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Shape of a substrate directly influences the residual stress in thin film coatings. In this study, a method involving Fiber Bragg Grating (FBG) was used to measure residual stress in a film deposited on a cylindrical surface. An FBG has a cylindrical surface and its Bragg wavelength shifts continuously when a film is being deposited on the sensor's surface. Herein, we calculated the residual strain in the film from the wavelength shift of the Bragg grating by studying the transfer of the residual strain of the cylindrical film to the core of the optical fiber substrate during deposition. By employing the energy method, we derived expressions that related the strain in the core of fiber to the residual strain in single layer films, bilayer films, and multilayer cylindrical films. As an example, we demonstrated a detailed process for testing the stress and the strain distribution across a nickel (Ni) film electrodeposited on the surface of a nickel-phosphorus (Ni-P) alloy-coated optical fiber. The results indicated that the measured strain repeatability was less than 500 μɛ and the strain sensitivity was more than −2 × 10 pm/μɛ, when the thickness of the film was less than 5 μm. The negative sign on the strain sensitivity indicated that the tensile strain in the film produced compressive strain in the core of the optical fiber. The FBG sensor system has high test speed, and integrates measurement and signal transmission. This method provides an effective and convenient approach to measure stress in a film deposited on a cylindrical surface. [ABSTRACT FROM AUTHOR]

This paper describes a residual stress measurement approach that determines a two-dimensional map of biaxial residual stress. The biaxial measurement is a combination of contour method and slitting measurements and a computation to determine the effects of out-of-plane stress on a thin slice. The measurement approach uses only mechanical stress release methods, which is advantageous for some measurement articles. The measurement approach is verified with a numerical experiment and validated with independent confirmation measurements. Biaxial mapping measurements are performed in a long aluminum bar (77.8 mm width, 51.2 mm thickness, and 304.8 mm length) that has residual stresses induced with quenching. The measured stresses are consistent with quench induced residual stress, having peak magnitude of 150 MPa and a distribution that is tensile toward the center of the bar and compressive around the boundary. The validating confirmation measurements showed good agreement with the biaxial map. An uncertainty assessment, performed for each step of the experimental procedure, shows that the overall combined uncertainty is low, maximum of 21 MPa for longitudinal stress and maximum of 6 MPa for transverse stress, indicating that the new biaxial mapping approach has excellent measurement precision. [ABSTRACT FROM AUTHOR]

Magneto acoustic emission (MAE) is a magnetic nondestructive testing (NDT) method that has been used in different fields for 30 years. MAE is a promising method for the early characterization of damage and the evaluation of the residual stress of ferromagnetic materials. However, this technique is still in its early stage and requires further development. The mechanism and influence factors of MAE are still under investigation. Quantitative NDT is difficult because of the lack of robust theoretical bases and models. In this study, we investigated the influence factors of MAE signals systematically and established a mathematical model to describe these influences. The special design of the specimen and the precise control of experimental conditions are the key points for obtaining reasonable experimental results and for developing the model. A methodology for stress assessment was developed on the basis of the proposed model and was verified by using the pure bending test. Results show that stresses within a measureable depth of 4 mm can be evaluated and that the maximum testing error is 30 %. [ABSTRACT FROM AUTHOR]

The ability to determine the individual components of displacement, strain and stress from measured values of a single component of displacement is demonstrated. Advantages of the method include its ability to significantly simplify the experimental technique by measuring only a single displacement component, to provide reliable results both full-field and at the edge of geometric discontinuities, and the lack of need to differentiate the recorded data physically. Results agree with those from thermoelastic stress analysis, strain gages and finite element analysis. [ABSTRACT FROM AUTHOR]

Strain gauges are used together with the corresponding gauge factor to relate the relative electrical resistance change of the strain gauge with the strain of the underlying material. The gauge factor is found from a calibration on a stiff material - steel. Nevertheless, the gauge factor depends on the stiffness of the calibration material and ideally the calibration should be done on a similar material as tested. In practice, the gauge factor found by the strain gauge manufacturer is often used. The paper documents that even for moderately stiff materials such as glass-fibre composites a significant error is found on the strain measurements obtained by the strain gauges. This is documented both experimentally and numerically. A stiffness, also test sample and strain gauge geometry dependent correction coefficient of the gauge factor is proposed. A correction coefficient covers material stiffnesses ranging from 1 GPa to 200 GPa. [ABSTRACT FROM AUTHOR]

The main purpose of the present work is to measure the stress-strain behavior under warm conditions (about 100 °C) of advanced high strength steel (AHSS) sheets up to large strains compared to uniaxial tension. The test equipment consists of two main parts, i.e., a hydraulic bulge tester and a heating device. A mechanical system is attached to the test equipment for measuring the membrane stress and thickness strain at the bulge pole. The stress-strain curves were measured for three kinds of AHSS sheets with the proposed test method for various initial temperatures (10, 50 and 100 °C). The proposed method does not provide isothermal stress-strain curves because the specimen temperature increases during the test due to the effect of deformation-induced heating. A numerical scheme using thermo-mechanical finite element (FE) simulations is suggested to deconvolute the isothermal stress-strain curves. [ABSTRACT FROM AUTHOR]