Method for evaluating stress relaxation of rubber material

Abstract

The present application relates to the technical field of automobile rubber seal sealing capacity test, and discloses a kind of evaluation method of rubber material stress relaxation, the compression of the compression deformation sample of rubber material is carried out in compression deformation device, is placed in aging oven under specified temperature condition and is aged, reaches specified time, takes out sample, tests compression stress under quasi-static condition, compares compression stress value with initial force test value, records the compression stress retention rate under each time point, makes graph to the common logarithm of compression stress retention rate to time, obtains a straight line with slope, which is a simple measure of the stress relaxation of the sample, a measure of testing the sealing capacity of rubber, and can effectively reflect the change of the sealing force of the sealant.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of automobile rubber seal sealing capacity test, in particular to a method for evaluating stress relaxation of rubber material. BACKGROUND

[0002] Generally, compression set method is a simple and commonly used test method for evaluating rubber sealing performance. The standard sample with known thickness is compressed (usually 25%), and after being placed in a given temperature environment for a certain time, the compression stress is removed, and the thickness of the sample is measured. The measured value is the compression deformation without recovery after stress removal, expressed in percentage. However, compression set cannot directly reflect the change of sealing force of the seal, at most it is an indirect measurement method of sealing capacity, which is easy to be misunderstood as a prediction of rubber sealing performance. Compression stress relaxation (CSR) test is a method for directly measuring sealing capacity, that is, measuring the decrease of resistance of rubber sample with time under constant compression strain. However, stress relaxation (CSR) test requires additional purchase of special compression stress relaxation instrument for measurement. SUMMARY

[0003] In order to overcome the defects of the prior art, the purpose of the present application is to provide a method for evaluating stress relaxation of rubber material, so as to solve the technical problem that the stress relaxation of rubber material cannot directly reflect the change of sealing force of the seal in the prior art.

[0004] The present application is realized by the following technical scheme:

[0005] A method for evaluating stress relaxation of rubber material, comprising the following processes:

[0006] The rubber compression sample is compressed to obtain a compression force value, and the compression force value is taken as an initial sealing force;

[0007] The compressed rubber compression sample is placed in an aging oven under a specified temperature condition for aging, and after a specified time, the rubber compression sample is taken out and tested under quasi-static condition to obtain a compression stress;

[0008] The compression stress value is compared with the initial force test value, the compression stress retention rate at each time point is recorded, and a straight line with a slope is obtained by plotting the common logarithm of time according to the compression stress retention rate at each time point, and the evaluation of stress relaxation of rubber material is completed according to the straight line.

[0009] Preferably, before the rubber compression sample is compressed, the rubber compression sample is preheated and heat adjusted, and after heat adjustment, the rubber compression sample is left standing at standard temperature for 16-48h.

[0010] Preferably, the rubber compression sample is placed on a compression force testing fixture installed on a material testing machine, the compression force testing fixture comprising a material testing machine joint, a shaft pin, a horizontal fastening ring, an upper compression plate and a lower compression plate; one end of the material testing machine joint is connected to the material testing machine, and the other end is connected to the connecting end of the upper compression plate through the shaft pin and the horizontal fastening ring; the rubber compression sample is placed on the lower compression plate and directly below the upper compression plate, and is compressed and set by moving the upper compression plate towards the lower compression plate.

[0011] Further, when the rubber compression sample is compressed, the material testing machine compression program is set, the compression speed is set, and the pre-load force displacement is cleaned; when the rubber compression sample is compressed to a strain threshold, the rubber compression sample is relaxed at the same speed, and the continuous compression and relaxation test is repeatedly performed; the compression force value of the last rubber compression sample when the strain threshold is reached is recorded, and the compression force value is taken as the initial sealing force.

[0012] Preferably, the compressed rubber compression sample is placed in an aging oven under specified temperature conditions for aging, wherein the specified temperature conditions include 75°C, 100°C, 125°C, 150°C, 175°C; and the rubber compression sample is taken out after a specified time point.

[0013] Preferably, the sealing force of the rubber compression sample at each time point is repeatedly tested under quasi-static conditions, and the compression force value of the last rubber compression sample when the strain threshold is reached is recorded.

[0014] Preferably, the formula for calculating the compression stress retention rate at each time point is as follows:

[0015] Compression stress value / initial sealing force=compression stress retention rate at each time point.

[0016] Preferably, the straight line with a slope is obtained by curve fitting the common logarithm of the compression stress retention rate at each time point versus time.

[0017] Preferably, the curve fitting stress relaxation formula is as follows:

[0018] The basic differential equation for setting the decay of the sealing force of the rubber compression sample with time is:

[0019] dF / dt=K / t;

[0020] Where K is a constant, and the rate of change of stress relaxation is inversely proportional to time;

[0021] After integral transformation of the basic differential equation, the curve fitting stress relaxation formula is obtained, wherein the curve fitting stress relaxation formula is as follows:

[0022] Stress relaxation percentage=100(Ft F(t) = A*logt + 100 - A*logt0

[0023] Wherein, F0 is initial sealing force, t0 is test time corresponding to initial sealing force F0, the rate of stress relaxation is inversely proportional to time.

[0024] Preferably, the straight line with slope is the percentage reduction rate of sealing force when time increases by a certain value, the evaluation of stress relaxation of rubber material is determined according to the straight line, when the absolute value of the straight line is larger, the sealing force of rubber decays faster, the sealing ability is worse, and vice versa.

[0025] Compared with the prior art, the present application has the following beneficial technical effects:

[0026] The present application provides an evaluation method of stress relaxation of rubber material, a compression deformation sample of rubber material is compressed in a compression deformation device, placed in an aging oven under a specified temperature condition, taken out after reaching a specified time, and tested under quasi-static condition. The compression stress value is compared with the initial force test value, the compression stress retention rate at each time point is recorded, the compression stress retention rate is plotted against the common logarithm of time, and a straight line with a slope of 100 / A is obtained. The straight line is a simple measure of stress relaxation of the sample, and is a measure of testing the sealing ability of rubber. The stress relaxation of rubber material can effectively intuitively reflect the change of sealing force of the sealing member. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The flow chart of the evaluation method of stress relaxation of rubber material in the present application;

[0028] Figure 2 The structural schematic diagram of the compression force test tooling in the present application;

[0029] Figure 3 The compression stress retention rate and time logarithmic curve schematic diagram in the present application;

[0030] In the figure: 1-material testing machine joint; 2-shaft pin; 3-horizontal fastening ring; 4-upper compression plate; 5-lower compression plate. DETAILED DESCRIPTION

[0031] In order to make the person in the art better understand the present application scheme, the technical scheme in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by the person skilled in the art without creative labor should belong to the protection scope of the present application.

[0032] The application will be described in further detail below with reference to the drawings:

[0033] The application aims to provide a method for evaluating stress relaxation of rubber material, so as to solve the technical problem in the prior art that stress relaxation of rubber material cannot intuitively reflect the change of sealing force of a sealing member.

[0034] Referring to Figure 1 In an embodiment of the application, a method for evaluating stress relaxation of rubber material is provided, including the following processes:

[0035] Step 1, compressing a rubber compression sample to obtain a compression force value, and taking the compression force value as an initial sealing force;

[0036] Specifically, before the rubber compression sample is compressed, the rubber compression sample is preheated at 70℃ for thermal adjustment, and after the thermal adjustment, the rubber compression sample is left to stand at a standard temperature for 16-48h.

[0037] Specifically, the rubber compression sample is placed on a compression force test tool installed on a material testing machine, and according to Figure 2 As shown in the figure, the compression force test tool includes a material testing machine joint 1, a shaft pin 2, a horizontal fastening ring 3, an upper compression plate 4 and a lower compression plate 5; one end of the material testing machine joint 1 is connected to the material testing machine, and the other end is connected to the connecting end of the upper compression plate 4 through the shaft pin 2 and the horizontal fastening ring 3, the rubber compression sample is placed on the lower compression plate 5 and directly below the upper compression plate 4, and the upper compression plate 4 is moved towards the lower compression plate 5 to compress and set the rubber compression sample.

[0038] Specifically, when the rubber compression sample is compressed, the material testing machine compression program is set, the compression speed is set, and the pre-load force displacement is cleaned, and when the rubber compression sample is compressed to a strain threshold, the rubber compression sample is released at the same speed, and the continuous compression and release test is repeatedly performed, the compression force value of the rubber compression sample when the strain reaches the threshold in the last time is recorded, and the compression force value is taken as the initial sealing force.

[0039] Step 2, placing the compressed rubber compression sample in an aging oven under a specified temperature condition for aging, and after a specified time, taking out the rubber compression sample and testing to obtain a compression stress under quasi-static conditions;

[0040] Specifically, the compressed rubber compression sample is placed in an aging oven under a specified temperature condition for aging, wherein the specified temperature condition includes 75℃, 100℃, 125℃, 150℃ and 175℃; and the rubber compression sample is taken out after a specified time point.

[0041] Specifically, the sealing force of the rubber compression sample at each time point is repeatedly tested under quasi-static conditions, and the compression force value at which the strain of the last rubber compression sample reaches the threshold value is recorded.

[0042] Step 3, compare the compression stress value with the initial force test value, record the compression stress retention rate at each time point, and plot the common logarithm of time according to the compression stress retention rate at each time point to obtain a straight line with a slope, and complete the evaluation of the stress relaxation of the rubber material according to the straight line.

[0043] Specifically, the calculation formula of the compression stress retention rate at each time point is as follows:

[0044] Compression stress value / initial sealing force = compression stress retention rate at each time point.

[0045] Specifically, the straight line with a slope is obtained by curve fitting the common logarithm of time according to the compression stress retention rate at each time point.

[0046] Specifically, the stress relaxation formula obtained by curve fitting is as follows:

[0047] The basic differential equation of the sealing force of the rubber compression sample decaying with time is set as:

[0048] dF / dt = K / t;

[0049] Where K is a constant, and the change rate of stress relaxation is inversely proportional to time.

[0050] Specifically, after integral transformation of the basic differential equation, the stress relaxation formula obtained by curve fitting is obtained, wherein the stress relaxation formula obtained by curve fitting is as follows:

[0051] Stress relaxation percentage = 100(F t / F0) = A*logt + 100-A*logt0

[0052] Where F0 is the initial sealing force, t0 is the test time corresponding to the initial sealing force F0, and the change rate of stress relaxation is inversely proportional to time.

[0053] Specifically, the straight line with a slope is the percentage reduction rate of the sealing force when the time increases by a certain value, and the evaluation of the stress relaxation of the rubber material is determined according to the straight line. The greater the absolute value of the straight line, the faster the sealing force of the rubber decays, and the worse the sealing ability, and vice versa.

[0054] Embodiment

[0055] The embodiment provides a method for evaluating the stress relaxation of a rubber material, and the specific process is as follows:

[0056] S1, prepare the Ф(29±0.5)mm, high(12.5±0.5)mm cylindrical rubber compression sample specified in GB / T 7759.1-2015 "Determination of compression set of vulcanized or thermoplastic rubber", heat the sample at 70°C for 3h(h represents hours) before the test to perform thermal conditioning, eliminate the mold stress existing in the sample, after thermal conditioning, allow the rubber sample to stand at standard temperature for not less than 16h and not more than 48h. According to Figure 1 Install the compression force test tool on the material testing machine, and place the rubber sample centrally between the two plates. Set the compression program of the material testing machine, the compression speed is 10mm / min, the pre-load force is 2N, the displacement is zero, and the compression is stopped when the strain reaches 25%(the compression amount of high hardness rubber can be selected as 15% or 10%), then relax the sample at the same speed, and repeat the compression and relaxation test continuously for a total of four times, the first three times are used for mechanical conditioning to improve the repeatability of the test results, and the fourth test is the formal test, record the compression force value when the strain reaches 25%, which is the initial sealing force F0.

[0057] S2, the true measurement of the sealing capacity of the rubber is the retention of the compression stress after a long period of time(such as days, weeks, months or years, not minutes), therefore, the compression test under long time combined with the use temperature of the part is more suitable for the actual use condition.

[0058] After the rubber sample with the measured initial sealing force F0 is compressed by 25% in the compression deformation clamp according to GB / T 7759.1-2015, it is placed in an aging oven at a specified temperature(determined according to the type of rubber material combined with the use requirements of the part, or according to the increasing temperature values of 75°C, 100°C, 125°C, 150°C, 175°C, etc.), and after reaching the specified time point(test time node is recommended to be 24h, 72h, 168h, multiples of 168h), the compression deformation clamp is taken out of the oven, cooled freely at room temperature for two hours, and the rubber sample is taken out, and the sealing force F t The compression stress value F t In addition to the initial sealing force F0, the compression stress retention rate at each time point can be obtained.

[0059] S3, the basic differential equation of the decay of the sealing force(F) of the sample with time(t) is:

[0060] dF / dt=K / t (1)

[0061] In the formula, K is a constant, and the change rate of stress relaxation is inversely proportional to time. After integral transformation of formula(1), the simple operation formula of the stress relaxation decay process can be obtained:

[0062] Stress relaxation percentage=100(Ft F(t) = A*logt + 100 - A*logt0 (2)

[0063] In the formula, F0 is the initial sealing force, t0 is the test time corresponding to the initial sealing force F0, and the rate of change of stress relaxation is inversely proportional to time. As can be seen from formula (2), a straight line with a slope of A can be obtained by plotting the common logarithm of the retention rate of the sealing force against time t, as shown in Figure 3 A is the percentage reduction rate of the sealing force when the time is increased by 10 times (for example, from 0.5 h to 5 h). Therefore, A is a simple measure of the stress relaxation of the sample and a measure of the sealing ability of the rubber. When the absolute value of A is larger, it indicates that the sealing force of the rubber decays faster and the sealing ability is poorer, and vice versa. By fitting the formula, the sealing force decay at each time can be inferred, and the service life of the rubber seal can be predicted.

[0064] In summary, the present application provides an evaluation method for stress relaxation of rubber materials. The compression deformation sample of the rubber material is compressed in a compression deformation device, placed in an aging oven under specified temperature conditions, taken out after a specified time, and tested for compression stress under quasi-static conditions. The compression stress value is compared with the initial force test value, the compression stress retention rate at each time point is recorded, and the common logarithm of the compression stress retention rate against time is plotted to obtain a straight line with a slope of A. The straight line is a simple measure of the stress relaxation of the sample and a measure of the sealing ability of the rubber. It can effectively reflect the change of the sealing force of the rubber material under stress relaxation.

[0065] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application and not to limit it. Although the present application has been described in detail with reference to the above examples, those skilled in the art should understand that the specific embodiments of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application. Any modification or equivalent replacement without departing from the spirit and scope of the present application should be covered within the protection scope of the claims of the present application.

Claims

1. A method for evaluating stress relaxation in rubber materials, characterized in that, The process includes the following: The rubber compression sample is compressed to obtain the compression force value, and the compression force value is used as the initial sealing force; The compressed rubber sample was placed in an aging chamber at a specified temperature for aging. After the specified time was reached, the rubber sample was taken out and the compressive stress was measured under quasi-static conditions. Compare the compressive stress value with the initial force test value, record the compressive stress retention rate at each time point, and plot the compressive stress retention rate at each time point against the common logarithm of time to obtain a straight line with a slope. The evaluation of stress relaxation of rubber material is completed based on the straight line. When compressing the rubber compression sample, the compression program of the material testing machine is set, the compression speed is set, and the preload displacement is cleared. When the rubber compression sample is compressed to the strain threshold, the rubber compression sample is relaxed at the same speed. The continuous compression and relaxation test is repeated. The compression force value at which the strain of the last rubber compression sample reaches the threshold is recorded, and the compression force value is used as the initial sealing force. Based on the common logarithm of the compressive stress retention rate with respect to time at each time point, a stress relaxation formula is obtained by curve fitting to obtain a straight line with a slope. The stress relaxation formula derived from curve fitting is as follows: The fundamental differential equation for the decay of the sealing force of a rubber compression specimen over time is defined as follows: dF / dt = K / t; Where K is a constant, and the rate of change of stress relaxation is inversely proportional to time; After performing an integral transformation on the fundamental differential equation, the stress relaxation formula is obtained by curve fitting, as follows: Stress relaxation percentage = 100 ( F t / F 0) = A *logt+100- A *logt0 in, F 0 represents the initial sealing force, and t0 represents the corresponding initial sealing force. F For a test time of 0, the rate of change of stress relaxation is inversely proportional to time; The straight line with a slope represents the percentage decrease in sealing force as time increases by a set value. The evaluation of stress relaxation of the rubber material is determined based on the straight line. The larger the absolute value of the straight line, the faster the sealing force of the rubber decays and the worse the sealing ability becomes, and vice versa.

2. The method for evaluating stress relaxation of rubber materials according to claim 1, characterized in that, Before compressing the rubber compression specimen, preheat the rubber compression specimen for heat conditioning, and after heat conditioning, let the rubber compression specimen stand at the standard temperature for 16-48 hours.

3. The method for evaluating stress relaxation of rubber materials according to claim 1, characterized in that, The rubber compression sample is placed on the compression force testing fixture installed on the material testing machine. The compression force testing fixture includes a material testing machine connector (1), a shaft pin (2), a horizontal fastening ring (3), an upper pressure plate (4), and a lower pressure plate (5). One end of the material testing machine connector (1) is connected to the material testing machine, and the other end is connected to the connection end of the upper pressure plate (4) through the shaft pin (2) and the horizontal fastening ring (3). The rubber compression sample is placed on the lower pressure plate (5) and located directly below the upper pressure plate (4). The upper pressure plate (4) moves toward the lower pressure plate (5) and presses the rubber compression sample.

4. The method for evaluating stress relaxation of rubber materials according to claim 1, characterized in that, The compressed rubber samples were placed in an aging chamber under specified temperature conditions, including 75℃, 100℃, 125℃, 150℃, and 175℃; and the rubber samples were removed from the chamber after a specified time point.

5. The method for evaluating stress relaxation of rubber materials according to claim 1, characterized in that, The sealing force of the rubber compression specimen was repeatedly tested at various time points under quasi-static conditions, and the compression force value at which the strain of the last rubber compression specimen reached the threshold was recorded.

6. The method for evaluating stress relaxation of rubber materials according to claim 1, characterized in that, The formulas for calculating the compressive stress retention rate at each time point are as follows: Compression stress value / initial sealing force = compression stress retention rate at each time point.

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