André Moreau

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-ultrasonic monitoring of phase transformations in steels

The ferrite-austenite transformations in A36 and in IF steels were monitored using laser-ultrasonics. Sudden variations of ultrasonic attenuation were observed at transformation temperatures. Dilatometry and standard metallographic observations support an interpretation of these attenuation variations in terms of nucleation and growth of the new phase. Laser-ultrasonics is a new technique to monitor microstructural changes that take place during phase transformations. This information is obtained in real-time and may greatly facilitate laboratory studies on phase transformations. Using ultrasonic scattering models, and with reliable values of the austenite elastic constants as a function of temperature, laser-ultrasonics could provide a quantitative evaluation of grain sizes during phase transformations.

Ultrasonic velocity measurements during phase transformations in steels using laser ultrasonics

This article presents accurate laser-ultrasonic measurements of longitudinal velocity in the 500–1000 °C temperature range in carbon steel samples for different conditions known to affect phase transformations such as cooling rate, carbon concentration, and rolling. Measurements were performed during continuous heating and cooling at rates varying between 0.1 and 20 °C/s. Carbon concentrations ranged from 0.0% to 0.72%. Hot-rolled and cold-rolled samples were measured. For the hot-rolled samples, a reproducible hysteresis was observed in the dependence of the ultrasonic velocity versus temperature of samples having a significant carbon concentration. This hysteresis is attributed to the combined effects of the phase transformation and of the ferromagnetic–paramagnetic transition. In particular, the rate of change of velocity with temperature during heating suddenly diminishes at the Curie temperature, and the velocity behavior during cooling shows clearly the start and end of phase transformations, even allowing discrimination between ferrite and pearlite nucleations. For the cold-rolled samples, significant drops in ultrasonic velocity were observed at the transformation temperatures when the samples were heated for the first time. However, the magnitude of these drops decreased for subsequent heating. These drops are attributed to irreversible crystallographic orientation changes caused by phase transformations. This effect was modeled using the Kurdjumov–Sach transformation relationship. The model calculation agrees well with the experimental data.

Ultrasound generated by a femtosecond and a picosecond laser pulse near the ablation threshold

We have investigated high-frequency ultrasound generated by single laser pulses in thin (50μm) aluminum foils as a function of the laser fluence. Laser-pulse durations of 80fs and 270ps were used to compare the ultrasound generated in two very different regimes: thermoelastic and ablation. The measured rear-surface displacement induced by the ultrasound pulse is similar after 50-μm propagation through the foils for the two laser-pulse durations in the fluence range of 0.1–0.7J∕cm2. For fluences greater than the ablation threshold (0.25 and 0.63J∕cm2 for the 80-fs and 270-ps pulses, respectively), the ultrasound amplitude generated by the 270-ps laser pulse is increased significantly due to absorption of laser energy by the ablating plasma. This is not observed for the 80-fs laser pulse since ablation is produced well after the laser-pulse irradiation of the target. The measured surface displacement as a function of laser fluence is compared to the calculations of a one-dimensional fluid code for both lase...

Laser‐Ultrasonic Measurements of Residual Stresses in a 7075‐T651 Aluminum Sample Surface‐Treated with Low Plasticity Burnishing

Low‐plasticity burnishing (LPB) is used to introduce deep compressive surface residual stresses that improve the durability of parts. A non‐destructive measurement of residual stresses, their anisotropy, and distribution as a function of depth is being sought to verify initial process quality and residual stress retention over time. Laser‐ultrasonic measurements of Rayleigh wave and surface skimming longitudinal wave (SSLW) velocities were used together to evaluate the magnitudes and directions of the two principal stresses independently of LPB‐induced texture variations. The results agree with x‐ray measurements at the surface. In addition, it was found that the laser‐ultrasonic pulse generation mechanism was surface‐process dependent.

Monitoring microstructure evolution of nickel at high temperature

Laser ultrasonics is used to monitor the microstructure evolution of commercial pure nickel from room temperature up to 1000 °C. The objective is to characterize the response of ultrasonic velocity, attenuation and absorption to recovery/recrystallization and grain growth. These parameters are measured during the applied thermal cycle and for different magnetic fields. Below the Curie temperature, the annealed microstructure shows strong magnetoelastic effects. Above the Curie temperature, the ultrasonic attenuation is dominated by grain scattering, allowing the characterization of grain growth. A relationship between the grain size and attenuation at high temperatures is established.

Laser-Ultrasonic Characterization of the Microstructure of Aluminium

Ultrasonic velocity and attenuation measurements are powerful tools to infer much information about the microstructure and properties of aluminum and its alloys. Laser-ultrasonics is a technology that enables doing these measurements remotely, in-situ or inline and in a fraction of a second. Therefore, it is possible to characterize the thermomechanical processing of aluminum alloys with unprecedented time resolution. This paper reviews the physical principles that allow relating velocity and attenuation measurements to various materials properties and microstructural features such as elastic moduli, crystallographic distribution orientation (texture), residual stresses, recrystallization and dislocations. In-situ (in laboratory furnaces) and in-line measurement examples from the Industrial Materials Institute research group are reviewed and presented.

Bimodal dwell-fatigue Weibull distribution of forged titanium IMI 834

This study addresses cumulative damage and its evolution during the cold dwell fatigue of a near-α titanium alloy. An experimental study was undertaken to examine the evolution of life, strain, strength and damage of 13 titanium IMI 834 samples cut from a single disk forging. The samples were tested in the same dwell-fatigue loading conditions. In the dwell phase, the load is maintained at 80% of ultimate tensile strength (824 MPa, 90% of yield strength) for 30 s. The secant Young’s modulus and inelastic strain at minimum load were measured in order to document the evolution of the irreversible damage against the number of cycles for all specimens. Experimental observations show significant differences in dwell-fatigue life and damage behavior. This mechanical analysis and an analysis of the cumulative Weibull reliability distribution suggest a bimodal dwell-fatigue failure process. Some features of the mechanical behavior can be used to sort the samples according to each of the two failure modes and improve the reliability of the fatigue test campaign.

Characterizing materials with laser-ultrasonics

Recent developments in laser-ultrasonic technology have made it possible to remotely inspect large and complex aeronautic structures for structural integrity as well as gauge hot steel products on the production line. These developments based on laser and interferometer technology have also resulted in a powerful tool for characterizing materials and their microstructure. In fact the same features that make laser-ultrasonics attractive for the inspection of complex structures, such as relaxed requirements of the normalcy of the transducer, a broad frequency bandwidth, and large standoff distances make this technology unique and extremely useful for characterizing materials both in the laboratory and on the production line. Examples of recent developments and applications are presented. These relate to the measurement of microstructural features such as texture, grain size, degree of cold deformation in metals, solid-liquid-interfaces. In particular, the real-time measurement of grain size during thermal p...

Laser‐Ultrasonic Characterization of the Annealing Process of Low‐Carbon Steel

Laser‐ultrasonic measurements are performed during the annealing of cold‐worked low‐carbon steel at temperatures between 590°C and 610°C. During annealing, the ultrasound velocity behaves in a peculiar way, first decreasing and then increasing to a final value. The recrystallized fraction evaluated by metallography indicates that recrystallization occurs during both the decrease and the increase in longitudinal velocity. Texture evaluated using laser ultrasonics suggests that the peculiar behavior of the velocity might be due to two different regimes of texture evolution. Internal friction, investigated through ultrasound absorption measurements, is also revealed to be very sensitive to the annealing process. This internal friction is attributed to magnetomechanical effects that are also known to decrease the ultrasonic longitudinal velocity and probably contribute to the observed peculiar behavior of the velocity. From this work, we conclude that, although there remains much work to do to explain our obs...