Research On Characteristics Of Weld Crack Signal Based On Metal Magnetic Memory Detection Technology

Due to impact,fatigue and other factors,cracks and stress damage often occur in the welding seam of long-distance oil and gas pipelines during operation,which is very easy to cause major safety accidents such as pipeline cracking,causing huge economic losses to the country.Therefore,in order to maintain the safe operation of long-distance oil and gas pipelines,internal inspection must be carried out.Traditional nondestructive testing techniques,such as eddy current method,magnetic powder method,X-ray method and ultrasonic method,do not meet the technical requirements of in pipe testing.For the widely used magnetic flux leakage testing technology,due to the limitation of testing principle,it can only detect the macro defects on the pipe surface.As an emerging non-destructive testing technology,Metal Magnetic Memory(MMM)has the advantages of stress sensitivity,high detection accuracy and non-contact on-line dynamic testing.It can detect the cracks and stress damage by abnormal stress concentration.Therefore,it has a good application prospect in the field of long-distance oil and gas pipeline weld crack detection.The weld has more complex metallographic structure and a large amount of thermal stress,but the traditional magnetomechanical model does not take these factors into account,so it is not suitable for the calculation of weld magnetomechanical relationship.Considering the limitations of the traditional model,this paper first analyzes the weld formation process by finite element technology,and studies the thermal stress distribution around the weld,which provides a basis for theoretical analysis.Then,considering the influence of metallographic structure on the quantitative calculation of weld magnetic force,a non-uniform magnetic charge model for internal inspection of long oil and gas pipelines is established.The generation mechanism of weld crack magnetic memory signal is analyzed,and the influence of weld width and thickness,crack depth and width,pipeline internal pressure and lift off value on the characteristic value of magnetic memory signal is quantitatively calculated,It provides a theoretical basis for the application of magnetic memory testing technology in the field of weld crack detection.In this paper,the correctness of the theoretical analysis is verified by systematic experiments,and the experimental results shows that the MMM signal is is mainly based on the characteristics of weld signal,while the crack signal is superimposed,and its axial component has an extreme value,the radial's contains two independent and zero-crossing peak-to-peak values,where the outer side corresponds to the weld and the inner side corresponds to the crack.The crack does not affect the thermal stress distribution of the whole weld,the stress concentration around the crack is much higher than the weld,and its gradient value has the maximum value.When the pipeline internal pressure increases,the weld thermal stress and internal pressure are superimposed,which increases the stress concentration of the weld.The stress around the crack is more sensitive to the change of internal pressure.When the internal pressure fluctuates,the stress concentration around the crack increases sharply,which is more serious to the pipeline.The MMM signal is positively correlated with the internal pressure,and the characteristic value of the signal increases linearly with the increase of the internal pressure.The signal eigenvalue decays exponentially with the increase of sensor lift off,and the axial component decays faster than the radial component.