Establish Target Specifications
Step 1. Prepare the List of Metrics
- Sample refers to a cross shape sample made of UltraFlex polymer material. Our spec is concerned with the over all footprint of the sample. This footprint determines how large the central region of the Biaxial adapter would need to be.
Step 2. Collect Competitive Benchmarking Information
There is no real commercial application for membrane testing such as organ tissue. There is a lot of literature trying to find an answer to material properties of lung material but there aren't any conclusive studies.
- Benchmark 1 (Uniaxial Load Test Stand)
- This is a simple loading machine. It applies either a compressive load or a tensile load to a specimen. We see these used in the mechanics lab using stainless steel test specimen. Each specimen is threaded and set up into the system. The system then pulls/pushes in one direction usually until failure. We read the stress being applied to the system via an indicator on the test stand and we measure the amount of elongation on the test specimen using two predetermined points on either end of the midpoint of the specimen.
- This would be the cheapest application of a membrane test stand. Mounting the test specimen would be questionable. You can not thread the end of lung tissue/ultraflex and molding the material into a normal tensile specimen is out of the question. A thin hour glass shaped specimen could be created and clamped at the ends but the side edge effects would create question in the results. Could use the current membrane indenter to determine whether this is an applicable solution or not.
- Benchmark 2 (Biaxial Load Test Stand)
- This would the equivalent of mounting two tensile test stands in one and pulling along two different axes. Mounting would be different than that of a uniaxial stand. No longer would you have two threaded ends on the test specimen but rather you would have a square shaped piece that would be pulled from either end and then elongation would be measured in either an area increase or the elongation of either side of the piece. Each side should deform equally assuming an equibiaxial loader.
- Edge effects once again come into play and we have yet to find a viable solution. Some of the literature speaks about using sutures along the edge of the test sample and pulling with equally spaced wiring. This could be a potential solution. Provided results in the literature need to be verified with further investigation.
- Benchmark 3 (Volumetric Test Stand)
- This system would inflate a test sample using a vacuum induced expansion, similar to how the diaphragm causes the expansion of the lungs. Specimen would be balloon shaped and could hopefully, be easily scalable to a smaller shape such as a rat lung casting. A system currently exists in the laser lab that could be a stepping stone to this idea.
- Each test stand can accomplish all of the customer needs except for one each
- With exception to edge effects, Biaxial and Uniaxial can accomplish the customer needs. Volumetric testing would avoid any sort of edge effects but the volumetric strain measurements necessary for ANSYS to correlate curve fitting to are unknown and no where to be found in the help files. Further investigation is ongoing.
The following tables reflect the accuracy of measurement and meeting some of the metrics of the customer. However, seeing as it was concluded that all of the load systems would work in most applications, concentration was primarily on mounting and measurement materials and applications in the breakdown of benchmarking. This will give a better understanding of what will be need to be ordered in the future and what can be expected as far as accuracy and applicability.
Step 3. Set Ideal and Marginally Acceptable Target Values
Given the customer needs, awareness of the marketplace, and resource limitations of the current project, assign preliminary engineering specifications on each of the metrics. In addition to setting the nominal or target value or each specification, provide guidance to the team on the ideal value or direction that the team should strive for, once the nominal target values have been realized.