CN108548719B

CN108548719B - Method for testing interlayer normal strength of composite material

Info

Publication number: CN108548719B
Application number: CN201810186065.8A
Authority: CN
Inventors: 黎增山; 王鑫; 王晓东; 关志东
Original assignee: Beihang University
Current assignee: Beihang University
Priority date: 2018-03-07
Filing date: 2018-03-07
Publication date: 2021-03-30
Anticipated expiration: 2038-03-07
Also published as: CN108548719A

Abstract

The invention discloses a method for testing the normal strength between layers of composite materials, which belongs to the technical field of composite materials and material performance testing. In the method of the present invention, the test piece is a square plate block. During the laying process, a polytetrafluoroethylene film that does not cover the test area is inserted between the upper and lower layers of the predetermined test interface, and then cured and formed; The device tests the interlaminar strength of composite materials. The invention is a method for testing the normal tensile strength between layers of composite materials, and the method breaks through the limitation of the interlayer performance of the test piece to be tested by the previous method. The size of the hollow area of the PTFE film can be designed according to the interlaminar properties of the composite material to be tested, so that the normal failure load between the layers in the test can be controlled, and the interlaminar failure of the composite material can be guaranteed before the metal block and the test. The adhesive between the parts is destroyed, thereby ensuring the failure mode.

Description

Method for testing interlayer normal strength of composite material

Technical Field

The invention belongs to the technical field of composite materials and material performance testing, and relates to a method for testing the interlayer normal strength of a composite material and a method for designing and manufacturing a test piece of the composite material.

Background

The composite material has the characteristics of light weight, high strength and the like, and is widely applied to the fields of sports, aviation, aerospace and the like. The advanced composite material is specially used for a main bearing structure or a secondary bearing structure, has rigidity and strength performance equivalent to or superior to those of aluminum alloy, and mainly refers to a reinforced composite material with high strength and modulus, such as boron fiber, carbon fiber, aramid fiber and the like at present. Although the performance of the composite material is equivalent to or stronger than that of metal, the interlaminar performance of the composite material is very low, and the interlaminar bearing only by the interlaminar interface is an area which is easy to damage, so that the interlaminar strength of the composite material must be measured for evaluating the quality and the performance of the composite material.

At present, no method for testing the interlayer normal strength of the carbon fiber composite material exists, although GB/T4944 can measure the interlayer tensile strength of the glass fiber laminate, the bonding strength of a test piece and a metal binder is required to be higher than the interlayer tensile strength of the test piece, and the interlayer performance of the advanced composite material is often higher than the bonding strength of the binder, so that the failure mode of the test piece is contrary to the expectation, and the measured performance is inaccurate.

Disclosure of Invention

The invention aims to provide a testing method capable of accurately testing the interlayer normal strength of all advanced composite materials. The invention provides a method for testing the interlayer normal strength of a composite material, which comprises the following two steps:

in the first step, films and test pieces were prepared.

The test piece is a square flat plate block, a polytetrafluoroethylene film which does not cover a test area is inserted between an upper layer and a lower layer of a preset test interface in the paving process (the polytetrafluoroethylene film can be properly adjusted in size according to requirements, and if the interlayer performance of the composite material to be tested is high, the test area can be properly reduced), and then the polytetrafluoroethylene film is cured and molded.

And secondly, testing the interlayer strength of the composite material by using a testing device.

The test device comprises two metal blocks with through holes and two load transmission joints, wherein the end faces of the metal blocks are respectively bonded with the upper layer and the lower layer of the test piece, one ends of the two load transmission joints are respectively connected with the testing machine, and the other ends of the two load transmission joints are respectively connected with the two metal blocks through the through holes in the metal blocks.

And applying unidirectional tensile load to the load-transmitting joints at the two sides until the test piece is subjected to interlaminar failure, and recording the failure load P (the unit is N). Calculating the interlaminar normal tensile strength sigma of the composite material_n：

Wherein, the inner and outer diameters R of the upper ring of the film₁、R₂And the width w of the spokes on the film is in mm, sigma_nIn Mpa.

The test piece and the film have the same shape and size.

The invention has the advantages that:

(1) the invention relates to a method for testing the normal tensile strength between layers of a composite material, which breaks through the limitation of the previous method on the interlayer performance of a test piece to be tested.

(2) According to the method for testing the interlayer normal tensile strength of the composite material, the polytetrafluoroethylene film is inserted into the test interlayer to manufacture the test piece, the metal blocks are adhered to the upper surface and the lower surface of the test piece, and the load transmission joint is used for loading, so that the test piece is subjected to interlayer normal tensile damage, and the interlayer normal tensile strength is measured.

(3) The test piece designed in the invention has originality and has the advantages that the size of the corresponding polytetrafluoroethylene film hollowed-out area can be designed according to the interlayer performance of the composite material to be tested, so that the normal damage load between layers in the test is controlled, the interlayer damage of the composite material is guaranteed to be prior to the adhesive damage between the metal block and the test piece, and the damage mode is further guaranteed.

(4) The invention is suitable for testing the interlayer normal tensile strength of all composite materials.

Drawings

FIG. 1 is a diagram showing a process for preparing a test piece according to the present invention.

FIG. 2 is a schematic diagram of the critical dimensions of a PTFE membrane.

FIG. 3 is an assembly drawing of a test piece and a test apparatus according to the present invention.

In the figure:

1. layering the upper part of the composite material; 2. Layering the lower part of the composite material; 3. A polytetrafluoroethylene film; 4. A test piece;

5. a test zone; 6. A thin film hollowed-out area; 7. A metal block; 8. And a load transmission joint.

Detailed Description

The present invention will be described in detail with reference to the accompanying drawings.

The invention provides a method for testing the interlayer normal strength of a composite material, which comprises the following steps:

in the first step, a film is prepared.

The membrane is made of 100mm x 100mm square polytetrafluoroethylene material. The thickness of the film is thin enough under the premise of process allowance, so that the completeness of the film can be ensured in the preparation process, the interface strength of two sides of the hollow area can be ensured not to be weakened, and the thickness is usually about 20% of the theoretical thickness of a single layer of the measured composite material. The film contains an annular hollow area corresponding to the composite material test area. As shown in fig. 1 and 2, the hollowInner and outer diameters R of the empty zone₁、R₂And the spoke width w needs to be adjusted according to the situation, so that the bonding peel strength sigma of the composite material and the metal_MNStrength σ of the adhesive itself_cohAnd the interlayer normal phase strength estimated value sigma of the composite material to be measured_{n estimation}In this regard, it is also influenced by the manufacturing process conditions and is necessary to ensure that the test is broken at the predetermined test interface. The method specifically comprises the following steps:

wherein sigma_{n estimation}The transverse tensile strength of the unidirectional ply of the same material can be used for estimation; strength σ of the adhesive itself in general_cohIs larger than the bonding peel strength sigma of the composite material and the metal_MNIf the binder used is relatively weak, i.e. σ_coh≤σ_MNThen, will be σ in the formula_MNSubstitution as sigma_coh。

And secondly, preparing a test piece.

The test piece is a square plate block body with the thickness of 100mm multiplied by 100mm, as shown in figure 1, in the paving process, a polytetrafluoroethylene film 3 which does not cover a test area is inserted between a composite material upper layer 1 and a composite material lower layer 2 of a preset test interface, and then the polytetrafluoroethylene film is solidified and molded to form the test piece 4.

And thirdly, preparing a testing device and testing.

As shown in fig. 3, the test device includes two metal blocks 7 with through holes and two load transmission joints 8, and the end surfaces of the two metal blocks 7 are respectively bonded to the upper and lower surfaces of the test piece 4. One end of each of the two load transmission joints 8 is connected with the testing machine, and the other end of each of the two load transmission joints is connected with the two metal blocks 7 through holes in the metal blocks 7. And applying unidirectional tensile load to the two side load transmission joints 8 at the speed of 2mm/min until the test piece 4 is subjected to interlaminar failure, and recording the failure load P (unit is N). After the test is finished, the adhesive on the surface of the metal block 7 is removed, and the testing device can be repeatedly used under the condition of ensuring the adhesive quality.

Fourthly, the interlayer normal direction pulling of the composite materialTensile Strength σ_nAnd (3) calculating:

wherein, the inner and outer diameters R of the upper ring of the film₁、R₂In units of mm, σ_nIn Mpa.

Claims

1. a method for testing the normal strength between layers of a composite material, comprising the following two steps,

The first step is to prepare films and test pieces;

The shape and size of the test piece and the film are the same, the test piece is the composite material to be tested, and a polytetrafluoroethylene film that does not cover the test area is inserted between the upper and lower layers of the composite material to be tested, and then curing molding;

The second step is to use a test device to test the interlaminar strength of the composite material;

The test device is two metal blocks with through holes and two load-carrying joints, the end faces of the metal blocks are respectively bonded to the upper and lower layup surfaces of the test piece, and one end of the two load-carrying joints is respectively connected to the test piece. The other end is connected to the two metal blocks through the through holes on the metal block; a unidirectional tensile load is applied to the load transfer joints on both sides until the interlayer failure of the test piece occurs, and the failure load P is recorded, the unit is N, Calculate the interlaminar normal tensile strength σ _n of the composite;

The prepared test piece is a square flat block body. During the laying process, a polytetrafluoroethylene (PTFE) that does not cover the test area is inserted between the upper layer (1) of the composite material and the lower layer (2) of the composite material at the predetermined test interface. The vinyl film (3) is then cured and formed into a test piece (4);

It is characterized by:

The film is made of 100mm×100mm square polytetrafluoroethylene material, the thickness of the film is 20% of the theoretical thickness of the single layer of the measured composite material, the film contains an annular hollow area, corresponding to the composite material test area, the inner and outer diameter of the hollow area on the film R _1. R ₂ needs to be adjusted according to the situation, which is related to the bonding peel strength σ _MN between the composite material and the metal, the strength of the adhesive itself σ _coh and the _{estimation of the interlaminar phase strength σ n} of the composite material to be tested. Destruction occurs in the predetermined test interface, specifically:

Among them, σ _{n is estimated} by the transverse tensile strength of the unidirectional laminate of the same material; usually, the strength of the adhesive itself, σ _coh , is greater than the bond peel strength σ _MN between the composite material and the metal, when σ _coh ≤ σ _MN , then replace σ _MN in the formula with σ _coh ;

The interlaminar normal tensile strength σ _n of the composite material:

The unit of the inner and outer diameters R ₁ , R ₂ of the annular hollow area on the film and the width w of the spokes on the film is mm, and the unit of σ _n is MPa .