Photoluminescence (PL) spectroscopy and x-ray diffraction measurements were employed to study biaxial strain in AlN epilayers grown on different substrates. X-ray diffraction revealed that AlN epilayers grown on AlN bulk substrates (or homoepilayers) have the same lattice parameters as AlN bulk crystals and are almost strain-free. Compared to the free exciton (FX) transition in an AlN homoepilayer, the FX line was 31meV higher in AlN/sapphire due to a compressive strain and 55 (69)meV lower in AlN∕SiC (AlN∕Si) due to a tensile strain. A linear relationship between the FX transition energy peak position and in-plane stress was obtained, and a value of 45meV∕GPa for the linear coefficient of the stress-induced bandgap shift in AlN epilayers was deduced. The work here establishes PL as another simple and effective method for monitoring the biaxial stress in AlN epilayers.

PurposeThe purpose of this study is to investigate the effects of stud height, stud diameter, ultimate stress of stud and concrete strength on the static behaviour of studs in push-off tests based on the ductile fracture theory.Design/methodology/approachPush-off tests of headed stud shear connectors with different heights and diameters used in concrete of various strengths were designed and implemented. A finite element model was established based on a ductile fracture criterion of ML15 cold-heading steel with stress triaxiality and Lode angle parameter. Based on the results of the parametric study of the numerical model, equations were proposed to evaluate the effect of stud height hs, stud area As, concrete strength fc and stud ultimate strength fsu used in concrete of various strengths on the static behaviour of studs.FindingsThe typical failure phenomenon observed among the test specimens was the fracture of the shank of studs. The microscopic images of the stud fracture surfaces and the verified finite element model indicate that the studs were fractured as a result of the combined action of tension and shear.Originality/valueA new method for calculating ultimate load Pu and ultimate slip Su is proposed in this paper. In the method, Pu is linearly related to fsu0.2143, As0.7790, hs0.0974, fc0.2065. Su is linearly related to fsu1.078, As0.4681, hs(−0.3135), fc(−0.3480).