Bringing out the best in testing

In a Q&A session, Ian McEnteggart, Instron’s composites marketing manager, tells Aerospace Manufacturing about the trends and demands associated with the current testing methods for composite materials.

Instron supplies a range of static, dynamic, and impact testing equipment to perform composite testing to all of the relevant ISO, ASTM and EN standards, as well as meeting the appropriate Nadcap requirements. These systems combine high performance with flexibility, allowing users to easily swap between test types and configurations. All of the systems are controlled by powerful, easy-to-use software, which include test libraries pre-loaded with the most popular testing standards.

Q) What types of composite testing performance demands are placed on you by today’s designers?

Designers require the full set of static properties for composite materials. For laminates this includes both in-plane and through thickness properties. Depending on the application the effects of the environment e.g. temperature and humidity on the properties may need to be quantified. Another important set of properties relates to damage resistance and durability.

Q) Are the thicknesses of the sections used in structures increasing, along with the increase in the strength of the composite materials themselves, which leads to larger test forces requiring higher capacity machines and fixtures?

Yes, the use of thicker sections and the increasing strength of composites materials means that the forces seen in mechanical testing are increasing and so are the shocks generated by the explosive failures. Testing machines used for testing composites need to be very stiff and robust.

Q) What are the most common examples of testing method types, i.e. Compression After Impact (CAI) for example?

Determination of static properties generally requires in-plane tension and compression tests along with shear tests. In some case tests are also required in the through thickness direction. Compression After Impact tests are used to quantify damage resistance and cyclic fatigue tests are conducted to determine durability.

Q) Are you being forced to look at different types of gripper/end effector/alignment methods as the shapes/sizes of composite test pieces become increasing more varied, i.e. no longer ‘dog bone’ shaped?

Gripping of composite laminates has always been a challenge because the faces of the grip jaws must not damage the composites as this will cause premature failures. The traditional approach to preventing gripping damage to composites with a thermoset matrix is to use a rectangular coupon along with bonded ‘tabs’. Bonding of tabs is labour intensive, and it is difficult to bond to the surface of the new generation of thermoplastic matrix composites. For these reasons, new waisted coupon designs and gripping methods using jaws with smooth high friction coatings are being developed and standardised.

Q) In your view, is there a lack of empiric data for T&M companies to dip into in the same way that traditional metallics testing companies can quickly access data on say, 316L/416L, EN14, etc.?

At the moment, composites material specifications are manufacturer specific. One of the keys to the widespread adoption of composite materials is the development of generic composites material specifications - as is the practice for most engineering materials. The use of generic material specifications for composite materials will simplify the design process and material selection as well as improving security of supply. From the point of view of a test equipment supplier, however, the lack of generic materials specification for composites is not a major issue.

Q) Is the composites community looking for design parity with metals which will mean development and acceptance of equivalent tests, both physically and in underpinning theory?

Composites blur the traditional boundaries between ‘materials’ and ‘components’ or ‘structures’ in that the properties of the final component are controlled not just by the properties of the fibres and matrix but also by the way in which the fibres are combined to build up the component (e.g. the fibre directions in the various layers). Using composites in an optimum manner requires a different approach to that taken with metals and thinking of a composite as ‘black metal’ will not produce the best designs

Q) In terms of your R&D, where is the main emphasis – more innovations in the hardware or the software?

We continue to make significant innovations in both hardware and software and in many cases, these go hand-in-hand. For example, Instron’s new 6800 series of electromechanical testing systems incorporate new hardware features and are controlled by Instron’s Bluehill Universal Touch operated software. Instron’s range of ElectroPuls dynamic test machines uses a combination of advanced linear motor and real time control software to produce an easy to use machine for fatigue testing without hydraulic power.

Another example is Instron’s easy-to-use, integrated Digital image Correlation system which uses a high-performance camera along with powerful analysis software to produce full 2D strain maps of test specimens. As well as these developments, the hardware in Instron test machines provides excellent axial and lateral stiffness and precision alignment. Hardware and software developments can bring small, but important benefits to the user, but major improvements are impossible without development of both hardware and software in synergy.

Q) How do you find the industry in terms of doing business and keeping pace with stringent (i.e. Nadcap) specifications and qualifications?

Obviously, the aerospace industry is heavily regulated and controlled. The requirements of say Nadcap are stringent and demanding, but they are well documented and clear. This means that the specifications of equipment are well defined and equipment suppliers and their customers can communicate in an unambiguous manner.

Q) By contrast, do you think certain metals applications are in a stage of much slower evolution and growth, where heavy reliance on legacy data can cause considerable inertia in adopting new methods as users need to prove results can be compared with decades’ worth of production and design data?

Qualifying a new material as a replacement for a material with an established history of use is challenging. Selecting the appropriate qualification tests requires careful analysis and testing of the material that is being replaced and may be required if a complete data set is not available. A comprehensive testing program for the new material is the only way to gain the confidence needed to deploy it.

Q) What composites-related testing developments will we be seeing in the future, i.e. a demand for more data on materials subject to multi-axial stresses?

We are seeing increasing interest in testing that more accurately simulates real world conditions. This includes multi-axial fatigue testing and testing under representative environmental conditions e.g. temperature and humidity. Also fatigue testing of realistic defects and after damage. The growth of thermoplastic composites driven by their improved processability, properties (e.g. toughness) and recyclability will drive changes to testing methods.

www.instron.co.uk