Micromechanical model of rough contact between rock blocks with application to wave propagation

This is the published version. Copyright 2008 De Gruyter Open.The relationship between effective stiffness of rough contacts of rock blocks and transmission of plane waves is well known. Effective stiffness of a rough contact may be related to the force-deformation behavior of the asperity contacts and the statistical description of rock joint surface topography through micromechanical methods. In this paper, a micromechanical methodology for computing the overall rock contact effective stiffness is utilized along with the imperfectly bonded interface model to investigate how transmitted and reflected wave amplitudes are affected by the incident wave frequency, rock joint closure and the existing rock joint normal stress conditions. As a result, expressions for reflected and transmitted wave amplitudes as well as group time delay of the wave-packets are obtained and parametrically evaluated

SCANNING ACOUSTIC MICROSCOPY MODELING FOR MICROMECHANICAL MEASUREMENTS OF COMPLEX SUBSTRATES

The Scanning Acoustic Microscope (SAM) is a powerful tool for understanding the mechanical characteristics of substrates with micro-scale near-surface graded layers. To interpret the SAM results from such substrates, a theoretical model was developed that incorporated the interaction of focused ultrasonic field, with a substrate having a near-surface graded layer. The focused ultrasonic field model was formulated in terms of spherical wave expansions. The substrate wave propagation was computed with a multilayered stiffness method. The bridging between the two models was accomplished by utilizing the angular spectrum. A commercial SAM was used to characterize a dentin substrate subjected to acid-etching. Calibration and a homotopic measurement protocol were developed for data accuracy and meaningful data comparison from pre and post etching states. The reflection coefficients from the SAM measurement for the etched dentin exhibited frequency dependent attenuation. The developed theoretical model was successfully applied to explain the observed frequency dependent phenomenon

Scanning Acoustic Microscopy Investigation of Frequency-Dependent Reflectance of Acid-Etched Human Dentin Using Homotopic Measurements

Composite restorations in modern restorative dentistry rely on the bond formed in the adhesive-infiltrated acid-etched dentin. The physical characteristics of etched dentin are, therefore, of paramount interest. However, characterization of the acid-etched zone in its natural state is fraught with problems stemming from a variety of sources including its narrow size, the presence of water, heterogeneity, and spatial scale dependency. We have developed a novel homotopic (same location) measurement methodology utilizing scanning acoustic microscopy (SAM). Homotopic measurements with SAM overcome the problems encountered by other characterization/ imaging methods. These measurements provide us with acoustic reflectance at the same location of both the pre- and post-etched dentin in its natural state. We have applied this methodology for in vitro measurements on dentin samples. Fourier spectra from acid-etched dentin showed amplitude reduction and shifts of the central frequency that were location dependent. Through calibration, the acoustic reflectance of acid-etched dentin was found to have complex and non-monotonic frequency dependence. These data suggest that acid-etching of dentin results in a near-surface graded layer of varying thickness and property gradations. The measurement methodology described in this paper can be applied to systematically characterize mechanical properties of heterogeneous soft layers and interfaces in biological materials

Physico-mechanical properties determination using microscale homotopic measurements: Application to sound and cariesaffected primary tooth dentin

Microscale elastic moduli, composition and density have rarely been determined at the same location for biological materials. In this paper, we have performed homotopic measurements to determine the physico-mechanical properties of a second primary molar specimen exhibiting sound and caries-affected regions. A microscale acoustic impedance map of a section through this sample was acquired using scanning acoustic microscopy (SAM). Scanning electron microscopy was then used to obtain mineral mass fraction of the same section using backscattered images. Careful calibration of each method was performed to reduce system effects and obtain accurate data. Resorption, demineralization and hypermineralization mechanisms were considered in order to derive relationships between measured mineral mass fraction and material mass density. As a result, microscale mass density was determined at the same lateral resolution and location as the SAM data. The mass density and the acoustic impedance were combined to find the microscale elastic modulus and study the relationship between microscale composition and mechanical properties

Viscoelastic and fatigue properties of model methacrylate-based dentin adhesives

The objective of the current study is to characterize the viscoelastic and fatigue properties of model methacrylate-based dentin adhesives under dry and wet conditions. Static, creep, and fatigue tests were performed on cylindrical samples in a 3-point bending clamp. Static results showed that the apparent elastic modulus of the model adhesive varied from 2.56 to 3.53 GPa in the dry condition, and from 1.04 to 1.62 GPa in the wet condition, depending upon the rate of loading. Significant differences were also found for the creep behavior of the model adhesive under dry and wet conditions. A linear viscoelastic model was developed by fitting the adhesive creep behavior. The developed model with 5 Kelvin Voigt elements predicted the apparent elastic moduli measured in the static tests. The model was then utilized to interpret the fatigue test results. It was found that the failure under cyclic loading can be due to creep or fatigue, which has implications for the failure criterion that are applied for these types of tests. Finally, it was found that the adhesive samples tested under dry conditions were more durable than those tested under wet conditions

Fatigue life prediction of dentin-adhesive interface using micromechanical stress analysis

Objectives The objective of this work was to develop a methodology for the prediction of fatigue life of the dentin-adhesive (d-a) interface. Methods At the micro-scale, the d-a interface is composed of dissimilar material components. Under global loading, these components experience different local stress amplitudes. The overall fatigue life of the d-a interface is, therefore, determined by the material component that has the shortest fatigue life under local stresses. Multiple 3d finite element (FE) models were developed to determine the stress distribution within the d-a interface by considering variations in micro-scale geometry, material composition and boundary conditions. The results from these models were analyzed to obtain the local stress concentrations within each d-a interface component. By combining the local stress concentrations and experimentally determined stress versus number of cycle to failure (S-N) curves for the different material components, the overall fatigue life of the d-a interface was predicted. Results The fatigue life was found to be a function of the applied loading amplitude, boundary conditions, microstructure and the mechanical properties of the material components of the d-a interface. In addition, it was found that the overall fatigue life of the d-a interface is not determined by the weakest material component. In many cases, the overall fatigue life was determined by the adhesive although exposed collagen was the weakest material component. Comparison of the predicted results with experimental data from the literature showed both qualitative and quantitative agreement. Significance The methodology developed for fatigue life prediction can provide insight into the mechanisms that control degradation of the bond formed at the d-a interface

Adhesive/Dentin Interface: The Weak Link in the Composite Restoration

Results from clinical studies suggest that more than half of the 166 million dental restorations that were placed in the United States in 2005 were replacements for failed restorations. This emphasis on replacement therapy is expected to grow as dentists use composite as opposed to dental amalgam to restore moderate to large posterior lesions. Composite restorations have higher failure rates, more recurrent caries, and increased frequency of replacement as compared to amalgam. Penetration of bacterial enzymes, oral fluids, and bacteria into the crevices between the tooth and composite undermines the restoration and leads to recurrent decay and premature failure. Under in vivo conditions the bond formed at the adhesive/dentin interface can be the first defense against these noxious, damaging substances. The intent of this article is to review structural aspects of the clinical substrate that impact bond formation at the adhesive/dentin interface; to examine physico-chemical factors that affect the integrity and durability of the adhesive/dentin interfacial bond; and to explore how these factors act synergistically with mechanical forces to undermine the composite restoration. The article will examine the various avenues that have been pursued to address these problems and it will explore how alterations in material chemistry could address the detrimental impact of physico-chemical stresses on the bond formed at the adhesive/dentin interface

Goa, India Micromechanics of Rough Interfaces

ABSTRACT: The mechanical behavior of rough interfaces is modeled from a micromechanical viewpoint. A kinematically driven mechanistic approach is adopted which explicitly considers the interaction of asperities on the fracture surface. The mating asperities are assumed to behave in accordance with contact mechanics postulates of non-conforming bodies. The roughness of the fracture surface is represented via statistical distributions of asperity contact normal and asperity heights. A directional distribution function of asperity contact orientations is introduced recognizing that the asperity contacts are not equally likely in all directions. Both the elastic deformation and frictional sliding under oblique loading are modeled at asperity contacts. Thus, the resultant model naturally accounts for the coupling between normal and shear behavior of rough joints, even though at asperity contact level there is no such coupling. An iterative procedure is implemented to obtain the asperity contact forces at each load increment, recognizing that the asperity contact force distribution is not always known a priori. The derived model is utilized to understand the effect of surface roughness and asperity friction on the initial normal and shear stiffness behavior of rough interfaces. The calculated initial normal and shear stiffness are then used to investigate the plane wave propagation behavior through interfaces.

Wrinkling patterns of electrospun nanofabrics in uniaxial tension

Electrospun nanofabrics are gaining popularity in a variety of applications [1,2]. One such application is the use of nanofabrics, as enhancers of the mechanical performance of fiber-reinforced polymers (FRPs). The enhancement is realized through a multi-scale structure comprising the polymer matrix, the fibers which form the layered macro scale reinforcement and the nano-scale reinforcement introduced as interlayers [3]. Candidate nanofabric systems, were investigated in uniaxial tension in order to evaluate their potential as interlayer reinforcements. This contribution, aims to bring forth wrinkling pattens that were observed in the transverse direction, during the performed tensile strength tests. The tests were performed with a custom-made tensile apparatus which provided force and displacement resolutions of 0.25N of 200 microns. The specimens used for tensile testing were two strips of nanofabric placed back to back, with gauge length between 122-125 mm, width 32 mm and thickness ranging between 9-16 microns per strip, depending on the nanofabric system. The testing was captured on video and the evolution of wrinkling patterns were inferred by video analysis. We find that out of plane wrinkling initiates at small linear strains at the direction normal to the loading axis. In the post-wrinkling stage, mode-jumping is observed with higher frequency wrinkles at lower amplitudes, which manifest beyond the nanofabrics’ yielding stress. Furthermore, wrinkling appears to be heterogeneous, in the sense that there are regions within the specimen that exhibit higher frequency wrinkles than others for the same loading increment, while there exist bands in between them that show none to minimal wrinkling. These phenomena will be juxtaposed to the measured stress-strain curves for a selection of the nanofabrics tested and will be discussed in relationship to existing analytical models [4] and the potential development of further analytical models to describe this response. [1] Z.-M. Huang, Y.-Z. Zhang, M. Kotaki, and S. Ramakrishna, “A review on polymer nanofibers by electrospinning and their applications in nanocomposites,” Compos. Sci. Technol., vol. 63, no. 15, pp. 2223–2253, 2003, doi: https://doi.org/10.1016/S0266-3538(03)00178-7. [2] N. E. Zander, “Hierarchically Structured Electrospun Fibers,” Polymers (Basel)., vol. 5, no. 1, pp. 19–44, 2013, doi: 10.3390/polym5010019. [3] V. Kostopoulos, A. Masouras, A. Baltopoulos, A. Vavouliotis, G. Sotiriadis, and L. Pambaguian, “A critical review of nanotechnologies for composite aerospace structures,” CEAS Sp. J., vol. 9, no. 1, pp. 35–57, 2017, doi: 10.1007/s12567-016-0123-7. [4] E. Cerda and L. Mahadevan, “Geometry and Physics of Wrinkling,” Phys. Rev. Lett., vol. 90, no. 7, p. 74302, Feb. 2003, doi: 10.1103/PhysRevLett.90.07430

Rock-Joint Micromechanics: Relationship of Roughness to Closure and Wave Propagation

The closure behavior of rock joints is intimately related to joint roughness. Here we utilize a micromechanical approach that explicitly considers asperity interactions on joint surfaces to study the rock-joint closure and wave propagation behavior. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. Rock-joint roughness is modeled through distributions of asperity heights and asperity contact orientations. The micromechanical approach developed in this paper establishes the link between the rock-joint closure behavior, the initial overlap of the joints, the asperity height distribution parameters, and the average asperity slope. The model is verified by comparison with experimental measurements. Subsequently a parametric study is performed. The results show that rock joints with the same roughness can exhibit a range of closure behavior depending upon initial overlap and rock intrinsic friction. Therefore, unique descriptions of rock-joint closure behavior are elusive. Finally, the model-predicted nonlinear normal and shear stiffness are used to investigate the reflection and transmission of plane waves at rock joints