JP2013217791A - Method of evaluating interfacial strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of evaluating interfacial strength of a matrix and a filler in a composite material separately from resin strength and with ease.SOLUTION: A method for evaluating strength of a composite material obtained by dispersing a filler in a resin, the method for evaluating interfacial strength by measuring acoustic emission generated by increasing a load applied onto a test piece of the composite material, includes: continuously measuring energy of the acoustic emission while increasing the load; and defining as the interfacial strength the load of when the integral value of the energy of the acoustic emission increases abruptly with respect to the increase in the load.

Description

  The present invention relates to a method for evaluating the interface strength between a matrix and a filler in a composite material.

  The composite material contains a filler for improving required properties. One of the required properties is mechanical strength. In order for the filler to contribute to the improvement of mechanical strength, not only the properties and shape of the filler itself, but also the adhesiveness of the interface between the filler and the matrix containing it is important. It is an indispensable element.

  However, measurement of interface strength is often difficult. The experimental measurement method includes a fragmentation method in which the critical fiber length is obtained by embedding a single fiber in the resin and applying a load to the entire resin in the fiber direction, or a micro drop in which a single fiber is attached with a resin drop and the fiber is pulled out of the drop. Although there is a let method, these are often measurements that do not reflect the actual situation, although they take a lot of time and effort. Therefore, the interface strength is often evaluated based on whether the strength test such as a bending test is good or bad, but the accuracy is low due to the strength of the matrix. Therefore, there has been a demand for a method for easily evaluating the interface strength between the matrix and the filler, excluding the influence of the strength of the matrix.

  As a method of detecting fracture simultaneously with the strength measurement, there is a method of measuring acoustic emission (hereinafter sometimes referred to as AE). AE is a sound generated when a material breaks down, and since the amplitude, frequency, and energy of this sound are different for each type of breakage, the breakage can be analyzed in detail by analyzing this. This technique is applied in various fields (see, for example, Patent Documents 1 to 3). As an evaluation method using AE energy, a method has been proposed in which a load at which the AE energy is maximized is estimated to be a crack generation load of high-strength fiber reinforced concrete (see Patent Document 4). However, these are methods for evaluating the starting point of the destruction of the material subjected to the measurement, and it is not distinguished whether it is the destruction of the matrix or the interface between the matrix and the filler. There was a need for a method to evaluate only.

  The inventor has disclosed the technical contents related to the present invention (see Non-Patent Document 1). This is considered to be applicable to Article 30 Paragraph 2 of the Patent Act.

JP-A-6-331534 JP-A-6-319442 JP 2000-258404 A JP 2010-223761

Yoshinori Watanabe, Masaro Nishimura "Analysis of Fracture Behavior of Phenolic Resin-Glass Fiber Composites Using AE Method", 61st Network Polymer Lecture Meeting, General 10-19-20, Synthetic Resin Industry Association, Heisei Issued on October 12, 2011 (Publications to be applied under Article 30 (2) of the Patent Law)

The present invention provides a method for easily evaluating the interfacial strength between a matrix and a filler in a composite material without the influence of the strength of the matrix.

Such an object is achieved by the present invention described in the following [1] to [2].
[1] A method for evaluating the strength of a composite material in which a filler is dispersed in a resin, wherein the interface strength is evaluated by measuring the acoustic emission generated by increasing the load on the test piece of the composite material. In this evaluation method, the energy of acoustic emission is continuously measured as the load increases, and the load when the integrated value of the energy of acoustic emission increases rapidly as the load increases is defined as the interface strength. An evaluation method characterized by that.
[2] The evaluation method according to [1], wherein the resin is a thermosetting resin.

  The evaluation method of the present invention has an effect that the interface strength between the matrix and the filler in the composite material can be easily evaluated without the influence of the strength of the matrix.

It is the graph which showed the relationship of the load and integration energy which were obtained in the Example and the comparative example.

Below, the evaluation method of this invention is demonstrated in detail.
<Test Method> The test method used in the present invention is a test method in which tensile stress acts on at least a part of the test piece. Such a test method is not particularly limited, but typical test methods are a tensile test, a bending test, and the like.

  <Test piece> Although it does not specifically limit as a test piece used for this invention, You may form a notch (notch) in the location where the said tensile stress acts. Thereby, since the generation | occurrence | production location of AE is limited, the reproducibility of evaluation can be improved markedly.

  <AE sensor> As an AE sensor used for this invention, it is preferable to use the AE sensor which can detect AE contained in the frequency band of 20-1000 kHz. The number of AE sensors is not particularly limited, but it is preferable that the number of AE sensors is two in consideration of the labor of attaching the AE sensor, measurement accuracy, and the like. The AE sensor can be attached with grease or the like, and is preferably attached to both sides across a portion where the test piece is expected to break. Thereby, the generation | occurrence | production position of AE can be pinpointed.

  <Measurement of AE> The measurement method of AE of the present invention will be described. First, generated AE is measured by an AE sensor. At this time, the output of the AE sensor is digitized into an AE waveform by, for example, amplifying the AE signal with a preamplifier, separating it from background noise with a threshold value and an analog filter, and digitally converting it with an A / D converter. Etc. can be performed. In the present invention, the preamplifier condition is preferably 20 to 60 dB, and more preferably 30 to 50 dB. The range of the analog filter is preferably 10 to 2000 kHz, more preferably 95 to 1000 kHz.

<Calculation of AE energy> The calculation method of the AE energy of the present invention will be described. The AE energy of each event in the AE waveform is calculated. Here, the event is a portion of the AE waveform from when the amplitude exceeds the threshold to less than the threshold, and the AE energy of the event is the integration of the AE waveform during the event. Value. In the present invention, in calculating the AE energy of each event, the threshold value of the amplitude of the AE waveform is preferably 20 to 50 dB. If the threshold value is less than 20 dB, even though many AE waves are continuously generated, there is a possibility that it is apparently determined as one event, and the measurement accuracy is lowered. On the other hand, when the threshold value exceeds 50 dB, a small AE signal below the threshold level is not taken into account, and the error of the AE energy increases, resulting in a decrease in measurement accuracy.

  <Evaluation of Interfacial Strength> The interfacial strength of the present invention is obtained by continuously measuring the AE energy calculated as described above with respect to an increase in load, and the integrated value of the AE energy rapidly increases with respect to the increase in load at that time. , Which can be easily found from a graph plotting the relationship between the applied load and the integrated value of the AE energy. The reason for this evaluation is based on the following crack observation results. First, when the integrated value of the AE energy is smaller than the load at which the integral value starts to increase rapidly, a minute crack is generated in the matrix of the composite material, but the crack stops at the interface with the filler. On the other hand, when the load is larger than the load at which the integrated value of the AE energy starts to increase rapidly, a large crack is generated at the interface between the matrix and the filler of the composite material, and the crack propagates along the interface between the matrix and the filler. doing. From this, it can be determined that the load at which the integrated value of the AE energy starts to increase rapidly is the interface strength, and is separated from the resin strength.

  <Matrix Resin of Composite Material> The matrix resin of the composite material of the present invention is preferably a thermosetting resin. This is because the resin with a relatively small breaking strain can clearly separate the resin from the interface.

  EXAMPLES Hereinafter, although this invention is demonstrated with reference to an Example and a comparative example, this invention is not limited to description at all in these Examples. Each raw material and apparatus used in Examples and Comparative Examples are as follows.

(1) Glass fiber 1: surface-treated chopped strand, average fiber diameter 11 μm
(2) Glass fiber 2: Untreated surface chopped strand, average fiber diameter 11 μm
(3) Novolac type phenolic resin: PR-51305 manufactured by Sumitomo Bakelite Co., Ltd.
(4) Hardener: Hexamethylenetetramine (5) Thickener: Magnesium oxide (6) Release agent: Calcium stearate (7) Colorant: Carbon black (8) AE sensor: AE144A manufactured by Fuji Ceramics

1. Preparation of composite material test piece [Example 1]
47% by weight of a mixture of novolak type phenolic resin and curing agent as matrix resin (40% by weight of novolac type phenolic resin, 7% by weight of curing agent), 50% by weight of glass fiber 1, and thickener for the entire composite material 1% by weight of a release agent and a colorant were blended and premixed. This mixture was melt-kneaded with 105 ° C. heating rolls having different rotational speeds, and the cooled sheet was pulverized to obtain a granular composite material.

[Example 2]
In Example 1, a composite material was obtained in the same manner as in Example 1 except that glass fiber 2 was blended instead of glass fiber 1.

[Comparative example]
In Example 1, a resin was obtained in the same manner as in Example 1 except that the glass fiber 1 was not blended.

2. Strength Property Evaluation The test piece molding method and evaluation method used for property evaluation are as follows.

  The test piece was produced by transfer molding using the molding materials obtained in the above Examples and Comparative Examples. The molding conditions were a mold temperature of 175 ° C. and a curing time of 3 minutes.

The measuring method is as follows.
Bending strength: Measured according to JIS K 6911.
3. Evaluation of AE

The evaluation method of AE is as follows.
Each test piece (length 80 mm, width 10 mm, thickness 4 mm) was formed with a notch having a depth of 2.0 mm in the width direction of the test piece at the center in the length direction. As the shape of the notch, the width was 0.2 mm and R was 0.1 mm. The evaluation was performed by a three-point bending test with a span of 64 mm, and the test piece was installed with the notched surface facing down and the notched portion directly below the indenter. The AE sensor was attached using grease on both sides at a position 25 mm from the indenter on the upper surface of the test piece. The descending speed of the indenter was measured at 0.1 mm / min. The output of the AE sensor is obtained by amplifying the AE signal with a preamplifier condition of 40 dB, and then digitizing it with an acoustic signal processing device, setting the threshold value to 30 dB, the analog filter range to 95 to 1000 kHz, AE energy was calculated.

  Table 1 shows the composition and characteristics, and FIG.

In Example 1 and Example 2, glass fiber is used as the filler, but only the surface treatment of the glass fiber is different, so that only the interface strength is different. In the bending test, in Example 1, the elastic modulus was substantially constant from the start of the test to the end of the test, but in Example 2, a decrease in the elastic modulus was clearly observed during the test, and interface peeling occurred. Considering that the bending test of Example 2 continued to be strained even when interface peeling occurred, it is clear that the result of the bending test does not reflect the influence of only the interface strength. On the other hand, in the evaluation method of the interfacial strength of the present invention, in the case of only the resin of the comparative example, it breaks at a stretch, so a rapid increase in AE energy is not measured, but it breaks immediately after the last AE event. The load at this time is low compared to the example. From this, as a fracture mechanism, cracks are first generated inside the resin, and if the filler is not contained, the crack progresses and breaks as it is, but if the filler is contained, the crack is stopped at the interface and the load is further increased. When the interfacial strength is reached, the cracks are connected and progress. This mechanism is considered to be the same even when the interface strength is different. Therefore, the load at which the integrated value of the energy of AE increased rapidly was the interface strength, and it could be evaluated that Example 1 had a higher interface strength than Example 2.

  From the above results, it was found that the interface strength between the matrix and the filler in the composite material can be easily evaluated by removing the influence of the strength of the matrix according to the present invention.

With the interface strength evaluation method of the present invention, the substantial effect of interface adhesion on the mechanical strength of the composite material can be evaluated. Thereby, it is possible to provide a composite material having excellent mechanical strength.

Claims (2)

  A method for evaluating the strength of a composite material in which a filler is dispersed in a resin, wherein the interface strength is evaluated by measuring acoustic emission generated by increasing a load on a test piece of the composite material. The acoustic emission energy is continuously measured as the load increases, and the load when the integrated value of the acoustic emission energy increases rapidly as the load increases is defined as the interface strength. Evaluation method. The evaluation method according to claim 1, wherein the resin is a thermosetting resin.