Patent Publication Number: US-2023135339-A1

Title: Sandwich panel and manufacturing method for sandwich panel

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2021-180363 filed in Japan on Nov. 4, 2021. 
     FIELD 
     The present disclosure relates to a sandwich panel and a manufacturing method for the sandwich panel. 
     BACKGROUND 
     It has been known that there is a delamination development prevention structure of a sandwich panel for preventing development of delamination of the sandwich panel (for example, refer to Patent Literature 1). The delamination development prevention structure of the sandwich panel has a structure in which a delamination development prevention piece is disposed to project from a face plate of the sandwich panel toward an inner side in a thickness direction as a core material side. The delamination development prevention piece is formed on an unbent surface, and has a substantially semicircular cross-sectional shape, for example. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2006-282046 
     SUMMARY 
     Technical Problem 
     Delamination of a sandwich panel occurs when external force caused by a vibration or an impact is applied to the sandwich panel. Herein, the external force includes a mode in which the external force is applied in an in-plane direction of a joint surface between the face plate and the core material (referred to as a lateral mode), and a mode in which the external force is applied in a thickness direction of the core material (referred to as a vertical mode). A conventional delamination development prevention piece is a composite material being in contact with the core material made of flexible foam material, so that development of delamination due to external force in the lateral mode can be prevented, but it is difficult to suppress occurrence and development of delamination due to external force with an impact in the vertical mode. 
     Thus, a problem of the present disclosure is to provide a sandwich panel and a manufacturing method for the sandwich panel that can preferably suppress delamination even in a case in which external force with an impact is applied in the thickness direction. 
     Solution to Problem 
     A sandwich panel according to the present disclosure includes: a core material having a plate shape; a pair of face plates that are formed using a composite material, and respectively disposed on both sides in a thickness direction of the core material; and a crack arrester that is formed using the composite material, disposed on at least one side in the thickness direction of the core material, disposed between the face plate and the core material, and disposed to project from the face plate toward the core material side. The crack arrester has flat side surfaces being in contact with the core material and extending along the thickness direction from a boundary surface between the face plate and the crack arrester. An angle formed by the side surface and the boundary surface is equal to or larger than 90 degrees. 
     A manufacturing method according to the present disclosure is for a sandwich panel for manufacturing a sandwich panel. The sandwich panel includes a core material having a plate shape; a pair of face plates respectively disposed on both sides in a thickness direction of the core material; and a crack arrester that is disposed on at least one side in the thickness direction of the core material, disposed between the face plate and the core material, and disposed to project from the face plate toward the core material side. The manufacturing method includes: forming a groove to have a shape complementary to the crack arrester on the core material; disposing a composite material to be the crack arrester in the groove; disposing the composite material to be the pair of face plates on both sides in the thickness direction of the core material; and joining the composite material with the core material to form the sandwich panel. The forming of the groove includes processing a working surface of the core material to form a groove having an opening and flat side surfaces extending along the thickness direction. An angle formed by the side surface and the working surface at the opening is equal to or larger than 90 degrees. 
     Advantageous Effects of Invention 
     According to the present disclosure, delamination can be preferably suppressed even in a case in which external force with an impact is applied in a thickness direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view of a sandwich panel according to a first embodiment. 
         FIG.  2    is an exploded perspective view illustrating constituent elements of the sandwich panel. 
         FIG.  3    is a flowchart related to a manufacturing method for the sandwich panel according to the first embodiment. 
         FIG.  4    is a diagram illustrating performance related to presence/absence of a crack arrester. 
         FIG.  5    is an explanatory diagram illustrating an analytic model of the crack arrester. 
         FIG.  6    is a diagram illustrating performance of an example corresponding to a type of the crack arrester. 
         FIG.  7    is a diagram illustrating performance of an example corresponding to a type of the crack arrester. 
         FIG.  8    is a diagram of a sandwich panel according to a second embodiment. 
         FIG.  9    is a cross-sectional view of a sandwich panel according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments according to the present disclosure in detail based on the drawings. Note that the present invention is not limited to the embodiments. Constituent elements in the following embodiments include a constituent element that is easily replaced by those skilled in the art, or substantially the same constituent element. The constituent elements described below can be appropriately combined with each other. In a case in which there are a plurality of embodiments, the embodiments can also be combined with each other. 
     First Embodiment 
     Sandwich Panel 
     A sandwich panel  10  according to a first embodiment is a panel that is to be disposed on a vehicle, for example, an amphibious vehicle, to prevent a flying object from passing therethrough.  FIG.  1    is a cross-sectional view of the sandwich panel according to the first embodiment. FIG. 
       2  is an exploded perspective view illustrating constituent elements of the sandwich panel.  FIG.  3    is a flowchart related to the manufacturing method for the sandwich panel according to the first embodiment. 
     Flying object is assumed to move toward a thickness direction of the sandwich panel  10 , so that a surface on one side of the sandwich panel  10  is an outer surface that the flying object enters, and a surface on the other side thereof is an inner surface from which the flying object is emitted. In  FIG.  1    and  FIG.  2   , a lower side is an outer side, and an upper side is an inner side. Thus, the flying object moves from the lower side toward the upper side in  FIG.  1    and  FIG.  2   . 
     As illustrated in  FIG.  1    and  FIG.  2   , the sandwich panel  10  includes a core material  11 , a pair of face plates  13 , and crack arresters  15 . As illustrated in  FIG.  2   , the sandwich panel  10  also includes a pair of adhesive films  17  respectively disposed between the core material  11  and the pair of face plates  13  at the time of molding. 
     The core material  11  is formed in a plate shape. The core material  11  is made of material having high rigidity, and has a shear modulus equal to or more than  50  MPa. The shear modulus of the core material  11  is more preferably in a range from not less than 136 MPa to not more than 362 MPa. As the core material  11 , for example, a balsa core is applied. The balsa core is made by wood with porous material. In the first embodiment, the balsa core is applied as the core material  11 , but the embodiment is not limited thereto. The core material  11  may be made of, for example, a resin-based foam material so long as such a material has high rigidity and achieves a shear modulus equal to or more than 50 MPa. 
     The core material  11  includes grooves  21  in which the crack arresters  15  are housed, the grooves  21  having a shape complementary to the crack arresters  15 . The grooves  21  are formed by performing cutting work on one surface of the core material  11  as a working surface  23 . The grooves  21  are formed to extend in one direction (longitudinal direction) within the working surface. As illustrated in  FIG.  2   , the grooves  21  are formed in parallel at predetermined intervals in the other direction (width direction) orthogonal to the one direction within the working surface. Each of the grooves  21  includes an opening  25 , a pair of side surfaces  26 , and a bottom surface  27 , and a cross section of the groove  21  cut along a surface orthogonal to the longitudinal direction has a substantially quadrangular shape. The opening  25  is a part where the groove  21  opens, and disposed along the longitudinal direction. The pair of side surfaces  26  are surfaces opposed to each other in the width direction, and are flat surfaces extending along the thickness direction from the working surface  23 . The bottom surface  27  is disposed across the pair of side surfaces  26 . Thus, an inner surface side in the thickness direction of the side surface  26  intersects with the working surface  23 , and an outer surface side in the thickness direction thereof intersects with the bottom surface  27 . 
     In the first embodiment, the cross section of the groove  21  has a substantially quadrangular shape, but the shape is not particularly limited thereto. Details will be described later, but for example, the cross section thereof may have a shape illustrated in  FIG.  9    as a third embodiment. 
     Each of the pair of face plates  13  is formed in a plate shape by using a composite material in which reinforced fiber is impregnated with resin. As the composite material, for example, a composite material such as carbon fiber reinforced plastics (CFRP) is used. The composite material is not limited to the CFRP, but may be any composite material containing reinforced fiber and resin. The pair of face plates  13  are joined to both surfaces of the core material  11  using an adhesive agent. 
     The crack arrester  15  is formed in a stick shape that is long in the longitudinal direction using a composite material in which reinforced fiber is impregnated with resin. Similarly to the face plate  13 , a composite material such as the CFRP is used as the composite material. The composite material is not limited to the CFRP, but may be any composite material containing reinforced fiber and resin. 
     The crack arrester  15  is disposed on at least one side (inner side) in the thickness direction of the core material  11 . Specifically, the crack arrester  15  is disposed between the core material  11  and the face plate  13  on one side (inner side) to project toward the core material  11  side from the face plate  13 . The crack arrester  15  is housed in the groove  21  formed on the core material  11 . The crack arrester  15  has a shape complementary to the groove  21 . The crack arrester  15  is formed to extend in one direction (longitudinal direction) within the working surface. Additionally, as illustrated in  FIG.  2   , a plurality of the crack arresters  15  are formed to be arranged in parallel at predetermined intervals in the width direction within the working surface  23 . 
     The crack arrester  15  has a pair of side surfaces  31  and a front end surface  32 , and a cross section of the crack arrester  15  cut along a surface orthogonal to the longitudinal direction has a substantially quadrangular shape similarly to the groove  21 . The crack arrester  15  also has a boundary surface  33  as a surface joined to the core material  11 . The boundary surface  33  is positioned at the opening  25  of the groove  21 , and formed along the longitudinal direction. The pair of side surfaces  31  are surfaces opposed to each other in the width direction, and are flat surfaces being in contact with the core material  11  and extending along the thickness direction from the boundary surface  33 . The front end surface  32  is disposed across the pair of side surfaces  31 . Thus, an inner surface side in the thickness direction of the side surface  31  intersects with the boundary surface  33  of the core material  11 , and an outer surface side in the thickness direction thereof intersects with the front end surface  32 . 
     An angle θ 1  formed by the side surface  31  and the boundary surface  33  of the crack arrester  15  is equal to or larger than 90 degrees. Specifically, in the first embodiment, the angle θ 1  is 90 degrees. That is, the pair of side surfaces  26  of the groove  21  and the pair of side surfaces  31  of the crack arrester  15  are in a state of being in vertical contact with the boundary surface  33 . The angle θ 1  is assumed to be 90 degrees in the first embodiment, but is not particularly limited thereto so long as the angle θ 1  is equal to or larger than 90 degrees and smaller than 180 degrees. If the angle θ 1  is equal to or larger than 90 degrees, load transfer due to shear or mechanical fitting between the crack arrester  15  and the core material  11  can be expected, and improvement in an effect of suppressing delamination can be expected. 
     Herein, a length in the width direction of the crack arrester  15  is assumed to be D, a length in the thickness direction thereof is assumed to be L, and a ratio of the length L in the thickness direction to the length D in the width direction on the boundary surface  33  is assumed to be L/D. In this case, the ratio L/D of the crack arrester is larger than ½. That is, the length L in the thickness direction is longer than a half of the length D in the width direction. 
     The adhesive films  17  are respectively disposed between the core material  11  and the pair of face plates  13  before molding of the sandwich panel  10 . The adhesive film  17  is a thermosetting resin, for example, and is thermally cured after viscosity thereof is lowered when being heated at the time of molding, thereby joining the core material  11  with the face plate  13 . Additionally, part of the adhesive films  17  is respectively disposed between the face plate  13  and the crack arresters  15 , and is thermally cured after viscosity thereof is lowered when being heated at the time of molding, thereby joining the face plate  13  with the crack arresters  15 . 
     Regarding the sandwich panel  10  as described above, when a flying object comes flying toward the sandwich panel  10 , external force with an impact of the flying object is applied from an outer side toward an inner side in the thickness direction of the sandwich panel  10 . That is, shearing force in the thickness direction is applied to the sandwich panel  10  due to the external force with the impact of the flying object. At this point, the side surface  26  of the core material  11  and the side surface  31  of the crack arrester  15  are joined to each other along the thickness direction, so that a structure having resistance against the shearing force is achieved, and occurrence and a developing range of delamination are suppressed. 
     Manufacturing Method for Sandwich Panel 
     Next, the following describes the manufacturing method for the sandwich panel  10  with reference to  FIG.  2    and FIG.  3 . In the manufacturing method for the sandwich panel  10 , described is a case of manufacturing the sandwich panel  10  illustrated in  FIG.  1    and  FIG.  2   . 
     First, in the manufacturing method for the sandwich panel  10 , the grooves  21  are formed on the core material  11  (Step S 1 ). At Step S 1 , for example, the grooves  21  are formed by performing cutting work on the working surface  23  of the core material  11  using a machining device for performing cutting work. At Step S 1 , the grooves  21  are formed to extend in the longitudinal direction, and formed to be arranged in parallel at predetermined intervals in the width direction. As described above, the grooves  21  formed at Step S 1  each have the opening  25 , the pair of side surfaces  26 , and the bottom surface  27 . At this point, at Step S 1 , each of the grooves  21  is formed so that an angle formed by the side surface  26  and the working surface  23  of the core material  11  at the opening  25  is equal to or larger than 90 degrees. 
     Subsequently, in the manufacturing method for the sandwich panel  10 , composite materials to be the crack arresters  15  are disposed in the grooves  21  (Step S 2 ). As the composite material, used is a composite material before curing in which reinforced fiber is impregnated with resin, specifically, used is a unidirectional material to be continuous fiber extending in one direction. At Step S 2 , the composite material is disposed so that the longitudinal direction of the groove  21  becomes a fiber direction. 
     Next, in the manufacturing method for the sandwich panel  10 , composite materials to be the pair of face plates  13  are disposed on both sides in the thickness direction of the core material  11  (Step S 3 ). As the composite material, used is a composite material before curing in which reinforced fiber is impregnated with resin, specifically, used is a fiber sheet. At Step S 3 , at the time of disposing the pair of face plates  13 , the adhesive films  17  are respectively disposed between the core material  11  and the face plates  13 . 
     In the manufacturing method for the sandwich panel  10 , the composite materials and the adhesive films  17  are heated and thermally cured to mold the pair of face plates  13  and the crack arresters  15 , and the pair of face plates  13  and the core material  11  are joined to each other to form the sandwich panel  10  (Step S 4 ). 
     Performance of Sandwich Panel 
     Next, the following describes performance of the sandwich panel  10  with reference to  FIG.  4    to  FIG.  7   .  FIG.  4    is a diagram illustrating performance related to presence/absence of the crack arrester.  FIG.  5    is an explanatory diagram illustrating an analytic model of the crack arrester.  FIG.  6    and  FIG.  7    are diagrams illustrating performance of examples corresponding to types of the crack arrester. 
     With reference to  FIG.  4   , damaged areas on the sandwich panel with crack arresters  15  and the sandwich panel without crack arresters  15  generated by collision with a flying object are compared with each other. As illustrated in  FIG.  4   , the damaged area has a numerical value normalized by an average damaged area in a case in which the crack arresters  15  are absent. In a case in which the crack arresters  15  are present, the damaged area is increased from an entering side toward an emitting side from the flying object. On the other hand, an average damaged area in a case in which the crack arresters  15  are present is smaller than an average damaged area in a case in which the crack arresters  15  are absent. 
     Next, with reference to  FIG.  5   , the following describes an analytic model for evaluating the crack arresters  15  of different types. A conventional crack arrester  15 A illustrated on an upper side of  FIG.  5    has a semicircular cross section cut along a surface orthogonal to the longitudinal direction. A conventional crack arrester  15 B illustrated in the middle of  FIG.  5    has an equilateral triangular cross section cut along a surface orthogonal to the longitudinal direction, a base of the cross section being the boundary surface  33 . A crack arrester  15 C according to the present embodiment illustrated on a lower side of  FIG.  5    has a quadrangular cross section cut along a surface orthogonal to the longitudinal direction, and the ratio L/D is ½. 
     A position P 1  and a position P 2  in  FIG.  5    are positions for evaluating delamination. The position P 1  is a predetermined position outside the crack arresters  15 A,  15 B, and  15 C on the boundary surface  33 . The position P 2  is a position at a predetermined depth in the thickness direction from the boundary surface  33 . 
     In  FIG.  6   , loads with which delamination develops at the position P 1  are compared with each other depending on shapes of the crack arresters  15 A,  15 B, and  15 C illustrated in  FIG.  5   . That is, evaluation is made for a load with respect to a lateral mode in which delamination triggered when external force is applied in an in-plane direction of the boundary surface  33  by the flying object develops in a lateral direction. As illustrated in  FIG.  6   , each load has a numerical value normalized by a load in a case in which the crack arresters  15  are absent. As materials of the crack arresters  15 A,  15 B, and  15 C, used are a composite material obtained by overlapping unidirectional materials while causing fiber directions thereof to be different by 90°, a composite material containing carbon fiber being short fiber, and a resin material obtained by curing an adhesive agent. 
     As illustrated in  FIG.  6   , the crack arrester  15 C having a quadrangular shape and made of a composite material containing reinforced fiber may be more load-bearing, and particularly, the crack arrester  15 C made of carbon fiber being short fiber may be the most load-bearing. Although having a quadrangular shape, the crack arrester  15 C made of a resin material has a load smaller than that of the crack arrester made of a composite material. The crack arrester  15 A having a semicircular shape and the crack arrester  15 B having an equilateral triangular shape each have a load smaller than that of the crack arrester having a quadrangular shape and made of a composite material. Thus, it has been confirmed by analysis that delamination hardly develops at the position P 1  in the crack arrester  15 C having a quadrangular shape and made of the composite material as compared with the other crack arresters  15 A and  15 B. 
     In  FIG.  7   , loads with which delamination develops at the position P 2  are compared with each other depending on the shapes of the crack arresters  15 A,  15 B, and  15 C illustrated in  FIG.  5   . That is, evaluation is made for a load with respect to a vertical mode in which delamination triggered when external force is applied in the thickness direction of the sandwich panel  10  by the flying object develops in a plate thickness direction. In  FIG.  7   , similarly to  FIG.  6   , the load has a numerical value normalized by a load in a case in which the crack arresters  15  are absent. The materials of the crack arresters  15 A,  15 B, and  15 C in  FIG.  7    are the same as those in  FIG.  6   . 
     As illustrated in  FIG.  7   , the crack arrester  15 C having a quadrangular shape and made of a composite material containing reinforced fiber may be more load-bearing, and particularly, the crack arrester  15 C obtained by overlapping unidirectional materials while causing directions thereof to be different by 90° may be the most load-bearing. Although having a quadrangular shape, the crack arrester  15 C made of a resin material has a load smaller than that of the crack arrester made of a composite material. The crack arrester  15 A having a semicircular shape and the crack arrester  15 B having an equilateral triangular shape each have a load smaller than that of the crack arrester having a quadrangular shape and made of a composite material. Thus, it has been confirmed by analysis that delamination hardly develops at the position P 2  in the crack arrester  15 C having a quadrangular shape and made of a composite material as compared with the other crack arresters  15 A and  15 B. 
     Second Embodiment 
     Next, the following describes a second embodiment with reference to  FIG.  8   . In the second embodiment, portions different from those in the first embodiment are described to avoid redundant description. A portion having the same configuration as that in the first embodiment will be denoted by the same reference numeral.  FIG.  8    is a diagram of the sandwich panel according to the second embodiment. 
     A sandwich panel  50  in the second embodiment includes a crack arrester  51  in place of the crack arresters  15  in the first embodiment. The crack arrester  51  in the second embodiment is formed in a lattice shape having intersecting portions  53  and side portions  54  within the boundary surface  33 . Due to this, on the core material  11  of the sandwich panel  50 , a lattice-shaped groove having a shape complementary to the crack arrester  51  is formed in place of the grooves  21  in the first embodiment. 
     In the crack arrester  51  having the lattice shape, the side portions  54  extend in the longitudinal direction and also extend in the width direction. The side portions  54  extending in the longitudinal direction are disposed in parallel in the width direction. The side portions  54  extending in the width direction are disposed in parallel in the longitudinal direction. A part where the side portion  54  extending in the longitudinal direction intersects with the side portion  54  extending in the width direction is the intersecting portion  53 . 
     The intersecting portion  53  includes, as reinforced fiber contained in a composite material, reinforced fiber being short fiber. That is, the composite material used for the intersecting portion  53  is a short-fiber reinforced resin. The side portion  54  is continuous fiber as reinforced fiber contained in a composite material in which a fiber direction of the reinforced fiber extends in a direction along the side. That is, the composite material used for the side portion  54  is a fiber-reinforced resin using a unidirectional material. Due to this, in the crack arrester  51 , the composite materials at the intersecting portion  53  are prevented from overlapping in the thickness direction of the reinforced fiber, so that the thickness of the intersecting portion  53  is enabled to be equivalent to that of the side portion  54 . 
     A cross section of the side portion  54  of the crack arrester  51  cut along a surface orthogonal to a direction along the side is the same as the cross section in the first embodiment. In the crack arrester  51  according to the second embodiment, the unidirectional material is used as the reinforced fiber for the side portion  54 , but the reinforced fiber for the side portion  54  may be short fiber similarly to the reinforced fiber for the intersecting portion  53 . That is, all reinforced fiber in the composite material may be short fiber. 
     Third Embodiment 
     Next, the following describes a third embodiment with reference to  FIG.  9   . In the third embodiment, portions different from those in the first and the second embodiments are described to avoid redundant description. A portion having the same configuration as that in the first and the second embodiments will be denoted by the same reference numeral.  FIG.  9    is a cross-sectional view of the sandwich panel according to the third embodiment. 
     In a sandwich panel  60  according to the third embodiment, a groove  61  formed on the core material  11  has a shape different from that of the groove  21  in the first embodiment. The groove  61  in the third embodiment is a groove on which cutting work is performed with an end mill having a rounded front end. The groove  61  has an opening  65 , a pair of side surfaces  66 , and a bottom surface  67 . The opening  65  is a part where the groove  61  opens, and disposed along the longitudinal direction similarly to the opening  25  in the first embodiment. The pair of side surfaces  66  are surfaces opposed to each other in the width direction, and are flat surfaces extending along the thickness direction from the working surface  23  similarly to the side surfaces  26  in the first embodiment. The bottom surface  67  is disposed across the pair of side surfaces  66 , and has a shape along the front end of the end mill. Specifically, the bottom surface  67  is a curved surface projecting downward with a predetermined curvature at a cross section cut along the longitudinal direction. Due to this, the front end surface  32  of the crack arrester  15  having a shape complementary to the groove  61  is also a curved surface with the predetermined curvature. The front end surface  32  of the crack arrester  15  is a surface continuous to the core material  11  side in the thickness direction of the side surface  31 . 
     In the manufacturing method for the sandwich panel  10 , in a case of forming the groove  61  according to the third embodiment, the groove  61  is formed by performing cutting work on the working surface  23  of the core material  11  using the end mill at Step S 1 . Specifically, at Step S 1 , the end mill is caused to abut on the working surface  23  of the core material  11  to perform cutting work while being rotated, and the end mill is moved relatively to the core material  11  along the longitudinal direction of the groove  61 . Due to this, in the third embodiment, the groove  61  that is long in the longitudinal direction illustrated in  FIG.  9    is formed by the end mill. 
     The shapes of the crack arresters  15  and  51  described in the first embodiment to the third embodiment are not particularly limited, but the crack arrester  15  may have any shape so long as the angle formed by the side surface and the boundary surface  33  is equal to or larger than 90 degrees. For example, the crack arrester  15  may be a dovetail projection the side surfaces  31  of which spread out toward the front end side in the thickness direction. 
     As described above, the sandwich panels  10 ,  50 , and  60 , and the manufacturing method for the sandwich panels  10 ,  50 , and  60  described in the present embodiments are grasped as follows, for example. 
     The sandwich panels  10 ,  50 , and  60  according to a first aspect include: the core material  11  having a plate shape; the pair of face plates  13  that are formed using the composite material and respectively disposed on both sides in the thickness direction of the core material  11 ; and the crack arresters  15  and  51  that are formed using the composite material, disposed on at least one side in the thickness direction of the core material  11 , disposed between the face plate  13  and the core material  11 , and disposed to project from the face plate  13  toward the core material  11  side. The crack arresters  15  and  51  include the flat side surfaces  31  being in contact with the core material  11  and extending along the thickness direction from the boundary surface  33  between the face plate  13  and the crack arresters  15  and  51 . The angle formed by the side surface  31  and the boundary surface  33  is equal to or larger than 90 degrees. 
     With this configuration, even in a case of a load (shearing force) in the vertical mode in which external force with an impact is applied in the thickness direction, a structure having resistance against occurrence and development of delamination due to the shearing force can be achieved. Due to this, delamination between the crack arresters  15  and  51  and the core material  11  at a joint part can be preferably suppressed. 
     As a second aspect, the crack arresters  15  and  51  include the two side surfaces  31  opposed to each other in the in-plane direction of the boundary surface  33 . Assuming that the direction in which the side surfaces  31  are opposed to each other is the width direction, the length in the width direction of the crack arresters  15  and  51  is D, and the ratio of the length L in the thickness direction to the length D in the width direction on the boundary surface  33  is L/D, the ratio of the crack arresters  15  and  51  is larger than ½. 
     With this configuration, the length L in the thickness direction of the crack arresters  15  and  51  can be increased, so that the resistance against the shearing force can be further increased. 
     As a third aspect, the crack arresters  15  and  51  include the two side surfaces  31  opposed to each other in the in-plane direction of the boundary surface  33 , and the front end surface  32  to be a surface connected to the core material  11  side in the thickness direction of the side surface  31 . 
     With this configuration, the crack arrester  15  having a simple shape can be formed by the two side surfaces  31  and the front end surface  32 . The shape of the front end surface is not particularly limited, and may be a flat surface, or a curved surface with a predetermined curvature. 
     As a fourth aspect, the crack arresters  15  are disposed to extend in the longitudinal direction as a predetermined direction within the boundary surface  33 , and disposed in parallel at predetermined intervals in the direction orthogonal to the longitudinal direction. The crack arrester  15  includes, as reinforced fiber contained in the composite material, a unidirectional material in which the fiber direction of the reinforced fiber is the longitudinal direction. 
     With this configuration, the composite material can be disposed so that the fiber direction thereof becomes the longitudinal direction of the crack arresters  15 , so that the crack arresters  15  can be molded to be strong against external force. 
     As a fifth aspect, the crack arrester  15  is formed in a lattice shape having the intersecting portions  53  and the side portions  54  within the boundary surface  33 . The intersecting portion  53  includes the reinforced fiber being short fiber as reinforced fiber contained in the composite material, and the side portion  54  includes the unidirectional material in which the fiber direction of the reinforced fiber is a direction along the side as the reinforced fiber contained in the composite material. 
     With this configuration, it is possible to prevent the thickness of the intersecting portion  53  from being increased due to overlap of fibers at the intersecting portion  53 . Due to this, the entire thickness of the crack arrester  15  can be made uniform. 
     As a sixth aspect, the core material  11  includes the groove  61  formed to have a shape complementary to the crack arrester  15 , and the groove  61  is a groove processed by using the end mill. 
     With this configuration, the groove  61  can be easily formed by the end mill. 
     As a seventh aspect, the core material  11  has a shear modulus equal to or more than 50 MPa. 
     With this configuration, even in a case in which the flying object collides with the sandwich panels  10 ,  50 , and  60 , a range of damage of the sandwich panels  10 ,  50 , and  60  caused by the flying object can be suppressed. 
     As an eighth aspect, the core material  11  is the balsa core. 
     With this configuration, the core material  11  that can suppress the range of damage caused by the flying object can be obtained by using an inexpensive material. 
     The manufacturing method for the sandwich panels  10 ,  50 , and  60  according to a ninth aspect is the manufacturing method for the sandwich panels  10 ,  50 , and  60  for manufacturing the sandwich panels  10 ,  50 , and  60  including: the core material  11  having a plate shape; the pair of face plates  13  respectively disposed on both sides in the thickness direction of the core material  11 ; and the crack arresters  15  and  51  that are disposed on at least one side in the thickness direction of the core material  11 , disposed between the face plate  13  and the core material  11 , and disposed to project from the face plate  13  toward the core material  11  side. The manufacturing method includes: Step S 1  for forming the grooves  21  and  61  to have a shape complementary to the crack arrester  15  on the core material  11 ; Step S 2  for disposing the composite material to be the crack arrester  15  on the grooves  21  and  61 ; Step S 3  for disposing the composite materials to be the pair of face plates  13  on both sides in the thickness direction of the core material  11 ; and Step S 4  for joining the composite material with the core material  11  to form the sandwich panels  10 ,  50 , and  60 . At Step S 1  for forming the grooves  21  and  61 , the grooves  21  and  61  each having the opening  25  and the flat side surfaces  26  extending along the thickness direction are formed by processing the working surface of the core material  11 , and the angle formed by the side surface  26  and the working surface at the opening  25  is equal to or larger than 90 degrees. 
     With this configuration, the side surfaces  31  of the crack arresters  15  and  51  having a shape complementary to the grooves  21  and  61  can be formed so that the angle formed by the side surface  31  and the boundary surface  33  is equal to or larger than 90 degrees. Due to this, even in a case of a load (shearing force) in the vertical mode in which external force is applied in the thickness direction, a structure having resistance against the shearing force can be achieved. Accordingly, delamination between the crack arresters  15  and  51  and the core material  11  at the joint part can be preferably suppressed. 
     REFERENCE SIGNS LIST 
       10  Sandwich panel 
       11  Core material 
       13  Face plate 
       15  Crack arrester 
       17  Adhesive film 
       21  Groove 
       23  Working surface 
       25  Opening 
       26  Side surface 
       27  Bottom surface 
       31  Side surface 
       32  Front end surface 
       33  Boundary surface 
       50  Sandwich panel (second embodiment) 
       51  Crack arrester (second embodiment) 
       53  Intersecting portion 
       54  Side portion 
       60  Sandwich panel (third embodiment) 
       61  Groove (third embodiment) 
       65  Opening (third embodiment) 
       66  Side surface (third embodiment) 
       67  Bottom surface (third embodiment)