JP2012202454A - Fiber-reinforced plastic spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber-reinforced plastic spring which prevents breakage due to compressive stress.SOLUTION: An FRP spring 1 is a leaf spring having a laminated structure 20, for instance. The laminated structure 20 is a three layered structure formed of a first fiber layer 21, a second fiber layer 22 on a compression side, and a second fiber layer 23 on a tension side, for instance. The first fiber layer 21 has tensile elastic modulus E. The second fiber layer 22 on a compression side and the second fiber layer 23 on a tension side have tensile elastic modulus Esmaller than the tensile elastic modulus E. The first fiber layer 21 consists of a compressive stress region 21A which is an upper region with respect to the neutral axis Sa and a tensile stress region 21B which is a lower region with respect to the neutral axis Sa. The neutral axis Sa is positioned on a compressive stress generation region side from the center of a plate thickness direction. Since the thickness from the neutral axis Sa to a surface on the compressive stress generation region side becomes smaller, compression deformation when being subjected to a pulsating bending load can be made smaller.

Description

  The present invention relates to a fiber-reinforced plastic spring to which a single swing bending load is applied, and more particularly to a technique for preventing breakage due to compressive stress.

  For example, in the automotive field, unidirectional springs (spiral springs, mainsprings, leaf springs, etc.) that are subjected to bending loads are used, and these springs are required to be lightweight and space-saving. For example, in order to reduce the weight, it has been proposed to use a fiber reinforced plastic spring (hereinafter referred to as an FRP spring) instead of a metal spring.

  For example, the technique of Patent Document 1 discloses an FRP tapered leaf spring as an FRP spring. In this technique, a plurality of sheets having different lengths are impregnated with glass fiber or carbon fiber, and the sheets are overlapped to form a tapered leaf. Manufactures springs. Further, the technology of Patent Document 2 discloses an FRP leaf spring as an FRP spring, and in that technology, the leaf center portion is made of carbon fiber, and the leaf surface portion is made of glass fiber, thereby having flexibility. Propose to manufacture leaf springs.

Japanese Examined Patent Publication No. 3-81022 JP-A-7-77231

  By the way, as shown in FIG. 4, when a one-way bending load P is applied to the leaf spring 51 supported by the support portion 52, compressive stress is generated on the upper surface on the load load side, which is opposite to the load load side. Tensile stress is generated on the lower surface. Reference sign S is a neutral shaft located at the center of the plate spring 51 in the plate thickness direction.

  For example, when a one-way bending load (load in the direction of the arrow in the figure) is applied to a carbon fiber reinforced plastic spring (CFRP spring) having a single layer structure as an FRP leaf spring, the compressive strength of the CFRP spring is 1 of the tensile strength. Since it is low at about / 2 to 1/3, buckling occurs due to compression, and breakage tends to occur in the compressive stress generation region. Since the fracture is caused by such a low load, the feature of CFRP excellent in tensile strength cannot be fully utilized, and the energy density of the available spring is substantially reduced.

  In such an FRP spring, an effective technique has not been developed for preventing breakage from the compressive stress side surface. For example, in the technique of Patent Document 1, a plurality of stacked sheets use the same fiber, and a technique for preventing fracture from the surface on the compression stress side is not disclosed. Moreover, in the technique of patent document 2, although the flexible leaf | plate spring is disclosed, it does not pay attention to the compression characteristic of a fiber. Further, the leaf surface portion is made of glass fiber, and it is not efficient to dispose glass fiber having lower tensile strength than carbon fiber on the leaf surface portion having high stress.

  Therefore, an object of the present invention is to provide a fiber-reinforced plastic spring that can prevent breakage due to compressive stress.

  The fiber-reinforced plastic spring of the present invention (hereinafter referred to as FRP spring) is a fiber-reinforced plastic spring to which a swinging bending load is applied, and the neutral shaft is located closer to the compression stress generation region than the center in the plate thickness direction. It is characterized by that.

The neutral axis in the present invention is an axis that does not generate tensile stress and compressive stress and does not stretch or contract. The tensile modulus of elasticity in the present invention is expressed by the following relational expression using the first straight part (the straight part passing through the origin or the tangent at the origin of the curve) in the tensile stress-strain curve obtained in the tensile test. (Reference: FRP Design Handbook, Japan Reinforced Plastics Association, 1979).
E = Δσ / Δε
E is the tensile modulus (unit: N / mm ² ), Δσ is the stress difference (unit: N / mm ² ) due to the average original cross-sectional area between two linear points, and Δε is the strain between the two points. It is a difference.

  In the FRP spring of the present invention, the neutral axis is located closer to the compressive stress generation region than the center in the plate thickness direction, and therefore the thickness from the neutral axis to the surface on the compressive stress generation region side is reduced. As a result, the compressive stress generation region is reduced, so that the compressive deformation at the time of a one-way bending load can be reduced. Therefore, destruction due to compressive stress can be prevented. In addition, since the tensile stress generation region becomes large, the tensile deformation at the time of applying a swinging bending load increases. However, since the FRP spring is strong against the tensile deformation, it is possible to prevent breakage due to the tensile stress. By effectively utilizing the characteristics of the FRP spring that is excellent in tensile strength in this way, it is possible to prevent breakage due to the generated stress. Therefore, since the breaking stress of the whole spring can be increased, the available energy density can be increased.

Various configurations can be used for the FRP spring of the present invention. For example, a laminated structure in which fibers having different tensile elastic moduli are laminated can be used. Specifically, the first fiber layer, the second fiber layer formed on each of the surface on the compressive stress generation region side and the surface on the tensile stress generation region side of the first fiber layer, the Nth fiber layer (N comprises a natural number of 2 or more) and a tensile modulus of the first fibrous layer E _1, the tensile modulus E _N of the tensile modulus E ₂ ,,, N th fiber layer of the second fiber layer is reduced in this order A mode set as described above can be used. In this aspect there the first N fibrous layer disposed in a surface portion of the spring having a minimum tensile modulus E _N, since the first N fibrous layer easy bending, effectively preventing the destruction of such breakage by buckling can do.

Such as tensile modulus stress strain diagram of a fiber reinforced plastic having a E _1, the tensile stress strain diagram of a fiber reinforced plastic having a stress-strain diagram ,,, tensile modulus E _N of the fiber-reinforced plastic having a modulus of elasticity E ₂ Is obtained by a tensile compression test, and a strain energy ratio V′j (= U′jt / U′jc, j) between the tensile strain energy U′jt and the compressive strain energy U′jc is obtained from each of the stress strain diagrams. Is a natural number satisfying 1 ≦ j ≦ N), and the tensile strain energy U1t of the tensile stress generation region of the first fiber layer and the compression stress generation region of the first fiber layer when a predetermined one-way bending load is applied. The strain energy ratio V1 (= U1t / U1c) with the compressive strain energy U1c is equal to the strain energy ratio V′1 and the tension is applied when a predetermined one-way bending load is applied. The strain energy ratio Vj between the tensile strain energy Ujt of the jth fiber layer on the generation region side and the compression strain energy Ujc of the jth fiber layer on the compression stress generation region side is strain (when Ujt / Ujc, j is 2 or more). A mode in which the position of the neutral axis is set to be equal to the energy ratio V′j can be used.

  In the above aspect, the position of the neutral axis can be set by setting the thickness of each fiber layer having different tensile elastic modulus corresponding to the stress distribution in the FRP spring at the time of the one-way bending load. Therefore, the characteristics of the FRP spring excellent in tensile strength can be used sufficiently effectively. Therefore, the breaking stress of the entire spring can be further increased, and as a result, the available energy density can be further increased.

For example, the compressive strain energy Ujc and the tensile strain energy Ujt are calculated based on the mathematical formulas 1 to 3, and the compressive stress is generated in the first fiber layer so that the strain energy ratio V′j is equal to the strain energy ratio Vj. The thicknesses h _1c and h _{1t of} the region and the tensile stress generation region, and the thicknesses h _jc and h _jt (when j is 2 or more) of the jth fiber layer on the compression stress generation region side and the tensile stress generation region side The set aspect can be used. In addition, since the strain energy due to the shear is smaller than the strain energy due to the bending in the mathematical formula 1, the strain energy due to the shear can be ignored.

なお、Ｍは曲げモーメント、Ｅは引張弾性率、Ｉは断面二次モーメント、κは形状係数、Ｑはせん断力、Ｇはせん断弾性係数、Ａはばねの断面積、ｌはばねの長さ、ｘは長さ方向の座標軸、ｂはばねの幅である。

M is a bending moment, E is a tensile modulus, I is a secondary moment of section, κ is a shape factor, Q is a shearing force, G is a shear elastic modulus, A is a sectional area of the spring, l is a length of the spring, x is the coordinate axis in the length direction, and b is the width of the spring.

なお、σｊｃは圧縮応力、ρは中立軸の曲率半径、ηは中立軸を原点としたときの厚さ方向の座標軸、ｈ_０ｃは０である。

Σjc is the compressive stress, ρ is the radius of curvature of the neutral axis, η is the coordinate axis in the thickness direction when the neutral axis is the origin, and h _0c is 0.

なお、σｊｔは引張応力、ρは中立軸の曲率半径、ηは中立軸を原点としたときの厚さ方向の座標軸、−ｈ_０ｔは０である。

Σjt is the tensile stress, ρ is the radius of curvature of the neutral axis, η is the coordinate axis in the thickness direction when the neutral axis is the origin, and −h _0t is 0.

  According to the FRP spring of the present invention, it is possible to prevent the breakage due to the generated stress, so that the breakage stress of the whole spring can be increased, and thereby the available energy density can be increased.

The structure of the fiber-reinforced plastic spring which concerns on one Embodiment of this invention is represented, (A) is a perspective view, (B) is a side view. It is a figure for demonstrating the method of determining the thickness of each fiber layer of the fiber reinforced plastic spring which concerns on one Embodiment of this invention, (A) is a figure for demonstrating numerical calculation, Comprising: The figure showing the state of bending, (B) is a stress strain diagram of the fiber reinforced plastic having the same tensile elastic modulus as the first fiber layer, (C) is the fiber reinforced plastic having the same tensile elastic modulus as the second fiber layer. It is a stress strain diagram. It is a sectional side view showing the partial composition of an example of the fiber reinforced plastic spring concerning one embodiment of the present invention. It is a figure for demonstrating the stress distribution in the conventional fiber reinforced plastic spring at the time of the bending load of one swing.

  Embodiments of the present invention will be described below with reference to the drawings. 1A and 1B show a configuration of a fiber-reinforced plastic spring (hereinafter referred to as an FRP spring) according to an embodiment of the present invention, in which FIG. 1A is a perspective view and FIG. 1B is a side view. FIG. 2 is a view for explaining a method for determining the thickness of each fiber layer of the fiber-reinforced plastic spring according to the embodiment of the present invention.

  The FRP spring 1 is a leaf spring having, for example, a leaf portion 11 and an eyeball portion 12. The FRP spring 1 includes a laminated structure 20 in which, for example, the neutral axis Sa is located closer to the compressive stress generation region than the center in the plate thickness direction. The neutral axis Sa is an axis in which tensile stress and compressive stress are not generated when a single swing bending load is applied, and the fiber does not stretch or contract. In FIGS. 1 and 2, the upper surface of the FRP spring 1 is a surface to which a one-way bending load (symbol P in FIG. 4) is applied, and the upper region with respect to the neutral axis Sa of the laminated structure 20 is a compressive stress that generates a compressive stress. The lower region with respect to the neutral axis Sa of the laminated structure 20 is a tensile stress region where tensile stress is generated. 2A indicates the plate thickness of the laminated structure 20 of the FRP spring 1, and η indicates the coordinate axis in the thickness direction when the neutral axis Sa is the origin.

  The laminated structure 20 is a three-layer structure having, for example, a first fiber layer 21, a compression-side second fiber layer 22, and a tension-side second fiber layer 23. The first fiber layer 21, the compression side second fiber layer 22, and the tension side second fiber layer 23 are, for example, UD (one direction) fiber layers in which fibers are oriented in the longitudinal direction of the spring.

The first fibrous layer 21 has a tensile modulus _{E 1.} The compression side second fiber layer 22 and the tension side second fiber layer 23 have a tensile elastic modulus E ₂ smaller than the tensile elastic modulus E ₁ . The neutral axis Sa is located in the first fiber layer 21, for example. The first fiber layer 21 has a compressive stress region 21A that is an upper region with respect to the neutral axis Sa and a tensile stress region 21B that is a lower region with respect to the neutral axis Sa.

As each fiber layer 21-23 of the laminated structure 20, a prepreg can be used, for example. The resin may be either thermosetting or thermoplastic. The fiber layers 21 to 23 of the laminated structure 20 may be formed by a filament winding method. As the fibers constituting the fiber layers 21 to 23 of the laminated structure 20, for example, carbon fibers, glass fibers, aramid fibers (Kevlar fibers), boron fibers, or the like can be used. As the carbon fiber, for example, both PAN-based and pitch-based can be used. A tensile modulus E ₁ of the first fibrous layer 21, to the tensile modulus E ₂ of the second fibrous layer 22 and 23 is set differently, for example it may be changing the type of fibers, the spring You may change the orientation direction of the fiber with respect to the longitudinal direction. Further, instead of the UD fiber layer, a cloth fiber layer in which fibers are arranged so as to intersect at a predetermined angle may be used.

  In the FRP spring 1 of the present embodiment, the compression-side second fiber layer 22 is disposed on the surface of the first fiber layer 21 on the compression stress generation region side, and the tensile fiber generation surface side of the first fiber layer 21 is on the tensile side. The 2nd fiber layer 23 is arrange | positioned and the FRP spring 1 is a case where N is 2 in the FRP spring of this invention. In the FRP spring 1, the position of the neutral axis Sa that is located closer to the compressive stress generation region than the center in the plate thickness direction is set as follows, for example.

  For example, in the simple bending state of the FRP spring 1 shown in FIG. 2A, a bending moment is generated in each of the fiber layers 21-23. The bending moment M1c due to the compressive stress σ1c in the compressive stress generation region 21A of the first fiber layer 21 is the case where j = 1 in the mathematical formula 2, and is represented by the mathematical formula 4. The bending moment M1t due to the tensile stress σ1t of the tensile stress generation region 21B of the first fiber layer 21 is the case where j = 1 in the mathematical formula 3, and is expressed by the mathematical formula 5.

  The bending moment M2c due to the compressive stress σ2c of the compression-side second fiber layer 22 is a case where j = 2 in the mathematical formula 2, and is represented by the mathematical formula 6. The bending moment M2t due to the tensile stress σ2t of the tension-side second fiber layer 23 is a case where j = 2 in the mathematical formula 3, and is represented by the mathematical formula 7.

  By substituting the bending moment M1c in the compressive stress generation region 21A of the first fiber layer 21 into Equation 1, the compressive strain energy U1c is calculated, and the bending moment M1t in the tensile stress generation region 21B of the first fiber layer 21 is calculated. By substituting into Equation 1, the tensile strain energy U1t is calculated. Thereby, a strain energy ratio V1 (= U1t / U1c) between the tensile strain energy U1t of the tensile stress generation region 21B of the first fiber layer 21 and the compressive strain energy U1c of the compression stress generation region 21A of the first fiber layer 21 is obtained. It is done.

  Further, by substituting the bending moment M2c at the compression-side second fiber layer 22 into Equation 1, the compression strain energy U2c is calculated, and the bending moment M2t at the tension-side second fiber layer 23 is substituted into Equation 1. Thus, the tensile strain energy U2t is calculated. Thereby, the strain energy ratio V2 (= U2t / U2c) between the tensile strain energy U2t of the tension side second fiber layer 23 and the compression strain energy U2c of the compression side second fiber layer 22 is obtained.

Stress-strain diagram is obtained by performing a tensile compression test to the respective tensile fiber reinforced plastic having a modulus of elasticity E ₁ and tensile fiber reinforced plastic having a modulus of elasticity E _2. FIG. 2 (B), the stress strain diagram of a fiber reinforced plastic having a tensile modulus E _1, FIG. 2 (C) is a stress-strain diagram of a fiber reinforced plastic having a tensile modulus E _2. In FIG. 2B, −ε _1c is a compressive strain at the time of breaking the fiber reinforced plastic, and ε _1t is a tensile strain at the time of breaking the fiber reinforced plastic. In FIG. 2C, −ε _2c is a compressive strain at the time of breaking the fiber reinforced plastic, and ε _2t is a tensile strain at the time of breaking the fiber reinforced plastic.

The compressive strain energy is calculated by integrating the stress strain diagram from the origin to the compressive strain. Specifically, the compression strain energy U′1c of the fiber reinforced plastic having the tensile elastic modulus E ₁ is the area of the mesh portion on the compression side in FIG. 2B, and the fiber reinforced plastic having the tensile elastic modulus E _2. The compressive strain energy U′2c is the area of the mesh portion on the compression side in FIG. The tensile strain energy is calculated by integrating the stress strain diagram from the origin to the tensile strain. Specifically, the tensile strain energy U′1t of the fiber reinforced plastic having the tensile elastic modulus E ₁ is the area of the mesh portion on the tensile side in FIG. 2B, and the fiber reinforced plastic having the tensile elastic modulus E _2. The tensile strain energy U′2t is the area of the mesh portion on the tension side in FIG.

The strain energy ratio V′1 (= U′1t / U′1c) between the tensile strain energy U′1t and the compressive strain energy U′1c of the fiber reinforced plastic having the tensile modulus E ₁ is calculated, and the tensile modulus E The strain energy ratio V′2 (= U′2t / U′2c) between the tensile strain energy U′2t and the compressive strain energy U′2c of the fiber reinforced plastic having ₂ is calculated.

In the design of the FRP spring 1, the strain energy ratio V1 obtained by numerical calculation is equal to the strain energy ratio V′1 obtained by the stress-strain diagram obtained by the tensile compression test, and the strain obtained by numerical calculation is obtained. The thickness h _1c of the compressive stress generation region 21A of the first fiber layer 21 and the generation of tensile stress so that the energy ratio V2 is equal to the strain energy ratio V′2 obtained from the stress-strain diagram obtained by the tensile compression test. The thickness h _1t of the region 21B is set, and the thickness h _{2c of} the compression side second fiber layer 22 and the thickness h _2t of the tension side second fiber layer 23 are set. In this case, each fiber layer is set by setting the sum of the thicknesses of all the layers 21 to 23 (= h _1c + h _1t + h _2c + h _2t ) to be a predetermined plate thickness H of the laminated structure 20. The specific value of the layer thickness is obtained.

  FIG. 3 is a side sectional view showing a partial configuration of an example of the FRP spring 1. For example, carbon fiber is used as the fiber, the tensile elastic modulus of the fiber of the first fiber layer 21 is 210 GPa, the tensile elastic modulus of the fibers of the compression side second fiber layer 22 and the tensile side second fiber layer 23 is 150 GPa, and the fiber volume content When Vf is set to 67% and the plate thickness H of the spring is set to 15 mm, the plate thickness of the first fiber layer 21 is set to 4 mm, and the neutral shaft is set to a position 5.6 mm from the surface on the compression generation region side. The plate thickness of the side second fiber layer 22 may be set to 2 mm, and the plate thickness of the tension side second fiber layer 23 may be set to 9 mm.

  In the present embodiment, since the neutral axis Sa is located closer to the compression stress generation region than the center in the plate thickness direction, the thickness from the neutral axis Sa to the surface on the compression stress generation region side is reduced. As a result, the compressive stress generation region is reduced, so that the compressive deformation at the time of a one-way bending load can be reduced. Therefore, destruction due to compressive stress can be prevented. Further, since the tensile stress generation region becomes large, the tensile deformation at the time of applying a swinging bending load becomes large. However, since the FRP spring 1 is strong against the tensile deformation, it is possible to prevent the breakage due to the tensile stress. As described above, by effectively utilizing the characteristics of the FRP spring 1 having excellent tensile strength, it is possible to prevent breakage due to generated stress. Therefore, since the breaking stress of the whole spring can be increased, the available energy density can be increased.

In particular, the compression-side second fibrous layer 22 and the tensile side second fibrous layer 23 is disposed in a surface portion of the spring, the second fibrous layer 22 and 23 because easy to bend with minimum tensile modulus E _2, the seat Breakage such as breakage due to bending can be effectively prevented. Further, the position of the neutral axis Sa is set by setting the thickness of each fiber layer 21 to 23 having different tensile elastic modulus corresponding to the stress distribution in the FRP spring 1 at the time of a one-way bending load. Therefore, the characteristics of the FRP spring 1 having excellent tensile strength can be sufficiently effectively used. Therefore, the breaking stress of the entire spring can be further increased, and as a result, the available energy density can be further increased.

  DESCRIPTION OF SYMBOLS 1 ... FRP spring (fiber reinforced plastic spring), 20 ... Laminated structure, 21 ... First fiber layer, 21A ... Compression stress region, 21B ... Tensile stress region, 22 ... Compression side second fiber layer, 23 ... Tensile side first 2 fiber layers, Sa ... neutral axis

Claims (4)

In a fiber reinforced plastic spring to which a single swing bending load is applied,
A fiber-reinforced plastic spring characterized in that the neutral axis is located closer to the compressive stress generation region than the center in the plate thickness direction. A first fiber layer; a second fiber layer formed on each of the surface on the compression stress generation region side and the surface on the tensile stress generation region side of the first fiber layer; and an Nth fiber layer (N is 2 or more) Natural number)
Tensile modulus E ₁ of the first fibrous _layer, the tensile elastic modulus E _N of the second fibrous layer of the tensile modulus E ₂ ,,, the first N fiber layer that is set to be smaller in that order The fiber-reinforced plastic spring according to claim 1. Stress strain diagram of fiber reinforced plastic having tensile modulus E ₁ , Stress strain diagram of fiber reinforced plastic having tensile modulus E ₂ , Stress strain of fiber reinforced plastic having tensile modulus E _N A diagram is obtained by a tensile and compression test, and a strain energy ratio V′j (= U′jt / U ′) between the tensile strain energy U′jt and the compressive strain energy U′jc is obtained from each of the stress strain diagrams. jc, j is a natural number satisfying 1 ≦ j ≦ N),
Strain energy ratio V1 between the tensile strain energy U1t of the tensile stress generation region of the first fiber layer and the compressive strain energy U1c of the compression stress generation region of the first fiber layer when a predetermined one-way swing load is applied. U1t / U1c) is equal to the strain energy ratio V′1 and the tensile strain energy Ujt of the j-th fiber layer on the tensile stress generation region side when the predetermined swinging bending load is applied and the compression stress generation region The position of the neutral axis is set so that the strain energy ratio Vj (= Ujt / Ujc, where j is 2 or more) with the compression strain energy Ujc of the j-th fiber layer on the side becomes equal to the strain energy ratio V′j. The fiber-reinforced plastic spring according to claim 2, wherein the spring is made of fiber-reinforced plastic. The compressive strain energy Ujc and the tensile strain energy Ujt are calculated based on mathematical formulas 1 to 3,
Thicknesses h _1c and h _{1t of} the compression stress generation region and the tensile stress generation region of the first fiber layer, and the compression so that the strain energy ratio V′j and the strain energy ratio Vj are equal. The fiber reinforcement according to claim 3, wherein thicknesses _jc and _hjt (when j is 2 or more) of the jth fiber layer on the stress generation region side and the tensile stress generation region side are set. Plastic spring.

M is a bending moment, E is a tensile modulus, I is a secondary moment of section, κ is a shape factor, Q is a shearing force, G is a shear elastic modulus, A is a sectional area of the spring, l is a length of the spring, x is the coordinate axis in the length direction, and b is the width of the spring.

Σjc is the compressive stress, ρ is the radius of curvature of the neutral axis, η is the coordinate axis in the thickness direction when the neutral axis is the origin, and h _0c is 0.

Σjt is the tensile stress, ρ is the radius of curvature of the neutral axis, η is the coordinate axis in the thickness direction when the neutral axis is the origin, and −h _0t is 0.

Images