Stress Reduction
Time-dependent release of internal mechanical strains within a material occurs without the application of external loads. In mechanical and micro-machined sensors, residual stress relaxation can alter the physical dimensions or structural stiffness of the sensing element. This mechanical drift affects the zero-point stability and baseline calibration of the instrument over its operating life by introducing subtle changes in the tension of the active membranes.
Material Mechanism
The driving force behind this phenomenon is the high elastic energy stored in the material during fabrication processes such as machining or deposition. Over time, plastic deformation occurs at the microscopic level through dislocation climb or grain boundary sliding. This residual stress relaxation occurs slowly at room temperature, but is significantly accelerated by elevated temperatures or mechanical vibrations.
Metrological Effect
In capacitive or piezoresistive pressure sensors, the relief of these internal forces manifests as a slow, continuous drift in the zero-pressure output. Since this change in stress alter the sensor’s spring constant, it also causes a shift in the sensitivity of the transducer. Re-calibration must be performed to adjust for these stress-driven sensitivity changes.
Stabilization Technique
Manufacturers often employ thermal annealing cycles to accelerate this material stabilization before the sensor is calibrated. By heating the components to a specific temperature and cooling them slowly, they achieve controlled residual stress relaxation in the metal or silicon structures. This thermal seasoning ensures that the transducer remains dimensionally stable when deployed in the field.
