Viscoelasticity describes the dual nature of materials that, under deformation, exhibit both viscous (time‐dependent flow) and elastic (instantaneous recovery) responses. In a purely viscous fluid, applied stress leads to continuous deformation (strain) over time, whereas in a purely elastic solid, deformation is immediate and fully recoverable upon removal of stress. A viscoelastic material bridges these behaviors: part of the deformation is recovered like a spring (elastic), and part flows like a dashpot (viscous). Quantitatively, this is characterized by parameters such as the storage modulus (G′), which measures energy stored elastically, and the loss modulus (G″), which measures energy dissipated viscously. The ratio G″/G′ (tan δ) provides insight into whether a material behaves more like a fluid (tan δ > 1) or more like a solid (tan δ < 1).
Different rheological test protocols probe various aspects of viscoelastic behavior:
Small‐Amplitude Oscillatory Shear (SAOS): A sinusoidal strain is applied at low amplitudes to remain within the material’s linear viscoelastic region. Frequency sweeps (varied ω) map G′(ω) and G″(ω), revealing relaxation spectra and molecular dynamics.
Strain Sweep: Conducted at a constant frequency while increasing the strain amplitude, this test determines the linear viscoelastic region (LVER)—the range in which the material behaves predictably without structural breakdown.
Frequency Sweep: Performed within the LVER at varying frequencies, it reveals how the material responds over different time scales. High-frequency data relate to fast deformations (e.g., during processing), while low frequencies reflect long-term stability or structural resilience.
Creep and Recovery: A constant stress is imposed and the resulting time‐dependent strain (creep) is recorded; upon stress removal, partial strain recovery is monitored. This test is sensitive to long‐term flow and structural rearrangements.
Stress Relaxation: A sudden, fixed strain is applied and the decay of stress over time is measured, elucidating the relaxation mechanisms governing recovery. This test is useful for materials that need to absorb shocks or relieve stress under constant deformation.
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