AN APPLICATION OF SPI TECHNIQUE FOR STUDING STRA DISTRIBUTION AT CRACK TIP

A computerized digital speckle-interferometry(SPI)system has been set up to determinethe in-plane distribution of displacement and strain at crack tip of CF specimens.Theprinciple and the experimental method of this technique are described in detail.Theeffect of corrosion factors on plastic deformation at crack tip in CF process has beenexamined by comparing the near-tip strain fields of steel A537 before and after hydrogencharging,and of pure copper before and after applying an anodic current in 3.5% NaCl.The results shows that the clasic finite element mathematical models are not suitable todescribe the near-tip deformation under the experimental conditions.Hydrogen chargingmade the crack tip plasticity decreased for steel A537 and the anodic dissolution enhancethe near tip deformation of pure copper specimen.     

Thermal Waves for Imaging of Defects with Lockin-Speckle Interferometry

ABSTRACT:  A temporal modulated deformation field was monitored by a phase shifted out-of-plane interferometer. The periodical deformation was generated by exposure to intensity modulated light which produces a thermal wave by absorption at the surface of the object. From the recorded sequence of speckle-interferograms time-varying absolute displacement maps are derived by an unwrapping process. The transformation in the frequency domain and the evaluation at the modulation frequency (lockin-technique) allows to improve the signal-to-noise ratio and to investigate the depth where the defects are located. The measuring time for depth localisation can be reduced when the object is simultaneously excited and evaluated at several frequencies. The potential of this new method is shown on different model samples.