Abstract:
An apparatus for testing the gas permeability on the inward facing direction of a pre-impregnated resin matrix composite material layer includes a mold, a loading apparatus, a temperature control apparatus, a vacuuming apparatus, and a flow detecting element. A test specimen is laid flat inside the mold between airtight sealing elements that leave exposed two ends of the test specimen on the inward facing direction of the layer. An air inlet and an air outlet respectively are arranged on the mold and correspond to the two exposed ends of the test specimen. The inlet is connected to the flow detecting element, and the outlet is connected to the vacuuming apparatus. Air is forced to move in a vacuumed cavity along the inward facing direction of the layer. Testing of air permeability of pre-impregnated material in solidification technique is allowed via adjustments of pressure and temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to International Application Serial No. PCT/CN2011/085123, filed Dec. 31, 2011, which claims priority to Chinese Application No. 201110030090.5, filed Jan. 27, 2011. International Application Serial No. PCT/CN2011/085123 is hereby incorporated herein for all purposes by this reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an apparatus and method for testing gas permeability, and more specifically, to an apparatus and method of testing gas permeability of a to-be-tested sample in a lateral direction of a resin matrix composite material layer by using gas as a fluid. 
       BACKGROUND OF THE INVENTION 
       [0003]    Permeability is a parameter characterizing the degree of difficulty of a fluid flowing through a porous medium under a pressure. The greater the permeability is, the smaller a resistance of the porous material against flow of the fluid is. During preparation of a resin-based composite material by using an autoclave or a vacuum bag process, permeability of a prepreg layer of resin matrix composite material is one of crucial factors affecting discharge of entrained air and volatile components under the action of vacuum, and thus affects the forming quality of members made of the composite material to a certain degree. A prepreg system comprises a fiber reinforcement and a resin substrate. Hence, the permeability of the prepreg layer of resin matrix composite material does not only depend on physical properties of the fiber and fiber network structure, but also relates to physical and chemical properties of the resin substrate. Currently, a method of testing permeability of a fiber layer by using a liquid as a fluid is getting mature, and there are documents and patents regarding the method. However, using the liquid to test the prepreg permeability has a lot of limitations: for example, (1) the prepreg system contains the resin substrate; if the liquid is used as a testing fluid, a flow front is difficult to be judged and thus permeability cannot be accurately tested; (2) the permeability of the prepreg layer is by far smaller than the permeability of the fiber layer. A flow of the liquid through prepreg is very small. On the one hand, the extremely small flow of the liquid between the layer and the testing mold during the testing procedure causes a substantial deviation in effect, and on the other hand, waste of time is caused. (3) the prepreg permeability in the composite material-forming process is mainly used to characterize the discharging capability of gases such as entrained air and volatile components under the action of vacuum, so there is a difference between use of the liquid as the testing fluid and actual situations. (4) During solidification, the prepreg layer is affected by temperature and pressure, so it is difficult for a conventional method to achieve on-line test of the permeability during the solidification. 
         [0004]    As can be seen from the above, the apparatus and method of using a gas as a fluid to test the permeability of the prepreg layer are of important theoretical significance and high application value, and confronted with a certain technical difficulty. Accurate measurement cannot be achieved unless influences from many factors are taken into consideration together. 
         [0005]    To this end, an object of the present invention is to provide a testing apparatus and method for testing gas permeability of a prepreg in the lateral direction of the layer, applied in the resin-based composite material. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a testing apparatus for testing gas permeability in a lateral direction of a resin matrix composite material layer. Preferably, the apparatus is applied to the test of permeability of a prepreg in a resin-based composite material. The apparatus achieves the test of gas permeability of the prepreg in the lateral direction of the layer by providing in a cavity a power enabling a gas to move in the lateral direction of the prepreg layer. Measurement of the gas permeability of the prepreg in a solidifying process is achieved by regulating a pressure and temperature. 
         [0007]    The present invention provides a testing apparatus for testing gas permeability in a lateral direction of a layer, comprising: a mold at least comprised of a cavity and a cover plate, a loading device for controlling a testing pressure, a temperature control device for controlling a testing temperature, a vacuum device for evacuating the cavity, and a flow detection element for detecting a gas flow, wherein the cover plate has an opening for receiving the loading device, the cavity and the cover plate forming an inner cavity, a sealing assembly being placed in the inner cavity and sealing the opening of the cover plate, a to-be-tested sample being laid flat between the sealing assembly and sealed airtight by the sealing assembly except both ends of the sample in the lateral direction of the layer. A gas inlet and a gas outlet are respectively arranged on the mold and correspond to said both ends of the sample not being sealed airtight, wherein the gas inlet is communicated with the flow detection element and the gas outlet is communicated with the vacuum device. After the vacuum device is started to make the pressure at the gas inlet higher than the pressure at the gas outlet, gas flows from the gas inlet, through the sample in the lateral direction of the layer of the sample, and then out of the gas outlet. 
         [0008]    Specifically, the control temperature device at least comprises a heating rod. 
         [0009]    Specifically, the vacuum device is a vacuum pump. 
         [0010]    Preferably, a vacuum gauge is provided between the vacuum pump and the gas outlet. 
         [0011]    Alternatively, the gas inlet and the gas outlet are provided on the cover plate, and the cover plate is fixed in the cavity. 
         [0012]    Specifically, the sealing assembly comprises an upper sealing sheet and a lower sealing sheet. 
         [0013]    Preferably, the sealing assembly comprises an upper sealing sheet, a lower sealing sheet and a pair of side sealing sheets located between the upper sealing sheet and the lower sealing sheet. 
         [0014]    More preferably, the mold further comprises a porous material which is provided between the upper sealing sheet and the lower sealing sheet in an up-down direction and corresponds to both ends of the sample not being sealed airtight in the lateral direction of the layer. The porous material is provided between the side sealing sheets in the lateral direction of the layer. 
         [0015]    More specifically, the porous material is a gas-permeable felt. 
         [0016]    Specifically, the loading device has a loading flat panel and a loading body, wherein the loading flat panel is received in the opening of the cover plate, and the loading body applies a pressure to the loading flat panel. 
         [0017]    More specifically, the loading body is a press. 
         [0018]    Specifically, a recess is provided respectively on both sides of the sealing assembly and the cover plate at a location adjacent to an inner wall of the cavity. 
         [0019]    Alternatively, the to-be-tested sample is a prepreg. 
         [0020]    Alternatively, the to-be-tested sample is laid between the sealing assembly in a uni-directional laying manner, orthogonal laying manner, or a quasi-isotropic laying manner. 
         [0021]    The testing apparatus for testing gas permeability of the prepreg in the lateral direction of the layer according to the present invention has the following advantages: (1) accurate test of the gas permeability of the prepreg in the lateral direction of the layer can be achieved under different pressure and temperature conditions; (2) the apparatus is adapted for a prepreg of various continuous fibers such as glass fiber, carbon fiber and aramid fiber as well as for various fabric prepregs; (3) the apparatus can achieve measurement of the gas permeability of the prepreg laid in different laying manners with different numbers of layers; (4) the apparatus exhibits a small measurement error and a high test reliability and a high repeatability; (5) the apparatus is simple, easily operable and time-saving. 
         [0022]    The present invention further provides a method of measuring gas permeability, comprising the following steps: 
         [0023]    (1) sealing the gas inlet of the mold and starting the vacuum device communicated with the gas outlet of the mold to inspect where there is gas leakage; 
         [0024]    (2) applying a testing pressure to the sealing assembly with and without the to-be-tested sample by the loading device respectively and measuring a thickness h of the sample under the testing pressure, if the inspection result in step (1) is no gas leakage; 
         [0025]    (3) setting a pressure of the vacuum device as P to form a differential pressure between the gas outlet and the gas inlet to enable the gas to flow, setting a temperature of the cavity as a testing temperature T via the temperature control device and obtaining a corresponding gas viscosity η, and detecting a gas flow rate Q via the flow detection element; 
         [0026]    (4) calculating the permeability in the lateral direction according to a permeability calculating formula. 
         [0027]    Specifically, the temperature-viscosity formula is 
         [0000]    
       
         
           
             
               
                 η 
                 
                   η 
                   0 
                 
               
               = 
               
                 
                   
                     ( 
                     
                       T 
                       
                         T 
                         0 
                       
                     
                     ) 
                   
                   
                     3 
                     2 
                   
                 
                  
                 
                   
                     
                       T 
                       0 
                     
                     + 
                     C 
                   
                   
                     T 
                     + 
                     C 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    wherein the parameters T 0  and η 0  respectively represent a reference temperature and a corresponding viscosity, and the parameter C represents a constant related to a gas type. 
         [0028]    Specifically, the permeability calculating formula is 
         [0000]    
       
         
           
             
               K 
               = 
               
                 
                   Q 
                    
                   
                       
                   
                    
                   η 
                    
                   
                       
                   
                    
                   l 
                 
                 Phb 
               
             
             , 
           
         
       
     
         [0000]    wherein the parameters l and b respectively represent a length and a width of the layer of the sample. 
         [0029]    Preferably, the step (1) further comprises a step of using a sealing adhesive tape to seal gaps between the sealing assembly and the cavity. 
         [0030]    Preferably, in the step (1), the vacuum device sets the pressure of the cavity as −0.1 MPa; if no pressure relief occurs after the vacuum device is switched off, there is no gas leakage. 
         [0031]    Specifically, in the step (2), if a thickness as measured in a way that the loading device applies a pressure to the sealing assembly without the sample is set as h1 and a thickness as measured in a way that the loading device applies a pressure to the cavity with the sample is set as h2, the thickness h of the sample is h2−h1. 
         [0032]    Specifically, in the step (3), a pressure P of the vacuum device is in a range of −0.1-0 MPa. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0033]    Exemplary embodiments of the present invention will be described hereunder with reference to the drawings to make the present application apparent. Among the figures, 
           [0034]      FIG. 1  is a structural view of a cavity of a testing apparatus for testing gas permeability in a lateral direction of a resin matrix composite material layer; 
           [0035]      FIG. 2  is a structural view of a sealing cover plate of the testing apparatus for testing gas permeability in a lateral direction of the layer; 
           [0036]      FIG. 3  is a view for assembling a testing mold of the testing apparatus for testing gas permeability in a lateral direction of the layer; 
           [0037]      FIG. 4  is an exploded view of  FIG. 3 ; 
           [0038]      FIG. 5  is a schematic view showing assembling of the testing apparatus for testing gas permeability in a lateral direction of the layer; 
           [0039]      FIG. 6  is a curve diagram showing a gas permeability of carbon fiber/epoxy  914  resin prepreg in a lateral direction of the layer, and a testing pressure; 
           [0040]      FIG. 7  is a curve diagram showing a gas permeability of T700 carbon fiber/bismaleimide resin uni-directional prepreg in a lateral direction of the layer, and a testing pressure. 
       
    
    
       [0041]    Wherein reference numbers in the figures denote the following features:
         1  testing mold     2  loading device     3  temperature control device     4  flow meter     5  vacuum gauge     6  vacuum pump     7  gas duct     21  loading flat panel     10  to-be-tested sample     11  cavity     111  threaded hole     112  locking bolt     113  heating rod hole     114  heating rod     12  sealing cover plate     121  through hole     122  recess     123  gas inlet     124  gas outlet     125  central opening     131  upper sealing sheet     132  lower sealing sheet     133  side sealing sheet     134  porous material       
 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0066]    The present invention will be further described in detail with reference to the figures. 
         [0067]    The present invention provides a testing apparatus for testing gas permeability of a to-be-tested sample  10  in a lateral direction of a resin matrix composite material layer. As shown in  FIG. 5 , the testing apparatus comprises a testing mold  1 , a loading device  2 , a temperature control device  3 , a flow detection element  4 , a vacuum detection element  5  and a vacuum device  6 . Specifically, the flow detection element  4  is a flow meter, the vacuum detection element  5  is a vacuum gauge, and the vacuum device  6  is a vacuum pump. In combination with  FIG. 1-FIG .  4 , the flow meter  4  is connected to a gas inlet  123  of the testing mold  1  via a gas duct  7 , a gas outlet  124  of the testing mold  1  is connected to the vacuum gauge  5  and the vacuum pump  6  via the gas duct  7 , a heating rod  114  of the testing mold  1  performs temperature control via the temperature control device  3 , and the temperature control device  3  is preferably a temperature control meter. The loading device  2  comprises a loading flat panel  21  and a loading body, wherein the loading body applies a pressure to a layer of the to-be-tested sample  10  via the loading flat panel  21 , and the loading body  21  is preferably a press machine. 
         [0068]    The testing mold  1  comprises a cavity  11  and a sealing cover plate  12 , wherein the cavity  11  is as shown in  FIG. 1 , a bottom of the cavity is provided with four threaded holes  111 , and two heating rod holes  113  are provided below. Referring to  FIG. 4 , in a receiving space of the cavity  11 , an upper sealing sheet  131 , a porous material  134  and side sealing sheets  133  and a lower sealing sheet  132  are received there one by one from upside to downside, wherein the loading flat panel  21  passes through a central opening  125  provided in the sealing cover plate  12  and flatly presses the upper sealing sheet  131  of the cavity  11 , and can transfer a loading pressure evenly to the to-be-tested sample  10  and effectively control a compression area of the to-be-tested sample  10 . The heating rods  114  are respectively provided in the cavity  11  through heating rod holes  113 . The upper sealing sheet  131  and the lower sealing sheet  132  are provided opposite to each other and have substantially the same shape. Two side sealing sheets  133  are longitudinal provided between the upper sealing sheet  131  and the lower sealing sheet  132  and laterally located respectively on one side and the other side relative to the upper sealing sheet  131  or the lower sealing sheet  132 . Gas tightness is provided between the upper sealing sheet  131  and the side sealing sheets  133  and between the side sealing sheets  133  and the lower sealing sheet  132 . Two porous materials  134  are provided at gaps of left and right ends formed by the upper sealing sheet  131 , the lower sealing sheet  132  and the two side sealing sheets. The to-be-tested sample  10  is laid flat in the space formed by the upper sealing sheet  131 , the lower sealing sheet  132 , side sealing sheets  133  and the porous material  134 . A gas inlet  123  and a gas outlet  124  are respectively provided at both ends of the sealing cover plate  12 , through holes  121  are provided on the sealing cover plate  12  and connected to the threaded hole  111  of the cavity  11  via a locking bolt  112 . The locking bolt  112  is used to apply compression to the sealing sheets to divide the cavity  11  into two independent subcavities. Recesses are provided on both sides of each of the sealing cover plate  12 , the upper sealing sheet  131 , side sealing sheets  133  and the lower sealing sheet  132  at a location adjacent to the wall of the cavity. Such design facilitates use of a sealing adhesive tape to eliminate the gaps between the sealing sheets and the cavity wall to ensure the two subcavities isolated from each other and prevent flow of gas through the gaps. 
         [0069]    The upper sealing sheet  131 , the lower sealing sheet  132  and side sealing sheets  133  constitute a sealing assembly of the present invention. However, those skilled in the art can appreciate that the sealing assembly may only have the upper sealing sheet and the lower sealing sheet. In such design, both of the upper and lower sealing sheets have side sealing ends which engage with each other to perform the function of the above side sealing sheets  133 , but the gas tightness effect of such design is worse than that of the sealing assembly having side sealing sheets  133 . 
         [0070]    In the present invention, the to-be-tested sample  10  is a pregreg applied to a resin-based composite material. Fibers in the prepreg may be glass fibers, carbon fibers, aramid fibers or basalt fibers; a prepreg reinforcement may be a uni-directional fiber or fiber fabric, wherein the fiber fabric may be plain fabric, twill fabric, satin fabric or the like; the resin in the prepreg may be epoxy resin, phenolic resin, cyanate ester resin, bismaleimide resin or the like. 
         [0071]    The present invention can be used to test gas permeability of a resin-based composite material pregreg in a lateral direction of a layer. Specific operations are as follows: 
         [0072]    Preparation of Sample 
         [0073]    At first, the pregreg is cut into pieces with a size 10×10 cm 2 , then the pieces are laid in a superimposed manner according to a laying manner and the number of layers to form a prepreg layer, wherein the laying manner may be uni-directional laying, orthogonal laying, quasi-isotropic laying or the like, and the number of layers is set according to the testing requirement. 
         [0074]    Packaging of Sample 
         [0075]    The lower sealing sheet  132  is laid flat in the cavity  11  first, then the duly-superimposed prepreg  10  is placed on the lower sealing sheet  132 , and then side sealing sheets  133  are placed close to the prepreg  10 . Although the prepreg is very close to the side sealing sheets  133  during the packaging, but complete sealing cannot be achieved between the prepreg and the side sealing sheets, and there is an edge effect. If measures are not taken, gas flowing along the edges of the pregreg  10  and side sealing sheets  133  will affect accuracy of the gas permeability. Hence, an organic silica gel is used here to bind the prepreg  10  together with the side sealing sheets  133  to effectively eradicate the edge effect and provide high precision of the testing results. A porous material  134  is provided at both ends of the layer of the prepreg  10 , wherein a thickness of the porous material  134  and the side sealing sheets  133  should be equivalent to the layer&#39;s thickness of the prepreg  10 , wherein the porous material  134  is preferably a gas-permeable felt with a certain thickness. Then, the upper sealing sheet  131 , the sealing cover plate  12  and the loading flat panel  21  are placed in turn as shown in  FIG. 4 , the locking bolts  112  are screwed tightly to fix the sealing cover plate  12  to the cavity  11 . The locking bolts  112  are used to apply compression to the upper and lower sealing sheets  131 ,  132  to divide the cavity  11  into two independent subcavities. Finally, a sealing adhesive tape is used to seal gaps between the sealing cover plate  12  and the cavity  11  and between the locking bolts  112  and the sealing cover plate  12 , and meanwhile the sealing adhesive tape is used to seal the gaps between the sealing sheets and the cavity  11  at the recess  122  to ensure no leakage of the cavity  11 . 
         [0076]    Cavity Leakage Testing 
         [0077]    The gas inlet  123  of the duly-packaged sealing cover plate  12  is sealed, and the gas outlet  124  is connected to the vacuum gauge  5  and the vacuum pump  6  via the gas duct  7 . Then, the vacuum pump  6  is started, and the sealing adhesive tape is inspected and pressed tightly. When the vacuum gauge  5  displays a value up to −0.1 MPa, the vacuum pump  6  is switched off, and whether the vacuum gauge  5  relieves pressure is checked. In the event of pressure relief, leakage point should be found out and sealed by the sealing adhesive tape. Such operations are repeated for many times until the vacuum gauge  5  does not relieve pressure after the vacuum pump  6  is switched off, and then the gas permeability testing can be performed. 
         [0078]    Testing of the thickness of the prepreg layer 
         [0079]    The loading flat panel  21  directly presses the upper sealing sheet  131  in the cavity  11 , and the upper and lower sealing sheets  131 ,  132  are pressurized by the loading body via the loading flat panel  21 . When a predetermined pressure is reached, a dial indicator is adjusted and fixed to allow its contact head to contact the upper surface of the loading flat panel  21 , and the dial indicator reads h 0 . 
         [0080]    After pressure relief, the loading flat panel  21  is moved away, the layer of the prepreg  10  is directly laid in the cavity  11  and the loading flat panel  21  is placed thereon. The prepreg  10  is pressurized by the loading body via the loading flat panel  21 . When a predetermined pressure is reached, the contact head of the dial indicator is allowed to contact the upper surface of the loading flat panel  21 , and the dial indicator reads h 1 . 
         [0081]    Under this pressure, the thickness h of the prepreg  10  is obtained by the formula h=h 1 −h 0 . 
         [0082]    Testing of permeability of the prepreg layer in the lateral direction 
         [0083]    Referring to  FIG. 5 , at first, the gas outlet  124  ( FIGS. 2 and 3 ) of the sealing cover plate  12  is connected to the vacuum gauge  5  and the vacuum pump  6  via the gas duct  7 , and the gas inlet  123  ( FIGS. 2 and 3 ) is connected to the flow meter  4  via the gas duct  7 . 
         [0084]    Secondly, the loading device  2  is adjusted to control the testing pressure applied to the layer of the prepreg  10 . In an embodiment of the present invention, the pressure is in a range of 0-0.6 MPa; 
         [0085]    Then, the temperature control device  3  is regulated to control a temperature T in the cavity  11 . In an embodiment of the present invention, the temperature should not be below the room temperature and also not above 200° C.; 
         [0086]    Then, the vacuum pump  6  is started. A gas valve is used to regulate a vacuum pressure (−0.1-0 MPa), and the vacuum meter  5  is used to accurately measure the vacuum pressure P; 
         [0087]    Under a pressure gradient, air flows through the flow meter  4 , the gas inlet  123 , the porous material  134 , the pregreg  10 , and the gas outlet  124  in turn. A volume flow rate Q of air is tested through the flow meter  4 , with a measure unit m 3 /s. 
         [0088]    Finally, permeability K in a lateral direction may be obtained according to Darcy law, and it has a measure unit m 2 . 
         [0089]    The permeability in the lateral direction of the layer can be calculated by the formula: 
         [0000]    
       
         
           
             
               K 
               = 
               
                 
                   Q 
                    
                   
                       
                   
                    
                   η 
                    
                   
                       
                   
                    
                   l 
                 
                 Phb 
               
             
             , 
           
         
       
     
         [0000]    wherein the parameter η represents gas viscosity with a measure unit Pa·s; the parameter P represents an indication of the vacuum meter  5  with a measure unit Pa; the parameters l and b respectively represent a length and a width of the layer of the prepreg  10 ; since the values of the parameters l and b are equal to each other, the formula for calculating the permeability in the lateral direction may be simplified as: 
         [0000]    
       
         
           
             K 
             = 
             
               
                 
                   Q 
                    
                   
                       
                   
                    
                   η 
                 
                 Ph 
               
               . 
             
           
         
       
     
         [0090]    By adjusting the loading device  2  and the temperature control device  3 , and changing the pressure and temperature acting on the layer of the prepreg  10 , the permeability of the prepreg  10  in the lateral direction is tested under different pressure and temperature conditions. 
         [0091]    Air viscosity η increases as the temperature T rises. When the temperature T is below 2000K, the gas viscosity may be calculated from Satran formula: 
         [0000]    
       
         
           
             
               η 
               
                 η 
                 0 
               
             
             = 
             
               
                 
                   ( 
                   
                     T 
                     
                       T 
                       0 
                     
                   
                   ) 
                 
                 
                   3 
                   2 
                 
               
                
               
                 
                   
                     
                       T 
                       0 
                     
                     + 
                     C 
                   
                   
                     T 
                     + 
                     C 
                   
                 
                 . 
               
             
           
         
       
     
         [0092]    Wherein in the formula, the parameter T represents the temperature under a testing condition with a measure unit K; the parameters T 0  and η represent a reference temperature (K) and a corresponding viscosity respectively; the parameter C represents a constant related to a gas type, and the constant of air C is 110.4K. Air viscosity under partial temperatures is listed in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Air Viscosity under Different Temperature Conditions 
               
             
          
           
               
                   
                 Temperature (° C.)  
                 Viscosity (10 −6  Pa·S) 
               
               
                   
                   
               
             
          
           
               
                   
                 0 
                 16.8 
               
               
                   
                 10 
                 17.3 
               
               
                   
                 15 
                 17.8 
               
               
                   
                 20 
                 18.0 
               
               
                   
                 40 
                 19.1 
               
               
                   
                 60 
                 20.3 
               
               
                   
                 80 
                 21.5 
               
               
                   
                 100 
                 22.8 
               
               
                   
                   
               
             
          
         
       
     
         [0093]    A volume of gas is greatly affected by the temperature. The flow meter  4  measures a gas flow under the measurement value of the temperature T 0 . When the temperature of the testing mold is T, the gas flow under this temperature condition may be calculated by using a modification formula 
         [0000]    
       
         
           
             Q 
             = 
             
               
                 T 
                 
                   T 
                   0 
                 
               
                
               
                 
                   Q 
                   0 
                 
                 . 
               
             
           
         
       
     
       Embodiment 1 
       [0094]    In the present embodiment, the carbon fiber/epoxy  914  resin fabric prepreg is used as a testing object. The pregreg is cut into pieces with a size 10×10 cm 2 , and seven layers of the pieces are laid. The loading device is used to apply a load to the prepreg layer, and then the gas permeability of the prepreg layer in the lateral direction is measured under different pressure conditions. At the testing temperature 20° C., the air viscosity is 18×10 −6  Pa·S. The testing results are shown in  FIG. 6 . 
       Embodiment 2 
       [0095]    In the present embodiment, the T700 carbon fiber/bismaleimide resin uni-directional prepreg is used as a testing object. The pregreg is cut into pieces with a size 10×10 cm 2 , and 16 layers of the pieces are laid. The laying manner is uni-directional laying. The loading device is used to apply a load to the prepreg layer, and then the gas permeability of the prepreg layer in the lateral direction is measured under different pressure conditions. At the testing temperature 20° C., the air viscosity is 18×10 −6  Pa·S. The testing results are shown in  FIG. 7 . 
       Embodiment 3 
       [0096]    In the present embodiment, the carbon fiber/epoxy  914  resin fabric prepreg is used as a testing object. The pregreg is cut into pieces with a size 10×10 cm 2 , and seven layers of the pieces are laid. The loading device is used to apply a 0.14 MPa pressure to the prepreg layer, and then the gas permeability of the prepreg layer in the lateral direction is measured under different temperature conditions. The testing results are shown in Table 2 below. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Gas Permeability of the Prepreg Layer in the Lateral  
               
               
                 Direction under Different Temperature Conditions 
               
             
          
           
               
                   
                 Temperature 
                 Gas permeability in the 
               
               
                   
                 (° C.) 
                 lateral direction (10-11 m 2 ) 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 2.45 
               
               
                   
                 30 
                 2.21 
               
               
                   
                 40 
                 2.10 
               
               
                   
                 50 
                 1.97 
               
               
                   
                 60 
                 1.81 
               
               
                   
                 70 
                 1.77 
               
               
                   
                 80 
                 — 
               
               
                   
                   
               
             
          
         
       
     
         [0097]    In the above embodiments, the range of the testing pressure varies. For example, in embodiment 1, the testing pressure may reach 1 MPa, while the testing pressure in embodiment 2 may reach 1.5 MPa. Those skilled in the art may appreciate that the pressure range upon application of the present invention for testing is not limited to 0-0.6 MPa, and the testing may be performed in a broader pressure range according to the loading capability of the loading device. In addition, the testing temperature range in the present invention is from room temperature to 200° C. This is because a cooling system is not additionally provided in the present invention and a temperature upper limit of the testing instruments is 200° C. However, those skilled in the art may appreciate that so long as the testing instrument permits, the present invention may test permeability in a broader temperature range. 
         [0098]    The present invention is not, in any form, limited to exemplary embodiments presented in the description and drawings. Many variations are possible within the scope of the present invention as generalized by the appended claim set. In addition, any reference signs in the claim set shall not be construed as limiting the scope of the present invention.