The film biaxial stretching apparatus precisely controls the stretching rate to simulate varying strain rates. The dynamic mechanical properties of a material are essentially its mechanical response under dynamic loading (a time-varying load). Strain rate, as a key parameter of dynamic loading, directly influences the presentation of test data and, therefore, is a crucial link in analyzing dynamic mechanical properties. Compared to static testing, which only reflects material performance under slow loading, the film biaxial stretching apparatus can cover a wide range of testing conditions, from low strain rates (e.g., 0.001 s⁻¹, simulating slow material bonding and molding processes) to high strain rates (e.g., above 1 s⁻¹, simulating high-speed packaging and impact loading scenarios). The output data, including stress-strain curves, yield strength, and fracture parameters, can characterize the core properties of a material under dynamic conditions from multiple dimensions, such as elasticity, plasticity, and toughness, providing a quantitative basis for understanding its dynamic behavior.
The changes in the stress-strain curve morphology at different strain rates provide a direct indicator of a material's dynamic elastic properties and stiffness response. In low-strain-rate tests, the stress-strain curves of thin films typically have a gentle slope and a smooth ascending portion. This indicates that when the material is subjected to slow stress, the molecular chains have ample time to adjust their conformation, resulting in a gentle stress increase during the elastic deformation phase and a correspondingly low dynamic elastic modulus (the initial slope of the curve). However, as the strain rate increases, the initial slope of the curve increases significantly, and the overall curve becomes steeper. This indicates that under high dynamic loads, the molecular chains have insufficient time to fully adjust, resulting in an instantaneous increase in the material's resistance to deformation and an increase in the dynamic elastic modulus. This directly reflects the dynamic property of increasing material stiffness with increasing strain rate. This difference in curve morphology provides direct data support for assessing a material's resistance to initial deformation under different dynamic conditions.
The changes in yield strength and yield behavior in the test data accurately reflect a material's resistance to plastic deformation under dynamic loads. When tested at low strain rates, most thin film materials (such as polyethylene and PET films) exhibit a distinct yield plateau on their stress-strain curves, with a low yield strength. This indicates that the material readily enters the plastic deformation stage when subjected to slow stress and exhibits weak resistance to plastic deformation. As the strain rate increases, the yield plateau gradually shortens or even disappears, and the yield strength shifts significantly upward, a phenomenon known as the "retarded yield point." This shift stems from the fact that at high strain rates, the rate of dislocation movement within the material cannot keep up with the rate of load change, requiring higher stress to initiate plastic deformation. This demonstrates that a material's resistance to plastic deformation increases with increasing dynamic loading rate, making it crucial for assessing its morphological stability in scenarios such as high-speed forming and rapid stretching.
Changes in test data for elongation at break and fracture strength are key indicators for analyzing a material's dynamic toughness and fracture resistance. During low strain rate testing, the film material has ample time for molecular chain slip and orientation, resulting in high elongation at break and relatively low fracture strength. This indicates that the material can withstand large deformations without breaking under slow loading, demonstrating good dynamic toughness. When the strain rate is increased to a certain level, the elongation at break decreases significantly, and some brittle materials may even experience "brittle fracture," while the fracture strength may increase slightly. This change in data reflects that under high dynamic loads, the internal crack propagation rate of the material accelerates, preventing the molecular chains from absorbing energy through slip, resulting in reduced toughness and increased brittleness. This directly demonstrates the correlation between a material's dynamic fracture resistance and strain rate, providing a key reference for selecting films for applications prone to fracture, such as impact and high-speed cutting.
The stress relaxation and hysteresis data recorded during the test further reveal the material's dynamic viscoelastic properties. Film materials often exhibit viscoelastic properties. In biaxial tensile testing, if the strain is held constant (stress relaxation testing mode), the stress decay rate varies at different strain rates: faster stress decay at low strain rates indicates more complete molecular chain relaxation under slow dynamic loading, resulting in a higher proportion of viscous deformation; slower stress decay at high strain rates indicates a higher proportion of elastic deformation under rapid dynamic loading, resulting in a delayed viscous response. Furthermore, changes in the hysteresis loop area (the area formed by the round trip of the stress-strain curve) at different strain rates during cyclic tensile testing can also reflect dynamic viscoelasticity. Larger hysteresis loop areas at low strain rates indicate greater internal energy loss in the material, while smaller areas at high strain rates indicate reduced energy loss. These data provide a basis for quantifying the dynamic viscoelastic properties of materials and are particularly important for assessing the durability of films subjected to repeated dynamic loading, such as the repeated opening and closing of packaging film.
The biaxial stress synergy data output by the film biaxial stretching apparatus can reflect the anisotropic response of a material's dynamic mechanical properties. During the production process, thin films are prone to anisotropy due to stretching and forming processes. The difference in stress between the longitudinal (machine direction) and transverse (perpendicular to the machine direction) directions varies at different strain rates. At low strain rates, the bidirectional stress difference is small, and the anisotropy is mild. At high strain rates, the bidirectional stress difference in some materials increases significantly, with stress increasing much faster in one direction than in the other, demonstrating enhanced anisotropy under dynamic loading. This variation in data reflects the different response rates of molecular chains in different directions when subjected to dynamic bidirectional loading. For thin films subjected to bidirectional dynamic loading (such as photovoltaic module encapsulation films and lithium battery separators), this is a key indicator for determining whether the material is uniformly stressed in practical applications and avoiding localized failure.
By integrating test data at different strain rates, a constitutive model of the material's dynamic mechanical properties can be constructed, providing support for performance prediction in practical application scenarios. By fitting stress, strain, and time data at different strain rates, mechanical response equations (such as the Zener model and the Maxwell model) can be established for the material under dynamic loading, accurately describing the performance changes of the material under different dynamic conditions. For example, the food packaging industry can use models to predict the tensile deformation of thin films in high-speed packaging machines. The electronics industry can predict the dynamic stress distribution of thin films during chip packaging, guiding material formulation optimization and process adjustments to ensure that products meet performance requirements in real-world dynamic applications. This closed loop, from test data to property analysis and application guidance, is the core value of the film biaxial stretching apparatus, which uses strain rate control to reflect the dynamic mechanical properties of materials.
