JP2016124125A - Method for producing fiber-reinforced plastic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing fiber-reinforced plastic including a shaping step with good reproductivity of a prepreg laminate in a shaping method which is referred to as hot forming.SOLUTION: A method for producing fiber-reinforced plastic includes: a laminate formation step of laminating a prepreg containing a reinforced-fiber impregnated with a thermosetting resin to form a flat plate-like prepreg laminate 1; a shaping step of pressing the prepreg laminate 1 to a male mold 5 and shaping the prepreg laminate; and a curing step of curing a thermosetting resin. In the shaping step, when a side fixing portion for fixing one side and its opposite side of a holding sheet 2 for holding the prepreg laminate by sandwiching it from both sides is connected to a first operation section for grasping and moving the side fixing portion, the first operation section is operated in a pushing direction of the prepreg laminate to the male mold without substantially causing no deflection, and thereby the prepreg laminate is shaped, an operation speed of the first operation section is controlled to be a range of 0.9-1.1 times of an average speed of the operation speeds.SELECTED DRAWING: Figure 9

Description

  The present invention relates to a method for producing a fiber reinforced plastic having a shaping step of shaping a prepreg laminate into a three-dimensional shape.

  Fiber reinforced plastics composed of reinforced fibers and matrix resins are attracting attention in industrial applications because they have high specific strength and specific elastic modulus, excellent mechanical properties, and high functional properties such as weather resistance and chemical resistance. Deployed in structural applications such as aircraft, spacecraft, automobiles, railways, ships, electrical appliances, sports, etc., the demand is increasing year by year. Among them, a high-quality fiber-reinforced plastic can be obtained by laminating a prepreg, which is an intermediate base material impregnated with a thermosetting resin, into continuous reinforcing fibers and performing pressure molding with an autoclave or the like.

  It is known that a shaping process for making a prepreg follow a three-dimensional shape before forming and curing by an autoclave or the like is an important process that determines the success or failure of member quality. Mainly in the lamination and shaping of aircraft parts, slit tape prepregs, which are obtained by cutting wide prepregs in the fiber direction and dividing them into narrow widths, are formed continuously using an automatic machine. A method called “auto fiber placement” is known (for example, Patent Document 1). By arranging the slit tape prepregs in a three-dimensional shape and shaping (tape layup), the narrow slit tape prepreg itself only needs to follow a two-dimensional shape, and can follow the shape even if it is a complicated shape. However, since the slit tape prepreg is thin, it takes time to arrange the slit tape prepreg in a desired shape, the productivity is low, the manufacturing cost is high, and the process for cutting the wide prepreg into a slit tape prepreg is increased. Therefore, there has been a problem that the material cost itself becomes high.

  Therefore, in order to achieve a high-productivity shaping process using an inexpensive wide prepreg, a prepreg laminate that has been laminated in a flat plate at high speed using an automatic machine is pressed against a mandrel with a bladder while applying heat. Thus, a shaping method called hot forming, which is shaped into a three-dimensional shape, has been developed (for example, Patent Document 2). However, there is a problem that wrinkles are often generated during shaping because it is sensitive to slight variations in process conditions and material variations. The wrinkles are inherited as defects even after curing, and the structural strength as a molded product may be reduced.

International Publication No. 2009/052263 International Publication No. 96/06725

  In view of the background art, an object of the present invention is to provide a method for producing a fiber reinforced plastic having a shaping step capable of shaping a prepreg laminate with good reproducibility under optimum conditions free from wrinkles.

  In order to solve such a problem, the present invention takes the following measures. That is, a laminate forming step of laminating a prepreg containing reinforcing fibers impregnated with a thermosetting resin to form a flat prepreg laminate, a shaping step of pressing the prepreg laminate against a male mold, and heat A fiber reinforced plastic manufacturing method including a curing step of curing a curable resin, and in a shaping step, one side of a grip sheet for gripping a prepreg laminate from both sides and a side fixing that fixes the opposite side Is connected to the first working part that moves by grasping the side fixing part, and the first working part is operated in a direction in which the prepreg laminated body is pressed against the male mold without substantial deflection. In forming the fiber-reinforced plastic, the operating speed of the first operating section is controlled to be in a range of 0.9 to 1.1 times the average speed of the operating speed.

  ADVANTAGE OF THE INVENTION According to this invention, a prepreg laminated body can be shape | molded with reproducibility on optimal shaping conditions, and a high quality fiber reinforced plastic can be manufactured.

It is a conceptual diagram of a state where a prepreg laminate is sandwiched between gripping sheets, the gripping sheet is fixed by a side fixing part, and the side fixing part is gripped by a first operating part. It is sectional drawing of the approximate center of the shaping apparatus using the set of FIG. It is the conceptual diagram which showed a mode that the edge fixing | fixed part and 1st action | operation part in FIG. 1 were made into the shape of a substantially male ridgeline, and the elastic body was provided between the 1st action | operation part and the edge fixation part. It is a conceptual diagram of the male type | mold which has a concave-shaped wall surface. It is a conceptual diagram of the male type | mold which has a convex-shaped wall surface. It is a conceptual diagram of the male type | mold which has the wall surface which combined concave shape and convex shape. It is sectional drawing of the approximate center of the shaping apparatus using the set of FIG. In the shaping using the guide A and the guide B, it is sectional drawing of the approximate center of the shaping apparatus which showed the mode before shaping. It is a conceptual diagram at the time of moving a 2nd operation part with the shaping apparatus shown in FIG. It is a conceptual diagram of the substantially central part cross section of a male type | mold and a 1st operation part. It is the schematic which shows the shape of guide B16-a which concerns on Example 1. FIG. 1 is a schematic diagram showing a male shape according to Example 1. FIG. 1 is a schematic view showing a prepreg laminate formed by the method according to Example 1. FIG. It is sectional drawing of the shaping apparatus which concerns on the comparative example 1.

  The present inventors stably produce high-quality preforms in a shaping process in which a prepreg laminate in which a plurality of prepregs containing reinforcing fibers impregnated with a thermosetting resin are laminated is pressed against a male mold to follow the shape. In order to do this, the mechanism of wrinkle generation was verified. As a result, in order to suppress wrinkles, it is important that the layers of the prepreg laminate slide smoothly, that the prepreg of each layer is easily shear-deformed in the plane, and that the prepreg of each layer has high bending rigidity. I found it. In addition, the prepreg is prone to shear deformation in the surface by raising the temperature, but there is a trade-off relationship such as lowering the bending rigidity, and wrinkles can occur depending on the shape and material characteristics to be shaped It has been found that there is an optimum process condition that satisfies the above prepreg characteristics in a well-balanced manner and minimizes the properties. In order to maintain optimum process conditions during shaping, it is preferred not to change the prepreg properties during the shaping process. Factors affecting the prepreg characteristics include the temperature applied to the prepreg, and the above-mentioned characteristics of the prepreg, which is a viscoelastic body, are speed-dependent, so the shaping speed is also important. Therefore, it is preferable to perform shaping while maintaining optimum temperature and speed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The prepreg is obtained by impregnating a reinforcing fiber such as carbon fiber with a thermosetting resin such as an epoxy resin. The reinforcing fibers may be arranged in one direction or may be in the form of a woven fabric. In general, a thermosetting resin is added with a curing agent in addition to the monomer component so that the thermosetting property is exhibited. Such prepregs are generally commercially available.

  A laminated body formation process is a process of laminating | stacking several prepregs and forming a flat prepreg laminated body. The thickness of the prepreg laminate may be constant, but the number of laminations varies depending on the location, and may include ply drops. When laminating prepregs, the adhesion between prepregs may be strengthened by placing the prepreg laminate in a space sandwiched between elastic bodies such as sheet-like rubber, and evacuating the space. Vacuuming may be performed every time one prepreg is stacked, or evacuation may be performed after all the prepregs are stacked. Since the prepreg laminate is flat, it is shaped into a preform having a desired shape through a shaping process, and a fiber-reinforced plastic is produced by a curing process.

  The shaping step is a step of obtaining a preform by pressing the prepreg laminate to a male mold and shaping. 1 and 2 show an example of a shaping process for shaping a prepreg laminate 1 obtained by a laminate forming process into a C-shaped cross-sectional preform such as one of aircraft structural members, such as a spar. Yes. The prepreg laminate 1 is held by a holding sheet 2 that sandwiches the prepreg laminate 1 from both sides. The gripping sheet 2 may be a general-purpose bag film. The space between the gripping sheets 2 is a sealed space, the pressure is reduced by degassing the sealed space with a vacuum pump or the like, and the prepreg laminate 1 is pressurized with a differential pressure from the atmospheric pressure. However, the shaping may be performed. The prepreg laminate 1 and the grip sheet 2 may be in contact with each other, but a secondary material such as a release film or silicone rubber may be inserted between the prepreg laminate 1 and the grip sheet 2.

  As shown in FIG. 1, the grip sheet 2 is fixed by a side fixing portion 3 that fixes one side of the grip sheet 2 and its opposite side. The opposite side means sides facing each other in a quadrilateral or the like. The side fixing part 3 may have a clip shape for holding the two holding sheets 2 sandwiching the prepreg laminated body 1, and may have a mechanism for fixing using a bolt or the like. The side fixing unit 3 is connected to the first operating unit 4 that moves by grasping the side fixing unit 3. A plurality of side fixing portions divided into each side may be provided and connected to a plurality of first operating portions.

  The prepreg laminate 1 thus gripped is shaped by moving the first working part 4 in the working direction 6 as shown in the sectional view of FIG. The operation direction 6 is an operation direction of the first operation unit 4 in which the prepreg laminate 1 is pressed against the male mold 5 without substantially bending. Here, the state having substantially no deflection means a state in which there is no deflection due to factors other than the weight of the prepreg laminate 1 and the grip sheet 2. Moreover, the 1st operation part 4 is an apparatus which can move the edge fixing | fixed part 3 to the operation direction 6, and may be a robot arm. By controlling the first working unit 4 in the working direction 6 to a substantially constant speed, that is, a speed in the range of 0.9 to 1.1 times the average speed, the adjustment at the speed selected as the optimum process condition is performed. You can execute the shape. In the present invention, the speed of the first operating part 4 is defined by the speed of the substantially central part of the first operating part 4.

  Furthermore, as a preferred embodiment of the present invention, when the first working part 4 is operated, a tensile load is applied to the prepreg laminated body 1 via the grip sheet 2, so that the prepreg laminated body 1 and the male contact part 8 are applied. It is preferable to apply a tensile load in the direction 9 toward the first working unit 4 from the grip sheet 2. The contact portion 8 shows a contact end portion with the male mold 5 in the prepreg laminate 1 during shaping (actually, the prepreg laminate 1 is in contact with the male mold 5 via the grip sheet 2). . Since the cause of wrinkle generation is a compressive load generated in the prepreg laminate, applying a tension is effective in suppressing wrinkles. Preferably, the tension should be kept substantially constant, i.e. 0.9 to 1.1 times the average value of tension, so as not to destroy the optimum process conditions.

  Further, as a preferred embodiment of the present invention, in order to apply a substantially constant tension to the gripping sheet 2, the side fixing part 10 and the first working part 11 are formed in a substantially male ridge line shape parallel to each other as shown in FIG. It is preferable that the elastic body 12 is provided between the side fixing portion 10 and the first working portion 11 and has a mechanism in which tension is applied to the grip sheet 2 when the elastic body 12 extends. The side fixing part 10 and the first working part 11 may have a linear shape or a curved shape in accordance with the ridgeline of the male wall surface 7. Specifically, for the concave male ridge line 13-a curved toward the inside of the male mold shown in FIG. 4, it is preferable to use the concave side fixing part 10 and the first working part 11. For the convex male ridge 13-b curved toward the outer side of the male shown in FIG. 5, the convex side fixing part 10 and the first working part 11 are preferably used. Further, for the male ridge line 13-c in which the convex shape and the concave shape are combined as shown in FIG. 6, the side fixing portion 10 and the first working portion having the shape in which the concave shape and the convex shape are combined are used. Is good. In order to increase the pressing force to the male mold, the concave part of the male ridge line is curved further inside the male mold than the male ridge line, and the male ridge line for the convex part of the male ridge line. Alternatively, the shape may be closer to a straight line. For example, when the male ridge line is represented by a curvature, the side fixing unit 10 and the first working unit 11 may have a curvature that is 0.9 to 1.1 times larger than the curvature. When the elastic body 12 is stretched, a tensile load is applied to the grip sheet 2 in the direction from the contact portion 8 of the prepreg laminate 1 and the male mold 5 toward the first working portion 11 as shown in FIG. It is preferable. The elastic body means a material that is elastic and easily stretched, such as rubber or a spring. If the elastic body 12 is in the form of a plate and the elastic modulus of the gripping sheet 2 itself is sufficiently high and the amount of elongation can be ignored, basically the tension is the product of the elastic modulus of the elastic body 12 and the amount of elongation of the elastic body 12. However, when the width of the elastic body 12 and the width of the grip sheet 2 are different, the tension can be calculated by the equation (1).

  When the load applied to the gripping sheet 2 depends on the number of springs, such as a spring, the tension per unit width applied to the gripping sheet 2 can be calculated by Expression (2). Such quantification of tension contributes to improving the reproducibility of shaping.

  Furthermore, as a preferred embodiment of the present invention, a guide B is fixed to a fixed guide A and a second operating part that operates at a constant speed in one direction toward the male mold, such as a press machine. It is preferable to guide the first working part at the intersection of Thereby, the speed which is an optimal shaping condition can be made constant during the shaping process without making a large investment such as a machine robot. Specifically, as shown in FIGS. 8 and 9, a guide A14 that guides and fixes the first working unit 10 and a guide B16 that is attached to the second working unit 15 that moves up and down (right side in FIGS. 8 and 9). The guide B16 is referred to as 16-a for convenience and the left side is referred to as 16-b). The guide A may be fixed to a male mold. As shown in FIGS. 8 and 9, the first operating unit 11 is designed to be guided to a place where the guide A14 and the guide B16 overlap. When the second operating unit 15 moves in the downward direction 17, as shown in FIG. 9, the overlapping portion of the guide A14 and the guide B16 moves along the guide A14, and the first operating unit 11 is also guided accordingly. The first operating unit may not have an operating mechanism in the operating unit itself. The shape of the guide A14 is preferably a circular arc or an elliptical arc. A plurality of guides A and guides B may be provided in order to stabilize the movement of the first working part, and the shape of all the guides A or guides B need not be the same.

  One of the advantages of this embodiment is that when pressing the prepreg laminate to the male mold, a jig corresponding to the bending angular velocity at the corner portion generated in the prepreg laminate and the speed of the second working portion can be freely designed. It is. FIG. 10 is a conceptual diagram of a cross-sectional view of a substantially central portion of the male mold 5 and the first working part 11, and in FIG. 10, an intersection of a straight line representing the male wall surface and a straight line 19 including the upper surface of the male mold, An angle formed by the straight line 20 passing through the first working unit 11 indicated by a point and the straight line 19 is a bending angle 21. The bending angular velocity is the angular velocity at the bending angle 21. For example, when the shape of the guide A14 is a circular arc having a radius L (mm), the guide B16 is obtained using the speed V (mm / min) of the second working part and the target bending angular speed C (° / min). The shape of −a can be determined by the shape represented by the equations (3) and (4) on the xy plane. Here, t represents time (min), and t ranges from 0 to 90 / C. When the male mold 5 and the prepreg laminate 1 to be shaped are symmetric, the shapes of the guide B16-b and the guide B16-a are preferably symmetric.

  Furthermore, as a preferred embodiment of the present invention, in order to further improve the reproducibility of shaping, it is preferable to keep the temperature of the entire prepreg laminated body uniform. Here, the uniform temperature throughout the prepreg laminate means that the difference between the maximum temperature and the minimum temperature within the prepreg laminate is within 20 ° C. For example, since the slipperiness between layers varies depending on the temperature, if there is a place where the temperature is locally inappropriate, the layers do not slip and wrinkles may occur. The shaping may be performed after heating for a certain time in the temperature-controlled chamber, or the temperature may be kept in the chamber during the shaping. You may shape while heating with a radiation heater, a contact-type heater, etc. The temperature may be measured indirectly from the outside of the grip sheet with a thermocouple, or a non-contact thermometer may be used.

  About the preform obtained through the shaping step, a fiber reinforced plastic is produced by passing through a curing step of curing the thermosetting resin. The curing step is performed by exposing the preform to a temperature at which the thermosetting resin is cured, and is usually performed by heating the preform. Heating may be performed while the preform is pressed against the male mold. The space surrounded by the grip sheet may be a sealed space connected to the vacuum pump, and may be cured while evacuating. The heat source may be a radiant heater, a contact heater, or the like, or may have a mechanism in which the male mold itself is heated.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to the invention as described in an Example.

Example 1
Shaping was performed by a method using guide A and guide B. First, a prepreg (P2352W-19) manufactured by Toray Industries, Inc. was cut at a size of 150 mm × 150 mm in the direction of 0 ° and 45 ° with respect to the fiber direction, and quasi-isotropic lamination [+ 45/0/90 / − one by one] 45] 2 s to produce a flat prepreg laminate. The holding sheet for holding the prepreg laminate was a general-purpose bag film, and silicone rubber and FEP (tetrafluoroethylene / hexafluoropropylene copolymer) film were prepared as auxiliary materials. The size of the bag film is 200 mm x 200 mm, the silicone rubber and FEP film are the same size as the prepreg laminate, and the order is bug film / silicone rubber / FEP film / prepreg laminate / FEP film / silicone rubber / bug film. A prepreg laminate was sandwiched between bag films. Adhesives called sealants were used to bond the bag films, and the space between the bag films was defined as a sealed space. However, at the center of one side of the bag film, a hose with a diameter of 3 mm is inserted between the sealant and the bag film and connected to a vacuum pump, and the pressure in the sealed space is reduced to 100 Pa until shaping is completed. The state was kept. The shape of the guide A is a circular arc with a radius of 100 mm, and the shape of the guide B corresponding to the guide B16-a is L = 100 mm, V = 20 mm / min, and C = 10 ° / min. The shape shown in FIG. 11 was calculated according to 4). By using the shape of the guide B, the angular velocity becomes 10 ° / min when the velocity of the second working part is 20 mm / min. Further, the angular velocity C when the velocity V of the second operating unit is changed can be calculated with C = 2V. The guide A and the guide B are respectively formed on a male surface 23 which is a surface different from the male wall 7 and a surface 24 which is opposite to the surface 23 in the same manner as the sectional view shown in FIG. Attached to both sides. Using this guide B, it was pressed into a stainless steel male mold shown in FIG. 12 and shaped into a C-shape so that the shaping was completed in C = 45 ° / min, that is, 2 minutes. The male wall surface of FIG. 12 has a concave and gently curved surface. The side fixing part 10 and the first operating part 11 are also in the shape of a curved bar shown in FIG. 13 in accordance with the shape of the male ridge line. When ten springs 22 having a spring constant of 0.49 N / mm are attached between the side fixing unit 10 and the first working unit 11 on each side, shaping can be performed with the springs 22 extended by 1.2 mm. At this time, a tension of 0.059 N / mm is applied to the bag film. Prior to shaping, the prepreg laminate was heated to 60 ° C. with a sheet heater, quickly moved to a male mold installed at room temperature, and shaped at the above speed. After shaping, the resulting C-shaped preform is moved into the chamber heated to 180 ° C together with the male mold, and maintained for 2 hours while maintaining the pressure in the sealed space sandwiched between bag films at 100 Pa. C-shaped fiber reinforced plastic was manufactured. When five prepreg laminates were prepared and shaped, four high-quality C-shaped fiber-reinforced plastics without wrinkles were obtained. For one sheet with wrinkles, arc-shaped wrinkles were generated on the surface in contact with the male mold of the prepreg laminate at the central corner in the width direction. There is a possibility that the temperature-controlled setting of the prepreg laminate to the male mold is delayed compared to the other four, and the prepreg laminate temperature has deviated from the optimum range.

(Example 2)
A fiber reinforced plastic was produced in the same manner as in Example 1 except for the heating means in the shaping step. That is, instead of the heating means for heating the prepreg laminate to 60 ° C. with a sheet heater before shaping, the male mold is heated in a chamber heated to 60 ° C. in advance, and the entire area of the prepreg laminate is 60 ° C. After adopting the heating means to keep in the chamber for 10 minutes, the same guide A, guide B and male mold as in Example 1 are used to form in the chamber, and after shaping, the temperature is increased to 180 ° C. Warmed and held for 2 hours to produce a fiber reinforced plastic. When five prepreg laminates were prepared and shaped, no wrinkles were produced on all the five sheets, and high-quality fiber-reinforced plastic was produced.

(Comparative Example 1)
With the apparatus shown in FIG. 14, the prepreg laminate obtained in Example 1 was shaped and cured. The sealed space 26 including the male mold and the prepreg shown in FIG. 12 is connected to a vacuum pump (not shown), and the pressure in the sealed space 26 is reduced to 100 Pa through the connection port 27 with the vacuum pump. The bag 25 is pressed against the male mold 28, and the prepreg laminate is shaped into a C shape. The temperature of the sealed space 26 can be adjusted by heating means (not shown). The prepreg laminate 1 is sealed with a gripping sheet (not shown), and is held in a state where the inside is evacuated. The vacuum pump was started, and shaping was performed so that shaping was completed in a C-shaped manner in 2 minutes in the sealed space 26 heated to 60 ° C. as in Example 1. As a result, at the same time when the vacuum pump was started, the prepreg laminate was pressed against the male mold and shaping started, but the bending angular velocity could not be controlled. After shaping, the inside of the sealed space 26 was heated to 180 ° C. with the vacuum pump started, and then held for 2 hours to cure the preform. All 5 out of 5 wrinkled wrinkles were generated inside the corners of the fiber reinforced plastic produced into a C-shape.

1: Pre-preg laminate 2: Grasping sheet 3: Side fixing part 4: First working part 5: Male mold 5-a: Male mold with concave wall surface 5-b: Male mold 5- with convex wall surface c: male mold 6 having a wall surface in which concave and convex shapes are combined 6: operating direction of the first working portion 7: male wall surface 8: contact portion 9 between the prepreg laminate and the male die 10: direction 10 in which tension is applied: Side fixing part of approximately male ridgeline shape
11: First male ridge line-shaped first working portion 12: Elastic body 13-a: Concave male ridge line 13-b: Convex male ridge line 13-c: Male combining concave and convex shapes Mold edge 14: Guide A
15: 2nd operation part 16-a: Guide B Right side 16-b: Guide B Left side 17: Movement direction of the 2nd operation part 18: Straight line 19 which shows the wall surface in the substantially center cross section of the male type | mold 5: Abbreviation of the male type | mold 5 Straight line 20 including the upper surface in the central cross section: intersection of straight line 18 and straight line 19, straight line 21 including first working part 11: bending angle 22: spring 23: one surface 24 of male mold 24: facing 25 of surface 23: expansion Bag 26: Sealed space 27: Connection port 28 with vacuum pump: Cross section of male mold in FIG.

Claims (6)

Laminate formation process for laminating prepregs containing reinforcing fibers impregnated with thermosetting resin to form a flat prepreg laminate, shaping process for pressing prepreg laminate to male mold, and thermosetting A method for manufacturing a fiber reinforced plastic including a curing step for curing a resin, and in the shaping step, a side fixing portion for fixing one side of the grip sheet for gripping the prepreg laminate from both sides and a side opposite to the side. The prepreg laminate is shaped by operating the first operative portion in a direction in which the prepreg laminate is pressed against the male mold substantially without bending. At the time, the manufacturing method of the fiber reinforced plastic which controls the operating speed of the 1st operating part in the range of 0.9 times to 1.1 times the average speed of the operating speed. The manufacturing method of the fiber reinforced plastics of Claim 1 which provides tension | tensile_strength to a holding | grip sheet | seat in the direction which goes to a 1st operation part from a contact part of a prepreg laminated body and a male type | mold. The method for producing a fiber-reinforced plastic according to claim 2, wherein the tension applied to the grip sheet is maintained in a range of 0.9 to 1.1 times the tension. The side fixing part and the first operating part have a substantially male ridge shape, and have an elastic body between the first operating part and the side fixing part, and the tension is the elastic modulus of the elastic body and the extension amount of the elastic body. The manufacturing method of the fiber reinforced plastics of Claim 2 or 3 calculated by a product. The first operating part is guided at the intersection of the fixed guide A and the guide B fixed to the second operating part operating at a constant speed in one direction toward the male mold. A method for producing the fiber-reinforced plastic according to claim 1. The method for producing a fiber-reinforced plastic according to any one of claims 1 to 5, wherein the temperature is adjusted so that the temperature of the entire prepreg laminate is constant.