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24th May 2017, Ulm, Germany

Composites testing – trends and challenges

The Zwick/Roell Group, headquartered in Ulm, Germany, is one of the top three global suppliers of materials testing solutions, with an order entry totalling €236 million in 2015. Amanda Jacob talks with David Phillips, VP Corporate Marketing, about current trends in materials testing.

Inside Composites: Please can you tell us a little about Zwick’s background in materials testing?

David Phillips: Zwick/Roell has been involved in materials testing for more than 160 years. The company can trace its roots back to a Swiss company selling hydraulic testing machines back in the 1890s. The current group was formed in 1992, when materials testing specialists Zwick, founded in 1935, and Roell, established in 1920, merged.

Today, Zwick is a still family-owned company, a global leader in static testing machines and a leading supplier of fatigue testing systems. We also offer hardness testing systems and instruments, extrusion plastometers, and products for impact testing. Zwick machines are used in R&D, production and quality control applications in more than 20 industries. As well as standard equipment our industry experts develop tailor-made solutions to meet customers' unique requirements. Innovation, quality and reliable test results are the cornerstones of our business.

David Phillips, VP Corporate Marketing, Zwick/Roell.

IC: What challenges are involved in testing composite materials and how are you addressing these?

DP: Unlike metals and polymers, which are normally treated as isotropic materials, composites generally exhibit anisotropic properties, which are determined by the orientation of the fibre reinforcement. The type of fibre, the format of the reinforcement, the matrix material and the processing method are further variables which can be tailored to provide the desired properties. Consequently a large number of standards have been developed for test methods to characterise all of the properties of the material in a wide range of loading situations. While there are well more than 100 international testing standards that apply to the characterisation of composites, we find that there are approximately 20 commonly applied types of tests.

In the past, in order to perform all the tests required companies and testing laboratories had to use a number of testing machine arrangements, some of which were very complex. To simplify the process of testing and to elevate throughput, Zwick has developed a modular design based on its Allround-Line testing machine which combines ease of operation with a wide range of configuration options. Available in 100 kN and 250 kN versions, a single machine can perform 21 different types of test, encompassing 115 standards. This means overall testing time is reduced. The machine also has two work spaces, which minimises the need for fixture changeovers, and temperature chambers are available to enable testing from -70°C up to +250°C.

Performance of these tests also requires precision fixtures and accurate force application with excellent alignment. Zwick’s extensive range of fixtures covers the whole materials characterisation spectrum for composite materials.

The Allround-Line testing machine for composites can perform 21 different tests, covering 115 standards.As industry trends drive the increased adoption of composite materials in new application areas, we are continually challenged to meet unique testing requirements. A big benefit of our modular approach is that our testing machines can easily be retrofitted for new types of tests in the future.

IC: Can you tell us about some of these trends and the solutions you have developed?

DP: In recent years, an important trend we have contributed to in the transportation sector is lightweighting, which is driving increased use of materials such as composites to reduce weight, improve fuel efficiency and lower CO2 emissions. These 'new' lightweight materials require extensive testing before they can replace conventional, well-characterised materials such as steel. In the automotive industry, for example, Zwick has been involved in a number of unique composite testing challenges. One recent project involved a solution to test composite suspension springs developed by Sara Composites, France, part of the Sogefi Group.

Sogefi's glass fibre reinforced epoxy resin springs are 40% lower in weight than steel coil springs, resulting in a weight reduction of 4-6 kg per car. These springs have been used in the Audi R8.

Zwick supplied multi-channel spring testing machines to Audi to test the composite springs. The objective was to evaluate the characteristics of the new springs with 'classical measurement' tools in the context of an application with strict requirements for measurement accuracy and precision. Multi-channel test platforms allow the line of action of coil compression springs (e.g. in MacPherson struts) to be determined. Standard testing systems equipped with a six or nine-component force-measurement platform enable specific determination of spring penetration points and the resulting load from the force components which occur when the spring is under compression. One important use of these characteristic values is in assessing quality attributes, such as friction, wear and durability.

Testing a glass fibre composite suspension spring.The multi-channel spring-testing device can be installed in a Zwick Allround-Line machine. The machine’s two test areas allow tensile and compression tests to be performed using a single machine. Eliminating the need for re-tooling ensures reproducible test results, as the same arrangement is always used for testing. The availability of two test areas also represents a considerable reduction in time.

A second trend we're seeing is the growing application of multi-material components and hybrid materials. As OEMs continue to adopt composites and novel alloys for use in series production, mixed material strategies that balance performance with cost have emerged. Joining of mixed materials using novel approaches for fastening has become an integral part of the design cycle. Manufacturers routinely explore emerging options for adhesive bonding and mechanical fastening to support joining of these materials. Characterisation of joined materials and components now represents a growing interest in quality control.

A third trend is the use of robotics to eliminate human error and enhance productivity, without sacrificing accuracy. Zwick has pioneered developments in automated and especially robotic testing over the past 35 years.

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