Bi-component microcapsule self-repairing epoxy resin composite material and preparation method thereof

Abstract

The invention discloses a bi-component microcapsule self-repairing epoxy resin composite material as well as a preparation method and application thereof, the bi-component microcapsule self-repairing epoxy resin composite material is of a core-shell structure, epoxy resin and pentaerythritol tetra (3-mercaptopropionate) are taken as a core, and the outer layer is coated with polymethyl methacrylate. In the aspect of tensile property, the tensile strength of the epoxy resin composite material is in a trend of firstly increasing and then decreasing along with the increase of the microcapsule content from 0 wt%. Due to the fact that the microcapsules with the thermoplastic polymethyl methacrylate as the shell are added, the overall tensile property of the material is improved; the microencapsulated repairing agent not only can heal cracks, but also can provide higher fracture toughness for the damaged part of the epoxy resin composite material.

Description

Technical Field

[0001] The invention belongs to the field of material science, and specifically relates to a two-component microcapsule self-repairing epoxy resin composite material and a preparation method thereof. Background Art

[0002] Polymer-based composites are a type of high-performance material that is composed of an organic polymer as a matrix and a variety of reinforcing materials. This material combines the advantages of the matrix and the reinforcing material. Compared with pure composite matrix, it has the characteristics of high specific strength, high specific stiffness, fatigue resistance, good vibration reduction, good safety under overload, and strong designability. Polymer-based composites are widely used in machinery manufacturing, transportation, aerospace and other fields. Epoxy resin is often used as a high-performance matrix polymer in composite materials due to its excellent physical and mechanical properties and electrical insulation. However, composite materials are difficult to detect or repair when they are subjected to internal microscopic cracks, which seriously reduces the service life and increases the cost of use.

[0003] Epoxy resin composites will inevitably produce internal micro crack damage during practical applications, but there is currently a lack of truly effective solutions to the problem of internal micro crack damage in epoxy resin composites. Most of the research on internal micro crack damage in epoxy resin composites focuses on discussing the influencing factors and microscopic changes of internal micro crack damage, but there is a lack of corresponding research on how to efficiently repair internal micro crack damage.

[0004] The components of the microcapsule repair agent in the epoxy resin composite material after curing are different from the matrix, which is equivalent to introducing impurities into the epoxy resin. Since the material and internal structure of the components of the microcapsule repair agent after curing are not exactly the same as the epoxy resin matrix, it will cause structural defects in the epoxy resin composite material, causing the mechanical properties such as the tensile strength of the epoxy resin composite material to decrease, and it is impossible to realize the widespread application of the existing microcapsule self-repair technology in the industrial field.

[0005] Existing microcapsule self-repairing technology has made great progress in the field of epoxy resin composite self-repairing, but the efficiency of traditional microcapsule self-repairing epoxy resin composites in self-repairing internal microcrack damage at room temperature is low, and it usually takes a long time to achieve effective self-repair. In order to achieve efficient self-repair of internal microcrack damage in epoxy resin composites, it is urgent to improve the existing technology and innovatively develop a highly efficient self-repairing microcapsule for internal microcrack damage in epoxy resin composites. Summary of the invention

[0006] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0007] In view of the above problems and / or the problems existing in the prior art, the present invention is proposed.

[0008] Therefore, the purpose of the present invention is to overcome the deficiencies in the prior art and provide a two-component microcapsule self-healing epoxy resin composite material.

[0009] In order to solve the above technical problems, the present invention provides the following technical solutions: a two-component microcapsule self-healing epoxy resin composite material, characterized in that: the two-component microcapsule self-healing epoxy resin composite material is a core-shell structure, wherein epoxy resin and pentaerythritol tetrakis(3-mercaptopropionate) are used as the core, and the outer layer is coated with polymethyl methacrylate.

[0010] As a preferred embodiment of the preparation method of the present invention, the epoxy resin is an epoxy prepolymer.

[0011] Another object of the present invention is to overcome the deficiencies in the prior art and provide a method for preparing a two-component microcapsule self-repairing epoxy resin composite material.

[0012] As a preferred embodiment of the preparation method of the present invention, wherein: epoxy prepolymer and polymethyl methacrylate are dissolved in dichloromethane to obtain an oil phase solution 1;

[0013] Pentaerythritol tetrakis(3-mercaptopropionic acid) ester and polymethyl methacrylate are dissolved in dichloromethane, and N,N-dimethylbenzylamine is added to obtain an oil phase solution 2;

[0014] With deionized water as the aqueous phase, the oil phase solution 1 is added into the aqueous phase and stirred at room temperature to obtain an oil-water emulsion 1;

[0015] With deionized water as the aqueous phase, the oil phase solution 2 is added into the aqueous phase and stirred at room temperature to obtain an oil-water emulsion 2;

[0016] Hexadecyltrimethylammonium bromide and the oil-water emulsion 1 are mixed and stirred, and after the dichloromethane is completely evaporated, the mixture is filtered and washed with methanol and deionized water respectively, and dried at room temperature to obtain microcapsules 1;

[0017] Sodium dodecyl sulfate and oil-water emulsion 2 are mixed and stirred, and after dichloromethane is completely evaporated, the mixture is filtered and washed with methanol and deionized water respectively, and dried at room temperature to obtain microcapsules 2;

[0018] Epikote TM828 and Epikure TM F205 is mixed as a matrix; microcapsules 1 and 2 are mixed with the matrix, the mixed sample is degassed and poured into a dumbbell-shaped silicone rubber mold, and then cured at room temperature and then cured at a high temperature to obtain the two-component microcapsule self-healing epoxy resin composite material.

[0019] As a preferred embodiment of the preparation method of the present invention, the mass ratio of the epoxy prepolymer to polymethyl methacrylate is 1.0-1.2:0.85-1; the mass ratio of pentaerythritol tetrakis(3-mercaptopropionate) to polymethyl methacrylate is 1.0-1.3:0.8-1.1.

[0020] As a preferred embodiment of the preparation method of the present invention, the mass ratio of the water phase to the oil phase is 19-22:11-15.

[0021] As a preferred embodiment of the preparation method of the present invention, the mass ratio of hexadecyltrimethylammonium bromide to oil-water emulsion 1 is 1.1-1.9:30-35; the mass ratio of sodium dodecyl sulfate to oil-water emulsion 2 is 1.2-1.6:30-37.

[0022] As a preferred embodiment of the preparation method of the present invention, wherein: the epikote in the matrix TM 828 and Epikure TM The mass ratio of F205 is 90~100:63~75.

[0023] As a preferred solution of the preparation method of the present invention, the total content of the microcapsules 1 and 2 is 3-12 wt%.

[0024] As a preferred embodiment of the preparation method of the present invention, the mass ratio of the microcapsules 1 and 2 is 1:1.

[0025] Another object of the present invention is to overcome the deficiencies in the prior art and provide a two-component microcapsule self-repairing epoxy resin composite material for use in micro crack damage.

[0026] Beneficial effects of the present invention:

[0027] In terms of tensile properties, as the content of microcapsules increases from 0wt%, the tensile strength of the epoxy resin composite material shows a trend of first increasing and then decreasing. The addition of microcapsules with thermoplastic polymethyl methacrylate as the outer shell increases the overall tensile properties of the material; the microencapsulated repair agent of the present invention can not only heal cracks, but also provide higher fracture toughness for the damaged parts of the epoxy resin composite material. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0029] Figure 1 This is a scanning electron microscope characterization image of the epoxy resin and thiol microcapsules with polymethyl methacrylate as the shell of the present invention.

[0030] Figure 2 The tensile strength curves of epoxy resin composite materials with different microcapsule contents in the examples and comparative examples of the present invention are shown.

[0031] Figure 3 The fracture toughness curves of epoxy resin composite materials with different microcapsule contents in the examples and comparative examples of the present invention are shown.

[0032] Figure 4 This is a crack expansion diagram around the microcapsules of the two-component microcapsule self-healing epoxy resin composite material before self-healing.

[0033] Figure 5 The graphs are the effects of different contents of repair agent microcapsules on the self-repairing efficiency of epoxy resin composite materials in the examples and comparative examples of the present invention. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

[0035] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0036] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0037] Unless otherwise specified, all raw materials used in the examples of the present invention are commercially available, as shown in Table 1 for details.

[0038] Table 1

[0039] name Commercial channels Part Number Epoxy prepolymer (EC157) Shanghai MacLean Biochemical Co., Ltd. EC157 Polymethyl methacrylate Sinopharm Chemical Reagent Co., Ltd. 9011-14-7 Pentaerythritol tetrakis(3-mercaptopropionate) Sinopharm Chemical Reagent Co., Ltd. 7575-23-7 Deionized water Sinopharm Chemical Reagent Co., Ltd. 7732-18-5 N,N-Dimethylbenzylamine Sinopharm Chemical Reagent Co., Ltd. 103-83-3 Hexadecyltrimethylammonium bromide Sinopharm Chemical Reagent Co., Ltd. 57-09-0 Sodium Lauryl Sulfate Sinopharm Chemical Reagent Co., Ltd. 151-21-3 Methanol Sinopharm Chemical Reagent Co., Ltd. 67-56-1 <![CDATA[Epikote TM 828]]> Sigma-Aldrich (Shanghai) Trading Co., Ltd. 1974-84-87 <![CDATA[Epikure TM F205]]> Sigma-Aldrich (Shanghai) Trading Co., Ltd. 684-32-1 chlorobenzene Sinopharm Chemical Reagent Co., Ltd. 108-90-7 Gum Arabic Sinopharm Chemical Reagent Co., Ltd. 9000-01-5

[0040] Example 1

[0041] (1) 3 g of epoxy prepolymer (EC157) and 3 g of polymethyl methacrylate were successively dissolved in 110 ml of dichloromethane to prepare an oil phase solution 1;

[0042] 3 g of pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 3 g of polymethyl methacrylate were dissolved in 110 ml of dichloromethane, and then 0.5 g of N,N-dimethylbenzylamine was added as a catalyst to obtain an oil phase solution 2.

[0043] (2) using deionized water as the aqueous phase, adding the oil phase solution 1 containing the epoxy prepolymer (EC157) into the aqueous phase, wherein the addition ratio of the aqueous phase to the oil phase is 19:11, and stirring at a high speed of 1200 rpm at room temperature for 45 min to obtain an oil-water emulsion 1 containing the epoxy prepolymer (EC157);

[0044] Deionized water is used as the aqueous phase, and the oil phase solution 2 containing pentaerythritol tetrakis(3-mercaptopropionate) ester is added into the aqueous phase, wherein the addition ratio of the aqueous phase to the oil phase is 19:11, and the mixture is stirred at a high speed of 1200 rpm for 45 min at room temperature to obtain an oil-water emulsion 2 containing pentaerythritol tetrakis(3-mercaptopropionate);

[0045] (3) mixing hexadecyltrimethylammonium bromide and the oil-water emulsion 1 in a ratio of 1.1:30, and after the dichloromethane is completely evaporated, filtering and washing with methanol and deionized water respectively, and drying at room temperature to obtain microcapsules 1;

[0046] Sodium dodecyl sulfate and oil-water emulsion 2 are mixed and stirred at a ratio of 1.2:30. After dichloromethane is completely evaporated, the mixture is filtered and washed with methanol and deionized water respectively, and dried at room temperature to obtain microcapsules 2.

[0047] (4) 90 copies of Epikote TM 828 and 63 Epikure TM F205 is mixed as a matrix; the microcapsules of the two components prepared in step (3) are mixed with the matrix, wherein the content of the microcapsules of the two components is 9wt%, and the mass ratio of the two microcapsules is 1:1, and the mixed sample is degassed and poured into a dumbbell-shaped silicone rubber mold, and cured at room temperature for 48 hours, and then cured at 75°C for 2 hours to obtain a two-component microcapsule self-healing epoxy resin composite material.

[0048] Example 2

[0049] The difference from Example 1 is that the content of the two-component microcapsules in step (4) is 12 wt%.

[0050] Example 3

[0051] The difference from Example 1 is that the content of the two-component microcapsules in step (4) is 6 wt %.

[0052] Example 4

[0053] The difference from Example 1 is that the content of the two-component microcapsules in step (4) is 3 wt %.

[0054] Comparative Example 1

[0055] The difference from Example 1 is that only pure epoxy resin samples are prepared. The pure epoxy resin samples are composed of 90 parts of Epikote TM 828 and 63 Epikure TM F205 mixed.

[0056] Example 5

[0057] The samples prepared in Examples 1 to 4 and Comparative Example 1 were subjected to tensile property tests:

[0058] The tensile properties were measured using a servo hydraulic material testing system at a speed of 5 mm / min. Six specimens were tested for each sample and the average value was calculated.

[0059] The ultimate tensile strength of the self-healing epoxy resin composite varies with the content of microcapsules. Figure 2 As shown. The tensile strength value is the average of 6 measurements. The tensile strength of pure epoxy resin is 60.2MPa. The results show that as the microcapsule content increases from 0wt% to 12wt%, the tensile strength of the epoxy resin composite first increases to 62.45MPa and then decreases to 49.58MPa. Compared with pure epoxy resin, the tensile strength of the self-healing epoxy resin composite initially increases, which may be due to the addition of microcapsules with thermoplastic polymethyl methacrylate as the shell gradually increasing the overall tensile properties of the material. The decrease in the tensile strength of the self-healing epoxy resin composite can be explained from two aspects. On the one hand, the microcapsules filled with more healing agents can be regarded as defects on the cross-section of the composite material. On the other hand, the interface between the microcapsule shell and the epoxy resin composite is an area with weaker material properties, which will also reduce the overall tensile properties of the material under tensile loading.

[0060] Example 6

[0061] The samples prepared in Examples 1 to 4 and Comparative Example 1 were subjected to fracture toughness test:

[0062] A blade was inserted into the sample and tapped gently to create a precrack. Under displacement control, a 6 μms -1The fracture test was carried out at room temperature at a displacement rate of . When the specimen was only about half broken, the cracked specimen was unloaded and the crack was allowed to re-contact and self-heal at room temperature for 48 hours. The self-healed specimen was tested again according to the above steps.

[0063] The effect of microcapsule content in the range of 3-12wt% on the fracture toughness of self-healing epoxy resin composites was studied through fracture tests. Figure 3 As shown in the figure, when the microcapsule content increases from 0wt% to 12wt%, the fracture toughness gradually increases from 0MPam1 / 2 to 1.3MPam1 / 2 and then decreases to 1.0MPam1 / 2. The relationship between the fracture toughness of the self-healing specimen and the microcapsule content also shows a trend of first increasing and then decreasing. The results show that the appropriate content of the repair agent can not only heal the cracks, but also provide higher fracture toughness for the damaged parts. The increase in toughness of epoxy resin composites due to the addition of microcapsules filled with an appropriate amount of liquid repair agent can be explained by the crack pinning mechanism. The crack pinning mechanism usually occurs in polymers with well-bonded high-hardness particles, such as silica. This mechanism also occurs in self-healing composites containing microcapsules, which indicates that after the microcapsules are ruptured, the crack pins around them will increase the resistance to crack growth, thereby increasing the fracture toughness of the material. Figure 4 Scanning electron microscope characterization of crack propagation around microcapsules of two-component microcapsule self-healing epoxy resin composite before self-healing.

[0064] Example 7

[0065] The samples prepared in Examples 1 to 4 and Comparative Example 1 were tested for self-repair efficiency:

[0066] The self-healing efficiency of the samples was evaluated based on the dumbbell-shaped sample fracture test.

[0067] The present invention uses epoxy resin and thiol microcapsules in a mass ratio of 1:1 to prepare a self-repairing composite material. The self-repairing efficiency of dumbbell-shaped epoxy resin composite materials is measured using the same parameters to compare the self-repairing efficiency of self-repairing epoxy resin composite samples with different microcapsule contents. Figure 5 This is the influence curve of different repair agent microcapsule contents on the self-healing efficiency of epoxy resin composites.

[0068] Figure 5It is shown that when the mass ratio of epoxy resin and thiol filled microcapsules is 1:1, the self-healing efficiency of epoxy resin composites increases significantly as the microcapsule content increases from 3wt% to 12wt%. When the content of the repair agent microcapsules is 12wt%, the self-healing efficiency of the epoxy resin composite is 95.67%. Obviously, the self-healing effect is related to the amount of repair agent released by the broken microcapsules on the crack surface. This rapid increase in repair efficiency is due to the release of enough repair agent by the microcapsules to fill the microcrack space. The two-component microcapsules embedded in the composite system provide sufficient epoxy resin and thiol for the self-healing of epoxy resin composites and promote the bonding of the new epoxy resin to the original matrix interface.

[0069] Comparative Example 2

[0070] (1) 6 g of epoxy prepolymer (EC157) and 6 g of polymethyl methacrylate were dissolved in 220 ml of dichloromethane to prepare an oil phase solution;

[0071] (2) using deionized water as the aqueous phase, adding the oil phase solution containing the epoxy prepolymer (EC157) into the aqueous phase, wherein the addition ratio of the aqueous phase to the oil phase is 19:11, and stirring at a high speed of 1200 rpm at room temperature for 45 minutes to obtain an oil-water emulsion containing the epoxy prepolymer (EC157);

[0072] (3) mixing hexadecyltrimethylammonium bromide with the oil-water emulsion in a ratio of 1.1:30, and after the dichloromethane is completely evaporated, filtering and washing with methanol and deionized water respectively, and drying at room temperature to obtain microcapsules;

[0073] (4) 90 copies of Epikote TM 828 and 63 Epikure TM F205 is mixed as a matrix; the microcapsules prepared in step (3) are mixed with the matrix, wherein the content of the microcapsules is 9wt%, the mixed sample is degassed and poured into a dumbbell-shaped silicone rubber mold, cured at room temperature for 48 hours, and then cured at 75°C for 2 hours to obtain a microcapsule self-healing epoxy resin composite material.

[0074] Comparative Example 3

[0075] (1) 6 g of pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 6 g of polymethyl methacrylate were dissolved in 220 ml of dichloromethane, and then 0.5 g of N,N-dimethylbenzylamine was added as a catalyst to obtain an oil phase solution.

[0076] (2) using deionized water as the aqueous phase, adding the oil phase solution containing pentaerythritol tetrakis(3-mercaptopropionate) to the aqueous phase, wherein the addition ratio of the aqueous phase to the oil phase is 19:11, and stirring at a high speed of 1200 rpm at room temperature for 45 minutes to obtain an oil-water emulsion containing pentaerythritol tetrakis(3-mercaptopropionate);

[0077] (3) mixing sodium dodecyl sulfate and the oil-water emulsion in a ratio of 1.2:30, and after the dichloromethane is completely evaporated, filtering and washing with methanol and deionized water respectively, and drying at room temperature to obtain microcapsules 2;

[0078] (4) 90 copies of Epikote TM 828 and 63 Epikure TM F205 is mixed as a matrix; the microcapsules prepared in step (3) are mixed with the matrix, wherein the content of the microcapsules is 9wt%, the mixed sample is degassed and poured into a dumbbell-shaped silicone rubber mold, cured at room temperature for 48 hours, and then cured at 75°C for 2 hours to obtain a microcapsule self-healing epoxy resin composite material.

[0079] Comparative Example 4

[0080] The difference between this embodiment and embodiment 1 is that dichloromethane is replaced with chlorobenzene, and the rest of the preparation process is the same as that of embodiment 1 to obtain microcapsules.

[0081] Comparative Example 5

[0082] The difference between this comparative example and Example 1 is that sodium lauryl sulfate is replaced with gum arabic, and the rest of the preparation process is the same as that of Example 1 to prepare microcapsules.

[0083] Table 2

[0084]

[0085]

[0086] from Figure 1 It can be seen that the microcapsules are evenly distributed, most of them are spherical or nearly spherical, and the surface morphology is relatively smooth. The outer surface of the microcapsules is smooth and has almost no protrusions, which is because the outer layer of the microcapsules is a polymethyl methacrylate shell.

[0087] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the present invention.

Claims

1. A two-component microcapsule self-repairing epoxy resin composite material, characterized in that: The two-component microcapsule self-repairing epoxy resin composite material is a core-shell structure, wherein epoxy resin and pentaerythritol tetrakis(3-mercaptopropionate) are used as a core and polymethyl methacrylate is coated on an outer layer.

2. The two-component microcapsule self-repairing epoxy resin composite material according to claim 1, characterized in that: The epoxy resin is an epoxy prepolymer.

3. The method for preparing the two-component microcapsule self-repairing epoxy resin composite material according to claim 1 or 2, characterized in that: The method comprises dissolving epoxy prepolymer and polymethyl methacrylate in dichloromethane to obtain an oil phase solution 1; Pentaerythritol tetrakis(3-mercaptopropionic acid) ester and polymethyl methacrylate are dissolved in dichloromethane, and N,N-dimethylbenzylamine is added to obtain an oil phase solution 2; With deionized water as the aqueous phase, the oil phase solution 1 is added into the aqueous phase and stirred at room temperature to obtain an oil-water emulsion 1; With deionized water as the aqueous phase, the oil phase solution 2 is added into the aqueous phase and stirred at room temperature to obtain an oil-water emulsion 2; Hexadecyltrimethylammonium bromide and the oil-water emulsion 1 are mixed and stirred, and after the dichloromethane is completely evaporated, the mixture is filtered and washed with methanol and deionized water respectively, and dried at room temperature to obtain microcapsules 1; Sodium dodecyl sulfate and oil-water emulsion 2 are mixed and stirred, and after dichloromethane is completely evaporated, the mixture is filtered and washed with methanol and deionized water respectively, and dried at room temperature to obtain microcapsules 2; Epikote TM 828 and Epikure TM F205 is mixed as a matrix; microcapsules 1 and 2 are mixed with the matrix, the mixed sample is degassed and poured into a dumbbell-shaped silicone rubber mold, and then cured at room temperature and then cured at a high temperature to obtain the two-component microcapsule self-healing epoxy resin composite material.

4. The preparation method according to claim 3, characterized in that: The mass ratio of the epoxy prepolymer to polymethyl methacrylate is 1.0-1.2:0.85-1; the mass ratio of pentaerythritol tetrakis(3-mercaptopropionic acid) to polymethyl methacrylate is 1.0-1.3:0.8-1.

1.

5. The preparation method according to claim 3, characterized in that: The mass ratio of the water phase to the oil phase is 19-22:11-15.

6. The preparation method according to claim 3, characterized in that: The mass ratio of the hexadecyltrimethylammonium bromide to the oil-water emulsion 1 is: 1.1~1.9:30~35; the mass ratio of sodium dodecyl sulfate to oil-water emulsion 2 is 1.2~1.6:30~37.

7. The preparation method according to claim 3, characterized in that: Epikote in the matrix TM 828 and Epikure TM The mass ratio of F205 is 90~100:63~75.

8. The preparation method according to claim 3, characterized in that: The total content of the microcapsules 1 and 2 is 3-12 wt %.

9. The preparation method according to claim 8, characterized in that: The mass ratio of the microcapsules 1 and 2 is 1:

1.

10. Use of the composite material according to claim 1 or 2 in treating micro crack damage in epoxy resin composite materials.

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