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Static fracture toughness characteristics are traditionally determined in tests of standard notched specimens using a P-V curve, where P is the load and
*V* is the notch-opening displacement. This curve has a characteristic point
*Q*. At the load
*P*
_{Q} corresponding to this point, the crack starts to propagate. For this load, the fracture toughness characteristics are then calculated. In brittle (elastic) fracture, the P-V curve at the onset of crack propagation has an extremum (or a local extremum), from whose ordinate
*P*
_{Q} is determined with sufficient accuracy. In ductile and elastic-ductile fracture, P-V curves are monotonically increasing, and
*P*
_{Q} is calculated using the 5% secant offset method without taking into account the characteristics of the material, so that the
*P*
_{Q} is determined inaccurately. To improve the accuracy of
*P*
_{Q} determination, we propose a thermographic method for determining the fracture toughness of metals. This method involves plotting the load P against the temperature change
Δ
*Т* over a relatively short period of time at the notch tip. This plot is then transformed to a
*P-ΔS* curve, where ΔS is the specific entropy increment at the notch tip, which is calculated through
ΔТ. This thermodynamic diagram has a characteristic step at the beginning of crack propagation, and from the ordinate of this step,
*P*
_{Q} can be determined much more accurately. Furthermore, in the thermographic method, the preparation of test specimens can be simplified by replacing the process of growing a fatigue crack at the tip of a notch by making a sharp cut, which provides significant time savings. Statistical processing and comparison of test results of steel 20 specimens using the conventional and thermographic methods have shown the advantages of the thermographic method in accuracy and complexity.

Optics-mechanical devices, being used in modern machines, might be exploited in extreme conditions under significant external loading. That is why the power components of these instruments must possess adequate strength characteristics.

This work considers a new (designed by the authors) thermographic method for definition of static crack resistance characteristics of materials.

These characteristics are defined experimentally on special samples with an incision (cut) created in advance. We use either flat samples with a crack in the center or in the edge, or cylinder samples with a ring crack. Before the test it is necessary to extend the cut by fatigue crack with the length not less than 1.5 mm.

At standard testing [_{Q}, on the basis of which all the crack resistance parameters are then calculated.

In case of brittle destruction nominal load is equal to either maximal, or local maximal of loading, i.e. crack initiation is managed to be fixed pretty exactly. Diagrams “P-V”, representing visco-elastic or viscous destruction, are characterized by the absence local maximums of loading, which makes it difficult to fix nominal load. For this purpose, it is recommended to make the following―to draw 5 % secant line, and nominal load P_{Q}_{ }is defined as the ordinate of this secant line crossing the curve “P-V” [

However at such traditional approach, the nominal load P_{Q}_{ }is not defined with enough accuracy, because individual features of tested materials are not taken into account, and also drawing a 5% secant line is rather difficult. Besides, when preparing the samples for test, it is necessary to make fatigue cracks 0.3 mm deep, which appears to be rather difficult an operation.

The objectives of this research were: the improvement of determination accuracy of crack resistance parameter in case of visco-elastic and viscous destruction; decreasing labour intensity when preparing samples for experiment. The research was conducted on flat samples from steel 20.

For more precise fixation of nominal load we used thermographic parameter

By comparison of these two thermodynamic diagrams with diagram “P-V” the correlation between their characteristic points was determined.

The sample sizes were chosen according to the recommendations in [

Crack resistance (fracture viscosity) parameters were determined on the results of single static tests of given samples on the universal hydraulic machine “УГ-20”. The temperature in the crack tip was fixed with help of thermo vision device“Aga-750” [

In testing the samples in crack tips occur elastic-plastic deformations with big plastic zone formation. In such cases as the most reliable crack resistance parameters are accepted:

Here,

Let us discuss the obtained results. First of all we will note that the diagram “-V” for the 2 group samples (

Let us also note, that diagrams “P-ΔT” and “P-ΔS” have three clearly defined zones [

And the very important thing is, that certain points of these diagrams correlate with certain points on diagram ”P-V”: ordinates of the point on the boundary between 1-st and 2-nd zones practically correspond with the ordinate of the point Q on diagram “P-V”, and points С, denoting maximal loads, are practically equal.

Then with the standard method all necessary crack resistance parameters for 1-st group samples were defined on the obtained in experiment reference points, taken from diagram “P-V”, and thermodynamic diagram “P-ΔS” defined the parameters for all the samples of both groups.

And in conclusion calculated parameters were statistically processed, and as a result were defined confidence intervals and average square errors of crack resistance parameters, obtained from diagrams ”P-V” and “P-ΔS”.

1) Fracture toughness characteristics are fairly accurately determined by the conventional method (based on P-V curves) in brittle (elastic) fracture. In tests of steel 20 specimens, ductile fracture was observed. Due to the complex nature of the crack initiation phenomenon in ductile fracture, it cannot be detected directly from P-V curves. Hellan [

2) From the calculated confidence intervals, it can be seen that the fracture toughness characteristics calculated by the P-ΔS curve are more concentrated than the characteristics calculated from the P-V curve, suggesting that the accuracy of their determination is higher. This result can be explained by the fact that the P-ΔS curve has a sharp break near the calculated point, from which this point is easy to determine. This break has a clear physical interpretation -it corresponds to the onset of intense plastic deformation, which, as noted in the literature and observed in our experiments, occurs suddenly [

3) The finding that P-∆Т and P-∆S curves do not depend on the type of specimen allows the time-consuming process of growing a fatigue crack in the preparation of test specimen to be replaced by the simple process of making a sharp cut from the top of a notch. This also suggests that thermodynamic diagrams accurately reflect the properties of the test material.

The list of comments and concerns on the use of the thermodynamic approach in static fracture toughness tests can be continued. However, it is undeniable that in the viscoelastic fracture tests of steel 20 specimens, P-∆S diagrams provided a more accurate and rapid determination of the load at the onset of crack propagation, which can then be used to calculate fracture toughness characteristics [

Kurilenko, G.A. and Ayrapetyan, V.S. (2016) Determination of the Fracture Toughness of Optomechanical Devices. Optics and Photonics Journal, 6, 298-304. http://dx.doi.org/10.4236/opj.2016.611030