Hochaufl?sende NMR an L?sungen mit Nitroxidradikalkomplexen

Ohne Zusammenfassung     

Influence of late-breaking ligaments on crack propagation in compact specimens—A photoelastic study

Dynamic photoelasticity and high-speed photography were utilized to study the influence of tough ligaments on crack propagation in compact specimens. The data obtained during the fracture experiments were analyzed to obtain crack velocity, instantaneous stress-intensity factor and the energy losses away from the crack tip. The results showed that the ligaments can behave as crack arrestors or decelerators. The crack if not arrested at the ligament slowly tunnels in between the ligaments without immediately rupturing them. The ligaments also tend to increase damping losses in the specimens.     

A critical review of methods for determining stress-intensity factors from isochromatic fringes

Two-parameter methods of fracture analysis for determining the stress-intensity factor from photoelastic isochromatic-fringe data were critically reviewed. The methods of Irwin, Bradley and Kobayashi, and Smith were developed in detail and differences in the three approaches were noted. Theoretical fringe loops were generated for a crack of length 2a in a semi-infinite plate with biaxial loading. These fringe loops were used to compare the three analysis methods and to determine the accuracy of each method. All three methods give a close estimate of the stress-intensity factor, with the Bradley-Kobayashi differencing procedure providing the most precise estimate ofK. However, if measurement errors become excessive (larger than 2 percent) the differencing procedure magnifies these errors and the original method proposed by Irwin is the recommended approach. The two-parameter methods can be employed to determineK to within ±5 percent, provided the angle of tilt of the isochromatic-fringe loop is 73 ≤θ _m < 139 deg. Ifθ _m is outside this range, the two-parameter methods should not be employed.     

Large-Eddy Simulation of a Jet-in-Hot-Coflow Burner Operating in the Oxygen-Diluted Combustion Regime

Large-eddy simulations of moderate and intense low-oxygen dilution (MILD) combustion of a jet-in-hot-coflow (JHC) burner are performed. This burner configuration consists of three streams, providing fuel, oxygen-diluted coflow, and air to the burner. To account for the mixing between the three reactant streams, a three-stream flamelet/progress variable (FPV) formulation is utilized. This model was previously applied to a condition corresponding to the upper range of MILD-combustion, and the objective of this contribution is to further investigate this model in application to highly diluted operating conditions. Comparisons of mean and conditional results show that the model accurately captures effects of increasing oxygen-depletion on the flame-structure and heat-release, and predictions for temperature and species mass fractions are in overall good agreement with experiments.     

An analytic separation method for photoelasticity

A method for separating principal stresses in photoelasticity is presented. This method is based upon the series solution of Laplace's equation and the determination of the unknown coefficients arising in this series by a least-squares numerical technique. By selecting an adequate number of terms in the series, the representation of the boundary values of the first stress invariant can be established as accurately as the initial photoelastic data. This form of representation of the first stress invariant at interior points in the region is moe accurate than the boundary values employed.     

Determination of mixed-mode stress-intensity factors K I and K II for a subsurface crack near a concentrated force

Two-dimensional photoelastic experiments were conducted to obtain isochromatic fringe fields in the region between a crack tip and a concentrated load on the boundary of a half plane. An analysis method is developed to determine the mixed-mode stress-intensity factors due to two interacting stress singularities. The results obtained showed the dominance of the opening mode in extending a delamination crack by sliding surface loads. Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 28–June 1.     

Crack arrest in duplex specimens

Dynamic photoelasticity with high-speed photographic recording is employed to study crack arrest and reinitiation in duplex specimens. The starter section of the duplex specimen is fabricated from Homalite 100, a brittle polyester and the arrest section is fabricated from a relatively tough epoxy. The two materials are joined with a tough, high shear-strength adhesive. The crack was observed to arrest abruptly with very short penetration into the adhesive layer. Crack tip decelerations of the order of 4 × 10⁷ g's were estimated. The fracture affected zone of the arrested crack exhibited a diameter of about 0.5 mm. After arrest the stress intensity factor K increases rapidly with time as the kinetic energy in the modified compact tension specimen is converted to strain energy. If K achieves a sufficiently high value, the crack will reinitiate in the adhesive and extend into the arrest section of the duplex specimen. Otherwise, the crack remains at arrest and the K field at the crack tip oscillates.