Dynamic mechanical properties and strain rate effect of bolts

In order to reveal the dynamic mechanical characteristics and strain rate effect of bolt support materials under impact load, so as to provide reference for the support design of different magnitude rock burst, and finally achieve the purpose of classified prevention and control of rock burst.The static and dynamic mechanical properties of different impact toughness bolts are tested in laboratory, the impact resistance of different toughness bolts is analyzed through meso, micro-scale, and the effect of strain rate on the dynamic mechanical response of bolts is also analyzed.The results show that the impact absorbing energy of CRM700 bolt is 1.30 times and 1.14 times of HRB500 bolt and CRM600 bolt respectively, the peak load is 1.18 times and 1.07 times of the two bolts, and the instantaneous deformation is 1.33 times and 0.91 times of the two bolts.The fracture morphology and metallographic structure of CRM700 bolt and HRB500 bolt are significantly different.The fracture source area of HRB500 bolt is dimple shape, the middle expansion area is quasi cleavage+a small amount of dimple, the final fracture area is dimple shape, and the metallographic structure is ferrite+pearlite, while the fracture area of CRM700 bolt is dimple shape, and the metallographic structure is tempered sorbite+bainite+ferrite.At the same time, the strain rate has a significant effect on the yield strength, tensile strength and strain of HRB500 bolt, but has little effect on CRM600 bolt, and has the least effect on CRM700 bolt.Compared with the dynamic mechanical properties of the three kinds of bolts, the load curve and impact energy curve of CRM700 bolt are relatively smooth, and the material shows the characteristics of slow fracture after impact load, which has a good adaptability to impact load.The CRM700 bolt strength has a poor sensitivity to strain rate, while its strain increases significantly with the increase of strain rate, which is well adapted to the impact load at a high strain rate.Under the impact load at a high strain rate, the energy absorption capacity can be improved by increasing the instantaneous deformation.The grain size of HRB500 bolt is grade 6, while that of CRM700 bolt is grade 10.The higher the grain grade, the smaller the grain size and the better the strengthening effect.At the same time, the decrease of grain size results in the increase of crystal interface, which effectively hinders the crack growth, increases the crystal interface area and decreases the inclusion concentration on the grain boundary.Thus it avoids the intergranular fracture and greatly improves the impact resistance of bolt.The smaller the grain size is, the lower the gradient of dislocation accumulation around the second phase, and the weaker the sensitivity of strain rate to the mechanical properties of the bolt.In a word, no matter from the characteristics of impact load curve and impact energy curve, or in the sensitivity of strain rate, CRM700 anchor rod shows a good adaptability to impact load, which can improve energy absorption force by increasing instantaneous deformation under high strain rate impact load. © 2020, Editorial Office of Journal of China Coal Society. All right reserved.