The authors developed an achromatic speckle pattern interferometer able to measure in-plane displacements in polar coordinates. It has been used to measure combined stresses resulting from the superposition of mechanical loading and residual stresses. Relaxation methods have been applied to produce on the surface of the specimen a displacement field that can be used to determine the amount of combined stresses. Two relaxation methods are explored in this work: blind hole-drilling and indentation. The first one results from a blind hole drilled with a high-speed drilling unit in the area of interest. The measured displacement data is fitted in an appropriate model to quantify the stress level using an indirect approach based on a set of finite element coefficients. The second approach uses indentation, where a hard spherical tip is firmly pressed against the surface to be measured with a predetermined indentation load. A plastic flow occurs around the indentation mark producing a radial in-plane displacement field that is related to the amount of combined stresses. Also in this case, displacements are measured by the radial interferometer and used to determine the stresses by least square fitting it to a displacement field determined by calibration. Both approaches are used to quantify the amount of bending stresses and moment in eight sections of a 12 m long 200 mm diameter steel pipe submitted to a known transverse loading. Reference values of bending stresses are also determined by strain gauges. The comparison between the four results is discussed in the paper.
A radial in-plane electronic speckle pattern interferometer (ESPI) is used to measure residual stresses in combination
with an indentation method. A semi-empirical mathematical model is developed to quantify the residual stresses from
the radial in-plane displacement component measurement around the indentation print. Several tests were made in a
specimen with different levels of residual stresses induced by mechanical loading. Correlation functions were fitted to
tests results and are used to predict the residual stresses levels. This paper briefly presents the measurement principle,
testing details and results of the performance evaluation. Finally, an uncertainty budget of the testing and measurement
process was carried out. The tests presented here are not complete since they are restricted to only one material, oneaxis
stress state, two indentation tip geometry and only one indentation force, but they are sufficient to encourage
further development.
A radial in-plane electronic speckle pattern interferometer (ESPI) has been developed by the authors’ group. This interferometer is used in this paper to measure residual stresses in combination with the indentation method. A semi-empirical mathematical model is developed to quantify the residual stresses. Several tests were made in a specimen with different levels of residual stresses imposed by a mechanical loading. Empirical constants were computed from those tests and are used in combination with the developed model to predict the residual stresses levels. The radial displacement field around a controlled indentation print is measured, processed and fitted to a mathematical model to predict residual stresses. Series of tests were designed and executed. Different indentation tip geometry and different loading conditions were involved. This paper presents the measurement principle, implementation details and results of the performance evaluation. The tests presented here are not complete since they are restricted to only one material, one-axis stress state, two indentation tip geometry and only one indentation force, but they are sufficient to encourage further development.
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