JP2011122885A - Method of evaluating interlaminar strength of composite material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To facilitate evaluating interlaminar strength of a composite material having a curved part in various mixed modes. <P>SOLUTION: The method of evaluating the interlaminar strength of the composite material 10 has a curved part in which a defect of a predetermined size has been introduced between the layers and two plane parts 12a and 12b on both sides of the curved part. The method of evaluating the interlaminar strength of the composite material 10 includes: a mixing ratio calculating step, wherein a fracture mechanics parameter is calculated by FEM analysis on the case where a face 14a of the one plane part 12a, which is on the same face as the outside curved face of the curved part, is fixed, and a load is applied to the other plane part 12b at a predetermined angle, and from the calculated fracture mechanics parameter, the mixing ratio of mode I and mode II and that of mode I and mode III at the predetermined angle are calculated respectively; a strength measuring step, wherein the interlaminar strength is measured by applying a load to the composite material 10 at the predetermined angle; and a strength acquiring step, wherein the interlaminar strength of the composite material 10 in a predetermined mixed mode is acquired. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for evaluating the delamination strength of a composite material.

As a composite material, for example, there is a resin-based composite material manufactured by laminating resin sheets reinforced with fibers such as carbon fibers. Resin-based composite materials are widely used as structural members for aircraft, automobiles, ships, and the like because they are lightweight and have high strength.
In the resin-based composite material, the interlayer strength is as small as about 1/10 of the in-plane strength. Further, in the manufacturing process of the resin matrix composite material, foreign substances may be sandwiched between the sheets when the sheet-like prepregs are stacked. Sheets are not bonded to each other at a location where foreign matter is present, and delamination occurs from the location where foreign matter is present, resulting in a decrease in interlayer strength. Therefore, it is important to evaluate the interlaminar strength of a composite material that is assumed to contain foreign matter.

  In JIS K 7086 (Non-Patent Document 1), mode I (opening type) and mode II (in-plane shearing type) deformation modes are applied to flat carbon fiber reinforced plastic with defects introduced between layers. A method for measuring the fracture toughness value of is specified.

  In addition, for example, a fiber reinforced plastic member having a curved portion bent in an L-shape is subjected to a mode I deformation test (waist bending test) and a mode II deformation applied to a defect introduced into the curved portion. It is common practice to perform a test (a tensile test in the horizontal direction) to measure the fracture toughness value in each mode.

  On the other hand, ASTM D6671 (Non-Patent Document 2) stipulates a method of measuring the fracture toughness value by applying a mixed mode of mode I and mode II to a flat fiber reinforced plastic substrate.

Japanese Industrial Standard JIS K 7086 "Test Method for Interlaminar Fracture Toughness of Carbon Fiber Reinforced Plastics" American Standards for Materials Testing ASTM D6671 “Standard Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites”

In the test method of Non-Patent Document 2, since the load condition setting and the jig setting are complicated, the test cannot be easily performed.
Moreover, about the member which has a curved part, the method of measuring the fracture toughness in mode III (out-of-plane shear type) and the method of measuring the fracture toughness in mixed mode are not established.

  The present invention provides a method for easily evaluating delamination strength in various mixed modes for a composite material having a curved portion.

  In order to solve the above-described problems, the present invention provides a composite material having a curved portion in which a defect of a predetermined size is introduced between layers and two planar portions sandwiching the curved portion, in one planar portion. When a surface that is in the same plane as the curved surface outside the curved portion is fixed and a load is applied to the other planar portion at a predetermined angle, Mode I and Mode II at the defect And mode III fracture mechanics parameters are respectively calculated using FEM analysis, and from the calculated fracture mechanics parameters, the mixing ratio of the mode I and the mode II at the predetermined angle, and the mode I And a mixing ratio calculation step in which a mixing ratio of mode III is calculated, and one plane portion of the composite material into which a defect having the same size as the defect applied to the FEM analysis is introduced. A strength measuring step in which a surface in the same plane as the curved surface outside the curved portion is fixed, and a load is applied to the other flat portion of the fixed composite material at a predetermined angle to measure an interlayer strength. And the value of the interlayer strength at the corresponding angle, the mixing ratio of the mode I and the mode II, and the mixing ratio of the mode I and the mode III, and the composite in a predetermined mixing mode There is provided a method for evaluating the interlayer strength of a composite material, comprising a strength acquisition step of acquiring the interlayer strength of the material.

  According to the present invention, by changing the angle of the load applied to the composite material having the curved portion, various mixing ratios for the mixed mode of mode I and mode II and the mixed mode of mode I and mode III are obtained. The delamination strength of the composite material can be evaluated by a simpler method than before.

According to the present invention, delamination strength in various mixed modes can be easily evaluated for a composite material having a curved portion into which a defect of an arbitrary size is introduced.
From the delamination strength in various mixed modes obtained by the present invention, an allowable range of fracture toughness values of structural members such as aircraft can be set, and allowable defect dimensions can be set.

It is the schematic which shows an example of the composite material used as evaluation object. It is the schematic showing each deformation mode of a defect. It is the schematic explaining the method to evaluate the delamination strength of the composite material of this invention. It is an example of the graph showing the load load angle dependence of a fracture mechanics parameter. Is a graph showing the load application angle dependence of fracture mechanics mixing ratio obtained from the parameter _{G II / (G I + G} II). It is an example of the graph showing the correlation with the initial stage peeling position of a plate | board thickness direction, and the fracture mechanics parameter calculated | required by FEM analysis.

An embodiment of a method for evaluating the delamination strength of a composite material of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram illustrating an example of a composite material to be evaluated. The composite material 10 is, for example, a resin-based composite material configured by laminating a plurality of fiber reinforced resin sheets. A composite material 10 shown in FIG. 1 has an L-shaped curved portion 11. In FIG. 1, the shape of the bending portion 11 is an L shape obtained by bending a flat plate at 90 ° (that is, the inner angle of the bending portion is 90 °), but is not limited thereto, and the inner angle of the bending portion 11 is an obtuse angle or It may be an acute angle.
Both sides of the composite material 10 across the curved portion 11 are formed as flat portions 12.

  The composite material 10 is configured by laminating a plurality of resin sheets (prepregs) mainly composed of fiber reinforced resin in which fibers such as carbon fibers and glass fibers are fixed with an epoxy resin or the like. A defect 13 having a predetermined size is introduced into the bending portion 11.

A method for evaluating the delamination strength of the composite material of this embodiment will be described below.
(1) Mixing ratio calculation process The dimensions of the composite material and the defect to be evaluated shown in FIG. 1 are determined. The composite material 10 is rectangular, the length of the side to be bent is L, and the length of the side parallel to the bending portion 11 is W.
The defect 13 is disposed inside the bending portion 11. It is preferable that the position where the defect 13 is arranged in the plate thickness direction is a place with the highest stress. The defect 13 is rectangular, the length of the side parallel to the side L is a, and the length of the side parallel to the width W is b. The defect 13 is disposed at the center with respect to the side W of the composite material 10 and is disposed at the center of the curved portion 11 in the cross section viewed from the side L. Further, the thickness direction of the position of the defect 13 is the distance t _i from the inside of the curved surface.

As shown in FIG. 2, the defect deformation mode is mode I in which the crack opens in the y direction, mode II in which the crack shifts in the x direction, and mode III in which the crack shifts in the z direction.
In the case of FIG. 1, the type in which a crack opens in the thickness direction in the defect corresponds to mode I, the deviation on side a corresponds to mode II, and the deviation on side b corresponds to mode III.

Fracture mechanics parameters in each deformation mode are calculated by FEM calculation for the composite material in which the defect is introduced. In the calculation, the following assumptions are made.
FIG. 3 is a schematic diagram illustrating a method for evaluating the delamination strength of the composite material of this embodiment. The surface shown in FIG. 3 is fixed to the composite material 10. Specifically, the fixed surface 14 a that is in the same plane as the curved surface outside the curved portion 11 is fixed to the jig 15 on one side 12 a of the plane portion where the side L is a straight line.

  A load is applied to the other flat portion 12b that is not fixed. The load is applied at a predetermined angle with respect to the flat surface portion 12b in the cross-sectional surface viewed from the side L. As shown in FIG. 3, the angle θ formed by the plane portion 12b and the load direction is minus (−) for compressing the plane portion 12b toward the curved portion, and plus (+) for pulling the plane portion 12b from the curved portion. ). In FIG. 3, the load angle with respect to the direction parallel to the side W is 0 °.

A mode in which a load of an arbitrary magnitude is applied at a predetermined angle θ by FEM analysis using the above L, W, a, b, t _i , the thickness t of the composite material, and the radius of curvature of the curved portion as parameters. Fracture mechanics parameters G _I , G _II and G _III for I, mode II and mode _III are calculated. In the analysis, the material properties of the composite material are known physical properties.

FIG. 4 is an example of a graph showing the load load angle dependence of fracture mechanics parameters. In the figure, the vertical axis represents fracture mechanics parameters (arbitrary scale), and the horizontal axis represents the load load angle.
FIG. 4 shows the fracture mechanics parameters G _I and G _II of mode I and mode II at the tip of the defect indicated by point A in FIG. 1 (a point on the side b opposite to the side on which the load is applied). Has been. The size of the defect was calculated on the assumption that a = b, a / L << 1, b / W << 1 (when the defect is sufficiently smaller than the composite material). Arbitrary material properties were applied for FEM analysis.
As shown in FIG. 4, the values of the fracture mechanics parameters G _I and G _II change by changing the load angle θ of the load.

From the fracture mechanics parameters obtained by the FEM analysis, the mixing ratio G _II / (G _I + G _II ) of mode I and mode II at each load angle is obtained. FIG. 5 is a graph showing the load angle dependency of the mixing ratio G _II / (G _I + G _II ) obtained from the fracture mechanics parameters shown in FIG. In the figure, the vertical axis represents the mixing ratio, and the horizontal axis represents the load load angle. The fracture mechanics parameters G _I and G _II vary depending on the load value, but the mixing ratio G _II / (G _I + G _II ) is a value that does not depend on the magnitude of the load.
As shown in FIG. 5, the mixing ratio changes as the load angle of the load changes. When the load angle of the load is + 45 °, the mixing ratio is G _II / (G _I + G _II ) = 0, and the deformation mode is mode I. When the load angle is 90 °, the mixing ratio is G _II / (G _I + G _II ) = 1, and the deformation mode is mode II.

In the same manner as described above, the fracture mechanics parameters G _I and G _{III in} mode I and mode III when loads are applied at various angles by FEM analysis are calculated. Furthermore, the load load angle dependency of the mixing ratio G _III / (G _I + G _III ) is acquired from the fracture mechanics parameters G _I and G _III .

By changing the defect ratio a / b set in the calculation by FEM analysis, the values of the fracture mechanics parameters G _I , G _II and G _III are changed. Thus, you are possible to change the balance between the mixing ratio _{G II / (G I + G} II) and _{G III / (G I + G} III).
For example, when a / L = 1 (a / b = ∞), a defect penetrating the composite material in the side L direction is introduced. In this case, the defect end on the side a is deformed by the mixed mode of mode I and mode III.
Further, when b / W = 1 (a / b≈0), a defect penetrating the composite material in the side W direction is introduced. In this case, the defect end on the side on which the load is applied and the opposite side b is deformed by the mixed mode of mode I and mode II.

(2) Strength measurement step The composite material 10 shown in FIG. 1 is produced. In the production of composite material 10, as a film that is a defect 13 in the interlayer corresponding to the distance t _i from the inside of the curved surface 11 is inserted, it is laminated prepreg. The film inserted here is, for example, a polyimide film, polytetrafluoroethylene, aluminum foil or the like. The size of the film is the same as that of the defect 13 applied to the FEM analysis in (1). That is, a film having sides a and b is inserted between prepregs.
The laminated prepreg is curved so as to have the shape shown in FIG. 1 (the same shape as the composite material subjected to FEM analysis), and then heated and cured.

The cured composite material 10 has the same surface 14a as (1) fixed with a jig 15 (see FIG. 3). If necessary, the composite material 10 is processed according to the specifications of the jig 15.
A load is applied to the flat portion 12b that is not fixed by the jig 15 at a predetermined angle. The magnitude of the load is, for example, about 50 kgf to 500 kgf.

The load load angle is the same as the load load angle in the FEM analysis.
The load angle with respect to the direction parallel to the side W is 0 °. When −90 ° <θ ≦ 0 °, a load is applied to a surface (16a in FIG. 3) that is in the same plane as the curved surface inside the flat portion 12b. In the case of 0 ° ≦ θ <+ 90 °, a load is applied in a direction of pulling a surface (16b in FIG. 3) that is in the same plane as the curved surface outside the flat portion 12b. When θ = −90 ° and + 90 °, a load is applied to the side surface (16c in FIG. 3) of the composite material 10.
The above is merely an example, and the load direction may be changed by the combination of the jig 15 and the composite material 10. However, when performing FEM analysis, it is necessary to set boundary conditions in consideration of the combination of the jig 15 and the composite material 10.

  The above load is applied to the composite material, and the load when the peeling (defect) starts to progress is measured as the delamination strength.

(3) Strength acquisition step The relationship between the load load angle and the mixing ratio obtained in (1) is associated with the delamination strength at each load load angle measured in (2). That is, when the load angle is the same, it is regarded as the delamination strength of the composite material in the mixed mode at the mixing ratio.

  The allowable range of the fracture toughness value of the structural member made of the composite material can be determined from the delamination strength at various mixing ratios (mixing modes) obtained by the present embodiment, and the allowable defect size is set. be able to.

An example of the allowable defect dimension setting method will be specifically described below.
FIG. 6 is an example of a graph showing the correlation between the initial peel (defect) position in the plate thickness direction and the fracture mechanics parameter obtained by FEM analysis. FIG. 6 shows the results under the conditions of a load load angle of 0 °, a = b = 12.7 mm, a / L << 1, b / W << 1. In the figure, abscissa peeling position in the thickness direction, the vertical axis represents the ratio of the fracture mechanics parameters G _I at defect position relative fracture mechanics parameters G _IC fracture limit _(fracture toughness value) for mode I. The release position in the thickness direction was expressed as a percentage of the distance t _i from the inside of the curved surface of the defect relative to the thickness t of the composite.

As shown in FIG. 6, when an arbitrary load is applied to the composite material having a curved portion, the fracture mechanics parameter value varies depending on the distance t _i in the thickness direction of the initial peeling. In FIG. 6, when t _i / t = 30%, G _I / G _IC ≈0.85, and when t _i / t = 50%, G _I / G _IC ≈0.70.
For example, when G _I / G _IC <0.9 is set in the design of the structural material, the allowable initial peeling dimension in mode I is larger than a = b = 12.7 mm used in the acquisition of FIG. . _Further, since the direction of t i / t = 50% is lower _G I _{/ G IC,} it is possible to increase the allowable size of the initial peel than t i / t = 30%.

  The graph showing the correlation between the initial peeling position in the plate thickness direction and the fracture mechanics parameter in each deformation mode is acquired for various load loading angles. By doing so, it is possible to set the allowable defect size in each deformation mode according to the position of the initial separation in the thickness direction and the load load angle.

DESCRIPTION OF SYMBOLS 10 Composite material 11 Bending part 12, 12a, 12b Plane part 13 Defect 14a Fixed surface 15 Jig 16a, 16b, 16c Surface where load is applied

Claims (1)

In a composite material having a curved portion in which a defect of a predetermined size is introduced between layers and two plane portions sandwiching the curved portion,
In the case where a surface in the same plane as the curved surface outside the curved portion in one of the planar portions is fixed and a load is applied to the other planar portion at a predetermined angle, the defect Mode I, mode II and mode III fracture mechanics parameters are respectively calculated using FEM analysis,
From the calculated fracture mechanics parameters, a mixing ratio calculating step in which a mixing ratio of the mode I and the mode II at the predetermined angle and a mixing ratio of the mode I and the mode III are calculated;
A surface that is in the same plane as the curved surface outside the curved portion in one planar portion of the composite material into which the defect having the same size as the defect applied to the FEM analysis is introduced is fixed,
A strength measurement step of measuring the interlayer strength by applying a load at a predetermined angle to the other plane portion of the fixed composite material;
The value of the interlayer strength at a corresponding angle, the mixing ratio of the mode I and the mode II, and the mixing ratio of the mode I and the mode III are related to each other, and the composite material in a predetermined mixing mode A method for evaluating an interlayer strength of a composite material comprising a strength acquisition step of acquiring an interlayer strength.

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