of crack (TC). That is why the using
*K* gives the considerable error. For overcoming this problem we proposed instead of
*K *to connect SCG with another diagnostic parameter, such as
ΔS^{(1c)}—increment of specific entropy for cycle (ISE) at the TC, which can be calculated with sufficient accuracy through passive temperature field on the surface of tested object. Parameter ISE can be obtained both simultaneously with building of a kinetic fatigue diagram and on the basis of measuring of temperature under exploitation of piece. In both cases the prediction of cyclic lifetime is much higher than with the help parameter
*K*. Besides parameter ISE allows to follow the crack development inside tested object. This means that suggested parameter ISE is more universal and convenient than traditional parameter
*K*.

Under cyclic deforming of pieces with macrocrack and certain conditions some plastic domain in top of crack is appeared. As a result the crack begins its movement. As it is known, under plastic deforming of metals the most part of mechanical energy is transformed in the heat energy. Therefore, a heat source arises at the top of a growing crack. Because of high heat conductivity of metals these heat sources form a passive thermal field on the surface of testing object, which characterizes the irreversible changing in the material and has a lot of information about damaging processes. This information can be received without contact with investigated object by using up-to-date infrared equipment allows to fix the kinetics of temperature distributions near top of crack with high precision. Thermographic method, based on that phenomenon, has some advantages in comparison with traditional approaches. These advantages consist in a higher accuracy and universality, because thermographic method sufficiently enlarged range of tested pieces [

The main problem in the thermographic method is correct choice of damage parameter. Such natural parameter is temperature. But there is essential moment. It is necessary to calculate not itself temperature in the domain of damage, but its change for sufficiently small interval of time, let us say, for one cycle of oscillation. In that case influence of background’s temperature is practically excepted.

As direct damage’s parameter we use

Traditionally this question is solved on the basis of Paris formula [

or its variety

which connect the speed of crack growth

In the Formulas (1) and (2)

Kinetic fatigue diagrams, built by testing of some samples under different loads on the basis Formulas (1) or (2), are characterized the material of tested samples.

Integration of Formula (2)

gives dependence n(l) allowing to calculate the lifetime of the piece

It must be noticed that using SIF as criterion of crack growth is often criticized at last years. SIF is attribute of linear elastic medium and using it as parameter of destruction not takes into account of many factors which have influence on the events near top of crack. It leads to essential, often unpredicted errors in cyclic life calculation.

Besides we notify the some limitations on using formula (3). These limitations connect with necessity of receipt function

These difficulties to some extent are softened and overcame under using as damage parameter

here

Our investigations show that under using thermographic method it is conveniently to keep structure of (2). Corresponding formula lead to

For prediction of cyclic life Formula (5) must be integrated and after that critical growth of crack

We tested some samples, produced from various materials under different loads and built kinetic fatigue diagrams (

On the

These values are taken as middle point on the linear middle part of diagram, which practically defined the cyclic life of samples.

Increment of crack

Dependence

where

Dependence (7) under constant oscillation’s amplitude can be received by two methods:

・ direct from diagram (

・ by testing givens ample under some small cycles of loading (so keep the cyclic life), or by observing for detail under its exploitation; in that case

All tested samples are brought to destruction, and as result its actual cyclic life

The results both experiment and calculation for one sample are showed in

For all samples tested under one-stage loading, divergence

Thermographic method allows predict cyclic life and many-stages loading. In that case Formula (6) become as