JP2008281429A - Bonding strength evaluating method of double wrap coupling - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bonding strength evaluating method of a double wrap coupling for evaluating a peel strength in a joint which can not clearly define an adhesive layer. <P>SOLUTION: The bonding strength evaluating method of the double wrap coupling 100 including three laminated plate members including a composite material 10 and bound on side faces, has the process of obtaining an allowable shear stress of the double wrap coupling 100 by a tension test using a plurality of specimens with different binding lengths L, the process of obtaining a shear stress of the double wrap coupling 100 by a finite element analysis method, and the process of comparing and evaluating the allowable shear stress and the shear stress. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for evaluating the joint strength of a double lap joint widely applied from the aerospace field to the shipbuilding field.

  Conventionally, a fastening structure for fastening a plurality of members using fasteners such as bolts and rivets is widely used not only in the aerospace field but also in the general structural field (particularly in the shipbuilding field). A joint recognized as a fastening structure fulfills its fastening function by taking charge of the tensile load and shearing force in the axial direction.

In recent years, in the fields described above, there are increasing examples of applying fiber reinforced composite materials in order to reduce the weight of structures such as airframes or hulls and reduce payloads. In addition, the application of a fiber reinforced composite material excellent in specific strength and specific rigidity is now indispensable, and the joining of fiber reinforced composite materials and the joining of fiber reinforced composite materials and metals are also increasing.
The joint structure includes a double wrap method / single wrap method. For example, as shown in FIG. 7, plate members 81 made of fiber reinforced composite material are bonded to both sides of a plate member 82 made of steel via an adhesive layer 83. Has been.

However, since the joint is the most frequently cited cause of structural destruction, such as peeling of the adhesive layer 83 at the adhesive interface, the strength of the joint can be evaluated to improve the safety and reliability of the entire structure. It is extremely important.
As a method for evaluating the bonding strength of a double lap adhesive joint / single wrap adhesive joint, a technique using a Hart-Smith chart (see FIG. 8) (see Non-Patent Documents 1 and 2) is widely supported. These papers take into account the effect of plasticity on the adhesive layer of the joint and physically explain the interface failure. That is, the relationship between the average shear stress acting on the adhesion interface and the side restraint length (adhesion length) can be expressed as shown in FIG. 8. When the adhesion length l is close to 0 and becomes infinitely long, Asymptotically approaching a certain value as in the equation.

As shown in FIGS. 9A and 9B, among the analysis models E, F, and G having different adhesion lengths, the analysis model E having the shortest adhesion length is broken when the entire bonding interface is plastically yielded. However, the analysis model G having the longest bond length is broken when the shear strain energy applied to the bond interface exceeds the limit. This indicates that when the bond length is short, the interface fracture is determined by plasticizing the entire bonded interface, and when the bond length is long, the interface fracture is determined by the shear strain energy acting on the bonded interface.
L, J.Hart-Smith; Adhesive-Bonded Double-Lap Joints; NASA CR-112235; 1973 L, J.Hart-Smith; Adhesive-Bonded Single-Lap Joints; NASA CR-112234; 1973

The above-described bonding strength evaluation method is often used in a molding method using a semi-cured prepreg, and evaluates based on the physical properties, thickness, etc. of the adhesive.
In recent years, a VaRTM molding method (vacuum pressure resin impregnation molding method) having excellent cost performance has appeared. This molding method is a composite-integrated molding method that expects the adhesive effect of the resin without using an adhesive, and impregnates the resin into a laminate of composite fibers and steel, so material costs and equipment costs Is advantageous in that it is reduced. However, when the bonding strength is evaluated, the Hart-Smith method for evaluating from the physical properties and thickness of the adhesive cannot be applied. The stress distribution of joints formed by the VaRTM molding method is given as a theoretical solution by Tsai-Oplinger. However, since the plastic effect of the resin instead of the adhesive cannot be taken into account, the behavior is only in the elastic range. No mention has been made and strength evaluation has not been reached. That is, since it is not possible to use the conventional strength evaluation method if the adhesive layer cannot be clarified, it is required to construct a new evaluation method.

  The present invention has been made in view of the above-mentioned problems of the prior art, and provides a method for evaluating the joint strength of a double lap joint capable of performing an appropriate peel strength evaluation on a double lap joint that cannot clearly define an adhesive layer. It is intended to provide.

In the method for evaluating the joint strength of a double lap joined body according to the present invention, the following means are employed in order to solve the above problems.
The present invention relates to a method for evaluating the bonding strength of a double lap joined body in which three plate-like members including a plate-like member made of a fiber reinforced composite material are overlapped and restrained on the side, and a plurality of test pieces having different side restraint lengths The step of obtaining the allowable shear stress (τc) of the double lap joint by a tensile test using, the step of obtaining the shear stress (τ) of the double lap joint by a finite element analysis method, the allowable shear stress (τc) and shear A step of comparing and evaluating the stress (τ);
It is characterized by having.

  In addition, the allowable shear stress (τc) is defined by the plasticized shear stress (τc1) and the limit shear stress (τc2) in a plurality of specimens.

  The shear stress (τ) is an average shear stress (τavg) of the double lap joined body.

  The average shear stress (τavg) is obtained from the shear stress at the end and center in the width direction of the double lap joined body.

  Further, the shear stress (τ) is obtained by elasto-plastic analysis.

  In addition, a Hill yield function is used in the elastic-plastic analysis.

  Moreover, it is characterized by comparing and evaluating whether or not the shear stress (τ) is smaller than the allowable shear stress (τc).

  The fiber-reinforced composite material is any one of FRP, CFRP, and GFRP.

According to the present invention, the following effects can be obtained.
In the present invention, it is possible to evaluate the strength of the double lap joined body by comparing the allowable shear stress obtained in the tensile test with the shear stress obtained by the finite element analysis method.
The allowable shear stress can be easily determined from two formulas obtained by performing a tensile test on a plurality of test bodies having different side constraint lengths.
Further, the shear stress can be calculated by performing a finite element analysis and averaging the shear stress generated at the interface in the direction of the lateral constraint length.
In the present invention, the shear stress acting on the interface can be obtained numerically using finite element analysis. At this time, the shear stress acting on the interface is set so as not to exceed the plasticized shear stress using the Hill yield function, and the plasticization at the interface is taken into consideration.
Thereby, appropriate strength evaluation can be performed with respect to the adhesive joint of the shear stress field whose adhesive layer is not clear. Therefore, a double lap joined body having sufficient strength can be obtained, and the safety and reliability of the entire structure can be improved.

  Embodiments of the present invention will be described below with reference to the drawings. In each drawing used for the following description, the scale of each member is appropriately changed to make each member a recognizable size.

  FIG. 1 is a cross-sectional view schematically showing a configuration of an analysis model according to a method for evaluating the joint strength of a double lap joined body according to the present invention.

As shown in FIG. 1, the double lap joined body 100 includes a pair of plate members (hereinafter referred to as a composite material 10) made of a fiber reinforced composite material and a plate member (hereinafter referred to as a steel material 11) made of a steel material. It has a double wrap structure in which the positions are different from each other in the longitudinal direction and the sides are constrained. The composite materials 10 and 10 and the steel material 11 are integrally formed by a VaRTM molding method described later.
Examples of the fiber reinforced composite material include FRP, CFRP (carbon fiber), and GFRP (glass fiber). Examples of the steel material include SUS (stainless steel).

Next, a method for evaluating the bonding strength of the double lap bonded body according to this embodiment will be described.
FIG. 2 is a flowchart showing a method for evaluating the bonding strength of the double lap bonded body according to this embodiment.
As shown in FIG. 2, the joint strength evaluation method of the double lap joined body 100 is based on a material interface 12 (see FIG. 1) by a tensile test using a plurality of analysis models (test bodies) having different side constraint lengths. The allowable shear stress τc at is determined (step S1). Next, the shear stress τ at the material interface 12 is obtained by FEM (finite element analysis method) (step 2). Then, the allowable shear stress τc obtained in step S1 and step 2 is compared with the shear stress τ to perform strength evaluation (step S3).

  Below, the joint strength evaluation method of the double lap joined body of this embodiment is explained in full detail. The description will be made in accordance with the flowchart of FIG. 2, with reference to FIGS.

[Tensile test]
In step S1, a tensile test of a plurality of specimens is performed according to the following procedure, and an allowable shear stress τc acting on the material interface is obtained. FIG. 3 is a perspective view schematically showing the configuration of the test body. FIG. 4 is a diagram showing a distribution of fracture strength by a tensile test.

(1) First, test bodies A, B, and C having a double wrap structure (see FIG. 3) in which a steel material 11 is sandwiched between a pair of composite materials 10 are prepared. The test bodies A, B, and C are molded by VaRTM molding method in which fibers made of the composite material 10 and the steel material 11 are laminated and then impregnated with a resin. In the present embodiment, a plurality of three types of test bodies A, B, and C having different side constraint lengths (hereinafter, constraint length L) are prepared for each type. As shown in FIG. 3, the restraint length L of each of the test bodies A, B, and C is set as follows: test body A (restraint length L = 25 mm), test body B (restraint length L = 50 mm), test body C (constraint length L). = 100 mm), the thickness t1 of each composite material 10 is set to 4.5 mm, the thickness t2 of the steel material 11 is set to 6 mm, and the width W is set to 30 mm.

  Examples of the resin include known materials (for example, epoxy) used in the resin impregnation forming method. Moreover, the same material is used for the composite materials 10 and 10 on both sides of the steel material 11. When the composite materials 10 and 10 are formed of different materials, an influence of bending occurs on the material interface. In this case, not only the shear stress τ but also the bending stress σ must be considered. In the embodiment, a double wrap structure using composite materials 10 and 10 made of the same material is used.

(2) A uniaxial load was actually applied to each specimen A, B, C, and the damage state at the material interface between the composite material 10 and the steel material 11 was verified by a static tensile test. The uniaxial load is a tensile load F acting in the opposite direction between the composite material 10 and the steel material 11. The tensile load F was increased linearly with time.

FIG. 4 shows a plot of the breaking strength by the tensile test for each of the specimens A, B, and C. In this figure, the x-axis is the constraint length L and the y-axis is the fracture strength Pcr.
The specimen A (constraint length L = 25 mm) yielded plasticity due to the yield of the entire material interface, and was destroyed earlier than the other specimens B and C. The plasticized shear stress τc1 when the entire material interface is plasticized is determined by the slope ΔPcr / Δl of the straight line connecting the fracture point (ΔPcr1, ΔL1) and the origin (0, 0). That is, the plasticizing shear stress τc1 is expressed by the following equation (2).

τｃ１：塑性化せん断応力
Ｐｃｒ：破壊強度
Ｌ：拘束長

τc1: Plasticized shear stress Pcr: Fracture strength L: Constraint length

  In the case of the test body B (restraint length L = 50 mm) and the test body C (restraint length L = 100 mm), the specimen was destroyed when the shear strain energy at the material interface exceeded the limit. At least the fracture prevention force Pu that can prevent fracture is determined by the fracture strength Pcr of the specimen B and the specimen C and the plasticized shear stress τc1. When the restraint area is A, the limit shear stress τc2 when the interface strain energy becomes the limit is expressed by the following equation (2).

τｃ２：限界せん断応力
Ｐｕ：破壊防止力
Ａ：拘束面積

τc2: Limit shear stress Pu: Fracture prevention force A: Restraint area

As described above, the allowable shear stress τc serving as the threshold is determined by the above formulas (1) and (2). With such a tensile test, using the same theoretical development as the Hart-Smith described above,
The allowable shear stress τc, which is a threshold value, can be obtained more simply than the theory.

  Thus, in the double lap joined body 100 which clamps the steel material 11 with a pair of composite materials 10 and 10, the influence of the allowable shear stress τc at each material interface of the test bodies A, B, and C having different restraint lengths L was examined. . The factor of the interface fracture varies depending on the constraint length L, and is roughly divided into plasticization at the material interface or shear strain energy at the material interface.

  In the above embodiment, the tensile test was performed using three test specimens A, B, and C having different constraint lengths L. However, the tensile test was not limited to this, and yield stress and fracture prevention at the material interface of each test specimen were performed. If the forces are different, the analysis can be performed even with two specimens having different constraint lengths L. That is, by clarifying the elastoplastic branch point, two types of threshold values (plasticized shear stress τc1 and limit shear stress τc2) are obtained by the above formulas (1) and (2).

[FEM analysis]
In step S2, the shear stress τ at each material interface of the specimens A, B, and C is obtained. In this embodiment, the shear stress τ at the material interface is performed using FEM (finite element method). Here, the analysis models A ′, B ′, and C ′ are used as the FEM analysis models of the specimens A, B, and C. The shear stress τ at each material interface of the analysis models A ′, B ′, and C ′ is an average shear stress τavg in the longitudinal direction of the material interface. This average shear stress τavg is obtained by FEM analysis. For example, ABAQUS (registered trademark) or ANSYS (registered trademark) is used as the analysis code.

(Elastic analysis)
First, an elastic analysis is performed using FEM, and it is confirmed whether the analysis method using FEM is appropriate.
FIG. 5 shows the distribution of the shear stress τ acting on each material interface of the analysis models A ′, B ′, and C ′.
The vertical axis represents shear stress (τ / τavg), and the horizontal axis represents the dimensionless axial position (x / L).

  According to FIG. 5, in each of the analysis models A ′, B ′, and C ′, the shear stress of the restraint portion 35 is particularly measured at the center in the width direction (position of the dimensionless restraint length x / L = 0.5). It can be seen that a peak (maximum shear stress τmax) occurs. The figure also shows the shear stress distribution at the material interface using a theoretical solution (Tsai-Oplinger) that does not consider the adhesive layer. According to the figure, it can be confirmed that the result of the elastic analysis by FEM generally matches the result of Tsai-Oplinger which is a theoretical solution. This proved that the technique using FEM is appropriate.

(Elasto-plastic analysis)
FIG. 6 shows the distribution of shear stress τ acting on the material interface of the analysis model by the elasto-plastic analysis of this embodiment. The vertical axis represents dimensionless shear stress (τ / (F / 2A)), and the horizontal axis represents dimensionless axial position (x / L). FIG. 6 simultaneously describes the distribution of shear stress by the elastic analysis described above, and shows a comparison between the elastic analysis and the elastic-plastic analysis. In FIG. 6, only the shear stress distribution of the analysis model C ′ is shown as an example, and the shear stress distributions of the other analysis models A and B are omitted.

  In this embodiment, elasto-plastic analysis is performed using FEM (hereinafter referred to as elasto-plastic FEM analysis), and a shear stress τ acting on each material interface of the analysis models A ′, B ′, and C ′ is obtained. However, when the elastic-plastic FEM analysis is performed, the Hill yield function, which is an anisotropic yield function, is used so that the shear stress value of the element adjacent to the material interface does not exceed the plasticized shear stress τc1 described above. Perform elasto-plastic analysis.

  As an analysis method, each analysis model A ′, B ′, C ′ is mesh-divided by a predetermined number of divisions, and the local shear stress τ (node stress on the composite material 10 side) at the material interface is averaged in the longitudinal direction. To calculate the average shear stress τavg. Here, the average shear stress τavg at both the end line and the center line in the width direction of the material interface is considered.

  The analytical models A ′, B ′, and C ′ limit only the upper limit of the shear stress τ. That is, in consideration of the influence of anisotropic plasticity, the stress in the restraining length direction (axial length) of the restraining portion 35 and the stress in the width direction length of the restraining portion 35 are not limited. In the present embodiment, the plasticized shear stress τc1 obtained in the tensile test is set as a reference value as the upper limit of the shear stress τ. That is, the elasto-plastic analysis is performed so as to be smaller than the stress when the material interface yields.

  In this way, in the double lap joined body 100 formed by the VaRTM molding method, the distribution of the shear stress τ (sometimes referred to as generated shear stress τ) applied to the material interface is numerically analyzed using FEM. Was able to ask for.

[Interfacial strength evaluation]
In step S3, peel strength evaluation in a uniaxial stress field (shear stress field) is performed using the following formula (3). Then, the shape dimension is set so that the generated shear stress τ acting on the material interface is within the range of the allowable shear stress τc.

  In this way, the comparative evaluation between the allowable shear stress τc and the shear stress τ is performed so that the shear stress τ applied to the material interface is within a range within the allowable shear stress τc. Thereby, it can be set as the structure which maintains the joining of the composite material 10 and the steel material 11 reliably, and the double lap joining body 100 which has sufficient intensity | strength can be obtained.

  In the present embodiment, a tensile test was performed on a plurality of specimens A, B, and C in which the constraint length ratio was varied, and the influence of plasticity and shear strain energy on each material interface was examined to obtain an allowable shear stress τc. Furthermore, numerical analysis using elasto-plastic FEM analysis was performed on multiple analytical models A ′, B ′, and C ′ with varying constraint length ratios, and the influence of shear stress τ at the material interface was investigated. .

  The allowable shear stress τc at the material interface can be easily calculated by the above formulas (1) and (2) obtained from the results of the tensile test. In addition, the elasto-plastic analysis using FEM is performed by determining the upper limit of the shear stress at the material interface using the Hill yield function, and considering the plastic effect of the resin constituting the double lap joint 100. It has become. By performing such an elasto-plastic analysis, the actual shear stress distribution at the material interface can be appropriately expressed. In addition, since numerical analysis using FEM is possible, the shear stress at the material interface can be easily obtained without performing an experiment.

  In addition, the tensile test only needs to be performed for at least two constraint length ratios, and since it is only necessary to examine the breaking strength when the detection item is also broken, the experiment is easy and the experiment cost can be reduced.

  According to the strength evaluation method of the double lap joined body 100 of this embodiment, the allowable shear stress obtained as described above is obtained even in the double wrap structure in which the joining interface formed by the VaRTM molding method is not clear. By comparing τc with the shear stress τ, an appropriate strength evaluation can be performed. Therefore, the safety and reliability of the entire structure using the double wrap joined body 100 integrally formed without using an adhesive can be improved.

  As mentioned above, although preferred embodiment which concerns on this invention was described referring drawings, it cannot be overemphasized that this invention is not limited to the said embodiment. Various shapes, combinations, and the like of the constituent members shown in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

  For example, in the said embodiment, although the steel material 11 was set as the double wrap structure clamped with a pair of composite materials 10 and 10, you may make it replace a material and clamp a composite material with a pair of steel materials. Moreover, you may make it comprise all by a composite material. When the constituent material of the double lap joined body is changed, the allowable shear stress is obtained by a tensile test according to the combination of materials constituting the joined body. The tensile test may be performed only when the constituent materials are changed.

It is sectional drawing which shows typically the structure of the analysis model which concerns on the joining strength evaluation method of the double lap joining body in one Embodiment of this invention. It is a flowchart which shows the joining strength evaluation method of the double lap conjugate | zygote in one Embodiment of this invention. It is a perspective view which shows the structure of an analysis model typically. It is a figure which shows distribution of the fracture strength by a tension test. It is a figure which shows distribution of the shear stress by the elastic analysis using FEM. It is a figure which shows distribution of the shear stress by the elastoplastic analysis using FEM. It is a figure which shows typically schematic structure of the conventional double lap joined body. It is a figure which shows the chart of Hart-Smith. (A), (b) is a figure which shows the peeling mechanism of the conventional joined_body | zygote.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... Double lap joined body, 10 ... Composite material (plate-shaped member), 11 ... Steel material (plate-shaped member), A, B, C ... Analytical model (test body)

Claims (8)

A method for evaluating the bonding strength of a double lap joined body in which three plate-like members including a plate-like member made of a fiber-reinforced composite material are overlapped and side-bounded,
Obtaining an allowable shear stress of the double lap joined body by a tensile test using a plurality of test bodies having different side constraint lengths;
Determining the shear stress of the double lap joint by a finite element analysis method;
A step of comparing and evaluating the allowable shear stress and the shear stress;
A method for evaluating the joint strength of a double-lap joined body, comprising:   2. The method for evaluating the joint strength of a double lap joint according to claim 1, wherein the allowable shear stress is defined by a plasticized shear stress and a limit shear stress in the plurality of test bodies.   The said shear stress is an average shear stress of the said double lap joined body, The joint strength evaluation method of the double wrap joined body of Claim 1 or Claim 2 characterized by the above-mentioned.   The double lap joint according to any one of claims 1 to 3, wherein the average shear stress is obtained from a shear stress at an end portion and a center portion in a width direction of the double lap joined body. Body strength evaluation method.   The said shear stress is calculated | required by the elastoplastic analysis, The joining strength evaluation method of the double lap joining body as described in any one of Claims 1-4 characterized by the above-mentioned.   6. The method for evaluating the joint strength of a double lap joint according to claim 5, wherein a Hill yield function is used in the elastic-plastic analysis.   The joint strength evaluation method for a double lap joined body according to any one of claims 1 to 6, wherein a comparative evaluation is made as to whether or not the shear stress is smaller than the allowable shear stress.   The said fiber reinforced composite material is any one of FRP, CFRP, and GFRP, The joining strength evaluation method of the double wrap joined body as described in any one of Claims 1-7 characterized by the above-mentioned.

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