Meeting the demands of composites testing

Gripping composite coupons can be challenging
Gripping composite coupons can be challenging

In a Q&A session, Composites in Manufacturing hears from Instron’s composites market manager, Ian McEnteggart about the kinds of trends and demands associated with the testing of composite materials.

The testing of composite materials presents specific challenges in terms of tensile, compression and bending properties. Their inherent strength and lightweight properties - and the production process required to make them - are being explored across many manufacturing sectors, but this also means testing these materials must keep pace with demand.

Leading test equipment specialists, Instron says its new 6800 Series range of high-capacity floor model universal testing machines are ideally suited to the demands of composites testing.

An example of composites tensile test explosive break
An example of composites tensile test explosive break

Comprising robust frames that are built to withstand the large shock loads experienced when testing composites, the 6800 Series are sealed to protect the internal electronics from the damaging effects of carbon fibre dust and debris. The frames incorporate precision guidance to achieve and maintain alignment capable of exceeding the levels needed to satisfy Nadcap requirements. The frames are controlled by Instron’s Bluehill Universal touch panel software, which includes a large library of composites test methods. They are compatible with Instron’s extensive range of grips, composites testing fixtures, and environmental test chambers.

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

Compared to traditional structural materials, composite materials offer designers much more performance and flexibility. However, these benefits come at the cost of increased complexity. Composite materials are anisotropic (i.e. their properties depend on direction) and many material parameters are needed to describe and model them. Determining these parameters requires a range of test modes e.g. tension, compression and shear - all potentially conducted over a range of environmental conditions. It is easy to overlook the challenges of producing high quality test data to support the needs of both design and materials development.

Q) Is increasing section thickness and the strength of composite materials themselves leading to larger test forces that require higher capacity machines and fixtures?

Continued development of both fibres and matrix materials is driving increases in the strength of composite materials. At the same time the thickness of the materials used in structures is increasing. Together these factors are resulting in the need for higher capacity test machines and fixtures. Another aspect of the use of thicker sections is the importance of testing in the ‘through thickness’ direction as well as the more common ‘in-plane’ direction.  

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

As already noted, characterisation of basic static composite materials properties requires a range of tests under different loading conditions e.g. tension, compression, shear and flexure. For the critical applications where composites now see service, additional, more complex tests are required to determine their durability under service conditions. Different industries focus most heavily on their own, specific, high-risk scenarios. CAI was developed by the aerospace industry to model the effects of, possibly invisible, impact damage on composite panels. Cyclic fatigue testing of turbine blade materials is important for the wind energy industry. High speed tension and compression testing is relevant for automotive crash protection.

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 composite coupons can be challenging. Laminate specimens are generally parallel sided with bonded tabs to prevent the grip jaws from damaging the material and causing premature failures. Gripping mechanisms include manual and hydraulic wedge grips. For demanding aerospace testing, hydraulic wedge gripping solutions are generally preferred because of their controllability and repeatability. However, well-designed mechanical wedge grips can also provide good levels of alignment. Hydraulic grip solutions for non-ambient testing often place the hydraulic components outside the temperature chamber for safety and reliability.

Meeting the demands of composites testing
Meeting the demands of composites testing

Unidirectional composite materials manufactured using a pultrusion process, such as the material used in composite rebar, are difficult to hold in conventional grips. These materials may require ‘potting’ in long metal sleeves prior to gripping.

Accurate alignment of the grips and specimen is very important when testing composite materials. Adjustable alignment fixtures are available to ensure that testing systems meet the alignment criteria required for reliable composites testing. When using well-aligned grips, it is recommended that the grips are permanently left in place on the testing machine and that, when required, test fixtures (e.g. compression platens, bend/ILSS fixtures, etc.) are mounted on the grips using adapters.

Q) In your view, is there still a lack of empiric data for T&M companies to dip into compared to that of metallics testing companies?

Unlike traditional metals there are no generic specifications for most composite materials and given that composites are relatively new and are still developing at a rapid pace this is likely to remain the case for some time. Also, since the fibre layup is usually tailored to the requirements of an individual part, the distinction between the ‘material’ and the ‘part’ is not always clear – an issue that is shared with additively manufactured parts. I think that it’s unlikely that we’ll see easy to access generic data on composite materials any time soon.

Q) Do certain metals applications have slower evolution and growth? Where heavy reliance on legacy data can cause considerable inertia in adopting new methods users need to prove results can be compared with decades’ worth of production and design data?

In aerospace and automotive industries, the trend has clearly been the replacement of traditional metals-based structures with lightweight composites structures. At the same time, the composites materials and associated production processes are evolving rapidly. Given the radical nature of changing from a metals-based solution to one based on composites, there is always a need to undertake extensive testing of the new composite materials and structures. 

Q) Does static testing provide the greatest volume of testing or is dynamic testing now taking the lead as the composites industry experiences another wave of dynamic characterisation needs?

The main volume of composites testing is static testing, however there is a very significant amount of dynamic fatigue testing being carried out. Fatigue testing of composites, to develop S-N curves, can be very time-consuming because the test frequency needs to be limited to prevent damage to the specimen due to overheating. Instron has developed an advanced Specimen Self-Heating Control (SSHC) module for its WaveMatrix software to optimise the speed of cyclic testing whilst preventing overheating during the test.   

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

We continue to develop both hardware and software products for composites testing and other industries. On the hardware side we have recently introduced our new 6800 Series universal testing machines, in addition to controller retrofit packages to extend the life of and upgrade existing frames.

Instron’s 68FM-100 test equipment
Instron’s 68FM-100 test equipment

On the software side we have introduced Bluehill Central software, a laboratory management tool that enables centralised, remote management of Bluehill Universal software applications associated with multiple Instron test frames. The software allows you to remotely manage all Bluehill Universal users, test templates, results, file revision approvals, and audit trail data from multiple Instron systems.

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

Working with the aerospace composites industry is demanding, however the requirements (e.g. Nadcap check list and various test standards) are well-documented, which helps enormously by making the requirements clear. Instron has a lot of experience working with aerospace and other sectors of the composites industry and can define and provide the appropriate equipment and support services such as training, maintenance, and calibration.     

Q) Do you have composites partnerships with any trade organisations and associations?

We have equipped the mechanical test labs at the National Composites Centre, the National Physical Laboratory and the National Composites Certification and Evaluation Facility at the University of Manchester. We are also members of Composites UK and we greatly value our relationships with all of these organisations. We actively participate in the development of ASTM and ISO standards and take part in Round Robin test programmes.

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

I expect to see developments regarding testing of thermoplastic matrix composites including the use of tabless test specimens for high volume testing in the automotive industry. Another likely area will be more testing of intermediate materials to optimise new manufacturing processes, e.g. bias extension tests on fabrics to evaluate formability and tack tests on prepreg. to evaluate handling. Finally, I expect to see growth in adhesive bond testing as we see more use of multi-material structures, especially in automotive.

Q) What do you feel are the most important assets of a company?

The quality and reliability of our products and services, our knowledge of static and dynamic composites testing, and the commitment and experience of our people. Furthermore, our multiple locations around the world mean that we have a global infrastructure capable of providing a broad range of critical service capabilities at the local level – including installations, calibrations, training, and on-site or remote support to minimise downtime.

www.instron.com

Company

Instron

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