Abstract:
The invention provides a test apparatus for providing axial stresses in a structure, comprising a first set of formations for abutting a first surface of the structure at a first plurality of locations, a second set of formations for abutting a second surface of the structure at a second plurality of locations, and a force actuator, and wherein each set of formations comprises at least three formations and wherein at least two formations in each set of formations are aligned with a notional alignment line along the structure and at least one formation in each set of formations is out of alignment along the structure with said notional alignment line, such that when the force actuator applies a force, loads are applied at different locations, causing the structure to bend biaxially and thereby providing biaxial stresses in the structure.

Description:
RELATED APPLICATIONS 
     The present application is based on, and claims priority from, British Application No. 1203104.3, filed Feb. 23, 2012, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention concerns a test apparatus for providing axial stresses in a structure. More particularly, but not exclusively, this invention concerns a test apparatus for providing axial stresses in a structure, the structure having a first surface on one side and a second surface on an opposite side. The invention also concerns a method of providing axial stresses in a structure. 
     An existing test arrangement is shown in  FIG. 1 . The test arrangement  1  is testing a sandwich structure  10 . The sandwich structure  10  has a top surface  11  and a bottom surface  12 . The structure  10  is made up of a top skin  13 , bottom skin  14  and a core  15 . The test apparatus comprises a base structure  20 , a top structure  30  and a load block  40 . 
     The base structure  20  comprises a base platform  21  from which two upwardly extending formations  22 ,  23  extend. The formations  22 ,  23  are curved at their tip to minimise the lengthwise distance along which the load is applied. The formations  22 ,  23  extend along the width of the structure  10 . On top of each formation  22 ,  23  grease  26  is applied to aid rotation, if large downward deflections of the structure  10  are expected. Then a Vee block  25  and then a rubber pad  26  are placed on top of the formations  22 ,  23 . The Vee block  25  and rubber pad  26  reduce the through thickness stress exerted on the structure  10  at the formations  22 ,  23 . The sandwich structure  10  is placed on top of the two rubber blocks  24 . 
     The top structure  30  comprises a top platform  31  from which two downwardly extending formations  32 ,  33  extend. The formations  32 ,  33  are curved at their tip to minimise the lengthwise distance along which the load is applied. The formations  32 ,  33  extend along the width of the structure  10 . On the end of each top formation  32 ,  33  grease  36  is applied to aid rotation, if large downward deflections of the structure  10  are expected. Then a Vee block  35  and then a rubber pad  36  are placed on the end of the formations  32 ,  33 . The Vee block  35  and rubber pad  36  reduce the through thickness stress exerted on the structure  10  at the formations  32 ,  33 . The top structure  30  is placed on top of the sandwich structure  10  so that the rubber pads  34  sit on the top surface  11  of the structure  10 . 
     In use, the sandwich structure  10  is placed in between the upwardly extending and downwardly extending formations  22 ,  23 ,  32 ,  33  and a load block  40  is placed on the top of the top platform  31 . This causes the structure  10  to bend downwards at all locations between the two upwardly extending formations  22 ,  23 . Maximum downward bending occurs between the two downwardly extending formations  32 ,  33 . Relative upward bending occurs at the outer portions of the structure  10  (either side of the two upwardly extending formations  22 ,  23 ). This creates uni-axial stresses (along a single axis) in the portion of the structure between the two downwardly extending formations  32 ,  33  and allows this portion to be tested. The maximum downwards deflection can be measured at the mid-point between the two downwardly extending formations  32 ,  33 . 
     This test apparatus is normally used for thin skinned sandwich panel structures. This is because in these structures, the test produces near uniform uni-axial compressive stress on the top surface  11  and near uniform uni-axial tensile stress in the bottom surface  12 . The bending moment and skin stresses are maximum and constant in between the two downwardly extending formations  32 ,  33 . Through thickness shear forces are only produced on either side of the downwardly extending formations  32 ,  33 . The test apparatus is normally used for sandwich structures with a lightweight (for example, honeycomb) core and thin composite (for example, fibre reinforced plastic or carbon prepreg) skins. In particular, the test works best if the core depth is over 6 times the skin thickness. 
     The test is used to investigate the strength of the sandwich skins. Therefore, ideally the load used and the formation positions are designed to ensure skin failure (prior to core shear failure). 
     The test rig is simple and can be used by using only a single downward load  40 . 
     However, it is often desired to test a structure by applying bi-axial loads. Currently, this can be done by attaching an actuator to each of four different legs of a cruciform structure, and applying a pulling or pushing force to each actuator. However, these bi-axial rigs require a complex calibration to confirm the biaxial loads in the structure compared to the forces applied at the actuators. This is because, the actuator on one leg, even if no force is applied to it, constrains the leg and affects the stress in the structure due to Poisson deformation being prevented. This makes bi-axial testing complicated, costly and time-consuming. 
     The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved test apparatus for and method of providing bi-axial stresses in a structure. 
     SUMMARY OF THE INVENTION 
     The present invention provides, according to a first aspect, a test apparatus for providing axial stresses in a structure, the structure having a first surface on one side and a second surface on an opposite side, wherein the test apparatus comprises a first set of formations for abutting the first surface of the structure at a first plurality of locations on the structure, a second set of formations for abutting the second surface of the structure at a second plurality of locations on the structure, the first and second plurality of locations defining different footprints, and a force actuator for applying a force which urges at least one of the first and second set of formations towards the other set, and wherein each set of formations comprises at least three formations and wherein at least two formations in each set of formations are aligned with a notional alignment line along the structure and at least one formation in each set of formations is out of alignment along the structure with said notional alignment line, such that when the force actuator applies the force which urges at least one of the first and second set of formations towards the other set, loads are applied at different locations over each notional alignment line and at a further location out of alignment with each notional alignment line, causing the structure to bend biaxially and thereby providing biaxial stresses in the structure. 
     Having out of alignment formations allows bi-axial bending to be produced whilst only applying load through a single force actuator. Hence, embodiments of the invention have the advantage of having the same simple load interaction of the test rig shown in  FIG. 1  whilst still providing bi-axial stresses in the structure. This could enable better understanding of bi-axial stress failure envelopes, refinement of bi-axial stress analysis and improve weight optimisation of, for example, composite structures. This is especially advantageous when sandwich panel structures or monolithic I-beam structures are being considered. The test apparatus is easier to maintain and set up than other apparatus for providing bi-axial stresses, as there is only one actuator and no bolting is required. It also means that open hole and impact damaged structures could be tested with bi-axial stresses. This could improve method refinement for calculating allowable damage under bi-axial loading and could lead to improved maintenance of, for example, composite structures. 
     Preferably, the footprints of the first and second sets of formations are entirely different. In other words, preferably, none of the first plurality of locations corresponds to any of the second plurality of locations. 
     Preferably, the force actuator is for applying a force which urges all formations in at least one of the first and second set of formation towards all formations of the other set. 
     Preferably, each set of formations comprises four formations and wherein the four formations comprise a first pair of formations aligned along a first notional alignment line and a second pair of formations aligned along a second notional alignment line, and wherein the first and second alignment lines are at an angle to each other. This provides a more stable arrangement. 
     More preferably, the first pair of formations of the first set of formations are aligned along the same first notional alignment line as the first pair of formations of the second set of formations and wherein the second pair of formations of the first set of formations are aligned along the same second notional alignment line as the second pair of formations of the second set of formations. This provides bi-axial bending along two defined axes. 
     Preferably, the first formation of the first pair of formations lies to one side of the second alignment line and the second formation of the first pair of formations lies to the other side of the second alignment line and wherein the first formation of the second pair of formations lies to one side of the first alignment line and the second formation of the second pair of formations lies to the other side of the first alignment line. This provides bi-axial loading in a cruciform/cross shape. 
     Preferably, the first alignment line is transverse, preferably, approximately perpendicular, to the second alignment line. This provides bi-axial loading in a cross shape, with 4 arms, each at 90 degrees to each other. 
     Preferably, each formation is rigid. In other words, preferably, each formation does not substantially depress under loading. 
     Preferably, each formation is elongate and arranged to provide a line of contact along the length of the formation. Preferably, each formation is arranged to provide a line of contact with the structure and wherein the line of contact of each formation is approximately perpendicular to its notional alignment line. This provides a more stable arrangement. 
     Optionally, for a first set of formations, the footprints of the formations of the first and second pairs of formations are arranged to be located around a first point, whereas, in the second set of formations, the footprints of the formations of the first and second pairs of formations are arranged to be located further away from that point. This provides equal sign biaxial bending, with the first surface of the structure under compression and the second surface of the structure under tension. 
     Alternatively, for a first set of formations, the footprints of the formations of the first pair of formations are arranged to be located around a first point, and the footprints of the formations of the second pair of formations are arranged to be located further away from that point, whereas, in the second set of formations, the footprints of the formations of the first pair of formations are arranged to be located further away from that point, and the footprints of the formations of the second pair of formations are arranged to be located around the first point. This provides opposite sign biaxial bending. Preferably, the first pair of formations of the first set of formations are aligned along the same notional alignment line as the second pair of formations of the second set of formations and wherein the second pair of formations of the first set of formations are aligned along the same notional alignment line as the first pair of formations of the second set of formations. 
     According to the first aspect of the invention, there is also provided a test rig for use with the test apparatus described above, wherein the test rig comprises a base, and a set of at least three formations attached to the base, wherein at least one formation in the set of formations is out of alignment along the base from other formations. 
     Preferably, the formations are movably mounted on the base such that the locations of the formations along the base are adjustable. This allows the load magnitudes in each direction to be tailored to the test required. 
     According to the first aspect of the invention, there is also provided a structure for testing with the test apparatus as described above or the test rig as described above, wherein the structure comprises a central test portion and at least three legs extending away from the central test portion. 
     Preferably, the structure is in the shape of a cruciform. 
     Preferably, the structure is a sandwich panel structure or an I-beam structure. 
     According to a second aspect of the invention, there is also provided a method of providing axial stresses in a structure, the structure having a first surface on one side and a second surface on an opposite side, wherein the method comprises the following steps providing a first set of at least three formations for abutting the first surface of the structure at a first plurality of locations on the structure, wherein at least two formations in the first set of formations are aligned with a first notional alignment line along the structure and at least one formation in the first set of formations is out of alignment along the structure with said first notional alignment line, providing a second set of at least three formations for abutting the second surface of the structure at a second plurality of locations on the structure, the first and second plurality of locations defining different footprints, wherein at least two formations in the second set of formations are aligned with a second notional alignment line along the structure and at least one formation in the second set of formations is out of alignment along the structure with said second notional alignment line, placing the structure to be tested in between the two sets of formations, applying a force to urge at least one of the first and second set of formations towards the other set, causing loads to be applied at different locations over each notional alignment line and at a further location out of alignment with each notional alignment line, causing the structure to bend biaxially and thereby providing biaxial stresses in the structure. 
     It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1  shows a side sectional view of a prior art test arrangement; 
         FIG. 2  shows a perspective view of a test arrangement according to a first embodiment of the invention; and 
         FIG. 3  shows a partly exploded perspective view of a test arrangement according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a perspective view of a test arrangement  2  according to a first embodiment of the invention. The test arrangement  2  is testing a sandwich structure  210 . The sandwich structure  210  has a top surface  211  and a bottom surface  212 . The structure  210  is in the form of a slender symmetrical cruciform shape with a central portion  213  and four legs  214 ,  215 ,  216 ,  217  at 90 degrees to each other. 
     The test apparatus comprises a base structure  220 , a top structure  230  and a load block (not shown). 
     The base structure  220  comprises a base platform  221  in the form of a slender symmetrical cruciform shape. The base platform  221  has a central portion  222  and four legs  223   a ,  223   b ,  223   c ,  223   d  at 90 degrees to each other. Towards the far end of each leg  223 , there is an upwardly extending formation  224   a ,  224   b ,  224   c ,  224   d  across the width of each leg  223 . The sandwich structure  210  is placed on top of the four upwardly extending formations  224  such that each leg is resting on one of the upwardly extending formations. The widths of the legs  214 ,  215 ,  216 ,  217  of the structure  210  are narrower than the length of the upwardly extending formations  224 . 
     The top structure  230  comprises a top platform  231  in the form of a squat symmetrical cruciform shape. The base platform  231  has a central portion  232  and four short legs  233   a ,  233   b ,  233   c ,  233   d  at 90 degrees to each other. Towards the far end of each leg  233 , there is a downwardly extending formation  234   a ,  234   b ,  234   c ,  234   d  across the width of each leg  233 . The top structure  230  is placed on top of the sandwich structure  210  so that the downwardly extending formations  234  sit on the top surface  211  of the structure  210 . The downwardly extending formations  234  rest on the structure  210  towards the central portion  213 , whereas the upwardly extending formations  224  rest on the structure  210  towards the far end of the legs  214 ,  215 ,  216 ,  217 . The widths of the legs  214 ,  215 ,  216 ,  217  of the structure  210  are narrower than the length of the downwardly extending formations  234 . 
     In use, the sandwich structure  210  is placed in between the upwardly extending and downwardly extending formations  224 ,  234  and a load block (not shown) is placed on the top of the top platform  231 . This causes the structure  210  to bend downwards in both directions between the upwardly extending formations  224  and upwards at its outer portions (in both directions outside of the upwardly extending formations  224 ). Maximum downward bending occurs in the central portion  213  (in both directions between the downwardly extending formations  234 ). This creates bi-axial stresses (along both the x and y axes) in the central portion  213  of the structure. In particular, in the central portion  213 , the upper skin of the structure  210  is exposed to equal sign biaxial compression (i.e. compression in both the x and y axes) and the lower skin of the structure  210  is exposed to equal sign biaxial tension (i.e. tension in both the x and y axes). 
       FIG. 3  shows a partly exploded perspective view of a test arrangement  3  according to a second embodiment of the invention. The test arrangement  3  is testing a sandwich structure  310 . The sandwich structure  310  has a top surface  311  and a bottom surface  312 . The structure  310  is in the form of a slender symmetrical cruciform shape with a central portion  313  and four legs  314 ,  315 ,  316 ,  317  at 90 degrees to each other. 
     The test apparatus comprises a base structure  320 , a top structure  330  and a load block (not shown). 
     The base structure  320  comprises a base platform  321  in the form of a rectangular shape with two long sides  323   b ,  323   d  along its length (in the direction of the y axis) and two short sides  323   a ,  323   c  along its width (in the direction of the x axis). On each long side  323   b ,  323   d  there is an upwardly extending elongate formation  324   b ,  324   d  extending along the middle portion of the long side  323   b ,  323   d . Towards each short side  323   a ,  323   c  there is an upwardly extending elongate formation  324   a ,  324   c  extending across the width of the base platform  321 . The sandwich structure  310  is placed on top of the four upwardly extending formations  324  such that each leg is resting on one of the upwardly extending formations, with opposite legs  314  and  316  being supported by upwardly extending formations  324   a ,  324   c  towards the end of the legs and opposite legs  315  and  317  being supported by upwardly extending formations  324   b ,  324   d  towards the central portion  313 . The widths of the legs  314 ,  315 ,  316 ,  317  of the structure  310  are narrower than the length of the upwardly extending formations  324 . 
     The top structure  330  comprises a top platform  331  in the form of a rectangular shape with two long sides  333   a ,  333   c  along its length and two short sides  333   b ,  333   d  along its width. On each long side  333   a ,  333   c  there is a downwardly extending elongate formation  334   a ,  334   c  extending along the middle portion of the long side  333   a ,  333   c . Towards each short side  333   b ,  333   d  there is a downwardly extending elongate formation  334   b ,  334   d  extending across the width of the top platform  331 . 
     The top structure  330  is placed on top of the sandwich structure  310  and base structure  320  such that the length of the top platform  331  is perpendicular to the length of the base platform  321  (i.e. the length of the top platform  331  is in the direction of the x axis) and so that each downwardly extending formation  334  rests on one of the legs  314 ,  315 ,  316 ,  317  of the structure  310 . The downwardly extending formations  334   a ,  334   c  rest on the structure  310  towards the central portion  313  on legs  314  and  316 , whereas the downwardly extending formations  334   b ,  334   d  rest on the structure  310  towards the far ends of legs  315  and  317 . The upwardly extending formations  324   b ,  324   d  support the structure  310  towards the central portion  313  on legs  315  and  317 , whereas the upwardly extending formations  324   a ,  324   c  support the structure  310  towards the far ends of legs  314  and  316 . The widths of the legs  314 ,  315 ,  316 ,  317  of the structure  310  are narrower than the length of the downwardly extending formations  334 . 
     In use, the sandwich structure  310  is placed in between the upwardly extending and downwardly extending formations  324 ,  334  and a load block (not shown) is placed on the top of the top platform  331 . This causes the structure  310  to experience bi-axial stresses (along both the x and y axes) in the central portion  313  of the structure. In particular, in the central portion  313 , the upper skin of the structure  310  is exposed to opposite sign biaxial compression (i.e. tension in the x axis and compression in the y axis) and the lower skin of the structure  310  is exposed to opposite sign biaxial tension (i.e. tension in the y axis and compression in the x axis). 
     A disadvantage of the embodiment shown in  FIG. 3  is that the test arrangement is “top-heavy” and may require a way of stabilising the top structure  320 . 
     In both embodiments, the cruciform sandwich structures  210 ,  310  could be made by forming a large square sandwich panel and cutting out squares from each corner of that large square to form the cruciform shape. The cut-off square could be used for a number of different uses, for example in process control tests, use as travelers or use in destructive examination quality checks. 
     Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
     The test apparatuses could be used to test other types of structures, such as thin webbed monolithic I-beams. This would enable bi-axial stress testing on a monolithic laminate. It could also be used for box beam structures or any other structure with similar behaviour. 
     The base structures  220 ,  230  and top structures  230 ,  330  may be modified to allow the formations  234 ,  334  to be moved. This allows for the possibility of varying the relative magnitude of the biaxial skin stress in one direction (x axis) compared to the other (y axis), whilst still using a single test apparatus. 
     Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.