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Dynamic testing of elastomeric materials.

Dynamic material property data on modern elastomers are more important than ever. Elastomeric components serve a crucial function in everything from automobiles and computers to golf balls. Predicting their vibration isolation and load-bearing performance over the life of the product is essential. To ensure an accurate model, dynamic material property data must be collected over a wide range of conditions, including dynamic loading conditions, temperature and fatigue history. This is especially true if the investment is made to use non-linear finite element analysis tools in new product development, since inaccurate material properties lead to inaccurate performance predictions.

Servo-hydraulic test systems have been one choice of industry for testing elastomeric components. The high force and displacement capabilities are well suited to measure component behavior, but there are drawbacks to the use of this technology for basic material characterization. The traditional DMA/DMTA test systems are the second choice, but they lack the range of force and displacement needed to characterize non-linear elastomers such as those used in modern pneumatic tire applications and vibration isolation devices.

A new electromagnetic technology that uses a moving magnet instead of a moving coil appears to be a perfect solution for the many opportunities in dynamic characterization of elastomeric material. Whether the need is to perform dynamic mechanical analysis, stress relaxation, creep, hyperelasticity, high cycle fatigue or crack growth testing, this new technology has the range of performance to ensure that models predict the real-world behavior.

Historical perspective

As mentioned, there have been two technologies used for dynamic testing of elastomeric material. Each technology has strengths and weaknesses and thus has found a niche. They include:

* Hydraulics, where high pressure (200-350 bar) oil provides the energy to move a piston; and

* electromagnetic, typically called voice coil, where fixed magnets interact with a moving coil (electromagnet) to create motion.

Hydraulic systems, similar to those provided by MTS Systems and Instron, provide a wide range of frequencies (static to 1,000 Hz) and high force (10,000 N or higher). This combination of capabilities has made this technology the traditional choice for dynamic testing.

However, hydraulic systems are expensive, and their many moving parts and seals lead to high maintenance costs. In addition, they have a high cost of energy, since the pump must run continuously, and the oil is considered a hazardous waste that must be replaced when contaminated. Resolution and fidelity may be limited by seal friction of the actuator and/or servo-valve, and the moving mass of the actuator is quite high. Side loading of the actuator also causes waveform distortion or seal wear due to friction in multiaxial applications.

Electromagnetic systems similar to those provided by Rheometric Scientific, TA Instruments, ThermoHaake and Mettler Toledo create high fidelity motion from static to high frequency (some greater than 400 Hz). They have been used extensively for measurement of the glass transition temperature of polymers, for modeling the curing process and for dynamic characterization of polymers under a wide variety of test conditions.

The systems are energy efficient; however, many use air bearings that require an air pressure source. They have a wide force range (nanoNewtons to hundreds of Newtons), but most systems configured for material testing are limited to 40 N or less maximum force. The displacement range is typically limited (6 mm or less). The maintenance costs of electromagnetic systems are typically low, although the systems used for maintaining alignment (mechanical or air bearings and diaphragms) often require maintenance to ensure optimum performance, and the leads to the moving coil have a finite fatigue life. The moving mass of the coil assembly is fairly high, which limits dynamic performance.

Why pursue a moving magnet motor?

Bose has developed technical strengths in electromagnetics, materials science and motion control that have culminated in high-performance linear actuators using moving coil motor designs (e.g., audio transducers). Recently, Bose introduced a very different type of linear actuator, known as a moving magnet motor, that offers far greater capabilities in dynamic material property testing, while eliminating several of the drawbacks of traditional material testing technology.

The linear motor design offers such material testing advantages as:

* The range of forces (nanoNewtons up to 6,000 N) and the range of displacements (nonometers to 25 mm) cover the majority of material test requirements;

* the high motor forces, coupled with the low magnet mass, means accelerations up to 1,500 meters/[second.sup.2], frequencies over 400 Hz and velocities greater than 3 m/sec.;

* the excellent dynamic performance, coupled with the frictionless flexure suspension, provides exceptional fidelity and precision;

* the linear motor is highly energy efficient, since electrical energy is directly proportional to the required force (no air or oil pressure source is required, and there are no mechanical fiction losses); and

* with no frictional wear in seals or bearings and no moving parts or flying leads, the linear motor has demonstrated excellent durability, with over 15 billion cycles without a failure or maintenance. There is no maintenance or oil or replacement of filters.

How does a moving magnet motor work?

A simple view of the moving magnet motor is illustrated in figure 1. The magnetic portion of the moving magnet motor is comprised of three basic elements, including the magnet, the coils and the core. The magnet moves left and right, while the coils and core remain stationary.

[FIGURE 1 OMITTED]

To meet the requirements of the testing market, Bose implemented the moving magnet motor using state-of-the-art neodymium iron boron (NdFeB) magnets. A simple way to describe the functionality is to view the core and coil combination as an electromagnet producing a north and a south pole as a function of current. When the current is applied, the appropriate poles of the magnet are either attracted or repelled, producing the force. The stronger the current, the stronger the resulting force.

How is the magnet suspended in the gap?

To keep the performance of the motor optimal, the gap between the magnet and core must be very small compared to the thickness of the magnet. 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 crashing against the face of the core. Contact with the core would produce undesirable friction and nonlinear behavior of the motor.

To meet these requirements, Bose created a low-mass flexure suspension with no friction, infinite life, high lateral stiffness and low axial stiffness. The suspension is fabricated with a special stainless steel alloy that has exceptional fatigue performance proven effective at more than 15 billion cycles.

A side benefit of the flexure design is that the flexure resists forces in the lateral and torsional directions of the main axis. The result is that multiaxis testing of specimens can be accomplished economically without damage to the linear motor or frictional effects of seals or bearings.

Integrating the linear motor into test instruments

Under a technical alliance, EnduraTEC Systems and Bose have integrated the Bose moving magnet linear motor technology with EnduraTEC's material testing systems to take advantage of the linear motor's superior range of performance. The outcome of this alliance is the ELectroForce (ELF) series of test instruments. These are available in tabletop versions with force capacity up to 4,500 N (Test-Bench, ELF 3200 and 3300), and a floor-standing model (ELF 3400) with force capacity up to 6,000 N. They are all powered from standard electrical outlets, requiring no additional infrastructure. They are air-cooled, clean room compatible and provide whisper-quiet operation in a compact, space-saving package. Multiple linear and torsional motors can be mounted on the same instrument for multi-axis applications.

With traditional electromagnetic technology, the range of forces and displacement as a function of frequency would be limited. Servohydraulic systems allow higher forces and displacements, but are costly to run and maintain, and their small amplitude excitation is limited by seal friction and moving mass. A test system driven by a moving magnet linear motor provides developers with a wider range of sinusoidal excitation than any other available test system, and the range exactly overlaps the requirements for dynamic testing of elastomeric materials.

The sinusoidal performance of the ELF 3230 as a function of displacement and frequency shows that full dynamic displacement of 12.5 mm is available well past 50 Hz, even with significant applied force. The precision of the linear motor allows testing at frequencies as low as 0.00001 Hz (approximately one cycle per day). The performance of the Bose linear motor has been verified by test data under realistic conditions, including grip mass and specimen stiffness.

Dynamic mechanical analysis

Dynamic mechanical analysis (MDA), sometimes known as dynamic mechanical thermal analysis (DMTA), is a technique for analysis and characterization of the properties of materials, usually polymers. A sinusoidal excitation is applied to a sample of the material, and the dynamic amplitude and phase of the force and displacement are measured. The dynamic properties of the material can then be determined as a function of changing frequency, temperature, strain amplitude, mean level or preconditioning.

Dynamic properties may be measured by an ELF test system, plotted as a function of the swept parameter, in one case, frequency. This test was designed to recreate the typical capabilities of a torsional DMA system. The example is four uncured rubber compounds with various degrees of branching tested at 5% dynamic strain in linear shear. Temperature was controlled to 120[degrees]C.

Data for non-linear material modeling

Most finite element material models require measurement of force and deflection behavior of materials under various conditions. While traditional technology focuses on the measurement of viscoelastic properties in the linear (small strain) range, this new motor is designed to measure the cyclic and time dependent properties of elastomeric materials under more realistic (large strain) conditions. The motor can test strain levels greater than 100% and frequencies from one cycle per day to 400 cycles per second to simulate the critical conditions for the particular component being modeled. The wide range of dynamic performance allows all required tests to be completed on the same system.

A typical tension grip configuration can be optimized for dynamic testing. Samples can be tested in simple tension, compression, shear, pure shear (planar tension), and three or four point bend configurations in single or multi-axial loading configurations to measure all critical responses of the elastomeric material.

Stress relaxation/creep

The low moving mass and high acceleration capabilities of the linear motor test technology provide optimum conditions to measure creep or stress relaxation properties of materials. The test condition can be applied in a matter of milliseconds and held with extreme accuracy. In one example, 15 mm of displacement is applied to the sample in 10 milliseconds, in comparison, the fastest time achieved by the servohydraulic system historically used for this test was 35 milliseconds.

Fatigue and fracture mechanics analysis

The linear motor offers a wide range of dynamic performance and durability, making it an excellent match for the requirements of fatigue and fracture studies of materials. The higher frequency capability of the Bose linear motor generates rapid crack growth in materials, or the application of millions of fatigue cycles in a short period of time. The advanced flexure of this linear motor means that the damage is applied to the specimen, not the seals and bearings of a standard test system, reducing maintenance costs and increasing product test time per system.

High fidelity ensures that test conditions are accurate and repeatable, thereby minimizing data scatter during testing. Tests can cover the full range of conditions needed to determine the fatigue limit or fracture toughness of a material.

Conclusions

Although the range of tests required to characterize the dynamic properties of elastomers can be performed with a combination of traditional servohydraulic and electromagnetic technologies, current economic conditions are driving companies to find ways to reduce costs--both operational and capital. Due to the superior performance of the moving magnet linear motor, researchers are able to accomplish all their testing needs with one test system. Capital costs to furnish a new laboratory have therefore been reduced by half to one-third. If you add to these capital savings the operational benefits, such as reduced maintenance and energy costs, reduced laboratory noise and reduced environmental costs associated with oil and cooling water disposal, the advantages of using the new linear motor technology can justify replacing existing equipment.

One test system can now provide all the material properties required to populate material models for advanced nonlinear finite element analysis. The system is ideal for performing dynamic mechanical analysis, stress relaxation, creep, hyperelasticity testing, high cycle fatigue and crack growth testing. The same system can be used to measure the dynamic properties of tire cord, nylon fiber and cured/ uncured rubber compounds. The patented advantages of stationary coils and a moving magnet, combined with a frictionless flexural suspension, deliver performance unavailable with voice coil or hydraulic based test systems.

Kirk Biegler, EnduraTEC Systems and Rie Carrereas, Bose
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Title Annotation:Process Machinery
Author:Carreras, Ric
Publication:Rubber World
Date:Sep 1, 2003
Words:2165
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