JPH04361126A - Method for measuring internal distortion of fiber-reinforced compound material lamination plate - Google Patents

Abstract

PURPOSE:To enable an internal distortion of a fiber-reinforced compound material lamination plate to be measured by burying a sensor portion of an optical fiber interferene meter into an interlayer of the fiber-reinforced compound material lamination plate and then measuring change in intensity of interference light. CONSTITUTION:Light from a laser light-emitting equipment 4 enters a single-mode optical fiber 6 through an objective lens 5. Then, light which is branched into 1:1 by a photo coupler 7 is reflected by an end face at a tip portion of two optical fibers 2a and 2b with a different length at a sensor portion 2. Then, it returns to the coupler 7 again and generates interference, the intensity is detected by a light-reception element 8, and then it is recorded by an XY recorder. By burying the sensor portion 2 into the fiber-reinforced compound material lamination plate 3, internal distortion which is generated at the interlayer of the lamination plate 3 is measured at high speed and with high sensitivity according to change in two gauge points at the sensor portion 2. Measurement of distortion is based on light which is transmitted through the fibers 2a and 2b and is not affected easily by electrical noise or interlayer residual stress etc.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring internal strain in fiber-reinforced composite laminates.

0002

[Prior Art] Fiber-reinforced composite laminates containing FRP, GFRP, CFRP, etc. are lightweight and durable, and can withstand large temperature differences, so they can be used in aircraft, ships, automobiles, civil engineering, etc. It is widely used as a structural material for buildings and as a corrosion-resistant coating material, but as its applications expand, detection of defects unique to composite materials, such as delamination and cracks, has become an important issue. In other words, fiber-reinforced composite laminates are made by laminating molding materials in which fibers are impregnated with resin in alternating directions so that the fibers are oriented in different directions and then melted and hardened. It has been confirmed that peeling and cracking occur. therefore,
It is important to understand the impact characteristics of fiber-reinforced composite laminates, such as the degree of impact that will cause delamination or cracks to occur, in terms of product quality. Establishing a detection method for delamination that cannot be performed is an extremely important issue.

As an example of a method for detecting delamination in a fiber-reinforced composite laminate, a method has been proposed in which the internal strain of the fiber-reinforced composite laminate is electrically measured using a resistance wire strain gauge. .

0004

[Problems to be Solved by the Invention] However, when measuring the internal strain of a fiber-reinforced composite laminate using the above-mentioned conventional electrical strain gauge, the fiber-reinforced composite laminate suffers from residual strain between layers with different lamination angles. Because of the presence of stress, there is a risk that the strain gauge will warp if it is embedded between layers.Also, when trying to measure the internal strain of CFRP as a fiber reinforced composite laminate, CFRP The drawback is that the measurement becomes unstable because it conducts electricity.

[0005] Therefore, the present invention provides a method for stably measuring the internal strain of a fiber-reinforced composite laminate, whether it is FRP, GFRP or CFRP. This paper attempts to provide a strain measurement method.

[0006]

[Means for Solving the Problems] In order to solve the above problems, the present invention uses a Michelson type optical fiber interferometer, and a sensor section of the optical fiber interferometer is placed between the layers of a fiber-reinforced composite laminate. A method for measuring internal strain in a fiber-reinforced composite laminate, characterized in that the internal strain of the fiber-reinforced composite laminate is measured by measuring the intensity change of interference light caused by the embedded sensor section.

[0007]

[Operation] When the sensor part of the optical fiber interferometer is embedded between the layers of the fiber-reinforced composite laminate, the change in the intensity of the interference light due to the sensor part is measured, so the internal strain of the fiber-reinforced composite laminate is measured. be done.

[0008]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

The method for measuring internal strain of a fiber-reinforced composite laminate according to the present invention uses a Michelson type optical fiber interferometer 1, as shown in FIGS. The sensor part 2, which has fibers 2a and 2b with different tip lengths (gauge point positions), is inserted between the layers of the fiber-reinforced composite material laminate 3 with the core part exposed.
It is embedded during the process of forming the laminate 3, and after the forming, the internal strain of the laminate 3 is measured by measuring the intensity change of the interference light by the sensor section 2.

In the optical fiber interferometer 1, light from a He-Ne laser emitter 4 is incident on a single mode optical fiber 6 via an objective lens 5, and an optical coupler 7 divides the light into a 1:1 ratio.
The branched light is reflected by the end faces of the tips of the two optical fibers 2a and 2b of different lengths constituting the sensor section 2, and returns to the optical coupler 7 again, causing interference and receiving light. The intensity is detected by an element (photodiode) 8 and recorded by an XY recorder 9.

In the present invention, as described above, an optical fiber interferometer 1 is used, which is composed of two optical fibers 2a and 2b having different tip lengths, an optical coupler 7 connecting them, etc. , the sensor section 2 is connected to a fiber-reinforced composite material laminate 3
By embedding it inside the fiber-reinforced composite material, the internal strain occurring between the layers of the laminate 3 can be measured at high speed and with high sensitivity by changes in the two gauge points of the sensor section 2. The internal strain of the laminate 3 can be measured. Thereby, delamination of the fiber-reinforced composite material laminate 3 can be detected. At this time, since the strain measurement is based on light passing through the optical fibers 2a and 2b, it is not easily affected by electrical noise, residual stress between layers, and the like. Therefore, it can also be applied to CFRP. In the above, the output of the sensor unit 2 is obtained as a periodic change in interference light intensity, and its strain sensitivity is calculated from the photoelastic constant of the quartz fiber as follows: Δφ/Lez = 2.32×107 (rad/m
) can be calculated as the theoretical value. Here, Δφ is the intensity change of the two optical fibers 2a and 2b observed as the intensity change of the interference light.
The phase difference (rad), ez is the axial strain of the optical fiber, and L is the difference in length (m) between the tips of the two optical fibers 2a and 2b as shown in Figure 2, which is the gauge length of the resistance wire strain gauge. corresponds to

Next, the results of an experiment (tensile test) conducted using the optical fiber interferometer 1 will be described. Each optical fiber has an outer diameter of 250 μm and a core diameter of 3.6 μm.
m was used. Moreover, the optical fiber 2a of the sensor section 2
, 2b was coated with paint instead of a mirror surface. The test piece of the fiber-reinforced composite laminate 3 was a GFRP laminate ([±45]s 250L x 30W x 0.8t
) was used. The sensor part 2 was embedded in the center of the test piece between the layers (between the second layer and the third layer) with a gauge length L of 12 mm.

The results of the tensile test were obtained as shown in FIG. 3, in which the horizontal axis represents the resistance wire type strain gauge output (sensor section output) and the vertical axis represents the interference light intensity. The interference light periodically repeats its strength and weakness as the strain of the test piece increases, and from this period, Figure 4
The relationship between the phase of the interference light intensity and the strain gauge output as shown in is obtained. According to FIG. 4, the phase of the change in the intensity of the interference light is proportional to the output of the strain gauge, and it can be seen that the amount of strain can be measured with high sensitivity from the intensity of the interference light. From the slope of the straight line in Figure 4, the strain sensitivity is Δφ/Lez = 2.14×107 (rad/m
) From similar tests for different gauge lengths (L = 25 mm), Δφ/Lez = 2.09 × 107 (rad / m
) was requested.

[0014] From these results, it was confirmed that the internal strain of a fiber-reinforced composite laminate can be measured with high sensitivity.

It should be noted that the present invention is not limited to the above-mentioned embodiments, and it goes without saying that various changes can be made without departing from the gist of the present invention.

[0016]

As described above, according to the method for measuring internal strain of a fiber-reinforced composite laminate of the present invention, the sensor portion of the Michelson type optical fiber interferometer is embedded between the layers of the fiber-reinforced composite laminate, and By measuring the strain in the sensor section, the internal strain of the fiber-reinforced composite laminate is measured, so whether the fiber-reinforced composite laminate is FRP, GFRP, or CFRP, electrical noise It is possible to accurately detect delamination in a stable state without being affected by residual stress between the layers, and therefore it can be used as a powerful method that contributes to elucidating the impact characteristics of fiber-reinforced composite laminates. It also exhibits excellent effects such as being able to be applied to large structures and strain measurements over long periods of time.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical fiber interferometer used to carry out the method for measuring internal strain of a fiber-reinforced composite laminate according to the present invention.

FIG. 2 is an enlarged view of a sensor section.

FIG. 3 is a diagram showing the relationship between interference light intensity and strain gauge output showing the results of a tensile test.

FIG. 4 is a diagram showing the relationship between strain gauge output and the phase of interference light.

[Explanation of symbols]

1 Michelson type optical fiber interferometer 2 Sensor part 3 Fiber-reinforced composite laminate

Claims (1)

[Claims] [Claim 1] By using a Michelson type optical fiber interferometer, embedding a sensor section of the optical fiber interferometer between the layers of a fiber-reinforced composite laminate, and measuring changes in the intensity of interference light due to the sensor section. A method for measuring internal strain in a fiber-reinforced composite laminate, the method comprising measuring the internal strain of the fiber-reinforced composite laminate.