Investigation On The Influence Of Hygrothermal Aging On Indentation Mechanical Behaviors Of PMMA With Strength Differential Effects

As a common engineering plastic,Poly?methyl methacrylate?i.e.PMMA has been widely used because its excellent physical properties,which results in the complex use conditions.A comprehensive understanding of the mechanical properties of PMMA is needed for better design of its structural component and prediction of its service life.On the other hand,micro-nano indentation method is an advanced method to examine the mechanical behaviors of materials.The indentation size effects are inevitable.Now times,the requirements i.e.high performance and miniaturization of new materials and test methods have been raised for industrial community and academic community for the reasons of lightweight design and energy conservation and emission reduction in the field of automobile,micro-electronics,aeronautics and astronautics.Taken above factors into comprehensive consideration,a combination method of experimental and theoretical is used to study the PMMA indentation size effects and its indentation mechanical properties as well as time dependent deformation of PMMA after artificial accelerated aging treatment under hygrothermal environment.As a consequence,the following aspects were conducted:?1?investigation on the PMMA indentation size effects based on asymmetry yield criterion and the theory of shear transformation plasticity,?2?effects of hygrothermal aging on indentation size effects and mechanical properties of PMMA,?3?influence of hygrothermal aging on indentation creep behaviors of PMMA.The yield criterion used to describe the yield behaviors of PMMA is established based on the experimental observation and the consideration of physical mechanism.The effects of hydrostatic pressure and sign of normal stress are also taken into consideration of the criterion and can be decoupled by introducing the parameters independently.The hydrostatic pressure sensitivity is introduced by mean stress P and controlled by b.And the asymmetry of yield strength in compression and tension directions is characterized by applying an odd function of principal value of deviatoric stress tensor.Furthermore,the parameters in yield criterion are calibrated through the complex loading experimental results.Combined with the theory of shear transformation plasticity which account for physical nature of plastic deformation of amorphous materials,a theoretical prediction model describes the indentation size effects of PMMA is build.The indentation size effects of PMMA are found can be attributed to the synthetic actions of discrete STz among deformed area under the indenter,loading conditions and mechanical properties of material.The STz size exhibits positive correlation with strain rate,which results in the characteristic deep increases with strain rate.The PMMA after hygrothermal aging is applied to the nano-indentation tests with different strain rates.And the differences between indentation responses of PMMA under different conditions have been analyzed.Similarly,the theory of shear transformation plasticity is utilized to analyze the indentation size effects of PMMA after hygrothermal aged.The STz sizes of PMMA under different conditions have been calculated and a function relates it with aging time and strain rate has been established.The characteristic deeps which represents the vanished of indentation size effects are obtained and exhibit increases with strain rate and aging time.Then the objective measurements i.e.elastic modulus and hardness represent the practical properties of aged PMMA can be calculated.Both elastic modulus and hardness are found decreases with aging time.The effects of aging on mechanical properties are found weaken as aging time increases which can be explained the micro-mechanism.Through relating?with aging time and strain rate,the prediction model can be build to explain the indentation size effects of PMMA after hygrothermal aging.The model is found can well predict the indentation mechanical properties of PMMA.The effects of aging time on indentation creep behaviors have been investigated and the creep displacement is found increases with aging time.However,the rate of change weakens as aging time increases.The generalized Calvin model consists of three Voigt elements is found can well describe the indentation creep behavior of PMMA during holding stage.Each Voigt element represents motions with different levels i.e.molecular chain,molecular chain segments,molecular chain linkage and branched-chain.The motions of molecular with different levels always have different delay time.The mechanism about the effects of aging time on the indentation creep responses is analyzed by comparing the contribution of different deformation types to total deformation.Furthermore,R_he decreases initial and then increases as aging time increases which indicates the weakening of elastic modulus results from hygrothermal aging.With the aging time increasing,R_{he v}?10?R_hv is first increasing and then decreasing which can be attributed to the different aging mechanisms at different stages.Further study on the effects of aging time on creep compliance and time delay spectrum of PMMA during holding stage indicates the different deformation levels of PMMA characterized by different Vogit elements.The change law of strain rate sensitive index is found consistent withR_{he v}?10?R_hv change caused by aging time.This illustrates the strain rate sensitivity of continuum solids is due to their inherent time-dependent deformation properties.