Non-axial loading is one of the most common causes of incorrect tensile measurements. Even very small off-center loading when using a load sensor or force gage can result in measurement errors of up to 0.5%. It is therefore important to ensure that the alignment of the testing string, i.e., the load cell, top test fixture, sample and bottom test fixture are perpendicular to one another.
Using a properly sized force gage or load cell sensor based on the expected load measurement is important to achieving accurate and repeatable results. A general rule is to use a sensor that is between 20% and 80% of the anticipated load measurement. This will avoid or minimize error results at the low end due to mechanical noise and will help prevent overloading conditions at the upper end of the measurement range. Since most sensors are calibrated and have their accuracy specification based on full scale, the closer you are to zero, the more influence the accepted error has on your measurement.
Having an incorrect test fixture is another common cause to inaccurate tensile measurements. A fixture that is too large for the sample or that applies too much gripping force to the sample during tensile movement can cause the sample to fracture outside the specified gage length area. The test fixture should be sized to the sample’s expected load characteristics. Wedge-action test fixtures work well on ductile samples but tend to be less reliable on brittle materials, since they apply load onto the sample as axial loading increases. Brittle materials tend to test more consistently when pneumatically-operated test fixtures are used that regulate the gripping force onto the sample.
Improper sample preparation can result in inconsistent and incorrect characterization. When testing to a specific international standard, the sample should be prepared to the prescribed dimensions. Force measurement applications will typically use the component in its finished state. Material testing, however, uses specially prepared specimens in various forms and shapes. Their cross-sections may be round, rectangular or square and they have a known standard gage length.
Testing at too fast or too slow a velocity is another common reason why tensile measurement may not be optimum. Where available, testing should be done in accordance to a recognized and accepted testing standard by ASTM, ISO, DIN or others. Test speed is clearly specified in these standards and will ensure a proper measurement is taken.
Tensile results may be significantly affected by temperature. Elasticity of a sample can decrease significantly as temperature increases.