Unconstrained Expansion
The alteration in the physical dimensions of a material occurring in the absence of external forces or boundary constraints defines a fundamental baseline for structural metrology. This phenomenon, referred to as free deformation, is typically driven by temperature changes or moisture absorption. Engineers measure this change to understand how materials behave before they are integrated into rigid assemblies.
It establishes the starting point for thermal stress calculations.
Thermal Strain
Temperature variations cause materials to expand or contract without generating internal mechanical stress when they move without restriction. This free deformation is quantified using the coefficient of thermal expansion. Sensors are designed to accommodate this change.
Measurement Reference
High-precision dilatometers are used to measure the displacement of a specimen under controlled heating. The free deformation is recorded by a displacement transducer, such as an LVDT, which must be calibrated against a certified reference material. These reference standards, often made of fused silica or platinum, provide a traceably measured expansion curve.
Any friction in the dilatometer pushrod will introduce a constraining force and corrupt the free deformation measurement.
Calibration Deviation
Strain gauges attached to a test specimen must be calibrated to account for the thermal expansion of both the gauge and the substrate. When the substrate undergoes free deformation under temperature cycles, the gauge registers an apparent strain. This apparent strain is subtracted from subsequent load measurements to ensure that only the mechanical strain is reported.
The temperature compensation curve is verified in a calibration chamber under zero-load conditions.
