Device for Determining the Load-Bearing Capacity of Cylindrical Shells

Abstract

A device is configured to allow for the load-bearing capacity of cylindrical shells of a composite fiber material that are at risk of buckling to be determined. The device allows for deriving improved reduction factors for analytical calculation of the design load, which enables an improved approximation to the actual load-bearing capacity. The device includes a load distribution head for applying an axially acting force to a cylinder shell introduced into the device; a dent actuator for producing a single dent in a surface of the cylinder shell in a predetermined dent direction with a predetermined dent depth; a fixed support for fixing the dent depth of the single dent produced by the dent actuator in the predetermined dent direction; a dent force sensor for determining a dent force in the predetermined dent direction of the single dent that is fixed in the predetermined dent direction when a steadily increasing, axially acting force is applied to the cylinder shell by the load distribution head; a control unit for actuating the load distribution head for applying a steadily increasing, axially acting force to the cylinder shell until a complete failure of the cylinder shell is detected as a load step by an axial force sensor; and a digital data memory for storing a current dent force in an event of a complete failure of the cylinder shell and the applied axially acting force on the cylinder shell at the current dent force for the predetermined dent depth.

Claims

1. A device for determining the load-bearing capacity of cylinder shells that are at risk of buckling of a composite fiber material, comprising: a load distribution head for applying an axially acting force to a cylinder shell introduced into the device; a dent actuator for producing a single dent in a surface of the cylinder shell in a predetermined dent direction with a predetermined dent depth; a fixed support for fixing the dent depth of the single dent produced by the dent actuator in the predetermined dent direction; a dent force sensor for determining a dent force in the predetermined dent direction of the single dent that is fixed in the predetermined dent direction when a steadily increasing, axially acting force is applied to the cylinder shell by the load distribution head; a control unit for actuating the load distribution head for applying a steadily increasing, axially acting force to the cylinder shell until a complete failure of the cylinder shell is detected as a load step by an axial force sensor; and a digital data memory for storing a current dent force in an event of a complete failure of the cylinder shell and the applied axially acting force on the cylinder shell at the current dent force for the predetermined dent depth.

2. The device as claimed in claim 1, wherein the dent actuator produces the single dent radially inwards into the cylinder shell, and wherein the fixed support is located in an internal space of the cylinder shell.

3. The device as claimed in claim 1 wherein the dent force sensor is disposed in a load path of the fixed support or in a load path of the dent actuator.

4. The device as claimed in claim 1 wherein the dent force sensor is a force measurement cell or a strain gauge.

5. The device as claimed in claim 1 wherein the dent actuator works in conjunction with a dent depth controller for setting the predetermined dent depth.

6. The device as claimed in claim 1 further comprising an analysis unit for determining a dent depth from the data memory at which a dent force at a complete failure of the cylinder shell is at a maximum compared to dent forces of other dent depths, wherein the axially acting force on the cylinder shell at which the complete failure of the cylinder shell has been detected for the determined dent depth is output by the analysis unit as a load-bearing capacity of the cylinder shell that is at risk of buckling.

7. The device as claimed in claim 1 further comprising a bending device for producing a bending stress in the cylinder shell by rotating an upper cross-sectional plane of the cylinder shell relative to a lower cross-sectional plane of the cylinder shell by a predetermined bending angle.

8. The device as claimed in claim 7, wherein the bending device has one or more adapter plates for setting a bending angle, the one or more adapter plates having an inclination corresponding to the predetermined bending angle for introduction into the device between the cylinder shell's upper cross-sectional plane and the load distribution head and/or between the cylinder shell's lower cross-sectional plane and a supporting base of the device.

9. The device as claimed in claim 7 wherein the bending device is designed for turning the load distribution head for rotating the cylinder shell's upper cross-sectional plane by the predetermined bending angle.

10. The device as claimed in claim 7 wherein the bending device is designed for turning the cylinder shell's upper cross-sectional plane relative to the cylinder shell's lower cross-sectional plane by the predetermined bending angle in the direction of the single dent that is produced.

11. The device as claimed in claim 7 wherein the digital data memory stores the current dent force and the axially acting force applied to the cylinder shell with the current dent force for the respective set dent depth in conjunction with the predetermined bending angle.

12. The device as claimed in claim 7 wherein the analysis unit is arranged to determine a minimum load path of the axially acting force at which a complete failure of the cylinder shell has been detected using various bending angles and from a number of axially acting forces at which a complete failure of the cylinder shell has been detected for predetermined bending angles and dent depths, and to determine the load-bearing capacity of the cylinder shell that is at risk of buckling depending on the minimal load path.

Description

DESCRIPTION OF THE DRAWINGS

[0025] The invention is described using the accompanying figures by way of example.

[0026] In the figures:

[0027] FIG. 1—shows a schematic representation of the device according to the invention for determining the load-bearing capacity;

[0028] FIG. 2—shows a diagram for determining geometric imperfections; and

[0029] FIG. 3—shows a diagram for estimating “boundary condition” imperfections.

DETAILED DESCRIPTION

[0030] FIG. 1 shows schematically the device 10 according to the invention, which has a tool bench 11 and a stop or base 12 connected thereto. A cylindrical shell 1 that rests with the lower end 2 thereof on the base 12 of the workbench 11 is placed on the base. On the upper end 3 opposite the lower end 2, the cylindrical shell 1 is contacted by a load distribution head 13, wherein the load distribution head 13 is designed for applying an axially acting force towards the cylindrical shell 1 (illustrated by the arrows at the upper end 3 of the cylindrical shell 1).

[0031] A mounting arm 15, on which a dent actuator 16 is disposed axially displaceably relative to the cylindrical shell 1, is removably disposed on the load distribution head 13 by means of a magnet arrangement 14. The dent actuator 16 is in this case designed for inducing a single dent 17 in the cylindrical shell 1 or the surface of the cylindrical shell in the direction B.sub.R, which is directed radially towards the interior of the cylindrical shell 1. The dent depth of the single dent 17 can be adjusted in this case using a dent depth controller 18.

[0032] Moreover, a fixed support 19 is provided within the cylindrical shell 1, which fixes the single dent 17 in the dent direction B.sub.R in the event of an axially acting force applied by the load distribution head 13, so that the dent depth essentially remains constant even while the axially acting force and the steadily rising axially acting force are acting.

[0033] Furthermore, the device 10 comprises a control unit 20 with which the load distribution head 13 as well as the dent actuator 16 can be suitably actuated.

[0034] Furthermore, in the exemplary embodiment of FIG. 1 a force sensor 21 is provided in the base 12, which is designed for measuring the axially acting force on the cylindrical shell 1 that is applied by the load distribution head 13.

[0035] Furthermore, the dent actuator 16 comprises a force measurement cell 22, with which the dent force in the dent direction B.sub.R can be measured while the axially acting force applied by the load distribution head 13 is acting. This enables conclusions to be drawn regarding the force conditions within the single dent, in particular the force conditions in the dent direction B.sub.R, in order for example to be able to make a statement about the geometric imperfections.

[0036] Alternatively or additionally, it is also conceivable that a strain gauge 23 is disposed on the fixed support 19 in order to be able to determine the bending direction while the axially acting force is acting.

[0037] The control unit 20 can furthermore comprise a data memory 24 in order to be able to suitably store the data and data correlations obtained during the individual measurements that are carried out using the device 10. Using an analysis unit 25, corresponding statements about the permitted load-bearing capacity of cylindrical shells that are at risk of buckling can then be derived.

[0038] FIGS. 2 and 3 show suitable diagrams for this, with which suitable boundary conditions for the load-bearing capacity can be calculated. In doing so, various series of measurements are carried out using the device 10, wherein the corresponding load-bearing capacity data can be determined very accurately from the data that are obtained as a result. In step 1 a single dent is introduced in the predetermined dent direction and with a defined dent depth and is fixed with a fixed support. In step 2, an axially acting force is then applied to the cylindrical shell, wherein during the step the dent force acting in the radial direction in the single dent that is induced is detected while the steadily rising axially acting force is acting on the cylindrical shell. This results in a continuous data pair between the axially acting force on the one hand and the dent force on the other hand, wherein using a load step the axially acting force at the time of a complete failure of the cylindrical shell can then be identified. The axially acting force on the cylindrical shell that is set at the point in time of the complete failure and the dent force acting at said point in time are of particular significance here.

[0039] During this, step 2 is repeated for various dent depths or impression depths of the single dent, so that a respective dent force can then be determined for various dent depths, which has been measured at the point in time of the complete failure of the cylindrical shell. This is illustrated in FIG. 2. It can be seen that the dent force initially increases for various impression depths or dent depths, while it has reached the maximum thereof at a dent depth of 3 mm. During this, the dent force plotted on the Y-axis is the dent force that was measured at the point in time of the complete failure of the cylinder structure. For larger dent depths, it is determined here that the dent force that was measured at the point in time of the complete failure of the cylinder structure decreases again, so that a maximum dent force can be measured at a dent depth of 3 mm.

[0040] The diagram shown in FIG. 2 is to be considered exemplary here and should only be used as an explanation of the measurement methodology. Depending on the cylindrical shell and the properties thereof, i.e. the materials used, the radius as well as the wall thickness, quite different diagrams result for this purpose.

[0041] The maximum of the dent force derived from said diagram is thus reached at a dent depth of 3 mm, wherein a stop point for the so-called geometric imperfections is now formed based on the axially acting force at the point in time of the complete failure of the cylinder structure, which has been measured for said dent depth of 3 mm for a corresponding dent force. Here it was recognized that said axially acting force, which was measured for the complete failure of the cylinder structure at the dent depth that correlates with the maximum dent force at the point in time of the complete failure (according to FIG. 2 a dent depth of 3 mm), gives a very good approximation to the possible load-bearing capacity the cylinder structure, and indeed in conjunction with the so-called geometric “mid-surface” imperfections (MSI).

[0042] In step 3, the deviation from the ideal homogenous loading, the so-called geometric “boundary condition” imperfections (BCI), is now sought based on the results of step 2. For this purpose, the dent depth determined from step 2 (3 mm in FIG. 2) is induced in the form of a single dent using the device according to the invention, wherein using the device a bending stress is then introduced into the cylindrical shell towards the induced single dent. This can for example take place owing to the fact that the load distribution head 13 of the device 10 is designed to be rotatable, tiltable or pivotable, whereby the upper end 3 can be rotated or tilted relative to the lower end 2 of the cylindrical shell (not in connection with a magnetic arrangement of the dent actuator on the load distribution head). If a suitable bending angle is set, then the cylindrical shell is axially loaded again using the load distribution head 13, wherein the single dent is fixed at the value determined in step 2 regarding the dent depth thereof. In this case the cylindrical shell with the set bending angle is axially loaded until a complete failure of the cylinder structure can be detected again. During this, the axial force applied at the point in time of the complete failure of the cylinder structure is stored in a data memory together with the set bending angle and possibly the set dent depth.

[0043] Said series of measurements is now repeated for various bending angles, wherein the bending angle is steadily increased here. In this case, it can be seen that for a defined bending angle a load step relating to the axially acting force relative to the preceding measurement with the small bending angle can suddenly be detected at the point in time of the complete failure, as marked in FIG. 3 by L. The subsequent measurements then show an essentially homogenous load path, which is reduced by the load step 11 to the preceding measurements before the load step. It has been found that the axially acting force at which the entire structure fails and that has been measured for the bending angles that lie after the load step represents a homogenous equilibrium path that indicates the maximum load-bearing capacity of said cylinder structure.

[0044] This can be verified, for example, by increasing the dent depth by 40% or 80%. It can be seen here that when the dent depth is increased by 40% for an almost identical bending angle, a corresponding load step can be detected. Furthermore, for an 80% increase in the impression depth or dent depth it can be determined that it moves essentially on the homogenous load path almost without a load step, so that a very accurate estimate of the possible load-bearing capacity of the cylinder structure can be output here.

[0045] In summary, it can therefore be said that with the new test method a specific instability point for geometric “mid-surface” imperfections of shell structures can be determined, wherein the structure can then be forced onto a particularly stable equilibrium path with said instability point, wherein the load-bearing capacity on said equilibrium path then provides a limit value for the real load-bearing capacity. Using the present device, a corresponding real measurement of cylindrical shells is necessary in order to carry out such a test method.

REFERENCE CHARACTERS

[0046] 10 device [0047] 11 tool bench [0048] 12 base [0049] 13 load distribution head [0050] 14 magnet arrangement [0051] 15 mounting arm [0052] 16 dent actuator [0053] 17 single dent [0054] 18 dent depth controller [0055] 19 fixed support [0056] 20 control unit [0057] 21 force sensor [0058] 22 force measurement cell [0059] 23 strain gauge [0060] 24 data memory [0061] 25 analysis unit [0062] B.sub.R dent direction