Viscoelastic Material Testing

Viscoelastic material testing mainly conducts scientific testing and analysis on dynamic mechanical properties, stress relaxation, creep properties, compression set, glass transition temperature and static mechanical properties, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Dynamic mechanical properties: This is the most core item for evaluating viscoelastic materials, mainly determining the storage modulus (E'), loss modulus (E'') and loss factor (tan δ) of the material under periodic stress or strain. Through these parameters, the proportion of elastic and viscous behavior of the material at specific frequencies and temperatures can be precisely analyzed, providing key data for product design.

Stress relaxation: Refers to the phenomenon that the stress inside the material gradually decays with time under constant strain conditions. This test is used to evaluate the stress retention ability of the material under long-term fixed deformation, and is crucial for applications such as seals and damping pads that need to bear preload for a long time. It is usually characterized by the stress relaxation rate or the residual stress percentage.

Creep properties: As opposed to stress relaxation, it refers to the behavior that the deformation of the material gradually increases with time under constant stress. This test is used to predict the dimensional stability and deformation amount of the material under long-term load, and is of decisive significance for the life evaluation of application scenarios such as structural support and load-bearing cushioning.

Compression set: This indicator reflects the extent to which the thickness of the viscoelastic material cannot return to its original state after the compression load is released. It is a key parameter for evaluating the elastic recovery ability and permanent deformation resistance of materials such as rubber and foam, directly affecting the service life and reliability of products such as seals and gaskets.

Glass transition temperature (Tg): Refers to the temperature point at which the material transitions from the glassy state to the rubbery state, and is one of the most important characteristic temperatures of viscoelastic materials. By determining Tg with DMA or DSC, the service temperature range of the material can be defined, and the dramatic change pattern of its modulus, damping and other properties with temperature can be understood.

Static mechanical properties: Including basic properties such as tensile strength, elongation at break, compressive strength and hardness (Shore A, D, IRHD, etc.). These data are the basis for material selection, reflecting the load-bearing, deformation and failure resistance of the material under quasi-static or single loading conditions.

Damping properties: Specially evaluates the ability of the material to dissipate mechanical vibration energy by converting it into heat energy, usually characterized by the peak value of the loss factor (tan δ) and the effective temperature/frequency range. High-damping materials are widely used in the fields of vibration reduction and noise reduction, and this test is the key to screening and optimizing materials.

Testing Scope

Rubber and elastomers: Including natural rubber, styrene-butadiene rubber, silicone rubber, fluororubber and polyurethane elastomers. Testing needs to cover their broad viscoelastic response, from low-temperature embrittlement to high-temperature softening, and evaluate their dynamic fatigue, heat generation and aging properties as products such as sealing rings, tires and conveyor belts.

Polymer foam materials: Such as polyurethane foam, polyethylene foam and rubber foam. The testing focus is on their compression resilience, energy absorption efficiency, performance attenuation after multiple impacts, and dimensional stability under different humidity/temperature environments, applicable to the fields of packaging, shoe materials and furniture.

Adhesives and sealants: Including adhesives of epoxy resin, silicone, polyurethane and acrylate systems. The viscoelastic changes of the adhesive before and after curing are tested, focusing on its shear modulus, peel strength retention rate, creep resistance and stress distribution on different substrates, to ensure long-term bonding reliability.

Asphalt and polymer-modified asphalt: As typical viscoelastic materials, the core of their testing is road performance. Through the dynamic shear rheometer (DSR), the rutting factor and fatigue factor are determined to evaluate its high-temperature deformation resistance, low-temperature crack resistance and medium-temperature fatigue resistance, for road engineering quality monitoring.

Biomedical polymer materials: Such as hydrogels, medical silicone and degradable polymers. Testing needs to be carried out under simulated physiological environments (temperature, pH and body fluids), focusing on the influence of their creep and stress relaxation behavior on the stability of tissue engineering scaffolds, the release rate of drugs and the long-term safety of implanted devices.

Composite damping materials: Such as constrained layer damping plates, foam sandwich structures and particle-filled polymer composites. The testing focus is on the vibration transmission loss and sound insulation of the material in a wide frequency band (such as 1-5000 Hz), as well as the overall damping efficiency under the composite structure, applied in the fields of aerospace and precision instruments.

Testing Methods

Dynamic mechanical analysis (DMA): This is the most powerful method for studying viscoelasticity. By applying a small-amplitude oscillatory force to the sample and synchronously measuring its stress and strain response and the phase difference, the spectrogram of the storage modulus, loss modulus and loss factor changing with temperature, frequency or time can be directly obtained. It is a powerful tool for studying glass transition, secondary relaxation and curing processes.

Stress relaxation test: Usually carried out on a universal material testing machine or a dedicated stress relaxation instrument. The sample is quickly stretched or compressed to a predetermined strain and held, and a high-precision sensor continuously records the curve of stress decaying with time. In data analysis, Prony series or Maxwell models are often used for fitting to predict long-term performance.

Creep test: Similar to the stress relaxation test equipment, but in the opposite mode. A constant stress is applied to the sample, and the curve of its strain increasing with time is monitored through displacement sensors or laser extensometers. The test period can range from several hours to thousands of hours, and the data are used to construct the creep compliance curve and evaluate the long-term dimensional stability.

Compression set test: In accordance with standards such as ASTM D395 and ISO 815. A standard specimen is compressed to a specified deformation (such as 25%) at a specific temperature (such as 70 ℃ and 125 ℃), kept for a certain time (such as 22 hours and 70 hours), then taken out and allowed to recover at room temperature for a specific time (such as 30 minutes), and its residual deformation is measured and the percentage is calculated.

Dynamic shear rheology (DSR) test: Mainly used for asphalt, molten polymers, gels and other materials. Using parallel plate or cone-plate fixtures, oscillatory shear is applied to the sample, and its complex shear modulus (G*) and phase angle (δ) are measured. For asphalt, its high-temperature performance can be further evaluated through the multiple stress creep recovery (MSCR) test.

Pendulum impact and rebound test: Used for qualitative or semi-quantitative evaluation of the damping properties of materials. The rebound rate is calculated by measuring the rebound height or angle of the pendulum after impacting the sample. The lower the rebound rate, the stronger the energy loss (damping) ability of the material, which is often used for rapid comparison of rubber and foam materials.

Testing Standards

Domestic Standards:

1. GB/T 528-2009, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties

2. GB/T 1685-2008, Rubber, vulcanized or thermoplastic — Determination of stress relaxation in compression at ambient and at elevated temperatures

3. GB/T 7759-2015, Rubber, vulcanized or thermoplastic — Determination of compression set — Part 1: At ambient or elevated temperatures

4. GB/T 2941-2006, Rubber — General procedures for preparing and conditioning test pieces for physical test methods

5. GB/T 531.1-2008, Rubber, vulcanized or thermoplastic — Determination of indentation hardness — Part 1: Durometer method (Shore hardness)

6. GB/T 9341-2008, Plastics — Determination of flexural properties

International Standards:

1. ISO 815-1, Rubber, vulcanized or thermoplastic — Determination of compression set — Part 1: At ambient or elevated temperatures

2. ASTM D395, Standard test methods for rubber property — Compression set

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Dynamic mechanical analyzer (DMA): The core equipment, with multiple deformation modes such as tension, compression, bending, shear and torsion. High-end models can realize multi-frequency scanning, time-temperature-frequency superposition, humidity and chemical atmosphere control. The temperature control range can be from -150 ℃ to above 600 ℃, and the frequency range from 0.001 Hz to several hundred Hz.

Universal material testing machine: Equipped with a high-precision load sensor and a temperature control chamber, it can be used for stress relaxation, creep and quasi-static tensile/compression tests. Modern models integrate a digital control system that can precisely program complex load-hold-unload sequences and collect data in real time.

Dynamic shear rheometer (DSR): Specially designed for fluid and semi-solid viscoelastic materials. It is driven by a strain-controlled or stress-controlled electromagnetic motor, and equipped with a precise temperature control Peltier system or a forced convection oven. It can carry out amplitude sweeps, frequency sweeps, temperature sweeps and time sweeps, and is an essential instrument for asphalt PG grading and polymer melt rheology research.

Compression set tester: Composed of a constant temperature chamber, dedicated compression fixtures and a thickness gauge. The compression fixtures are usually parallel plate limiters, which can ensure that multiple specimens are precisely compressed to the same deformation in a constant temperature environment. The equipment needs to meet the strict requirements of the standards for temperature uniformity and stability (such as ±1 ℃).

Hardness tester: Including Shore durometers (type A and type D), international rubber hardness testers (IRHD) and ball indentation hardness testers. It is used for rapid, non-destructive evaluation of the softness and hardness of materials. Its results are correlated with the initial modulus of the material, and it is an important tool for quality control at the production site.

Thermal analysis combined system: Such as the combination of DMA with FTIR (infrared spectroscopy) or mass spectrometry. While the DMA tests the change of the mechanical properties of the material, the released gas components or surface chemical changes are analyzed through the combined interface, thereby directly correlating the evolution of the mechanical properties with the microscopic chemical structure (such as degradation and crosslinking), and is used for in-depth research on mechanisms such as aging and curing.