In this paper, the change of Barkhausen Noise has been measured in the tension process of 09CuPTi steel. The relationship between Barkhausen Noise intensity and tension damage was obtained. The damage state of material and the change of damage in tension process can be shown by using of the intensity of Barkhausen Noise.
Under action of external magnetic field, irreversible and abrupt motion of magnetic domain walls and Barkhausen Noise are created simultaneously. The variation of Barkhausen Noise strength and fettered motion of the domain walls are related to size and distribution of pinning point, i. e. It is relatively sensitive to micro structure (dislocation, surface defect, anisotropy) of ferromagnetic material and local stress state (stress level and direction). Under loading higher than yield stress of metal material, plastic deformation and damage may be created. The damage of the material may be lead to an exchange of microstructure of the material. The exchange of microstructure shall have an influence on the magnetic domain structure and its motion and on the size and the distribution of the pinning point. And then, Barkhausen Noise strength should be affected. And so, we can measure the damage of the ferromagnetic material in tension process through measuring the variation of Barkhausen Noise strength. And through the variation of Barkhausen Noise strength, damage expansion and variation in the tension process are indicated. In the paper, evolution of material damage in the tension process is measured by measuring the variation of Barkhausen Noise strength of climate resisting steel in one-axis tension process.
EXPERIMENTAL METHOD AND SPECIMENS
In this experiment, specimens made of climate resisting steel (09CuPTi). The specimens are flat plate type and are sampled on rolling direction. The sizes of the specimens are shown in Fig 1. In order to eliminate working residual stress, the specimens are carried on tempering treatment, i. e. Heat restraining at 200oC times of 8 hours and slowly cooling to room temperature in furnace. And then, the specimens surface have been carried on CSS!!)) type electronic universal testing machine at room temperature and its loading speed is 1mm/min.
By using STRESSCAN 500C type elastic-magnetic instrument, Barkhausen Noise strength which is indicated by MP are measured. For the sake of getting the maximum resolving power of output MP, we optimized some controlled magnetic parameters of the instrument. In the experiment, measuring depth is 0.02mm, and magnetization parameter is 45. The detecting head is fixed on the specimen surface and it is measured along loading direction. By using PHLIPHS 3365A type digital store oscillograph, waveforms of Barkhausen Noise are recorded in different steps with sampling time is 2msce. Deformation in loading and residual deformation after unloading is measured by strain gage stuck on the specimen surface. The experiment is carried on in many steps. When the specimen has been tensioned to a fixed deformation, Barkhausen Noise strength is measured, and when unloading to zero, Barkhausen Noise strength is measured again. And so, the variation of MP value and apparent elastic modulus can be observed in the process from initial loading to fracture.
RESULTS AND DISCUSSION
Fig 1 shown a relationship diagram of retaining load, MP-e
and apparent stress-strain curve. Fig2 had shown a relationship between MP and e after unloading to zero. As shown in Fig3, Fig4, before material yield, as load increasing MP is increasing monotonously in the retaining load condition and after unloading to zero, MP is return to initial value. After material is reached to yield, MP is increasing monotonously with increasing of loading, but it is approach to saturation gradually in the retaining load condition, i. e as load increasing the variation of MP is not obvious. After unloading to zero, the variation of MP is obvious and its value is raised as strain increasing. It is demonstrated that after material yield, the plastic strain is created in the specimen and so the damage is occurred. After unloading, microstructure of the specimen is varied, and so MP value is larger than its initial value.
Fig 1: relationship between MP and e in retaining load condition
Fig 2: Relationship between MP and e after unloading
And then the material yield can be determined through measuring the variation of MP value. Ductile damage of the metal material is associated with its large deformation intrinsically. After the material is damaged, micro defects are growing, propagating and combination, and so the apparent elastic modulus is decreasing. According to damage mechanics, after i th step loading, the damage variable D can be expressed as following:
Fig 3: relationship between elastic modulus and residual plastic strain
Fig 4: relationship between damage variable D and residual plastic strain
where E0 is initial elastic modulus of material, Ei is the elastic modulus after i th step loading. By virtue of material damage is occurred after tension yield, the residual plastic deformation may be existed. And so, as a result of the variation of the residual deformation ep in ductile damage, elastic modulus of the material should be varied, its damage can be expressed as
where ss is yield stress of the material, E0 is initial elastic modulus of non-damage material, S is a constant of the material, eo is threshold value of damage strain. Fig6 shows a relationship between the apparent elastic modulus and the residual strain.
Fig4 shows the relationship between the damage variable D and the residual plastic strain ep . As shown in Fig7, the coefficient in formula (2) S = 0.54 ´10-3, ep = 0 can be estimated by using linear regression analysis. As the plastic strain increasing, the damage is enlarged, the elastic modulus is decreased and Barkhausen Noise is increased continuously. Fig5 shows the relationship between Barkhausen Noise strength and the residual plastic strain. As shown in Fig8, MP0 is initial Barkhausen Noise strength value of the specimen and MP is measured instantaneous Barkhausen Noise strength value.
According to Fig5, the relationship between MP and plastic strain ep can be obtained as follow
Fig 5: relationship between Barkhausen Noise strength and residual plastic strain
By using the regression analysis, it can be obtain as follow
substituting Eq(4) into Eq(2), we get
And then, Barkhausen Noise strength MP can be used as a parameter to measure the tension damage variable. The variation of the material damage can be monitored through measuring the variation of MP.
Measuring the damage of the ferromagnetic material by using of Barkhausen Noise method is a new nondestructive method. Barkhausen Noise strength is relatively sensitive to ductile damage. And so , by using Barkhausen Noise method, we can be measured the damage of the material in tension process directly and nondestructive. The damage parameter indicated by Barkhausen Noise characteristic value can be responded to the damage state of the material and its variation in the whole tension process.
This work was supported by the National Natural Science Foundation of Chain.
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