Study On The Key Issues In The Engineering Application Of Photomechanics

In experimental research, higher requirements are brought forward to various measurement methods with the rapid development of modern science and technology. Existing contact measurement techniques, such as resistance strain gauge, extensometer, dial gauge etc, are limited due to their own characteristics. As the sub discipline of experimental mechanics, modern optical testing technology, that has been widely used, owns the advantages of non-contact, full-field measurement, high resolution. Among all the measurement techniques, digital image correlation and shearography are especially suitable for engineering application for their high precision, relaxed requirement for experimental environment, and simple optical setup. However, modern optical testing technology is faced with challenges in the process of practical application to engineering. We have studied some key issues, and propose novel methods to achieve higher measurement resolution and reliability.1. A novel strain measuring method based on the principle of optical extensometer is proposed to achieve higher resolution in engineering application. The proposed optical extensometer employs two identical image sensors and a large format lens to record images, leading to the increase of gauge length, and subsequently the resolution of strain result is improved. Moreover, a special experimental setup called "correction sheet" is used to eliminate the virtual strain induced by out-of-plane motions. Uni-axial tensile experiments are performed to validate the reliability and resolution of the optical extensometer, and the measurement results demonstrate that the resolution of the optical extensometer achieves 2-3??. Then the proposed method is applied to the measurement of Young's modulus of bamboo material and the evaluation of the concrete residual strain based on ring-core method.2. According to the low measurement resolution of non-uniform deformation, we propose a super-resolution method to acquire higher-resolution images, that are then used for digital image correlation calculation. The light field could be shifted through rotating the optical flat positioned in front of the telecentric lens; subsequently the acquired images translate as the shift of the light field. A series of recorded images with translation are recorded, and then assembled to form an image with high resolution. Afterwards, digital image correlation is performed with the assembled images, and higher displacement and strain resolution can be achieved. Simulation and experiment are conducted to validate the reliability the proposed method.3. Various interferometry techniques are unsuitable for on-spot measurement because of the requirement for shock isolation; accordingly we propose a high-resolution non-uniform out-of-plane displacement field measurement method based on shearography. Shearography combines high-resolution interferometry with the advantage of relaxing the environmental stability requirements, has great potential for in situ measurement. In order to apply shearography to precision measurement of out-of-plane displacement field, the modified shearography system is proposed to measure the out-of-plane displacement gradients along the orthogonal directions, and subsequently the two-step integration method is employed to extract the relative out-of-plane displacement field. Simulations and experimental results show that the measurement result is immune to the position of the seed point and initial integration direction, and the experimental results are compared with electronic speckle pattern interferometry to validate the proposed method.4. The software based on shearography which is used for recording images and controlling the PZT is developed. In the software, the multithread technology is used to avoid the interruption with each other of such functions:phase shifting, image capturing and real-time display. Combined the software with the self-developed portable shearography interferometry, the out-of-plane displacement field of the vacuum plate glazing is obtained, and experimental results are compared with simulation.