_{1}

^{*}

Through the static tensile test of Q690 high - strength steel, the relevant mechanical parameters are obtained and the maximum fatigue load is determined. The fatigue life is measured by the fatigue test under the load. According to the fatigue cumulative damage method, the number of fatigue pre-damage vibration is designed in proportion. Then the fatigue pre-damage test is carried out on the high-strength steel, the stress-strain curve and the variation of residual mechanical property reduction coefficient with fatigue damage were drawn. The results show that: compared with the undamaged specimens, the yield strength and tensile strength of Q690 steel are less affected by fatigue damage, but the elongation changes more significantly, and the elastic modulus is not significantly affected. Finally, through the change of mechanical properties of Q690 high-strength steel with different fatigue damage, it provides a scientific basis for the performance evaluation of existing Q690 high-strength steel structure after fatigue damage.

High-strength steel refers to structural steel with a yield strength of ≥460 MPa. Compared with ordinary steel, high-strength steel has great differences in material properties and chemical composition [

Up to now, scholars have carried out a series of tests on the mechanical properties of steel after fatigue damage. Zhang et al. [

Therefore, in order to explore the effect of alternating loads on high-strength steel structures, it is necessary to study the mechanical properties of high-strength steels after fatigue damage. However, at present, domestic scholars have more comprehensive research on the mechanical properties of steel after corrosion [

In this test, Q690 high-strength steel was selected. According to the regulations in the literature [

This test was carried out in the Sichuan Key Laboratory of “Engineering Materials and Structural Shock and Vibration”, Southwest University of Science and

Technology. First, in accordance with the specification GB/T 228.1-2010 “Tensile Test of Metal Materials Part 1: Test Method at Room Temperature” [

This test consists of three parts: fatigue test, fatigue pre-damage test and static tensile test. Three groups of Q690 tensile specimens were first stretched to obtain their related mechanical property parameters, as shown in _{y} is the yield strength, it refers to the stress strength corresponding to the yield phenomenon of metal materials. f_{u} is the ultimate strength, after entering the strengthening stage, the steel will have unrecoverable plastic deformation, and with the increase of tension, there will be obvious necking phenomenon, and the final specimen will fracture. At this time, the corresponding stress is the ultimate strength, which is the maximum stress that the steel can bear before fracture. E is the elastic modulus, it represents the ability of metal materials to resist elastic deformation. and δ is the elongation, it refers to the ratio of the extension length of the original gauge distance to the original gauge distance after tensile fracture of steel.

According to the literature [_{max} in this test is 0.66 f_{u}, that is, 584.6 MPa. According to the relevant regulations of the literature [

group | f_{y}/MPa | f_{u}/MPa | E/GPa | δ/% |
---|---|---|---|---|

1 | 798.28 | 886.00 | 208.70 | 22.00 |

2 | 792.54 | 885.42 | 206.41 | 23.00 |

3 | 796.67 | 886.00 | 204.85 | 23.00 |

mean | 795.83 | 885.81 | 206.65 | 22.67 |

S_{max}/MPa | N_{1} | N_{2} | N_{3} | N_{4} | N_{5} | N_{6} | mean |
---|---|---|---|---|---|---|---|

584.6 | 117,536 | 93,216 | 122,481 | 131,844 | 103,578 | 118,532 | 114,531 |

mechanical property parameters of Q690 high-strength steel with fatigue damage was summarized.

Based on the above test methods, static tensile tests were carried out on the three groups of damaged specimens to obtain stress-strain curves, as shown in

Three groups of Q690 high-strength steel specimens with different damage degrees were subjected to static tensile tests to obtain the yield strength, ultimate strength, elastic modulus and elongation, and the average value of the mechanical parameters of each group of specimens, as shown in

working condition | Specimen | f_{y}/MPa | f_{u}/MPa | E/GPa | δ/% |
---|---|---|---|---|---|

15,000 times | S_{1}-1 | 760.78 | 863.83 | 209.03 | 22.00 |

S_{1}-2 | 769.03 | 868.58 | 209.60 | 21.50 | |

S_{1}-3 | 784.20 | 864.67 | 210.00 | 21.50 | |

mean | 771.34 | 865.69 | 209.54 | 21.67 | |

30,000 times | S_{2}-1 | 760.00 | 873.17 | 207.40 | 19.00 |

S_{2}-2 | 727.37 | 832.92 | 203.30 | 20.70 | |

S_{2}-3 | 760.83 | 835.14 | 204.30 | 19.10 | |

mean | 749.40 | 847.08 | 205.00 | 19.58 | |

45,000 times | S_{3}-1 | 724.62 | 815.21 | 204.20 | 15.50 |

S_{3}-2 | 724.79 | 807.92 | 204.80 | 17.50 | |

S_{3}-3 | 733.71 | 805.81 | 203.50 | 13.00 | |

mean | 727.71 | 809.65 | 204.17 | 15.33 |

D | f_{y,D}/MPa | f_{y,D}/f_{y} | f_{u,D}/MPa | f_{u}_{,D}/f_{u} | E_{0}/GPa | E_{D}/E | δ_{D}/% | δ_{D}/δ |
---|---|---|---|---|---|---|---|---|

0 | 795.83 | 1 | 885.81 | 1 | 206.65 | 1 | 22.67 | 1 |

0.13 | 771.34 | 0.969 | 865.69 | 0.977 | 209.54 | 1.014 | 21.67 | 0.956 |

0.26 | 749.4 | 0.942 | 847.08 | 0.956 | 205.00 | 0.992 | 19.58 | 0.864 |

0.39 | 727.71 | 0.914 | 809.65 | 0.914 | 204.17 | 0.987 | 15.33 | 0.676 |

and the reduction coefficient is the ratio of the mechanical parameter values after fatigue damage to the parameter values without damage. It can be seen from

The variation law of the yield strength of Q690 high-strength steel after different fatigue vibration times is shown in _{1}, S_{2} and S_{3} specimens is reduced by 3.08%, 5.83% and 8.56% respectively. It can be seen that the yield strength of the specimens decreased after different fatigue damages, and with the increase of fatigue damage, the yield strength of the specimens showed a decreasing trend, but the decrease was not significant. The reason is that the fatigue damage causes different degrees of micro-cracks inside the steel. When the steel is subjected to tensile stress, the micro-cracks continue to develop. When the micro-cracks develop to be completely unstressed, the uncracked steel around the micro-cracks is borne. The stress and yield strength gradually decrease; The existence of internal micro-cracks reduces the plasticity of the steel so that there is no obvious yield plateau in the stress-strain curve.

As shown in _{1}, S_{2} and S_{3} specimens decreased by 2.27%, 4.37% and 8.60% respectively. It can be seen that the change law is basically similar to the above yield strength change law, and the ultimate strength of the specimen gradually decreases with the increase of fatigue damage, and the reduction range is not significant. The reason for this is the same as the reason for the decrease in yield strength mentioned above, which is affected by internal micro-cracks.

After the specimen is subjected to different fatigue damage, the elastic modulus of S_{1} specimen is only increased by 1.40% compared with the elastic modulus of undamaged steel under static tension, while the elastic moduli of the S_{2} and S_{3} specimens decreased by 0.80% and 0.90% respectively. The elastic modulus of

Q690 steel is less affected by fatigue damage, the change is extremely insignificant, and there is no obvious rule (

The change of elongation of the specimen is shown in _{1}, S_{2} and S_{3} specimens was 4.41%, 13.63% and 32.38% lower than that of non-damaged steel, respectively. It can be seen that the elongation of Q690 steel decreases with the increase of fatigue damage, and the greater the fatigue damage value, the more obvious the decreasing trend.

In the mechanical property analysis in the previous section, it can be clearly observed from the figure that the yield strength, ultimate strength and elongation of S_{1}, S_{2} and S_{3} specimens change linearly with the increase of fatigue damage, because fatigue damage will change the arrangement between grains in the metal, thus reducing various mechanical properties. Therefore, according to its changing trend, using the least squares method, the mathematical expressions of the yield strength, ultimate strength, elastic modulus and elongation reduction coefficient of Q690 high-strength steel after fatigue damage are obtained respectively with fatigue damage, such as Formulas (1), (2), (3) and (4) (0 ≤ D ≤ 0.39), and the comparison between the experimental value of the reduction coefficient and the mathematical model, as shown in

f y , D f y = 0.044 D 2 − 0.237 D + 1.0 ( R 2 = 0.99969 ) (1)

f u , D f u = − 0.281 D 2 − 0.105 D + 0.999 ( R 2 = 0.9934 ) (2)

E D E = 4.021 D 3 − 2.633 D 2 + 0.382 D + 1 ( R 2 = 1 ) (3)

δ D δ = − 2.13 D 2 + 0.012 D + 0.998 ( R 2 = 0.99814 ) (4)

In this paper, through experiments and theoretical analysis, the change law of fatigue properties of Q690 steel after fatigue damage is deeply studied. The main conclusions are as follows:

1) After the fatigue damage of Q690 high-strength steel, micro-cracks are generated inside the material, which reduces the plasticity of the material, and its stress-strain curve has no obvious yield plateau.

2) The yield strength and ultimate strength of Q690 high-strength steel after fatigue damage decreased with the increase of fatigue damage, but the decrease range was small. When the fatigue damage value is 0.13, 0.26 and 0.39, the yield strength is reduced by 3.08%, 5.83% and 8.56% respectively compared with the non-damaged steel. The ultimate strength is reduced by 2.27%, 4.37% and 8.60% respectively compared with the non-damaged steel; the elastic modulus of Q690 high-strength steel is less affected by fatigue damage. When the fatigue damage value is 0.13, 0.26 and 0.39, the elastic modulus does not change by more than 2% compared with the non-damaged steel, and the change has no obvious regularity, and the variation has no obvious regularity.

3) Fatigue damage makes Q690 high-strength steel appear fatigue cycle softening, elongation decreases with the increase of fatigue damage, and the greater the fatigue damage value, the more obvious the decreasing trend. When the fatigue damage value is 0.13, 0.26 and 0.39, the elongation is reduced by 4.41%, 13.63% and 32.38% respectively compared with the non-damaged steel.

4) The analysis of yield strength, ultimate strength, elastic modulus and elongation of Q690 high-strength steel after fatigue damage can refer to the mathematical expressions in the text for corresponding calculations. The degree is poor, and a large number of experimental studies are still needed to be done in the follow-up.

The author declares no conflicts of interest regarding the publication of this paper.

Luo, R. (2022) Experimental Study on Mechanical Properties of Q690 High-Strength Steel after High Cycle Fatigue Damage. Open Journal of Applied Sciences, 12, 243-255. https://doi.org/10.4236/ojapps.2022.122018