Innovations In Composites Testing

Interview conducted by G.P. ThomasMar 6 2013

In this interview, Ian McEnteggart, Composites Market Manager for Instron, talks to AZoM regarding the important processes involved in modern composites testing and how this is opening up new avenues of application for composites.

Could you please provide a brief introduction to Instron and how the company is involved in composite testing?

Instron is a worldwide market leader in the materials testing industry. Instron products are used to evaluate the mechanical and physical properties of a wide range of materials and components.

Instron equipment is in widespread use for testing composite materials. The most common types are polymer composites with short, long and continuous fibres.

But other composite materials such as Metal Matrix and Ceramic Matrix types are also being tested. Instron equipment can perform over 100 different test types on composites and have a host of testing solutions and videos portraying our involvement.

Please could you briefly provide a definition of composite materials?

In general, composite materials are materials made up of 2 or more phases or constituent parts.

The most common type of high-performance composites are made up of continuous fibres held together with a thermosetting or thermoplastic polymer matrix.

How are these created and where are they used?

There are many different routes used to produce a composite component. Traditionally the fibres (usually carbon or glass) are processed into sheets.

These sheets may then be impregnated with resin to form “prepreg” These sheets are then cut, layered into a mould and cured in an autoclave.

Alternatively dry fibres are infused with resin in the mould and then cured. These traditional processing methods are inefficient, slow and labour intensive but there is a lot of work on improved manufacturing techniques for composites e.g. automatic fibre placement, or rapid forming using thermoplastic matrix materials.

The main user of composite materials has been the aerospace industry. However, composite materials are now finding applications in many different areas, especially general ground transportation, construction, and marine sectors. The most exciting growth area is volume automotive applications.

Why is it important to have unique testing methods for composites?

Traditional materials, say metallic or polymer are isotropic (i.e. properties are the same in all directions), composite materials are often highly anisotropic (i.e. their properties will vary with direction).

These anisotropic properties can have a huge advantage when designing composite structures because the material properties (e.g. stiffness and strength) can be aligned with the various stresses in the structure. A number of unique mechanical tests are needed to determine the properties of such anisotropic materials

Because of their multiphase composition, some properties are governed by the fibre characteristics, some by the matrix, and some by the fibre/matrix bonding.

Again, unique tests are required to elucidate these different properties.

Why is it important to have specialized instrumentation and fixturing to measure the mechanical properties of composites?

Mechanical tests on composites require specimens and fixtures designed to produce a range of specific stress conditions in the material.

The measurement of strain in the material may require either a specialised extensometer or multiple strain gauges.

Testing is often conducted at non-ambient temperatures and so instrumentation and fixtures must be suitable for the test temperatures used.

How is compression testing utilised and how do these tests conform to national standards?

Composite compression testing is a very common test because it provides its unique information on the nature of the bonds between the fibres and the matrix.

Composite laminate compression test specimens all have a short gauge length to prevent buckling, and tests are conducted to a variety of standard test methods using fixtures designed to minimise unwanted specimen bending.

We provide composite compression fixtures that comply with all international compression standards over a full range of temperatures.

Which of your large range of testing methods are routine in every test?

Tension testing to establish the axial modulus and strength of the material is the most commonly used test, with additional elastic properties being determined by measuring both axial and transverse strain.

Compression testing is also in widespread use and both tests can be conducted on open and hole specimens, as well as on plain specimens.

There are also a number of shear tests commonly used; the Iosipescu Shear test is used for the determination of both shear modulus and strength, whereas the In Plain Shear test (IPS) can also provide shear properties.

The Interlaminar Shear Test (ILSS), which gives an indication of shear strength, is also a very common quality control test.

How is rheology testing employed at Instron?

The Rheology that we do is actually more associated with thermoplastic polymers. >Thermoplastic polymers are finding increasing use in composite materials - traditionally it has been thermosetting resins that have been used with high-performance composite materials.

But now with the big push coming from the automotive industry, and the need to reduce cycle times, thermoplastic materials are very attractive.

So in that case, looking at the properties of the polymers themselves in isolation is often something that would be done in research.

Are there environmental benefits to using thermoplastics in automotive industry?

There are, in that one of the key benefits is that they are easier to recycle than the thermosetting materials.

With the thermosetting materials, all you can do is grind them up into small pieces and use them as filler, but with thermoplastics you can melt it, you can extract the fibres, and you can reuse it. That is a gross simplification, but that’s the idea!

Could you tell us a little bit about some of the organisations that you work with?

We have partnerships with a couple of major organisations; one is the NCCEF – the National Composites Certification and Evaluation Facility at the University of Manchester.

They use multiple Instron systems for their testing and we have a very close working relationship with the staff.

We also have a partnership with an organisation called TPRC, which is the Thermoplastics Composites Research Centre, in the Netherlands.

We are not only their equipment supplier, but we are also on the advisory boards and contribute to what they are doing.

Has Instron recently developed any new equipment for composite testing?

One of the key industry requirements, especially in the aerospace industry, is to have very accurate and verifiable alignment of the testing machines when performing composite tests. This is something that is generally policed by an organisation called NADCAP.

We’ve recently introduced a set of manual tensile grips which have a very good alignment characteristic that satisfies the NADCAP requirements.

We have also recently introduced the AutoX 750, which is a high-resolution, automatic extensometer for automatic strain measurements with accuracy of ±1µm The gauge length of the extensometer can be changed from the test software and it is rugged construction allows it to be attached to the specimen through failure.

The use of the automatic extensometer improves both the accuracy of test results and productivity.

Additionally we have expanded the number of standard test methods included in our BlueHill 3 Software Composites Module which now covers the majority of common composite tests.

How is Instron unique in the field of composite testing?

I think we offer system solutions where the uniqueness doesn’t come from a single magic bullet - it comes from the fact that we offer high-performance, integrated and very ergonomic systems.

One of the aspects of composites testing is that because it requires many different test types and fixturing, the time required to reconfigure the machine can be significant. So we’ve done a lot of work on making it easy to change from one test configuration to another.

How do you see composite testing advancing over the next decade and why will it continue to be of importance?

Most industry sectors are seeking to reduce the mass of structures. We’ve seen this in aerospace, and now it is starting in the automotive industry as they move towards electric vehicles.

There will be an ever-growing demand to reduce the weight of the rest of the car due to the increased battery weight. Composites offer a unique opportunity to do that.

I think we’re going to see composites moving from specialist application into the volume car market – becoming an everyday material.

About Ian McEnteggart

Ian McEnteggart is currently the Composites Market Manager for Instron, having worked with the company for over thirty years, and is a specialist in this growing market.

He holds a degree in Physics from the University of Birmingham, as well as a Diploma in Systems Engineering, and has a strong background in the development of materials testing machines and transducers.

He is also active in the development of international standards for the calibration of transducers and the testing of metals, plastics and composites, and has had several papers on materials testing published worldwide.