Fracture Failure Analysis Of A Compressor Bolt Connection

Bolt fracture failure is a common problem in engineering under fatigue loading. In this paper, a typical bolt fracture failure problem was chosen as the research object, the loading conditions, material properties and fracture bolt connection failure mode, the combination of macro and micro analysis and theoretical research were considered to explore the intrinsic factors of the bolt fracture failure. The corresponding improvement measures were put forward to prevent the failure behavior of similar bolt connection. The main contents and conclusions were listed as follows:The cracked bolt material is 35CrMo. The alloy element content, the yield strength and section shrinkage rate, elongation and impact toughness meet the requirements, while the tensile strength and Rockwell hardness HRC are less than those of defined in the standard. Moreover, the bolt surface hardness is significantly higher than that of the core hardness due to the effect of surface carbon. The surface of the bolt is hard and brittle, plastic was significantly lower than the internal, which may reduce the hot and cold fatigue resistance and the bolt fatigue strength.The microstructure of the cracked bolt is typical tempered sorbite, and contains a small amount of ferrite, which meets the characteristics of the material microstructure. The tensile fracture surface has obvious radial crack, and the fracture surface is relatively plane. Moreover, some micro-hole can be observed on the fracture surface. This indicates that tensile fracture has the obvious characteristics of brittle cracking. Dimples and secondary crack can be found on the impact specimen fracture surface. This indicates that the impact fracture has both characteristics of ductile fracture and brittle cracking. The strip inclusions caused by the smelting process are the main factors for fracture porosity, and the main inclusion is CaO-SiO2-Al2O3-MgO system, followed by MnS inclusion.Considering the connection of nonlinear geometric structure and nonlinear contact condition of the screw, the stress distribution of bolt connection structure subjected to the pre-tightening force and operating pressure based on finite element method was investigated. Results showed the stress on the stud is most obvious, the maximum stress take place at the first thread of the stud contacted with the nut near the flange. This indicates the first thread engagement area of the stud and nut is the most dangerous region of the bolt connection.The inclusions caused by the smelting process decreases the local material properties of the first thread engagement area of the stud and nut. Moreover, the finite element analysis showed that this local area is the maximum stress region of threaded connection. Bites and wear under fatigue loadings is easy to occur, which is the fatigue crack source. The initial fatigue crack source can be further expanded under the fatigue loadings. Furthermore, the formation of internal strip inclusions can be regarded as the internal micro cracks. Extension and aggregation also occurs under fatigue conditions. Finally, brittle cracking occurs when the actual bearing area is not enough to resist the external loading.