Method for measuring internal residual strain and sensor for measuring the strain

Abstract

To provide a method for measuring an accurate residual strain even when a cutting piece is deformed, and to provide a sensor for providing a method for measuring the strain.SOLUTION: The present invention relates to a method for measuring the internal strain of a resin product, in which a strain gauge is set in a surface of a measurement site of the product and a sheet-like soft material is set on the strain gauge, and then, another strain gauge is further set on the soft material and a bridge circuit is connected between both gauges, and thereafter, the internal residual strain of the resin product is measured by the change in the electric resistance of the strain gauges before and after a part of the product is cut off.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for measuring residual strain inside a resin molded product and a sensor for measuring the residual strain. [Background technology]

[0002] Conventionally, methods for measuring residual strain inside a resin molded product have been known, including the hole drilling method and the surface layer sequential removal method, which are methods for measuring while destroying the resin molded product, as well as the photoelastic method, which does not destroy the resin molded product.

[0003] The hole drilling method is a method for measuring residual strain by first attaching a strain gauge to the area around the part of the resin molded product where the strain is to be measured, and then measuring the change in strain when a hole is drilled through the resin molded product in the thickness direction. This method is primarily used to measure residual strain in the flat parts of plate-shaped products or three-dimensional shaped products.

[0004] Patent Document 1 describes a residual stress calculation method in which an improvement on the hole drilling method is made to measure internal residual stress (residual strain) at a predetermined position of a resin molded product by the hole drilling method to obtain a residual stress result including a component derived from molding shrinkage in the thickness direction and a component derived from the molded product shape, and a cutting piece is cut out of only the peripheral portion of the predetermined position from the resin molded product to obtain an internal residual stress result at the predetermined position by the hole drilling method to obtain a residual stress result of only the component derived from molding shrinkage in the thickness direction, and then the residual stress derived from the molded product shape is calculated from the difference between the two results, thereby determining the residual stress derived from the molded product shape. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-46379 A Summary of the Invention [Problem to be solved by the invention]

[0006] According to this method, the residual stress inside a resin molded product can be calculated for each of the molded product shape-derived and molded shrinkage-derived residual stresses in the thickness direction at a given position of the resin molded product, regardless of the shape, location, or size of the resin molded product. Therefore, by modifying the product design to take into account the contributions of each of these, it is possible to effectively reduce the internal residual stress of the molded product.

[0007] However, with this measurement method, the cut pieces may be deformed when cut, which may cause errors in the measurement and may result in unstable measurements.

[0008] The first object of the present invention is to provide a method for measuring accurate residual strain even if the cut piece is deformed, and the second object is to provide a sensor for implementing the measurement method. [Means for solving the problem]

[0009] The present inventors have found that the above object can be achieved by:

[0010] 1. A method for measuring internal strain in a resin molded product, in which a strain gauge is placed on the surface of the molded product at the measurement location, a sheet-like soft material is placed on top of that, and another strain gauge is then placed on top of that, and a bridge circuit is then connected to both gauges, and the internal residual strain of the resin molded product is measured from the change in electrical resistance of the strain gauge before and after cutting a part of the molded product. 2. The measuring method according to 1 above, wherein the resin molded product is an insert molded product. 3. A sensor for measuring internal residual strain in plastic molded products, consisting of a sheet of soft material sandwiched between two strain gauges. Effect of the Invention

[0011] According to the present invention, it is possible to provide a method for measuring residual strain accurately even if the cut piece is deformed, and to provide a sensor for implementing this measuring method. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram showing a model of a resin molded product of the present invention. [Diagram 2] FIG. 2 is a diagram showing a model of strain that the resin molded product of the present invention has. [Diagram 3] 1A is a schematic diagram showing a general method for measuring strain in a resin molded product, and FIG. 1B is a schematic diagram showing a method for measuring strain in a resin molded product assuming the two-gauge method. [Figure 4] FIG. 1 illustrates a strain sensor according to the present invention. [Diagram 5] 1 is a photograph of a strain sensor of the present invention. [Figure 6] FIG. 2 is a schematic diagram showing a model shape A of the resin molded product of the present invention. [Figure 7] FIG. 2 is a schematic diagram showing a model shape B of the resin molded product of the present invention. [Figure 8] 1 is a graph comparing the results of the measurement method of the present invention and the results of the existing measurement method (model shape 1). [Figure 9] 1 is a graph comparing the results of the measurement method of the present invention and the results of the existing measurement method (model shape 2). [Figure 10] 4 is a conceptual diagram showing distortion that occurs after a rib is removed from the resin molded product of the present invention. FIG. [Figure 11] FIG. 2 is a conceptual diagram showing the positions and dimensions of a resin molded piece and a sensor according to the present invention. [Figure 12] FIG. 2 is a conceptual diagram showing a case where a flexible sheet is deformed due to dimensional changes and deformation changes caused by cutting a rib of the resin molded piece of the present invention. [Figure 13] FIG. 2 is a conceptual diagram showing the relationship between a resin molded piece and a soft sheet of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail. Figure 1 shows a model of a restrained resin molded product. Assuming a two-stage resin molded product with ribs on both ends, as shown in Figure 1(a), this model. When this resin molded product is produced by injection molding, even if the cooling of the mold is ideal, the crystallization will differ in each part due to the difference in thickness of the two stages and the thickness of the ribs on both ends. In addition, because the orientation is different, the difference in crystallization in each part causes internal strain due to the difference in shrinkage rate.

[0014] By cutting the ribs on both ends of Figure 1(a), the constraint of the resin molded product (Figure 1(a)) is released, and Figure 1(a) deforms in Figure 1(c), resulting in a distortion in a form that combines the dimensional change (Figure 2(d)) and deformation change (Figure 2(e)), as shown in Figures 2(d) and (e).

[0015] When measuring the strain in Figure 1(c), a single strain gauge cannot obtain individual measurement values ​​because it can only obtain the combined strain due to dimensional change and strain due to deformation, as shown in Figure 3. On the other hand, the two-gauge method and four-gauge method, which use bridge circuits, are known as methods for measuring strain in plate-shaped molded products.

[0016] In this method, strain gauges are placed on both the front and back of a plate-shaped resin molded product as shown in Figure 3(b), and when the resin molded product is subjected to tension (compression) or bending, only bending strain or only tensile (compression) strain is measured. It is not known to apply this method to measuring strain before and after cutting, but it can measure strain due to dimensional changes after cutting and strain due to deformation changes.

[0017] Measurements using the two-gauge method and four-gauge method are effective when strain gauges can be placed on both sides of a resin molded product, but cannot be used when strain gauges cannot be placed on either side. For example, they cannot be applied when the resin molded surface has an uneven surface, a step makes it impossible to attach a strain gauge, or there is no space to attach it. Also, in the case of resin molded products made by insert molding with metal, etc., it is not possible to attach a strain gauge in advance to the resin on the side that comes into contact with the insert metal, so they cannot be applied.

[0018] The present invention proposes a new measurement method that can obtain the same results as when strain gauges are placed on both the front and back sides of the resin molded product.

[0019] <Sensor for measuring internal residual strain in plastic molded products> The strain measurement sensor of the present invention is characterized by having a structure in which a sheet-like soft material (hereinafter also referred to as a soft sheet) is sandwiched between two strain gauges, as shown in Figure 4. Figure 5 is a photograph of the actual sensor.

[0020] The strain gauges may be any commonly used strain gauge. The two strain gauges may be the same or different, but it is preferable that they are the same.

[0021] It is important that the soft sheet sandwiched between the two strain gauges has a lower elastic modulus than the resin to be measured, and a material with a modulus of elasticity of preferably 1000 MPa or less, more preferably 100 MPa or less, and most preferably 50 MPa or less can be used.

[0022] Examples of materials for the soft sheet include thermoplastic elastomers such as olefin-based elastomers, styrene-based elastomers, polyester elastomers, and silicon-based elastomers, and synthetic rubbers such as styrene butadiene rubber, butadiene rubber, chloroprene rubber, acrylonitrile butadiene rubber, isoprene rubber, butyl rubber, and ethylene propylene rubber. If the above materials cannot be used due to melting during measurements at high temperatures, a heat-resistant thermoplastic resin having an elastic modulus of 1000 MPa or less at the measurement temperature may be used.

[0023] The area of ​​the soft sheet is preferably larger than the strain gauge as shown in Figure 5. The thickness is preferably 1 / 4 or less of the thickness of the resin product since it does not affect the strain value, and a thickness of about 50 to 1000 μm can be selected as desired in consideration of workability.

[0024] The sensor in Figure 5 was made using a strain gauge manufactured by Tokyo Measuring Instruments Co., Ltd. and a soft sheet made of ENEOS NUC's EEA NUC-6570 with a thickness of 0.7 mm (hereinafter referred to as sensor 1).

[0025] <Measurement principle> Next, the measurement principle is shown in Figures 10 to 13. Figure 10 shows the strain that occurs after the ribs are removed from a resin molded product to produce a resin molded piece. Sa and Sb are assumed to be strains caused by dimensional changes due to shrinkage, while Da and Db are assumed to be strains caused by deformation due to differences in orientation, etc. The combined strain amounts of the strain caused by the dimensional changes and the deformation changes that occur at this time are εa and εb, respectively. In this case, we assume that Sa = Sb = S, and Da + Db = 0. As a result, the total strains εa and εb can be expressed as follows. εa=S+Da Equation (1) εb=S+Db Formula (2) Therefore, if εa and εb are obtained as measured values, the strain due to dimensional change can be calculated using equation (3). S = (εa + εb) / 2 Equation (3) In addition, the strain caused by the deformation can be calculated using equation (4). Da = (εa-εb) / 2 Equation (4)

[0026] Next, consider the case where sensor 1 is used in FIGS. t is the thickness of the resin molded piece, t' is the thickness of the soft sheet, and ε0 is the combined strain of the strain due to deformation changes and dimensional changes in the soft sheet. Figure 12 shows the case where the soft sheet is deformed due to dimensional changes and deformation changes caused by cutting the ribs of the resin molded piece. Da' indicates the strain on one side of the resin molded soft sheet, and D0 indicates the strain on the opposite side. The deformation of the resin molded piece shown in Figure 12 also causes deformation of the soft sheet, with Da' generating tensile strain and D0 generating compressive strain, which can be expressed as D0 = -Da', just like in Figure 10. In addition, the total strain of the soft sheet can be expressed as ε0 = S + D0 and εa = S + Da', as in Figure 10. Therefore, the strain caused by the deformation of the soft sheet is Da'=-(ε0-εa) / 2 (5) It can be expressed as:

[0027] Next, the relationship between the resin molded piece and the soft sheet is shown in FIG. Because the thickness ratio t / t' and deformation ratio Da / Da' of the resin molded piece and the soft sheet are the same, Da = t / t' × Da' (6) It can be expressed as: Therefore, substituting equation (5) Da' = -(ε0 - εa) / 2 into equation (6), we get Da=t / t'×-(ε0-εa) / 2 (7) From this, the strain due to the deformation change of the resin molded piece can be calculated. In addition, the strain Sa=S due to the dimensional change of the resin molded piece is εa=S+Da according to Figure 10. S=εa-Da=εa+t / t'×(ε0-εa) / 2 (8) It can be calculated as follows.

[0028] <How to measure strain> The model shape shown in Figure 6 For A, a resin molded product having the shape shown in Table 1 was produced. ,The model shape shown in Fig. 7 B About Table 2 A resin molded product with the shape shown in is manufactured, and the distortion before and after cutting the rib shown in Figure 1 is measured using a contour shape measuring machine, which is an existing distortion measurement method. Compared to The results were compared (n=3). r is the radius of curvature of the step.

[0029] The values ​​in Figures 6 and 7 and Tables 1 and 2 are in mm. The strain measurement sensor is A Place the sample in the center of the thick side. B In the experiment, the sensor was placed in the center. The resin used was polyacetal resin (Duracon M90-44, manufactured by Polyplastics Co., Ltd.). The sensor used was sensor 1. The measurement atmosphere was 23°C and 50% RH.

[0030] [Table 1]

[0031] [Table 2]

[0032] The results are shown in Figures 8 and 9. In Figures 8 and 9, CDC is the result of the sensor of the present invention, and SCR is the strain value calculated from the existing contour shape curvature radius (equivalent to the measured value shown in Figure 3). In Figures 8 and 9, flow means the XX' direction, and perpendicular means the Y-Y' direction.

[0033] As can be seen from FIGS. 8 and 9, the measurement method using the sensor 1 of the present invention can obtain the same results as measurements using existing double-sided strain gauges, even though only one side is measured.

[0034] The measuring method of the present invention can be applied to strain measurement of metal-inserted resin molded products and the like, on which it is not possible to attach strain gauges on both sides.

Claims

1. A method for measuring internal strain in a resin molded product, comprising the steps of placing a strain gauge on the surface of a measurement portion of the resin molded product, placing a sheet-like soft material on top of that, placing another strain gauge on top of that, connecting a bridge circuit to both strain gauges, and measuring the internal residual strain in the resin molded product from the change in electrical resistance of the strain gauge before and after cutting a portion of the resin molded product (except in the case where the soft material has a function of blocking the strain detected by one of the two strain gauges from being transmitted to the strain detected by the other of the two strain gauges).

2. 2. The strain measuring method according to claim 1, wherein the resin molded product is an insert molded product.

3. An internal residual strain measurement sensor for a resin molded product, which has a sheet-like soft material sandwiched between two strain gauges (except for the case where the soft material has the function of blocking the strain detected by one of the two strain gauges from being transmitted to the other of the two strain gauges).

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