Two substrates with surface roughness(Ra) of about 1.6 and 0.8,respectively,were employed to fabricate two NiCoCrAl/YSZ microlaminates by using EB-PVD method. The average ceramic-layer thicknesses of the two NiCoCrAl/YSZ microlaminates are different,about 0.9 μm and 1.2 μm,respectively,but their average metal-layer thicknesses are equal,about 5 μm. The microstructures and fractographs were examined by SEM. Uniaxial tensile testing was performed to determine the mechanical properties. The results show that the microlaminate deposited on the relatively coarse substrate(MDCS) contains wavy layer interfaces and larger flaws,while the microlaminate deposited on the relatively smooth substrate(MDSS) has relatively flat layer interfaces and no larger flaws. The tensile specimens of the two microlaminates display obvious difference in tensile strengths and fracture modes. The ratio of strength of MDCS to that of MDSS is 0.5 at room temperature,0.67 at 700 ℃ and 1.33 at 1 000 ℃,increasing with increasing temperature. The factors which caused the variation of the strength ratio were discussed. It is found that the larger flaws in MDCS result in the relatively low strength ratio at room temperature and 700 ℃,and the wavy layer interface in MDCS is responsible for the relatively great strength ratio at 1 000 ℃.
In this paper, the dynamic interaction of two parallel cracks in functionally graded materials （FGMs） is investigated by means of the non-local theory. To make the analysis tractable, the shear modulus and the material density are assumed to vary exponentially with the coordinate vertical to the crack. To reduce mathematical difficulties, a one-dimensional non-local kernel is used instead of a twodimensional one for the dynamic problem to obtain stress fields near the crack tips. By use of the Fourier transform, the problem can be solved with the help of two pairs of dual integral equations, in which the unknown variables are the jumps of displacements across the crack surfaces. To solve the dual integral equations, the jumps of displacements across the crack surfaces are expanded in a series of Jacobi polynomials. Unlike the classical elasticity solutions, it is found that no stress singularity is present at the crack tips. The non-local elastic solutions yield a finite hoop stress at the crack tips. The present result provides theoretical references helpful for evaluating relevant strength and preventing material failure of FGMs with initial cracks. The magnitude of the finite stress field depends on relevant parameters, such as the crack length, the distance between two parallel cracks, the parameter describing the FGMs, the frequency of the incident waves and the lattice parameter of materials.
Jun Liang Shiping Wu Shanyi Du Center for Composite Materials and Structure,Harbin Institute of Technology,Harbin 150001,China
The behavior of a Mode-I finite crack in functionally graded materials is investigated using the non-local theory. To make the analysis tractable, it is assumed that the shear modulus varies exponentially with coordinate vertical to the crack. The problem in this paper can be solved through the Fourier transform with the help of two pairs of dual integral equations, in which the unknown variables are jumps of dis- placements across crack surfaces. To solve dual integral equations, the jumps of displacements across crack surfaces are directly expanded in a series of Jacobi polynomials. Unlike the classical elasticity solutions, it is found that no stress singularities are present at crack tips. The non-local elastic solu- tions yield a finite stress at crack tips, thus allowing us to use the maximum stress as a fracture crite- rion. Numerical examples are provided to show the effects of the crack length, the parameter describ- ing the functionally graded materials, the lattice parameter of materials and the materials constants upon the stress fields near crack tips.
LIANG Jun Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150001, China
