Abstract: A sensor unit is disclosed for use in monitoring in situ changes in dynamic and static stress properties of any material to be monitored, such as monolithic concrete or underground rock structures. The sensor unit includes a hollow, waterproof tubular body to be inserted into a cylindrical borehole in the material to be monitored. A transducer is mounted within the body, which by means of strain gauges, provides an output signal responsive to deflection of the sidewall of the borehole. The transducer is coupled to the sidewall by means of pressure buttons. A monitor is also disclosed, which comprises the sensor unit, a data collection unit and a data analyzing unit. A method for sensing dynamic and static changes in material stresses is also disclosed.