RU2192000C2 - Procedure of nondestructive test of thermal-physical characteristics of materials - Google Patents

Abstract

FIELD: thermal-physical measurements. SUBSTANCE: heat source in the form of circle is positioned on heat-insulated surface of tested material. Surface of tested material is subjected to action of heat pulses of equal energy fed at moments of onset of specified relation of integral values of temperatures in points of location of temperature-sensitive elements. Heat pulse repetition frequency and temperature in one test point are recorded at moment of termination of heat test and thermal-physical characteristics of tested material are computed on basis of formulas given in description of invention. EFFECT: enhanced accuracy of determination of thermal-physical characteristics thanks to increase of excess temperature in test points. 3 dwg

Description

 The invention relates to the field of thermophysical measurements and can be used to determine the thermophysical characteristics of materials.

 There is a method of non-destructive testing of thermophysical characteristics (TPC), in which a pulsed thermal effect is applied in a straight line to the thermally insulated surface of the investigated material (product), and the time is fixed when the ratio of excess temperatures at two points of the surface of the material different from the source reaches the predetermined value in advance ( copyright certificate 834480 of the USSR, MKI G 01 N 25/18, 1979).

 The disadvantage of this method is the relatively low accuracy of the determination of TPC due to the low excess temperature at the control points.

 To increase the excess temperature at the control points is possible by repeatedly supplying thermal pulses. For this, a linear heater is pulsed on the heat-insulated surface of the material under study to pulse equal amounts of energy at times when the temperature ratios at the control points correspond to a given series of numbers (copyright certificate 1728755 USSR, MKI G 01 N 25/18, 1992). The TLC of the test material is calculated on the basis of the recorded thermal pulse repetition rate and excess temperature at one of the control points.

 The disadvantage of this method is that during thermal tests the accuracy of temperature registration (for example, when using thermocouples) can be significantly affected by external electromagnetic fields, which leads to an increase in the error in the determination of TFC.

 In the known technical solution, which is closest to the proposed one (patent for the invention of the Russian Federation 149387, MKI G 01 N 25/18, 2000), a multiple pulsed thermal effect of a point heat source is carried out on the heat-insulated surface of the test material. Thermal pulses are applied when the specified ratios of the integral temperature values are reached, recorded on the surface of the material under study at given distances from the heat source. During thermal tests, the pulse repetition rate and the integral temperature values at the control points are recorded.

 Registration of integral temperature values significantly reduces the influence of external electromagnetic fields on the accuracy of determining the TFC, however, this method does not allow for large excess temperatures at control points, because at the same time, the temperature rises at the location of the heater, which can lead to destruction of the material under study.

 The technical result of the invention is to increase the accuracy of determining the TPC by increasing the excess temperature at the points of placement of the temperature sensors.

 The essence of the proposed method consists in multiple pulsed thermal effects on the thermally insulated surface of the test material by the heater in the form of a circle and registration of the repetition rate of the thermal pulses applied at the moments of reaching the given ratios of the integral temperature values at the points of placement of the temperature sensors.

On the insulated surface of the test material, a heater is arranged in the form of a circle of radius r and two temperature sensors (thermocouples) T _p1 and T _p2 , at the points of placement of which integral values of temperatures I ₁ (τ) and I ₂ (τ) are recorded. The thermocouple T _{p1 is} installed on the circumference of the heater, and the thermocouple T _p2 is installed in the center of the circle. At the moment of the start of the thermal tests τ = 0, the heater pulsedly emits energy Q per unit length of the heater, after which the moment of the equality of the ratio I ₁ (τ) / I ₂ (τ) of the given value h _{1 is} recorded and a second thermal pulse is applied. The condition for giving the third impulse is the occurrence of the equality I ₁ (τ) / I ₂ (τ) = h ₂ , etc. In total, N thermal pulses are supplied.

где ε - заданная постоянная;

табличный интеграл.The values of h _i , (i = 1,2, ..., N) are calculated using the expression:

where ε is a given constant;

table integral.

а коэффициент теплопроводности из соотношения

(3)
где Q - энергия, выделяемая единицей длины нагревателя; r - радиус окружности нагревателя; ε - заданная постоянная; F - регистрируемая частота следования тепловых импульсов; N - количество подаваемых тепловых импульсов; I₂ - температура в точке размещения термопары Т_п2 в момент наступления равенства I₁(τ)/I₂(τ) = h_N;

табличный интеграл.The calculation of the values of h _i in accordance with (1) allows us to provide a constant period of repetition of thermal pulses τ ₀ , depending on the thermal characteristics of the material under study. During the monitoring process, the thermal pulse repetition rate F = 1 / τ ₀ and the integral temperature value I ₂ at the point of placement of the thermocouple T _p2 at the moment of equality I ₁ (τ) / I ₂ (τ) = h _{N are recorded} .
Based on the data obtained, the thermal diffusivity is determined by the formula

and the coefficient of thermal conductivity from the ratio

(3)
where Q is the energy released by the unit length of the heater; r is the radius of the circumference of the heater; ε is the given constant; F is the recorded pulse repetition rate; N is the number of supplied thermal pulses; I ₂ is the temperature at the point of placement of the thermocouple T _p2 at the time of equality I ₁ (τ) / I ₂ (τ) = h _N ;

table integral.

Если коэффициент температуропроводности исследуемого материала находится в пределах от a_min до а_max, то на сновании (4) можно определить наибольший и наименьший периоды следования тепловых импульсов

и наибольшее время проведения тепловых испытаний, которое составит

или

где N - количество подаваемых тепловых импульсов; r - радиус окружности нагревателя; ε - заданная постоянная; а_min - минимальное значение коэффициента температуропроводности исследуемого материала.In view of (2), we can write

If the thermal diffusivity of the studied material is in the range from a _min to a _max , then on the basis of (4) it is possible to determine the largest and smallest periods of the following thermal pulses

and the longest thermal test time, which will be

or

where N is the number of supplied thermal pulses; r is the radius of the circumference of the heater; ε is the given constant; and _min is the minimum value of the coefficient of thermal diffusivity of the investigated material.

 Thus, the choice of ε is advisable to take into account the range of thermal characteristics of the materials subjected to control, and the requirements of control efficiency.

 Figure 1 shows the layout of the heater and thermocouples on the surface of the test material.

а на расстоянии r от нагревателя как

где

энергия, выделяемая точечным источником тепла; τ - текущее время;

и а - соответственно коэффициенты тепло- и температуропроводности исследуемого материала.When one heat pulse is supplied from a point heater, the excess temperature at the source location can be found as

and at a distance r from the heater as

Where

energy released by a point heat source; τ is the current time;

and a are, respectively, the coefficients of thermal and thermal diffusivity of the test material.

а в центре окружности как

где Q - количество теплоты, выделяемое единицей длины нагревателя в виде окружности.When using a heater in the form of a circle of radius r, the temperature at the point of the circle will be determined by the expression

and in the center of the circle as

where Q is the amount of heat released by the unit length of the heater in the form of a circle.

где τ_и - длительность теплового импульса; T(r,τ) - температура, определяемая в зависимости от формы источника тепла равенствами (6)-(9).When calculating the temperature, taking into account the finite duration of the thermal pulse, the relation

where τ _and is the duration of the thermal pulse; T (r, τ) is the temperature determined depending on the shape of the heat source by equalities (6) - (9).

 When conducting thermal tests, the temperature at the heating point (when using a point heater) and on the line of action of the heater in the form of a circle at the time of the heat pulse should not exceed the maximum allowable temperature at which the destruction of the test material occurs. If, by choosing the appropriate heating energy, the equality of excess temperatures is ensured at the time of the heat pulse at the point of action of the point heater and on the line of action of the heater in the form of a circle, then on the basis of (6) - (10) it can be shown that at subsequent times the heater The shape of a circle provides a large excess temperature at the control points compared to a point heater.

Дж; Q= 178 Дж/м; τ_и = 0.1 с; r=2.5•10^-3 м; λ = 1.2 Дж/(м•К); а= 3.5•10^-6 м²/с.In FIG. Figure 2 shows the graphs of temperature changes at the heating point under the action of a point heater (line 1) and on the line of action of the heater in the form of a circle of radius r (line 2) at the same temperature at the time of the heat pulse, constructed taking into account equalities (6), (8) and (10) for

J; Q = 178 J / m; τ _and = 0.1 s; r = 2.5 • 10 ^-3 m; λ = 1.2 J / (m • K); a = 3.5 • 10 ^-6 m ² / s.

 In FIG. Figure 3 shows graphs of temperature changes at a distance r from a point heater (line 3) and in the center of the heater in the form of a circle of radius r (line 4), calculated on the basis of (7) and (9).

Claims (1)

A method of non-destructive testing of the thermophysical characteristics of materials, which includes multiple pulsed thermal effects on the thermally insulated surface of the studied material and registration of the repetition rate of thermal pulses supplied at the moments of the set temperature ratios at two control points on the surface of the studied material, characterized in that the heater is used in the form of a circle and two temperature sensors that record the temperature on the circumference of the heater and in its center, and the coefficients of heat and thermal diffusivity are calculated by the formulas:

where Q is the amount of heat released by the unit length of the heater;
r is the radius of the circumference of the heater;
ε is the given constant;
N is the number of supplied thermal pulses;
F is the recorded pulse repetition rate;
I ₂ is the temperature in the center of the circumference of the heater at the end of the heat test.

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