In a number of industries, it is important to determine and map out-of-plane displacements of structural panels during loading. This is particularly important in the aerospace industry where it is important to determine displacements of load-sensitive structures such as composite panels.
Traditionally, the methods used to predict buckling failures of composite structures have been largely empirical. The predictions were based on finite element analysis and a large number of failing load tests performed on stiffened shear panels, compression panels, and beams in universal test machines. The tests provide very little information on the deformation of the structures as a function of applied load magnitude. Analysts and designers have virtually no test correlation of any parameter except failing load.
The prior art has suggested the utilization of the projection Moire mapping technique for determining out-of-plane displacements of structural surfaces. Typically, this is done by projecting a line ruling on the surface of the structure to be examined. The surface should have a color and texture which permits the line ruling to be imaged on it. Ideally, the surface should be painted with flat white paint. The image of the ruling is viewed by a lens of a recording station and imaged on another ruling located at a recording station image plane. If the image of the projected grille on the recording station image plane and the recording station master grille have different pitches, Moire fringes are produced. If they are of the same pitch, no fringes will be produced at the recording station. In the latter situation as a test surface is deformed away from its initial position, Moire fringes proportional to out-of-plane displacement will be created at the recording station image plane. The resulting recorded image will be a contour map of the deformed surface and sequential photographs may be taken at the recording station image plane as the load is varied on the structural member being observed. The information derived from the resulting maps are useful in analyzing the effects of load, particularly shear loads which tend to cause buckling in composite structural elements.