A Novel Measurement Method For The Residual Stress In Aircraft Canopy

Aircraft canopy is a pivotal component of aircraft. Recently, the crack of aircraft canopy brings a sudden disastrous burst. By researching the crack of canopy, residual stresses in materials have been found as the key factor. In this paper, Photoelastic theory,digital phase-shifting technique and phase unwrapping method are employed to complete calculating the residual stresses in aircraft canopy, and realizing a full-field , real-time and automatic measuring.Six-step phase shifting technique has been used to obtain the separate information of isoclinic fringe and isochromatic fringe, but phase shifting can't completely separate isoclinic information from isochromatic fringe pattern. By using digital image processing technique, and basing on the rule of phase continuous variation, a new method is found that solves the problem of isoclinic-isochromatic interaction, and improves the precision of stress calculation. To get the full-field phase information, a new path-discriminance phase unwrapping algorithm is presented, which considers the objection of traditional phase unwrapping algorithm and the characteristic photoelastic fringe pattern. The algorithm can obviously remove"line-draw"and eliminate random noise.The experimental method of photoelastic separation of principal stresses is oblique incidence. After oblique incidence, the size of image is the size of specimen's projection in the non-vertical direction of optical axis. The problem of phase restoration with same position point during vertical incidence, has been settled by designing a interpolation and accurately locating the model edge which is obtained by automatic region-identification single edge method. This matching algorithm realizes the auto-separation of principal stresses, has actual significance.Finally, researching the cause of residual stresses in aircraft canopy, and a digital photoelasticity is used to analyze residual stresses of the plane board and bore circumjacent, especial of bore which was broken during machining. Consequently, machining stresses in materials will be increasing by improper machining, this maybe brings some accident. So in this thesis, a new nondestructive testing method, which base on the digital photoelasticity, is provided to calculate the inner stresses of canopy.