CN103542894B - Reinforcement stresses, strain measurement method under high temperature change loading speed - Google Patents

Abstract

The invention relates to a method for measuring the stress and strain of steel bars under variable loading rates at high temperatures, and the invention relates to a method for testing the stress and strain of building materials in the field of civil engineering. The invention aims to solve the problem that the existing high-temperature strain extensometer cannot directly obtain the full strain of the steel bar at high temperature and the measurement result is inaccurate, and provides a method for measuring the stress and strain of the steel bar under high-temperature variable loading rate. Obtain the stress and strain curves and corresponding stress and deformation curves of steel bars at high temperatures through the steel bar strain test system at high temperatures; when measuring displacement, calculate the strain of steel bars at high temperatures based on the displacement coordination equation; assume the stress and strain curves of steel bars at high temperatures Composed of a "sticky pot" and an inviscid elastoplastic element, the viscous characteristic value k/α is obtained; according to formula (3), the stress and strain curves obtained by the two-stage loading rates v ₁ and v ₂ are transformed into arbitrary loading rates Stress and strain curves under v. The invention is applied to the high-temperature stress and strain measurement of steel bars in the field of civil engineering.

Description

Measuring method of steel bar stress and strain under variable loading rate at high temperature

technical field

The invention relates to a method for testing building material stress and strain in the field of civil engineering.

Background technique

The ultimate tensile strain of steel bars used in building structures at high temperatures can reach 20%, but the range of high-temperature strain extensometers in the steel bar strain test system at high temperatures is limited (generally ±0.03), and the existing high-temperature strain extensometers cannot directly obtain the steel bars at high temperatures. Full strain. At the same time, the strength of the steel bar varies with the loading rate at high temperature, resulting in inaccurate measurement results.

Contents of the invention

The invention aims to solve the problem that the existing high-temperature (20-1100°C) strain extensometer cannot directly obtain the full strain of the steel bar at high temperature and the measurement results are inaccurate, and provides a method for measuring the stress and strain of the steel bar under high-temperature variable loading rates.

1. Obtain the stress and strain curves and corresponding stress and deformation curves of steel bars at high temperature through the steel bar strain test system at high temperature;

Wherein, the maximum value of strain in the stress and strain curve is ε ₀ , and ε ₀ is less than the ultimate tensile strain of the steel bar at high temperature;

When the steel bar strain is less than ε ₀ , the loading rate is v ₁ ; the steel bar strain test system simultaneously measures the stress, strain, stress and displacement of the steel bar at high temperature;

When the steel bar strain exceeds ε ₀ , the high-temperature strain extensometer in the steel bar strain test system reaches the measuring range, and the steel bar strain test system automatically switches to displacement measurement, that is, measures the total deformation Δ _T of the steel bar, and the loading rate is v ₂ ;

2. When performing displacement measurement, based on the displacement coordination equation, the calculation formula for the strain of the steel bar at high temperature is as follows:

ε _T =[Δ _T -(l ₀ -l ₁ )σ _T /E _s ]/l ₁ (1)

In the formula, ε _T represents the strain of the steel bar at the temperature of T°C, Δ _T represents the total elongation of the steel bar at the temperature of T°C, σ _T represents the measured stress of the steel bar at the temperature of T°C, E _s represents the elastic modulus of the steel bar at room temperature, l ₀ means the total length of the steel bar, l ₁ means the length of the steel bar after equivalent treatment at T℃, and T ₀ means the room temperature;

Wherein, the equivalent processing steps are:

Draw the temperature distribution diagram of the steel bar at high temperature, the equivalent temperature distribution diagram of the steel bar at high temperature, and perform equivalent processing on the temperature distribution diagram of the steel bar at high temperature and the equivalent temperature distribution diagram of the steel bar at high temperature according to the equal temperature area of the steel bar before and after the equivalent;

Wherein, the calculation steps of ₁₁ are:

According to the principle that the steel bar strain is equal at the change of strain measurement and deformation measurement, iteratively calculate the length l ₁ of steel bar temperature T°C after equivalent calculation:

①. Determine the total elongation Δ _T and stress σ _T of the corresponding steel bar when the strain (T) is ε ₀ at high temperature;

②. Find the same strain as σ _T stress in the stress and strain curve at room temperature (T ₀ );

③. Given an initial value of l ₁ , put it into formula (1) to find ε _T ;

④. Adjust the initial value of l ₁ ;

⑤. Repeat steps ③ and ④, iteratively calculate until ε _T is equal to ε ₀ , and record the l ₁ value when ε _T is equal to ε ₀ ;

⑥. Calculate the strain ε _T in the high temperature zone of the steel bar by using the l ₁ value and the stress σ _T in the stress and deformation curve greater than ε ₀ , and then convert the stress and deformation curve of the steel bar with a strain greater than ε ₀ into a stress and strain curve;

3. Assuming that the stress and strain curve of the steel bar at high temperature is composed of a "sticky pot model" and an inviscid elastic-plastic element, define the ratio of the loading rate v to the strain change rate When the loading rate v ₁ , the stress provided by the "sticky pot model" σ ₁ =k×v ₁ /α; when the loading rate is v ₂ , the stress provided by the "sticky pot model" σ ₂ =k×v ₂ / α, thus the viscous eigenvalue k/α can be obtained:

$\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

From the stress and strain curves obtained in step 1, the stress change when the loading rate changes, that is, (σ ₂ -σ ₁ ), the two-stage loading rates v ₁ , v ₂ and (σ ₂ -σ ₁ ) are substituted into the formula ( 2), the viscosity characteristic value k/α corresponding to each specimen at a certain temperature can be obtained, and k is the viscosity coefficient;

4. Transform the stress-strain curves obtained by the two-stage loading rates v ₁ and v ₂ into stress and strain curves at any loading rate v according to formula (3):

σ=σ _i +k/α(vv _i )i=1,2(3)

That is to say, the method of measuring the stress and strain of steel bar under high temperature and variable loading rate has been completed.

Effect of the invention: Based on the deformation coordination method, a small-scale high-temperature strain extensometer is realized to measure the ultimate tensile strain of the steel bar. Based on the principle of viscoelasticity, the influence of the change of the loading rate at high temperature on the strength of the steel bar is eliminated. This method can realize different loading at high temperature Rate steel stress, strain full curve measurement. By applying the invention, the measuring range of tensile strain of steel bar under high temperature can be extended from 0.03 to 0.2, and the measuring range of strain can be enlarged by 6.7 times. At the same time, the influence of the change of loading rate on the stress measurement accuracy is solved, and the improvement of the measurement accuracy varies with the change of the loading rate. When the type of steel bar is determined, the greater the change in the loading rate, the higher the measurement accuracy can be after adopting the present invention, and the stress measurement accuracy can be improved by 5-10% in general.

Description of drawings

Fig. 1 is a flowchart of the present invention;

Fig. 2 is the actual temperature distribution of steel bar under high temperature in the embodiment;

Fig. 3 is the temperature distribution equivalence figure of reinforcing bar under high temperature in the embodiment;

Fig. 4 is certain steel bar stress, strain curve under different loading rates in the embodiment;

Fig. 5 is reinforcing bar stress, strain curve under high temperature in the embodiment;

Fig. 6 is reinforcing bar stress, displacement curve under high temperature in the embodiment;

Fig. 7 is a flow chart of converting the stress and displacement curves of steel bars under high temperature into stress and strain curves in the embodiment;

Fig. 8 is the full stress and strain curve of the steel bar at high temperature with the elimination of the influence of the loading rate obtained after applying the present invention in the embodiment.

detailed description

Specific implementation mode 1: The steel bar stress and strain measurement method under the high temperature variable loading rate of the present implementation mode includes the following contents:

1. Obtain the stress and strain curves and corresponding stress and deformation curves of steel bars at high temperature through the steel bar strain test system at high temperature;

Wherein, the maximum value of strain in the stress and strain curve is ε ₀ , and ε ₀ is less than the ultimate tensile strain of the steel bar at high temperature;

When the steel bar strain is less than ε ₀ , the loading rate is v ₁ ; the steel bar strain test system simultaneously measures the stress, strain, stress and displacement of the steel bar at high temperature;

When the steel bar strain exceeds ε ₀ , the high-temperature strain extensometer in the steel bar strain test system reaches the measuring range, and the steel bar strain test system automatically switches to displacement measurement, that is, measures the total deformation Δ _T of the steel bar, and the loading rate is v ₂ ;

2. When performing displacement measurement, based on the displacement coordination equation, the calculation formula for the strain of the steel bar at high temperature is as follows:

ε _T =[Δ _T -(l ₀ -l ₁ )σ _T /E _s ]/l ₁ (1)

In the formula, ε _T represents the strain of the steel bar at the temperature of T°C, Δ _T represents the total elongation of the steel bar at the temperature of T°C, σ _T represents the measured stress of the steel bar at the temperature of T°C, E _s represents the elastic modulus of the steel bar at room temperature, l ₀ means the total length of the steel bar, l ₁ means the length of the steel bar after equivalent treatment at T℃, and T ₀ means the room temperature;

Wherein, the equivalent processing steps are:

Draw the temperature distribution diagram of the steel bar at high temperature, the equivalent temperature distribution diagram of the steel bar at high temperature, and perform equivalent processing on the temperature distribution diagram of the steel bar at high temperature and the equivalent temperature distribution diagram of the steel bar at high temperature according to the equal temperature area of the steel bar before and after the equivalent;

Wherein, the calculation steps of ₁₁ are:

According to the principle that the steel bar strain is equal at the change of strain measurement and deformation measurement, iteratively calculate the length l ₁ of steel bar temperature T°C after equivalent calculation:

①. Determine the total elongation Δ _T and stress σ _T of the corresponding steel bar when the strain (T) is ε ₀ at high temperature;

②. Find the same strain as σ _T stress in the stress and strain curve at room temperature (T ₀ );

③. Given an initial value of l ₁ , put it into formula (1) to find ε _T ;

④. Adjust the initial value of l ₁ ;

⑤. Repeat steps ③ and ④, iteratively calculate until ε _T is equal to ε ₀ , and record the l ₁ value when ε _T is equal to ε ₀ ;

⑥. Calculate the strain ε _T in the high temperature zone of the steel bar by using the l ₁ value and the stress σ _T in the stress and deformation curve greater than ε ₀ , and then convert the stress and deformation curve of the steel bar with a strain greater than ε ₀ into a stress and strain curve;

3. Assuming that the stress and strain curve of the steel bar at high temperature is composed of a "sticky pot model" and an inviscid elastic-plastic element, define the ratio of the loading rate v to the strain change rate When the loading rate v ₁ , the stress provided by the "sticky pot model" σ ₁ =k×v ₁ /α; when the loading rate is v ₂ , the stress provided by the "sticky pot model" σ ₂ =k×v ₂ / α, thus the viscous eigenvalue k/α can be obtained:

$\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

From the stress and strain curves obtained in step 1, the stress change when the loading rate changes, that is, (σ ₂ -σ ₁ ), the two-stage loading rates v ₁ , v ₂ and (σ ₂ -σ ₁ ) are substituted into the formula ( 2), the viscosity characteristic value k/α corresponding to each specimen at a certain temperature can be obtained, and k is the viscosity coefficient;

4. Transform the stress-strain curves obtained by the two-stage loading rates v ₁ and v ₂ into stress and strain curves at any loading rate v according to formula (3):

σ=σ _i +k/α(vv _i )i=1,2(3)

That is to say, the method of measuring the stress and strain of steel bar under high temperature and variable loading rate has been completed.

The effect of this embodiment: the small-scale high-temperature strain extensometer is realized to measure the ultimate tensile strain of the steel bar, and the influence of the change of the loading rate at high temperature on the strength of the steel bar is eliminated. This implementation mode can realize the stress and strain of the steel bar at different loading rates at high temperature. curve measurement. By applying this embodiment, the measurement range of tensile strain of the steel bar at high temperature can be extended from 0.03 to 0.2, and the strain measurement range can be expanded by 6.7 times. At the same time, the influence of the change of loading rate on the stress measurement accuracy is solved, and the improvement of the measurement accuracy varies with the change of the loading rate. When the type of steel bar is determined, the greater the change in the loading rate, the higher the measurement accuracy will be after adopting this embodiment. In general, the stress measurement accuracy can be improved by 5-10%.

Embodiment 2: This embodiment is different from Embodiment 1 in that: the high temperature in the step 1 is 20-1100°C. Other steps and parameters are the same as those in Embodiment 1.

Embodiment 3: The difference between this embodiment and Embodiment 1 or 2 is that the high temperature in the step 1 is 50-1000°C. Other steps and parameters are the same as those in Embodiment 1 or Embodiment 2.

Embodiment 4: This embodiment is different from Embodiment 1 to Embodiment 3 in that: the high temperature in step 1 is 500°C. Other steps and parameters are the same as those in Embodiments 1 to 3.

Embodiment 5: This embodiment is different from Embodiment 1 to Embodiment 4 in that: the room temperature in the step 2 is 20°C. Other steps and parameters are the same as in one of the specific embodiments 1 to 4.

Example:

Using the steel bar strain test system at high temperature, the measured stress and strain curves of a steel bar at high temperature (300°C) are shown in Figure 5, the maximum strain ε ₀ corresponding to the curve, and the loading rate v ₁ ; , the displacement curve is shown in Figure 6, the loading rate v ₂ ;

The actual temperature distribution of steel bars is shown in Fig. 2.

When performing displacement measurement, based on the displacement coordination equation, the strain calculation formula of the steel bar at high temperature is as follows:

ε _T =[Δ _T -(l ₀ -l ₁ )σ _T /E _s ]/l ₁ (1)

In the formula, ε _T represents the strain of the steel bar at the temperature of T°C, Δ _T represents the total elongation of the steel bar at the temperature of T°C, σ _T represents the measured stress of the steel bar at the temperature of T°C, E _s represents the elastic modulus of the steel bar at room temperature, l ₀ means the total length of the steel bar, l ₁ means the length of the steel bar after equivalent treatment at T℃, and T ₀ means the room temperature;

Wherein, the equivalent processing steps are:

Draw the temperature distribution diagram of the steel bar at high temperature, as shown in Figure 2, and the equivalent temperature distribution diagram of the steel bar at high temperature, as shown in Figure 3, according to the equal temperature area of the steel bar before and after the equivalent, the temperature distribution diagram of the steel bar at high temperature is compared with that at high temperature Equivalent treatment of the equivalent temperature distribution diagram of steel bars;

Wherein, the calculation steps of ₁₁ are:

According to the principle that the steel bar strain is equal at the change of strain measurement and deformation measurement, iteratively calculate the length l ₁ of steel bar temperature T°C after equivalent calculation:

①. Determine the total elongation Δ _T and stress σ _T of the corresponding steel bar when the strain (T) is ε ₀ at high temperature;

②. Find the strain at the same time as the σ _T stress in the stress-strain curve at room temperature (T ₀ );

③. Given an initial value of l ₁ , put it into formula (1) to find ε _T ;

④. Adjust the initial value of l ₁ ;

⑤. Repeat steps ③ and ④, iteratively calculate until ε _T is equal to ε ₀ , and record the l ₁ value when ε _T is equal to ε ₀ ;

⑥. Calculate the strain ε _T in the high temperature zone of the steel bar by using the l ₁ value and the stress σ _T in the stress and deformation curve greater than ε ₀ , and then convert the stress and deformation curve of the steel bar with a strain greater than ε ₀ into a stress and strain curve; as shown in Figure 7 shown;

Applying formula (1), the stress and displacement curves at steel temperature (300°C) can be transformed into stress and strain curves, as shown in Figure 4.

The viscous characteristic value k/α is determined by formula (2), and the stress-strain curve obtained by the two-stage loading rates v ₁ and v ₂ is transformed into the stress-strain curve when the loading rate is 0 by formula (3), as shown in Figure 8 shown.

In this example, applying the invention, the maximum point of the measured steel stress and strain curve is expanded from about 0.0297 to about 0.0883, and the strain measurement range is expanded about 3 times. After eliminating the influence of the loading strain rate, the peak stress is reduced from 1346MPa to 1243MPa, and the accuracy An increase of 8.3%.

Claims (5)

1. under the high temperature variable loading rate, the steel bar stress, the strain measurement method is characterized in that under the high temperature variable loading rate, the steel bar stress, the strain measurement method comprises the following steps: 1. Obtain the stress and strain curves and corresponding stress and deformation curves of steel bars at high temperature through the steel bar strain test system at high temperature; Wherein, the maximum value of strain in the stress and strain curve is ε ₀ , and ε ₀ is less than the ultimate tensile strain of the steel bar at high temperature; When the steel bar strain is less than ε ₀ , the loading rate is v ₁ ; the steel bar strain test system simultaneously measures the stress, strain, stress and displacement of the steel bar at high temperature; When the steel bar strain exceeds ε ₀ , the high-temperature strain extensometer in the steel bar strain test system reaches the measuring range, and the steel bar strain test system automatically switches to displacement measurement, that is, measures the total deformation Δ _T of the steel bar, and the loading rate is v ₂ ; 2. When performing displacement measurement, based on the displacement coordination equation, the calculation formula for the strain of the steel bar at high temperature is as follows: ε _T ＝[Δ _T -(l ₀ -l ₁ )σ _T /E _s ]/l ₁ (1) In the formula, ε _T represents the strain of the steel bar at the temperature of T°C, Δ _T represents the total elongation of the steel bar at the temperature of T°C, σ _T represents the measured stress of the steel bar at the temperature of T°C, E _s represents the elastic modulus of the steel bar at room temperature, l ₀ means the total length of the steel bar, l ₁ means the length of the steel bar after equivalent treatment at T℃, and T ₀ means the room temperature; Wherein, the equivalent processing steps are: Draw the temperature distribution diagram of the steel bar at high temperature, the equivalent temperature distribution diagram of the steel bar at high temperature, and perform equivalent processing on the temperature distribution diagram of the steel bar at high temperature and the equivalent temperature distribution diagram of the steel bar at high temperature according to the equal temperature area of the steel bar before and after the equivalent; Wherein, the calculation steps of ₁₁ are: According to the principle that the steel bar strain is equal at the change of strain measurement and deformation measurement, iteratively calculate the length l ₁ of steel bar temperature T°C after equivalent calculation: ①. Determine the total elongation Δ _T and stress σ _T of the corresponding steel bar when the strain (T) is ε ₀ at high temperature; ②. Find the strain at the same time as the σ _T stress in the stress and strain curve at room temperature (T ₀ ); ③. Given an initial value of l ₁ , put it into formula (1) to find ε _T ; ④. Adjust the initial value of l ₁ ; ⑤. Repeat steps ③ and ④, iteratively calculate until ε _T is equal to ε ₀ , and record the l ₁ value when ε _T is equal to ε ₀ ; ⑥. Calculate the strain ε _T in the high temperature zone of the steel bar by using the l ₁ value and the stress σ _T in the stress and deformation curve greater than ε ₀ , and then convert the stress and deformation curve of the steel bar with a strain greater than ε ₀ into a stress and strain curve; 3. Assuming that the stress and strain curve of the steel bar at high temperature is composed of a "sticky pot model" and an inviscid elastic-plastic element, define the ratio of the loading rate V to the strain change rate When the loading rate v ₁ , the stress provided by the "sticky pot model" σ ₁ =k×v ₁ /α; when the loading rate is V ₂ , the stress provided by the "sticky pot model" σ ₂ =k×V ₂ / α, thus the viscous eigenvalue k/α can be obtained: $\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$ From the stress and strain curves obtained in step 1, the change of stress when the loading rate changes, that is, σ ₂ -σ ₁ , the two-stage loading rates v ₁ , v ₂ and σ ₂ -σ ₁ are substituted into formula (2), namely The viscosity characteristic value k/α corresponding to each specimen at a certain temperature can be obtained, and k is the viscosity coefficient; 4. Transform the stress-strain curves obtained by the two-stage loading rates V ₁ and v ₂ into stress and strain curves at any loading rate V according to formula (3): σ=σ _i +(k/α)(vv _i ), i=1, 2(3) That is to say, the method of measuring the stress and strain of steel bar under high temperature and variable loading rate has been completed. 2. The method for measuring steel bar stress and strain under high-temperature variable loading rates according to claim 1, characterized in that the high temperature in step 1 is 20-1100°C. 3. The method for measuring steel bar stress and strain under high-temperature variable loading rate according to claim 2, characterized in that the high temperature in step 1 is 50-1000°C. 4. The method for measuring steel bar stress and strain under variable loading rates at high temperatures according to claim 3, characterized in that the high temperature in step 1 is 500°C. 5. The method for measuring steel bar stress and strain under high temperature variable loading rate according to claim 1, characterized in that the room temperature in the step 2 is 20°C.