Study On Non-destructive Measurement And Evaluation Of Internal Stress In Aluminum Alloy By Using Ultrasonic

As aluminum alloy has features, e.g., small density, high specific strength, low price and excellent corrosion resistance, it has wide application in aerospace, transportation, and etc. The internal stress, known as residual stress, exists deeply in the material to keep internal force balance without external factors. It is often generated when the body is subjected to non-uniform plastic deformation and thermo-mechanical processing. It seriously influences the mechanical properties and dimensional stability of aircraft aluminum alloy components as well as service life of those materials. Therefore,it is necessary for us to measure and evaluate internal stress of the material of interest.Measurement of residual stress has been a focused problem for many years. Most of existing method for residual stress measurement can hardly test the internal residual stress non-destructively. The new methods including short-wave X ray diffraction and neutron diffraction are in the very beginning. These methods are also not suitable to practical application for their low efficiency and high costs and high demanding on protection control or environment.By investigating regulation of ultrasonic propagating in stressed materials, a new method to non-destructive evaluate internal stress for engineering application was developed in this paper. By using the new technique, internal stress within 7075 pre-stretched aluminium alloy plate and LC4 aluminium alloy bar was measured and evaluated non-destructively. By using critically refracted longitudinal(LCR) wave transducers with frequency of 1MHz and 5MHz and 5MHz polarized shear wave transducer, we study the relationship between ultrasonic wave and internal stress in 7075 pre-stretched aluminium alloy plate and LC4 aluminium alloy respectively. Besides, the result of stress measurement using this method was compared to that using destructive method. The experimental results from this study are shown as follow:First, based on the LCR wave, stress components along the direction of wave propagation can be measured within several millimeters for the sub-surface of material. When specimen is axially tensioned and the angle between the direction of stress and the propagation orientation of LCR wave increases, stress has decreasing effect on LCR wave velocity. Besides, the velocity of longitudinal wave propagating perpendicular to axial hardly changes in stressed state and this demonstrates that the velocity of LCR wave propagating perpendicular to the direction of stress has no relation with the applied stress. Therefore, LCR ultrasonic technique has high potential to measure the stress component along the propagation direction of LCR wave in the stressed material.Second, by using normal incidence polarized shear wave, plain stress field in the whole propagation route can be evaluated non-destructively. There is a little difference of velocity of shear wave polarizing between parallel and perpendicular to the direction of extrusion in unstressed state, and anisotropy of the bar has little effect on the velocity of the polarized shear wave in different directions. When the specimen is submitted to an increasing axial loading, the time of flight of shear wave polarizing parallel to the direction of stress increases, the time of flight of shear wave polarizing perpendicular to the direction of stress decreases; after eliminating the influence of change of the propagation length of the ultrasonic wave brought by the elastic deformation of material, the results find that velocity of shear wave polarizing parallel to stress orientation decreases, velocity of shear wave polarizing perpendicular to stress orientation increases; the effect of stress which is parallel to polarization direction on velocity of the shear wave is more than that of stress which is normal to polarization direction on ultrasound velocity. The characters and magnitude of internal stress within the material can be evaluated based on ultrasonic birefringence. This experiment provides a non-destructive method for measuring the internal plane stress field of material. This method can be used for evaluating internal stress within the material of interest non-destructively.Third, the result of stress measurement using ultrasonic method has a good consistency with the result using hole drilling method. However, confirmation for absolute magnitude of stress is needed to the improvement of measurement precision. Ultrasonic non-destructive evaluation was verified by using hole drilling method. The trend of residual stress measurement using ultrasonic method is very similar to the trend of stress measured by using hole drilling method, therefore it illustrates that stress component along the propagation direction can be given or calculated by using ultrasonic method. The difference of the absolute value of residual stress by using those two methods mentioned above is caused by the difference of measuring range. Accuracy of non-destructive stress measurement using ultrasonic method is verified by using hole drilling method and it indicates that ultrasonic method for measuring stress is feasible and effective.