Improved test specimens and related tests for characterizing type II interfacial fracture toughness.
By designing test specimens with thickness transition zones and offset load application points, and by implementing different loading methods, the problems of crack propagation instability and friction effects in ENF and 4ENF tests were solved, and the critical energy release rate of type II crack propagation at the interface was accurately measured.
Patent Information
- Application Number
- CN202480065468.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2023-10-13
- Filing Date
- 2024-09-26
- Publication Date
- 2026-05-26
AI Technical Summary
Existing ENF and 4ENF tests, when measuring the critical energy release rate during type II crack propagation at the interface, show that crack propagation is unstable and easily affected by frictional effects, leading to an overestimation of the critical energy release rate.
A test specimen was designed with a three-point bending loading method, featuring a thickness transition zone and an offset load application point, to ensure the stability of crack propagation and reduce friction effects by introducing thickness variations into the specimen.
Stable crack propagation during type II crack propagation was achieved, reducing the influence of friction effects, thereby enabling accurate measurement of the critical energy release rate and expanding the stable propagation region.
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Figure CN122095235A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a novel test for characterizing the critical energy release rate during the propagation of a type II crack at an interface, the test being based on the bending of a notched specimen. Background Technology
[0002] Fracture toughness is a material's ability to resist crack propagation. There are three types of crack propagation modes, such as […]. Figure 1 The diagram in the image shows the second crack propagation mode, known as Type II or planar shear mode. It is important to note that when a crack propagates between two substrates (and thus at the interface), this is called delamination. To experimentally determine the fracture toughness of a material or interface, a pre-notched specimen (also known as a notched bar) of the material is used, which is broken by bending it to estimate its resistance to such defect propagation. The specimen is typically cuboid in shape.
[0003] ASTM D7905 / D7905M-14 describes a test for characterizing type II crack propagation at interfaces. The purpose of this test is to characterize the relationship between the critical energy release rate during type II crack propagation at an interface and its length. This is also known as interfacial fracture energy. This test is also called the ENF test (End Notch Buckling Test). [See...] Figure 2 As illustrated in the figure, the ENF test involves a three-point bending load on a notched specimen 10', wherein the specimen 10' is placed on two simple bearings 20' and subjected to a load 30' applied from the top of the middle of the specimen 10'.
[0004] The energy required to increase the area of crack 40' by one unit dS corresponds to the portion of energy GdS that will be elastically released by the specimen during crack propagation. G is called the energy release rate and can be estimated using the following formula:
[0005]
[0006] in:
[0007] - 'a' is the length of the crack (assuming the edge is a straight line).
[0008] - B is the width of the specimen.
[0009] - C represents the system's flexibility or supremacy.
[0010] - P is the load that the system bears.
[0011] Cracks can propagate stably or unstably. For a crack to propagate stably, the load must be continuously increased to allow it to grow. In this case, under a constant load, the energy release rate decreases with increasing crack length. Conversely, unstable propagation corresponds to, under a constant load, the energy release rate increasing with increasing crack length. In other words, a crack can propagate "spontaneously" without increasing the load.
[0012] In the case of ENF, the propagation of crack 40' is generally unstable, especially when the length of the pre-notch is less than approximately 0.7 times the distance between the location of the first load 20' and the point of application of load 30'. In fact, in the case of ENF, crack 40' can only stably propagate to a maximum length of 0.3 times the aforementioned distance, but a frictional effect exists near the point of application of load 30'.
[0013] The system's compliance C, or flexibility, is given by: C(a) = U / P, where U corresponds to the specified displacement. When propagation is unstable, compliance and crack length cannot be measured during testing. In fact, the stable region—that is, the length range of a crack that propagates stably—is strictly limited, optimally 15% of the length between the two loads 20', or 30% of the distance between the position of the first load 20' and the point of application of the load 30. Therefore, experimental compliance for different initial crack lengths (15 mm, 25 mm, and 35 mm) is estimated by offsetting the specimen without allowing crack propagation. For this purpose, as [ Figure 3 As shown in the diagram, the specimen is moved sequentially between loads.
[0014] To assess the change in the critical value of energy release rate with crack length, achieving stable crack propagation is highly desirable.
[0015] Other tests are known to be available for characterizing type II crack propagation at interfaces, but these tests are not standard. Therefore, [ Figure 4 The ONF test (over-notch buckling test) and [ ] are illustrated in the figure. Figure 5 The 4ENF test (four-point bending end-notch buckling test) illustrated in the figure allows for stable crack propagation under three-point bending and four-point bending loads, respectively. However, because the tip 42' of the initial crack 40' is located near or after the point of application of the load 30', these tests are very sensitive to the frictional effects between the lips 41' of the crack 40', which can also lead to an overestimation of the critical value of the critical energy release rate.
[0016] Therefore, in order to accurately measure the critical value of the critical energy release rate, a test is needed to characterize the propagation of type II cracks from the interface, which improves the stability of crack propagation while limiting the frictional phenomena between the crack lips. Summary of the Invention
[0017] The present invention provides a solution to the previously discussed problem by providing a test specimen having a thickness transition zone between a first bearing point located on the side of the initial notch and the point of application of the load, and offsetting the point of application of the load (typically located at the center) toward a second bearing point located opposite the first bearing point and away from the initial notch.
[0018] One aspect of the invention relates to a test specimen for characterizing the energy release rate during type II crack propagation, the specimen extending longitudinally along an X-X' axis in a plane P and having an initial notch extending along an X-X' axis parallel to plane P throughout the entire width of the specimen, the specimen comprising three longitudinally continuous portions adjacent to each other, namely:
[0019] - A thin section having a non-zero constant thickness H0 and a non-zero length L0;
[0020] - A thick portion having a non-zero constant thickness H2 and a non-zero length L2, where H2 > H0; and
[0021] - A thickness transition intermediate portion having a non-zero thickness H1 and a non-zero length L1 that is longitudinally variable along the X-X' axis, located between the thin portion and the thick portion, wherein H1 = H0 in the thin portion and H1 = H2 in the thick portion; and wherein
[0022] - When the specimen is subjected to three-point bending, the initial notch is located at least in the thin part and can extend along the X-X' axis parallel to the P plane into the middle part, and possibly into the thick part.
[0023] At a given displacement, stable propagation corresponds to the positive second derivative of stiffness (the inverse of compliance) as a function of crack length. Therefore, increasing the specimen thickness along the crack propagation path compensates for the decrease in stiffness with increasing crack length, thus slowing the rate of stiffness decrease as the crack length increases. Increasing the thickness advantageously allows for an expansion of the region of stable crack propagation, such as […]. Figure 8 As shown in the figure, increasing the specimen thickness along the crack propagation path also allows the stable region to move away from the point of application of the load, thus limiting the frictional effect.
[0024] In addition to the features just discussed in the preceding paragraphs, a specimen according to one aspect of the invention may also have one or more of the following additional features, which may be considered individually or in all possible combinations:
[0025] - The thickness H1 of the middle part changes linearly along the X-X' axis from thickness H0 to thickness H2;
[0026] - The initial notch extends along the entire length of the thin section, especially to avoid instability at the start of testing and to maximize the extended length;
[0027] - The initial notch extends partially into the middle portion, which does not diminish the advantages of the invention in terms of stability and friction, but only shortens the possible extension length;
[0028] - The initial notch divides the thin section into two lips, each with the same thickness h0;
[0029] - 2 x h0 < H2 < 5 x h0, preferably H2 = 4 x h0;
[0030] - L1 > L2;
[0031] The specimen is formed from two layers of identical material assembled together, wherein an initial notch extends parallel to plane P at the interface between the two layers; and
[0032] - Two layers of the same material each have the same thickness h2 in the thick part, where H2 = 2 x h2.
[0033] Another aspect of the invention relates to a test method for characterizing the energy release rate during the propagation of a type II crack at an interface, the test method being based on a three-point bending of a notched specimen, wherein the notched specimen is the test specimen as described above.
[0034] According to one aspect of the invention, the specimen is horizontally positioned, placed on two simple supports, and subjected to a load applied to the top of the specimen, wherein:
[0035] - The first simple support is located below the thin section.
[0036] - The second simple support is located below the thick part.
[0037] - A load is applied to the thick portion at a longitudinal position along the X-X' axis between the longitudinal position of the first load and the longitudinal position of the second load.
[0038] The load application point is off-center and offset towards the second bearing point, which advantageously amplifies the two aforementioned benefits: expanding the stability region and limiting frictional effects. The crack edge is located upstream of the load application point (no longer downstream as in 4ENF or ONF tests), which further limits the frictional effects between the crack lips. To estimate G using the relation [Equation 1], the compliance, a function of crack length, needs to be measured experimentally. However, since propagation is stable, it can be measured during testing. Therefore, changing the specimen position is no longer necessary.
[0039] A better understanding of the invention and its various applications will be gained after reading the following description and reviewing the accompanying drawings. Attached Figure Description
[0040] The accompanying drawings are for illustrative purposes and not for limiting the scope of the invention.
[0041] [ Figure 1 The illustration shows three types of cracks.
[0042] [ Figure 2 The illustration shows the ENF test of the ASTM D7905 / D7905M-14 standard.
[0043] [ Figure 3 The figure illustrates experimental measurements of compliance for three different lengths of the initial crack in the ENF test.
[0044] [ Figure 4 The diagram illustrates the ONF test.
[0045] [ Figure 5 The diagram illustrates the 4ENF test.
[0046] [ Figure 6 The illustration depicts a test according to the present invention.
[0047] [ Figure 7 The illustration shows a test specimen according to the present invention.
[0048] [ Figure 8 [Illustration] is a diagram illustrating the stability region of crack propagation for the ENF test and for the test according to the present invention.
[0049] exist[ Figure 2 ]arrive[ Figure 6 The outline of the sample can be seen in the image, and the crack propagation surface is represented by dashed lines. Detailed Implementation
[0050] Unless otherwise stated, the same element appearing in different figures has the same reference label.
[0051] As is customary in this application, the term "thickness of a part or component" refers to the height of the part or component along an axis perpendicular to plane P (in which the specimen 10 extends); and the term "length of a part or component" refers to the distance along the X-X' axis within plane P, where the X-X' axis is the longitudinal axis of the specimen 10.
[0052] Similarly, by convention, "three-point bending" refers to a standard mechanical test in which the specimen 10 is placed on two simple bearings 20 and subjected to a load 30 applied to the top of the specimen 10 through simple contact. As a reminder, "simple bearing" or "simple contact" refers to a point bearing or point contact represented by a spherical connection.
[0053] The test specimen 10 according to the present invention is intended for use in testing to characterize the energy release rate during the propagation of type II cracks at the interface.
[0054] The test specimen 10 extends longitudinally along the X-X' axis in plane P.
[0055] The specimen 10 according to the invention consists of three continuous portions along the X-X' axis: a first portion 11, a second portion 13 with a thickness greater than that of the first portion 11, and an intermediate portion 12 connecting the first portion 11 and the second portion 13 to achieve a thickness transition. For simplicity, the first portion 11 will be referred to as the "thin portion 11" and the second portion 13 as the "thick portion 13".
[0056] like[ Figure 7 As indicated in the diagram, the thin portion 11 has a non-zero constant thickness H0 and a non-zero length L0, the thick portion 13 has a non-zero constant thickness H2 and a non-zero length L2, where H2 > H0, and the intermediate portion 12 has a non-zero thickness H1, which varies longitudinally along the X-X' axis, and its length L1 is also non-zero. Preferably, the length L1 of the intermediate portion 12 is greater than the length L2 of the thick portion 13. Similarly, the length L2 of the thick portion 13 is preferably greater than the length L0 of the thin portion 11.
[0057] According to a preferred embodiment of the invention, L0 represents 15% to 20% of the length L of the specimen 10, L1 represents 45% to 55% of the length L, and L2 represents 30% to 40% of the length L. For example, the specimen 10 has a length L of 150 to 250 mm and a width of 15 to 30 mm. Of course, the specimen 10 can have many other combinations of dimensions.
[0058] Since the intermediate portion 12 is located between the thin portion 11 and the thick portion 13, it has a thickness H1 = H0 at its junction with the thin portion 11 and a thickness H1 = H2 at its junction with the thick portion 13. The thickness H1 of the intermediate portion 12 increases along the X-X' axis from the value H1 = H0 to the value H1 = H2. This increase in the thickness H1 of the intermediate portion 12 along the X-X' axis is provided so that the thickness transition allows for stable expansion, regardless of its form. It is not necessarily linear.
[0059] According to a preferred embodiment of the invention, thickness H0 represents 2% to 5% of the length L of the sample 10, and thickness H2 represents 5% to 10% of the length L. For example, H0 can be between 8 and 10 mm, while H2 can be between 15 and 20 mm. Of course, the sample 10 can have many other combinations of dimensions.
[0060] Typically, the specimen 10 has an initial notch 40 that extends across the entire width of the specimen 10 parallel to the plane P. This initial notch 40 extends along the X-X' axis into the thin section 11, with its open end portion 43 located at the free end of the thin section 11 (left side in the figure). Opposite to its open end portion 43 along the X-X' axis, the initial notch 40 terminates at the tip 42, referred to as a crack.
[0061] According to a preferred embodiment of the invention, the initial notch 40 is provided over the entire length L0 of the thin portion 11, without extending into the intermediate portion 12. Therefore, the tip 42 of the crack is preferably located at the junction between the thin portion 11 and the intermediate portion 12.
[0062] When the specimen 10 is subjected to three-point bending, the initial notch 40 is provided to extend into the middle portion 12 along the X-X' axis parallel to the plane P, and may extend into the thick portion 13.
[0063] The initial notch 40 divides the thin portion 11 into two lips 41, each lip preferably having the same thickness h0. Therefore, the thickness H2 is preferably between two and five times H0, more preferably about four times h0. For example, h0 can be between 4 mm and 5 mm.
[0064] The test method according to the invention for characterizing the energy release rate during type II propagation of crack 40 is applicable to all types of test specimens 10 used, wherein the two substrates of the test specimen 10 are identical in geometry and constituent materials, and the two substrates are symmetrical about the interface being characterized. The initial notch 40 extends parallel to plane P at the interface, which substantially coincides with the crack propagation surface 50. Preferably, the two layers of material each have the same thickness h2 in the thickness portion 13, where H2 = 2 × h2.
[0065] The test method according to the invention is based on the three-point bending of the specimen 10 according to the invention.
[0066] like[ Figure 6 As indicated in the diagram, in this test method, the specimen 10 is horizontally positioned and placed on two simple supports 20, 21, and 22, and subjected to a load 30 applied to the top of the specimen 10. The first simple support 21 is located below the thin section 11, at a distance D1 from the free end of the thin section 11. Figure 6[Left side); The second simple support 22 is located below the thick part 13, and the distance from the free end of the thick part 13 is D2 ([ Figure 6 (Right side). Distances D1 and D2 are, for example, slightly beyond the required length, which ensures that the specimen 10 is firmly placed on supports 20, 21, and 22 during testing. Distances D1 and D2 are preferably equal, but they may also be different.
[0067] Load 30 is applied from the top of specimen 10 onto the thick portion 13, making simple contact at a point D3 away from the free end of the thick portion 13 (right side in the figure). Load 30 is applied at a point located between the longitudinal positions of the first bearing 21 and the second bearing 22. For example, the distance D3 makes the ratio D3 / L between 0.2 and 0.3.
[0068] Therefore, the main difference between the test method according to the invention and known test methods (especially the ENF test and ONF and 4ENF tests in the ASTM D7905 / D7905M-14 standard) is that the specimen 10 has a thickness transition, the thickness of which increases along the extended surface 50 of the initial notch 40; and the point of application of the load 30 is not located in the center, but is biased toward the second load 22.
[0069] According to the invention, the initial notch 40 is located at least in the thin portion 11. Depending on the extent of the stability region, the initial notch 40 may also extend partially into the intermediate portion 12.
[0070] During the test according to the invention, the initial notch 40 extends at least partially into the intermediate portion 12. Depending on the materials(s) used to make the specimen 10, the initial notch 40 may also (but not necessarily) extend at least partially into the thick portion 13 depending on the test. This extension occurs along the extended surface 50, along the X-X' axis, and in a plane parallel to plane P.
[0071] To demonstrate the advantages of the testing method according to the present invention, [ Figure 8 This diagram illustrates the stability region of crack propagation for an ENF test (thick dashed line) and for a test according to the invention (thick solid line). The stability region refers to the length range of the crack that propagates stably. The stability region is characterized by the fact that, for a given load (here the load is applied by a specified displacement), the larger the crack length, the lower the energy release rate. In the example provided for plotting (sample length 200 mm), the solution of the invention produces DS... INV The stability region is 100 mm, while the stability region DS for the reference test is... ENF It is 30 mm. In [ Figure 8In the diagram, these stability regions are indicated by shaded lines. In this figure, the end on the right side of each curve corresponds to the location where the load is applied.
[0072] It should be pointed out that, Figure 8 [Illustration] is an example of a stability region that exists for a given specimen dimension, and allows for the illustration of the potential of the provided test. If the specimen dimension changes, this region may increase or decrease slightly.
[0073] It can be noted that the solution of the present invention allows the stability region to be far from the point of application of the load, and also allows for the limitation of frictional effects, which is different from the extended unstable reference test.
[0074] Although described by way of a number of examples, alternatives and embodiments, the specimens according to the invention and the test methods according to the invention using the specimens include various alternatives, modifications and improvements, which will be apparent to those skilled in the art, and it should be understood that such alternatives, modifications and improvements all fall within the scope of the invention.
Claims
1. A test method for characterizing the energy release rate during type II crack propagation at an interface, the test method being based on a three-point bending test specimen (10) extending longitudinally along an X-X' axis in a plane P and having an initial notch (40) extending along an X-X' axis parallel to the plane P through the entire width of the specimen (10), characterized in that, The sample (10) comprises three longitudinally continuous parts that are adjacent to each other, namely: - Thin section (11), which has a non-zero constant thickness H0 and a non-zero length L0; - Thick portion (13), which has a non-zero constant thickness H2 and a non-zero length L2, where H2 > H0; and - A thickness transition intermediate portion (12) having a non-zero thickness H1 and a non-zero length L1 that are longitudinally variable along the X-X' axis, this intermediate portion (12) being located between the thin portion (11) and the thick portion (13), wherein H1 = H0 at the thin portion (11) and H1 = H2 at the thick portion (13); and, - When the specimen (10) is subjected to three-point bending, the initial notch (40) of the test specimen (10) subjected to three-point bending is located at least in the thin part (11) and can extend along the X-X' axis parallel to the plane P to the middle part (12) and possibly to the thick part (13).
2. The test method according to claim 1, characterized in that, The initial notch (40) of the test specimen (10) subjected to three-point bending extends along the entire length of the thin section (11).
3. The test method according to claim 2, characterized in that, The initial notch (40) of the test specimen (10) subjected to three-point bending extends along the entire length of the thin section (11) and partially extends into the middle section (12).
4. The test method according to any one of the preceding claims, characterized in that, The initial notch (40) of the test specimen (10) subjected to three-point bending divides the thin section (11) into two lips (41), each of which has the same thickness h0.
5. The test method according to the preceding claims, characterized in that, The test specimen (10) subjected to three-point bending is such that 2 x h0 < H2 < 5 x h0, preferably, H2 = 4 x h0.
6. The test method according to any one of the preceding claims, characterized in that, The test specimen (10) subjected to three-point bending results in L1 > L2.
7. The test method according to any one of the preceding claims, characterized in that, The test specimen (10) subjected to three-point bending is formed of identical material layers assembled together, wherein an initial notch (40) extends parallel to plane P at the interface between the two material layers.
8. The test method according to the preceding claims, characterized in that, The test specimen (10) subjected to three-point bending has two identical layers of material with the same thickness h2 in the thick part, and H2 = 2 x h2.
9. The test method according to any one of the preceding claims, wherein the specimen (10) is horizontally positioned, placed on two simple supports (20, 21, 22), and subjected to a load applied to the top of the specimen (10), characterized in that: - The first simple support (21) is located below the thin section (11), - The second simple support (22) is located below the thick part (13), - At a longitudinal position along the X-X' axis between the longitudinal position of the first bearing (21) and the longitudinal position of the second bearing (22), the load (30) is applied to the thick part (13).