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 MTS 810 material testing machine a fatigue testing apparatus for prosthetic feet, and a prosthetic foot loading apparatus. Also, we have 8 miniature Kulite pressure transducers (Model XTM-190), 8 Measurements Group signal conditioning amplifiers, and miscellaneous mechanical equipment. Below is a description of the three main pieces of equipment.

Material Testing Machine

The mechanical properties of materials and components are determined using an MTS 810 (MTS Systems Corporation) material testing system. 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, 1000 pound, or 110 pound load transducer. A data acquisition program connected to the MTS acquires the loading response information and creates stress-strain diagrams for the material being tested. During loading, specimen slippage is prevented through the use of either Advantage Screw Action Grips or custom designed fixtures.

Fatigue Testing Apparatus

Knowledge of the functional lifespan of a prosthetic component is important because unexpected failure could lead to injury for the wearer of the device.  To address these concerns, a fatigue testing machine was built 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 slight modifications were made to keep costs down. The fatigue machine, which uses two pneumatic cylinders, is designed to operate 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 cycles completed.

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).

Prosthetic Foot Loading Apparatus

The Prosthetic Foot Loading Apparatus (PFLA) is a class two force-multiplying lever system used to load prosthetic feet. The device’s design is such that a force applied at the end of the beam is roughly doubled at the contact point with the prosthetic foot. The prosthetic foot is positioned in neutral alignment, meaning that the top attachment surface of the foot is always orthogonal to the longitudinal axis of the pylon.

The pylon connecting the prosthetic foot to the device is instrumented with a set of strain gauges. An aluminum plate attached to the beam is also instrumented with a pair of strain gauges. The PFLA strain gauges are calibrated using a rigid block in the place of the foot. Known weights are incrementally set on the weight trays, which are connected to the end of the loading beam. The weight of the beam, weight trays, and ball bearing interface plate are included in the calibration. Using the geometries of the PFLA and the rigid block, the known moments being applied at the sites of the strain gauges are calibrated to the voltage outputs of the strain gauges. The PFLA potentiometer for deflection measurements is calibrated using a dial indicator. The output from both sets of strain gauges and the potentiometer are linear. The foot interacts with the aluminum plate through a ball-bearing interface plate, which acts to eliminate shear forces on the foot. The shear components are removed to insure a controlled loading situation and to simplify the calculation of the force applied to the foot. A spring-loaded potentiometer is mounted on the beam and is used to determine beam movement. The loading beam deflects a small amount under the maximum loads used in our studies in comparison to the deflections of the prosthetic feet. Using the measurements from the potentiometer and from the two sets of strain gauges, the force applied to the foot, the center of pressure along the foot, and the foot deflection can be calculated .