In Bradley v. Ameristep, Inc., 800 F.3d 205 (6th Cir. 2015) (No. 14-6087), plaintiff sued defendants over the failure of polypropylene polymer ratchet straps used to secure a tree stand for hunting while up in the trees.  The ratchet straps broke causing plaintiff to fall and sustain injuries.  The district court had excluded plaintiff’s proffered expert on the grounds that his expertise focused on metallurgy, which was not relevant analyzing the failure of polypropylene straps, and dismissed the case because it held that Tennessee law required expert testimony to establish causation for the product failure.  The Sixth Circuit reversed.  First, the court held that, while plaintiff’s expert had background training in metallurgy, he also had lengthy experience more generally in materials-failure analysis on all types of materials, including polymer materials.  Thus, the Sixth Circuit held that district court had erred in excluding his opinions.  Further, the Sixth Circuit concluded that the Tennessee Supreme Court had adopted the consumer expectation test for judging whether products are defective, and rejected defendant’s argument that polypropylene chemical composition and breakdown should not be subject to the consumer expectation test because the subject matter is too complex.  Instead, the court held that people have a great deal of experience with polymer materials in a large variety of products in the market today, and that while they may not understand the chemistry of these products, they have developed reasonable expectations as to the performance of such products based on years of experience with it.