A review of the Effect of a/W ratio on fracture toughness (II)—experimental investigation in LEFM

In part I of this series, experimental investigation in EPFM （elastic-plastic fracture mechanics） had been discussed. In this paper, experimental investigation in LEFM （ linear elastic fracture mechanics） is given. Fracture toughness tests had been carried out on three different strength steels, using both through-cracked specimens with different α/W ratio and semi-elliptical cracked specimens with variable crack size and shape. Results show that the fracture toughness KIC increases with decreasing α/W when α/W 〈 0.3 for three-point-bend specimens, and that for α/W 〉 0.3, it is independent of α/W. Shallow crack specimens, both through-cracked and surface-cracked, gave markedly higher values than deeply notched specimens. However, the effect of crack shape on fracture toughness is negligible. Results also show that the LEFM approach to fracture is not tenable for design stresses where αc is often very small, far less than 2.5 （ KIC/σy）^2.

Strong,tough,and thermally conductive nacre-inspired boron nitride nanosheet/epoxy layered nanocomposites

Thermally conductive polymer nanocomposites integrated with lightweight,excellent flexural strength,and high fracture toughness(KIc)would be of great use in many fields.However,achieving all of these properties simultaneously remains a great challenge.Inspired by natural nacre,here we demonstrate a lightweight,strong,tough,and thermally conductive boron nitride nanosheet/epoxy layered(BNNEL)nanocomposite.Because of the layered structure and enhancing the interfacial interactions through hydrogen bonding and Si–O–B covalent bonding,the resulting nacre-inspired BNNEL nanocomposites show high fracture toughness of~4.22 MPa·m^(1/2),which is 7 folds as high as pure epoxy.Moreover,the BNNEL nanocomposites demonstrate sufficient flexural strength(~168.90 MPa,comparable to epoxy resin),while also being lightweight(~1.23 g/cm^(3)).Additionally,the BNNEL nanocomposites display a thermal conductivity(κ)of~0.47 W/(m·K)at low boron nitride nanosheet loading of 2.08 vol.%,which is 2.7 times higher than that of pure epoxy resin.The developed nacre-inspired strategy of layered structure design and interfacial enhancement provides an avenue for fabricating high mechanical properties and thermally conductive polymer nanocomposites.

Anisotropic alumina ceramics with isotropic optical properties

Excellent mechanical and isotropic optical properties are achieved simultaneously from fully dense polycrystalline advanced alumina ceramics with a hexagonal crystal structure,which are optically anisotropic.A small amount(240 ppm)of ZrO_(2) additive is used to synthesize the transparent alumina ceramics with an average grain size of 0.7µm,and locally detected optical distortion(birefringence)is extremely minimized,less than 20 nm/pass.Total transmittance(86%)of the alumina ceramics(thickness=0.3 mm)in ultraviolet(UV)to infrared(IR)regions is similar to that of commercial c-axis sapphire single crystals produced by Czochralski(CZ)method,while the extinction ratio in the visible wavelength area is over 25 dB.Like glass and the c-axis sapphire single crystals,the alumina ceramics can clearly display texts and images on liquid crystal display(LCD)screens.A grain boundary phase of nano-sized Al_(2)O_(3)-ZrO_(2) composition(amorphous)is formed at the grain boundary of the advanced alumina ceramics,which enhances four-point bending strength and fracture toughness(KIC)simultaneously to 921 MPa and 6.8 MPa·m^(0.5),respectively,and hence the mechanical properties are superior to those of the sapphire single crystals.

Mechanical property enhancements and amorphous thermal transports of ordered weberite-type RE_(3)Nb/TaO_(7) high-entropy oxides

A_(3)BO_(7)-type(A=rare earth(RE),B=Nb or Ta)oxides have been studied as protective coating materials because of their low thermal conductivity;however,their hardness,toughness,and stiffness are insufficient,particularly for members with webeirte-type structures.In this work,we have synthesized two high-entropy oxides(HEOs)of weberite-type RE niobates/tantalates(RE_(3)Nb/TaO_(7)),i.e.,(Nd_(1/7)Sm_(1/7)Eu_(1/7)Gd_(1/7)Dy_(1/7)Ho_(1/7)Er_(1/7))_(3)NbO_(7)(7HEOs-Nb)and(Nd_(1/7)Sm_(1/7)Eu_(1/7)Gd_(1/7)Dy_(1/7)Ho_(1/7)Er_(1/7))3(Nb_(1/2)Ta_(1/2))O_(7)(7HEOs-NbTa),to overcome the mechanical deficiencies.The short-and long-range ordered arrangements of RE cations in the A-site and Nb/Ta cations in the B-site were identified by the X-ray diffraction(XRD),scanning electron microscopy equipped with energy-dispersive spectrometry(EDS),and transmission electron microscopy.The enhancements in hardness(H=9.4 GPa)and fracture toughness(KIC=2.0 MPa·m^(1/2))were realized by grain refinement,solid solution strengthening,and high stiffness(K).The exceptional phase stability at 25-1500℃,amorphous thermal conductivity(k=1.5-1.7 W·m^(−1)·K^(−1) at 25-900℃),and high thermal expansion coefficients(TEC>11.0×10^(−6) K^(−1) at 1500℃)further supported their potential application as protective coating materials.