Patent Publication Number: US-7222515-B2

Title: Impact test apparatus and impact test method

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an impact test apparatus and an impact test method that can obtain an impact stress-strain characteristic of a test piece. 
   2. Description of Related Art 
   An Izod impact test or a Charpy impact test has been conventionally known as a method of measuring the impact strength of a material (for example, see Japanese Unexamined Patent Publication No. H2(1990)-13825). According to these methods, when an impact energy (impact force) is applied to a test piece to break a test piece, the impact strength of the test piece can be measured from the impact energy. However, in the case of a test piece formed of an impact absorbing material of a foaming material, a viscoelastic material, or a combination of these materials, when the impact force is applied to the test piece, the test piece is elastically deformed to absorb the impact energy (impact force). Hence, it is difficult to evaluate an impact force applied to the test piece and an impact stress-strain characteristic of the test piece when the impact force is applied to the test piece. For this reason, in the impact absorbing material, the characteristic of the impact absorbing material has been evaluated by measuring an impact absorbing property or an repulsion property that are found by a Lubke impact resilience test method or a Trypso impact resilience test method (see, for example, Japanese Standards Association JIS Handbook Rubber (issued on Apr. 1, 1999). 
   By the way, a foaming material, a viscoelastic material, or a combination of these materials has been used as the impact absorbing material of various kinds of machines and electric appliances. Portable machines and electric appliances (for example, cellular mobile phone) have been increased in number in recent years. In these various products, an impact force is applied to them in a manner that has never been hitherto conceivable. 
   Moreover, these impact absorbing materials have different impact absorbing properties according to material, internal form, or the like. Hence, it becomes difficult to evaluate the impact absorbing materials by the conventional impact tests or repulsion elasticity test on the same basis. Therefore, these impact absorbing materials are placed on actual products and impact drop tests or the like are conducted on the actual products. In this manner, a durability evaluation test or the like for impact is conducted on the product. 
   SUMAMRY OF THE INVENTION 
   However, as product diversification has been increased and product development has been accelerated in recent years, for the purpose of controlling the quality of an impact absorbing material without conducting an impact test on an actual product, a method of evaluating an impact absorbing material has been demanded by which an impact force is applied to a test piece formed of the impact absorbing material to obtain an impact stress-strain characteristic and the like. 
   The present invention has been made to solve the above demand. The object of the invention is to provide an impact test apparatus and an impact test method that can obtain an impact stress-strain characteristic and the like when an impact force is applied to an impact absorbing material including a foaming material, a viscoelastic material, or a combination of these materials. 
   To achieve the above purpose of the invention, there is provided an impact test apparatus comprising: a holding device for holding a test piece at an arbitrary holding force; an impact applying device for applying an impact force to the test piece held by the holding device; an impact force sensing device for sensing the impact force applied to the test piece by the impact applying device; a displacement detecting device for detecting a displacement of the test piece when the impact force sensing device senses the impact force; and an output device for outputting impact stress-strain characteristic information expressing a relationship between an impact stress and a strain corresponding to the impact stress of the test piece on the basis of the impact force sensed by the impact force sensing device and the displacement detected by the displacement detecting device. 
   With the above structure, the impact test apparatus can hold a test piece at an arbitrary holding force and apply an impact force to the test piece. Moreover, impact stress-strain characteristic information expressing the relationship between an impact stress and a strain to the impact stress of the test piece can be obtained on the basis of the impact force, which is applied to the test piece and is sensed by an impact force sensing device, and the displacement of the test piece, which is detected by a displacement detecting device when the impact force is applied to the test piece. 
   According to another aspect of the invention, there is provided an impact test method comprising the steps of: holding a test piece at an arbitrary holding force by a holding device; applying an impact force to the test piece held at the holding step by an impact applying device; sensing the impact force applied to the test piece at the impact applying step by an impact force sensing device and of detecting a displacement of the test piece when the impact force sensing device senses the impact force by a displacement detecting device; and outputting impact stress-strain characteristic information expressing a relationship between an impact stress and a strain corresponding to the impact stress of the test piece on the basis of the impact force and the displacement, which are sensed and detected at the detecting step, by an output device. 
   In the above-described impact test method, at a holding step, a test piece is held at an arbitrary holding force, and at an impact applying step an impact force can be applied to the test piece. Moreover, at a detecting step, the impact force applied to the test piece is sensed and the displacement of the test piece when the impact force is sensed is detected. Then, at an output step, impact stress-strain characteristic information expressing the relationship between an impact stress and a strain to the impact stress of the test piece can be obtained on the basis of the sensed impact force and the detected displacement. 
   Further, according to another aspect of the invention, there is provided an impact test method comprising the steps of: applying an impact force by an impact applying device in a state where a test piece is not set in a holding device; sensing the impact force at the first impact applying step by an impact force sensing device; holding the test piece at an arbitrary force by the holding device; applying an impact force to the test piece held at the holding step by the impact applying device; sensing the impact force applied to the test piece at the second impact applying step by the impact force sensing device; and outputting an impact absorption rate of the test piece by dividing an impact force, which is found by subtracting the impact force sensed at the second sensing step from the impact force sensed at the first sensing step, by the impact force sensed at the first sensing step to find an impact absorption rate. 
   In the above-described impact test method, first, at the first sensing step, an impact force is sensed which is applied by the impact applying device in a state where a test piece is not set in a holding device; next, a test piece is held at an arbitrary holding force by the holding device, and then at the second sensing step an impact force applied to the test piece by the impact force applying device is sensed. Then, an impact force, which is obtained by subtracting the impact force sensed at the second sensing step from the impact force sensed at the first sensing step, is divided by the impact force sensed at the first sensing step, whereby the impact absorption rate of the test piece can be obtained. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view showing a schematic construction of an impact test apparatus in a first embodiment; 
       FIG. 2  is a front view illustrating a state where an impact force is applied to a test piece in the impact test apparatus in the first embodiment; 
       FIGS. 3A to 3D  are a plan view, a front view, a left side view, and a right side view showing the schematic construction of the impact test apparatus in the first embodiment; 
       FIG. 4  shows the schematic construction of an output device that detects the impact force and displacement of a test piece and outputs impact stress-strain characteristic information in the impact test apparatus in the first embodiment; 
       FIG. 5  is a graph showing one example of an impact stress-strain characteristic curve obtained by the impact test apparatus in the first embodiment; 
       FIG. 6  is a graph showing one example of an impact absorption rate with respect to a swing angle of a hammer obtained by the impact test apparatus in the first embodiment; 
       FIG. 7  is a front view showing a schematic construction of an impact test apparatus in a second embodiment; 
       FIG. 8  is a left side view showing the schematic construction of the impact test apparatus in the second embodiment; 
       FIG. 9  shows the schematic construction of an output device that detects the impact force and displacement of a test piece and outputs an impact stress-strain characteristic information in the impact test apparatus in the second embodiment; 
       FIG. 10  is a graph showing respective impact stress-strain characteristic curves obtained by the impact test apparatus in the second embodiment at set temperatures of −20° C., +20° C., and +60° C. and at a set humidity of 60%; 
       FIG. 11  is a partial enlarged view showing a state where an acceleration sensor is fixed to the supporting plate of a holding device of the impact test apparatus in another embodiment; and 
       FIG. 12  illustrates a method of measuring the displacement of a metal foil placed on the supporting plate of a holding device of the impact test apparatus in another embodiment by a laser Doppler vibrometer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereafter, a first and a second embodiments in which an impact test apparatus and an impact test method in accordance with the present invention are embodied will be described with reference to the drawings. 
   Embodiment 1 
   First, the schematic construction of an impact test apparatus in a first will be described with reference to  FIGS. 1 to 4 . 
   As shown in  FIGS. 1 to 4 , an impact test apparatus  1  in the first embodiment is constructed of: a holding device  3  for holding a test piece  2  at an arbitrary force; an impact applying device  4  for applying an impact stress to the test piece  2 ; a force sensor  5  as an impact force sensing device for sensing an impact force applied to the test piece  2  by the impact applying device  4 ; a high-speed camera  6  as a displacement detecting device for detecting the displacement of the test piece when the impact applying device  4  applies an impact force to the test piece  2 ; and an output device  7  for synchronizing a signal from the force sensor  5  with a signal from the high-speed camera  6  and outputting an impact stress-strain characteristic curve as impact stress-strain characteristic information expressing the relationship between an impact stress of the test piece and a strain to the impact stress when an impact force is applied to the test piece  2 . 
   Further, the holding device  3  for holding the test piece  2  at an arbitrary holding force is constructed of a fixing jig  11  fixed to a table  10  and a pressing member  12  that is opposed to the fixing jig  11  to sandwich the test piece  2  between them and can slide so as to be able to hold the test piece  2 . The pressing member  12  is slidably placed on a base  13  sliding on the table  10  and can slide on the table  10  along with the base  13 . Further, the base  13  is provided with a lever  14  for sliding the base  13  and a fixing member  15  for fixing the base  13  to the table  10  after the base  13  is slid by the lever  14  and is temporarily positioned. Still further, the pressing member  12  is provided with a pressing pressure adjusting unit  16 . The pressing pressure adjusting unit  16  is preferably provided with a digital gage or the like moving in response to the pressing member  12 . The pressing pressure adjusting unit  16  can finely adjust the pressing member  12  on the base  13  fixed at a temporary position in the back-and-forth direction (in the left-and-right direction in  FIG. 1 ) with respect to the test piece  2 , thereby being able to adjust a holding force applied to the held test piece  2 . 
   Further, an impact applying device  4  for applying an impact force to the test piece  2  held by the holding device  3  is constructed of a shaft  23  having one end  22  that is turnably journaled by a support column  20  and a hammer  24  at the other end, and an arm  21  that lifts the hammer  24  to a predetermined angle and holds it. The arm  21  can be fixed and held at the predetermined angle. Here, the hammer  24  is made of a steel ball and can be lifted to the predetermined angle along with the arm  21  by an electric magnet  25  fixed to one end of the arm  21 . In this regard, the electric magnet  25  is used here but any other part can be used which can lift the hammer  24  along with the arm  21  and can easily separate the hammer from the arm  21 . Moreover, the hammer  24  is not limited to a ball but may be, for example, a block. 
   Still further, the force sensor  5  for sensing an impact force applied to the test piece by the impact applying device  4  is fixed to such a surface of the fixing jig  11  that is opposite to a surface in contact with the test piece  2 . A force sensor constructed of a piezoelectric device is preferably used as the force sensor  5  in the first embodiment. When the hammer  24  of the impact applying device  4  applies an impact force to the test piece  2 , the force sensor  5  can sense the impact force with reliability. Here, in place of the force sensor  5 , a strain gage can be also employed. 
   Still further, the high-speed camera  6  for detecting the displacement of the test piece  2  when the impact force is applied to the test piece  2 , as shown in  FIG. 4 , a high-speed camera capable of directly observing the displacement of the test piece  2  is preferably used. Here, it is preferable that the high-speed camera can take pictures at intervals of 200 μsec or less, more preferably, at intervals of 100 μsec or less, and still more preferably, at intervals of 75 μsec or less. 
   Still further, as shown in  FIG. 4 , the test piece  2  is sandwiched between the fixing jig  11  and the pressing member  12  via a supporting plate  28  constructed of a highly elastic plate such as a resin plate made of acrylic resin or the like or a metal plate made of aluminum or the like. 
   Still further, as shown in  FIG. 4 , the output device  7  for outputting an impact stress-strain characteristic curve is constructed of: the first control section  32  for outputting a displacement detecting signal (image signal) from the high-speed camera  6  serving as the displacement detecting device; synchronizing section  35  for synchronizing a signal from the force sensor  5  serving as the impact force sensing device with a displacement detecting signal (image signal) from the first control section  32 ; and the second control section  30  that analyzes an image of the displacement detecting signal (image signal) from the first control section  32 , divides a displacement by the thickness of the test piece  2  when load is not applied thereto to calculate a strain, analyzes a signal from the force sensor  5  synchronized by the synchronizing section  35  and calculates an impact force, divides the impact force by the area of such surface of the test piece  2  that abuts against the supporting plate  28  to calculate an impact stress, and outputs an impact stress-strain characteristic curve. Here, the synchronization between the signal from the force sensor  5  and the displacement detecting signal (image signal) from the first control section  32  is achieved by the simultaneous detection of the respective signals by the first control section  32  and the synchronizing section  35  at a timing of a signal from a speed meter  33  outputted when the hammer  24  passes before a speed sensor  34  disposed before the test piece  2 . 
   Next, an impact test method using the impact test apparatus  1  constructed in the manner described above will be described. 
   (Holding Step) 
   First, the test piece  2  is not inserted between the fixing jig  11  and the pressing member  12  of the holding device  3  but only the supporting plate  28  made of acrylic resin is inserted. Then, the base  13  is slid to the fixing jig  11  side by the lever  14 , thereby being into nearly close contact with the supporting plate  28  and then being temporarily fixed there. Next, the base  13  is finely adjusted by the pressing pressure adjusting unit  16  to put the fixing jig  11  into close contact with the supporting plate  28 . At this time, a memory provided in the pressing pressure adjusting unit  16  is reset and the position of the pressing member  12  is made a base point. 
   Next, the thickness of the test piece  2  is previously measured. The pressing pressure adjusting unit  16  is adjusted to move the pressing member  12  from the base point by the thickness of the test piece  2  to set the test piece  2  between the fixing jig  11  and the supporting plate  28 . At this time, in the case of setting the test piece  2  made of an impact absorbing material, which is made of a foaming material, a viscoelastic material, or a combination of these materials and damps and absorbs an impact force when the impact force is applied to the test piece  2 , in the holding device  3 , a pressing pressure is adjusted while checking the gage of the pressing pressure adjusting unit  16  and considering the thickness of the test piece  2 . In this manner, the test piece  2  can be fixed at a compression rate close to a compression rate in the actual state of placement of a product. Further, when an adhesive agent or the like is applied to a portion or the whole of one surface of the test piece  2 , the test piece  2  can be placed on the fixing jig  11  and hence can be fixed to the fixing jig  11  with reliability. 
   (Impact Applying Step) 
   After the test piece  2  is set, the switch of the electric magnet  25  fixed to one end of the arm  21  is turned on to fixedly hold the hammer  24  by the arm  21 . The arm  21  is swung up to an arbitrary angle and is fixed there and then the switch of the electric magnet  25  is turned off to bring the hammer  24  into collision with the supporting plate  28  held by the holding device  3  (see  FIG. 2 ). 
   (Detecting Step) 
   Moreover, an impact force at this time is measured by the force sensor  5 . Here, the fixing jig  11  is put into close contact with the supporting plate  28  before the test piece  2  is inserted between them and then the hammer  24  is similarly brought into collision with the supporting plate  28  and an impact force at this time is measured. Then, the impact absorption rate (%) of the test piece  2  can be measured by calculating the following equation (1). 
   
     
       
         
           
             
               
                 
                   Impact 
                   ⁢ 
                   
                       
                   
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                   absorption 
                   ⁢ 
                   
                       
                   
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                   rate 
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                   ⁢ 
                   
                     ( 
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                 = 
                 
                     
                   
                     
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                             ( 
                             
                               
                                 Impact 
                                 ⁢ 
                                 
                                     
                                 
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                                 force 
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                                 ⁢ 
                                 of 
                                 ⁢ 
                                 
                                     
                                 
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                                 only 
                                 ⁢ 
                                 
                                     
                                 
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                                 supporting 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 plate 
                               
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                                 Impact 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 force 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 when 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 test 
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                                 piece 
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                                 inserted 
                               
                             
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                           Impact 
                         
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                         force 
                         ⁢ 
                         
                             
                         
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                         supporting 
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                     × 
                     100 
                   
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                 ( 
                 1 
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   Next, a method of measuring the displacement of the test piece  2  when the hammer  24  is brought into collision with the supporting plate  28  will be described. 
   The impact force applied to the test piece  2  is sensed by the above-described method by the force sensor  5  when the hammer  24  is brought into collision with the supporting plate  28 . A signal from the force sensor  5  is inputted to the synchronizing section  35 . On the other hand, the displacement of the test piece  2  at this time is taken by the high-speed camera  6  and an image pickup signal from the high-speed camera  6  is inputted to the synchronizing section  35  via the first control section  32 . Here, the synchronization between a signal from the force sensor  5  and a signal from the high-speed camera  6  is achieved by the synchronizing section  35  in the following manner: as shown in  FIG. 4 , when the hammer  24  passes by the speed sensor  34  disposed just before the holding device  3 , a signal is sent out by the speed meter  33 , thereby being inputted to the first control section  32  and at the same time a synchronizing signal is inputted to the synchronizing section  35  via the first control section  32 ; then, at the timing of the signal from the speed meter  33 , the high-speed camera  6  starts picking up an image and the synchronizing section  35  starts sending a signal of the force sensor  5 ; with this, the picking-up of the image by the high-speed camera  6  and the measuring of the impact force by the force sensor  5  are started at the same time and hence the displacement of the test piece  2  at each measuring time can be measured in synchronization with the impact force applied to the test piece  2 . 
   (Outputting Step) 
   The signal from the force sensor  5  and the image pickup signal from the high-speed camera  6 , which are synchronized with each other by the synchronizing section  35 , are inputted to the second control section  30 . In this second control section  30 , the signal from the force sensor  5  is converted to an impact force and the image pickup signal from the high-speed camera  6  is subjected to an image processing, thereby being converted to a displacement. Here, the conversion of the image pickup signal from the high-speed camera  6  to the displacement by the image processing is performed by converting the path of a mark put on the end surface of the test piece  2  or the supporting plate  28  into numerals. Further, the second control section  30  acquires data of the impact force and the displacement at each measuring time and then divides the impact force by the area of such surface of the test piece  2  that abuts against the supporting plate  28  and divides the displacement by the thickness of the test piece when load is not applied to the test piece  2 , thereby being able to output an impact stress-strain characteristic curve. For example, when the second control section  30  is constructed of a personal computer or the like having a liquid crystal display screen, the impact stress-strain characteristic curve can be displayed on the liquid crystal display screen. 
   In this manner, even if the test piece  2  is made of a foaming material, a viscoelastic material, or a combination of these materials, that is, made of an impact absorbing material that damps or absorbs an impact force when the impact force is applied to the test piece  2 , the impact test apparatus  1  can output an impact stress-strain characteristic curve when the impact force is applied to the test piece  2 . Moreover, the impact test apparatus  1  can output also the impact absorption rate of the test piece  2  at the same time. 
   Here, one example of an impact test will be described which was conducted on a test piece  2 , which was formed of a foaming material made of polypropylene and was set in the impact test apparatus  1  of the above construction. 
   This test piece  2  was cut out of an impact absorbing material formed of a foaming material so as to have a size of 20 mm×20 mm×1 mm and then was held by the holding device  3  via the supporting plate  28  made of acrylic resin having a size of 90 mm×80 mm×3 mm. At this time, the pressing member  12  was slightly pressed in from the base point to hold the test piece  2  in a state where the test piece  2  was compressed by 30% in the direction of thickness. Then, the hammer  24  was set in a state where the hammer  24  was swung up at 45° with respect to the support column  20  and then the hammer  24  was separated from the arm  21 , thereby being brought into collision with the supporting plate  28 . At this time, the displacement image of the test piece  2  was picked up on a needed basis by the high-speed camera  6  (for example, FASTCAM manufactured by Photron). This impact test was conducted under environment of room temperature of 20° C. and humidity of 60%. 
   An impact stress-strain characteristic curve  45  obtained under the test conditions described above is shown in  FIG. 5 . As shown in  FIG. 5 , even if the test piece  2  was made of the impact absorbing material such as a foaming material of polypropylene, the impact stress-strain characteristic curve  45  could be obtained. 
   Further, the test piece  2  was held in a state where it was compressed by 30% in the direction of thickness and was set in a state where the hammer  24  was swung up to angles of 30°, 45°, 60°, 75°, and 90° and then the hammer  24  was separated from the arm  21 , thereby being brought into collision with the supporting plate  28 . The impact absorption rates of the test piece  2 , which were obtained by the above equation (1), are shown in  FIG. 6 . As shown in  FIG. 6 , even if the test piece  2  was made of the impact absorbing material such as a foaming material of polypropylene, the impact absorption rates of the test piece  2  with respect to the respective angles to which the hammer  24  was swung up could be obtained. 
   Embodiment 2 
   Next, the schematic construction of an impact test apparatus in a second embodiment will be described with reference to  FIGS. 7 to 9 . Here, in the following description, the same reference symbols as in the construction or the like of the impact test apparatus  1  in the first embodiment shown in  FIGS. 1 to 4  denote the same or equivalent parts as in the construction or the like of the impact test apparatus  1  in the first embodiment. 
   The schematic construction and control construction of an impact test apparatus in the second embodiment are nearly equal to those of the impact test apparatus  1  in the first embodiment. 
   However, as shown in  FIGS. 7 to 9 , an impact test apparatus  51  in the second embodiment is different from the construction of the impact test apparatus  1  in the first embodiment in that the holding device  3  having the force sensor  5  fixed thereto and the impact applying device  4 , which construct the impact test apparatus  1  in the first embodiment, are disposed in a thermal insulating housing  54  of a thermo-hygrostat  52  as a temperature keeping device and humidity keeping device. 
   As shown in  FIG. 7  and  FIG. 8 , the thermo-hygrostat  52  is constructed of: the thermal insulating housing  54  having a square opening  53  on the front surface; a thermal insulating door  55  turnably supported by the end portion on one side of the opening  53  so that it may pivot and close the opening  53 , and mounted so that it can be freely opened or closed; and a refrigerator unit  56  disposed under the thermal insulating housing  54 . Further, in the back of the thermal insulating housing  54  are disposed: a humidifier  58  having water supplied from a water supply tank  57  disposed on the front side of the refrigerator unit  56 ; a cooling-dehumidifying device  59  disposed above the humidifier  58  and cooled by the refrigerator unit  56 ; a heater  60  disposed above the cooling dehumidifying device  59 ; and a blower  61  disposed above the heater  60 . Moreover, a humidity sensor and a temperature sensor (both not shown) are disposed near the blowoff port of the blower  61 . 
   Further, an observation window  62  is formed in the lower central portion of the side wall of the thermal insulating housing  54  and hence the displacement of the test piece  2  held by the holding device  3  disposed in the thermal insulating housing  54  can be observed through the observation window  62 . Still further, a protecting plate  64 , which is formed of a clear acrylic plate or the like and has a vertically elongated through hole formed nearly in the center and is formed nearly in a square, is provided outside the observation window  62 . The high-speed camera  6  and an illuminating device  66  are disposed outside the protecting plate  64 . With this, the displacement of the test piece  2  held by the holding device  3  can be observed. 
   Further, a wire pull-out port  67  used for pulling out wires from the electric magnet  25  of the impact applying device  4 , the speed meter  33 , and the force sensor  5 , which are disposed in the thermal insulating housing  54 , is provided on the side of the observation window  62 . 
   Still further, an operating part  68 , which is provided with a control circuit (not shown) for driving and controlling the refrigerator unit  56 , the humidifier  58 , the cooling dehumidifying device  59 , the heater  60 , and the blower  61 , is provided on the right side of the thermal insulating door  55  and can set temperature and humidity in the thermal insulating housing  54  at a predetermined temperature and at a predetermined humidity. Here, in the thermo-hygrostat  52 , temperature in the thermal insulating housing  54  can be set at an arbitrary temperature within a range of from −40° C. to 100° C. and humidity in the thermal insulating housing  54  can be set at an arbitrary humidity within a range of from 0% to 100%. 
   Still further, as shown in  FIG. 9 , the speed sensor  34  provided before the test piece  2  and the speed meter  33  connected to the speed sensor  34  are disposed in the thermal insulating housing  54 . 
   Still further, as shown in  FIG. 9 , the output device  7  for outputting an impact stress-strain characteristic curve is disposed outside the thermal insulating housing  54  and is constructed of the first control section  32  for outputting a displacement detecting signal (image signal) from the high-speed camera  6  serving as the displacement detecting device; the synchronizing section for synchronizing a signal from the force sensor  5  serving as the impact force sensing device with a displacement detecting signal (image signal) from the first control section  32 ; and the second control section  30  that analyzes an image of the displacement detecting signal (image signal) from the first control section  32 , divides a displacement by the thickness of the test piece  2  when load is not applied thereto to calculate a strain, analyzes a signal from the force sensor  5  synchronized by the synchronizing section  35  and calculates an impact force, divides the impact force by the area of such surface of the test piece  2  that abuts against the supporting plate  28  to calculate an impact stress, and outputs an impact stress-strain characteristic curve. Here, the synchronization between the signal from the force sensor  5  and the displacement detecting signal (image signal) from the first control section  32  is achieved by the simultaneous detection of the respective signals by the first control section  32  and the synchronizing section  35  at the timing of a signal from the speed meter  33  outputted when the hammer  24  passes before the speed sensor  34  disposed before the test piece  2 . 
   Next, an impact test method using the impact test apparatus  51  constructed in the manner described above will be described. 
   (Holding Step) 
   First, the test piece  2  is not inserted between the fixing jig  11  and the pressing member  12  of the holding device  3  disposed in the thermal insulating housing  54  but only the supporting plate  28  made of acrylic resin is inserted. Then, the base  13  is slid to the fixing jig  11  side by the lever  14 , thereby being brought into nearly close contact with the supporting plate  28  and then being temporarily fixed there. Next, the base  13  is finely adjusted by the pressing pressure adjusting unit  16  to put the fixing jig  11  into close contact with the supporting plate  28 . At this time, a memory provided in the pressing pressure adjusting unit  16  is reset and the position of the pressing member  12  is made a base point. 
   Next, the thickness of the test piece  2  is previously measured in the room. Then, the pressing pressure adjusting unit  16  is adjusted to move the pressing member  12  from the base point by the thickness of the test piece  2  to set the test piece  2  between the fixing jig  11  and the supporting plate  28 . At this time, in the case of setting the test piece  2  made of an impact absorbing material, which is made of a foaming material, a viscoelastic material, or a combination of these materials and damps and absorbs an impact when the impact is applied to the test piece  2 , in the holding device  3 , a pressing pressure is adjusted while checking the gage of the pressing pressure adjusting unit  16  and considering the thickness of the test piece  2 . In this manner, the test piece  2  can be fixed at a compression rate close to a compression rate in the actual state of placement of a product. Further, when an adhesive agent or the like is applied to a portion or the whole of one surface of the test piece  2 , the test piece  2  can be placed on the fixing jig  11  and hence can be fixed to the fixing jig  11  with reliability. Then, after the test piece  2  is set, the switch of the electric magnet  25  provided at one end of the arm  21  is turned on and the hammer  24  is fixed to and held by the arm  21 . Then, the arm  21  is swung up to an arbitrary predetermined angle and is fixed there. 
   (Temperature Keeping Step and Humidity Keeping Step) 
   Then, after the arm  21  is set at the arbitrary predetermined angle, the thermal insulating door  55  is closed and then the set temperature and the set humidity of the thermo-hygrostat  52  are set at a predetermined temperature and at a predetermined humidity by the operating part  68  and is operated for a predetermined time (for example, approximately from 30 minutes to two hours). With this, the temperature of the test piece  2  can be increased to the set temperature with reliability and the test piece  2  can be held in the atmosphere of the set humidity. 
   (Impact Applying Step) 
   Thereafter, the switch of the electric magnet  25  is turned off to bring the hammer  24  into collision with the supporting plate  28  held by the holding device  3  (see  FIG. 9 ). 
   (Detecting Step) 
   Moreover, an impact force at this time is measured by the force sensor  5 . 
   Here, the fixing jig  11  is put into close contact with the supporting plate  28  before the test piece  2  is inserted between them and then the hammer  24  is similarly brought into collision with the supporting plate  28  and an impact force at this time is measured. Then, the impact absorption rate (%) of the test piece  2  can be measured in the atmosphere of the respective set temperatures and humidities by calculating the above-described equation (1). 
   Next, a method of measuring the displacement of the test piece  2  when the hammer  24  is brought into collision with the supporting plate  28  will be described. 
   The impact force applied to the test piece  2  is sensed by the above-described method by the force sensor  5  when the hammer  24  is brought into collision with the supporting plate  28 . A signal from the force sensor  5  is inputted to the synchronizing section  35 . On the other hand, the displacement of the test piece  2  at this time is picked up through the observation window  62  by the high-speed camera  6  and an image pickup signal from the high-speed camera  6  is inputted to the synchronizing section  35  via the first control section  32 . Here, the synchronization between the signal from the force sensor  5  and the signal from the high-speed camera  6  is achieved by the synchronizing section  35  in the following manner: as shown in  FIG. 9 , when the hammer  24  passes by the speed sensor  34  disposed just before the holding device  3 , a signal is sent out by the speed meter  33 , thereby being inputted to the first control section  32  and at the same time a synchronizing signal is inputted to the synchronizing section  35  via the first control section  32 ; then, at the timing of the signal from the speed meter  33 , the high-speed camera  6  starts taking an image and the synchronizing section  35  starts sending the signal of the force sensor  5 ; with this, the taking of the image by the high-speed camera  6  and the measuring of the impact force by the force sensor  5  are started at the same time and hence the displacement of the test piece  2  at each measuring time can be measured in synchronization with the impact force applied to the test piece  2  in the atmosphere of the set temperature and the set humidity. 
   (Outputting Step) 
   Then, the signal from the force sensor  5  and the image pickup signal from the high-speed camera  6 , which are synchronized with each other by the synchronizing section  35 , are inputted to the second control section  30 . In this second control section  30 , the signal from the force sensor  5  is converted to an impact force and the image pickup signal from the high-speed camera  6  is subjected to an image processing, thereby being converted to a displacement. Here, the conversion of the image pickup signal from the high-speed camera  6  to the displacement by the image processing is performed by converting the path of a mark put on the end surface of the test piece  2  or the supporting plate  28  into numerals. 
   Further, the second control section  30  acquires data of the impact force and the displacement at each measuring time and then divides the impact force by the area of such surface of the test piece  2  that abuts against the supporting plate  28  and divides the displacement by the thickness of the test piece when load is not applied to the test piece  2 , thereby being able to output an impact stress-strain characteristic curve in the atmosphere of the set temperature and set humidity. For example, when the second control section  30  is constructed of a personal computer or the like having a liquid crystal display screen, the impact stress-strain characteristic curve in the atmosphere of the set temperature and set humidity can be displayed on the liquid crystal display screen. 
   In this manner, the impact stress-strain characteristic curve in the atmosphere of the set temperature and set humidity can be outputted. Hence, for example, even if the test piece  2  is made of a foaming material, a viscoelastic material, or a combination of these materials, that is, made of an impact absorbing material that damps or absorbs an impact force when the impact force is applied to the test piece  2 , the impact test apparatus  51  can output an impact stress-strain characteristic curve when an impact force is applied to the test piece  2  in the atmosphere of the arbitrary set temperature and set humidity. Moreover, at the same time, the impact test apparatus  51  can output also the impact absorption rate of the test piece  2  in the atmosphere of the arbitrary set temperature and set humidity. 
   Here, one example of an impact test will be described which was conducted on a test piece  2 , which was formed of a foaming material made of polypropylene and was set in the impact test apparatus  51  of the above construction, at set temperatures of −20° C., +20° C., and +60° C. and at a set humidity of 60%. 
   This test piece  2  was cut out of an impact absorbing material formed of a foaming material of polypropylene so as to have a size of 20 mm×20 mm×1 mm and then was held by the holding device  3  via the supporting plate  28  made of acrylic resin and having a size of 90 mm×80 mm×3 mm. At this time, the pressing member  12  was slightly pressed in from the base point to hold the test piece  2  in a state where the test piece  2  was compressed by 30% in the direction of thickness. 
   Then, the hammer  24  was set in a state where the hammer  24  was swung up at 45° with respect to the support column  20  and then was left at the set temperatures of −20° C., +20° C., and +60° C. and at the set humidity of 60% for approximately one hour. Then, the hammer  24  was separated from the arm  21 , thereby being brought into collision with the supporting plate  28 . At this time, the displacement image of the test piece  2  was picked up on a needed basis by the high-speed camera  6  (for example, FASTCAM manufactured by Photron). 
   Impact stress-strain characteristic curves  71 ,  72 , and  73  obtained under the test conditions described above at the set temperatures of −20° C., +20° C., and +60° C. and at the set humidity of 60% are shown in  FIG. 10 . As shown in  FIG. 10 , even if the test piece  2  was made of the impact absorbing material such as a foaming material of polypropylene, the impact stress-strain characteristic curves  71 ,  72 , and  73  at the set temperatures of −20° C., +20° C., and +60° C. and at the set humidity of 60% could be obtained. 
   Here, needless to say, the present invention is not limited to the above-described first and second embodiments but may be variously improved and modified without departing from the scope of the present invention, which is set forth in the appended claims. For example, the invention may be modified as follows. 
   Embodiment 3 
   As shown in  FIG. 11 , in place of measuring the displacement of the test piece  2  by the use of the above-described high-speed camera  6 , for example, an acceleration sensor  81  is fixed in a position close to a portion where the hammer  24  is brought into collision with the supporting plate  28  and an acceleration when the hammer  24  is brought into collision with the supporting plate  28  is integrated twice. With this, the displacement of the test piece  2  can be also found. 
   Embodiment 4 
   As shown in  FIG. 12 , in place of measuring the displacement of the test piece  2  by the use of the above-described high-speed camera  6 , a metal foil  82  is placed on the supporting plate  28  and a laser is applied to the metal foil  82  from a laser Doppler vibrometer to vibrate the metal foil. With this, the displacement of the test piece  2  can be also measured by the vibration of the metal foil  82 . 
   Embodiment 5 
   Further, in the above first and second embodiments, cases where the test piece  2  is held via the supporting plate  28  have been described as examples. However, for example, an impact force is uniformly applied to the whole surface of the test piece  2  made of impact absorbing material of a foaming material, a viscoelastic material, or a combination of these materials, by using a block-shaped part as the hammer  24 , without using the supporting plate  28 . With this, the impact stress-strain characteristic curve and the impact absorption rate of the test piece  2  can be also obtained.