By Landon Goldfarb, Senior Applications Engineer, Instron
Universal testing systems are most commonly known for performing tensile or compressive tests on raw materials, often with specimens in simple geometries. These tests are usually driven by testing standards, and insights are typically limited to what can be found in a material data sheet. Raw materials are not the whole story, however, and the role of universal testing machines is not limited to these basic (though critical) measurements. For example, in the development of medical devices, material selection is only the first in a long and arduous process from design conception to product release. From these materials, components and sub-components are prototyped and manufactured. Components introduce a myriad of new variables that can affect device performance, all of which need to be assessed through testing.
Manufacturing Process or Design Defects
Manufacturing defects typically affect individual components off a production line and can manifest in many different ways. For example, the laser cutting process for machining arterial stents can induce thermal damage, which is invisible to the naked eye but can result in premature failure. Inclusions can occur during the forming process for certain alloys, creating microscopic cracks in the material over time. Design defects are slightly different and will affect every component equally, as the actual design is flawed in some way.
Most medical devices are composed of many different materials which interact with one another, such as a silicon lubricant used inside a syringe barrel or the plastic liner and femoral head of a hip implant. An immense amount of testing is required in order to identify the correct combinations of materials or surface treatments needed to achieve the required level of friction and product durability.
Non-Uniform Stresses & Strains
Typical raw material tests increase repeatability by looking at constant rectangular or circular cross-sectional areas. With products intended to work in conjunction with or mimic biological structures, the product geometry will most likely be more complex. Depending on how the load is applied to the component, the resulting stresses will vary from point to point. Identifying the location of the failure is crucial to adequately evaluating the weakest points of the component.
Instron® offers a range of both hardware and software solutions to provide greater analytic capabilities for customers. Statistical analysis of testing results is a first defense for catching potential outliers or evaluating specific design variables. The standard Bluehill® Universal software allows users to incorporate defined acceptance criteria into their test protocol. Clear visual indicators will be applied to the test results table, directly indicating in which direction the result is out of tolerance. Additionally, a highly visible Pass/Fail graphic will alert the operator to any potential issues. Results can be analyzed in various graphical formats including control charts to identify outliers for specific results within the standard deviation. For larger scale analysis, Instron’s Trendtracker software provides the ability to sort through and compare data across samples, with all the data hosted on a SQL server. To add to the scope of the analysis, networked connections to the database allow multiple systems across different labs to be analyzed, meaning that variables specific to a location or lab can be addressed. Intuitive search and visualization functions offer the user unparalleled visibility into their collected data – for example, they can be used to evaluate multiple silicone compounds used in syringes, highlighting the break loose and glide force statistics across the different samples and determining the compound with the lowest statistical deviation.
While visualizing statistical data can provide a bird’s eye view from sample to sample, Bluehill Universal’s Testcam module gives operators the ability to record and save video of each specimen. After a test is complete, a scanning cursor allows the operator to select particular points of interest on the stress-strain or force-displacement plot while viewing the associated video frame. They can also replay the test to compare specimens. This functionality is ideal for components which exhibit high-energy failures which can easily be missed, and for components that exhibit multiple modes of failure. This feature is extremely useful for medical devices that feature ultrasonic welds, because it is important to know not only the force at which it failed, but also whether the failure was in the weld itself or the surrounding material.
Performing uniaxial tests on components with complex geometries will always result in non-uniform stresses and strains. Standard measuring techniques will only provide a piece of the puzzle, and this is especially true with traditional point-to-point extensometry. Intron’s AVE 2.0 non-contacting extensometer can be used in conjunction with Digital Image Correlation (DIC) software to create full-field strain and displacement maps. In other words, you can see an FEA-style picture to visualize strain and displacement over the full two-dimensional surface of the test specimen. This is an important analytical tool to compare FEA analysis to real world loading conditions, validating your models. The strain mapping can specifically highlight the weakest points of the components, trace the path of crack propagation, and even uncover manufacturing defects which cause premature failure. Within the software, virtual extensometers and strain gauges can be applied anywhere on the specimen surface as well.
These analytical tools enable customers to better assess the performance of their components through providing greater insights at the laboratory, sample, and specimen levels. Instron’s suite of hardware and software accessories allow customers to push the limits of what it is possible to achieve with a universal testing system by adding the functionality necessary for efficient and comprehensive evaluation of components.
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