Stephen R. Schaps

Fatigue load monitoring in steel bridges with Rayleigh Waves

Fatigue load monitoring is a useful tool for safety assessment of highway bridges. Monitoring has been conventionally done using strain gages. Installation of these gages is labor-intensive and requires safety precautions. Noncontact electromagnetic-acoustic transducers (EMATs) offer an attractive alternative. EMATs were used to transmit and receive Rayleigh Waves (RW). Changes in time of flight of RW due to the acoustoelastic effect can in principle be used to monitor stresses resulting from vehicular traffic. We have performed proof-of-concept experiments to demonstrate the feasibility of this approach. Specimens were subjected to bending to simulate the load environment in bridges. RW EMATs were used to measure the relatively low stresses (less than 14 MPa) typically experienced by bridge girders. The signal-to-noise ratio achievable with our system should allow adequate stress resolution for fatigue load monitoring. Factors which could impede technology transfer were considered. The primary obstacle appears to be variability in time of flight (TOF) due to magnetostriction. If the magnetic state is changed (e.g., by scanning of the EMATs) the TOF can change, even at constant stress. We have characterized this effect. If a proper installation procedure is followed, fatigue load monitoring with RW EMATs is feasible.

Methods to improve the accuracy of on-line ultrasonic measurement of steel sheet formability

We report here on an ultrasonic system which quantifies factors which could degrade the resolution of the plastic strain ratio, orr-value, in sheet steel. We also present means to suppress these artifacts. We have developed a moving sheet device (MSD) to be used as a test bed to demonstrate the feasibility of on-line measurement ofr-value in a steel mill. The device can move specimens at speeds comparable to those in industrial practice. An automated velocity measurement system has also been developed and integrated with the MSD. This allows ultrasonic measurements to be made with an array of transducers. Measurements were made in both static and dynamic mode. Artifacts due to sheet motion were small, and should not significantly degrader-value resolution.

Noncontact measurement of the exit temperature of sheet metal in an operating rolling mill

In order to monitor the temperature at the exit end of a hot rolling mill, the Aluminum Association and NIST entered into a Cooperative Research and Development Agreement to use the sheet itself as the basic element of a resistance thermometer. The non-contacting requirement can be satisfied by using eddy current techniques, and the temperature can be deduced from the basic relationship between electrical resistivity and temperature. To convert the measured resistivity to temperature, the temperature dependence of the resistivity of pure aluminum is obtained from laboratory calibration measurements and a temperature independent contribution characteristics of the alloy being produced. The latter is determined from the nominal alloy composition or by making a direct measurement of the resistivity and the temperature at an upstream location when the particular alloy being processed is temporarily held stationary for a contact thermocouple measurement of its temperature. The results of on-line measurements with the eddy current noncontacting thermometer in an operating rolling mill are discussed. These measurements consist of data taken from an entry eddy probe, a contact thermometer, a precise resistivity meter, an exit eddy probe, and a thickness gauge. The entry eddy probe was designed, built, and calibrated by the NIST.

Production line NDE of welds in air-bag inflators

NIST, in joint venture with a manufacturer of automotive air-bags, investigated and developed a system using EMATs to test all production parts and to reject weak welds in them without slowing the production line. After qualification tests were completed, the transducers and associated instrumentation were packaged for operation under mass production conditions in a factory environment by a commercial supplier of EMAT systems. This system was designed to inspect parts immediately after the weld stage while they may still be hot from the welding process.

APPLIED STRESS MEASUREMENT IN STEEL FOR FATIGUE LOAD MONITORING

Over 40% of the nation’s bridges have been classified as structurally deficient or functionally obsolete, with the estimated cost of repairs running in the billions of dollars [1]. It would not be economically feasible to replace all of these structures. Therefore, quantitative assessment of bridge condition is needed to determine which structures need replacing and which are safe.