RU2139515C1 - Method determining susceptibility of loaded material to injury and its service life - Google Patents

Abstract

FIELD: evaluation of susceptibility of structures to injury and determination of their service life under conditions of constant and variable loads. SUBSTANCE: some samples of metal are subjected to annealing, thermal or strain hardening. All samples are tensile-tested and tested under mode of cyclical load to failure under fixed rate of loading. Deformation curves are plotted for all states of samples with allowance for change of cross-section area of samples. Yield points, yield strength in annealed state, ultimate strength corresponding to moment of failure of samples are found. Dependencies of present measure of injury and criterion of injury are calculated per each state of metal by these characteristics. Comparing values of present measure of injury and criterion of injury which is constant value one determines margin of service life and measure of metal injury at specified time moment by mathematical dependence. Total service life or remaining share of service life of structure from given material are found by comparison of calculated value of measure of injury with criterion of injury. EFFECT: enhanced authenticity of evaluation of measure of susceptibility to injury and service life of structure at stage of its manufacture and in process of its usage. 2 cl, 6 dwg, 1 tbl

Description

 The invention relates to the field of structural strength testing and can be used to assess their damage and determine the temporary service life under long-term exposure to constant and variable loads.

 A known method for determining the stress-strain state of a structural element, RF patent N 2082141 10/03/91, G 01 N 3/00, according to which a sample is cut out of the structural element, subjected to compressive load until it is destroyed and a deformation curve σ = f (ε) is built . The structural element is loaded with an additional load, the corresponding stress Δσ and the relative deformation Δε are determined. The Δσ / Δε ratio is determined and the stress σ ′ and the strain ε ′ corresponding to this relation are determined by the deformation curve, which are selected as parameters by which the stress-strain state of the material of the structural element is judged and the corresponding strength and deformability resources at the time of testing. The method is based on the relationship between the stress-strain state of the structure and the measured characteristics of the fracture process. However, this method does not make it possible to determine the residual resource, since it does not take into account the damage to the material due to both internal defects and accumulated during operation.

Closest to the claimed method in essence is the "Method for determining the damageability of the loaded material." RF patent N 2077046, cl. G 01 N 3/00. The method includes determining the parameters of damage and evaluating the measure of damage by calculation. The number of cracks n forming in the loaded material during time t is measured, the dependence n = f (t) is built, extrapolated to the moment t at which the measure of damage is evaluated, the average length of cracks r and the volume V of the crack formation region are measured, and the limit number of cracks r is calculated ^* and determine the measure of damage to the material at time t as the probability Q (t) of the formation of a cluster of i initial cracks by the formula.

 The advantages of this method compared to the analogue are that it considers a physically justified measure of the damageability of the material, which can serve as a measure of the long-term determination of the time before the destruction of the structure. The method is non-destructive and can be applied to various materials and operating conditions.

 For the first time, localization of crack formation (growth of a main crack) is associated with the number of initial cracks, and non-destructive testing methods such as X-ray scattering, light scattering, acoustic emission, etc. are used to measure the number of initial cracks, but the main disadvantage of physical metols is that they do not measure damage proper, and the physical effects corresponding to them, therefore, as applied to metals, the accuracy of these metols, from the point of view of the relation of damage to measured physical parameters, is and small. Thus, it is not possible to determine the number of cracks experimentally and, therefore, in this work, the measure of damage is a random probability value. There is no criterion for assessing damage, therefore, and its quantitative assessment, without which it is impossible to reliably determine the service life of the structure.

 The objective of the present invention is to increase the reliability of assessing the measure of damage and determining the serviceability resource both at the stage of manufacturing the structure and during operation by substantiating and taking into account the totality of factors that cause mechanical failure and the laws of their interaction, which allows us to derive the problem of destruction from the field of random events and move from a qualitative to a specific quantitative assessment of the measure of damage, resource and remaining service life.

а критерий повреждения определяют по формуле

сравнивая значение текущей мер повреждений P_σ с критерием повреждения P(r)= const определяют долю ресурса, а меру повреждения в момент времени t определяют по формуле

где

ядро повреждений в начальный момент времени;
t - время;
σ(t) - функция процесса нагружения реальной конструкции, выраженная через напряжение;
σ^° - напряжение - задается как постоянная величина в пределах эксплуатационных напряжений,
сравнивая расчетное значение меры повреждений P(t) с критерием повреждения P(r)=const, определяют полный ресурс или оставшуюся долю ресурса конструкции; в процессе механических испытаний, а также в промежутках между циклами нагружения измеряют физические параметры образцов методами неразрушающего контроля и получают тарировочные характеристики шкалы приоров в зависимости от значений текущей меры повреждений P_σ и меры повреждений при разрушении P_r исследуют конструкцию методами неразрушающего контроля, по показаниям приоров и тарировочным характеристикам определяют значения текущей меры повреждений P_σ и критерий повреждения P(r)=const, которые используют для расчета меры повреждения P(t) и соответственно определения ресурса на любой стадии эксплуатации конструкции.The problem is solved in that in a method that includes determining the parameters of damage and assessing the degree of damage to the material, take samples of the metal of this melting, some of the samples are subjected to annealing, heat hardening or curing operations, all samples are subjected to tensile tests and to the cyclic fracture mode of failure at a fixed loading rates according to the results of mechanical tests will build the strain curves σ (ε) for all states of the samples, taking into account changes in the cross-sectional area of the sample and determine yield yield strength -σ _s , yield strength in annealed condition -σ $\genfrac{}{}{0ex}{}{\text{°}}{\text{s}}$ , the strength limits σ _in , corresponding to the moment of destruction, and according to these parameters, the dependences of the current damage measure for each state of the metal are built:

and the damage criterion is determined by the formula

comparing the value of the current damage measure P _σ with the damage criterion P (r) = const determine the share of the resource, and the measure of damage at time t is determined by the formula

Where

core damage at the initial time;
t is the time;
σ (t) is the function of the loading process of a real structure, expressed in terms of stress;
σ ^° - stress - is set as a constant in the range of operational stresses,
comparing the calculated value of the damage measure P (t) with the damage criterion P (r) = const, determine the total resource or the remaining fraction of the design resource; in the process of mechanical tests, as well as in the intervals between loading cycles, the physical parameters of the samples are measured by non-destructive testing methods and the calibration characteristics of the prior scale are obtained depending on the values of the current damage measure P _σ and damage measures during destruction P _{r the} structure is examined by non-destructive testing, according to the testimony of prior and calibration characteristics determine the values of the current damage measure P _σ and the damage criterion P (r) = const, which are used to calculate the damage measure P (t ) and, accordingly, determining the resource at any stage of operation of the structure.

 Of the many factors affecting structural failure: total stresses (temperature-force, technological, stresses from cracks, etc.), local heterogeneities of the structure, it is difficult to single out the determining one. Therefore, at present, there are no methods that allow reproducing real damage to the structure of the material in laboratory conditions, as well as estimating their number, taking into account the entire process of damage accumulation. There is a need to search for such a method for assessing damage, which would have a direct relationship with the physicomechanical strength parameters, which reflects real structural changes in the metal.

 In developing the experimental-theoretical method for assessing the strength of metals, we used the methods of continuum mechanics, which is based on the development of processes of mechanical (structural) damage that occur on the surface of products under prolonged static and vibrational loading, reflecting the entire process of damage accumulation.

где φ(t) - ядро повреждений.As a basic theory of long-term strength, we used the tensor linear theory of A. Ilyushin, according to which the theoretical measure of damage P (t) is

where φ (t) is the damage core.

 According to the theoretical measure of damage (1), physical and mechanical parameters are taken as indicators of the damage measure.

где E₀ и σ_s - модуль упругости и предел текучести в неповрежденном материале;
E(ε,t) и σ(ε,t) - модуль упругости и напряжение упрочнения соответственно, измеренные после процесса нагружения ε = ε(t).
Меры повреждения (2) через модуль упругости и напряжение прямо связаны с характеристиками структуры металла. При этом модуль упругости в основном отражает межатомные связи в металле, а характер изменения напряжения в пластической области прямо зависит от развития процесса структурных несовершенств металла. Из формулы (2), которая отражает функциональную связь между теоретической мерой повреждений P(t) и ядром повреждений φ(t), следует, что если экспериментальные показатели повреждений P_σ и P_E действительно отражают уровень структурных несовершенств металлов, то они должны быть подобны, отличаясь на величину масштабного коэффициента.

where E ₀ and σ _s are the elastic modulus and yield strength in the undamaged material;
E (ε, t) and σ (ε, t) are the elastic modulus and hardening stress, respectively, measured after the loading process ε = ε (t).
Damage measures (2) through the elastic modulus and stress are directly related to the characteristics of the metal structure. In this case, the elastic modulus mainly reflects interatomic bonds in the metal, and the nature of the change in stress in the plastic region directly depends on the development of the process of structural imperfections of the metal. From formula (2), which reflects the functional relationship between the theoretical measure of damage P (t) and the damage core φ (t), it follows that if the experimental damage indicators P _σ and P _E really reflect the level of structural imperfections of metals, then they should be similar , differing by the magnitude of the scale factor.

Illustration of the experimental dependences obtained by tensile carbon steel and chromium-nickel steel, shown in Fig. 1 and 2, respectively, show that the graphs of the dependences P _σ and P _E are similar and differ by the value of the numerical coefficient, which is close to 10. An adequate coincidence of the nature of the functions P _σ and P _E allows one of them to be used for assessing the long-term resistance of metals, preferably P _σ , which is more sensitive to structural imperfections of metals associated with heat hardening and deformation modes than the elastic modulus, the variation range of which rarely exceeds 10% from the original value. The legitimacy of the use of the characteristics of P _σ is based on the main fundamental position of the theory of damage Ilyushin AA, which consists in the fact that for any heat treatment of the metal, the damage criteria for a fixed stress state is constant.

где
П(O,U) = P_σ/σ^o - ядро повреждений в начальный момент времени;
U(τ) - функция процесса нагружения реальной конструкции, которая однозначно может быть выражена через напряжение;
σ^o - напряжение - постоянная величина в пределах эксплуатационных напряжений;
t - время.To assess the long-term structural strength, a variant of the non-linear theory of A. Ilyushin, as applied to the plastic state, in which the physical processes of loading can be expressed through deformation, the measure of damage, has the form:

Where
P (O, U) = P _σ / σ ^o - damage core at the initial time;
U (τ) is a function of the loading process of a real structure, which can be unambiguously expressed in terms of stress;
σ ^o - stress - a constant in the range of operational stresses;
t is time.

To experimentally confirm this position, we constructed the deformation curves σ = f (ε) of chromium-silicon-manganese steel samples for three states, shown in FIG. 3, where:
1 - the sample is hardened to the highest strength (minimum ductility);
2 - sample in the initial state;
3 - the sample is heat treated to one annealing (soft state).

σ(ε) - истинное напряжение (с учетом изменения площади образца);
σ_s - предел текучести металла в термообработанном состоянии (для каждого состояния свой предел текучести), σ $\genfrac{}{}{0ex}{}{\text{o}}{\text{s}}$ - предел текучести в отожженном состоянии (принимается за начало отсчета).The current measure of damage in various states of hardening is calculated by the formula

σ (ε) is the true stress (taking into account changes in the area of the sample);
σ _s is the yield strength of the metal in the heat-treated state (for each state its yield strength), σ $\genfrac{}{}{0ex}{}{\text{o}}{\text{s}}$ - yield strength in the annealed condition (taken as the reference point).

A graphical representation of function (3) for three states is shown in FIG. 4, where:
1 - sample hardened;
2 - sample without heat treatment (delivery status);
3 - sample in annealed condition.

It can be seen from it that at the moment of fracture (or the formation of a “neck”) all the curves have approximately the same damage value P _r - const, which was required to confirm experimentally.

 This experimental-theoretical method, based on real physical quantities used in assessing strength, in combination with non-destructive testing (MNC) methods, allows us to establish a relationship between the indicators of mechanical damage to metal and MNC data. The conducted studies have confirmed the correspondence between the indicators of mechanical damage to the metal structure and the signals of the prior used in OLS. Thus, using the proposed experimental-theoretical method and equipment for OLS, a closed system of parameters is obtained that is sufficient for use in testing finished structures, to assess the residual strength and resource during operation.

Example. Samples of one heat from 30KhGSA steel were used. One part of the samples was annealed, quenched, some samples were left in the initial state, the samples were subjected to tensile tests to failure at a fixed loading rate, according to the test results and the deformation curves σ (ε) for all states of the samples, taking into account changes in the cross-sectional area of the sample, and determined yield strength -σ _s , yield strength in annealed condition -σ $\genfrac{}{}{0ex}{}{\text{o}}{\text{s}}$ , the strength limits σ _in , corresponding to the moment of destruction, and according to these characteristics, dependencies of the current measure of damage for each state of the metal are constructed.

и определен критерий повреждения для каждого состояния:

(графическое изображение этого построения представлено на фиг. 5),
где 1 - образец закален в воде;
2 - образец закален в масле;
3 - образец отожжен;
4 - образец без термообработки.

and a damage criterion is defined for each condition:

(a graphic image of this construction is presented in Fig. 5),
where 1 is a sample hardened in water;
2 - the sample is hardened in oil;
3 - the sample is annealed;
4 - sample without heat treatment.

 In the process of mechanical testing, the physical parameters of the samples were measured by non-destructive testing; relative magnetic permeability, ultrasonic inspection, microhardness (see Fig. 6).

 The results of mechanical tests, calculations, and the correspondence of the parameters of mechanical damage with the physical values of the readings of devices used in OLS are summarized in the table.

ядро повреждений в начальный момент времени;
t - время;
σ(t) - функция процесса нагружения реальной конструкции, выраженная через напряжение;
σ^o - напряжение задается как постоянная величина в пределах эксплуатационных напряжений.From the table and FIG. 5 it can be seen that, for example, in a sample after quenching in oil, the current measure of damage at the time of failure is P _σ = 0.6, the difference between it and the damage criterion P _r -P _σ = 0.85-0.6 = 0.25, it follows that steel hardened from oil quenching, which amounted to 34% of the limit value or damage criterion P _r , if the damage criterion P _r = 0.85 is taken as 100%, in other words, the resource margin of steel after quenching in oil is 34% The value of the damage measure for the structure made of this steel was obtained by calculation taking into account the time (t) according to the formula:

core damage at the initial time;
t is the time;
σ (t) is the function of the loading process of a real structure, expressed in terms of stress;
σ ^o - the voltage is set as a constant value in the range of operational stresses.

If τ = 3 years, and P (t) = 0.29, then comparing P (t) with the magnitude of the universal damage criterion) P _r = 0.85 - const for a given steel, we obtain the total estimated life reserve of the design working capacity equal to 0.85 x 0.29 = 8.7 years.

Since the physical values of the readings of prior non-destructive testing are related to the damage index P _σ and, therefore, to P (t), the readings of the prior MNCs can be used to evaluate the level or proportion of damage in structures accumulated during operation and comparing them with the ultimate damage index P _r = 0.85, determine the residual life of the structure of this steel, as described above.

 Similar measurements using OLS will allow normalizing the permissible levels of metallurgical and technological damage that affect the resource of the product.

 Thus, the application of the proposed method makes it possible to determine the damage to the structural material accumulated at the stages of the metallurgical, technological processes, i.e. during factory quality control and at the installation stage; To calculate the resource of a new product, taking into account loading conditions during operation, diagnose a working product to determine accumulated damage during operation and residual life (residual strength). If the test procedure for this method uses samples of the product destroyed during operation, it is possible to identify the cause of the destruction (metallurgical, technological, operational, etc.).

Claims (3)

1. A method for determining the damageability of a loaded metal and a service life resource, including determining the damageability parameters and evaluating a metal damage measure, characterized in that some of the metal samples are annealed, heat strengthened, or hardened, all samples are subjected to tensile tests and under cyclic loading to failure under fixed loading speed, according to the results of mechanical tests, strain curves σ (ε) are constructed for all states of the samples taking into account changes in area operechnogo sectional samples are determined for them yield stress σ _s, the yield stress in the annealed condition σ $\genfrac{}{}{0ex}{}{\text{°}}{\text{s}}$ , the strength limits σ _in corresponding to the moment of destruction, for each state of the metal according to these characteristics calculate the dependence of the current measure of damage P _σ :

determine damage criteria

comparing the values of the current damage measure P _σ with the damage criterion P _r = const, determine the margin by resource, and the measure of metal damage at time t is determined by the formula

Where

core damage at the initial time;
P - core damage;
t is the current time;
σ (t) is the function of the metal loading process;
σ ^° is the specified constant voltage;
σ (τ) is the calculated voltage;
τ is the integration time,
comparing the calculated value of the damage measure P (t) with the damage criterion P _r = const, the total resource or the remaining fraction of the resource of a structure made of a given metal is determined. 2. The method according to claim 1, characterized in that during the mechanical tests, as well as between the loading cycles, the physical parameters of the metal samples are additionally measured by non-destructive testing methods and the dependences of the physical parameters on the current damage measure P and damage criterion P _{r are} determined. 3. The method according to claim 1, characterized in that non-destructive testing methods examine the structure of this metal to determine the current damage measure P and damage criterion P _r .

Images