Pulsed holographic interferometry for the study of bending wave propagation in paper and in tubes

Sammanfattning: Pulsed Holographic Interferometry (HI), is used to record propagating transient bending waves in plates and shells. These waves are generated mechanically by impact of a small steel sphere or optically by focusing a high energy laser pulse at the surface of the tested object. Bending waves propagate with different speeds in different directions if the object material is anisotropic. The wave pattern is also affected by thickness and density variations as well as by defects present in the material. These phenomena are used in non destructive testing of products. In part one of this thesis, pulsed HI is applied for the study of mechanical properties of sheets of paper and corrugated board. This method can be used to detect anisotropy, to determine principle directions, and to examine local variations and defects in paper. The local dynamic behaviour of the board is different from what can be expected from static testing. Mechanical defects in board can be identified. A new impact method is developed, where transient bending waves are generated and recorded by the same pulsed laser. The proposed method can be developed into a commercial measuring device of mechanical properties of paper. In part two, HI is applied to wave propagation in tubes and rings. These are made out of steel, aluminium and fibre reinforced epoxy. A conical mirror is placed axially inside the tube. Axial illumination and axial observation directions make it possible to view the circumference of the tube with a high and constant sensitivity to radial deformation. The resulting interferograms, which have an unusual perspective, are captured with a CCD-camera connected to a computer. The images are thereafter numerically transformed into an unwrapped strip of the tube wall and displayed as a 3D-displacement map. This makes the interpretation of the interferograms simple. The method can be used in non-destructive testing of tubes. In part three, the impact from a laser pulse on a plate, is studied more closely. A high energy laser pulse focused at a steel plate is transferring both mechanical impulse and local thermal energy to the plate. The mechanical impulse creates propagating bending waves in the plate. The heated spot at the plate surface creates thermal stresses which give rise to an out-of-plane deformation of the plate. These deformations are superimposed. The effects are studied using HI to record the deformation. The possibility of creating mechanical waves optically has many advantages. The method is of noncontact character, the laser produces an impulse with short duration and the technique can be applied on-line in the production control.