nondestructive characterization of fatigue processes in cyclically ...

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2 nd International Workshop on Structural Health Monitoring, Stanford University, 1999 6 The analysing frequency of the evaluating unit in the 3MA-system directly influences the depth to where the micromagnetic signals can be detected, the so-called interaction depth. The excitation frequency of the power supply could be varied between 1 and 160 Hz and also has to be adapted to the maximum field strength and the material properties. From former parameter studies, an excitation frequency of 100 Hz and an analysing frequency of 0.4 MHz was found to be at an optimum leading to a interaction depth of ≈10 µm, which is similar to the penetration depth of the X-rays used in the residual stress measurements. The commercial 3MA-testing system used here is shown in Fig. 7 The raw data from the 3MA device could be transferred into a computer for further data evaluation. Using a specially designed software, the shape of the Barkhausen noise curve and the time signal of the tangential field strength could be analysed to derive further parameters [1, 10, 11]. 3 SIMULATION OF THE DEFORMATION BEHAVIOUR OF THE WELDED JOINT Depending on the load stress, often the greatest magnitude of local plastic derformations and corresponding residual stress relaxation in a welded joint can be detected after the first load cycle and thus determines the further fatigue process to a great extent. For this reason, it is essential to investigate this static load step for an assessment of the fatigue behaviour. Fig. 8 shows the calculated distribution of the total residual strains after loading including the elastic strains resulting from residual stresses at the surface of the weld for different maximum nominal loading stresses. According to the material properties (see Fig. 2 and 3), the hard weld bead of the final pass exhibits very small plastic deformations in comparison to the soft base material. At the weld toe, the geometrical notch at the transition from weld material to HAZ leads to a local stress concentration and thus high local (plastic) strains. A similar effect can be observed at the transition zone from HAZ to base material. The 400 MPa 500 MPa 550 MPa 400 MPa 500 MPa 550 MPa Base Mat. HAZ Weld Base Mat. HAZ Weld Fig. 8 Calculated strains (elastic and plastic) after different loading stresses across the surface of the weld FEM-model (left: final pass, right: cap pass)
2 nd International Workshop on Structural Health Monitoring, Stanford University, 1999 7 strain gradient from hard weld bead to soft base material is much less severe in the softer cap pass where no geometrical notch is present, the weld bead of the cap pass plastically deforms proportional with growing nominal stress with a magnitude approximately half of that of the base material. 4 BEHAVIOUR OF NONDESTRUCTIVE PARAMETERS IN CYCLICALLY LOADED WELDED JOINTS Fig. 9 shows the distributions of the tranverse residual stresses and the micromagnetic parameters coercivity H cm and the Barkhausen noise amplitude M max across the weld determined after different decades of pure pulsating tension loading with an upper stress of 400 MPa. The investigated 6- layer GTA double vee weldment was welded with a filler metal of a higher strength than the base material (see hardness distribution in Fig. 2). Despite the fact that compressive residual stresses are found in the weld seam and the load stress is of a pure tension type, a relaxation of these compressive residual stresses can be observed. This relaxation indicates local plastic deformation in the weld seam, which is in agreement with the result of the FEM-simulation in Fig. 8. Until 10 5 load cycles, the compressive residual stresses further relax but begin to increase again after σ RS t [MPa] Mmax [V] Hcm [A/cm] 300 200 100 0 -100 -200 -300 -400 S 355, GTA-DV-Weld, Cap Pass, σ o = 400 MPa, N = 10 5 until fracture. On the Distance from weld centerline [mm] other side, in the base material, tensile residual stresses up to 200 MPa continously arise during fatigue loading. Analogous to the behaviour of the residual stresses, both micromagnetic parameters change significantly after the first load cycle. Additionally, a continous change of H cm and M max throughout the fatigue process can be observed. The Barkhausen noise amplitude M max continuously increases across the whole weld area until 10 5 load cycles and decreases to an intermediate level at fracture of the specimen. In opposite to the behaviour of M max , the coercivity decreases in the weld seam and the HAZ until shortly before fracture. 7.5 7 6.5 6 5.5 5 4.5 4 9.5 9 8.5 8 7.5 7 Weld toe Weld HAZ Base Mat. Weld HAZ Base Mat. 0 5 10 15 20 N = 0 N = 1 N = 1000 N = 100000 N = 102234 (cracked) Fig. 9 Distribution of the transverse residual stresses and the micromagnetic parameters H cm and M max at different load cycles until fracture (steel S355, pure tension loading with maximum stress of 400 MPa)
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