Method for testing fatigue crack propagation of rubber

Abstract

The invention provides a rubber fatigue crack propagation test method, and relates to the technical field of rubber materials, the method comprises the following steps: obtaining a test piece of a to-be-tested rubber sample and a calendaring direction of the test piece; determining a test stretching direction based on the rolling direction; and performing crack tensile test on the test piece based on the test tensile direction to obtain a test result. The method comprises the following steps: determining a stretching direction which is the same as a calendering direction or a stretching direction which is different from the calendering direction according to test pieces made of different rubber materials, or cutting a plurality of test pieces with the same size on a rubber sample to be tested, and respectively stretching the test pieces with the same size in the calendering direction or stretching the test pieces with the same size in the calendering direction or stretching the test pieces with the same size in the calendering direction; according to the present invention, the multi-direction crack stretching test of the rubber material test piece is achieved, the stretching direction of the optimal stretching test can be selected, and compared with the prior art, the method can well illustrate the detected rubber material crack growth rate condition, and can improve the accuracy.

Description

Technical Field

[0001] The invention relates to the technical field of rubber materials, in particular to a method for testing fatigue crack extension of rubber. Background Art

[0002] Rubber is an important component material and matrix of various tire components. The fatigue performance of rubber materials has a significant impact on the service life of the tire. Therefore, accurately testing the crack growth rate of rubber is crucial to evaluating its fatigue properties.

[0003] Calendering is one of the important molding methods in the rubber material processing process. During the calendering process, when the material passes through the gap between the calendering rollers, it will be subjected to shear force and tensile stress at the same time. The rubber molecular chain will be oriented along the calendering direction, resulting in obvious differences in the physical and mechanical properties of the product in the horizontal and vertical directions. The tensile strength along the calendering direction is large, the elongation is small, and the shrinkage is large, while the tensile strength along the direction perpendicular to the calendering direction is small, the elongation is large, and the shrinkage is small. This difference in physical and mechanical properties will affect the test results of the fatigue crack rate of the rubber.

[0004] However, in the prior art, when conducting crack growth rate tests on rubber materials, rubber samples are mostly cut into pure shear test pieces, and the relationship between the calendering direction and the cutting angle and the stretching angle is not paid attention to. Due to the differences in physical and mechanical properties caused by calendering, the test results in different directions will be very different, resulting in the current crack growth rate test being difficult to fully reflect the overall fatigue performance of the material. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a method for testing rubber fatigue crack growth, so as to alleviate the problem in the prior art that the crack growth test of rubber materials cannot fully explain the crack growth rate, and the test results of the crack growth rate of rubber materials have poor accuracy.

[0006] In a first aspect, an embodiment of the present invention provides a method for testing fatigue crack growth of rubber, the method comprising: Obtain a test piece of the rubber sample to be tested and the calendering direction of the test piece; Based on the calendering direction, determine the test stretching direction; Based on the test tensile direction, a crack tensile test is performed on the test piece to obtain the test results.

[0007] In combination with the first aspect, the embodiment of the present invention provides a first possible implementation of the first aspect, before the step of obtaining the rubber sample to be tested and the calendering direction of the rubber sample to be tested, the method further includes: Obtain the sample and the rolling direction of the sample; Cutting the specimen to obtain multiple test pieces of the same shape; For each test piece, a crack tensile test is performed on the test piece according to a preset test direction to obtain a test result.

[0008] In combination with the first aspect, an embodiment of the present invention provides a first possible implementation manner of the first aspect, wherein the test piece includes a target test piece, and a testing direction and a rolling direction of the target test piece are the same; For each test piece, a crack tensile test is performed on the test piece according to a preset test direction to obtain the test result, including: cutting a midpoint of a short side of the target test piece to form a target crack on the target test piece; The target test piece is clamped at both sides of the target crack and stretched along the rolling direction to obtain the first crack growth rate of the target test piece.

[0009] In combination with the first aspect, the embodiment of the present invention provides a first possible implementation manner of the first aspect, wherein the test piece further includes a comparison test piece, and the test direction of the comparison test piece forms an angle with the calendering direction; For each test piece, a crack tensile test is performed on the test piece according to a preset test direction to obtain the test result, including: Cutting the midpoint of the short side of the comparative test piece to form a target crack on the comparative test piece; The parts on both sides of the target crack of the comparative test piece are clamped and stretched along the test direction of the comparative test piece to obtain the second crack growth rate of the comparative test piece.

[0010] In combination with the first aspect, an embodiment of the present invention provides a first possible implementation of the first aspect, wherein the step of cutting the sample to obtain a plurality of test pieces of the same shape includes: The thickness of the specimen is less than or equal to 2 mm; The ratio of the length to the width of the target test piece and the comparison test piece is greater than or equal to 10.

[0011] In combination with the first aspect, the embodiment of the present invention provides a first possible implementation of the first aspect, wherein the step of cutting the sample to obtain a plurality of test pieces of the same shape further includes: The extending direction of the short side of the target test piece is the same as the rolling direction, and the extending direction of the long side of the target test piece is perpendicular to the rolling direction; The extension direction of the short side of the comparison test piece is perpendicular to the extension direction of the short side of the target test piece.

[0012] In combination with the first aspect, the embodiment of the present invention provides a first possible implementation of the first aspect, wherein the lengths of the target crack of the target test piece and the target crack of the comparison test piece are both 25 mm.

[0013] In combination with the first aspect, the embodiment of the present invention provides a first possible implementation manner of the first aspect, wherein for each test piece, a crack tensile test is performed on the test piece according to a preset test direction to obtain a test result, further comprising: Applying a cyclic tensile strain in the form of a sine wave with a strain range of 10%-50% to the target specimen and the comparison specimen; Record the change in length of the target crack of the target specimen and the target crack of the comparison specimen after each increase in strain; Based on the initial sizes of the target test piece and the comparison test piece and the length changes of the cracks of the target test piece and the comparison test piece, a first crack growth rate and a second crack growth rate are calculated; Based on the first crack growth rate and the second crack growth rate, a comparison chart of the test results of the expansion rates of the target test piece and the comparison test piece is drawn.

[0014] In combination with the first aspect, the embodiment of the present invention provides a first possible implementation manner of the first aspect, wherein the step of obtaining a sample and a rolling direction of the sample includes: Repeatedly knead the mixed rubber in the same direction to obtain a first-class colloid; The primary colloid is vulcanized to obtain a test sample.

[0015] The embodiments of the present invention bring the following beneficial effects: The present invention provides a method for testing fatigue crack extension of rubber, the method comprising: obtaining a test piece of a rubber sample to be tested and a rolling direction of the test piece; determining a test stretching direction based on the rolling direction; and performing a crack stretching test on the test piece based on the test stretching direction to obtain a test result.

[0016] In the present invention, before the test is carried out, a test piece of the rubber sample to be tested and the calendering direction of the test piece are obtained, and the test stretching direction in the subsequent tensile test is determined according to the calendering direction. The stretching direction that is the same as the calendering direction or the stretching direction that is different from the calendering direction can be determined according to the test pieces of different rubber materials, or a plurality of test pieces of the same size are cut from the rubber sample to be tested and respectively stretched in the same direction as the calendering direction or in a direction different from the calendering direction, thereby realizing a multi-directional crack tensile test on the test piece of the rubber material, and being able to select the optimal stretching direction for the tensile test. Compared with the prior art, the method provided by the present invention can better illustrate the crack propagation rate of the rubber material being tested, and improve the accuracy.

[0017] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention are realized and obtained by the structures particularly pointed out in the description, claims and drawings.

[0018] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 A schematic flow chart of a method for testing rubber fatigue crack growth provided in Embodiment 1 of the present invention; Figure 2 A diagram showing the relationship between the cutting positions of the target test piece and the comparison test piece in the method for testing the rubber fatigue crack growth provided in the first embodiment of the present invention; Figure 3 The following is a comparison of the crack growth rate test results of A7 and E9 in Example 3 of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0022] In order to facilitate the understanding of this embodiment, the application scenarios and design concepts of the embodiments of the present application are briefly introduced below.

[0023] Rubber crack growth refers to the process in which tiny cracks inside or on the surface of rubber materials gradually expand during repeated loading or long-term use. This phenomenon has a significant impact on the performance and life of rubber products, especially in key applications such as tires and seals.

[0024] Calendering is one of the important molding methods in the rubber material processing process. During the calendering process, when the material passes through the gap between the calendering rollers, it will be subjected to shear force and tensile stress at the same time. The rubber molecular chain will be oriented along the calendering direction, resulting in obvious differences in the physical and mechanical properties of the product in the horizontal and vertical directions. The tensile strength along the calendering direction is large, the elongation is small, and the shrinkage is large, while the tensile strength along the direction perpendicular to the calendering direction is small, the elongation is large, and the shrinkage is small. This difference in physical and mechanical properties will affect the test results of the fatigue crack rate of the rubber.

[0025] Under the action of mechanical external force, the freely moving macromolecular chain structure in the rubber material moves first, and the molecular chain structure is stretched, resulting in a weakening of the force between rubber molecules. When the external force is removed, the macromolecular chain returns to its original free curled structure. Therefore, in the early stage of fatigue, the rubber structure resists the damage of external force through the freely moving macromolecular chain structure. As the external force further increases, the rubber layer on the surface of the carbon black particles in the rubber system is stretched, the molecular chain will tend to a more orderly structural arrangement, and the interaction between molecules will increase. After repeated tensile fatigue, the rubber material breaks and fails.

[0026] Therefore, the crack propagation of rubber materials is the breakage of the molecular chain at the crack tip, which is closely related to the strength and movement of the molecular chain. During the calendering process, the rubber molecular chain has a preliminary orientation. When the stretching direction is parallel to the calendering direction, that is, the stretching direction is the same as the molecular chain elongation direction, its elongation at break will be relatively large.

[0027] Example 1 See also Figure 1 This embodiment provides a method for testing rubber fatigue crack growth, the method comprising: S100, obtaining a test piece of the rubber sample to be tested and a calendering direction of the test piece.

[0028] S200, determining a test stretching direction based on the calendering direction.

[0029] S300, performing a crack tensile test on the test piece based on the test tensile direction to obtain a test result.

[0030] Specifically, before the test is performed, the present embodiment obtains a test piece of the rubber sample to be tested and the calendering direction of the test piece, and determines the test stretching direction in the subsequent tensile test according to the calendering direction of the test piece.

[0031] Furthermore, when testing the rubber sample to be tested, multiple test pieces of the same size and specification can be cut from the same sample, and different test pieces can be tested in the same stretching direction as the calendering direction or in a stretching direction different from the calendering direction, thereby realizing a multi-directional crack tensile test on the rubber material to be tested, and being able to select the optimal tensile test direction. Compared with the prior art, the method provided in this embodiment can better illustrate the crack growth rate of the rubber material being tested and improve accuracy.

[0032] In this embodiment, before the step S100 of obtaining the rubber sample to be tested and the calendering direction of the rubber sample to be tested, the following steps are also included: S110, obtaining the sample and the rolling direction of the sample.

[0033] S120, cutting the sample to obtain a plurality of test pieces with the same shape.

[0034] S130, for each test piece, perform a crack tensile test on the test piece according to a preset test direction to obtain a test result.

[0035] Specifically, in this embodiment, the rubber sample to be tested is extracted to obtain a sample with an area sufficient to cut out two test pieces, and the calendering direction of the rubber sample to be tested to which the sample belongs is recorded at the same time, so that the calendering direction of each test piece can be known during cutting, so as to judge the angular relationship between the stretching direction and the calendering direction of the test piece during stretching.

[0036] Subsequently, for each cut test piece, a crack tensile test is performed in a direction that is the same as the calendering direction and in a direction that is different from the calendering direction, thereby obtaining the test results of multi-angle tensile testing of the sample to be tested.

[0037] In this embodiment, the test piece includes a target test piece, and the testing direction and the rolling direction of the target test piece are the same.

[0038] Specifically, step S130, for each test piece, performs a tensile test on the test piece according to a preset test direction to obtain a test result, includes: S131, cutting the midpoint of the short side of the target test piece to form a target crack on the target test piece.

[0039] S132 clamps the parts on both sides of the target crack of the target test piece and stretches them along the rolling direction to obtain a first crack growth rate of the target test piece.

[0040] In order to accurately measure whether the calendering direction of the rubber sample to be tested has an effect on the crack growth rate, the target test piece in this embodiment uses the same direction as the calendering direction as the crack stretching direction. When cutting, it is necessary to leave space for making cracks between the two clamping parts of the stretching equipment, and the connecting line of the two clamping parts must be parallel to the direction, so as to ensure that the stretching direction is the same as the calendering direction.

[0041] In addition, the cutting position should be close to or at the midpoint of the side of the target test piece. At this position, the forces on both sides of the crack are more uniform, and the test results are more accurate.

[0042] In this embodiment, the testing direction of the comparative test piece forms an angle with the calendering direction.

[0043] In addition, in this embodiment, step S130, for each test piece, performs a tensile test on the test piece according to a preset test direction to obtain a test result, further includes: S133, cutting the midpoint of the short side of the comparative test piece to form a target crack on the comparative test piece.

[0044] S134, clamping portions on both sides of the target crack of the comparative test piece and stretching the comparative test piece along a test direction of the comparative test piece to obtain a second crack growth rate of the comparative test piece.

[0045] Specifically, in order to improve the accuracy of the test, the consistency of the conditions of the comparison test piece and the target test piece must be ensured, and at the same time, the tensile direction of the comparison test piece is different from the calendering direction of the comparison test piece. Therefore, in this embodiment, the cutting position, length and shape of the target crack on the comparison test piece are the same as those of the target crack on the target test piece, thereby further ensuring the accuracy of the rubber fatigue crack growth test.

[0046] In this embodiment, step S120, the step of cutting the sample to obtain a plurality of test pieces of the same shape, includes: The thickness of the specimen is less than or equal to 2 mm.

[0047] The ratio of the length to the width of the target test piece and the comparison test piece is greater than or equal to 10.

[0048] When the aspect ratio of the target test piece and the comparison test piece is greater than or equal to 10 mm and the thickness is less than or equal to 2 mm, it can be ensured that the effective areas of the target test piece and the comparison test piece are in a pure shear state, which can further improve the accuracy of the test.

[0049] Furthermore, in this embodiment, step S120, the step of cutting the sample to obtain a plurality of test pieces of the same shape, further includes: The extending direction of the short side of the target test piece is the same as the rolling direction, and the extending direction of the long side of the target test piece is perpendicular to the rolling direction.

[0050] The extension direction of the short side of the comparison test piece is perpendicular to the extension direction of the short side of the target test piece.

[0051] See also Figure 2 The target specimen is Figure 2 A, the comparison test piece is Figure 2 B. The comparison test piece and the target test piece in this embodiment are perpendicular to each other when they are cut, so that the short side of the comparison test piece is perpendicular to the calendering direction. When the comparison test piece is subjected to crack tensile test, the tensile strength and the calendering direction can be perpendicular to each other, thereby obtaining a comparison example with the largest angle difference compared with the target test piece, the data difference between the two is the largest, and the comparison is more obvious. It can be clearly concluded whether the calendering direction has an impact on the result of the crack tensile test of the rubber material, and it can be concluded whether the tensile direction of the optimal tensile test is the same as the calendering direction.

[0052] In this embodiment, the lengths of the target crack of the target test piece and the target crack of the comparison test piece are both 25 mm.

[0053] Specifically, the target test piece and the comparison test piece in this embodiment are both rectangular, and the extension direction of the short side of the rectangle is the stretching direction. The target crack is set at the midpoint of the short side of the rectangle, and the extension direction of the target crack is the same as the extension direction of the long side of the rectangle. Under this structure, it is easy to ensure the consistency of the test of the target test piece and the comparison test piece.

[0054] Step S130, for each test piece, performing a crack tensile test on the test piece according to a preset test direction to obtain a test result, further includes: S135, applying a periodic tensile strain in the form of a sine wave with a strain range of 10%-50% to the target test piece and the comparison test piece.

[0055] S136, recording the change in length of the target crack of the target test piece and the target crack of the comparison test piece after each increase in strain.

[0056] S137, calculating a first crack growth rate and a second crack growth rate based on the initial sizes of the target test piece and the comparison test piece and the length changes of the cracks of the target test piece and the comparison test piece.

[0057] S138, based on the first crack growth rate and the second crack growth rate, draw a comparison chart of the test results of the expansion rates of the target test piece and the comparison test piece.

[0058] Specifically, in this embodiment, a periodic tensile strain in the form of a sine wave is applied to the target test piece and the comparison test piece, and the strain range of the test is 10%-50%.

[0059] During the cyclic loading process, the strain was increased every 500 cycles (1 circle), the initial strain peak was 10%, the strain peak in the same circle remained unchanged, the experiment lasted 600 circles, and the strain peak in the last circle was 50%. At the same time, the stress-strain curve in each cycle was recorded, and a crack tip photo was taken after each circle.

[0060] Then, based on the photos taken, the length change of the crack is identified and recorded, the effective data source is intercepted, and the power function is used to fit the crack extension length and the number of cycles to calculate the crack extension rate (da / dN). Finally, according to the theory of Rivlin and Thomas, the tearing energy (T) is calculated, and the relationship between the crack extension rate and the tearing energy is established. The tearing energy calculation formula is T=W (W is the strain energy density of the sample determined from the stress-strain curve of the unloading section, and h0 is the initial height of the sample).

[0061] S110, the step of obtaining a sample and a rolling direction of the sample, comprises: The mixed rubber is kneaded repeatedly in the same direction to obtain a first-class colloid.

[0062] The primary colloid is vulcanized to obtain a test sample.

[0063] It should be noted that, in this embodiment, the vulcanization temperature is 130° C., the pressure is 10 MPa, and the vulcanization time is 30 min.

[0064] Embodiment 2 In this embodiment, rubber material E5 is used as a sample, and the rubber fatigue crack growth test method provided in the first embodiment is used to perform the test, and the comparison between the target test piece and the comparison test piece under different fixed extension states is shown in Table 1.

[0065] Table 1 is the comparison between the target test piece and the reference test piece under different fixed extension conditions

[0066] It can be seen from Table 1 that the greater the tensile strength, the more obvious the difference between the target test piece and the comparison test piece, that is, the calendering direction has an effect on the rubber properties.

[0067] Embodiment 3 In this embodiment, two rubber materials, E9 and A7, are used as samples to carry out the rubber fatigue crack growth test provided in the first embodiment.

[0068] Among them, the formula of E9 is natural rubber / butadiene rubber = 90 / 10.

[0069] The formula of A7 is natural rubber / styrene-butadiene rubber = 80 / 20.

[0070] Furthermore, in this embodiment, the target test piece made with E9 as the sample is E9_A, and the comparison test piece made with E9 as the sample is E9_B.

[0071] Correspondingly, the target test piece made with A7 as the sample is A7_A, and the comparison test piece made with A7 as the sample is A7_B.

[0072] The test results are shown in Table 2.

[0073] Table 2

[0074] According to Table 2, a comparison chart of the crack growth rate test results of A7 and E9 is drawn, as shown in Figure 3 .

[0075] Combined with Table 2 and Figure 3 It can be seen that the effect of the cutting direction of the specimen on the crack growth rate of E9 is significantly smaller than that on A7. The maximum difference between the crack growth rates of A and B of E9 is about 17.42%, and the maximum difference between the crack growth rates of A7 is about 307.90%.

[0076] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0077] Finally, it should be noted that the above embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The protection scope of the present invention is not limited thereto. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that any person skilled in the art can still modify the technical solutions recorded in the above embodiments within the technical scope disclosed by the present invention, or can easily think of changes, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for testing rubber fatigue crack growth, characterized in that: The method comprises: Obtaining a test piece of the rubber sample to be tested and a calendering direction of the test piece; Based on the calendering direction, determining a test stretching direction; Based on the test tensile direction, a crack tensile test is performed on the test piece to obtain a test result.

2. The method according to claim 1, characterized in that Before the step of obtaining the rubber sample to be tested and the calendering direction of the rubber sample to be tested, the method further includes: Obtaining a sample and a rolling direction of the sample; Cutting the sample to obtain a plurality of test pieces having the same shape; For each of the test pieces, a crack tensile test is performed on the test piece according to a preset test direction to obtain a test result.

3. The method according to claim 2, characterized in that The test piece includes a target test piece, and the testing direction of the target test piece is the same as the calendering direction; The step of performing a crack tensile test on each test piece according to a preset test direction to obtain a test result comprises: Cutting the midpoint of the short side of the target test piece to form a target crack on the target test piece; The parts on both sides of the target crack of the target test piece are clamped and stretched along the rolling direction to obtain a first crack growth rate of the target test piece.

4. The method according to claim 3, characterized in that The test piece also includes a comparative test piece, wherein the testing direction of the comparative test piece forms an angle with the calendering direction; The step of performing a crack tensile test on each test piece according to a preset test direction to obtain a test result comprises: Cutting the midpoint of the short side of the comparative test piece to form a target crack on the comparative test piece; The parts on both sides of the target crack of the comparative test piece are clamped and stretched along the test direction of the comparative test piece to obtain a second crack growth rate of the comparative test piece.

5. The method according to claim 4, characterized in that The step of cutting the sample to obtain a plurality of test pieces of the same shape comprises: The thickness of the sample is less than or equal to 2 mm; The ratio of the length to the width of the target test piece and the comparison test piece is greater than or equal to 10.

6. The method according to claim 4, characterized in that The step of cutting the sample to obtain a plurality of test pieces of the same shape also includes: The extending direction of the short side of the target test piece is the same as the rolling direction, and the extending direction of the long side of the target test piece is perpendicular to the rolling direction; The extension direction of the short side of the comparison test piece is perpendicular to the extension direction of the short side of the target test piece.

7. The method according to claim 4, characterized in that The lengths of the target crack of the target test piece and the target crack of the comparison test piece are both 25 mm.

8. The method according to claim 4, characterized in that The step of performing a crack tensile test on each test piece according to a preset test direction to obtain a test result also includes: Applying a periodic tensile strain in the form of a sine wave with a strain range of 10%-50% to the target test piece and the comparison test piece; Recording the change in length of the target crack of the target test piece and the target crack of the comparison test piece after each increase in strain; Calculating the first crack growth rate and the second crack growth rate based on the initial sizes of the target test piece and the comparison test piece and the length changes of the crack of the target test piece and the crack of the comparison test piece; Based on the first crack growth rate and the second crack growth rate, a comparison graph of test results of the expansion rates of the target test piece and the comparison test piece is drawn.

9. The method according to claim 2, characterized in that: The step of obtaining a sample and a rolling direction of the sample comprises: Repeatedly knead the mixed rubber in the same direction to obtain a first-class colloid; The primary colloid is vulcanized to obtain the sample.

Images