How to get dynamic tensile properties experimentally | High Rate tensile testing video
Dear Reader,
Good day. I hope you are doing well. I want to offer a quick reflection on some things that I have been working on this week.
Technical Reflections
How to get dynamic tensile properties experimentally?
Often, it is relatively straightforward extracting mechanical properties of engineering materials. This will require some kind of experiments. Some of these properties are: how stiff the material is under loading (Young's Modulus), how strong the material is before it breaks (Ultimate tensile strength) and how much it will stretch before fracture (failure or fracture strain). All these properties are often reported in a manufacturer's datasheet documentation of a material and those are referenced by a design engineering during the design stage of the project. All these properties are often generated under low speed (quasi-static) conditions.
Traditionally, the speed of testing is not considered but for certain types of materials described as rate-dependent materials, the speed at which such testing is done will affect the properties generated. These materials are very sensitive to how much fast a test specimen is tested. The property that drive this rate-dependent behaviour is diverse but most of it originate from crystalline (for most metals) or macromolecular (for most polymers) structure of the test material. A feature that drive this for polymers is viscoelasticity which describes the behaviour of the material as both a viscous (flowing like a liquid) and elastic (stretching like an elastic band) behaviour.
So, when a material is to be tested at high speed, one of the equipment to do so is called a split Hopkinson pressure bar. It involves two cylindrical metallic bars squashing (mainly in compression) a specimen placed in between them. The first bar (called incident bar) will be set in motion by a striker bar (projected onto the incident bar by a gas gun). As the incident bar travels under the effect of loading from the striker bar, it experiences stress waves through it which is recorded using a strain gauge which measures the corresponding strains (deformation) of the incident bar under the effect of the stress wave.
The same stress wave travels to compress the test specimen in-between the two bars. Some of the stress way is reflected away from the face of the test specimen whilst the other get transmitted through the specimen and then into the transmitter bar. The transmitter bar records the residual stress wave following compression of the specimen. Based on differentiation of the stress waves in input and output bars, you are then able to determine the stress-history of the test specimen during the deformation.
Unfortunately, this design only works in compression. What happens for a tensile deformation? To solve this, a slight modification is introduced where the input bar is attached to a loaded chamber which imposes tensile (pulling apart) load on the input bar which translates the same to the test specimen. The figure below shows a schematic of a modified split Hopkinson pressure bar (that results in tensile deformation of the specimen).
The above have been the subject of my work this week and I have been responding to some queries from people along the lines of this reflection. If you want to follow my thoughts in this YouTube video, then do check it out below. Please let me know if you agree with me or there are things I have missed or misrepresented which you would like to highlight.
Thanks for reading the above. We will catch up with more detailed newsletter next week. This is just a quick note to reflect on what I have been doing this week.
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