July 31, 2012, Accutek Engineering Team
By: Andrew Smith, Project Engineer
Question: What’s more important than starting tests quickly?
Answer: Starting tests correctly. And, having the proper fixturing is essential to proper test setup.
Fixture design begins with a 3D model of the specimen. We import the design into CAD, and design a custom form-fitting fixture around it. Using CAM software, we convert the fixture design into a custom machining program for our CNC equipment in our in-house machine shop. The result: a custom fixture that holds and loads your test parts properly.
Different tests have different setup requirements. Many standards for medical implant testing, such as ASTM F2077, F2582, and F2777, require or recommend testing in an in-vitro environment. Saline solution at elevated temperatures can do wonders to corrode stuff. The last thing you want is to corrode your test fixtures in the middle of a 10,000,000 cycle test. To prevent this, Accutek designs and manufactures in-vitro test fixtures out of a variety of plastics and 300-series stainless steels. We recommend either Type 316 Stainless Steel, or Type 304 if 316 is not available.
In addition to preventing reactions between fixtures and the test environment, it’s important to hold the test specimen correctly. Poor fixture design leads to improper loading and misalignment, both of which will impact test results. We have the equipment and the expertise to create custom form-fitting fixturing.
For example, say you want to test a tibial tray according to ASTM F1800, the specification for cantilever compression bending. ASTM F1800 allows for half of the tibial implant to be supported while a dynamic load is applied to the free end. You have two fixture options: a clamping fixture or a cemented fixture.
If you plan to test several implants at the same time, we recommend cemented fixtures. We can simultaneously pour and set 5+ specimens in test fixtures using either PMMA bone cement or a high strength epoxy. We can then begin testing all specimens in parallel.
If you want to test one or two specimens at a time, but aren’t yet sure how many specimens you want to test in total, we recommend clamping fixtures. We can make set of permanent, reusable test fixtures that mate with the test article’s geometry. You can test a few parts at a time. If you need to test more of the same design, we have the fixtures ready and can get your subsequent tests started quickly.