13.7 FORCE/MOMENT

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These values cannot currently be measured directly, and count on indirect measurements based on deflections or strain.

13.7.1 Strain Gages

These devices are attached to surfaces. As the surfaces experience stress/strain the devices are stretched and the resistance changes.

The basic theory is based on a stretched wire.

Changes in strain gauge values are typically small (large values would require strains that would cause the gauges to plastically deform). As a result the resistance values are also small, so we can use resistor bridges (eg whetstone bridge) to amplify the effect. In this circuit the variable resistor R2 should be turned until the circuit is balanced for no strain.

If the strain gauge is placed in the direction of the strain it will read the full strain. If the gauge is perpendicular, the reading will be zero.

uniaxial - the direction is critical

rosette - two gauges at 90deg. to each other will measure strain components in two direction (and can measure shear).

In some machines (etc.) a strain gauge is often mounted on a narrowed member to measure force. This is typically known as a load cell.

Strain gauges are normally made on thin films that are attached (mounted) to surfaces through a process that involves surface preparation and attachment with adhesives.

13.7.2 Piezoelectric

These are ceramic and crystal materials that will generate a small amount of charge when deformed (the capacitance also changes).

If the deformation is linear,