1. When describing the tensile test of Ф22 steel bar, please choose the appropriate range according to the testing machine used in your laboratory. How should load rate be controlled during stretching?
(1) Select the range of 0~300kN (testing machine with a maximum capacity of 300kN or 600kN) or the range of 0~500kN (testing machine with a maximum capacity of 1000kN). to set the force value to be measured between 20% and 80% of the selected range. As in this example, the maximum tensile load of Ф22 steel bar is estimated to be 200~220kN, and the range of 0~300kN or 0~500kN can be selected, and the yield point load is 120~150kN, which is not the case less than 20% of the selected area. Therefore, the range selected is reasonable.
(2) The cross-sectional area of Ф22 is 380mm2, and the loading speed before yielding during stretchingns should be regulated to 2.3~23kN/s (the strain rate is 6-60MPa/s, ie between 0.006*380~0.06*380=2.3~23kN/s). The load rate of the steel sample before yielding should be controlled to 6-60MPa/s, and the strain rate during yielding should be between 0.00025-0.0025/s (assuming that the distance between the upper and lower chuck of the testing machine before stretching). 150 mm, should the separation speed be between 0.035 and 0.35 mm/s), the speed of the testing machine is no longer adjusted during yielding. After yielding, the strain rate should not exceed 0.008/s (assuming the distance between the upper and lower chucks of the testing machine before straining is 150mm, the separation speed should be less than 0.008*150=1.2mm/s be).
(3) The strain rate of the cold-rolled twisted bar should be nicht be greater than 2 kN/min.
2. Briefly describe the two main problems and points to be considered in steel bar tensile testing?
Answer: Tensile strength During testing To obtain accurate results,
mainly consider the two aspects of the specimen and the instrument :
A: Aspect of the sample:
(1) The number of the sample should be clear and unambiguous and use the steel number print mark;
(2) Check carefully that the sample meets the requirements, including: the smoothness of the sample surface, whether there are transverse or longitudinal cracks in the sample itself or through manufacture and processing, and accurately measure the size of the sample;
(3) Set the gauge length according to the needs, and scrape lightly to avoid new artificial surface gaps;
(4) When clamping the sample, it should be carefully centered to avoid additional stress.
B: In BeTension on the test equipment:
(1) Adjust the tonnage of the test machine according to the size of the specimen and the expected tensile strength;
(2) Set the pointer to a zero before starting the machine;
(3) Use the extensometer to plot the load-strain curve;
( 4) Stop the machine to cool down during the test.