A.
L. Ross, C. T. Wang, and E. L. Reiss
New York University
New York University
Before
the advent of modern high speed aircraft, wrinkling of the covering of aircraft
components due to local buckling under flight loads did not introduce any
serious problems because such wrinkling does not have
a material effect on the performance of the aircraft. Present day high speed
aircraft, however, often operate under conditions in which the local airstream
Mach number adjacent to the aerodynamic surfaces is nearly unity; consequently
even slight surface irregularities, such as the appearance of wrinkles due to
buckling, may cause the local velocity to become supersonic. Such changes from
subsonic to supersonic flow are accompanied by the formation of shock waves in
the flow which materially increase the drag of the aircraft. It is therefore
desirable for these aircraft to employ a covering which will not buckle at
flight loads and yet will not introduce any undue increase in the weight of the
structure.
One type of such structure is sandwich construction. Sandwich construction consists of two thin external or face layers of high-strength material, such as aluminum alloy sheet, bonded to a thick internal layer or core of light-weight material, such as balsa wood or cellular cellulose acetate. The core serves to separate the strong faces a fixed distance apart, thus giving the structure a high bending and therefore buckling strength, but without a substantially increase in weight.
In this report an attempt is made to introduce another type of construction where buckling is delayed by a method of prestressing. It is proposed that flat plates can have their buckling strength increased by prestressing. The prestressing is accomplished by first cold rolling the plates into cylindrical form and then opening the plate by riveting onto or clamping into a flat frame. This prestressing will produce membrane stresses in the plane of the plate of such a magnitude and distribution as to raise its buckling load in the direction of the generator of the cylinder. The present investigation was restricted to the case of all four sides clamped.
The stresses produced by this process are measured and their effect calculated by the Rayleigh-Ritz method. The analysis shows that small stresses (1000 psi maximum) could raise the buckling load almost 50%, and probably more. These stresses would also change the buckling mode in some of the cases examined from antisymmetrical to symmetrical modes. A testing frame was constructed to produce, and measure the effect of the proposed prestressing. The tests showed that the buckling load was raised in some cases over 100% while the average for all tests was 38%.
A theoretical evaluation of the membrane stresses and their effect was carried out vhich involved the solution of the non-linear plate equations of Von Karman. Because the necessity of using the rather approximate assumptions, the analysis could be subjected to questioning. Nevertheless, the analysis showed that the buckling load at one particular type of prestressed plate would be raised approximately 15%.
One type of such structure is sandwich construction. Sandwich construction consists of two thin external or face layers of high-strength material, such as aluminum alloy sheet, bonded to a thick internal layer or core of light-weight material, such as balsa wood or cellular cellulose acetate. The core serves to separate the strong faces a fixed distance apart, thus giving the structure a high bending and therefore buckling strength, but without a substantially increase in weight.
In this report an attempt is made to introduce another type of construction where buckling is delayed by a method of prestressing. It is proposed that flat plates can have their buckling strength increased by prestressing. The prestressing is accomplished by first cold rolling the plates into cylindrical form and then opening the plate by riveting onto or clamping into a flat frame. This prestressing will produce membrane stresses in the plane of the plate of such a magnitude and distribution as to raise its buckling load in the direction of the generator of the cylinder. The present investigation was restricted to the case of all four sides clamped.
The stresses produced by this process are measured and their effect calculated by the Rayleigh-Ritz method. The analysis shows that small stresses (1000 psi maximum) could raise the buckling load almost 50%, and probably more. These stresses would also change the buckling mode in some of the cases examined from antisymmetrical to symmetrical modes. A testing frame was constructed to produce, and measure the effect of the proposed prestressing. The tests showed that the buckling load was raised in some cases over 100% while the average for all tests was 38%.
A theoretical evaluation of the membrane stresses and their effect was carried out vhich involved the solution of the non-linear plate equations of Von Karman. Because the necessity of using the rather approximate assumptions, the analysis could be subjected to questioning. Nevertheless, the analysis showed that the buckling load at one particular type of prestressed plate would be raised approximately 15%.
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