Meaning
The elastic deformation of a testing machine structure under an applied force alters the apparent displacement of the specimen being compressed or tensioned. Correcting for load frame deflection is necessary to obtain accurate strain values from crosshead position measurements. When high forces are applied, the frame itself stretches or compresses, which registers as specimen strain if left uncorrected.
System stiffness values are used to subtract this error from the raw data.
Error Calibration
Running a test with a high-stiffness dummy specimen allows the software to record the deflection of the frame across the entire force range. In high-capacity testing, load frame deflection can reach several tenths of a millimeter at maximum capacity. This baseline curve is then subtracted from subsequent test runs of real specimens.
Mechanical testing of brittle materials depends on this calibration for accurate modulus calculations.
Structural Stiffness
Machine manufacturers design the columns and crossheads with high-rigidity steel to minimize structural bending. Double-column configurations exhibit less load frame deflection than single-column designs. The alignment of the columns also reduces off-axis twisting during compression tests.
Measurement Precision
Using extensometers mounted directly on the specimen bypasses the deflection error completely. This practice is preferred for high-precision strain measurement.
