Study on Simple Sensor for Stress History Measurement of Structural Member using Piezoelectric Element

Abstract

In this paper, a stress sensor using piezoelectric element is developed to monitor the feature of stress history of structural member. The sensor is a rectangular box (90mm long, 60mm width and 25mm thick) made of thin aluminum plate, in which piezoelectric element, electric circuit including integrating circuit, button type battery, make and break switch, stress level meter using light emitting diodes (LED) and stress level cross counter are facilitated. The stress level meter can turn on and off different color diodes according to the change of stress level. The stress level cross counter can display the number of cycles which exceeds a specific stress level during a monitoring period. The sensor is bonded onto the structural member by using adhesive agent. The monitoring period can be determined by the attached switch.In order to see the influence of the installation method on the performance of the sensor, experiments are carried out using a fatigue testing machine. The sensor is bonded onto a smooth specimen and multiple repeated stresses are applied to the specimen. The output signals of the sensor are investigated by changing wave shape, stress amplitude and frequency. Followings are made clear from the experiments.1) The thickness of the sensor box influences on the measuring performance of the sensor.2) The sensor can operate until the yield point of the mild steel. Upper limit of the measuring by using piezoelectric element is about 260 JVIPa or 1200 micro strain.3) It is shown the possibility to measure stress of structures which have curved surface by using piezoelectric film (PVDF).