Cyclic Compression Deformation And Performance Characterization Of Gaskets Under High Temperature

The mechanical properties of material will deteriorate at high temperature.As,creep and relaxation properties of material will be aggravated at high temperature.Fatigue damage or cumulated deformation will be produced under cyclic loading.High temperature gaskets are the key sealing elements of bolted flange joints,their mechanical properties under repeated or vibration conditions affect the tightness of flange joints directly.Therefore,in this paper,the deformation behavior of reinforced graphite gaskets(RGG)and Non-asbestos fiber composite(NAFC)under high temperature cyclic compression load was studied systematically,and the compression-resilience properties and creep properties was characterized.The main contents are as follows.We designed the high temperature gasket repeated compression creep fatigue test device.The influences of insert type,temperature,stress amplitude,stress rate and loading sequence under multiple load steps are considered systematically.The research showed,Crescent-shaped stress-strain relationships can be observed for reinforced graphite gaskets under cyclic pulsating loads.It is similar to the stress-strain curve of traditional stainless steel materials under small stress amplitude due to the previous compression hardening effect.the mean compressive moduli under the peak compressive stress of 32 MPa between 500? to 600? for Type 3540,Type 3885,Type 3560 and Type 3865 gaskets are approximately 64.83 MPa,85.62 MPa,64.14 MPa and 64.98 MPa,respectively.The accumulated deformations of RGG under small stress amplitude of 2MPa,4MPa or 8MPa at 500? increase with number of cycles before about the first 25 cycles,and then gradually become constant during the subsequent cycles.It always enhances with increasing number of cycles for pulsating loading with the peak stress of 32 MPa at 500? or higher.The first cycle compression module of Non-asbestos fiber composite gasket showed obvious temperature and rate correlation.Results obviously indicate that the compressive modulus obviously decreases with increasing the temperature when it is less than 150?.However,the compressive modulus changes slightly when the temperature is greater than 150?.The compressive modulus greatly enhances with the increment of stress rate when the stress rate is less than 16MPa/s.however,the compressive modulus changes are not obviously when the stress rate is greater than 16MPa/s.During about the first 75 cycles,the ratcheting strain rate per cycle reduces promptly under different temperature and stress rate,while it almost remains steady with a very tiny increment per cycle in the subsequent cycles.It indicates the occurrence of shakedown after 75 cycles.The ratcheting strain and creep properties of NAFC under fluctuating loads with small stress amplitude are studied.Significant ratchet deformation can be observed at stress ratio of 0.9 under 100? and 150? test conditions.As the temperature increase,the ratchet strain increases correspondingly.At different temperature,the ratchet strain rate per cycle always decreases with the increase of cycles.When the number of cycles is 10,the ratchet strain of 100? and 150? is 6.9% and 7.8% respectively.When the number of cycles is 1000,the ratchet strain of 100? and 150? is 9.1% and 10.1% respectively.The creep strain rate decreases rapidly during the first 1 hour,and then tends to a steady state with a very slow creep strain rate.The creep strain rate is less than 0.01% per hour at 100? and 0.1% per hour at 150?,respectively.The evolution of ratcheting strain with testing time very approaches to the creep strain at the same testing time.This indicates that the ratcheting strain of NAFC under fatigue loads with small stress amplitude can be evaluated equivalently by the creep deformation with corresponding peak stress when the stress amplitude is small(R>0.9).