In this work, quasistatic crack initiation under mixed mode loading in planar (two-dimensional plane stress) functionally graded materials (FGMs) is studied. The goal of this work is to directly compare experiments and simulations so as to evaluate the applicability of the maximum tangential stress (MTS) criterion in predicting crack kinking in FGMs. Initially, crack initiation in the homogeneous material, which forms the basis of our FGM—polyethylene—is studied. The (generalized) maximum tangential stress is applied through the use of finite elements to determine crack initiation angles in the same graded configurations studied experimentally. Computational results of fracture parameters (stress intensity factors and T-stress), and crack initiation angles are compared to experimental results and good agreement is obtained. It is seen that the MTS criterion is applicable to FGM crack initiation prediction if the inherent material gradient length scale is larger than the fracture process zone.

In this study, the stress distribution in a nonhomogeneous anisotropic cylindrical body is investigated. Using equilibrium equations, HookeÕs law and strain-displacement relations, a system of equations is obtained in cylindrical coordinates in terms of stress potentials where elastic properties change in radial direction. YoungÕs and shear moduli are expressed as power functions of r and PoissonÕs ratios are kept constant. Closed-form solutions for stress potentials and stress distribution are obtained for an axisymmetric, orthotropic cylinder. Results are checked with FE results. A pressurized thick walled cylinder example is studied in details. Stresses in radial, tangential and axial directions and Von Mises stresses are plotted for different powers of r.

In polycrystalline metals, microstructural features such as grain boundaries (GBs) influence fatigue crack initiation. Stress and strain heterogeneities, which arise in the vicinity of GBs, can promote the nucleation of fatigue cracks. Because of variations in grain size and GB types, and consequently variations in the local deformation response, scatter in fatigue life is expected. A deeper quantitative understanding of the early stages of fatigue crack nucleation and the scatter in life requires experimental and modelling work at appropriate length scales. In this work, experiments are conducted on Hastelloy X under fatigue conditions, and observations of fatigue damage are reported in conjunction with measurements of local strains using digital image correlation. We use a recent novel fatigue model based on persistent slip band–GB interaction to investigate the scatter in fatigue lives and shed light into the critical types of GBs that nucleate cracks. Experimental tools and methodologies, utilizing ex situ digital image correlation and electron backscatter diffraction, for high resolution deformation measurements at the grain level are also discussed in this paper and related to the simulations.

In this work, the GursonTvergaardNeedleman model, commonly used for metallic materials, is applied to the failure of a polymeric material specifically a polyethylene carbon monoxide copolymer, which is an enhanced photodegradable material. GursonTvergaardNeedleman model parameters for this material are obtained using the NelderMead simplex method when correlating experimental and numerical results of both tensile and fracture specimens. Results show that the GursonTvergaardNeedleman model can also be used for polymeric materials with selection of proper parameters that are quite different from the ones proposed for metallic materials.

This study presents a novel computational framework to improve life prediction capabilities for hypersonic aerospace platforms where evaluating the performance of these structures in extreme environments remains a challenge. Here, thermo-mechanical loading histories determined from analyses that couple aerodynamic loads and structural deflections drive high-fidelity continuum models of the structural member, and results from the continuum model in turn drive critical-plane models of fatigue crack nucleation and growth. This approach readily enables complex features of the loading, geometry, and material response to be incorporated by the structural response and life predictions. Results shown here demonstrate the capabilities of this framework, including: representative thermomechanical and acoustic loadings from ascent to cruise conditions through to descent, the full structural response history, and damage indications that incorporate the full thermocyclic history. Preliminary studies on a challenge structural panel indicate that the ascent and descent phases of the flight profile represent the primary drivers for large residual deflections (on the order of the skin thickness), that remain present in subsequent flights and that may degrade aerodynamic and structural performance. Furthermore, the highly cyclic and transient response present in the acoustic loading phase contributes strongly to localized fatigue damage. Nomenclature b= compressibility exponent C p = pressure coefficient F c = compressible flow transformation function H = enthalpy k = parameter representing compressibility and heat transfer effects M = Mach number m = viscosity power law exponent, η/η e = (T /T e ) m n = velocity power law exponent, U/U e = (y/δ) (1/n) P r = Prandtl number p = pressurẽ p = rms fluctuating pressure q = ρU 2 /2, dynamic pressure r = P r 1/3 , turbulent flow recovery factor T = temperature U = velocity y = normal distance into boundary layer from wall γ = ratio of specific heats δ = boundary layer thickness δ 1 = boundary layer displacement thickness η = viscosity λ = viscous/velocity power law exponent ρ = density φ(ω) = power spectral density ω = frequency, rad/s Subscripts a = attached flow aw = adiabatic wall e = edge of boundary layer s = separated flow w = wall Superscripts * = reference enthalpy condition