A modified single edge V-notched beam method for evaluating surface fracture toughness of thermal barrier coatings

The surface fracture toughness is an important mechanical parameter for studying the failure behavior of air plasma sprayed(APS)thermal barrier coatings(TBCs).As APS TBCs are typical multilayer porous ceramic materials,the direct applications of the traditional single edge notched beam(SENB)method that ignores those typical structural characters may cause errors.To measure the surface fracture toughness more accurately,the effects of multilayer and porous characters on the fracture toughness of APS TBCs should be considered.In this paper,a modified single edge V-notched beam(MSEVNB)method with typical structural characters is developed.According to the finite element analysis(FEA),the geometry factor of the multilayer structure is recalculated.Owing to the narrower V-notches,a more accurate critical fracture stress is obtained.Based on the Griffith energy balance,the reduction of the crack surface caused by micro-defects is corrected.The MSEVNB method can measure the surface fracture toughness more accurately than the SENB method.

Quantitative characterization of defects in stereolithographic additive manufactured ceramic using X-ray computed tomography

Ceramics have been widely fabricated by additive manufacturing(AM).Compared to conventional technologies,the strength of additive manufactured ceramic is relatively low owing to the formation of defects during manufacturing process.These defects have significant effects on the microstructure and mechanical properties of additive manufactured ceramics.However,systematic research on defects,including defect geometrical features,quantitative statistics,and formation mechanism,as well as the intrinsic relationship with mechanical properties,need to be studied in depth.In this work,Al2 O3 ceramics were prepared from photosensitive slurries with different solid loadings by using stereolithographic(SL)additive manufacturing.The defects,including their sizes and distributions,in both green and sintered bodies were investigated by using scanning electron microscopy(SEM)and X-ray computed tomography(X-CT).Geometrical features and quantitative statistics of the defects were evaluated and discussed to reveal their formation mechanism.Moreover,the intrinsic relationship between defects and mechanical properties of the additive manufactured ceramic was revealed.This study can give some fundamental understanding of the defects in additive manufactured ceramics.

Rapid heating thermal shock study of ultra high temperature ceramics using an in situ testing method

In this paper,the rapid cooling thermal shock behaviors of ZrB_(2)-SiC ceramics were measured using traditional water quenching method,and the rapid heating thermal shock behaviors of ZrB_(2)-SiC ceramics were investigated using a novel in situ testing method.The measured critical thermal shock temperature difference for rapid cooling thermal shock was 373.6℃;however,the critical thermal shock temperature difference for rapid heating thermal shock of ZrB_(2)-SiC ceramics was measured to be as high as 1497.2℃.The thermal stress distribution states after rapid cooling thermal shock and rapid heating thermal shock testing were analyzed using finite element analysis(FEA)method.The FEA results showed that there is a tensile stress existed on the surface for rapid cooling thermal shock,whereas there is a compressive stress existed on the surface for rapid heating thermal shock.The difference of thermal stress distribution resulted in the difference of the critical temperature difference for rapid cooling thermal shock and rapid heating thermal shock.

Review on mechanics of ultra-high-temperature materials

Ultra-high-temperature materials have applications in aerospace and nuclear industry.They are usually subjected to complex thermal environments during service.The mechanical properties of materials in ultra-high-temperature environments have been attracted increasing attentions.However,the characterization and evaluation of ultra-high-temperature mechanical properties of materials are still challenging work.This article presents a review on the mechanical properties of materials at elevated temperatures.The experimental results and techniques on the ultra-high-temperature mechanical properties of materials are reviewed.The constitutive models of materials at elevated temperatures are discussed.The recent research progress on the quantitative theoretical characterization models for the temperature-dependent fracture strength of advanced ceramics and their composites is also given,and the emphasis is placed on the applications of the force-heat equivalence energy density principle.The thermal–mechanical-oxygen coupled computational mechanics of materials are discussed.Furthermore,the outlook and concluding remarks are highlighted.