Strain measurement is becoming ever more popular in fatigue testing; however it can be incredibly difficult to acquire this data in the biomedical industry. Typically, in the past, companies had to rely on the strain transducer within the material testing system or a contacting measuring device; both of which have their disadvantages including inconsistency, premature specimen failure, and time-consuming setups. This resulted in many companies simply ignoring strain and concentrating on load and displacement. The confidence in validating results was low and these companies struggled to benefit from the strain data they acquired.
Using a testing system to measure strain has two main issues: 1) it does not solely measure the strain on the specimen, it takes into account the compliance of the testing system and fixtures, and 2) it doesn’t allow the user to concentrate on a specific area of their specimen or component, which is critical when trying to acquire new data on a material or how a component reacts under different conditions.
Previously, the only way to overcome these challenges was to use a contacting device that measures directly on the specimen where the surface area is uniform. However, this introduced different issues related to how the device connects to the specimen, and maintain contact whilst cycling up to 20 Hz. Many users lacked confidence in their data due to the movement of the device slipping on the specimen. A more detrimental outcome was premature specimen failure induced via the knife edges of the device. This again resulted in many companies choosing to ignore strain measurement, relying solely on load and displacement.
Today, we are in a new era where not only is it possible to acquire strain data, but it is now possible to use this as a method of strain control when testing. Non-contacting measuring devices allow users to measure strain directly on the specimen, removing the risk of premature specimen failure (which was previously induced via the knife edges of contacting devices). Furthermore, non-contacting devices allow for improved repeatability, which is essential for validation of test data, and offer quick and easy set-up of a device, which can cater for a variety of gauge lengths and strains. This results in significant time and cost savings compared to requiring a variety of contacting devices greatly reducing time scales to market.
Non-contacting devices instil more confidence in users when collecting both axial and transverse data during cyclic testing and high-speed monotonic tests. Their seamless integration with the testing software allows for strain and load data to be acquired simultaneously. With the growing need to acquire more information about materials and components used in the biomedical market, this development is a giant step forward.
Nitinol is a metal alloy composed of nickel and titanium, for which the medical industry has found use for in a widespread of applications. Nitinol exhibits two unique properties: shape memory and super-elasticity. ASTM F2516-14 is an international standard for tensile testing of nickel-titanium wire. This standard seeks to address the required mechanical testing to determine upper plateau strength, lower plateau strength, residual elongation, tensile strength, and elongation at break. The major challenge in testing nitinol wire is accurate measurement of strain. For nitinol wire with a diameter of greater than 0.2 mm, ASTM F2516-14 requires the use of an extensometer with an ASTM E83-10a class C calibration or better.
In general, there are three extensometer options for testing nitinol wire: 1) a clip-on extensometer, 2) a video extensometer, or 3) an automatic contacting extensometer.
Clip-on extensometers are often used for nitinol wire testing. However, the weight of the clip-on extensometer may cause premature failure of the specimen. In addition, leaving the clip-on extensometer on the specimen through failure could result in direct damage to the extensometer. While clip-on extensometers are advantageous for measuring strain at body temperature in a chamber or temperature-controlled enclosure, using a video or automatic contacting extensometer tend to be more robust solutions.
Video extensometers, also known as non-contacting extensometers, do not directly attach to the material being tested. Instead, video extensometers use two dots to track strain, allowing for a lower likelihood that the extensometer will cause premature specimen failure. In addition, video extensometers can easily be used for measuring strain at elevated temperatures and in fully-hydrated conditions such as testing in a bath. With this said, there are also challenges associated with using a video extensometer to measure strain. Depending on the diameter of the specimen, marking the nitinol wire may be difficult. In addition, ASTM F2516-14 requires the specimen to be loaded and unloaded in a cyclic fashion. Occasionally, during unloading, the wire may bow outwards or inwards causing the dots to move in and out of the video extensometer’s calibrated plane. It may be very difficult to see this with the naked-eye. However, one indication will be an unexpected result for residual elongation and excess noise in the stress vs. strain curve.
If testing in vitro is not a requirement, measuring strain with an automatic contacting extensometer is by far the best option. Automatic contacting extensometers with counter-balanced arms operate on a near frictionless guide system; preventing premature specimen failure. Choosing a contacting extensometer with counter-balanced arms is especially important when testing delicate material such as wire. Another benefit of using an automatic contacting extensometer is accurate strain measurement that is not effected by specimen bowing or twisting during loading and unloading. This type of extensometer tends to provide the most repeatable strain measurement as gauge length and extensometer starting position is automatically set, which removes potential operator variation. In addition, accurate strain measurement can be captured throughout the test and, unlike traditional contacting clip-on extensometers, automatic contacting extensometers can be left on the specimen through failure.
For ASTM F2516-14 nitinol wire tests, strain measurement is critical for determining results such as residual elongation and elongation at break. While there are benefits and limitations of all extensometers, automatic contacting extensometers tend do the best at minimizing measurement errors associated with premature specimen failure and specimen bowing or twisting.
Finite Element Analysis (FEA) has become an increasingly important tool, among all industries, especially biomedical, to analyze the stress profiles of components under various loading scenarios. FEA software is able to create stress profiles by meshing the component into simpler subcomponents, creating a set of elemental equations for each subcomponent, and then recombining the equations in order to produce a full solution. Despite its technical accuracy, there are certainly discrepancies between the results provided and those found in actual testing. For example, inconsistencies in component machining, material variations, and environmental testing conditions can all affect results. For this reason, there has been a recent push for real-world comparison of data found through FEA.
As of now, an accepted method for this comparison has been through the use of strain gauges. In fact, ASTM F04 released a new standard in March of this year, F3161, which discusses specifically the testing of knee femoral components. The standard ran independent studies that found the FEA and strain gauge analysis to correlate within 10%. Strain gauges can be difficult to setup, and require significant time and experience to properly apply them to a component. Instron has sought an alternative solution for FEA comparison through the use of Digital Image Correlation (DIC) Software. DIC Software utilizes a video extensometer to map the strain fields on a specimen under loading. Within the software virtual strain maps can be created to visually compare with the results from FEA.
Figure 1: Speckled Specimen Pre-Test
To run a comparative test between FEA and DIC, a custom component was designed and machined, with the purpose of representing a generic scaled spinal fracture fixation plate. Using the modeling software embedded in SolidWorks, stress maps of the material were created, mimicking the conditions experienced under typical axial loading, at a specified load. The specimens were then speckled, which is required for the DIC software to track strain on the specimen, by measuring incremental movements in speckle locations. The testing was performed until failure, reaching a peak load of nearly 28 kN. Following testing, post-processing was completed in DIC.
SolidWorks’ FEA package has limitations compared to other FEA software packages and is only capable of producing stress mapping as opposed to DIC, which produces strain mapping. They are directly correlated and in most cases, will align considering straining is the direct cause of stress in specimens. In future testing of this concept, a more robust FEA solution would likely to be used. Both maps were found at a load of 25 kN, after specimen yielding in order to provide the most conclusive results.
Figure 2: Strain Mapping from DIC Analysis
Figure 3: Stress Mapping from SolidWorks Analysis
After post-processing, the resulting mappings were shown to have comparable stress/strain concentrations as seen above. Uniform patterns were found on both analyses, driven by the specimen geometry. It is interesting to note that the highest stress concentrations seen on the 2D mapping of DIC were only apparent when looking at the 3D mapping done in SolidWorks. The testing done was exploratory in nature, to identify the degree to which DIC could conform to computer modeling software. This initial testing has proven the concept, and future testing will be done to further investigate the capabilities of DIC.
Join us for a free webinar where Ian McEnteggart and Elayne Gordonov will explore Digital Image Correlation (DIC) applications for testing composites in the automotive industry and testing medical devices for new product development.
With a keen interest in advanced materials, not just from an academic approach, but also in real, end-use situations, Ghent University focuses on research to cover a vast range of materials, new manufacturing processes and materials applications. The lab's need was for evaluation testing of key biomedical components for industrial customers, in ways that were as close to real-life usage as possible. Critical to this was making sure that they could meet the industry's needs for dynamic testing in a variety of fields.
Read more on how the benefits of a system turned into one of the team’s biggest surprises.
We recently partnered with BORS to discuss various solutions in the biomedical field. This day-long workshop involved both theoretical and practical approaches, as well as hands-on demonstrations in our High Wycombe applications lab. All of the attendees attended the following discussions:
- Three-dimensional measurements of bone strain and displacements" by Dr. Gianluca Tozzi from the University of Portsmouth
- "Measuring the material properties of musculoskeletal soft tissues" by Prof. Helen Birch (UCL) & Dr. Chavaunne Thorpe (QMUL)
- "Measuring Bone Quality" by Dr. Richie Abel (Imperial College)
Thank you to all whom attended our BORS Bio Workshop!
Webinar: Common Sources of error in biomedical testingIn the medical device and pharmaceutical industries, data accuracy is incredibly important. Watch the webinar below for the most common areas overlooked in testing that could lead to inaccurate or misleading results.
About Strain Measurement
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