Stress measurement using ultrasonic techniques has emerging as one of the simplest, low cost, and fast technologies for integrity evaluation. The method consists of relating the ultrasonic wave speed with mechanical stress. Critically refracted longitudinal wave, also called Lcr wave, is the kind of wave which is most sensitive to strain and stress. Taking such technique to the field requires the knowledge about how non-controllable factors influence the measurement. Previous works identified the temperature as the main influence variable. This work deals with the magnitude of such influence. Samples of API 5L x70 steel, which is one of the most used materials for pipelines, were cut from an as-manufactured sheet and tested inside a room with controlled temperature. The magnitude of the temperature was allowed to change between 20 and 25°C. Care was taken to control the remaining influence factors. The measurement of the time-of-flight was performed using two ultrasonic transducers placed in a probe specially built for stress measuring with Lcr waves. For such probe, wave speed is proportional to time-of-flight. The same kind of transducers was used in all measurements, but their natural frequencies varied: 2.25, 3.5 and 5.0 MHz. The results showed that the time-of-flight is highly influenced by the temperature and that the coefficient of variation is between 14 and 18 ns/°C or equivalent to 50 MPa/°C. Considering that the method can measure stresses as low as 25 MPa, the temperature effect is very important and must be taken account for measurements in the field.

This content is only available via PDF.
You do not currently have access to this content.