Thermomechanical Coupling Mechanism And Stability Application Research Of Partially Crystalline Poly-Ether-Ether-Ketone Thin Film

This research project comes from the Chinese Academy of Sciences-PetroChina strategic cooperation special project "Engineering technology research on wellbore stabilization and exploitation after completing drilling shale gas".The morphology of the wellbore after drilling is an important basis for subsequent construction and stability assessment,which is necessary to use the in-situ drilling TV to image and detect the wellbore in a complex downhole environment.Dustproof and waterproof protective film could effectively protect lenses in in-situ drilled TVs,which could be made of Poly-Ether-Ether-Ketone potentially.Research on the thermodynamic coupling response of Poly-Ether-Ether-Ketone in the thermoforming and application processes is helpful to accurately grasp the highly nonlinear mechanical behavior of this type of material in complex processing and application scenarios,significantly promote the optimization of the thermoforming process parameters and accelerate the performance improvement of the product.Wrinkles during the thermoforming process and application will seriously affect the light transmittance and the imaging performance of the in-situ drilled TV.Therefore,the study of the mechanical behavior of Poly-EtherEther-Ketone in the thermoforming process and high-temperature application can better serve the in-situ drilling TV imaging system,which has important theoretical and practical significance.The research on unsaturated crystalline Poly-Ether-Ether-Ketone will split into three aspects based on the above background:(1)Material characterization:uniaxial tension and planar tension tests under different temperatures and strain rates were conducted based on the universal testing machine while equal biaxial tension and constant-width tension tests were conducted based on the biaxial tension machine.A method for determining the heating time and soaking time during tension tests was proposed based on the one-dimensional heat transfer model,which ensured that the temperature of the film satisfied the setting conditions during high-temperature tests.The influence of inertia effect on stress-strain curves under uniaxial tension was pointed out and a correction method based on the inertial force was proposed.A planar tension test fixture based on the universal testing machine was proposed,which was verified to satisfy the plane strain state.The unsaturated crystalline Poly-Ether-Ether-Ketone film was characterized at different temperatures and strain rates based on these test methods and the following characteristics were revealed:a)More significant temperature effect and strain rate effect than saturated crystalline Poly-Ether-Ether-Ketone;b)Significant temperature and strain rate dependent strain softening and strain hardening effect;c)Crystallization hardening effect induced by cold crystallization,indicating that the material property measured during tests with crystallization is not intrinsic property but external property d)Deformation mode and temperature dependent anisotropic plastic behavior.(2)Constitutive model:a new thermo-mechanical constitutive model describing the nonlinear mechanical behaviors was proposed based on the Oxford-Glass-Rubber model and the strong nonlinear mechanical behavior of Poly-Ether-Ether-Ketone was successfully captured:a)An anisotropic hyperelastic model based on the strain invariant I4 was proposed to describe the anisotropic plastic deformation;b)Polynomial equations of temperature and strain rate was proposed to describe the temperature and strain rate dependent strain softening phenomenon,and the phenomenon of strain softening amplitude negtive dependent of temperature was explained by the progressive hardening model that rejuvanation occurs in the material;c)A strain hardening model was proposed to decompose the strain hardening behavior into a weak entropy-elastic strain hardening part and a strong viscous hardening part,wherein the former was described using a hyperelastic model and the latter was established with a linear connection between the hardening modulus and the viscosity of the bond streching part;d)A crystallinity evolution model based on the Nakaruma equation to describe the nonisothermal crystallization kinetics of unsaturated crystalline PEEK was proposed,with which a linear relationship between crystallinity and modulus was established and the crystallization hardening phenomenon was accurately predicted.(3)Wrinkle:the local unstable wrinkling deformation caused by necking of the film material was pointed out,and a thermo-mechanical coupled numerical simulation model of film stretching was established by implementation of the nonlinear constitutive model into the finite element analysis software Abaqus.The evolution of strain and stress during the process of necking of uniaxial tension were discussed and the mechanism of generation and evolution of wrinkles was proposed by the lateral contraction induced by necking.The degree of necking was proposed as the main factor affecting wrinkling while softening parameters were proposed as influencing the degree of necking.Based on the above conclusions,the optimization of size and the process parameter of the waterproof and dustproof film were discussed to avoid wrinkling.The wrinkle wavenumber was found to increase linearly with the decrease in thickness,coinciding with the linear elastic buckling theory.The critical stress required for the material was pointed out as greater than the yield stress and no wrinkle generates after a certain thickness.The evolution of the wrinkle area was pointed out as dependent on aspect ratios,which will guide the shape design of the waterproof and dustproof film by avoiding the wrinkle area.Numerical simulations of tensile tests under different strain rates were conducted and the mechanism that the degree of heat generation and necking are aggravated under higher strain rate tension tests,thereby further affecting the amplitude and amplitude of wrinkle.A solution to retard wrinkling by decreasing the strain rate near yield strain was proposed.Finally,an anti-wrinkle method of pretreatment was pointed out when the pretreatment process will significantly reduce the degree of strain softening and strain hardening during secondary stretching.In conclusion,characterization of the mechanical behavior of PEEK film under multiaxial stretching was conducted and the complex nonlinear mechanical behaviors were revealed,through which an anisotropic thermodynamic coupling constitutive model was successfully established.Numerical simulation and buckling theory were combined to explain the mechanism of wrinkle,which offers significant guidance for the thermoforming process parameter design of the waterproof and dustproof film in engineering applications of the drilling imaging television.