Biomechanics Part 5 Applying loads · Materials testing machines · Sleds · Pendulums/impactors/drop towers · Other Materials testing machines · Also called universal testing machines (UTMs) · Used to test medical devices and biological tissues . Apply loads in compression, tension, or torsion · Rate of loading can be quasi-static or dynamic . Load is measured with load cell · Displacement is measured with a sensor . Many different manufacturers Instron or MTS are common in biomechanics labs Sleds · Commonly used to apply acceleration to a segment, volunteer, cadaver, or anthropometric test device (ATD) · Simulates dynamic loading typical in a car crash or other high-rate loading Impactors, pendulums and drop towers · Often have a load cell used to measure load applied · Velocity can be measured by video analysis or photo sensors Load Cells . Load cell: A transducer that converts force into an electrical signal . The electrical signal is proportional to the load applied · Common types are strain gauge, piezoelectric, pneumatic, or hydraulic · Can measure force and moments about x-, y- and z-axis
Strain gauge (SG) load cells · Strain gauges (SGs) are sensors that change their resistance based on change in shape . For SG load cells, SGs are attached to a metal piece that behaves elastically under loading · Inexpensive · Can be damaged by overload Piezoelectric load cells · A piezoelectric material (such as quartz) produces an electric charge in response to mechanical deformation · A piezoelectric load cell is calibrated to relate the electrical signal to the applied load · Cannot be used for static loading · Charge amplifiers are expensive Force Plates · Typically constructed with pylons at each corner containing either strain gauge or piezoelectric sensors · Measure ground reactions forces (GRFs) · Forces (normal, shear) · Moments · Types: · Strain gauge platforms · Strain gauges attached on pylons on corners · Gauges must be powered and amplified · AMTI and Bertec · Piezoelectric platforms : . No power supply to load cells required · Kistler Considerations with force plates
· Stepping up onto the force platform can lead to unnatural gait . Aiming to try to get the foot to land on the force plate can also lead to unnatural gait. Why do we want to measure forces in injury biomechanics · Tolerances How much force can a person/object withstand? · Stiffnesses Designing accurate human representations Computational models Anthropometric test devices · Effectiveness of safety devices What reduction in force does the safety device provide?