In a Q&A session, Instron’s senior applications specialist Peter Bailey tells Aerospace Manufacturing about the kinds of trends and demands associated with both static and dynamic testing of composite materials.
Q) What types of carbon and glass fibre composite testing performance demands are placed on you by today’s aerospace designers?
There is an ever-increasing volume of static testing to the ‘normal’ requirements i.e. a wide range of different test types (tension, compression, shear, ILSS, G1c, G2c, bearing load) on coupons. These tests need to be carried out under a wide range of temperatures (-80 to 250°C). The thickness of the sections used in structures is continually increasing, along with the increase in the strength of the composite materials themselves, this is leading to larger test forces, which require higher capacity machines and fixtures.
Composite materials are susceptible to invisible damage due to impacts, resulting in an established and on-going need for testing to evaluate the effects of damage on the strength of composite materials. The most common example of this type of test is Compression After Impact (CAI), a test sample in the form of a rectangular plate is subject to a defined amount of impact damage using an impact tester, the plate is then subject to longitudinal compression to determine the residual strength.
Producing reliable data from mechanical testing of composite materials requires very accurate specimen manufacture and very good test alignment. Industry audit requirements e.g. Nadcap AC 7122, for alignment are very demanding.
The greatest volume of testing is still undoubtedly static, but in recent years’ dynamic properties beyond ‘simple’ impact resistance has become vitally important to industrial design and development. The first new wave of commercial dynamic testing arrived some 8-10 years ago, in the form of fatigue life; driven by the wind turbine industry, where reliable fatigue performance makes a significant difference in material usage and cost. The composites industry is currently experiencing another wave of dynamic characterisation needs, as the automotive industry, with its emphasis on catastrophic failure conditions (i.e. crash safety), moves quickly to adopt composites. This means that both OEMs and raw material suppliers need to generate data on materials behaviour at high strain rates, requiring sophisticated test equipment capable of running monotonic tests, but at 10m/s or even 20m/s, and equally careful data analysis.
Q) In terms of your R&D, where is the main emphasis – more innovations in the hardware or the software?
We are working on both hardware and software for composites testing. For quasi-static testing, the key efforts have been to improve the usability of test machines, and of course to maintain a full range of fixtures and accessories to match evolving test methods. Recently we have introduced a new set of Precision Manual Grips for gripping composite test coupons which meet the alignment requirements of Nadcap AC 7122 and can work over a wide temperature range and allow a wide range of composite test fixtures to be mounted “piggy back” fashion on the grip body. We have introduced an all-new fully automatic extensometer (AutoX) capable of providing very accurate strain measurement for composites testing. We’ve also introduced a new video extensometer (AVE2), with best-in-class performance for quasi-static testing and the capability to deliver real-time non-contact strain measurement during fatigue testing, including transverse strain data.
On the software side, we have recently introduced a major new version of our established ‘Bluehill’ testing software. This version is called ‘Bluehill Universal’ and it’s the first piece of materials testing software to use, full, touch operation. Making widespread use of standard touch gestures ‘Bluehill Universal’ is quick and easy to use ensuring high test productivity. Standard libraries of common composites test method minimise the time needed to set up tests and powerful data export capabilities along with a database tool called ‘TrendTracker’ to help customers manage their test data.
In fatigue testing, customers are making increasing use of our proven live calculations to monitor modulus degradation and damping during tests. Software features such as these may not seem glamourous or novel, with options for step-by-step workflow make a huge difference to many users. Just this year we released a new touchscreen optimised computer interface for static testing equipment – currently receiving a great response from all sectors of materials testing because of its ruggedised yet convenient and intuitive interface.
Q) How do you find the industry in terms of doing business and keeping pace with stringent specifications and qualifications?
Keeping up to date with composite test standards is a key part of working in the composites market and there is no sector which is immune to that. We participate in the development of ASTM and ISO standards and keeps in contact with key players in the industry. In addition, we have two major partnerships with The National Composites Certification and Evaluation Facility (NCCEF) in the UK and the Thermoplastics Research Centre (TPRC) in the Netherlands.
The industry for large scale composites is still relatively young, so no design methods are yet “set in stone” and new developments in processing and raw materials are still coming to maturity quite frequently. As a result, this means the industry as a whole is quite dynamic and quick adoption of new technologies is key to competitive edge. By contrast, 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.
Q) What composites-related testing developments will we be seeing in the future?
As the predictive power of design tools gets more powerful, there will be a need for more comprehensive materials data. This will drive the need for more realistic fatigue testing in realistic environments. At the moment, most composites fatigue testing is tension-tension, but in the real-world materials are subject to multi-axial stresses.
As yet, there are no widely accepted analogues to such criteria as critical fracture toughness values or strain rate sensitivity curves, which are long-established tools in design of critical structures in metals. The composites community needs design parity with metals and that will mean development and acceptance of equivalent tests, both physically and in underpinning theory.
The composites world would definitely benefit from some rationalisation in the testing standards. At present, we see a lot of duplication between ATM, EN, ISO, prEN, as well as manufacturers’ own standards.
Q) Finally, where do you see the biggest business challenge coming from this year and how are you adapting to an ever-changing and unpredictable economic climate?
The biggest challenge is to continue to identify and produce cost-effective standard solutions in a diverse market like the composites market. By working closely with key partners and customers, we are able to track the market needs and produce effective solutions. It’s true that the overall economic climate is challenging, but at the moment we see the composites sector growing strongly and bucking the trend.
