JP2007147348A - Fracture toughness testing method for light-weight sandwich panel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To establish a test method capable of quantifying the fracture toughness value of a sandwich panel, which becomes the planning index of the fracture behavior and energy-absorbing behavior of a sandwich panel member, governing the safety of an automobile hood, having head protective function for a pedestrian or the like, under a condition that is close to practical conditions. <P>SOLUTION: To the two skins, made of a fiber reinforced plastic and the rectangular sandwich panel test piece, of which the core having a specific gravity lower than that of the skins is present between two skins via an adhesive layer, pre-cracking reaching from one skin to the other skin to completely dividing the core or pre-cracking for dividing 80% or more of the core is provided. The cracking is developed so as to peel the core and the skins by a three-point or four-point bending test; the load and the displacement of a load point and the development quantity of the cracking length at the development of cracking are measured; and the energy used in fracture is calculated from a load-displacement curve and is divided by the product of the average crack length and width to calculate the fracture toughness value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a lightweight sandwich panel made of a fiber reinforced plastic (hereinafter abbreviated as FRP) skin and a core of foam material or honeycomb material having a specific gravity lower than that of the skin, used for sports members and automobile parts (see FIG. 1). It relates to the fracture toughness test method.

  Lightweight sandwich panels consisting of FRP skins and foam and honeycomb cores with a lower specific gravity than skins have high specific rigidity (stiffness per weight), so there is a strong demand for weight reduction, including automobiles. Although application is expanding to transportation equipment members and sports members, it is important to accurately predict fracture behavior and to design members in order to ensure the safety of the members.

As shown in FIG. 1, a sandwich panel usually has a three-layer structure (a structure in which a core is sandwiched (sandwiched) with a skin) composed of two upper and lower skin plates and a core positioned therebetween, and has higher rigidity. Material on the outside of the panel (skin) and lighter material on the inside of the panel (core)
Thus, there is a feature that the bending rigidity can be increased with light weight. A metal sheet or a thin plate made of FRP is used for the skin, and a honeycomb material or foam material that is lighter than the skin is used for the core, which is used as a structural member such as a hull or deck of a yacht or boat. In recent years, in the automobile field where there is a great demand for improving fuel consumption, full-scale application of lightweight sandwich panels, such as hoods and roofs, using FRP as a skin, foam material and core as a means for weight reduction is being studied. .

  The hood is a cover that covers the engine room. The functions of the hood include rain protection and air rectification, as well as functions such as surface rigidity to give a high-class feel and bending deformation during a frontal collision. In recent years, in addition to these functions, pedestrian head protection performance is required and a head impact test has been conducted. In Japan, the pedestrian head protection standard has been introduced by the Ministry of Land, Infrastructure, Transport and Tourism in September 2005. In order to secure the pedestrian head protection performance with the conventional design using steel, it is necessary to increase the clearance with the engine in order to increase the stroke of deformation of the hood at the time of collision, and the bonnet position becomes high. If the bonnet position is raised, the field of view deteriorates, and it is necessary to raise the entire vehicle height in order to secure the field of view. Increasing the vehicle height has an impact on performance such as a decrease in fuel consumption due to weight increase. When a foam core sandwich panel using a plastic foam material with foamed plastic as a core is applied to an automobile hood, it is expected to reduce the weight of the hood alone, but the degree of freedom in rigidity design and energy absorption design is increased. It becomes possible to secure the pedestrian head protection performance while suppressing the height of the bonnet position.

On the other hand, when a sandwich panel is used as a structural member, first, strength and rigidity tests are performed in accordance with standards such as Japanese Industrial Standards (JIS). For example, the static stiffness and strength of a sandwich panel are measured according to the three-point or four-point bending test shown in FIGS. 2 and 3 (see Non-Patent Document 1 and Non-Patent Document 2). Know the fracture toughness value, which is the amount of energy absorbed when the sandwich panel breaks, as in FRP, when designing more practical materials such as energy absorption or when evaluating the ease of progress of skin and peeling cracks, etc. From this point of view, there is a need for a fracture toughness measurement method for designing the amount of energy absorption of a sandwich structure member. However, a test standard for evaluating the fracture toughness value of sandwich panels has not been established so far. Although it is conceivable to measure the fracture toughness value of a sandwich panel using a test standard for evaluating the fracture toughness value used for FRP (see Non-Patent Document 3 and Non-Patent Document 4), Non-Patent Documents 3 and 4 are sandwiches. As is clear from the fact that it is not specified that it can be applied to panels, sandwich panels that exhibit complex fracture behavior may exhibit fracture behaviors that are different from those expected for the operation of members. In some cases, it is not feasible, and it is difficult to accurately obtain the toughness value necessary for performing the energy absorption design.
JIS K7074-1988 "Bending test method for carbon fiber reinforced plastic" ASTM C393-00 “Standard Test Method for Flexural Properties of Sandwich Constructions” JIS K7086-1993 "Test method for interlaminar fracture toughness of carbon fiber reinforced plastics" ASTM D5528-94a “Standard Test Method for Mode I Interlaminar Fracture Toughness of Uniform Fiber-Reinforced Polymer Matrix

  The present invention quantifies the fracture toughness value of sandwich panels, which is a design index for sandwich panel members that control safety, such as automobile hoods with a pedestrian head protection function, under practical conditions. The purpose is to propose a test method that can be used. By establishing a test method for accurately measuring the fracture toughness of the skin, core, and adhesive layer constituting the sandwich panel, a lightweight sandwich panel member with higher reliability and safety can be realized.

  (1) Two skins made of fiber reinforced plastic and a sandwich panel test piece having a specific gravity core lower than the skin through an adhesive layer between the two skins. Reach the other skin and insert a pre-crack that completely divides the core, or a pre-crack that divides the core by 80% or more, so that the core and skin are peeled off by 3-point bending or 4-point bending test. Measure the load, displacement of load point, and crack length when the crack progresses, calculate the energy used for fracture from the load-displacement curve, and calculate the average crack length and width. The fracture toughness test method for lightweight sandwich panels, which calculates the fracture toughness value by dividing by the crack area, which is the product of

  (2) The fracture toughness test method for a lightweight sandwich panel according to (1), wherein the pre-crack is formed by a three-point bending test or a four-point bending test.

  According to the present invention, it becomes possible to quantify the fracture toughness value of the sandwich panel under conditions close to the load assumed in practical use of the member, so that the fracture behavior prediction and energy absorption design of the member are facilitated. For example, by applying the fracture toughness test method of the present invention to an automobile hood member using a sandwich panel, the optimum design of pedestrian protection performance is facilitated. Moreover, by designing based on such data, the reliability of the member is improved and the safety of the member itself is remarkably improved.

  The fracture toughness test and the test for introducing a precrack in the present invention can be performed by either a three-point bending test (FIG. 3) or a four-point bending test (FIG. 4). The embodiment will be described by a test method using a three-point bending test.

  First, the lightweight sandwich panel 1 is lower than two FRP skins 2 made of fiber reinforced plastic (hereinafter abbreviated as FRP: Fiber Reinforced Plastic), and a skin made of a plastic foam material or a honeycomb made of plastic. It is composed of a core 3 having a specific gravity and an adhesive layer 4. The core 3 is located between the FRP skins, and the adhesive layer is located between the skin and the core (FIG. 1).

  As shown in FIG. 2, the sandwich panel test piece 5 used in the present invention is a rectangular sandwich panel having a length 6, a width 7 and a thickness 8, and a pre-crack 22 is inserted before a fracture toughness test described later. (FIG. 8). The pre-crack serves as a trigger for generating a crack described later.

  The FRP skin 2 is a flat plate made of carbon fiber reinforced plastic (usually called CFRP), glass fiber reinforced plastic (usually called GFRP), and aramid fiber reinforced plastic (usually called AFRP). Yes, plastics such as epoxy resins and polyester resins are reinforced with carbon fibers that are reinforcing fibers. In addition to the above-described epoxy resin and polyester resin, thermosetting resins such as vinyl ester resins and phenol resins, and thermoplastic resins such as polypropylene resins, polyamide resins, polycarbonate resins, and polyetherimide resins are used as plastics. The reinforcing fibers are inorganic fibers such as carbon fibers and glass fibers, organic fibers such as aramid fibers (Kevlar, Twaron, etc.), high-strength polyethylene fibers, PBO fibers, etc. The specific gravity of the reinforcing fibers is that of aluminum, which is a light metal. Less than 7. Moreover, since the specific gravity of resin is in the range of 1.0 to 1.5, the specific gravity of FRP is also lower than that of aluminum. Among them, CFRP in which carbon fiber and epoxy resin are combined at a weight ratio of 4: 6 to 7: 3 has the highest rigidity and strength per weight and is the most preferable material in the present invention. Incidentally, the elastic modulus of carbon fiber is usually 200 to 800 GPa, the strength is 3 GPa to 8 GPa, and greatly exceeds the elastic modulus 70 GPa and strength 0.2 to 0.8 GPa of aluminum. Moreover, the specific gravity of carbon fiber is about 1.5 to 2.0, and is lighter than the specific gravity of 2.7 of aluminum. Furthermore, the specific gravity of the epoxy resin is 1.2 to 1.3, and the specific gravity of CFRP in which the resin is reinforced with carbon fiber is much lower than that of aluminum.

  The core 3 is a low density material such as a honeycomb material such as an aluminum honeycomb, a paper honeycomb, an aramid honeycomb or a carbon honeycomb, a plastic foam material obtained by foaming a resin such as urethane, polymethacrylimide, acrylic, phenol or polystyrene, or a balsa material. Is used. Specific examples of cores include aluminum honeycomb from Showa Airplane Co., Ltd., Nomex honeycomb using DuPont's Nomex, Rohmel from Rohm, and formal from Sekisui Chemical. The specific gravity of the core 3 is lower than that of the FRP skin 2 described above, and is 0.6 or less. Sandwich panels combined with the above skins are significantly lighter than metal materials. In addition, the specific gravity of the sandwich panel preferable in the present invention is in the range of 0.03 to 0.6, and the above-described plastic foam material is preferably used as the core.

  The thickness of the core 3 is 3 mm to 20 mm, and the thickness of the FRP skin 2 is 0.2 mm to 4 mm. In the sandwich panel 1, the skin and the core are integrated with the adhesive layer 4 as shown in FIG. By increasing the thickness of the sandwich core or by using a skin having high rigidity, the rigidity of the sandwich panel can be improved, and the material has high bending strength and bending rigidity per weight.

  The fracture toughness value in the present invention means that energy is required when a material breaks and a crack progresses, but the energy required for this crack progress divided by the crack progress area, the unit area required for crack progress It shows the energy per hit.

  The interlaminar fracture toughness value of FRP laminates can be measured by the DCB test, ENF test, etc. described in JIS K7086-1993. In the DCB test, a crack is opened in a direction perpendicular to the crack as shown in FIG. 6 (mode I) In the ENF test, as shown in FIG. 7, a load 20 called mode II in which a shearing force in the crack direction is applied is applied to the crack tip.

  In the fracture toughness test method of the present invention, the energy used when the skin and core of the sandwich panel peel and the crack progresses is divided by the crack propagation area as the fracture toughness value. Since both mode II stresses are applied, a mixed mode of mode I and mode II is obtained. Here, the crack growth area is obtained by multiplying the average crack growth length and the width of the test piece. The average crack growth length is the average of the crack growth lengths at both ends. The crack growth length may be measured along the crack or measured along the skin, but it is more preferable to calculate along the skin when evaluating the performance as a sandwich material. Cracks may develop at the interface between the skin and the adhesive layer, or may progress inside the core. If the crack propagates inside the core, it avoids these if there is a mass of foreign matter or adhesive in the core. To do. Calculation of energy used for destruction is obtained from the load displacement diagram. FIG. 13 shows an example of a load displacement diagram. If a crack develops between point A and point B in FIG. 13 and the crack area increases by a, the shaded area ΔU corresponds to the energy used to advance the crack, and ΔU is divided by a. This is the fracture toughness value Gc. In the DCB test described in JIS K7086-1993, the mode I fracture toughness value (GIc) is measured. In the ENF test, the mode II fracture toughness value (GIIc) is measured. In this test method, mode I and mode II are measured. The fracture toughness value (Gc) in the mixed mode is measured. As the crack progresses, the mixed ratio of the mode I and mode II stresses at the crack tip changes, so that the apparent fracture toughness value changes as the crack progresses.

  In the present invention, it is preferable to use a sandwich panel having a plastic foam material as a core (foam core sandwich panel). Foam core sandwich panel is autoclave molding method, hand lay-up molding method, pultrusion molding method, RTM molding (Resin Transfer Molding), Va-RTM molding (VaRTM: Vacuum Assisted Resin Transfer Molding, RTM molding method using vacuum pressure) However, in order to prevent the resin from being impregnated deep inside the core during molding, it is preferable that the core is closed-cell foamed. Even in this case, when cut out from a larger plastic foam material, the cells on the surface are connected to the outer space, and the cells on the surface may be impregnated with the resin (FIG. 5). Although the resin layer impregnated on the core surface functions as the adhesive layer 4, it is considered that this resin layer affects the fracture toughness value of the sandwich panel.

  The pre-crack 22 serves as a trigger for generating a crack described later. The pre-cracking method consists of dividing the core in advance at the pre-cracking position and molding it with a highly releasable film, or making holes in the core of the sandwich panel after molding, There is a method of making a pre-crack with such a blade, etc., but a method of making a pre-crack 22 by breaking the core by a shearing force by a three-point bending test (FIG. 3) or a four-point bending test (FIG. 4) with a short span. Is preferred. In this case, it is preferable to set the span so that the distance 12 between the fulcrums is 2 to 4 times the thickness 8 of the sandwich panel. The shape of the pre-crack 22 is such that the core is divided by 80% or more and each divided core is connected to the skin on both sides. As a preferred shape (when the cross section viewed from the width direction of the sandwich panel test piece is observed), the portion where the core is divided is linear, and the angle between the straight line and the two skins is 30 degrees or more and less than 60 degrees. A shape is mentioned (FIG. 8). Furthermore, when considering the destruction assumed in the operation of the member, particularly when a plastic foam material is used as the core, it is preferably within a range of 40 to 50 degrees where the shearing force is maximized. Further, the pre-crack 22 can be tested by a method such as a quadrangular shape (FIG. 9) or a triangular shape (FIG. 10). Furthermore, it is also preferable to make the precrack 22 using a three-point bend (or a four-point bend) close to the fracture load condition assumed in the operation of the member (FIG. 11).

  In the fracture toughness test according to the present invention, the above-mentioned fracture toughness test specimen is subjected to a three-point bending test (or a four-point bending test), and a crack is developed from the tip of the precrack 22 by gradually applying a load (FIG. 12). Quantifying the fracture toughness value when the crack progresses from the crack area, load, and displacement of the load point obtained by multiplying the crack length 23 when the crack progresses by the width 7 of the test piece (FIG. 2) . Although the fracture toughness test is possible in both the three-point bending test and the four-point bending test, the following describes the three-point bending test. The same test can be performed for the 4-point bending test.

  As for the span, the distance between the fulcrums needs to be 4 times or more of the thickness of the sample, and preferably 10 times or more. If it is less than 10 times, it is difficult to secure a space for cracks to propagate. However, if the span is too long, the crack may progress at once without gradually progressing (unstable fracture), and it may be difficult to evaluate the absolute value of the toughness value. The extent to which unstable fracture occurs is dependent on the sample configuration. Moreover, it is most preferable to carry out under the load condition assumed in the operation of the member. The position of the precrack can be tested at any location between the fulcrums, but it is preferable that the tip of the precrack is in contact with the center fulcrum in order to secure a space for the crack to propagate. This is because in the case of an impact load, cracks often occur at locations where the impact load is applied. However, when the crack occurrence position assumed in the operation of the member is different from this position, it is preferable to arrange the pre-crack at the expected crack position. Although it is necessary to change the speed of the testing machine depending on the size of the span and the rigidity of the test piece, it is preferable to perform the test with a span of 200 to 300 mm and 0.5 to 50 mm / sec as a guide. A preferable test condition is that carbon fiber cloth CO6343B (carbon fiber elastic modulus is 230 GPa) above and below a core (specific gravity 0.1) made of an independently foamed acrylic foam material (formac HR1000 manufactured by Sekisui Chemical Co., Ltd.) having a thickness of 6 mm. (Strength is 3.53 GPa) 2 layers at a time, bag the core and carbon fiber cloth, and vacuum the inside of the bag, then inject a mixture of liquid bisphenol A type epoxy resin and acid anhydride curing agent Then, when 90 ° C. × 40 minutes heat is applied in the oven and cured (RTM molding method), and the cured sandwich panel thickness is 7.2 mm, the span is 200 mm. From above to 300 mm or less, the speed of the testing machine was 0.5 mm.

Example 1
Carbon fiber cloth BT70-30 (carbon fiber elastic modulus is 230 GPa, strength is 4) on the top and bottom of a core (specific gravity 0.1) made of an independently foamed acrylic foam material (formac HR1000 made by Sekisui Chemical Co., Ltd.) having a thickness of 6 mm. .9 GPa) are placed in two layers, the core and carbon fiber cloth are bagged, the inside of the bag is vacuumed, and a mixture of liquid bisphenol A type epoxy resin and acid anhydride curing agent is injected into the oven. It was cured by applying heat at 90 ° C. for 40 minutes (VaRTM: Vacuum Assisted Resin Transfer Molding, RTM molding method using vacuum pressure). The sandwich panel after curing (size 500 mm × 500 mm) had an average thickness of 51 points of 7.21 mm (maximum thickness 7.4 mm, minimum thickness 7.0 mm).
From this sandwich panel, using a diamond cutter, cut out five sandwich panels for a bending fatigue test having a width of 22 mm and a length of 300 mm, using an Instron testing machine 5565, a load cell 2525-805 (maximum load 5 kN), A three-point bending load such as 9 was applied to pre-crack. The span at this time was 60 mm, the radius of curvature of the support side indenter was 2 mm, and the radius of curvature of the load side indenter was 5 mm. Further, since the skin breaks due to the concentrated load of the indenter on the load side, the silicon rubber 24 having a thickness of 1 mm is sandwiched between the test pieces of the indenter as shown in FIG.
As shown in FIG. 15, the pre-crack 22 was a pre-crack on the side surface of the test piece so that the part where the core was divided was linear, and the angle between the straight line and the two skins was 45 degrees. Both ends of the pre-crack extended near the interface with the skin inside the core, and when the pre-crack length 27 was measured in the direction along the skin, the average was 15.9 mm (minimum 8.18 mm, maximum 18.73 mm). Measure the toughness value of the sandwich panel by increasing the span and applying a three-point bending load to the specimen with pre-crack as described above, gradually developing the crack, recording the load, displacement, and crack length. did. The span at this time was 200 mm, the diameter of the indenter on the support side was 25 mm, and the diameter of the indenter on the load point side was 4 mm. Silicon rubber was not sandwiched between the load point side indenter and accurate displacement of the point could not be measured, but silicon rubber was not sandwiched, but the skin was not cracked by concentrated load. The cracks extended near the interface with the skin inside the core, and a load was applied until the average crack length finally reached 83.3 mm (minimum 78.9 mm, maximum 85.8 mm). An outline of the load displacement diagram is shown in FIG. Although the apparent value of the fracture toughness value increases as the crack progresses, it is considered that the mixing ratio of mode I and mode II has changed. When the average value of fracture toughness values in each sample (the average value of fracture toughness values from the start to the end of crack growth in the graph of FIG. 13) is further averaged by the number of samples 5, it is 1.23 J / m ² (minimum 0.90 J / M ² , maximum 1.64 J / m ² ).

(Example 2)
A sandwich panel is formed in the same manner as in Example 1 using a core obtained by cutting grooves 28 having a width of 1.5 mm and a depth of 1.5 mm on the surface of the core used in Example 1 at intervals of 10 mm. Then, as shown in FIG. 18, the test piece was cut out so that the crack direction and the length direction of the test piece were perpendicular to each other. When a fracture toughness test was conducted in the same manner as in Example 1 using three test pieces, the fracture toughness values (average values from the start to the end of crack growth) were 2.61 kJ / m ² and 1.58 kJ /. m ² , 1.53 kJ / m ² and an average of 1.91 kJ / m ² were obtained. Compared with Example 1, it is increased by 0.68 kJ / m ^2, and it seems that groove processing has an influence. When observing the crack propagation location, it is considered that the crack progresses while bypassing the groove as shown in FIG. 19 and the fracture area of the core is increased.

(Comparative Example 1)
Using a diamond cutter from the same sandwich panel as in Example 1, a sandwich panel for bending fatigue test having a width of 22 mm and a length of 300 mm was cut out, the skin on one side was peeled off 75 mm from the end of the test piece, and ENF described in JIS K7086-1993 With reference to the test method, as shown in FIG. 16, the skin on the peeling side was set to the load point side, and a fracture toughness test was conducted by a three-point bending with a span of 200 mm. 17) A significant decrease in rigidity occurred, making it impossible to continue the experiment. The core cracked in the same manner even when the skin on the peeling side was the supporting point side.

It is a perspective view which shows an example of the sandwich panel of the to-be-measured body in this invention. It is a perspective view which shows the sandwich panel test piece of the to-be-measured body in this invention. It is a figure which shows an example of a 3 point | piece bending test method. It is a figure which shows an example of a 4-point bending test method. It is an enlarged view near the skin of a foam core sandwich panel. It is a schematic diagram of the DCB test described in JIS K7086-1993. It is a schematic diagram of the ENF test described in JIS K7086-1993. It is a figure which shows an example of the precrack of the to-be-measured object test piece in this invention. It is a figure which shows an example of the precrack of the to-be-measured object test piece in this invention. It is a figure which shows an example of the precrack of the to-be-measured object test piece in this invention. It is a figure which shows an example of the method of putting a precrack in the to-be-measured object test piece in this invention. It is a figure which shows one embodiment of the fracture toughness test method of this invention. It is a figure which shows an example of a load displacement diagram required in order to calculate the fracture toughness value in this invention. It is a figure which shows an example of the method of putting a precrack in the to-be-measured object test piece in this invention. It is a figure which shows an example of the precrack of the to-be-measured object test piece in this invention. It is a figure which shows the fracture toughness test method performed with reference to the ENF test. It is a figure which shows the result of the fracture toughness test method performed with reference to the ENF test. It is a side view which shows an example of the fracture test piece used for the fracture toughness test method of this invention. It is a side view which shows an example of what the crack extended to the test piece as a result of performing the fracture toughness test method of this invention.

Explanation of symbols

1: Sandwich panel 2: FRP skin 3: Core
4: Adhesive layer 5: Sandwich panel test piece 6: Length (for sandwich panel test piece) 7: Width (for sandwich panel test piece) 8: Thickness (for sandwich panel test piece) 9: Upper fulcrum 10: Lower fulcrum 11: Bending test jig 12: Distance between supporting points 13: Load 14: CFRP skin 15: Cell of foam core 16: Cell of foam core impregnated with resin 17: Test piece (ENF, DCB)
18: Load 19: Mode I deformation direction 20: Load 21: Mode II deformation direction 22: Precrack 23: Crack length 24: Silicon rubber 25: Linear crack 26: Precrack 27: Crack length 28: Groove 29: Groove 30: Groove 31: Crack 32: Crack circumventing the groove

Claims (2)

  Two skins made of fiber reinforced plastic and a rectangular sandwich panel specimen in which a core having a specific gravity lower than the skin exists between the two skins through an adhesive layer, from one skin to the other skin , And a pre-crack that completely divides the core or a pre-crack that divides the core by 80% or more is added, and the crack progresses so that the core and the skin are peeled by a three-point bending or four-point bending test. Measure the load when the crack propagates, the displacement of the load point, and the crack length, calculate the energy used for the fracture from the load-displacement curve, and calculate this as the product of the average crack length and width. A fracture toughness test method for lightweight sandwich panels that calculates the fracture toughness value by dividing by a certain crack area.   The method for testing the fracture toughness of a lightweight sandwich panel according to claim 1, wherein the pre-crack is formed by a three-point bending test or a four-point bending test.

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