The accuracy of the universal testing machine is the core to ensure the reliability of material mechanical performance testing data. Its influencing factors can be divided into three categories: equipment hardware factors, test operation and method factors, and environmental and sample factors, as follows:
Hardware factors of the device itself
Sensor accuracy and calibration status: Force sensors, displacement sensors, and extensometers are the core components of accuracy. The accuracy level of the force sensor (0.3/0.5) directly determines the measurement error of the force value; The resolution and linearity of displacement sensors (grating rulers are superior to photoelectric encoders) affect the accuracy of displacement/deformation data; The clamping coaxiality and elastic claw state of the extensometer will directly interfere with the test results of high-precision indicators such as elastic modulus and yield strain. Sensors that are not regularly calibrated (recommended to be calibrated every six months to one year) or used beyond their expiration date can accumulate system errors and cause data to deviate from true values.
The servo motor and ball screw transmission chain of the electronic universal testing machine for driving and transmission system stability. If the screw lacks lubricating grease, is severely worn, or the belt is loose, it will cause the crossbeam to move and lag, and the speed fluctuation will affect the loading stability; The viscosity changes of hydraulic oil, wear of hydraulic valves, and leakage of oil cylinders in hydraulic universal testing machines can cause pressure transmission lag and large fluctuations in force values.
The compatibility between fixtures and tooling: Wear of the fixture clamping surface and aging of the anti slip pad can cause the sample to slip and the force curve to undergo sudden changes; The coaxiality deviation of the fixture can cause lateral force on the specimen, leading to premature fracture or incorrect determination of the yield point; The insufficient flatness of the compression/bending test pressure plate and support can lead to uneven stress on the specimen and high variability of the test data.
Experimental operation and method factors
Incorrect normative stretching rate settings for parameter settings (such as hard plastics mistakenly set to 50mm/min) can directly lead to high tensile strength; Improper range selection (using a 50kN sensor to measure plastic film below 5kN) can result in "weighing small objects with a large scale" and insufficient force resolution; Unreasonable shutdown conditions (such as not selecting "fracture shutdown") may result in equipment overload.
The accuracy of clamping and zeroing: If the sample clamping is skewed or the clamping length does not meet the standard, additional stress will be introduced; Failure to zero the force value, displacement, and extensometer before the experiment may result in initial deviation; Failure to accurately clamp the extensometer in the gauge length section of the specimen can result in distortion of deformation measurement data.
Early manual intervention (such as removing the extensometer in advance) and frequent emergency stop operations during standardized testing can affect data integrity; Failure to clean the fixture debris in a timely manner after sample fracture will affect the accuracy of the next clamping.
Environmental and Sample Factors
Laboratory environmental conditions with temperature deviation from the standard value (23 ℃± 2 ℃) can cause thermal drift of sensors and alter the mechanical properties of materials such as plastics and rubber; Excessive humidity (>60%) will accelerate the oxidation of sensor circuits and rusting of fixtures; External vibrations and strong electromagnetic fields can interfere with sensor signals, causing data jumps.
The preparation and state adjustment of the sample size deviation (thickness and width exceeding tolerances), as well as the presence of gaps or bubbles at the edges, can lead to stress concentration and high variability of test data; If the sample is not adjusted according to the standard (such as the plastic not being placed for more than 16 hours to eliminate internal stress), the test results will not reflect the true performance of the material.