CompositesWorld
Issue link: https://cw.epubxp.com/i/944580
MARCH 2018 12 CompositesWorld DESIGN & TESTING » Adhesive bonding is a critical part of the structural designer's toolkit, whether components are metallic or composite. e avail- ability of high-quality, reliable and reproducible data is central to the ability to design and deliver cost-effective bonded assemblies. Although standard test methods are available, those methods typi- cally specify the use of data acquisition tools that were considered state-of-the-art at the time the method was established. It can be useful to consider the opportunities made available by re-envi- sioning established test methodologies, using more recently devel- oped measurement technologies. is column examines how the application of a video metrology technology can offer more effective measurements than conven- tional means within ASTM standards and extend the range of data available without any changes to the test specifica- tion. ASTM D 5656 is used here as an example of the advantages of the new technology, which can be applied across a range of both standard and novel test procedures. Extensometry update In the ASTM D 5656 test method, thick adherend lap shear samples are adhesively bonded, and a tensile load is applied to place the adhesive in shear. e use of thick, rigid adherends minimizes peel forces, ensuring that the test will obtain data on the shear stress/strain properties in the adhesive — the data useful when designing or analyzing adhesively bonded joints. e current solution used to measure extensions on this specimen is the KGR extensometer, and it has been adopted as the standard by ASTM. e extensometer uses the conven- tional strain-gauged beam approach and is attached to the sample using either three or four pins, depending on the specific implementation. e relative movement between the two plates of the thick adherend shear test fixture is then tracked during the test. Exten- someters are attached to both edges of the specimen and the readings are averaged to give the reported strain. Problems have been reported with these extensometers in terms of slippage at the pin connections, the impact of any rotation on the reported strains, damage to the extensometers when samples fail with the extensometers attached, and the common issues of reliability and the potential for baseline drift with strain gauge systems. Application of video extensometry to ASTM D 5656 The video metrology alternative In place of the standard extensometer (Fig. 1, above), during a demonstration ASTM D 5656 test, a video metrology-equipped extensometer developed by Imetrum (Flax Bourton, UK) was used. Imetrum's systems are designed to yield traceable data across most testing and monitoring requirements for materials, components, assemblies and structures. e system is fully commercial and routinely used to determine material proper- ties, quality control, model verification, and to perform structural testing and in-service assessments. e system's trademarked Video Gauge technology ensures that the system meets the most demanding requirements for strain, elongation, modulus and Reliable, reproducible data is central to the design and delivery of cost-effective bonded assemblies. FIG. 1 An electro-mechanical KGR extensometer, the current ASTM standard test device, is shown in overview (top) with a close-up of the contact pin detail (bot- tom). Source | Imetrum