Material testing instruments and finite element analysis.

There was a time when field testing was the principal method for testing products and gauging performance success. Today, product development is a faster and far less expensive proposition, thanks to computer simulation and computer-aided engineering (CAE) software. Much of the testing that once took place in the "real world" now takes place on a desktop. But the accurate simulation of an event on a computer is completely reliant on an accurate understanding of the physical materials involved in the event. Actual physical testing of materials is integral to supplying the material models and data that make computer simulation as realistic as possible. Only through such material testing can the behavior of the materials being tested be completely understood.

This is where companies such as Axel Products of Ann Arbor, MI, enter the picture. This provider of material testing services for engineers and analysts offers characterization of nonlinear materials such as elastomers and plastics as its primary area of expertise. Axel receives materials from customers, and subjects the materials to an extensive series of structural or thermal experiments. The resulting experimental data, in the forms of graphs or digital data sets, allow Axel's customers to either make engineering decisions directly or to construct material models used in CAE software.

Inside finite element analysis (FEA) software, parts are broken down into many little elements, and the effects of temperature, forces or strains on the overall part are examined by analyzing the effects on the smaller elements. This analysis is based on a math description of the material properties measured in the laboratory.

Most of Axel's test instruments, and all of its major structural test instruments, were designed and manufactured by Instron, headquartered in Canton, MA. This company's testing equipment is used to test the mechanical properties and performance of various materials, components and structures in a wide array of environments. At Axel, the focus of their Instron instruments is on testing for sealing and fatigue problems associated with plastic and rubber materials. These instruments range from custom servohydraulic testers for high strain rate experiments to electromechanical instruments.

Servohydraulic systems are designed for a wide range of strain rate testing, including tensile, puncture, compression and flexure of materials. Instron uses high performance actuators, seismic reaction masses and low inertia/high resonant frequency load measurement systems to achieve strain rates of up to 1,000[s.sup.-1]. These systems offer the ability to perform quasi-static and fatigue tests, providing a degree of flexibility not typically associated with high strain rate test equipment.

Electromechanical equipment is said to feature the industry's most accurate force measurement specification, cutting edge control and data acquisition electronics and superior frame design.

To better understand the role that material testing instruments and FEA play in product testing and development, testing services that Axel recently provided to its customer, General Motors Powertrain Division, will be examined.

GM was working on designing a complex elastomer seal that would sit between an engine's rocker arm cover and a cylinder head; the purpose of the seal was to maintain a constant pressure with no oil leaking out and no air seeping in. There were some difficulties inherent to the placement of this seal that would require it to have certain material traits. The seal had to be squeezed into a groove in between a variety of bolts and other engine parts.

GM worked with Axel to predetermine the elastomer's material during this simulation. Both stress and strain problems inherent to the specific sealing application and the complex nature of the elastomer meant that there would be a lot of variables to work into any material tests.

The first complexities to consider involved the installation of the seal and the subsequent reaction of the seal to different forces. In other words, tests needed to determine if the seal could remain properly compressed between the top and bottom surfaces while maintaining the desired pressure.

The elastomer itself was a complicated material. Elastomers retain their volume, even while undergoing great stresses. This means that if you push on one side of an elastomer, it wants to pop out somewhere. Given that this elastomer was to be squeezed into a groove, there was an array of strains involved, including compressive, shear and tensile, and combinations of all three.

To accurately gauge the material behavior of the elastomer and GM's seal, Axel conducted experiments that included tension, pure shear and biaxial extension. These represent the three basic deformational modes of strain on an elastomer. Each of the tests was performed as a series of stretch and hold experiments, and all were conducted at the range of temperatures experienced by an elastomer in this application.

Instron's servohydraulic instruments were used in these experiments, and are very programmable, allowing specific loadings related to specific analysis to be input. They also allow any transducer to be used as a control variable, such as position or strain or force. And these instruments were able to measure across a very wide range.

Instron

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