A unified phenomenological model for tensile and compressive response of polymeric foams

Tensile and compressive stress-strain responses were obtained for various densities of polymer foams. These experimental data were used to determine relevant engineering parameters (such as elastic moduli in tension and compression, ultimate tensile strength, etc.) as a function of foam density. A phenomenological model applicable for both compressive and tensile responses of polymeric foams is validated by comparing the model to the experimentally obtained compression and tensile responses. The model parameters were analyzed to determine the effect of each parameter on the mechanical response of the foam. The engineering parameters were later compared to the appropriate model parameters and a good correlation was obtained. It was shown that the model indeed captures the entire compressive and tensile response of polymeric foams effectively

Apparatus and method for determining the dynamic indentation hardness of materials

An apparatus and method for determining dynamic indentation hardness values of a material using a propagating stress wave to make an indentation in the material. The invention provides such values without any prior knowledge of the material properties and enables the dynamic indentation hardness values to be directly compared to static indentation hardness values for the material.https://digitalcommons.mtu.edu/patents/1060/thumbnail.jp

Strain-induced formation of carbon and boron clusters in boron carbide during dynamic indentation

The authors found that the level of amorphization or structural disorder in boron carbide is higher when induced by dynamic indentation compared to static indentation. Visible and uv Raman spectroscopies indicate that sp2-bonded aromatic carbon clusters were formed, consistent with the detected photoluminescence spectra. Infrared absorption shows that amorphous boron clusters were created by dynamic indentation which has strain rates ∼108 order higher than that introduced by static indentation. The decreased intensity of infrared stretching mode of carbon-boron-carbon (CBC) chains also suggests that amorphization is due to the collapse of B11C(CBC) unit cells, which reorganize into the energetically favorite carbon and boron clusters

High fidelity frictional models for MEMS.

The primary goals of the present study are to: (1) determine how and why MEMS-scale friction differs from friction on the macro-scale, and (2) to begin to develop a capability to perform finite element simulations of MEMS materials and components that accurately predicts response in the presence of adhesion and friction. Regarding the first goal, a newly developed nanotractor actuator was used to measure friction between molecular monolayer-coated, polysilicon surfaces. Amontons law does indeed apply over a wide range of forces. However, at low loads, which are of relevance to MEMS, there is an important adhesive contribution to the normal load that cannot be neglected. More importantly, we found that at short sliding distances, the concept of a coefficient of friction is not relevant; rather, one must invoke the notion of 'pre-sliding tangential deflections' (PSTD). Results of a simple 2-D model suggests that PSTD is a cascade of small-scale slips with a roughly constant number of contacts equilibrating the applied normal load. Regarding the second goal, an Adhesion Model and a Junction Model have been implemented in PRESTO, Sandia's transient dynamics, finite element code to enable asperity-level simulations. The Junction Model includes a tangential shear traction that opposes the relative tangential motion of contacting surfaces. An atomic force microscope (AFM)-based method was used to measure nano-scale, single asperity friction forces as a function of normal force. This data is used to determine Junction Model parameters. An illustrative simulation demonstrates the use of the Junction Model in conjunction with a mesh generated directly from an atomic force microscope (AFM) image to directly predict frictional response of a sliding asperity. Also with regards to the second goal, grid-level, homogenized models were studied. One would like to perform a finite element analysis of a MEMS component assuming nominally flat surfaces and to include the effect of roughness in such an analysis by using a homogenized contact and friction models. AFM measurements were made to determine statistical information on polysilicon surfaces with different roughnesses, and this data was used as input to a homogenized, multi-asperity contact model (the classical Greenwood and Williamson model). Extensions of the Greenwood and Williamson model are also discussed: one incorporates the effect of adhesion while the other modifies the theory so that it applies to the case of relatively few contacting asperities

The constitutive behavior of refractory metals as a function of strain rate

The constitutive response and deformation mechanisms in refractory metals with a hexagonal close packed (hcp) crystal structure are reviewed. The research focuses on the high-strain-rate stress-strain response under uniaxial compressive loading and the associated microstructural deformation mechanisms. Some aspects relating to dynamic failure such as adiabatic shear banding in refractory metals and subsequent failure processes are also discussed. © 1995 TMS

Mechanics of mixed-mode ductile material removal with a conical tool and the size dependence of the specific energy

The mechanics of material removal during a single-grit rotating scratch has been investigated both analytically and experimentally. The models for cutting, plowing and mixed modes of material removal are analyzed based on the pressure and the frictional resistance. The mixed-mode model takes into account the contribution of built-up edge (BUE) ahead of the tool. To validate the model, single-grit rotating scratch experiments were conducted with a conical diamond tool on pure titanium. It was noticed that the adhesion between the tool and the deformed material, and the hardening properties of material play active roles in the scratching process and provide a driving force to the formation of the BUE. The overall frictional coefficient was found to oscillate strongly on both ends of the scratch but increases steadily over the central span of the scratch length. It is shown that the mixed-mode model captures the salient features of material removal and the size dependence of specific energy during the formation of a rotating scratch. The size dependence of specific energy may be attributed to the size effect of the yield pressure in titanium. © 2002 Elsevier Science Ltd. All rights reserved

Ductile to brittle transition depth during single-grit scratching on alumina ceramics

Variable-depth single-grit scratch experiments have been conducted on three different grain size alumina ceramics. The extent of induced damage as a function of depth of groove was measured. At low depth, the scratch groove appeared smooth with minimal brittle damage, indicating a ductile mode of deformation. With increased depth, brittle cracking extended beyond the scratch groove. The transition depth from the predominantly ductile mode of deformation to the predominantly brittle mode was measured and compared with an analytical model that estimates the plastic zone size surrounding a scratch in brittle materials. It was found that the ductile to brittle transition depth increases with decreasing grain size. © 2007 The American Ceramic Society

Grain size dependence of scratch-induced damage in alumina ceramics

Variable depth single and double scratch experiments were conducted on alumina ceramics of three distinct grain sizes. Utilizing laser profilometry, SEM, and force transducers, an in-depth analysis of volume of material removed, microstructural damage features, and applied force, respectively, was performed as a function of grain size. In single scratches, for a given depth of cut the extent of lateral cracking and material removal volume increased with grain size. In interacting double scratches, a critical separation distance was observed where the extent of damage reached a maximum. For all three grain sizes, the volume of material removed at this critical separation distance reached four times the volume removed by a single scratch. This critical separation distance decreased with decreasing grain size. It was also noticed that the force required to create the second scratch in the vicinity of the first scratch reduced considerably depending on the separation distance. These results are rationalized on the basis of material properties and inherent flaw size. Intergranular fracture and grain dislodgement were found to be the dominant modes of lateral crack propagation. © 2007 Elsevier B.V. All rights reserved

Influence of lateral confinement on dynamic damage evolution during uniaxial compressive response of brittle solids

A dynamic damage growth model applicable to brittle solids subjected to biaxial compressive loading is developed. The model incorporates a dynamic fracture criterion based on wing-crack growth model with a damage evolution theory based on a distribution of pre-existing microcracks in a solid. Influences of lateral confinement pressure (dynamic or static) as well as frictional coefficient on the rate dependence of fracture strength of basalt-rock are investigated systematically. It is found that the failure strength, damage accumulation and wing-crack growth rate are strongly influenced by the nature and the magnitude of confinement pressure. It is also verified that the effect of strain rate on fracture strength of brittle solids is independent of confinement pressure in a certain range of strain rate. © 2003 Elsevier Science Ltd. All rights reserved