text: It's not temperature, but this quantity has a linear relationship with the relative wavelength shift of a fiber Bragg grating.
full_question: It's not temperature, but this quantity has a linear relationship with the relative wavelength shift of a fiber Bragg grating. Tuning this quantity can greatly increase the spontaneous polarization and transition temperature of a ferroelectric thin film. This quantity's time derivative is the symmetric component of the gradient of the velocity field. This quantity's value is constrained by the Saint-Venant compatibility equations. This quantity is the independent variable of Hollomon's equation, in which it is raised to a namesake "hardening" exponent. A common device for measuring this quantity consists of a patterned strip of constantan alloy connected to a Wheatstone bridge, which gives this quantity as the relative change in resistance divided by the gauge factor. This quantity is localized when necking occurs, and it lies on the x-axis of a curve whose slope equals Young's modulus in the elastic region. For 10 points, name this tensor quantity that measures the local deformation of a body in response to stress.
first_sentence: It's not temperature, but this quantity has a linear relationship with the relative wavelength shift of a fiber Bragg grating.
raw_answer: strain tensor [prompt on "deformation"]