Residual stress effects and damage tolerance behaviour of integral lightweight structures manufactured by FSW and HSM

Estágio realizado na empresa Airbus Operations GmbH, orientado pelo Mr. Marco PacchioneTese de Programa Doutoral. Engenharia Mecânica. Universidade do Porto. Faculdade de Engenharia. 201

Analysis of the crack growth behavior in a double cantilever beam adhesive fracture test using digital image processing techniques

Digital image processing (DIP) techniques offer interesting possibilities in various fields of science.Automated analyses may significantly reduce the necessary manpower for certain cumbersometasks. The analysis of large series of images may be done in less time, since automatedimage processing techniques are able to work efficiently and with constant quality 24h per day.In this work, a series of images obtained by a high-speed camera is analyzed in order to determinethe crack growth behavior during a double cantilever beam (DCB) test [1]. The presentwork represents a contribution to the effort of automatizing the crack growth measurement,comparing various different techniques which may later be optimized for a specific task.Detecting cracks automatically from test images obtained by a digital camera is a difficult task,since the quality of crack images depends on the test conditions. The roughness of the specimensurface, luminance condition, and the camera itself may influence the detection quality.The specimens tested in this work where painted with white colour since this was found to leadto the best contrast for crack detection. High accuracy may only be expected if a sufficientlyhigh resolution is acquired by the camera and if the available lens setup is optimized for thespecific task.The DCB test is performed in order to obtain the experimental compliance-crack length curveof a polymeric adhesive. Accurate and reliable crack length measurement is indispensable forthe generation of the previously mentioned compliance-crack length curves. It should be notedthat due to the lenses used, unlike shown by Ryu [2], the distance to the specimen is higher than800 mm. This distance has to be reduced by the use of a different lens setup in order to get abetter accuracy of the results. Nevertheless a comparison between different DIP methods is possible.Four different algorithms were developed using The MathWorks MatLab, Massachusetts[3] in order to automatically measure the crack length and a comparison of the obtained resultsis made.Algorithm A is based on thresholding [4] each image of the sequence in order to detect thewhite painted region around the crack. In algorithm B, the image sequence is processed by afilter which reinforces horizontal lines such as the crack, and then isolated pixels are removedfrom the images using morphological cleaning [4]. In algorithm C, the first of two consecutiveimages is subtracted from the second one in order to detect the crack as a difference betweenboth images. Algorithm D is based on the optical flow concept developed by Horn [5]. Thebasic idea is to determine the velocity of each pixel in the image when this changes its positionfrom one image to the next in the analyzed sequence, and relate this information to the growing crack

Characterization of different aluminium alloys of the series 6000 and of their joining processes

Tese de mestrado integrado. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 200