Damping Metric
Dissipation of mechanical vibrational energy into heat within the interior of a solid material under cyclic stress defines the intrinsic damping capacity of that material. In physical metrology, internal friction serves as the quantitative measure of phase lag between applied stress and resulting strain, governed strictly by anelastic relaxation and microscopic defect movement. The domain encompasses metals, ceramics, single crystals, and polymers undergoing small-amplitude cyclic deformation, ceasing to apply where energy loss is governed by external friction, aerodynamic drag, or macroscopic crack propagation.
Dissipation Mechanisms
Sub-microscopic dislocation motion, point-defect migration, and grain boundary sliding govern energy dissipation across specific temperature and frequency domains. The metric is commonly expressed as the tangent of the mechanical loss angle, inverse quality factor, or logarithmic decrement of free decay oscillations. In interstitial alloys, the Snoek effect produces sharp internal friction peaks at temperatures where carbon or nitrogen atoms hop between preferred lattice sites under external strain fields.
Polymers exhibit mechanical dissipation via localized side-group relaxations or main-chain cooperative motions. Thermoelastic damping occurs simultaneously in thin beams as cyclic flexure drives reversible heat flow across thermal gradients.
Instrumentation Protocol
Measurements utilize inverted torsion pendulums, vibrating reed fixtures, or resonant acoustic spectroscopy systems operating under high vacuum. Evacuation below ten to the minus four millibar eliminates aerodynamic drag, isolating true internal material damping. Laser vibrometers or electrostatic pickups measure vibration amplitudes without adding mechanical mass or external damping constraints.
Specimen mounting requires nodal clamping, where grip contact occurs strictly at displacement nodes to eliminate energy transmission into supporting frames. Temperature sweeps running from cryogenic levels up to melting points map absorption peaks against vibration frequency.
Metrological Application
Sensor designers utilize internal friction measurements to qualify ultra-low-loss materials for high-stability acoustic wave resonators and microelectromechanical gyroscope cantilevers. Silicon and fused quartz demonstrate exceptionally low internal damping, enabling high quality factors in resonant sensing applications. High internal loss, conversely, is screened for and selected in structural elastomers used for vibration isolation and mechanical shock packaging.
Material purity, annealing states, and irradiation damage directly alter internal friction peak heights, making the measurement a reliable tool for microstructural non-destructive evaluation.
