Dynamic Testing of Elastic Materials

Dynamic material performance data for modern elastomers is more important than ever before. Elastomer components play a key role in all products, from automobiles to computers to golf balls. Predicting shock and load performance over the life of the product is critical. To ensure an accurate model, dynamic material performance data must be collected under a variety of operating conditions, including dynamic loading conditions, temperature history, and fatigue history, especially when investing in nonlinear finite element analysis tools for new product development, as inaccurate material properties can lead to inaccurate performance predictions.

Servohydraulic test systems have long been used to test elastomeric components, and their high load capacity and long travel are ideal for measuring the behavior of individual components, but this technology has its drawbacks when used to characterize base materials. Conventional DMA/DMTA test systems are a second choice, but they lack the load and travel ranges required to characterize nonlinear elastomers, such as those used in pneumatic tires and vibration isolation devices.

A new electromagnetic technology that uses a moving magnet instead of a moving coil appears to be the perfect choice for dynamic characterization of many elastomeric materials. Whether dynamic mechanical analysis, stress relaxation, creep, superelasticity, high cycle fatigue, or crack growth testing is required, the performance range of this new technology ensures that the model predicts real-world behavior.

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As mentioned above, there are two technologies used for dynamic testing of elastomeric materials. Each technology has its own advantages and disadvantages, and therefore has its own niche market segments. These include:

◆ Hydraulic technology, where high-pressure oil (200-350 bar) provides the energy to move the piston;
◆ Electromagnetic technology, commonly known as voice coil, where a fixed magnet and a moving coil (electromagnet) interact to produce motion.

Hydraulic systems, such as MTS systems and Instron, can provide a wide frequency range (static to 1000 Hz) and high loads (10,000 Newtons or more). These capabilities combine to make this technology a common choice for dynamic testing.

However, hydraulic systems are very expensive and have many moving parts and seals, making maintenance expensive. In addition, their energy costs are high because the pump must run continuously and the oil is considered a hazardous waste and must be replaced if contaminated. Resolution and fidelity can be limited due to friction in the seals of the actuator and/or servo valve, and the moving masses of the actuator are very high. Side loads on the actuator can also cause waveform distortion or seal wear due to friction in multi-axis applications.

Electromagnetic systems such as those from Rheometric Scientific, TA Instruments, ThermoHaake and Mettler Toledo can create high-fidelity motion from static to high frequencies (some exceeding 400 Hz). They have been used extensively to measure the glass transition temperature of polymers, simulate the curing process and characterize the dynamic properties of polymers under a variety of test conditions.

These systems are very energy-efficient: however, most use air bearings and require an air pressure source. Load ranges are wide (from nanonewtons to hundreds of newtons), but most systems for materials testing are limited to maximum loads of 40 newtons or less. Travel ranges are also usually limited (6mm or less). While systems used to keep electromagnetic systems calibrated (mechanical, air bearings or diaphragms) usually require maintenance to ensure optimal performance, maintenance costs are usually low, which also results in a limited fatigue life for the active coil. The active mass of the coil assembly is quite high, which limits its dynamic performance.

Why Active Electromagnetic Motors

Bose has technical advantages in electromagnetics, materials science and motion control, and it has finally developed high-performance linear actuators using active coil motor designs (such as acoustic transducers...). Recently, Bose introduced a completely different type of linear actuator, called an active electromagnetic motor, which has much higher capabilities in dynamic material performance testing and does not have some of the drawbacks of traditional material testing technology. The

linear motor design has the following advantages for material testing:

◆ The load range (from nanonewtons to up to 6000 newtons) and stroke range (nanonewtons to 25 mm) covers most areas where material testing is required;
◆ The high motor output force, coupled with the low magnet mass, means accelerations of up to 1500 meters per square second, frequencies exceeding 400 hertz, and speeds exceeding 3 meters per second;
◆ Excellent dynamic performance, coupled with a flexible suspension system without friction resistance, provides excellent fidelity and accuracy;
◆ Linear motors are highly energy-efficient because the electrical energy is proportional to the required load (no air or oil pressure source is required, and there is no mechanical friction loss);
◆ With no seals or bearings to wear out, no moving parts and no floating heads, linear motors have proven to have exceptional durability, operating for more than 15 billion cycles without failure or repair. There is no maintenance, oil or filter changes required.

How does an active electromagnetic motor work?

Figure 1 is a simple illustration of an active electromagnetic motor. The magnetic portion of an active electromagnetic motor consists of three basic elements: magnet, coil and core. The magnet moves left and right, while the coil and core remain stationary.

To meet the needs of the test market, Bose uses the latest technology magnets containing neodymium iron boron (NdFeB) in its moving magnet motors. A simple way to describe this function is to think of the core and coil combination as an electromagnet that generates north and south poles in response to an electric current. When an electric current is applied, the appropriate poles of the magnets attract or repel each other, generating a force. The stronger the current, the greater the force generated.

How are the magnets suspended in the gap?

For motor performance to remain optimal, the gap between the magnet and the core and the ratio of the magnet thickness must be very small. The mechanical suspension performs a number of important functions. First, the suspension allows the magnet to move along the desired axial path with minimal resistance. Second, the suspension keeps the magnet from colliding with the surface of the core. Contact with the core would create undesirable friction and nonlinear behavior of the motor.

To meet these requirements, Bose created a low-mass flexure suspension system that is frictionless, has unlimited life, is highly rigid in the lateral direction, and has low rigidity in the axial direction. The suspension is made of a special stainless steel alloy that has excellent fatigue properties and has been proven to be effective at more than 15 billion cycles.

An additional benefit of the flexure design is that the flexure resists loads in both the lateral and torque directions of the spindle, allowing multi-axis testing of samples to be completed economically without compromising the friction effects of linear motors, seals and bearings. [page]

Incorporating Linear Motors into Test Instruments

EnduraTEC Systems and Bose have formed a technology alliance to combine Bose's active electromagnetic linear motor technology with EnduraTEC's material testing systems to fully exploit the excellent performance range of linear motors. The result of the alliance is the ElectroForce (ELF) series of testers. The testers include benchtop models with load capacities up to 4500 Newtons (test bench, ELF3200 and 3300) and floor models with load capacities up to 6000 Newtons (ELF3400). They are all powered by standard power outlets and do not require any additional infrastructure. They are compact, space-saving, air-cooled, cleanroom-compliant, operate quietly, and a variety of linear and torque motors can be installed on the same instrument for multi-axis applications.

With traditional electromagnetic technology, the load and travel range as a function of frequency are also limited. Servohydraulic systems allow for higher loads and strokes, but are expensive to operate and maintain, and seal friction and moving masses can limit their smaller amplitude excitations. Test systems driven by active electromagnetic linear motors offer developers a wider range of sinusoidal excitations than other test systems, and the range overlaps exactly with the needs of dynamic testing of elastic materials. The

ELF3230's sinusoidal performance is a function of stroke and frequency, which means that full dynamic stroke of 12.5 mm can be achieved even with significant applied loads at well in excess of 50 Hz. The accuracy of the linear motor allows testing at frequencies as low as 0.00001 Hz (approximately one revolution per day). The performance of Bose linear motors has been confirmed by test data in real-world conditions, including fixture mass and sample stiffness.

Dynamic Mechanical Analysis

Dynamic Mechanical Analysis (DMA), sometimes referred to as Dynamic Mechanical Thermal Analysis (DMTA), is a technique used to analyze and characterize the properties of materials, usually polymers. A sinusoidal excitation is applied to a material sample, and the dynamic amplitude, phase, and stroke of the load are measured. The dynamic properties of the material are then expressed as a function of frequency, temperature, strain amplitude, average content, and preconditioning.

The ELF test system can measure dynamic properties, such as the acceleration parameter plotted against frequency in one case. This test is designed to reproduce the conventional capabilities of the torque DMA system, and an example is the testing of four uncured rubber compounds with different branching degrees under linear shear, 5% dynamic strain, and temperature control at 120°C.

Nonlinear Material Simulation Data

Most finite element material models require the load and deflection behavior of the material to be measured under different conditions. Traditional techniques focus on viscoelastic measurements in the linear (small strain) range, while the new motor is designed to measure the cyclic and time-dependent properties of elastic materials under more realistic (large strain) conditions. The motor can test strain levels in excess of 100% and frequencies from one revolution per day to 400 revolutions per second to simulate critical conditions for modeling specific components. Its wide range of dynamic performance allows all required tests to be completed on the same system.

A conventional tensile fixture configuration can be optimized for dynamic testing. Samples can be tested in simple tension, compression, shear and pure shear (in-plane tension), three- or four-point bending configurations in uniaxial or multiaxial load configurations to measure all the key responses of elastic materials.

Stress Relaxation/Creep

Linear Motor Testing Technology's low moving mass and high acceleration capabilities provide the best conditions for measuring creep and stress relaxation properties of materials. Tests can be performed in milliseconds with great accuracy. For example, a sample travels 15 mm in 10 microseconds. In comparison, the fastest time previously achieved with servo-hydraulic systems was 35 microseconds.

Fatigue and Fracture Mechanics Analysis

Linear motors offer a wide dynamic performance range and durability, making them ideal for studying fatigue and fracture properties of materials. The higher frequencies of Bose linear motors allow materials to grow cracks quickly or fatigue over millions of cycles in a short period of time. The advanced flexure design of the linear motors means that damage is applied to the sample, not the seals or bearings of standard test systems, reducing maintenance costs and increasing production test time per system.

High fidelity ensures that test conditions are accurate and repeatable, thus minimizing data scatter in the test. Tests can cover all conditions needed to determine a material's fatigue limit or fracture toughness.

Conclusion

Although tests to characterize the dynamic properties of elastomers can be performed with a combination of traditional servohydraulic systems and electromagnetic technology, modern economic conditions are driving companies to find new ways to reduce costs - both operating and capital costs. Due to the superior performance of active electromagnetic linear motors, researchers can use a single test system to perform all their testing needs. The capital required to build a new laboratory can also be reduced to one-half or one-third. When operational advantages such as reduced maintenance and energy costs, reduced laboratory noise, and reduced environmental costs due to oil and cooling water disposal are considered, it is well worth replacing existing equipment with this new linear motor technology.

Now a single test system can provide all the required material properties, with the addition of advanced nonlinear finite element analysis material models. The system is particularly suitable for performing dynamic mechanical analysis, stress relaxation, creep, hyperelastic testing, high cycle fatigue, and crack growth testing. The same system can also be used to measure the dynamic properties of tire cord, nylon fibers, and cured/uncured rubber compounds. The patented advantages of stationary coils and moving magnets, combined with a frictionless flexible suspension system, provide performance unmatched by voice coils and hydraulic test systems.