JPH0862112A - Method for testing fatigue crack propagation of composite material - Google Patents

Abstract

PURPOSE: To obtain a method for testing fatigue crack propagation of a composite material in which the length of fatigue crack can be evaluated accurately for a composite material in which the tip end position of crack can not be determined definitely. CONSTITUTION: In the fatigue crack propagation test for a composite material in which the tip end position of crack can not be determined definitely, the relationship between the length (a) of a notch 2 and the strain ε is determined previously for a simulation test piece 1 made of the same material as a fatigue test piece. The crack length ε is estimated from the strain ε obtained from the fatigue test piece thus evaluating the fatigue crack length correctly.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for fatigue crack propagation testing of composite materials.

[0002]

2. Description of the Related Art Since a composite material is woven with a reinforcing fiber such as glass fiber (GFRP) or carbon fiber (CFRP) in a resin, fatigue cracks are different from isotropic materials such as metal. 7 As shown in the explanatory diagram, it progresses in a zigzag manner without going straight, or damage spreads over a wide range of the reinforcing fiber and resin, and it is difficult to clearly determine the tip position of the crack. However, in fatigue crack propagation tests of such composites, the crack propagation rate per cycle: da / dN and the stress intensity factor range:
In order to obtain ΔK = ΔS√πa · f (ΔS: working stress range, a: crack length, f: correction coefficient), it is necessary to accurately measure the crack length: a. Since it is exclusively by visual observation, it is difficult to determine the crack tip position clearly as described above, and due to the peculiarity of the fatigue crack tip, automatic measurement using a crack gauge used for metal materials is impossible. Yes, the crack length measurement accuracy is low. Therefore, highly accurate fatigue crack propagation characteristic data (crack propagation velocity: d
a / dN mm / cycle and stress intensity factor range: ΔK Kgf / mm ^3/2
The Paris rule da / dN = C · ΔK ^mm ) that expresses the above relationship cannot be obtained.

[0003]

SUMMARY OF THE INVENTION The present invention has been proposed in view of the above circumstances, and can accurately evaluate the fatigue crack length of a composite material whose crack tip position cannot be clearly determined. An object is to provide a fatigue crack propagation test method for composite materials.

[0004]

Therefore, according to the present invention, in a fatigue crack propagation test of a composite material in which a crack tip position cannot be clearly determined, a notch length and a notch length are previously prepared in a simulated test piece of the same material as the fatigue test piece. Accurately evaluate the fatigue crack length by estimating the crack length from the strain obtained from the fatigue test piece, and the fatigue resistance of the composite material where the crack tip position cannot be clearly determined. In the crack propagation test, the relationship between the notch length and the crack opening amount was obtained in advance with a simulated test piece of the same material as the fatigue test piece, and the crack length was estimated from the crack opening amount obtained from the fatigue test piece. Accurate evaluation of fatigue crack length is characterized by each.

[0005]

In the first aspect of the fatigue crack propagation test method for a composite material of the present invention, a strain gauge is provided on the back surface of the notch of the simulated fatigue test piece (the position where the correlation between the notch length and the strain change is most obtained). After cutting, the notch with a sharp tip is machined step by step from the initial state, and the strain gauge output, that is, the strain is measured when the actual fatigue test load is applied at each notch length in each notch area, and the notch Create a master curve for length and strain. Then, a strain gauge is attached to the actual fatigue test piece at the same position as the simulated test piece, the strain during the fatigue test is intermittently measured, and the crack length is obtained from the master curve.

In the second aspect of the invention, a displacement gauge for measuring the crack opening amount is attached to the notch of the simulated fatigue test piece,
A notch with a sharp tip is processed stepwise from the initial state, and an actual fatigue test load is applied to measure the opening amount to create a master curve for the notch length and the opening amount.
The actual fatigue test piece is also equipped with a displacement meter that measures the opening amount at the same position as the simulated test piece, the opening amount is measured during the fatigue test, and the crack length is obtained from the master curve.

[0007]

EXAMPLE An example of a fatigue crack propagation test method for a composite material of the present invention will be described with reference to the drawings. FIG. 1 is a plan view of a simulated test piece of the first invention, and FIG. 2 is a notch length or a crack length. 3 is a plan view of the fatigue test piece. FIG. 4 is a plan view of a simulated test piece of the second invention, FIG. 5 is a diagram showing a relation between a notch length or a crack length and an opening amount, and FIG. 6 is a plan view of a fatigue test piece. First, in FIGS. 1 and 3 of the first invention, a simulated test piece 1 and a fatigue test piece 11 are glass fiber reinforced GFRP materials, which are usually called compact tension type test pieces. As shown in FIG. 1, leave affixed strain gauge 3 on the back of the 2-out initial notch practice test piece 1, a _{1, 2,} notch ... ₇ and the tip sharp from the length a ₀ notches Was processed and an actual fatigue test load P was applied at each notch length a, and the relationship between the notch length a and the strain ε as shown in FIG. 2 was obtained. As shown in FIG. 3, the strain gauge 13 is attached to the actual fatigue test piece 11 at the same position as that of the simulated test piece 1, and the strain ε during the fatigue test in which the fatigue crack 12 occurs is intermittently applied. The crack length was measured and the crack length was determined from the master curve as shown by the arrow in FIG. In FIG. 2, the data obtained by measuring the fatigue cracks by the conventional visual method is plotted to confirm the effectiveness of the present invention. All four fatigue test data are scattered around the master curve, and this method is effective. Was confirmed. A closer look at the plot points reveals that it is biased slightly below the master curve, and that there is a tendency for cracks to be read a little longer by the conventional visual inspection method.

In FIGS. 4 and 6 of the second invention, the simulated test piece 21 and the fatigue test piece 31 are GFRP materials and are usually called compact tension type test pieces.
As shown in FIG. 4, a clip gauge 23 is attached to the cutout of the cutout 22 of the simulated test piece 21, and the cutout length a ₀
To a ₁ , ₂ ... ₇ and a notch having a sharp tip is machined and an actual fatigue test load P is applied at each notch length a, and the notch length a and opening as shown in FIG. The relationship of the quantity δ was determined. Then, as shown in FIG. 6, the clip gauge 33 is attached to the actual fatigue test piece 31 at the same position as the simulated test piece 21, and the opening amount δ during the fatigue test in which the fatigue crack 32 propagates is measured, As shown by the arrow in FIG. 5, the crack length was determined from the master curve. In FIG. 5, the data obtained by measuring the fatigue cracks by the conventional visual method is plotted to confirm the effectiveness of the present invention. All four fatigue test data are scattered around the master curve, and this method is effective. Was confirmed. A closer look at the plot points reveals that they are biased slightly below the master curve, and that there is a tendency for cracks to be measured over a long period by the conventional visual inspection method.

Thus, according to the first aspect of the invention, the fatigue of the composite material fatigue test piece 11 in which the fatigue crack 12 propagates in a zigzag manner and the crack tip portion is damaged in a wide range and the crack tip cannot be clearly determined. It has become possible to accurately evaluate the crack length by strain, and thus the accuracy of fatigue crack life prediction can be improved. In addition, it is necessary to create a master curve using the simulated test piece 1, but it is not necessary to measure cracks for the same material, only strain changes need to be measured, and unmanned continuous operation is possible, which is a significant labor saving. Became possible.

According to the second aspect of the invention, the fatigue crack length of the composite fatigue test piece 31 in which the fatigue crack 32 propagates in a zigzag manner and is damaged in a wide range of the crack tip and the crack tip cannot be clearly determined. It has become possible to accurately evaluate cracks by the amount of crack opening. Therefore, the accuracy of fatigue crack life prediction has been improved. In addition, it is necessary to create a master curve using the simulated test piece 21, but it is not necessary to measure cracks with the same material, and it is sufficient to measure only the change in the opening amount, and unmanned continuous operation is possible, which is a significant Labor saving has become possible.

[0011]

In summary, according to the present invention, in a fatigue crack propagation test of a composite material in which the crack tip position cannot be clearly determined, the relationship between the notch length and the strain is previously obtained from a simulated test piece of the same material as the fatigue test piece. In advance, the fatigue crack length is accurately evaluated by estimating the crack length from the strain obtained from the fatigue test piece, and the fatigue crack propagation test of the composite material in which the crack tip position cannot be clearly determined. In advance, the relationship between the notch length and the crack opening amount was obtained in advance with a simulated test piece of the same material as the fatigue test piece, and the fatigue crack length was estimated by estimating the crack length from the crack opening amount obtained from the fatigue test piece. By accurately evaluating the fatigue crack propagation test method of the composite material that can accurately evaluate the fatigue crack length of the composite material whose crack tip position cannot be clearly determined, respectively, the present invention Is the highest in the industry It is useful to investors.

[Brief description of drawings]

FIG. 1 is a plan view of a simulated test piece in a first invention of a fatigue crack propagation testing method for a composite material of the present invention.

FIG. 2 is a diagram showing a relationship between notch length or crack length and strain.

FIG. 3 is a plan view of a fatigue test piece.

FIG. 4 is a plan view of a simulated test piece according to a second invention.

FIG. 5 is a diagram showing a relationship between a notch length or a crack length and an opening amount.

FIG. 6 is a plan view of a fatigue test piece.

FIG. 7 is an explanatory diagram of a fatigue crack propagation situation.

[Explanation of symbols]

 1 Simulated test piece 2 Notch 3 Strain gauge 11 Fatigue test piece 12 Fatigue crack 13 Strain gauge 21 Simulated test piece 22 Notch 23 Clip gauge 31 Fatigue test piece 32 Fatigue crack 33 Clip gauge a Notch length ε Strain δ Opening amount

Claims (2)

[Claims] 1. In a fatigue crack propagation test of a composite material in which a crack tip position cannot be clearly determined, a relationship between the notch length and strain is obtained in advance from a simulated test piece of the same material as the fatigue test piece, and the fatigue test is performed. A fatigue crack propagation test method for a composite material, which comprises accurately estimating the fatigue crack length by estimating the crack length from the strain obtained from the piece. 2. In a fatigue crack propagation test of a composite material in which the crack tip position cannot be clearly determined, the relationship between the notch length and the crack opening amount is obtained in advance from a simulated test piece of the same material as the fatigue test piece. , A fatigue crack propagation test method for a composite material, which comprises accurately estimating the fatigue crack length by estimating the crack length from the crack opening amount obtained from a fatigue test piece.