Northwestern University Feinberg School of Medicine

Northwestern University Prosthetics-Orthotics Center

Mechanical Testing Lab

Mechanical Testing LabThe Mechanical Testing Lab contains equipment that tests the mechanical properties of components and materials used in our research projects. This lab contains an Instron 8800 Materials Testing Machine and a fatigue testing apparatus for prosthetic feet. The three main pieces of equipment are described below. 

Material Testing Machine

The mechanical properties of materials and components are determined using an Instron 8800 (Instron Corp, Norwood, MA) Materials Testing Machine. This system uses hydraulic actuators to apply loads in cyclical or linear patterns to the specimens. These applied loads, which are controlled using feedback loops, are measured using either a 10,000-pound, 1,000-pound, or 110-pound load transducer. A data acquisition program connected to the Instron system acquires the loading response information and creates stress-strain diagrams for a material. Using custom adaptors and fixtures, this system can also perform all of the mechanical tests detailed in the American Orthotic and Prosthetic Association Prosthetic Foot Project Report (http://www.aopanet.org/wp-content/uploads/2013/12/Prosthetic_Foot_Project.pdf). 

 

Fatigue Testing Apparatus

Knowledge of the functional lifespan of a prosthetic component is important because unexpected failure could lead to injury for an individual who wears the device. To address these concerns, we built a fatigue-testing machine to test components developed in the lab. The machine follows the ISO (International Organization for Standardization) 10328 specifications for the testing of lower limb prostheses, but with slight modifications to reduce costs. The fatigue-testing machine uses two pneumatic cylinders and operates without a computer; instead, it uses a logic loop that is implemented through the wiring to cycle indefinitely. A counter tallies the total number of completed cycles.
The loading of the prosthetic foot simulates walking because the heel and the forefoot are loaded sequentially. According to ISO specifications, the cylinder at the heel is positioned at an angle of 20 degrees, and the forefoot cylinder is set at 15 degrees; both angles are set relative to the pylon axis. Each pneumatic cylinder exerts a force of approximately 1250 N (i.e., level A80 in ISO 10328).