Damage Behavior And Fracture Mechanism Of Lithium Hydride Ceramics Under Quasi-static Loading At Elevated Temperatures

Space nuclear reactor power system(SNRP)is key to deep space detection,and it is also the development direction for future space power system with high-power,long life and high reliability.Shielding is necessary for reducing nuclear radiation such as neutron and gamma rays to the safe level.However,due to limited shielding space,high temperature and complicated radiation conditions in SNRP,the mass and volume of shielding need to be strictly restricted and the properties of shielding materials are strict.Lithium hydride(LiH)has advantages of high hydrogen capacity,low density and high neutron absorption cross-section,which is the preferred neutron shielding material for SNRP.For space neutron shielding,LiH needs to be prepared into LiH ceramic materials.However,LiH ceramics have the characteristics of low strength,high brittleness/low toughness and show sensitivity to temperature and environmental conditions.In the process of production and service,it will inevitably be subjected to complex thermal-mechanical loading conditions,which may lead to damage of crack initiation/propagation.The cracks can cause reliability degradation and even severe damage to fracture.As LiH is highly reactive and has a certain toxicity,the related research is difficult and lacking.Therefore,the damage behavior and fracture mechanism are major concerns in safe design and application of LiH ceramics.In this dissertation,the damage behavior and fracture mechanism of LiH ceramics were studied systematically under quasi-static loading at elevated temperatures.The damage behavior and fracture toughness of LiH ceramics were characterized,analyzed and investigated.The fracture mechanism was determined by combining the macroscopic fracture behaviors and the analysis of microscopic fracture surfaces.In addition,the damage constitutive model of LiH ceramics was established according to the test results.The model was further analyzed,verified and improved.Firstly,the short-term high temperature damage and dynamic fatigue/creep damage of LiH ceramics were characterized by analyzing the mechanical responses and macroscopic mechanical properties under quasi-static loading at elevated temperatures.The increase in temperature will lead to short-term high temperature damage,the loaddeflection curves show nonlinearity before the peak load and softening behavior after the peak load.In addition,the bending modulus decreases with increasing temperature.The bending strength decreases slightly from room temperature(RT)to 200℃,then increases to the maximum between 200 and 300℃,and finally continues to decrease over 300℃.At RT,the strength of the notched specimens decreases with decreasing stress rate,showing obvious dynamic fatigue behavior.Slow crack growth(SCG)occurs before unstable fracture of the notched specimens.With increasing temperature and decreasing stress rate,creep damage gradually becomes significant.SCG of inherent defects and introduction of creep micro-cracks are two simultaneous and competing damage processes,one of which dominates the macroscopic fracture.Secondly,the resistance curves(R-curves)of LiH ceramics at different temperatures were obtained with the single edge V-notched beam(SEVNB)specimens,and temperature effect on fracture toughness was investigated.When brittle fracture occurs,the crack growth resistance remains constant,and there is a horizontal R-curve.With increasing temperature,LiH ceramics have sufficient plastic deformation capacity,and the crack growth resistance increases with crack propagation,showing rising Rcurve behaviors.The flaw tolerance increases greatly.Under the loading rate of 0.5 mm/min,the fracture toughness decreases slightly from RT to 200℃,and then increases from 200 to 400℃ substantially,and finally decreases slightly over 400℃.The trends of fracture toughness and bending strength of LiH ceramics with temperature show some similarities.Compared to strength,plasticity is a more important factor affecting fracture toughness of LiH ceramics.Furthermore,the fracture mechanism of LiH ceramics under quasi-static loading at elevated temperatures was analyzed and investigated by combining the analysis of macroscopic fracture behavior and microscopic fracture surfaces.At RT and under high loading rates at 200 and 300℃,the fracture mechanism is SCG-dominated cleavage fracture.When exceeding the fatigue threshold,SCG of inherent flaws will occur.The specimen will fail to unstable fracture when an inherent flaw reaches the critical size.The crack gives priority to propagate along a certain cleavage plane.With decreasing loading rate,the development time of SCG becomes longer,and the depth of SCG zone on the fracture surfaces increases.Under low loading rates at temperatures ≥200℃,the fracture mechanism is creep rupture dominated by creep process.There are rough creep damage zone on the fracture surfaces and a large number of creep related microcracks can be observed.With increasing temperature and decreasing loading rate,creep is activated and continuously accumulates into creep micro-cracks,which further grow,coalesce and connect into visible creep cracks and eventually lead to fracture.Under relatively high loading rates at temperatures ≥ 400℃,creep is negligible,and the fracture mechanism is ductile fracture dominated by microporous polymerization.The specimens have obvious plastic deformation.The crack propagation path is accompanied by crack deflection,bridging and grain extraction.The fracture surfaces are rough and the dimple features are obvious.The external load leads to separation of the material to form micro-voids.Under the action of dislocation motion,the microvoids continue to grow,merge and connect to form macroscopic cracks.Finally,the damage model of LiH ceramics was established based on the theory of statistics and damage mechanics.In addition,the model was further analyzed,verified and improved,and the damage constitutive model of LiH ceramics considering softening effect was established.The theoretical stress-strain curves obtained by the damage constitutive model considering softening effect are basically the same as the test results,indicating that the model can accurately describe the mechanical behaviors,damage evolution processes and deformation characteristics of LiH ceramics at elevated temperatures and different loading rates.All the research results of this dissertation provide key data for the preparation,processing,storage,transportation and application of LiH ceramics under quasi-static loading at specific temperatures,and provide reference for the analysis of damage and fracture mode.In addition,it is expected to be the experimental and theoretical guidance for further application design,performance improvement and service life of LiH ceramics in the future.