Determining the load history of a structural member is important for predicting fatigue life and preventing mechanical failures. It is also important for simulating service loads, evaluating stress, and accelerating durability testing.

The load history of a structural member is best acquired through direct measurement. Application of strain gage technology enables measurement of service loads for a wide variety of situations in the field and in the test lab. Depending on the specific application, there are several approaches for measuring load history with strain gages.

Uni-axial Strain Measurement

Measuring uni-axial strain is acceptable when a uni-axial strain (stress) state exists and the load path is well known. A significant weakness of uni-axial strain measurement is that significant measurement error can result from poor strain gage placement and orientation.

Principal Strain Measurement

Measuring principal strain is the preferred method when direction and or orientation of the principal strain is unknown. Principal strain measurement is also used to detect bi-axial strain states. Principle strain measurements requires the use of three uni-axial stain gages with fixed relative orientations. When strain measurements occur within the linear elastic region of a material, principal stresses can be calculated.

Service Load Measurement

Service loads include axial force, bending moments, torque, and shear force. Such loads are often used when predicting and optimizing performance through analysis or FEA. Service loads are also very important for developing accelerated test methods that correlate to intended use.  Services loads are measured using strain gages in various configurations of the Wheatstone bridge to isolate specific forces of interest.

Contact engineering helper to assist with your next strain gage measurements.