As a key equipment for evaluating the mechanical properties of metal materials, the metal tensile testing machine's test results are influenced by multidimensional factors. Analyze the sources of uncertainty and control points of experimental data from the eight core elements as follows:
1、 Specimen preparation standardization
1. Geometric dimension deviation: If the cross-sectional area error of the sample is greater than 1%, it will directly lead to stress calculation deviation. It is necessary to strictly follow the GB/T 6397 standard for processing, and the length of the parallel section should be ≥ 5 times the diameter.
2. Surface treatment quality: Turning marks, oxide scales, or machining lines can cause stress concentration. It is recommended to use grinding and polishing technology to treat the gauge length section, and control the Ra value below 1.6 μ m.
3. Distribution of metallurgical defects: The differences in segregation zones and inclusion positions caused by different sampling locations of ingots may result in a performance dispersion of more than 15% for samples from the same batch.
2、 Reliability of clamping system
1. Fixture adaptability: Wedge shaped fixtures are suitable for cylindrical specimens, while hydraulic flat push fixtures are more suitable for flat plates. Incorrect selection can lead to slippage or fracture displacement.
2. Neutral adjustment: When the coaxiality deviation of the upper and lower fixture axes is greater than 0.5mm, additional bending moments will be generated, resulting in a 5% -8% decrease in the measured tensile strength.
3. Clamping force control: Insufficient pre tension can cause slippage, while excessive compression can lead to premature cracking of the surface hardening layer. It is recommended to monitor the optimal clamping state through acoustic emission.
3、 Experimental parameter setting
1. Loading rate effect: For every order of magnitude increase in strain rate, the yield strength increases by about 10% -15%, and a constant displacement rate (such as 1-5 mm/min) needs to be set according to the standard.
2. Sampling frequency matching: Dynamic testing should ensure that the number of sampling points per second is ≥ 20 times, and lagging behind the deformation rate will result in the loss of key inflection point data.
3. Termination condition selection: The fracture criterion should adopt a dual trigger of "sudden drop in force value+sudden increase in displacement" to avoid misjudging local tearing as the final failure.
4、 Equipment characteristics
1. Sensor linearity: Strain gauge load sensors have excellent linearity in the range of 20% -80%. If it exceeds the range, the range needs to be replaced.
2. Insufficient rigidity of the crossbeam: When the span is greater than 1 meter and the self weight sag is greater than 0.1mm/m, deflection compensation correction is required.
3. Electrical noise interference: The electromagnetic pulse generated by the servo motor commutator may cause an instantaneous jump in A/D conversion, and a filtering module needs to be installed.
5、 Human error in operation
1. Calibration operation specifications: Equipment that has not undergone regular weight calibration can have an indication error of ± 2% after long-term use.
2. Observation angle of field of view: When manually judging the yielding platform, a 30 ° angle between the line of sight and the normal of the dial can reduce parallax.
3. Fracture positioning deviation: The minimum cross-sectional area should be taken for post fracture gauge measurement, and the resolution of the measuring tool should reach 0.02mm
6、 Data processing algorithm
1. Calculation of elastic modulus: The secant method (corresponding to the displacement of F_max/2) is more stable than the tangent method and is suitable for automated processing.
2. Correction of elongation after fracture: The elastic recovery amount needs to be deducted, and the true plastic strain=total elongation - elastic rebound amount.
3. Abnormal data processing: Outlier determination follows the Grubbs criterion and excludes data points with a probability of P<0.05.
8、 Material anisotropy
1. Grain orientation effect: The longitudinal yield strength of rolled steel plates can be 20% higher than the transverse direction, and the sampling direction should be clearly marked.
2. Texture influence: After annealing, cold drawn steel wire exhibits silk texture, resulting in a 10% difference in radial and axial performance.
By establishing standardized operating procedures (SOP), implementing regular equipment measurement traceability, and using automated extensometers instead of visual observation, the comprehensive uncertainty of test results can be controlled within ± 1%. The key is to identify the dominant error source, take targeted preventive measures, and ensure that data reproducibility meets the requirements of ISO 7500-1 standard.