Martin Lord

On-line measurement of texture, thickness and plastic strain ratio using laser-ultrasound resonance spectroscopy

Laser-ultrasound resonance spectroscopy, a non-contact ultrasonic technique, was used to determine reliably and rapidly the crystallographic texture, the average plastic strain ratio, and the thickness of sheet metal on the production line. As with laser-ultrasonics, a short laser pulse is used to generate a wide-band pulse of ultrasound and a laser interferometer is used for its detection. In this paper, a large number of echoes are collected and analyzed together using Fourier techniques to measure the natural resonance frequencies in the thickness of the sheet. One longitudinal and two shear resonance frequencies were measured together with their harmonics. From these frequencies, two crystallographic orientation distribution coefficients, W(400) and W(420), are obtained, as well as a highly accurate measurement of the sheet thickness that is corrected for changes in ultrasonic velocity caused by texture variations. Using these coefficients, the average and in-plane twofold and fourfold variations of the plastic strain ratio, respectively r delta(2)r, and delta(4)r, can be evaluated. These parameters are indications of the formability of metals sheets, which is of industrial interest. Measurements on 1 mm thick, low carbon steel sheets have shown the following measurement accuracies: r to within +/-0.08, delta(2)r, and delta(4)r to within +/-0.1, and thickness to better than +/-1 microm. On-line tests at LTV Steel Company showed that the sensitivity of the apparatus is sufficient to detect systematic variations in texture along the length of similar production coils and that the on-line repeatability for r was of order +/-0.02.

Laser‐Generated Surface Skimming Longitudinal Wave Measurement of Residual Stress in Shot Peened Samples

Shot peening is a surface enhancement technique that produces a compressive residual stress in a thin surface layer to extend the fatigue life and prevent stress corrosion cracking of structural components. A laser‐ultrasonic non‐contact technique is used for determining residual stresses produced by shot peening. The method is essentially based on the acoustoelastic effect of the laser‐generated surface skimming longitudinal wave (LSSLW) propagating just below the surface. The LSSLW is found to be more sensitive to residual stress than the Rayleigh surface acoustic wave. When used in combination with the Rayleigh surface wave, this laser ultrasonic technique is shown to be quite effective and provides results in good agreement with x‐ray data. Contributing factors such as surface roughness, crystallographic texture and dislocations have been examined and corrections for a satisfactory assessment of residual stress are proposed.

Laser Ultrasonic Thickness Measurements of Very Thick Walls at High Temperatures

Laser‐ultrasonics presents many advantages compared to conventional ultrasonics, but is, generally, considered as less sensitive. As a consequence, laser‐ultrasonics should not be adequate for ultrasonic measurements in coarse microstructure materials or measurements of large thicknesses. However, since the generated waves extend to very low frequencies, measurements in such conditions can be successfully performed if a photorefractive interferometer sensitive also to these low frequencies and properly balanced is used for detection. This is demonstrated by measurements of thicknesses up to 100 mm (4″) for various steel grades and at temperatures up to 1250 °C.

Inspection of thick welded joints using laser-ultrasonic SAFT

The detection of defects in thick butt joints in the early phase of multi-pass arc welding would be very valuable to reduce cost and time in the necessity of reworking. As a non-contact method, the laser-ultrasonic technique (LUT) has the potential for the automated inspection of welds, ultimately online during manufacturing. In this study, testing has been carried out using LUT combined with the synthetic aperture focusing technique (SAFT) on 25 and 50mm thick butt welded joints of steel both completed and partially welded. EDM slits of 2 or 3mm height were inserted at different depths in the multi-pass welding process to simulate a lack of fusion. Line scans transverse to the weld are performed with the generation and detection laser spots superimposed directly on the surface of the weld bead. A CCD line camera is used to simultaneously acquire the surface profile for correction in the SAFT processing. All artificial defects but also real defects are visualized in the investigated thick butt weld specimens, either completed or partially welded after a given number of passes. The results obtained clearly show the potential of using the LUT with SAFT for the automated inspection of arc welds or hybrid laser-arc welds during manufacturing.

RESIDUAL STRESS MEASUREMENT IN FRICTION STIR WELDS USING LASER ULTRASONICS

Friction stir welding (FSW) has been gaining acceptance and has found various applications in aerospace, automotive and naval industries. As with other welding process, thermal residual stresses induce distortion in thin section structures resulting in buckling. In the present work, laser ultrasonics is used to measure residual stresses induced by the FSW process. The proposed method is based on monitoring the small velocity change of the laser generated surface skimming longitudinal wave. This wave is found much more sensitive to stress than Rayleigh wave. The residual stress profile measured perpendicular to the weld line is in agreement with results from numerical simulations and strain gauge measurements.

Laser ultrasonic measurements of scattered waves in steel

A laser ultrasonic system has been used to measure backscattered waves with the intention of evaluating microstructures of steels. The frequency spectrum of the backscattered signal from samples with different grain sizes is evaluated and compared with a simple model that accounts for single scattering and a first order correction to multiple scattering. This model predicts a maximum in the spectrum of the backscattered signal at the frequency near the transition between the single and multiple scattering. Accordingly, the experimental results show that for larger grain sizes, the scattered signal has a lower frequency content. A good quantitative correlation between the grain size and a spectrum parameter is presented.

AISI/DOE Advanced Process Control Program Vol. 4 of 6: ON-LINE, NON-DESTRUCTIVE MECHANICAL PROPERTY MEASUREMENT USING LASER-ULTRASOUND

The goal of this project was to demonstrate the feasibility to measure the mechanical properties, such as yield strength, tensile strength, elongation, strain hardening exponent and plastic strain ratio parameters, of low carbon steel sheets on the production line using laser ultrasound. The ultrasound generated by the developed apparatus travels mostly back and forth in the thickness of the steel sheet. By measuring the time delay between two echoes, and the relative amplitude of these two echoes, one can measure ultrasound velocity and attenuation. These are governed by the microstructure: grain size, crystallographic texture, dislocations, etc. Thus, by recording the time behavior of the ultrasonic signal, one can extract microstructural information. These microstructural information together with the modified Hall-Petch equation allow measurement of the mechanical properties. Through laboratory investigations with a laboratory laser ultrasound system, followed by the installation of a prototype system at LTV Steel Companys No.1 Inspection Line in Cleveland, all target mechanical properties of ultra low carbon (ULC), low carbon (LC) and high strength low alloy (HSLA) steel sample lots were measured meeting or nearly meeting all the target accuracies. Thus, the project realized its goal to demonstrate that the mechanical properties of low carbon steel sheets can be measured on-line using laser ultrasound

Inference of Hardness from Magnetic Measurements in Pearlitic Steels

The possibility of monitoring material properties during the early stages of processing is an area of growing interest which offers potential not only for improving quality but also productivity. Of particular interest is the monitoring of mechanical properties on-line either during intermediate stages of processing or prior to shipping[1, 2].

Use of the Hugoniot elastic limit in laser shockwave experiments to relate velocity measurements

The US National Nuclear Security Agency has a Global Threat Reduction Initiative (GTRI) with the goal of reducing the worldwide use of high-enriched uranium (HEU). A salient component of that initiative is the conversion of research reactors from HEU to low enriched uranium (LEU) fuels. An innovative fuel is being developed to replace HEU in high-power research reactors. The new LEU fuel is a monolithic fuel made from a U-Mo alloy foil encapsulated in Al-6061 cladding. In order to support the fuel qualification process, the Laser Shockwave Technique (LST) is being developed to characterize the clad-clad and fuel-clad interface strengths in fresh and irradiated fuel plates. This fuel-cladding interface qualification will ensure the survivability of the fuel plates in the harsh reactor environment even under abnormal operating conditions. One of the concerns of the project is the difficulty of calibrating and standardizing the laser shock technique. An analytical study under development and experimental tes...