Peter Bailey, Senior Applications Specialist for Instron Dynamic Systems, talks to AZoM about the benefits and applications of composites fatigue testing and how Instron's unique Specimen Self-Heating Control can be utilised within this.
Could you briefly explain the need for fatigue testing of composites and polymers?
The answer is quite simply that for demanding applications we cannot design both safely and efficiently using only static material properties, since real structures also experience dynamic loading.
Let me ask you a question in return: should fatigue testing of composites be any less important than for metals? The answer must surely be “no”, and metals fatigue is well known and accepted as an essential consideration.
Actually, I do wonder whether using the term “fatigue” of composites is a little misleading for some, since the physical processes which occur in the material are so radically different to those in metals.
Nonetheless, there is no doubt that applying load to a composite causes some microscopic damage and that under cyclic loading this damage accumulates, degrading strength and stiffness on the macroscopic scale.
What industries benefit the most from these tests?
In the immediate case, many wind turbine blade manufacturers are already using these tests to inform their design parameters; those who do are successfully delivering products that push performance envelopes, yet with minimal numbers of in-service failures. This also means that raw material suppliers now need to be able to offer fatigue test data on their composite products for this sector.
However, the drive for commercial aerospace and automotive manufacturers to use composites more effectively, more efficiently, and in ever-more critical structures, means that they now recognise the need to incorporate fatigue data into their material selection and design. We anticipate significant growth in demand for composites fatigue testing in these sectors over the next 5 years.
Why do testing standards today specify testing at a fixed frequency?
I believe that at the time these standards were first drafted, this originated from caution over specimen heating, strain rate effects, and machine control. However, in real terms, we now know that unless the strain rate is changing by orders of magnitude, its effect on performance in structural composites is largely insignificant.
Similarly, modern machine control is usually able to adapt to changing frequency and specimen performance in a way that was difficult to achieve with analogue electronics.
How does Instron’s WaveMatrix software assist with these fatigue tests?
WaveMatrix dynamic test software provides user-friendly yet sophisticated test control, allowing the material scientist to easily and quickly build test methods, recording the key data in concise format, monitoring and controlling the test flow.
The basic package automatically corrects the controlled waveform (as mentioned above) to achieve the specified peaks, and latent add-on modules allow live calculation of derived parameters and sophisticated outer loop controls.
How does the Specimen Self-Heating Control (SSHC) enhance this software?
The Specimen Self-Heating Control is unique to Instron, designed specifically for testing of composites and polymers. The module automatically optimises the test frequency throughout the test, achieving a consistent specimen temperature while ensuring the shortest possible test duration.
What are the time-saving benefits of the SSHC add-on?
For production of a typical S-N curve (Wohler curve), we would expect at least a 25% saving in machine time compared with a single fixed frequency method.
In one case study testing 4 specimens at each of 5 stress levels, at 4 Hz, the whole sample would have taken 55 days of continuous running.
By using Specimen Self Heating Control to control absolute temperature to 30 °C, this sample actually took a total of 40 days, and only a modest increase to 6.5 Hz test frequency was required.
Are there any further benefits to this add-on?
Consistent specimen temperature can be maintained for all tests. Case studies indicate that this often results in reduced scatter in cycles-to-failure.
Composites fatigue test practitioners regularly observe in excess of 20 °C or 30 °C temperature rise on more highly loaded specimens (at the ISO/ASTM recommended 3 Hz to 5 Hz).
Meanwhile specimens subjected to lower load amplitudes may run near ambient temperature. Surely any scientist or engineer would agree that such variation, in what should be a controlled variable, is highly undesirable?
And don’t forget just putting your test inside a temperature chamber will not solve this, because the specimen temperature achieved is an offset from ambient conditions.
How is a stable temperature maintained throughout tests?
The software continuously monitors a specimen temperature input, based on which it automatically adjusts the test frequency in order to first attain, then maintain, a target temperature specified by the user.
The user can also specify a target temperature tolerance; in most test environments a tolerance as tight as ± 0.5 °C is easily achieved.
Could you give a brief overview of the recent verification case study conducted with the NCCEF?
The National Composites Certification and Evaluation Facility at the University of Manchester have worked in partnership with Instron since their inception, and have built up considerable expertise in fatigue testing of composites for both research and contract testing. During development of the new software module, we asked them to do a “road-test” on representative S-N curve generation.
Staff at NCCEF provided a sample of woven tow reinforced epoxy resin (from prepreg), then ran two complete datasets; one using their usual methodology at 4 Hz fixed frequency; a second using the same test parameters and the SSHC module to optimize frequency for a specimen temperature of 30 °C. Staff found that it resulted in a 27% saving in machine hours, and results showed a near-perfect correlation between the results of fixed frequency and adaptive frequency methods.
We currently have a similar case study in progress, again at NCCEF, in collaboration with a major raw material manufacturer. On a special glass fibre reinforced material for wind turbine applications, data so far indicates similar time savings, in addition to slightly higher cycles-to-failure with noticeably reduced scatter.
Can the Specimen Self Heating Control add-on be incorporated with all Instron testing systems?
It can be easily incorporated with any dynamic testing system that applies cyclic loading and is driven by an Instron 8800 series controller. If the system isn’t currently running WaveMatrix™ Software then a software update will be required, as well as a specimen temperature transducer.
WaveMatrix supports an affordable thermocouple data acquisition system from National Instruments, connected directly by USB. Alternatively any suitable temperature transducer can be connected via an additional analogue input channel on the controller.
For systems using older frame control electronics (or for other manufacturers), it is necessary to retrofit our 8800MT controller that will support PC control by WaveMatrix, as well as providing improved data acquisition, precision, and operator security.
About Peter Bailey
Peter Bailey is currently the Senior Applications Specialist for Instron Dynamic Systems, researching the current and developing needs in dynamic materials testing, as well as guiding the development of new products.
He holds a BEng degree in Materials Science and Engineering from Imperial College London, and holds a doctorate in the same discipline from the University of Sheffield.
Peter joined Instron in 2012 bringing with him a diverse range of experience in materials testing and laboratory analysis, ranging from industrial production support for marine defence industry, to research and development of a patented range of bio plastics formulations.
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