JP7381264B2 - How to detect cracks - Google Patents

Description

The present invention relates to a crack detection method. In particular, the present invention relates to a method for detecting cracks in resin molded products containing polyacetal resin.

Polyacetal resin has an excellent balance of mechanical strength, chemical resistance, sliding properties, and abrasion resistance, and has good processability, so it is used as a typical engineering plastic for electrical and electronic equipment. It is used in a wide range of mechanical parts, automobile parts, and other mechanical parts. Furthermore, as the fields of use for polyacetal resin expand, the required performance is becoming higher and higher.

On the other hand, it is known that when a solid deforms or breaks, the strain energy that has been stored up to that point is released and a portion of it is radiated as elastic waves (AE waves). Such a phenomenon is called acoustic emission (AE). This AE detection method is described in Non-Patent Document 1 and the like.

Journal of the Japan Society for Precision Engineering Vol.78, No.10, 2012, Fundamentals of Acoustic Emission Measurement

Polyacetal resin is being considered for use in vehicle components, particularly fuel pump modules. Vehicles are often cleaned using detergents at gas stations and the like. However, it has been found that when a detergent containing acid is used as a detergent, cracks may occur when stress is applied to parts inside the vehicle. If such a component inside the vehicle is a fuel pump module, the damage can be serious.
Until now, the presence or absence of cracks in molded articles formed from polyacetal resin has generally been confirmed by visual inspection. Specifically, a polyacetal resin or a resin composition containing a polyacetal resin is molded into an ISO dumbbell test piece, etc., the ISO dumbbell test piece is artificially distorted, and acid is dropped in that state to prevent drying. They were left undisturbed for several days and checked to see if any cracks had formed.
However, this method has a problem in that it takes time to determine whether or not cracks have occurred. In addition, the occurrence of cracks includes minute cracks that are difficult to visually confirm, and there is also a problem in that it is difficult to detect such cracks.
The present invention aims to solve this problem, and aims to provide a detection method that can appropriately detect the occurrence of cracks in a resin molded product containing polyacetal resin.

Based on the above-mentioned problem, the present inventor conducted studies and found that the occurrence of cracks can be appropriately detected by confirming the occurrence of predetermined vibrations in the AE signal. Specifically, the above problem was solved by the following means.
<1> A method for detecting cracks caused by contact with chemical substances that may occur in a resin molded product, wherein the resin molded product contains polyacetal resin as a main component, and the resin molded product is processed by an acoustic emission method. The method includes detecting a crack by confirming the presence of a peak value A of an acoustic emission signal observed at a frequency of 0.01 to 2.5 MHz, and the peak value indicates that the resin molded product before coming into contact with the chemical substance. A detection method in which the value is 1.7 times or more the peak value B of an acoustic emission signal in the frequency range of 0.01 to 2.5 MHz.
<2> A method for detecting cracks that may occur in a resin molded product, wherein the resin molded product has a polyacetal resin as its main component, and the resin molded product is heated at a frequency of 0.01 to 2.5 MHz by an acoustic emission method. A detection method comprising detecting a crack by confirming that at least one of the acoustic emission signals observed in the acoustic emission signal is greater than or equal to 11.4 mV.
<3> The detection method according to <1> or <2>, wherein the crack is a crack resulting from contact with an acid.
<4> The detection method according to <3>, wherein the acid is hydrochloric acid.
<5> The detection method according to any one of <1> to <4>, wherein the resin molded product is artificially strained and the presence or absence of potential cracks is confirmed.

The present invention has made it possible to provide a detection method that can appropriately detect the occurrence of cracks in polyacetal resin molded products. In particular, it has become possible to provide a detection method that can appropriately detect the occurrence of cracks resulting from contact with acids.

FIG. 1 is a schematic diagram showing an example of an AE measurement system.

The content of the present invention will be explained in detail below. In addition, in this specification, "~" is used to include the numerical values described before and after it as a lower limit value and an upper limit value.
In this specification, various physical property values and characteristic values are assumed to be at 23° C. unless otherwise stated.

The detection method of the present invention is a method for detecting cracks resulting from contact with chemical substances that may occur in a resin molded product, wherein the resin molded product has polyacetal resin as its main component, and the resin molded product is made of an acoustic material. The method includes detecting cracks by confirming the presence of a peak value A of an acoustic emission signal observed at a frequency of 0.01 to 2.5 MHz by an emission method, and wherein the peak value is detected before contact with the chemical substance. It is characterized by a value that is 1.7 times or more the peak value B of the acoustic emission signal in the frequency range of 0.01 to 2.5 MHz of the resin molded product.
With such a configuration, it becomes possible to appropriately detect the occurrence of cracks in the polyacetal resin molded product. In particular, it becomes possible to appropriately detect the occurrence of cracks that are difficult to detect visually.
In particular, it becomes possible to appropriately detect the occurrence of cracks resulting from contact with chemical substances, and furthermore, cracks resulting from contact with acids.
In the present invention, the term "cracks that may occur in a resin molded product" includes not only cracks that actually occur in the resin molded product, but also cracks that could not be visually confirmed until now, such as minute cracks.

For example, as shown in Figure 1, the AE measurement system includes an AE sensor that detects AE waves and converts them into AE signals, an amplifier (preamplifier) that amplifies weak AE signals, an AE analyzer, and a computer that displays and stores data. It consists of an oscilloscope and an oscilloscope. The AE analysis device has a built-in filter that extracts a signal in a necessary frequency band, and after filtering, the AE signal is amplified again by an amplifier and converted into each AE evaluation parameter through signal processing.
In the present invention, it has been discovered that the occurrence of cracks in a polyacetal resin molded product can be detected by measuring the AE signal at a predetermined AE frequency.
In addition, the AE sensor is a broadband AE sensor AE-900M-WB or AE-900H-200-WP manufactured by NF Circuit Design Block, and the AE analysis device is an AE tester AE9501A or AE9922 manufactured by NF Circuit Design Block. Can be used. AE testers typically measure the number of AE signals that occur during a period of 0.1 ms. As the oscilloscope, Picoscope manufactured by Pico technology can be used. Moreover, other than these devices, devices having equivalent performance may be used.

Specifically, by frequency-analyzing the AE signal waveform, the main frequency of the AE signal can be evaluated. It is known that the AE frequency differs depending on the phenomenon that contributes to the generation of the AE signal, and by analyzing the AE frequency of cracks, it is possible to explore the behavior that leads to the generation of cracks.

The AE signal waveform is an alternating current signal whose voltage changes between positive and negative. By half-wave rectifying the negative portion of this signal waveform, an envelope detection waveform is obtained. A pulse wave is output only for the time that exceeds this envelope detection waveform. By counting these pulse waves, an AE counting parameter can be obtained.
The AE count rate (times/sec) is used to evaluate the number of AE occurrences per unit time, that is, the frequency of AE occurrence. The total number of AE counts (times) is a parameter for evaluating the number of AE occurrences during the total test time.

In the present invention, cracks are detected by confirming the presence of a peak value A of an acoustic emission signal observed at a frequency of 0.01 to 2.5 MHz. The peak value A is the acoustic emission in the frequency range of 0.01 to 2.5 MHz of a resin molded product before contact with a chemical substance, that is, after being molded into a resin molded product and not in contact with a chemical substance. This value is 1.7 times the peak value B of the signal.
Peak value A and peak value B each refer to the average value of the peak of the acoustic emission signal in the frequency range of 0.01 to 2.5 MHz of the resin molded product. This peak value is the average value of the peaks calculated after noise values are removed by the AE analyzer.
The value of "1.7 times" is based on the peak value B of the AE signal in the frequency range of 0.01 to 2.5 MHz of a resin molded product formed from the same polyacetal resin as the inventor of the present invention. This is a value obtained empirically as a result of measuring the peak value A of the AE signal in the frequency range of 0.01 to 2.5 MHz after applying a chemical substance such as hydrochloric acid to a resin molded product.
The peak value A exceeding the peak value B may be observed once at a frequency of 0.01 to 2.5 MHz, but it is preferably observed twice or more. The peak value B is, for example, 11.4 mV.
In one embodiment of the detection method of the present invention, an acoustic emission signal observed at a frequency of 0.01 to 0.3 MHz is 11.4 mV or more, and an acoustic emission signal observed at a frequency of 0.3 to 2.5 MHz is Cracks are detected by confirming that the voltage is 11.4 mV or more.
In particular, by confirming that the acoustic emission signal observed in the frequency range of 0.01 to 0.3 MHz is 11.4 mV or higher, initial fine cracks can be detected, and acoustic emission signals observed in the frequency range of 0.3 to 2.5 MHz can be detected. It is presumed that by confirming that the signal is 11.4 mV or higher, it is possible to detect a crack just before breaking.
In the present invention, the occurrence of cracks can be confirmed without visual inspection because the AE signal is used to accurately grasp the misalignment between molecules. In particular, in the past, cracks had to be visually confirmed, which required the generation of cracks to a level that could be visually confirmed, which took time. Furthermore, no fine cracks could be detected. The present invention solves these problems.

The resin molded product whose cracks are to be detected in the present invention has polyacetal resin as its main component.
Polyacetal resin is a polymer having a repeating acetal structure -(-O-CRH-) _n - (where R represents a hydrogen atom or an organic group), and is usually an oxymethylene group where R is a hydrogen atom. The main structural unit is (-CH ₂ O-). As the polyacetal resin, the descriptions in paragraphs 0009 to 0013 of JP2019-123836A, the descriptions in paragraphs 0016 to 0029 of JP2019-099654A, and the descriptions in paragraphs 0013 to 0021 of JP2019-065233A, The contents thereof are incorporated herein by reference.
A molded article whose main component is polyacetal resin is 50% by mass or more, preferably 60% by mass or more of the molded article, more preferably 70% by mass or more, still more preferably 80% by mass. The above means that more preferably 90% by mass or more is polyacetal resin. Moreover, 100% by mass of a molded article whose main component is polyacetal resin may be polyacetal resin.
Moreover, a molded article containing polyacetal resin as a main component may contain components other than polyacetal resin. Specifically, the description in paragraph 0019 of JP2019-123836A and paragraphs 0080 to 0108 of JP2019-065233A can be referred to, and the contents thereof are incorporated into the present specification.

The detection method of the present invention can be used to evaluate the performance of resin compositions (for example, pellets) containing polyacetal resins that are in the middle of development, are planned to be marketed, or are on the market. More specifically, a resin composition containing a polyacetal resin is molded into a test piece, and detection can be performed by the detection method of the present invention.
The test piece is preferably an ISO dumbbell test piece. However, the shape of the test piece may be any shape as long as the AE signal can be measured.
Furthermore, in the present invention, depending on the application, it is preferable to artificially create distortion and check for potential cracks. A latent crack is a crack that has not yet occurred as a crack, but may occur due to the presence of a crack seed or stress applied depending on the application. With such a configuration, it is possible to check whether or not cracks have occurred, assuming the form after actually being used as a resin molded product.
The present invention is suitable for measuring cracks resulting from contact with chemical substances. Contact with a chemical substance refers to cracks that occur as a result of damage to a resin molded article caused by the resin molded article coming into contact with a chemical substance. Examples of chemical substances include acids, alkalis, and solvents. Further, specific examples include fuel, detergent, and the like. The present invention is suitable for measuring the occurrence of cracks caused by acid, and is particularly suitable for measuring the occurrence of cracks caused by hydrochloric acid. For example, it is possible to artificially adjust the strain to 0.5 to 2.0% and check the occurrence of cracks when acid is applied. Preferably, the acid is a 5-20% by weight aqueous solution.

Resin compositions that pass the detection method of the present invention are widely used in applications where cracks are likely to occur and where acid resistance is required. Specifically, as mentioned above, it is preferably used for vehicle-related parts, particularly fuel pump modules. In addition to the above, cams, sliders, levers, arms, clutches, felt clutches, idler gears, pulleys, rollers, rollers, key stems, key tops, shutters, reels, shafts, joints, shafts, bearings, guides, etc. Mechanical parts: Outsert molded resin parts, insert molded resin parts, chassis, trays, side plates, printers, HDD internal parts (e.g. lamps, latches, etc.), and for office automation equipment such as copiers. Parts: VTR (Video Tape Recorder), video movie, digital video camera, camera, and parts for cameras and video equipment such as digital cameras; cassette player, DAT, LD (Laser Disk), MD (Mini Disk), CD (Compact Disk) [including CD-ROM (Read Only Memory), CD-R (Recordable), CD-RW (Rewritable)], DVD (Digital Video Disk) [DVD-ROM, DVD-R, DVD+R, DVD- RW, DVD+RW, DVD-R DL, DVD+R DL, DVD-RAM (Random Access Memory), including DVD-Audio], Blu-ray (registered trademark) Disc, HD-DVD, and other optical disk drives; MFD, MO , music, video or information equipment such as navigation systems and mobile personal computers; parts for communication equipment such as mobile phones and facsimiles; parts for electrical equipment; parts for electronic equipment, etc.

The present invention will be explained in more detail with reference to Examples below. The materials, usage amounts, ratios, processing details, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below.

Experimental example 1
A dumbbell multi-purpose test piece (ISO dumbbell A test piece) was obtained. Five ISO dumbbell test pieces were obtained in the same manner.
The obtained ISO dumbbell test piece was fixed to a three-point bending jig in which the spacer was adjusted so that the strain was 1.5%. When the average peak amplitude of the acoustic emission signal in the frequency range of 0.01 to 2.5 MHz was measured using a resonant acoustic emission (AE) device, the maximum amplitude was 6.7 mV. Therefore, the peak value B was set to 6.7 mV, and the peak value A was set to 11.4 mV, which is 6.7×1.7. Then, we measured the number of times an AE signal with a peak value A (11.4 mV) or more occurred at a frequency of 0.01 to 0.3 MHz and the number of times an AE signal with a peak value A of a peak value A occurred at a frequency of 0.3 to 2.5 MHz. .
The above measurements were performed in the same manner on five ISO dumbbell test pieces, and the average value of the five measurements was taken as the number of AE signal occurrences.
The AE measurement conditions were an amplification factor of 69 dB and an AE filter of 100 kHz, and AE occurring during a period of 0.1 ms was measured.
The AE sensor used was a broadband AE sensor AE-900M-WB manufactured by NF Circuit Design Block Co., Ltd., the AE analysis device used an AE tester AE9501A manufactured by NF Circuit Design Block Co., and the oscilloscope used was Picoscope manufactured by Pico technology.
Incidentally, during the AE measurement, no cracks were visually confirmed in any of the ISO dumbbell test pieces. In addition, the presence or absence of cracks in the ISO dumbbell test piece after 72 hours was visually confirmed. No cracks were visually confirmed in any of the five sheets.
The results are shown in Table 1.

Experimental example 2
Five ISO dumbbell test pieces were obtained in the same manner as in Experimental Example 1.
The obtained ISO dumbbell test piece was fixed to a three-point bending jig with a spacer adjusted so that the strain was 1.5%, a 10% by mass hydrochloric acid aqueous solution was dropped on the strained part, and a cover was placed to prevent drying. covered with glass. Using a resonant acoustic emission (AE) device, the number of times an AE signal with a peak value A (11.4 mV) or more occurs at a frequency of 0.01 to 0.1 MHz and the peak value that occurs at a frequency of 0.3 to 2.5 MHz. The number of times an AE signal of A (11.4 mV) or higher was generated was measured.
The above measurements were performed in the same manner on five ISO dumbbell test pieces, and the average value of the five measurements was taken as the number of AE signal occurrences.
Incidentally, during the AE measurement, no cracks were visually confirmed in any of the five ISO dumbbell test pieces.

In addition, the remaining five ISO dumbbell test pieces obtained were fixed to a three-point bending jig with a spacer adjusted so that the distortion was 1.5%, and a 10% by mass hydrochloric acid aqueous solution was dropped onto the distorted parts. , covered with a cover glass to prevent drying. After 72 hours had elapsed, the presence or absence of cracks in the ISO dumbbell test piece was visually confirmed. The occurrence of cracks was visually confirmed in all five sheets.
The results are shown in Table 1.

As is clear from Table 1 above, in Experimental Example 1 in which no acid was added dropwise, no AE signal with a peak value A (11.4 mV) or higher was observed at a frequency of 0.01 to 2.5 MHz. Furthermore, no cracks were visually confirmed in either the test piece at the time of the AE signal measurement or after 72 hours had elapsed.

On the other hand, in Experimental Example 2 in which acid was dropped onto the ISO test piece, an AE signal with a peak value A or higher was observed at a frequency of 0.01 to 2.5 MHz. Further, at the time of measuring the AE signal, no cracks could be visually confirmed, but the occurrence of cracks was visually confirmed in the test piece after 72 hours had elapsed.
In addition, in Experimental Examples 1 and 2, when the polyacetal resin was changed to F10-01 and F20-61 (both manufactured by Mitsubishi Engineering Plastics) and the same procedure was carried out, the same tendency was observed in the AE signal. The occurrence was confirmed, and similar results were confirmed by visual inspection.

Claims (3)

A method for detecting cracks caused by contact with chemical substances that may occur in resin molded products, the method comprising:
The resin molded product has polyacetal resin as a main component,
The resin molded product is processed using the acoustic emission method.
Detecting a crack by confirming the presence of a peak value A of an acoustic emission signal observed at a frequency of 0.01 to 0.3 MHz, wherein the peak value A indicates that there is no crack before contact with the chemical substance. A detection method in which the value is 1.7 times or more the peak value B of the acoustic emission signal in the frequency range of 0.01 to 0.3 MHz of the resin molded product. The detection method according to claim 1 , wherein the crack is a crack resulting from contact with an acid. The detection method according to claim 2 , wherein the acid is hydrochloric acid.