Research On Strain Concentration Of Weld Joint By Using Electric Speckle Partten Interferometry Technology

This paper brings forward a new method to obtain the strain concentrationinformation of weld joint, in order to meet the urgent need for accuratemeasurement of stress and strain concentration level of weld joint in high-levelscientific research and engineering structures. Text system has been developed tomeasure the strain distribution of weld joint under the work load. This system hasmany advantages such as whole-field, non-contact, digitized, real-timemeasurements, and the advanced optical technology, computer image processingtechnology as well as mechanical engineering technology have been used to buildup the system. As a result, the strain concentration of weld joint can be measuredwith high precision.Based on the feature of stress and strain of the weld joint under work load, theproposal of optical programs with two coherent laser beams symmetrically isproposed, and optical function is brought out. Based on fiber optic transmissionprinciple, the laser coherent device has been built by one single-mode laser device,one fiber coupler device, single-mode fiber, one optic splitter and two fibercollimators, and its role is to obtain the two collimating paralleled coherent laserbeams. However, this device is very flexible and convenient. The imageacquisition device includes an optical camera, an imaging camera and a computer,and it is used for collecting the specklegram of the texting surface and storing thespecklegram into computer with the form of image file. Therefore, by usingadvanced computer image processing technology, the strain concentrationinformation can be got in the computer automaticly. A small but accurate loadingdevice has been built up which is used to give tensile test load of the textspecimen.From the optical function of the specklegram, the method of obtaining thespeckle interference stripe image by using “reduction mode” in computer has beenbrought forward, and programming combining OpenCV and Matlab have beenused in this paper to obtain the speckle interference stripe image with highly stripecontrast and littel noise signal. After that, programmed the computer to obtain thestrain concentration information from the image of speckle interference stripe anddraw the cloud image of the strain field based on the stripe centerline method. Theprograms contain image binaryzation, Stripe Skeleton Extraction andinterpolation.Finally, experiments have been carried out to test the strain concentration ofthe specimens with weld reinforcement or typical welding defects. These experiments are made by the system of Digital Laser Speckle PatternInterferometry method which has been established. Also finite element methodcalculations have been used to test and verify the results of the experiments.Comparing the results of the experiment using Digital Laser Speckle PatternInterferometry method with the results obtained by finite element method, we cansee that by using the Digital Laser Speckle Pattern Interferometry method can getresults with high accuracy. Thus, the Laser Speckle Interference method can beused to obtain the strain field distribution and strain concentration status of weldjoint with high precision.