Damage Mechanisms And Characteristics Of Metal And Alloy Subjected To Different Kinds Of Impulse Currents

Metal and alloy(hereinafter,short as‘metal’)are basic materials used in oil tanks,aircraft,electric power system,and other systems.Catastrophic and immeasurable losses are easily caused by lightning when a flash strikes the metal.Experimental research is the main method used in the investigation of metal damage caused by lightning.70%of cloud-to-ground flashes consist of multiple lightning currents.Different lightning current components with obviously different parameters will lead to different damage characteristics and physical responses in the stricken metal.Investigation of metal damage mechanisms and characteristics caused by lightning currents with taking into account actual lightning current characteristics is of considerable importance for the lightning protection.Based on the newly-developed Multi-Waveform Multi-Pulse Impulse Current Generator,this study is concerned with the metal damage experimental method and its influence factors,metal temperature rise model induced by lightning currents,metal damage mechanisms and characteristics caused by single and multiple lightning current components.The main contents and conclusions are shown as follows,1)The current parameters of natural lightning are summarized and compared with different standards and organizations to establish the parameters of simulated lightning currents.The electrode’s material is improved in this research.Then,the influences of electrode shape,diameter of igniting copper wire,and experimental distance are investigated by simulated lightning current experiments with different lightning current components,which are further explained by the proposed quasi 2-D discharge model for the short gap discharge in the experiment.Then the experimental method with the ability of solving the problems of electrode jet and arc energy restriction is established for the following investigations.2)A numerical inversion model based on the measured rear-face temperature rise of metal in the experiment is proposed to acquire the temperature rise at lightning attachment zone on the metal front-face caused by the long continuing current after strokes.In this model,metal is divided into numerous discrete control-volume units,forming four different kinds of nodes by the control volume finite element method(CVFEM).Then,temperature governing equations for different nodes are established and solved by the finite difference method(FDM)in time and spatial domain.The thermal sink method and equivalent heat capacity method are adopted to analyze the temperature distribution of material phase transition process in the time domain.The results show that subjected to long continuing current with amplitude 404 A and duration520 ms,the highest front-face temperature rise of Al alloy 3003 is 10800 K.The damaged area,depth and energy consumed in all processes calculated by the proposed model are 298.7 mm~2,3.1 mm,and 6710.1 J,respectively,of which the errors are 4.9%,6.1%,and 6.0%in comparison with the experimental results.3)Damage response and temperature rise of two kinds of typical metal used in oil tanks,Al alloy 3003 and steel alloy Q235B,subjected to four different lightning current components(first return stroke current component,continuing current in the stroke intervals component,long continuing current after stroke component,and subsequent stroke current component)are investigated by the metal damage experimental method established in 1).The influences of different current components and their parameters on metal damage characteristics and temperature rise are concluded.The quantitative relationships between metal damage results and lightning parameters are calculated by the numerical fitting method and product-moment correlation coefficient method.4)The high-resolution scanning electron microscope(SEM),energy dispersive spectrometer(EDS),and micro-hardness tester are adopted to investigate the metal damage morphology on surface,microstructure in cross section,and through documentation of changes in element composition and micro-hardness,respectively.Metal damage mechanisms are further clarified by analyzing the experimental results,lightning current parameters and thermal-electric-force conjoint effects.The metal with a strong relationship between microstructure and temperature is further investigated with the simulated lightning experiment of long continuing current after stroke.The response of metal damage characteristics and microstructure to temperature rise is analyzed to achieve the understanding of the metal-lightning interactions.5)Metal damage response to multiple lightning current components,which are metal damage morphology on surface,microstructure in cross section,and through documentation of changes in element composition and micro-hardness,are investigated using SEM,EDS,and micro-hardness tester,and then compared with the results caused by single lightning current component acquired in(3)-(4).Metal damage characteristics and mechanisms under multiple lightning current components are further analyzed and compared with the results under the single lightning current component.The typically remarkable effects caused by multiple lightning current components are concluded at last.