Non-Contacting Strain Measurement for Composites Coupon Testing
Strain measurement is a key requirement in many tests on composite coupons. Traditional approaches to strain measurement in composites coupon testing have relied on bonded strain gauges or clip-on extensometers. Recent developments in high-resolution image sensors and image processing algorithms mean that non-contacting extensometers offering similar performance to traditional methods are now available for both quasi-static and high speed / cyclic testing. These non-contacting extensometers are more robust than mechanical extensometers and can offer other significant benefits to the operator such as ease of use and the ability to provide full-field strain maps.
Non-Contacting Video Extensometers
Non-Contacting Video Extensometers utilize high-resolution digital cameras and real-time image processing to track the movement of contrasting marks on a test coupon. Strain is determined from the change in the distance between the marks divided by the initial mark separation.
Non-Contacting Video Extensometers are simple to use and reduce the influence of the operator, ultimately improving the consistency of test results. The absence of contact with the coupon means that there is no possibility of the extensometer influencing the material’s behavior or of the extensometer being damaged by the energy released when the coupon fails. When testing with a video extensometer at non-ambient temperatures in an environmental chamber it is usual to locate the camera outside of the chamber and to view the specimen through the chamber window. This means that the device is completely isolated from the hostile environment within the chamber and that it cannot be damaged by the energy/debris released when a coupon fails.
Full-Field Strain Measurement
An exciting development in non-contact strain measurement is the availability of systems capable of measuring full-field strain distributions. The ability to measure full field strain distributions is particularly valuable when investigating the behavior of anisotropic composite materials subject to complex loading e.g. tests on open-hole coupons.
The most widely used approach to full-field strain measurement is Digital Image Correlation (DIC). This technique works by applying a random pattern to the surface of a test specimen, capturing a series of images of a specimen during a test and then analyzing the images with an algorithm that determines; first the displacement field and then the strain field for each image. The number of images captured during a test depends on time, speed, and the sample, but usually 50-100 images are adequate. The first image – also known as the reference image – is captured when there is no strain on the sample. The image is then split into small subsets and the patterns within each subset of subsequent images are compared to the reference image and displacements are calculated. From these displacements, a strain map is calculated. The strain maps of all the strain components (axial, transverse, shear strain), along with maximum and minimum normal strains can be determined.
Compared to traditional methods of local strain (e.g. strain gauges) or average strain over a large gauge length (e.g. Extensometers) measurement, full-field strain measurement yields an enormous amount of additional information that can help engineers and scientists better understand test behavior in situations where the strain is not uniform e.g. Open Hole, Compression After Impact, Vee-notch shear tests.
Composite is a very popular material for many different applications, such as aerospace, automotive, or structural constructions, because it is usually lighter, stronger, and, in most cases, less expensive than traditional material. A good example is the Boeing 787 Dreamliner – the first passenger airplane to use composite materials as the primary material in the construction of its airframe.
The tricky part with composite is the inspection. The structure is so complex – usually fiber-reinforced polymers, carbon-fiber reinforced plastic, or glass-reinforced carbon – that it makes it difficult to inspect its internal structures with traditional methods. Even Ultrasonics and Shearography have some limits. Typical composite manufacturers are looking after delamination, porosities, wrinkles, fiber orientation problems, and lack of material.
This is where advanced technology becomes important. Computed Tomography can not only inspect the internal of the composite structure non-destructively, but it is able to represent a 3D model of the structure with relatively high resolution. 3D has become an important part of composites testing due to the material’s constituted layers, often with fibers distributed in the three dimensions, and based on the directions, the material can show different properties.
|Composite Cone-Shaped Part: Nose of a Commercial Jet Plane. Dimensional Analysis Using VGStudio. Photo Courtesy of North Star Imaging.
||Complex Composite Fiber Part. 3D Results Obtained with a 1-Minute CT.
Photo Courtesy of North Star Imaging.
Using a CT system from North Star Imaging (NSI) – typically the X5000 as it allows for large part inspection and flexible resolution for composite – 3D analysis of a composite structure becomes possible. The 3D rendering capabilities of the NSI software efX-CT allows for multiple virtual cross sections through the part in multiple axes; and the resolution often allows seeing individual composite fibers that are only a few micrometers in size. Delaminations and wrinkles are detectable. Porosities are quantifiable and measurable.
One of the new trends and a largely unexplored capability of CT is to be able to measure the fiber orientations. Based on the distribution and orientation of fibers, the composite part’s properties can rapidly change, and most manufacturers and users of composites are interested in quantifying this. CT is a very promising technology, and with the help of powerful 3D software, this analysis becomes fast, precise, and automatic.
|Fiber Orientation Analysis of a Composite Part. The Colors in the Middle Picture Shows the Angle of the Fibers and Indicate the Orientation Strength of the Part. Photo Courtesy of North Star Imaging.
*Article is courtesy of North Star Imaging
For many years now, the composites research community has considered methods of monitoring the structural health of materials and structures during their life; that is to say, the accumulation and location of damage, and the ongoing fitness-for-purpose of a part. Owing to the challenges of non-destructively analyzing large composite structures, this has often taken the form of “smart” technologies of embedded sensors. When working with carbon fiber-reinforced materials, there is clearly an opportunity to utilize the conductivity of the carbon fiber, correlating changes in resistivity with mechanical changes in the test piece. Various workers have successfully exploited this, particularly in locating impact damage (for example, industrially co-funded work by Hayes, Swait, & co-workers at the University of Sheffield).
Fatigue performance has grown to be a major area of investigation. In research terms, it is interesting phenomenologically, but there is also strong demand from designers and manufacturers to have clear summary data to enable them to “design for fatigue” in composite structures, as reliably and efficiently as they can for metals. These demands often play a large role in choosing the right materials testing equipment manufacturer. For that reason we work hard to maintain an awareness of emerging test methods and how we can help customers with new requirements. Our customers often ask about different methods to monitor composite specimens during fatigue tests.
We set out to investigate how easily we could use a typical fatigue testing system to turn a woven carbon fiber composite specimen into a transducer itself. It is quite simple to connect a specimen as part of a full bridge or potential divider, then measure the fluctuation in output voltage with resistance, using our conventional sensor conditioning channels. Perhaps it should be no surprise that the signal response from cyclic loading is a lot more difficult to interpret…
With a tensile, sinusoidal load on a single-ply specimen, at low frequency, we can see a reasonably clean response of output voltage with load or displacement. A point to note is that with increasing tension, the signal decreases, so specimen resistance must also be decreasing. Carbon fibers are known to be piezoresistive, so one might expect the resistance to go up instead, but clearly some micro-structural effects are having a much more dominant effect.
A significant area of interest is in more complex loading, such as flexure, where direct mechanical measurements are even more difficult. One of our experiments used a hybrid composite specimen with just 2 plies of carbon fiber-reinforcement at the surface, insulated from one another by 10 plies of glass fiber. Flexural loading allowed us to measure the behavior of surface plies in tension and compression simultaneously, revealing some more surprising electrical responses.
Firstly, resistance does not directly track stress or strain, on either surface. Secondly, the polarity of response is the same in both tension and compression, although the correlation is different. Thirdly, there is a significant time-dependence on the electrical response – it seems plausible that this is caused by time dependent mechanical effects at a micro-structural level.
This is just a taster of some effects we uncovered. Dr. Peter Bailey, Senior Applications Expert wrote about and presented the results of our investigation so far, at ICCM20 conference, held in Copenhagen earlier this year.
Composite materials require massive amounts of mechanical testing in all phases of use and application – from development, to qualification, to production, and to quality control. Their anisotropy (mechanical properties different in all directions) and their susceptibility to the surrounding environment (temperature, humidity) increase the likelihood for errors. The various combinations of testing, along with the sheer number of tests required to make statistically meaningful conclusions, may point to the need for some sort of automation somewhere along the testing process.
The requirements for testing composite materials involve a combination of:
- Accuracy: Composite materials are stiff, often hard, and fail at comparatively low strains
- Robustness: Composites often explode upon failure, which is very hard on the equipment
- Repeatability: Given the tightness of controls, any source of external error must be minimized
- Safety: It is common for carbon fiber reinforced plastics (CRFP) to launch projectiles when they fail, requiring significant shielding and protection for the operator
- Fixturing: Many carbon/glass fiber reinforced plastics (CRFP/GRFP) tests involve fixtures – anti-buckling, guidance, constraints – that need careful preparation prior to every test
Based on these requirements, it is not uncommon for labs to pursue ways that best remove the operator from the test cycle without compromising accuracy and repeatability, but yet produce the kinds of results necessary to support production or R&D.
When exploring the various levels of automation in your lab, the key areas to focus on are a combination of process and product:
- Specimen identification and data entry (bar-coding)
- Specimen measurement (digital measurement devices with direct input to the computer)
- Gripping (automatic pneumatic and hydraulic powered grips)
- Extensometry (automatic hands-free strain measurement devices, contacting, and non-contacting)
- Specimen Handling Systems (Cartesian, robotic)
The AT3 Cartesian specimen handling system presenting a plastic specimen to the Automated Specimen Measurement Device (ASMD) to the right of the frame while another specimen is under test in the machine.
Combining process and product into an automated solution contributes toward minimizing variability of results, making better use of skilled labor in the lab, increasing safety, and increasing laboratory throughput.
Downloadable White Papers
- Composites Gripping Guide
- Characterizing Complex Materials with Impact Testing
- A Review of Contacting Strain Measurement Techniques for Composites Laminate Testing
- The Effect of Frame Alignment on Tensile Test Data
- A Review of Current In-Plane Composites Compression Testing Methods & Standards
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As Featured In
- AM&P Magazine: Using Digital Image Correlation to Measure Full Field Strain
- Aerospace Manufacturing: Taking the Strain