Drill rods are an integral part of the drilling process as they act to transmit the impact force and guide the drill bit. However, during this process, the drill rod is subjected to several stresses that have a critical effect on its structure and performance.
In drilling, drill rod needs to bear three kinds of stress: axial anxiety, torsional stress, and bending stress.
Axial anxiety
In drilling, drill rod mainly bears axial tensile and compressive stress under the impact of rock drill piston and rock action. It is reciprocally transmitted between the piston and the rock in the form of stress waves so that drill rod is in a state of tension and compression fatigue.
Torsional stress
When the piston retreats, drill rod is driven by the rock drill sleeve to twist the angle. Drill rod bears torsional stress due to the frictional resistance of rock powder and hole wall.
Bending stress
Drill rod is elongated. The slenderness ratio of the B22mm hollow hexagonal conical rod is 1:100, and the slenderness ratio of threaded rod is larger. Therefore, additional bending stress is inevitably generated by natural vibration during the axial advance of multiple percussion drilling.
In the past, it was considered that the bending stress was the core stress on the drill rod in drilling, and the bending fatigue caused by the bending stress was the main factor for the fracture of drill rod. Although the brazed steel designed with this idea improves the bending fatigue strength of drill rod, it ignores the cyclic toughness, so drill rod cannot have a high drilling life.
The maximum stress of drill rod at each moment should be the superposition of axial tensile and compressive stress and bending stress. Hollow steel is the main material for making drill rods. It is of great significance for the design of hollow steel to know what kind of stress the drill rod bears in the drilling from the aspect of the performance of rock impact drilling to hollow steel. They all require steel with high purity, low-notch sensitivity, good microscopic plasticity, and sufficient fatigue resistance. But different stress states should have different needs. If the stress on the drill rod is mainly axial compressive stress in drilling, the design of the brazed steel should focus on high-impact fatigue resistance. If the stress on the drill rod is mainly bending stress in drilling, the surface fatigue strength of the drill rod should be improved as much as possible based on ensuring that the matrix steel of drill rod has a definite fatigue strength.
In summary, the stresses endured by the drill rod during the drilling process may cause damage such as wear, cracking, and deformation. It is of great significance to understand the mechanism and influencing factors of these stresses to optimize the material selection, structural design, and use method of drill rods. At the same time, through stress numerical simulation and analysis, the service life of drill rods can be predicted and evaluated so that maintenance and replacement can be carried out in advance to improve drilling efficiency. In future research, drill rod manufacturers can continue to explore and utilize advanced material technologies and numerical simulation methods to optimize the design and use of drill rods to better cope with various complex geological conditions and working environments. In addition, studying the effects of different materials and heat treatment processes on the stress resistance of the drill rod is also an important research direction, which is promising to provide new solutions for improving the durability and efficiency of the drill rod.
In short, the stress the drill rod bears during the drilling process has an important impact on its service life and efficiency. Understanding these stress types and mechanisms and how to deal with them through optimized design and management is the key to improving drilling efficiency and ensuring work safety.
