Study On Microstructure And Properties Of Carbon Fiber Reinforced Epoxy Resin Composites Under Electrothermal And Hygrothermal Conditions

The T300/E51 composites were treated by electrothermal treatment,hygrothermal ageing and electrothermal moisture treatment.The samples were tested by universal testing machine,infrared spectroscopy(IR),differential scanning calorimetry(DSC),optical microscope(OM),scanning electron microscope(SEM)and electrochemical workstation.The influence mechanism of electrothermal treatment,hygrothermal ageing and electrothermal treatment on the hygrothermal ageing was respectively revealed from bending properties,moisture absorption rate,chemical structure of the matrix,glass transition temperature,bending fracture and etc.In addition,the effect of electrothermal treatment,hygrothermal ageing on dielectric properties was analyzed by electrochemical method.After the T300/E51 composite was subjected to electrothermal treatment,the samples processed with 4A and 6A electrothermal treatment had an obvious postcure effect,which leaded to the Tg rise.The heat resistance of materials was improved.Because of the compressive stress generated by the electric heating,many micro cracks appeared in the matrix,and the cracks becomed more and the volume of the pores becomed larger with the increase of the current.The bending performance test showed that the bending strength of the samples was improved due to the effect of post curing and small current,and the flexural strength of 6A was lower than that of 4A.The variation law of bending modulus and bending strength were completely opposite.The bending modulus of the samples after electrothermal treatment was lower than that of the original sample,in which the bending modulus of the 6A sample was higher than that of 4A.The fracture surfaces of 4A electrothermal and 6A electrothermal specimens were smooth and showed brittle fracture.The angle between the fracture surface and the principal stress direction of the original sample was 45 degrees,which was a ductile fracture.The dielectric properties showed that the dielectric constant of 4A and 6A samples was smaller than that of the original sample,and the dielectric constant of 6A samples was larger than that of 4A samples.In accordance with the law of dielectric constant,the dielectric loss of 4A and 6A samples was smaller than that of the original sample,and the dielectric loss of 6A samples was larger than that of 4A samples.After the hygrothermal ageing treatment of T300/E51 composites,the samples treated for 6 days were not cured completely.The sample after hygrothermal ageing treatment had a postcure effect and the matrix was partially hydrolyzed.With the increase of the moisture absorption time and the amount of moisture absorption,the Tg of the sample decreased gradually.There were crack growth in the matrix of the 6d sample.There were not only crack propagation in the matrix of the sample after hygrothermal ageing treatment,but also new cracks.The flexural strength and flexural modulus decreased gradually with the increase of moisture absorption time and moisture absorption.The dielectric constant of the samples after hygrothermal ageing treatment was much larger than that of the 6d sample and the original sample,and the dielectric constant of the 6d sample was only slightly larger than that of the original sample.In the 10~3-10~6 Hz frequency range,the dielectric loss of the original sample,the 6d sample and the hygrothermal aging sample increased with the increase of moisture absorption.After the electrothermal moisture treatment of T300/E51 composites,the original sample was hydrolyzed partially after the hydrothermal treatment.The samples processed with electro-thermal and hydrothermal treatment had postcure effect,and the post curing of 4A electrothermal moisture treatment samples was obvious.The moisture absorption rate of 4A heated sample was decreased due to the decrease of the size of interface cracks caused by postcure effect.The variation of glass transition temperature was similar to that of bending behavior.The detailed analysis revealed that,the damage mechanism of T300/E51 composites after electrothermal treatment was different from that of traditional moisture.The glass transition temperature becomed a primary factor to measure the performance of composite materials,instead of moisture absorption.Microstructure observations showed that the micro-cracks were increased owing to the temperature gradient and compressive stress generated by the effect of electrothermal treatment.Additionally,the swelling occurs throughout the matrix after the subsequent hydrothermal treatment,which eventually leaded to the decrease of the bending property.The repair effect of current and the postcure caused by electrothermal treatment maked the surface defects of materials reduce,which hindered the diffusion of water molecules further and well prevented the effects of hygrothermal aging on the dielectric properties.It leaded that the dielectric constant and dielectric loss of 4A and 6A electrothermal moisture treatment samples were similar to that of 4A and 6A electrothermal treatment samples.