Fatigue Behavior Of High Strength Steel Research Articles

Ultra low cycle fatigue behavior of Q690 high-strength steel welded T-stub joints: Experiments and fracture prediction analysis

The paper presented an experimental and numerical investigation of the ultra low cycle fatigue behavior of high-strength steel welded T-stub joints. The normal and funnel T-stub joints in Q690 high-strength steel were tested under monotonic tensile loading and ultra low cyclic loading. The cyclic response of normal T-stub joints varying from bolt diameter, flange size and weld form were studied. It is found that fracture mode of the T-stub joints is basically the same under tensile loading and cyclic loading, i.e., cracking behavior occurs at the weld toe of the flange and tensile-shear failure is observed. The different configuration parameters of normal T-stub joints in the study show no significant influence on the fatigue life of the joints. Comparing the ultra low cycle fatigue results between normal and funnel T-stub joints indicates that the funnel T-stub joints have better energy dissipation capacity. In addition, the ultra low cycle fatigue performance of Q690 steel and ER80-G weld metal under various stress states were analyzed and discussed. In order to numerically generate the overall fracture behavior of the T-stub joins, the Lode parameter enhanced cyclic void growth model was adopted in the numerical simulation and the model parameters were calibrated according to the test results of Q690 steel and weld metal. Comparison with the experimental monotonic and cyclic tests of welded T-stub joints shows that the Lode parameter enhanced cyclic void growth model can provide accurate predictions of the ultra low cycle fatigue of the joints.

Corrosion and Fatigue Behavior of High-Strength Steel Treated with a Zn-Alloy Thermo-diffusion Coating

High and low cycle fatigue tests were conducted on high-strength steel using four-point bending. The materials tested were ASTM A723 steel in the as-machined condition, grit-blasted condition, MIL-DTL-16232 heavy manganese phosphate-coated condition, and ASTM A1059 Zn-alloy thermo-diffusion coated (Zn-TDC). The ASTM A723 steel base material exhibits a yield strength of ~1000 MPa. The effects of the surface treatments versus uncoated steel were examined. The fatigue life of the Zn-TDC specimens was generally reduced on as-coated specimens versus uncoated or phosphate-coated specimens. Several mechanisms are examined including the role of compressive residual stress relief with the Zn-TDC process as well as fatigue crack initiation from the hardened Zn-Fe alloy surface layer produced in the gas-metal reaction. Additionally, the effects of corrosion pitting on the fatigue life of coated specimens are explored as the Zn-TDC specimens exhibit significantly improved corrosion resistance over phosphate-coated and oiled specimens.

Effect of toughness and inclusion on fatigue strength of high strength steel.

Rotating bending fatigue tests and the measurement of inclusion size distribution using the quantitative metallography technique were carried out on a quenched and tempered SAE9254 steel, and discussions were made on the relationship among the fatigue limit, inclusion size and toughness. Results showed that the high fatigue limit over 1000 MPa is easily obtained when the induction-hardened SAE9254 steel is tempered at the temperature just below the temper brittleness zone, indicating that such a condition of tempering gives the combination of hardness and toughness preferable for high fatigue resistance. Existence of an inclusion having maximum size (∼30 μm in dia. for present material) below the specimen surface is a primary cause for the occurrence of subsurface fracture associated with a fish eye on the fatigue fracture surface. The role of the inclusion with extraordinarily large size in the fatigue behavior of high-strength steel has also been discussed on the basis of the results of the fatigue test of the specimen having a surface micropit artificially introduced to simulate a surface inclusion.