TRANSIENT THERMOELASTIC FRACTURE OF INTERFACE CRACKS

Abstract

The transient behavior of an interface crack at the center and edge of two finite dissimilar materials free to bend and subjected to a transient thermal load was studied. It was first assumed that the crack was insulated. The effect of allowing heat to conduct through the crack upon closing was also investigated. The effects of the mechanical and thermal material property ratios as well as the thickness ratio on the crack deformations and the transient strain energy release rate were calculated.     

A NUMERICAL STUDY ON THERMAL FRACTURE OF PRECRACKED CERAMIC COATINGSIN HIGH-HEAT-FLUX ENVIRONMENT

The thermomechanical fracture and interface delamination of thermal barrier coatings (TBCs) in a high-heat-flux environment is the result of large surface temperature and thermal gradient across the coating thickness and the resulting viscoplastic deformations induced in the ceramic material. The maximum coating surface temperature has been used as the key loading parameter in previous studies. The current study explores the effects of several other thermal loading parameters on thermomechanical response and fracture behavior of TBCs with or without preexisting surface cracks. Results show that for a constant maximum surface temperature, the thermal fracture of the coating is increased by (i) an increased temperature difference across the coating, (ii) longer heating duration, and (iii) more aggressive coating surface cooling after heating. These results provide insights into TBC thermal fracture mechanisms and can potentially improve the design of the morphology of preexisting cracks in the coating to reduce fatal interface fracture.     

Influence of Abrasive Properties on Residual Stresses in Lapped Ferrite and Alumina

The effect of abrasive-particle properties on surface-finishing residual stresses, surface finish, and material removal rates during the lapping of ferrite and aluminium oxide is quantified experimentally. It is shown that lapping with softer abrasives and smaller particles results in lower compressive residual stresses near the surface and improved surface finish. These results demonstrate possible methods for controlling surface finishing residual stresses in ceramics while at the same time achieving dimensional accuracy.     

Nucleation and Expansion During Extrusion and Microwave Heating of Cereal Foods

Expansion of biopolymer matrices is the basis for the production of a wide variety of cereal foods. A limited number of manufacturing processes provide practical solutions for the development of an impressive variety of expanded products, just by changing process variables. It is therefore essential that the mechanisms involved in expansion are well known and controlled. This paper summarizes the knowledge of nucleation and expansion in extruded and microwaved products available to date. The effect of processing conditions and properties of the biopolymeric matrix on nucleation and expansion are discussed. Moisture content enables the glassy polymeric matrix to turn into rubbery state at process temperatures, which allows superheated steam bubbles to form at nuclei and then expand, expansion being governed by the biaxial extensional viscosity of the matrix. Nucleation and expansion theories are presented along with quantitative data that support them.     

Thermal fracture mechanisms in ceramic thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) represent an attractive method of increasing the high-tempera-ture limits for systems such as diesel engines, gas turbines, and aircraft engines. However, the dissimilari-ties between ceramics and metal, as well as the severe temperature gradients applied in such systems, cause thermal stresses that can lead to cracking and ultimately spalling of the coating. This paper reviews the research that has considered initiation of surface cracks, initiation of interfacial edge cracks, and the effect of a transient thermal load on interface cracks. The results of controlled experiments are presented together with analytical models. The implications of these findings to the differences between diesel en-gines and gas turbines are discussed. The importance of such work for determining the proper design cri-teria for TBCs is underlined.     

Comparison of observed rheological properties of hard wheat flour dough with predictions of the Giesekus-Leonov,White-Metzner and Phan-Thien Tanner models

The measured rheological behavior of hard wheat flour dough was predicted using three nonlinear differential viscoelastic models. The Phan-Thien Tanner model gave good zero shear viscosity prediction, but overpredicted the shear viscosity at higher shear rates and the transient and extensional properties. The Giesekus-Leonov model gave similar predictions to the Phan-Thien Tanner model, but the extensional viscosity prediction showed extension thickening. Using high values of the mobility factor, extension thinning behavior was observed but the predictions were not satisfactory. The White-Metzner model gave good predictions of the steady shear viscosity and the first normal stress coefficient but it was unable to predict the uniaxial extensional viscosity as it exhibited asymptotic behavior in the tested extensional rates. It also predicted the transient shear properties with moderate accuracy in the transient phase, but very well at higher times, compared to the Phan-Thien Tanner model and the Giesekus-Leonov model. None of the models predicted all observed data consistently well. Overall the White-Metzner model appeared to make the best predictions of all the observed data.     

Effects of Desolvating Agent Types,Ratios, and Temperature on Size and Nanostructure of Nanoparticles from α‐Lactalbumin and Ovalbumin

In this study, we compare the preparation of ovalbumin (OVA) and α‐lactalbumin (α‐LA) nanoparticles using different desolvating agents (ethanol, acetone, and methanol) and water: desolvating agent volume ratios (1:3, 1:4, 1:5, 1:10, and 1:20). Also the effects of protein solution temperature (25, 50, and 80 ℃) on the size of nanoparticles and the stability of crosslinked nanoparticles for 30 d were studied. OVA and α‐LA were shown to be good candidates for nanoparticulation and nanoparticles in the range of 60 to 230 nm were obtained. The comparison between the 2 proteins offers guidance to optimize OVA and α‐LA nanoparticle fabrication and to efficiently obtain nanoparticles with desired characteristics. The particle sizes of OVA nanoparticles were found to be in the range of 60 to 160 nm, and the particle sizes of α‐LA were between 150 and 230 nm. The sizes varied with different desolvating agents: for OVA, ethanol, and methanol both produced nanoparticles smaller than 100 nm; for α‐LA, methanol produced the smallest nanoparticles. Water: desolvating agent ratios, in the studied range, did not show a significant effect on the particle sizes for both OVA and α‐LA nanoparticles. The size and morphology of the nanoparticles were found to change when the protein solutions were heated up to 50 and 80 ℃ and cooled down before nanoparticulation and most nanoparticles had a smaller diameter.     

Synthesis and properties of corn zein/montmorillonite nanocomposite films

The objectives of this study were to apply fabrication techniques for the zein montmorillonite (MMT) nanocomposite films and characterize the obtained nanocomposite films. Zein MMT nanocomposite films were successfully produced from solvent casting and blown extrusion methods. The two methods could mix the zein MMT resulting in partially exfoliated nanocomposite structures according to X-ray diffraction and transmission electron microscopy. The thermal resistant of the zein nanocomposite films fabricated from both methods improved as the MMT content increased. However, the mechanical and barrier properties showed non-linear relationships with the MMT loadings. The impact of MMT on properties of zein films strongly depended on the preparation techniques. This can be the good starting point to further study in depth insight of the controllable MMT rearrangement in zein films which will remarkably improve zein film properties for packaging applications.     

Free edge thermal stress singularities in finite concentric cylinders

The thermal stresses at the free edge of finite cylinders made of dissimilar materials are analyzed using the finite element method. The results of the analysis show that the radial stress and the hoop stress exhibit a logarithmic singularity. The region of influence of the singularity is determined to be a small one.     

Small intestinal submucosa as a vascular graft: a review

Continuing investigations of vascular graft materials suggest that unacceptable graft complications continue and that the ideal graft material has not yet been found. We have developed and tested a biologic vascular graft material, small intestine submucosa (SIS), in normal dogs. This material, when used as an autograft, allograft, or xenograft has demonstrated biocompatibility and high patency rates in aorta, carotid and femoral arteries, and superior vena cava locations. The grafts are completely endothelialized at 28 days post-implantation. At 90 days, the grafts are histologically similar to normal arteries and veins and contain a smooth muscle media and a dense fibrous connective tissue adventitia. Follow-up periods of up to 5 years found no evidence of infection, intimal hyperplasia, or aneurysmal dilation. One infection-challenge study suggested that SIS may be infection resistant, possibly because of early capillary penetration of the SIS (2 to 4 days after implantation) and delivery of body defenses to the local site. We conclude that SIS is a suitable blood interface material and is worthy of continued investigation. It may serve as a structural framework for the application of tissue engineering technologies in the development of the elusive ideal vascular graft material.