RU2167421C2 - Method determining safety assurance factor of loaded material - Google Patents

Abstract

FIELD: analysis of materials by way of determination of their physical properties. SUBSTANCE: invention can find use for determination of excess of breaking stress above level of active mechanical stresses, for detection of flaws-stress concentrators in materials. Expansion f range of tested materials, reduced labor input and increased accuracy of determination of safety assurance factor of loaded material are achieved thanks to recording of pulses of acoustic emission in tested material and to measurement of their quantitative characteristics. Tested material is loaded by two loads P1 and P2, rates N1 and N2 of counting of acoustic emission are measured under these loads and safety assurance factor S1 under load P1 is found from relation S1= (P2-P1)lgA/P1 lg(N2/N1), lgA=(Uo/2.3 RT)-13, where Uo is initial energy of activation of breakage; T is absolute temperature of tested material; R is universal gas constant. EFFECT: expanded range of tested materials, reduced labor input and increased accuracy of determination of safety assurance factor.

Description

 The invention relates to the field of analysis of materials by determining their physical properties, more specifically to diagnosing the stress state of loaded materials, in particular to determining the excess of breaking stress (tensile strength) over the level of existing mechanical stresses, and can be used to detect defects - stress concentrators, to assess the durability and residual life of pipelines, pressure vessels, machine parts and mechanisms, etc.

The strength margin S of the loaded material is customarily called the excess of the tensile strength of the material σ ^{* by} the stress level σ acting in the material due to the application of the load P, i.e.
S = σ ^* / σ. (1)
Usually, when determining S, they find σ as a function of P from the calculation [1] or from measurements in the loaded strain material [2], and σ ^{* is} measured independently when the laboratory samples are destroyed. The study of the physical nature of destruction leads to an analytical expression
σ ^* = (U _o -RTlnτ _o / τ ^* ) / γ (2)
where U ₀ is the initial activation energy of the destruction of the material (a constant independent of its defective structure), T is the absolute temperature, R is the universal gas constant, τ _o = 10 ^-13 s, τ ^* = 1 s, γ is the characteristic reflecting the state of the defective structure of the material [3]. In calculating σ ^{*, the} value in brackets (2) is known and the problem is reduced to finding the value of γ. Correspondingly, the definition of S reduces to finding γ, since the procedure for finding σ is standard.

The known method [4], in which the material is loaded with a constant voltage σ, the times τ _{i of} arrival of discrete signals of acoustic emission (AE) are measured and the value is calculated
γ _i = (U _o -RTlnτ _i / τ _o ) / σ.
In this method, the value of γ in the expression for the ultimate strength (2) is proposed to be found by extrapolation, the algorithm of which, however, is not justified.

 The prototype is a method for determining the adhesion of a polymer to a metal [5], which allows, on the basis of formula (2), to determine the tensile strength and thereby the margin of safety of a material loaded with voltage σ, including the effect on the material of a voltage that increases uniformly with time t, measuring the dependence of the number of signals AE N (t) and determination of the value of γ from the slope of the graph ln N - t. The prototype method contains an algorithm for determining the value of γ, but is limited by material (composite polymer-metal). When transferring the prototype method to steel and other metallic materials, there is a danger of destruction of the object during testing. In addition, the use of the prototype method requires loading with a constant growth rate of stress. However, it is time-consuming or practically impossible to implement such a loading regime of an industrial structure.

A common drawback of existing approaches to determining the safety factor (1) is the separate determination of stress σ and ultimate strength σ ^* . In this case, a localized defect-stress concentrator, responsible for the real strength of the structure, may be skipped, since the S determination procedure does not imply the presence of such a defect in the calculation: the measurements are averaged over a considerable length (of the order of 1 cm), and the tensile strength is measured on defect-free samples . These circumstances reduce the accuracy of determination of S.

 The objectives of the invention are to expand the range of the investigated materials, reducing the complexity and improving the accuracy of determining the margin of safety of the loaded material.

This is achieved by the fact that in the known method for determining the margin of safety of a loaded material, according to which acoustic emission pulses are recorded in the studied loaded material and their quantitative characteristics are measured, according to the claims, the material is loaded with two loads P ₁ and P ₂ , the count rate of AE N _{1 is} measured and N ₂ at these loads and the safety factor at load P ₁ is determined from the ratio
S ₁ = (P ₂ - P ₁ ) log A / P ₁ log (N ₂ / N ₁ ),
logA = (U _o / 2.3 RT) - 13,
where U _о is the initial activation energy of fracture, T is the absolute temperature of the studied loaded material, R is the universal gas constant.

 The essence of the method.

The kinetic theory of fracture prediction [6] shows that fracture is caused by the accumulation of a critical concentration of initial stable delocalized cracks C ^* , so that the time τ to fracture at a constant stress σ is
τ = C * / C, (3)
where C is the rate of accumulation of cracks, and has the form of the Zhurkov formula
τ = τ _o exp [(U _o -γσ) / RT] (4)
Given the expressions for the safety factor (1) and ultimate strength (2), formula (4) is transformed to
τ = τ ^* A ^{(s-1) / s} , A = (τ _o / τ ^* ) exp (U _o / RT). (5)
When registering a discrete AE corresponding to the generation of initial cracks (starting from the microscopic size), the value of C is proportional to the count rate of the AE N, so that expressions (3) - (5) can be rewritten in the form
N = N ^* / τ ^* A ^{(S-1) / S} , (6)
where N ^* is an analogue of the value of C ^* . With two voltage values leading to safety factors S ₁ and S ₂ , the ratio of the corresponding two counting rates will be
N ₂ / N ₁ = A ^{1 / S2-1 / S1} (7)
Given the definition of safety margin (1)
1 / S ₂ -1 / S ₁ = (σ ₂ -σ ₁ ) / σ ₁ S ₁ . (8)
Finally, since the stress σ is proportional to the load P, we finally find from (7) and (8)
S ₁ = (P ₂ - P ₁ ) logA / P ₁ (N ₂ / N ₁ ) (9)
For the first time, the author was able to establish a quantitative universal relationship between the margin of safety of a real (containing defects) loaded material and the rate of generation of discrete AE pulses in it, corresponding to the preparation of the material for failure at the stage of accumulation of diffuse damage (delocalized initial cracks).

 Unlike the prototype, the proposed method is valid for any materials (in which it is possible to register crack formation by the AE method) is less time consuming (since setting two fixed constant load values is simpler than loading with a constant voltage growth rate), it is more accurate (since it does not contain the transition from load to voltage and does not require separate determination of the effective voltage and tensile strength). The implementation of the proposed method does not require conversion of the load on the voltage.

The method is as follows. According to literature data or on samples of the studied material, the value of the initial fracture activation energy U ₀ is determined and lgA (5) is calculated for a given temperature T. A constant load is set at the object, discrete acoustic emission is recorded and its count rate is determined. Then the same is repeated for a different load and according to the claims, the margin of safety of the test material at the first load is calculated.

 An example implementation of the method.

The margin of safety of the zinc sample was determined under uniaxial tension at a temperature of 90 ^o C. According to the literature [6], for zinc U _о = 130 kJ / mol, that is, at a research temperature lgA = 5.84. AE was recorded in the loaded sample and its counting rate was measured, which at a load of P ₁ = 3000 H was equal to logN ₁ = 2.75, and at a load of P ₂ = 6000 H, logN ₂ = 4.5. According to the claims, with these data for the load P _{1, the} safety factor is S ₁ = 3.33. To verify this calculation, the specimen was destroyed and the breaking load was equal to P ^* = 9000 H, that is, the load P ₁ = 3000 H corresponded to a safety margin of 3. Thus, the determination of the safety margin by the proposed method is in satisfactory agreement with the assessment of the safety margin in independent experience.

 The proposed method can be used to study various metal structures, products from composite materials, to study the stress state of a rock mass in mines, etc. in those cases when the AE method can be used to register the process of crack formation in a loaded material.

Literature
1. Fedoseev V.I. Strength of materials. High School, 1963.

 2. Sukharev I.P. Experimental methods for the study of deformations and strength, M .: Engineering, 1987, p. 27, 88.

 3. Regel V. P., Slutsker A. I., Tomashevsky E.E. The kinetic nature of the strength of solids. M .: Nauka, 1974.

 4. Donin A. P. Flaw detection, 1981, N 9, p. 11-17.

 5. USSR author's certificate N 1467458, cl. G 01 N 19/04, BI N 11.1989.

 6. Petrov V.A., Bashkarev A.Ya., Vettegren V.I. Physical basis for predicting the durability of structural materials, St. Petersburg, Polytechnic, 1993.

Claims (1)

A method for determining the margin of safety of a loaded material, according to which acoustic emission pulses are recorded in the test material and measuring their quantitative characteristics, characterized in that the test material is loaded with two loads P1 and P2, the acoustic emission count rates N1 and N2 and the margin of safety S1 are measured at these loads at load P1 is determined from the ratio
S1 = (P2 - P1) logA / P1 log (N2 / N1),
logA = (Uo / 2,3RT) -13,
where Uo is the initial activation energy of destruction;
T is the absolute temperature of the test material;
R is the universal gas constant.