Optical Strain Measurement Glossary

Wavelength is the measured peak wavelength of a fiber Bragg grating sensor. It is normally expressed in nanometers (nm).

The reference wavelength is the peak wavelength of a fiber Bragg grating sensor at a reference condition (zero strain, at reference temperature, and so forth.. It is normally expressed in nanometers (nm).

The wavelength variation (also commonly referred to as shift or as change) is the difference between the wavelength and the reference wavelength (reference value): Δλ= λ- λ₀. It is normally expressed in nanometers (nm).

Strain is a dimensionless value that represents the relative change in the length of a material to its initial length. It is normally of a very small value, and hence is represented by µm/m, ppm or 10^-6.

The sensitivity of an optical strain sensor is the direct ratio between the measured strain and the change in the Bragg wavelength: Δε/Δλ= S. It is normally stated as value in micro-strain per nanometer [(µm/m)/nm)] and is different for every sensor, as it depends on its initial base wavelength, that is: S=1/(k. λ₀).

The temperature cross-sensitivity is a sensor measurement drift caused by temperature variation. It is the strain that is wrongly measured when there is a change of 1ºC (or 1ºK) in temperature. The value is given in (µm/m)/ºC [or (µm/m)/ºK] and can be used to compensate the effect of temperature on the optical strain sensor (not considering the compensation for the thermal expansion of the specimen).

Mechanical stress is expressed by the quotient of the force F and the cross-sectional area A of the stressed material, σ=F/A. It is normally represented in KPa.

The modulus of elasticity, or Young’s modulus, is the ratio between stress and strain in a linear elastic material. It is given by Hooke’s Law (σ=E.ε). It is normally represented in GPa (10⁹ Pa) to correlate strain in µm/m (10^-6) with stress in KPa (10³ Pa).

Poisson's ratio is defined by the division of the transverse strain ε_t and the longitudinal strain ε_l. For aluminum alloys, ν = 0.33, for example.