Showing papers by "John W. Hutchinson published in 2011"

Periodic patterns and energy states of buckled films on compliant substrates

Abstract: Thin stiff films on compliant elastic substrates subject to equi-biaxial compressive stress states are observed to buckle into various periodic mode patterns including checkerboard, hexagonal and herringbone. An experimental setting in which these modes are observed and evolve is described. The modes are characterized and ranked by the extent to which they reduce the elastic energy of the film–substrate system relative to that of the unbuckled state over a wide range of overstress. A new mode is identified and analyzed having nodal lines coincident with an equilateral triangular pattern. Two methods are employed to ascertain the energy in the buckled state: an analytical upper-bound method and a full numerical analysis. The upper-bound is shown to be reasonably accurate to large levels of overstress. For flat films, except at small states of overstress where the checkerboard is preferred, the herringbone mode has the lowest energy, followed by the checkerboard, with the hexagonal, triangular, and one-dimensional modes lowering the energy the least. At low overstress, the hexagonal mode is observed in the experiments not the square mode. It is proposed that a slight initial curvature of the film may play role in selecting the hexagonal pattern accompanied by a detailed analysis. An intriguing finding is that the hexagonal and triangular modes have the same energy in the buckled state and, moreover, a continuous transition between these modes exists involving a linear combination of the two modes with no change in energy. Experimental observations of various periodic modes are discussed with reference to the energy landscape. Discrepancies between observations and theory are identified and open issues are highlighted.

Lifetime Assessment for Thermal Barrier Coatings: Tests for Measuring Mixed Mode Delamination Toughness

Abstract: Mechanisms leading to degradation of the adherence of thermal barrier coatings (TBC) used in aircraft and power generating turbines are numerous and complex. To date, robust methods for the lifetime assessment of coatings have not emerged based on predictions of the degradation processes due to their complexity. In the absence of mechanism-based predictive models, direct measurement of coating adherence as a function of thermal exposure must be a component of any practical approach toward lifetime assessment. This paper outlines an approach to lifetime assessment of TBC that has taken shape in the past few years. Most TBC delaminations occur under a mix of mode I and mode II cracking conditions, with mode II delamination being particularly relevant. Direct measurement of TBC delamination toughness has been challenging, but recent progress has made this feasible. This paper surveys a range of potentially promising tests for measuring the mode dependence of delamination toughness with particular emphasis on toughness under mode II conditions.

Basic strain gradient plasticity theories with application to constrained film deformation

Abstract: A family of basic rate-independent strain gradient plasticity theories is considered that generalize conventional J2 deformation and flow theories of plasticity to include a dependence on strain gradients in a simple way. The theory builds on three recent developments: the work of Gudmundson (J. Mech. Phys. Solids 52 (2004), 1379‐1406) and Gurtin and Anand (J. Mech. Phys. Solids 57 (2009), 405‐421), proposing constitutive relations for flow theories consistent with requirements of positive plastic dissipation; the work of Fleck and Willis (J. Mech. Phys. Solids 57 (2009), 161‐177 and 1045‐1057), who clarified the structure of the new flow theories and presented the underlying variational formulation; and observations of Evans and Hutchinson (Acta Mater. 57 (2009), 1675‐1688) related to preferences for specific functional compositions of strains and strain gradients. The starting point in this paper is the deformation theory formulation of Fleck and Hutchinson (J. Mech. Phys. Solids 49 (2001), 2245‐2271) which provides the clearest insights into the role of strain gradients and serves as a template for the flow (incremental) theory. The flow theory is constructed such that it coincides with the deformation theory under proportional straining, analogous to the corresponding coincidence in the conventional J2 theories. The generality of proportional straining is demonstrated for pure power-law materials, and the utility of power-law solutions is illustrated for the constrained deformation of thin films: the compression or extension of a finite layer joining rigid platens. Full elastic-plastic solutions are obtained for the same problem based on a finite element method devised for the new class of flow theories. Potential difficulties and open issues associated with the new class of flow theories are identified and discussed.

A Stretch/Bend Method for In Situ Measurement of the Delamination Toughness of Coatings and Films Attached to Substrates

Abstract: Substrates stretch/bend method for the in situ measurement of the delamination toughness of coatings attached to substrates is described. A beam theory analysis is presented that illustrates the main features of the test. The analysis is general and allows for the presence of residual stress. It reveals that the test produces stable extension of delaminations, rendering it suitable for multiple measurements in a single test. It also provides scaling relations and enables estimates of the loads needed to extend delaminations. Finite element calculations reveal that the beam theory solutions are accurate for slender beams, but overestimate the energy release rate for stubbier configurations and short delaminations. The substantial influence of residual stress on the energy release rate and phase angle is highly dependent on parameters such as the thickness and modulus ratio for the two layers. Its effect must be included to obtain viable measurements of toughness. In a companion paper, the method has been applied to a columnar thermal barrier coating deposited onto a Ni-based super-alloy. DOI: 10.1115/1.4001938