Showing papers by "John W. Hutchinson published in 2002"

The characterization of telephone cord buckling of compressed thin films on substrates

Abstract: The topology of the telephone cord buckling of compressed diamond-like carbon (lms (DLC) on glass substrates has been characterized with atomic force microscopy (AFM) and with the focused ion beam (FIB) imaging system. The pro(les of the several buckles have been measured by AFM to establish the symmetry of each repeat unit, revealing similarity with a circular buckle pinned at its center. By making parallel cuts through the buckle in small, de(ned locations, straight-sided buckles have been created on the identical (lms, enabling the residual stress in the (lm to be determined from the pro(le. It has been shown that the telephone cord topology can be e;ectively modeled as a series of pinned circular buckles along its length, with an unpinned circular buckle at its front. The unit segment comprises a section of a full circular buckle, pinned to the substrate at its center. The model is validated by comparing radial pro(les measured for the telephone cord with those calculated for the pinned buckle, upon using the residual stress in the (lm, determined as above. Once validated, the model has been used to determine the energy release rate and mode mixity, G( ). The results for G( ) indicate that the telephone cord con(guration is preferred when the residual stress in the DLC is large, consistent with observations that straight-sided buckles are rarely observed, and, when they occur, are generally narrower than telephone cords. Telephone cords are observed in many systems, and can be regarded as the generic morphology. Nevertheless, they exist subject to a limited set of conditions, residing within the margin between complete adherence and complete delamination, provided that the interface has a mode II toughness low

Influence of substrate compliance on buckling delamination of thin films

Abstract: A thin film subject to in-plane compressive stress is susceptible to buckling-driven delamination. This paper analyzes a straight-sided delamination buckle with a focus on the effects of substrate compliance, following earlier work by B. Cotterell and Z. Chen. The critical buckling condition, the energy release rate and the mode mix of the interface delamination crack are calculated as a function of the elastic mismatch between the film and substrate. The average energy release rate at the curved end of a tunneling straight-sided blister is also determined. The more compliant the substrate, the easier for the film to buckle and the higher the energy release rates. The effect becomes significant when the modulus of the substrate is appreciably less than that of the film. When the substrate modulus is comparable to that of the film, or higher, the usual assumption is justified to the effect that the film is clamped along its edges. When the substrate is very compliant the energy release rate at the curved front exceeds that along the straight sides.

On the delamination of thermal barrier coatings in a thermal gradient

Abstract: Thermal barrier coating (TBC) systems are susceptible to delamination failures in the presence of a large thermal gradient. These failures, which occur within the TBC layer, are very different in character from those associated with the thermally grown oxide. Three possible causes of internal delamination are analyzed. In all cases, the thermomechanical properties of the TBC are allowed to vary because of sintering. (a) One mechanism relates to exfoliation of an internal separation in the TBC due to a through thickness heat flux. (b) Another is concerned with edge-related delamination within a thermal gradient. (c) The third is a consequence of sintering-induced stresses. The results of these analyses, when used in combination with available properties for the TBC, strongly suggest that the second mechanism (b) predominates in all reasonable scenarios. Consequences for the avoidance of this failure mode are discussed.

A Fundamental Model of Cyclic Instabilities in Thermal Barrier Systems

Abstract: Cyclic morphological instabilities in the thermally grown oxide (TGO) represent a source of failure in some thermal barrier systems. Observations and simulations have indicated that several factors interact to cause these instabilities to propagate: (i) thermal cycling; (ii) thermal expansion misfit; (iii) oxidation strain; (iv) yielding in the TGO and the bond coat; and (v) initial geometric imperfections. This study explores a fundamental understanding of the propagation phenomenon by devising a spherically symmetric model that can be solved analytically. The applicability of this model is addressed through comparison with simulations conducted for representative geometric imperfections and by analogy with the elastic/plastic indentation of a half space. Finite element analysis is used to confirm and extend the model. The analysis identifies the dependencies of the instability on the thermo-mechanical properties of the system. The crucial role of the in-plane growth strain is substantiated, as well as the requirement for bond coat yielding. It is demonstrated that yielding of the TGO is essential and is, in fact, the phenomenon that differentiates between cyclic and isothermal responses.

Particle impact on metal substrates with application to foreign object damage to aircraft engines

Abstract: Foreign object damage (FOD) occurs when hard, millimeter-sized objects such as gravel or sand are ingested into aircraft jet engines Particles impacting turbine blades at velocities up to about 300 m=s produce small indentation craters which can become sites for fatigue crack initiation, severely limiting the lifetime of the blade A framework for analyzing FOD and its e5ect on fatigue cracking is established in this paper Finite element analysis is used to determine the residual stresses and geometric stress concentration resulting from FOD The roles of material rate sensitivity and inertia are delineated The most important non-dimensional parameters governing impact indents are identi7ed, signi7cantly reducing the set of independent parameters The second step in the analysis focuses on the potency of cracks emerging from critical locations at the indents The results have been used to address the question: When and to what extent do the residual stresses and stress concentration caused by FOD reduce the critical crack size associated with threshold fatigue crack growth? For deep indents, it is found that elastic stress concentration is the dominant factor in reducing critical crack threshold when the applied cyclic load ratio, R, is large, otherwise the residual stresses are also important Comparisons with a set of experiments conducted in parallel with the theory show that the numerical approach can account for various phenomena observed in practice ? 2002 Elsevier Science Ltd All rights reserved

Mechanical relaxation of localized residual stresses associated with foreign object damage

Abstract: Foreign-object damage associated with the ingestion of debris into aircraft turbine engines can lead to a marked degradation in the high-cycle fatigue life of turbine components. This degradation is generally considered to be associated with the premature initiation of fatigue cracks at or near the damage sites; this is suspected to be due to, at least in part, the impact-induced residual stress state, which can be strongly tensile in these locations.

Are Lower-Order Gradient Theories of Plasticity Really Lower Order?

Abstract: An explicit example of one-dimensional shearing is used to illustrate the necessity of extra boundary conditions for a class of incremental theories of plasticity regarded as otherwise conventional apart from a dependence of the tangential moduli on gradients of plastic strain. ©2002 ASME

Simulation of indentation fracture in crystalline materials using mesoscale self-assembly.

Abstract: A new physical model based on mesoscale self-assembly is developed to simulate indentation fracture in crystalline materials. Millimeter-scale hexagonal objects exhibiting atom-like potential functions were designed and allowed to self-assemble into two-dimensional (2D) aggregates at the interface between water and perfluorodecalin. Indentation experiments were performed on these aggregates, and the stresses and strains involved in these processes were evaluated. The stress field in the aggregates was analyzed theoretically using the 2D elastic Hertz solution. Comparison of the experimental results with theoretical analysis revealed that fracture develops in regions subjected to high shear stress and some, albeit low, tensile stress. The potential for the broader application of the model is illustrated using indentation of assemblies with point defects and adatoms introduced at predetermined locations, and using a two-phase aggregate simulating a compliant film on a stiff substrate.

Mechanical relaxation of localized residual stresses associatedwith foreign object damage

Abstract: Foreign-object damage associated with the ingestion ofdebris into aircraft turbine engines can lead to a marked degradation inthe high-cycle fatigue life of turbine components. This degradation isgenerally considered to be associated with the premature initiation offatigue cracks at or near the damage sites; this is suspected to be dueto, at least in part, the impact-induced residual stress state, which canbe strongly tensile in these locations.

Annoucement of a round robin on the analysis of the peel test

Abstract: There is some controversy over the analysis of the peel test when there is significant plastic deformation. To resolve this controversy a round robin on its analysis is announced contributions to which are invited. Details are given of test cases to be analysed.