Abstract:
A fatigue test device for thin plates can reliably perform tests. The fatigue test device has a rod having ends in a longitudinal direction thereof; a vibration source for excitating a longitudinal end of the rod in the longitudinal direction thereof so as to form a standing wave having node portions and antinode portions formed therebetween and a mounting means for mounting a test piece having a longitudinal direction at one of the antinode portions, the test piece being positioned such that the longitudinal direction thereof is perpendicular to the longitudinal direction of the rod.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a fatigue testing device for thin plates and to a fatigue test method for the same, and specifically relates to a fatigue test technique in which the fatigue test can be reliably performed.  
           [0003]    2. Description of the Related Art  
           [0004]    Thin plates which form metal diaphragms and metal gaskets must have sufficient elasticity and flexibility against pressure and temperature, and must maintain necessary seal against pressure in order to prevent leakage at joined surfaces. Therefore, excellent fatigue characteristics are required in these thin plates. It is important to perform fatigue tests for the thin plates forming metal diaphragms to verify fatigue characteristics when metal diaphragms are produced.  
           [0005]    As examples of fatigue tests for thin plates proposed heretofore, there are the “Plane bending fatigue test method for metallic plate” according to JIS Z 2275 and “Fatigue test method and fatigue test device using resonance” described in Japanese Patent Unexamined (KOKAI) Publication No. 8-54331, etc.  
           [0006]    In the above-mentioned plane bending fatigue test method for metallic plates, bending moment is set to perform the fatigue test. In this method, the bending moment must be extremely small when the thickness of a test piece is extremely thin, for example 0.5 mm or less, or else the fatigue test cannot be performed. In the test, both ends of the test piece are bent by providing moment with a mechanical structure, so that the cycle frequency is very low, for example, about 50 Hz. Therefore, the fatigue test requires 20 days or more in a case of an ultra-high cycle, for example, 10 8  or more, and the fatigue test cannot be performed quickly.  
           [0007]    In contrast, “Fatigue test method and fatigue test device using resonance” described in Japanese Patent Unexamined (KOKAI) Publication No. 8-54331 is a technique in which the above-mentioned disadvantages in the technology are overcome. That is to say, in this technique, the fatigue test can also be performed on ultrathin plates and a quick fatigue test is realized with application of the resonance. However, in the technique, reliable fatigue tests cannot be performed since control of amplitude is difficult due to energy absorption by resonance in which a part of the energy generated by a vibration source is absorbed in the test piece. In particular, it is more difficult to resonate the test piece reliably when the frequency is high and the amplitude is low. Therefore, the reliability is extremely reduced in the case in which a fatigue test at high frequency is performed.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a fatigue test device for a thin plate in which the fatigue test can be reliably performed, and to provide a fatigue test method for the same.  
           [0009]    The present invention provides a fatigue test device comprising a rod having ends in a longitudinal direction thereof, a vibration source for excitating the longitudinal end of the rod in the longitudinal direction thereof so as to form a standing wave having node portions and antinode portions formed therebetween, and a mounting means for mounting a test piece having a longitudinal direction at one of the antinode portions, the test piece being positioned such that the longitudinal direction thereof is perpendicular to the longitudinal direction of the rod.  
           [0010]    The device having the above-described structure can make the test piece resonate, the test piece being secured such that the longitudinal direction thereof is perpendicular to the longitudinal direction of the rod in antinode portions of a standing wave generated by the excitation of the rod by exiting a longitudinal end of the rod by the vibration source. In this device, when a standing wave is generated in the rod by driving the vibration source, the amplitude of the standing wave can be easily controlled by the vibration source. Therefore, the amplitude of the antinode portion, at which the test piece is mounted, of the standing wave of the rod, can be controlled. For this reason, even if a part of energy is absorbed in the test piece, a reliable amplitude can be obtained, since this absorbed energy is smaller than the energy accumulated as a standing wave of the rod. Therefore, according to the invention, fatigue tests for thin plates which are excellent in reliability can be obtained.  
           [0011]    In the present invention, the test piece is preferably mounted at the antinode portion except for the free end of the rod. When the standing wave is generated in the rod, the shape of the free end of the rod must be formed in a flat surface in order to generate reflected waves at the free end. In one case, the test piece must be supported at an edge of the rod in order to position the test piece at the antinode portions of the standing wave of the rod. Therefore, it is considered that the mounting means should be made to be adhesive. However, in actual tests, operations for mounting and removing the test piece with respect to the rod are difficult, and the adhesion surface may exfoliate in the fatigue test. It is desirable that the test piece be mounted at the antinode portions except for the free end of the rod in order to avoid such problems. For example, it is considered that the rod is divided into parts in the longitudinal direction thereof, and the parts hold the test piece.  
           [0012]    In addition, it is desirable to have a measuring device for measuring displacement of the free end of the test piece and displacement of the free end of the rod so as to calculate amplitude stress given in the test piece. According to such an embodiment, the displacement of the free end of the rod is assumed to be the displacement of the excitation end of the test piece, and the displacement of the free end of the rod and displacement of the free end of the test piece are subtracted from each other, whereby bending deflection given to the test piece is calculated, thereby easily calculating amplitude stress given in the test piece.  
           [0013]    As mentioned in the above, the present invention relates to a fatigue test device and also relates to a fatigue test method with the above-mentioned features. That is to say, a fatigue test method of the present invention comprises securing a test piece at an antinode portion of a standing wave generated by excitation of a rod having both ends in a longitudinal direction thereof such that a longitudinal direction of the test piece is perpendicular to the longitudinal direction of the rod and excitating the longitudinal end of the rod in the longitudinal direction thereof so as to resonate the test piece. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 shows an example of a device of the present invention and a condition of a standing wave generated in the rod.  
         [0015]    [0015]FIG. 2 shows another example of a device of the present invention and another condition of a standing wave generated in the rod.  
         [0016]    [0016]FIG. 3 shows an example in which a test piece is mounted in a device of the present invention and a condition of a standing wave generated in the rod.  
         [0017]    [0017]FIG. 4 shows another example in which a test piece is mounted in a device of the present invention and another condition of a standing wave generated in the rod.  
         [0018]    [0018]FIG. 5 is an exploded view of the rod, vibration source and test piece shown in FIG. 4.  
         [0019]    [0019]FIG. 6 is a cross-section showing a holding portion of the rod and a cross section showing the test piece.  
         [0020]    [0020]FIG. 7 shows another example in which a test piece is mounted in a device of the present invention and another condition of a standing wave generated in the rod.  
         [0021]    [0021]FIG. 8 shows an example of a vibration pattern of the test piece.  
         [0022]    [0022]FIG. 9 shows another example of the vibration pattern of the test piece.  
         [0023]    [0023]FIG. 10 shows an excitation end and a free end of the test piece in which the vibration pattern and amplitude of a test piece are measured.  
         [0024]    [0024]FIG. 11 shows an example of a waveform in an excitation end and a free end of the test piece.  
         [0025]    [0025]FIG. 12 shows another example of a waveform in the excitation end and the free end of the test piece.  
         [0026]    [0026]FIG. 13 shows another example in which a test piece is mounted in a device of the present invention and another condition of a standing wave generated in the rod.  
         [0027]    [0027]FIG. 14 is a block diagram showing a practical example.  
         [0028]    [0028]FIG. 15 is a sectional view of a test piece used in the practical example.  
         [0029]    [0029]FIG. 16 shows the relationship between amplitude stress in the test piece and cycle. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    In the following, forms of examples of the present invention are explained according to the figures.  
         [0031]    A device such as is shown in FIG. 1 is proposed as a device in which excitation can be performed with reliable amplitude even if energy is absorbed in a test piece.  
         [0032]    As shown in this figure, vibration source  11  which excitates a rod  10  in axial direction X is disposed at an end of the rod  10 . In this figure, a mounting means for mounting the test piece such that the longitudinal direction thereof is perpendicular to the longitudinal direction of the rod is not shown. It is simple for the structure of the device to make an end at the opposite side of rod  10  to be a free end. However, it is also possible to make the end to be a secured end as shown in FIG. 2. The standing wave which vibrates in axial direction X is generated in rod  10  by operating vibration source  11  at a resonant frequency of rod  10 . In this case, although antinode portions of the standing wave strongly vibrate in the axial direction X of rods  10 , node portions are not shown. The amplitude of this standing wave is easily controllable by vibration sources  11 . Thus, the displacement of antinode portions of the standing wave can be easily controlled.  
         [0033]    A test piece of flat plate is placed at antinode portions of the standing wave in this rod  10 , such that the longitudinal direction thereof is perpendicular to the longitudinal direction of rod  10 . The test piece is formed to have a shape and dimensions in which the test piece resonates at frequency of the standing wave generated in rod  10 . By doing so, even if a part of the energy is absorbed in the test piece, reliable amplitude can be given to the test piece, since this absorption energy is less than energy accumulated as standing wave of rod  10 .  
         [0034]    As the simplest embodiment for a test piece in such a device, an embodiment in which the test piece  12  is mounted in the free end of rod  10  as shown in FIG. 3 may be mentioned. However, in order to produce a standing wave in the rod  10 , the free end must be a flat surface so as to produce reflected waves at the free end. Therefore, it is necessary to support the test piece  12  at the edge of rod  10  in order to dispose test piece  12  at antinode portions of standing wave. Therefore, rod  10  and test piece  12  must be secured by a technique such as adhesion, and in actual tests, operations for mounting and removing the test piece  12  with respect to the rod are difficult, and the adhesion surface may exfoliate in the fatigue test.  
         [0035]    Therefore, it is possible to avoid the above-mentioned problem by disposing test piece  12  in an antinode portion except for the free end of rod  10  as shown in FIG. 4. For example, as shown in FIG. 5, rod  10  is longitudinally divided from each other into rod part  10   a  and rod part  10   b , and the test piece  12  is held by these parts. The test piece can be easily mounted or removed by providing an male screw portion  10   c  in rod part  10   a  and providing a female screw portion  10   d  in rod part  10   b , and by providing mounting hole  12   a  in test piece  12  to form a mounting means, as shown in FIG. 4,  
         [0036]    In this case, as shown in FIG. 6, when rod  10  has a holding portion of which the cross-section is identical to a cross-section of test piece  12  which is held by rod  10 , a standing wave can be more reliably generated in the test piece. Therefore, the fatigue test can be progressively reliably performed. Reference numeral  12   c  in FIG. 6 indicates a test portion of test piece  12 .  
         [0037]    Although the bending fatigue test for a thin plate can be performed by the above-mentioned method, vibration frequency must be increased in the case of performing fatigue at higher cycles in a short time. However, the output of the vibration source is constant, the amplitude is low when the vibration frequency is high, and so the fatigue test is difficult. In contrast, when the output of the vibration source is increased, the resonant frequency of vibration source is reduced. In order to overcome such problems, it is effective to provide horn  10   e  having a conical trapezoidal shape between the mounting position of vibration source  11  of rod  10  and the mounting position of test piece  12  of rod  10  as shown in FIG. 7. By increasing the amplitude of the standing wave by such a horn  10   e , displacement of large amplitude can be obtained even if the frequency is high, whereby the fatigue test at high frequency can be realized.  
         [0038]    By the above-mentioned composition of the fatigue test device, fatigue test of ultrathin plates can be performed. However, the actual load given to the test piece must be calculated, since the load parameter in the fatigue test is excitation displacement. Furthermore, although the test piece resonates, the form of the vibration changes according to delicate conditions, so that these conditions must be confirmed. FIG. 8 and FIG. 9 show typical examples of vibration patterns of the test piece  12  which is mounted at the rod  10 . In order to measure such vibration patterns and amplitudes of test piece  12 , it is necessary to measure aging variation in displacement of excitation end A of test piece  12  and displacement of free end B of the same, as shown in FIG. 10. This measurement results are waveforms of the coordinate phase shown in FIG. 11 in the case of FIG. 8, and waveforms of the opposite phase shown in FIG. 12 in the case of FIG. 9. Actually, amplitude stress is calculated by subtracting the displacement of excitation end A of test piece  12  and the displacement of free end B of test piece  12  from each other, to calculate bending deflection given to the test piece.  
         [0039]    It is difficult to measure displacement of excitation end A when test piece  12  is disposed at the antinode portion except for the free end of rod  10 . However, since the amplitudes of the standing wave generated in the rod  10  are constant at each antinode portion, the amplitude measured at the free end of the rod  10  is used as the displacement which is input in the test piece. Therefore, as shown in FIG. 13, it is possible to easily judge the displacement of excitation end A of test piece  12  by measuring the displacement in the free end of rod  10 . However, it should be noted that the standing wave in excitation end A of test piece  12  and the free end of rod  10  may be of opposite phase.  
       EXAMPLES  
       [0040]    Next, an example of actually performing a fatigue test using the above-mentioned device is described.  
         [0041]    A block diagram of a practical example is given in FIG. 14. Reference numeral  11  in FIG. 14 shows a piezoelectric actuator as a vibration source, reference numeral  13  in this figure shows an oscillator, reference numeral  14  in this figure shows a vortex style of displacement gage, reference numeral  15  in this figure shows a laser displacement gage and reference numeral  16  in this figure shows an oscillator. Excitation frequency was 20 kHz, and shape and dimensions of test piece  12  and rod  10  were designed to resonate at 20 kHz. In such conditions, as shown in FIG. 15, a fatigue test was performed for test piece  12  made of a stainless steel (SUS304). Thickness of the test piece was 0.2 mm, and notch  12   d  was formed at a test portion  12   c . Amplitude stress given in test piece  12  was calculated as a stress which was generated in the case of supplying relative displacement between excitation end A and free end B of test piece  12  to a cantilever beam.  
         [0042]    S-N chart (relationship between amplitude stress and cycle given in the test piece) obtained as a result of the test is shown in FIG. 16. As shown in FIG. 16, the fatigue test using the ultrathin plates at high frequency could be performed with small dispersion, that is to say, with reliability. The fatigue test could also be performed quickly, since it was confirmed that the fatigue test up to 108 cycles was completed in about 1.4 hours and the fatigue test up to 10 9  cycles was completed in about 14 hours in the case of a test frequency of 20 kHz used in the practical example.