Page 193 - Aircraft Stuctures for Engineering Student

6.1 1 Failure stress in plates and stiffened panels 177

then given by Eq. (6.58), viz.
uCR = 12( rlkn2E - "2) M2

1
where the values of k, t and b depend upon the particular portion of the panel being
investigated. For example, the portion of skin between stiffeners may buckle as a plate
simply supported on all four sides. Thus, for a/h > 3, k = 4 from Fig. 6.16(a) and,
assuming that buckling takes place in the elastic range
uCR = 12(1 - "2) (E)
2
47r2 E

A further possibility is that the stiffeners may buckle as long plates simply supported
on three sides with one edge free. Thus
uCR = 12(1 - "2) (2)
0.43x2E
2

Clearly, the minimum value of the above critical stresses is the critical stress for the
panel taken as a whole.
The compressive load is applied to the panel over its complete cross-section. To
relate this load to an applied compressive stress cA acting on each element of the
cross-section we divide the load per unit width, say N,., by an equivalent skin
thickness i, hence
NX
UA = T
t
where

and A,, is the stiffener area.
The above remarks are concerned with the primary instability of stiffened panels.
Values of local buckling stress have been determined by Boughan, Baab and Gallaher
for idealized web, Z- and T- stiffened panels. The results are reproduced in Rivello7
together with the assumed geometries.
Further types of instability found in stiffened panels occur where the stiffeners are
riveted or spot welded to the skin. Such structures may be susceptible to interrivet
buckling in which the skin buckles between rivets with a wavelength equal to the
rivet pitch, or wrinkling where the stiffener forms an elastic line support for the
skin. In the latter mode the wavelength of the buckle is greater than the rivet pitch
and separation of skin and stiffener does not occur. Methods of estimating the
appropriate critical stresses are given in Rivello7 and the Handbook of Aeronautics4.

The previous discussion on plates and stiffened panels investigated the prediction of
buckling stresses. However, as we have seen, plates retain some of their capacity to