Stress relaxation behavior of low carbon steel at different temperatures

In this paper, the stress relaxation behavior of Q235 with the initial tensile stress of 70, 85 and 100 MPa were investigated at different temperature. Based on the thermal activation theory, the stress relaxation model of Q235 steel was established, and the physical mechanism and deformation process in the stress relaxation process were revealed. The results shows that with the increase of temperature or initial stress, the nominal activation volume decreases, but the strain rate and the strain rate sensitivity coefficient increase. The repeated stress relaxation test shows that the stress release amount decreases with the increase of the number of cycles, and the higher the temperature, the smaller the effect of the number of cycles. Under the action of temperature and stress, the dislocation starts to move from the disordered bending shape in the original sample to the flat shape gradually. Moreover, the dislocation density decreases to less than 47.8% of the initial sample as the temperature increases and the initial stress decreases. It can be concluded that the dislocation motion is the core mechanism of stress relaxation of Q235 steel.


Introduction
The Q235 steel is a typical low-carbon steel that is widely used in the manufacture of structural components [1].Although its composition and microstructure are basic, and scholars have achieved a lot of research results in other aspects, there is relatively less theoretical research on its stress relaxation, and there is a certain degree of blindness in the stress regulation process of its structural components.
Generally, the stress relaxation has been studied using power-law or logarithmic models [2].Among them, the Zener-Wert-Avrami empirical model based on material thermodynamics is widely used.However, its parameters are not extensive, so the application is limited.Moreover, the model is mainly based on the fitting of empirical data, which cannot make a reasonable explanation for the residual stress relaxation under the temperature field in theory, and the model mainly describes the stress relaxation in the temperature field and does not involve the concept of time.
By introducing inelastic strain rate and the dislocation evolution, Bai [3] combined dislocation density and plasticity theory to build a residual stress relaxation model for predicting different initial dislocation density, annealing temperature and time.Lu [4] obtained the apparent activation volume and the physical activation volume using stress relaxation tests, thus separating the contribution of mobile dislocation density and dislocation velocity to the plastic strain rate.
Based on the stress relaxation behavior, mechanical properties and microstructure evolution, Li [5] established a stress relaxation model considering the evolution of dislocation density and the volume fraction of the precipitated phase.Chen [6] is of the opinion that that the Chaboche model established by the traditional Armstrong Frederick model could not reflect the influence of the loading strain rate on the descending stress during the strain holding period because the static recovery item was ignored.Based on this, a modified model based on Chaboche's unified viscoplastic theory is proposed, which can realize the simulation and prediction of uniaxial stress relaxation under different stress peaks, to realize the generalized description of stress relaxation.From the perspective of thermodynamics and damage mechanics, Lian [7] proposed a stress relaxation prediction model for metal materials by studying the change law of entropy increase rate in the process of damage.
Based on the results of Galindo [8,9], Zhao [10] developed a stress relaxation model for describing HSLA steel after high temperature deformation.By tracking the evolution of average dislocation density during stress relaxation, the model provides a detailed description of the stress relaxation process.Guo [11,12] describes the stress relaxation behavior of copper wires with different diameters by the thermal activation theory, realizes the connection between the physical mechanism and the deformation process in the stress relaxation process, and analyzes and confirms that the interaction between dislocation and grain boundary is the dominant mechanism of stress relaxation.Based on the thermal activation theory and repeated stress relaxation experiments, Li [13] proposed a new constitutive model to predict the stress relaxation behavior of AA7B04 under different temperature and loading conditions.
To further study the stress relaxation behavior of Q235 steel, it is necessary to link the physical mechanism and deformation process in the stress relaxation process, to have a better understanding of the deformation mechanism in the relaxation process and provide theoretical guidance for the stress elimination.

Theoretical background
The stress relaxation is the process which the material undergoes elastic strain under temperature and initial stress, and then the stress/strain gradually decreases with the extension of time.Its essence is that the dislocation inside the material moves under the action of temperature and stress, resulting in the gradual conversion of elastic strain caused by residual stress into plastic strain [14,15].Figure 1 is a diagram of the stress relaxation process, showing the evolution process of stress and strain in the relaxation process.The elastic strain and plastic strain exist: ( ) The strain rate is proportional to the sliding area of dislocations in unit time and unit volume, and is the product of the density of mobile dislocations and the average velocity of dislocation, expressed by using Orowan's equation [16]: Where,  g is the strain rate during stress relaxation, r m is the density of mobile dislocation, v is the average dislocation velocity, b is the Berg vector of steel, = b n m 0.2841 .The possibility of mobile dislocation crossing the barrier is [15]: Where, v id is the vibration frequency of mobile dislocation, represents the energy barrier that blocks the dislocation motion.Then the average velocity of dislocation is expressed as: Where, ¢ A represents the area swept by the dislocation segment.Combined equations (3) and (4), the strain rate in equation (2) can be expressed as: is the Gibbs free energy required for a mobile dislocation to move a certain distance.
According to the law of thermodynamics: T is the activation volume, which is related to dislocation motion.
When the activation volume remains unchanged, DG can be expressed as [17]: Where, DG 0 is the activation energy at zero stress.So, equation (5) can be converted to According to the equation (1), the strain rate and corresponding stress relaxation rate in the stress relaxation process exist: The amount of stress relaxation is logarithmic with time, and its expression is: Where, C r is a time constant, V a is the apparent activation volume, which is related to the area swept by dislocation, and the expression is µ a V b . 3The value of activation volume is related to the deformation mechanism.
Substituting equation (10) into equation (9), the expression of strain rate in stress relaxation is: In repeated stress-relaxation tests, after relaxation 1, the sample is immediately loaded for relaxation 2. So, the change in the density of the mobile dislocation is negligible [4].The relationship between physical activation volume * V and repeated stress relaxation stress is [18]: Where, * V is the physical activation volume, indicating the relationship between dislocation velocity and stress,  g i2 and  g f 1 represents the strain rate at the beginning of the next stress release and at the end of the last stress release, t D - i f 2 1 represents the nominal elastic load of two adjacent stress release processes.m is the strain rate sensitivity coefficient, reflecting the effect of stress on strain rate, and its expression is: Mobile dislocation density is an important parameter for determining stress relaxation [9].In the process of stress relaxation, unit dislocation spontaneously annihilates or forms a dipole, and then the dipole disappears through spontaneous annihilation or dissolves and disappears through climb/cross-slipping, resulting in the decrease of dislocation density.In the process of thermal activation, the evolution equation of mobile dislocation density with time is [4]: Where, r m0 is the initial mobile dislocation density, b is a dimensionless parameter: Where, According to Mises yield criterion, the relationship between shear stress t and normal stress s is: Therefore, the normal stress, the normal strain and the physical activation volume can be expressed as: Where, M is the nominal elastic modulus, = = M E 198.7 GPa.

Stress relaxation test
The elevated temperature tensile testing machine is used to tensile test Q235 steel at 25 °C, 450 °C, 500 °C, 550 °C, 600 °C and 650 °C respectively, which provides a design standard for the initial stress of the stress relaxation test.The high-temperature tensile test results are shown in figure 2, which shows that the yield strength of the material is reduced from 316.4 MPa to 64.9 MPa with the heating temperature rising from 25 °C to 650 °C.Therefore, the stress relaxation test temperature is set at 450 °C, 500 °C, 550 °C and 600 °C, and the initial tensile stress (σ 0 ) is set at 100 MPa, 85 MPa and 70 MPa.
According to the requirements of GB/T 10120-2013 standard, The stress relaxation sample of j10 × 100 mm was made, and its surface roughness was 0.8, as shown in figure 3. Due to the large stress relaxation rate of ferrite (BCC structure), the initial stress loading rate is set at 500 N/s.First, Three K-type thermocouples are installed on the sample to monitor the temperature, as shown in figure 4(a).Then, the sample is heated to the nominal temperature by the resistance furnace, and kept for 15 min.After the measurement deviation of the three thermocouples is less than 3 °C, the initial stress is applied to the sample, as shown in figure 4(b).Finally, the clamp of the device was fixed and the data was collected at every 0.05 MPa reduction.In order to clarify the evolution law of mobile dislocations during stress relaxation, the repeated stress relaxation test was carried out with the initial stress set at 100 MPa.The diagram is shown in figure 4(c).

Microstructure
After the relaxation test is completed, the samples (8 mm × 6 mm × 4 mm) were cut from the center of the sample along the axial direction by the electric spark wire cutting, and then polished with sandpaper.The EBSD  samples are prepared by electrolysis with 32 V for 15 s, and the electrolyte is 4% perchloric acid alcohol solution cooled by drikold.After prepared, the GeminiSEM-500 electronic backscatter scanning electron microscope equipped with Nordlys HKL-Oxford EBSD is used to observe microstructure with voltage of 20 kV and a scanning step of 0.5 μm, and the EBSD data is analyzed by using HKL channel5 software.For TEM samples, a round slice with a thickness of 300 μm is cut horizontally along the sample using wire cutting, polished to 100 μm using metallographic sandpaper, and then a j3 mm round slice is flushed out.Then, the ion thinning is performed in a 5% perchloric acid alcohol solution at −30 °C.After the preparation of the sample, the FEI-Tecnai G2 transmission electron microscope is used to observe the dislocation morphology of the specimen at 200 kV.

Results and discussion
4.1.Analysis of the stress relaxation 4.1.1.Single stress relaxation Figure 5 shows the variation law of stress release over time.The relaxation behavior consists of two distinct characteristic stages: transient relaxation and secondary relaxation.In the first stage, the initial stress decay is fast, the stress relaxation amount is large, and the stress decline rate gradually decreases in the whole process, and the proportion of time is small.In the second stage, the stress decay is slow and gradually approaches the relaxation limit s ¥ .Figure 5 shows that with the same initial stress, the relaxation limit decreases with the increase of temperature.
The thermal activation theory is applied to explain the single stress relaxation result, and the apparent activation volume is obtained by fitting the relationship between the stress release value and time, as shown in figure 5.When s = 70 MPa, 01 the apparent activation volume decreases from 252.7b 3 to 156.1b 3 , with a decrease of 38.2%.When s = 85 MPa, 02 the apparent activation volume decreases from 231.1b 3 to 132.7b 3 , with a decrease of 42.6%.When s = 100 MPa, 03 the apparent activation volume decreases from 214.6b 3 to 126.6b 3 , with a decrease of 41.0%.At the same temperature, the reduction of nominal activation volume is: 450 °C−15.08%,500 °C−18.58%,550 °C−19.37%,600 °C−18.90%.Compared to the same initial stress, the apparent activation volume changes more with temperature.The results show that V a decreases as s 0 increases, decrease with the increase of temperature, and temperature has a stronger effect on the apparent activation volume.
Based on table 1, the influence of temperature and initial stress on the stress relaxation limit was calculated by the analysis of variance, as shown in equation (18).The results show that the initial stress has little effect on the stress relaxation limit, and the relaxation limit is mainly controlled by temperature.The essence of the change in activation volume with stress is the evolution of deformation microstructure [19].The activation volume decreases with the increase of stress, because at low stress, the deformation microstructure is weakly restricted and rapidly evolves during stress relaxation.However, at high stress, the microstructure evolution is limited within the same time period.
Figure 6 shows the change law of strain rate with time.As shown in the figure, the range of strain rate in stress relaxation behavior at different initial stress and temperatures is 10 −9 ~10 −4 s −1 .At the beginning stage of stress relaxation, the strain rate decreases sharply with time.Subsequently, the strain rate varies less with the extension of time.This segment is called quasi-static strain rate, which increases with the increase of temperature, consistent with Conrad's findings [20].The strain rate sensitivity coefficient increases with the increase of temperature and the initial stress.When the temperature is 600 °C, when the curve reaches a certain value, the curve shows an obvious turning point, which is bilinear.The bilinear relationship observed in the stress relaxation process can be attributed to the interplay between various factors that affect dislocation behavior.At 600 °C, high temperature and atomic vibrations facilitate the rapid motion of mobile dislocations, leading to a quick reduction in stress.However, as the mobile dislocations are gradually dissipated, the remaining immobile dislocations become increasingly important in defining the deformation behavior of the material.At this point, the rate of stress relaxation slows down, resulting in a change in the slope of the relaxation curve.
According to the thermal activation theory, the calculation of stress relaxation behavior under different initial stress and temperature shows that with the increase of heating temperature or initial stress, the apparent activation volume decreases, and the strain rate and strain rate sensitivity coefficient increase.This is because at low stress or high temperature, the dislocation structure limit is weak and the stress release rapidly.But, at high stress or low temperature, the deformation microstructure is limited and the stress release is slow.Compared to the initial stress, temperature has a stronger effect on stress relaxation.

Repeated stress relaxation
In order to study the evolution of mobile dislocation density in the process of stress relaxation, the repeated stress relaxation test is carried out with the initial stress of 100 MPa loaded on the sample.The amount of stress relaxation is shown in figure 8.The figure shows that the initial stress is 100 MPa, the stress relaxation amount of the second cycle is less than that of the first cycle at different heating temperatures, and the stress relaxation limit is also higher.Table 2 shows that when the relaxation time is 2000 s, the relaxation amount of the second cycle is smaller than that of the first cycle.In summary, the stress relaxation process is less affected by the number of cycles, and the effect of repeated stress relaxation is less obvious with the increase of heating temperature.Equation ( 17) is used to fit the stress release process at different temperatures, and the fitting results are shown in figure 8.The figure shows that the apparent activation volume of the first cycle varies with temperature from 116.6b 3 to 236.6b 3 and the apparent activation volume of the second cycle varies with temperature from 111.8 b 3 to 197.3b 3 .The calculation results show that the apparent activation volume decreases with the increase of heating temperature, and the apparent activation volume of the second cycle is less than that of the first cycle.
Figure 9 shows the evolution of repeated stress relaxation strain rate with time.The figure shows that the strain rate ranges from 10 −9 s −1 to 10 −4 s −1 .The strain rate increases with the increase of temperature, and the steady-state strain rate increases with the increase of the number of cycles.Figure 10 shows the relationship between strain rate and stress during repeated stress release.The sensitivity coefficient of the first cycle strain rate is 0.03723 ∼ 0.4543, and the sensitivity coefficient of the second cycle strain rate is 0.04224 ∼ 0.3044.The strain rate sensitivity coefficient increases with the increase of temperature, and the strain rate sensitivity coefficient of the second cycle is less than that of the first cycle.According to the repeated stress relaxation data in figure 8, the physical activation volume is obtained using equation (17)(c).Table 3 shows the variation law of nominal activation volume and physical activation volume with temperature.As shown in the table, the apparent activation volume range is: 111.8b 3 ∼ 236.6b 3 , the physical activation volume decreases from 89.3b 3 to 57.7b 3 with temperature, and the apparent activation volume and physical activation volume decrease with the increase of heating temperature.
Through the above calculation, it is found that compared with the first cycle, the second cycle has a higher stress relaxation limit and strain rate, and lower nominal activation volume.In order to study this phenomenon, the evolution of mobile dislocation density during stress relaxation process is calculated according to  Table 3. Nominal activation volume and physical activation volume under repeated stress relaxation.When the relaxation temperature is 450 °C, the mobile dislocation density decreases to the original 30% at 600 s.For 500 °C, 550 °C and 600 °C, the density of mobile dislocations decreases to 10% at about 100 s.This phenomenon reflects that the atom has low vibration entropy at 450 °C, the dislocation motion is not active, and it is difficult to break through the barrier to produce greater annihilation, which ultimately leads to a lower amount of stress release.

Dislocation evolution
TEM technology can directly observe the dislocation substructure and obtain intuitive microstructure [21].Figure 12 shows that the dislocation in the original sample presents disorderly curved shape, which is caused by multiple cross slip during the rolling process of the sample [22].Figure 13 shows the dislocation morphology after stress relaxation.Figure 13(a) shows the dislocation morphology of the stress relaxation sample at 450 °C, and the dislocation is severely entangled.Figures 13(b)-(d) show that with the increase of relaxation temperature, the dislocation gradually changes from disorderly curved shape in the original sample to a straight shape, and the entanglement phenomenon also disappears.
Unlike ultrafine grains, which achieve stress relaxation through grain boundary slip and dislocation motion [23], the dominant mechanism of stress relaxation in macroscopic materials is dislocation slip [24].As shown in figure 13(a), when the heating temperature is 450 °C, the atomic energy is low, the vibration is relatively gentle, and the dislocation cannot cross the barrier, forming many dislocation locks.As shown in figures 13(b) and (c), when the heating temperature is higher, the atom gains higher energy.Under the effect of vacancy diffusion and thermal activation energy, the dislocation continues to slip across the barrier.As shown in figure 13(d), when the temperature is higher, the thermal activation energy of dislocations is greater, the external force required to cross the barrier is reduced, and the mobile dislocations will be more completely annihilated.
In order to quantitatively study the evolution of dislocation density, figure 14 shows the calculation process of dislocation density of stress relaxation specimen by the Modified Williamson Hall (MWH).The relation between the diffraction peak width and the dislocation density is: Where, q l = K 2 sin , x DK is diffraction peak width, q q l D = ⋅D K 2 cos , x θ is the diffraction angle, l x is the wavelength of diffraction wave 0.1540 nm, d is the average grain size, b is the Burgers vector, 1 2 1 2 M is a constant related to dislocation density of effective cut-off radius of dislocation, r = M d , Ch00 is the dislocation reflection contrast factor of (h00) crystal plane, C h00 = 0.285 [25], h, k, l is the Miller factor of the diffraction plane, Table 4 shows the dislocation density after stress relaxation.The dislocation density of the original sample is 3.41 × 10 13 m −2 .According to the table, the dislocation density decreases by over 47.8% after stress relaxation at 450 °C, by approximately 55% at 500 °C, by around 66% at 550 °C, and to about 70% at 600 °C.The results demonstrate that the decrease of dislocation density is positively correlated with the decrease of initial stress at the same temperature.Moreover, the decrease of dislocation density decreases significantly with the increase of temperature under the same stress.Therefore, temperature plays a crucial role in stress relaxation compared to the effect of stress.
In the process of stress relaxation, the evolution of mobile dislocation density is a comprehensive manifestation of dislocation proliferation and annihilation, and dislocation dissipation is the main process.For the macroscopic sample, the grain boundary area is much larger than the sample surface area, and the dissipation of dislocation is mainly caused by the interaction between the mobile dislocation and the obstacles and dislocations.As the temperature increases, the atoms obtain higher energy, the probability of moving away from the fixation increases, and finally annihilates at the grain boundary.

Deformation mechanism
The essence of stress relaxation is the external manifestation of the increase of plastic strain caused by dislocation motion.On the one hand, higher temperature promotes the diffusion of atoms and vacancies, and accelerates the dislocation motion, while the external driving force brought by the initial stress also promotes the dislocation motion [26].Under the action of temperature and stress, the dislocation in the material starts to move.With the extension of relaxation time, the density of mobile dislocation decreases continuously, and finally the stress gradually tends to be stable.The activation volume is an effective kinetic signature of deformation mechanisms [27].In this experiment, the activation volume range is 111.8b 3 ∼ 252.7b 3 , indicating that the stress relaxation mechanism of Q235 steel in the process of is the action of mobile dislocation and forest dislocations.
The corresponding activation energy can be obtained by the stress relaxation of the material at the temperature field.The existence of strain rate and activation energy during stress relaxation [17]: G is linear with s, and the expression is: The negative slope of equation (21) indicates that the stress is negatively correlated with the activation energy, that is, the activation energy increases with the continuous decay of stress.At the same time, the intercept is 5.11, indicating that the activation energy is 5.11 eV at zero stress, that is: D = > G 5.11eV 1eV 0 [28].It indicates that dislocation dominates the deformation during stress release.
Considering the calculation results of strain rate sensitivity coefficient, activation volume and activation energy in the process of stress relaxation, it can be inferred that the deformation mechanism in the process of stress relaxation is the interaction between mobile dislocation and dislocation forest and grain boundary slip, and the effect of grain boundary slip becomes more and more prominent with the increase of temperature.

Conclusions
In this paper, the stress relaxation behavior of Q235 steel under different temperature and initial stress was studied.Based on the thermal activation theory, the stress relaxation model of Q235 steel is constructed to reveal the physical mechanism and deformation process in the stress relaxation process.The results can be summarized as follows: (1) The stress relaxation is the result of the joint effect of temperature and initial stress, and temperature has a stronger effect on stress relaxation.With the increase of temperature or initial stress, the apparent activation volume decreases, and the strain rate and strain rate sensitivity coefficient increase.
(2) Repeated stress relaxation shows that the stress relaxation limit is less affected by the number of repetitions.
The activation volume and strain rate of the second time are smaller than that of the first time.The calculation of movable dislocation density shows that the density dissipation rate of the movable dislocation is low at 450 °C, and the density of the movable dislocation rapidly decreases to zero with the increase of temperature.(3) Under the effect of heating, the dislocation gradually changes from a disorderly curved shape in the original sample to a straight shape and the dislocation density also decreases.With the increase of temperature, the decrease of dislocation density increases from 47.7% to 71.8%.
(4) The calculation of activation volume and activation energy shows that the main physical mechanism of stress relaxation of Q235 steel is the interaction between dislocation and forest dislocation.Moreover, under the action of thermal activation, the atoms obtain more energy, break through obstacles, undergo more thorough annihilation, and thus obtain greater stress relaxation.

Figure 1 .
Figure 1.Schematic of variation of different strain components during stress relaxation.

Figure 2 .
Figure 2. The elevated temperature tensile test of Q235 steel.(a) The elevated temperature tensile test system, (b) Yield strength and tensile strength of Q235 steel at different temperatures.

Figure 3 .
Figure 3. Structure diagram of stress relaxation specimen.

Figure 4 .
Figure 4. Experimental equipment and technology.(a) Stress relaxation equipment, (b) The single stress relaxation process diagram, (c) The repeated stress relaxation process diagram.

Figure 7
Figure7shows the relationship between strain rate and stress in stress release.According to equation(13), linear fitting is performed for  e s ln ln , p and the strain rate sensitivity coefficient m is the slope of the equation.As shown in the figure, the value range of strain rate sensitivity coefficient is: 0.04845 ∼ 0.3063.The figure shows that when the relaxation temperature is 600 °C,  e ln p and s ln are bilinear.In stage I, strain rate changes sharply with stress relaxation and it mainly occurs at the early stage of stress relaxation.In stage II, the strain rate changes gently with the decrease of stress relaxation relative to the first stage.The strain rate sensitivity coefficient increases with the increase of temperature and the initial stress.When the temperature is 600 °C, when the curve reaches a certain value, the curve shows an obvious turning point, which is bilinear.The bilinear relationship observed in the stress relaxation process can be attributed to the interplay between various factors that affect dislocation behavior.At 600 °C, high temperature and atomic vibrations facilitate the rapid motion of mobile dislocations, leading to a quick reduction in stress.However, as the mobile dislocations are gradually dissipated, the remaining immobile dislocations become increasingly important in defining the deformation behavior of the material.At this point, the rate of stress relaxation slows down, resulting in a change in the slope of the relaxation curve.According to the thermal activation theory, the calculation of stress relaxation behavior under different initial stress and temperature shows that with the increase of heating temperature or initial stress, the apparent activation volume decreases, and the strain rate and strain rate sensitivity coefficient increase.This is because at low stress or high temperature, the dislocation structure limit is weak and the stress release rapidly.But, at high stress or low temperature, the deformation microstructure is limited and the stress release is slow.Compared to the initial stress, temperature has a stronger effect on stress relaxation.

Figure 8 .
Figure 8. Stress released at different temperatures.(a) The first cycle, (b) The second cycle.

Figure 9 .
Figure 9. Evolution of strain rate with time during repeated stress relaxation.(a) The first cycle, (b) The second cycle.

Figure 10 .
Figure 10.Relationship between  e ( ) ln p and s ( ) ln during stress release.(a) The first stress relaxation, (b) The second stress relaxation.

Figure 11 .
Figure 11.Evolution of mobile dislocation density of the sample in the first cycle.
Figure 11 shows the evolution of mobile dislocation density with temperature in first cycle during stress relaxation.The figure shows that the density of mobile dislocations dissipates with time, and the density of mobile dislocations decreases rapidly, which is also the reason why the apparent activation volume in the second cycle of figure 8(b) is less than that in the first cycle.

0
Based on figures 8(a) and 10(a), corresponding  e ( ) ln p at different stress relaxation levels are collected and the curve of  e ( ) ln p and T 1 k B is constructed, as shown in figure 15(a).The figure shows that when the heating temperature is 500 °C, 550 °C and 600 °C,  e ( ) ln p and T 1 k B have a linear relationship, and the slope is s D ( ) G by fitting.The change of s D ( ) G with stress is shown in figure 15(b).As shown in the figure, s D ( )

Figure 15 .
Figure 15.The change law of activation energy in stress relaxation.(a) The law of activation energy changing with temperature, (b) Variation of activation energy with stress.

Table 1 .
The stress relaxation limits under different initial stress.

Table 2 .
The stress relaxation of the first cycle and the second cycle (MPa).