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From the same paper as the previous two sliides.
Figure 11 shows stiffness curve of load sub-step value vs. edge displacement of a nodal point at the loading edge taken from the cylindrical shell of size D=700 mm, L=340 mm, t=1.25 mm with a circular dent of size Dd=137.5mm and td=3.75mm
The normalized load sub-step value is the ratio of the actually applied load at that time step of non-linear buckling analysis to the first eigen buckling strength of the perfect cylindrical shell.
From the Figure, it can be concluded that stiffness of the cylindrical shell decreases as the load applied increases and at limit load condition, the stiffness becomes zero. On further loading the stiffness of cylindrical shell decreases resulting catastrophic failure.
In all the cases, it can be found that the load on the cylindrical shell is shared by DER and perfect cylindrical portion other than DER. The load above the DER is supported by local deformation formed in and around the dent tips. The major portion of the load above the DER is transferred to the dent tips via trough surfaces, only less amount of load is transferred to dent geometry through bridge surfaces. Because of this, dent tip regions subjected to high stress condition and the bridge surfaces subjected to low stress condition. Further, only limited portion of DER is involved in load carrying action and hence the load shared by the DER is less than that of equivalent size of perfect cylindrical shell portion.
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