Nanomechanical characterization of asphalt

In this study, asphalt binder and asphalt concrete (AC) materials are characterized using laboratory nanoindentation testing and mechanical models. Traditionally, laboratory nanoindentation test data is analyzed using the Oliver-Pharr method to determine elastic modulus and hardness of materials. In a nanoindentation test, a test sample surface is indented or loaded by a hard indenter tip and then unloaded. In the past, several studies of the polymer materials area have selected a loading rate and dwell time (i.e., the peak load is kept constant for a few seconds before unloading) to avoid or minimize the viscous effect of a material. No studies have attempted to examine the effects of dwell time and loading rate on viscous materials such as asphalt binder, which is the main topic of discussion in this study. This study focuses on determination of mechanical properties such as the elastic modulus and the hardness of viscoelastic materials, like asphalt, from nanoindentation load-displacement data. An existing spring-dashpot-rigid (SDR) element model developed by Oyen and Cook is employed as well as the well-established Oliver Pharr method. The SDR model uses the loading, holding and unloading time-displacement data to predict the modulus, hardness and viscosity of the material. The model has shown excellent agreement with the laboratory indentation data of asphalt binder. Further, the SDR model is calibrated for nanoindentation test data of polymer modified asphalt binder. In addition, mechanical models such as the Voigt model and the Burger model are fitted to creep displacement and time data from nanoindentation tests to predict viscosity, retardation time and creep compliance for asphalt binder. All the models are found to fit very well with an average R2-value of 0.99 for the Voigt model and R2-value of 0.99 for the Burger model. Lastly, the nanoindentation test is performed on an AC (solid) sample to understand the aging in AC. Nanoindentation is done on two different parts of the AC sample: one on the mastic part (mix of asphalt binder and fines) and the other on the pure aggregate part. One hundred indentations were made in a single test on the mastic part to capture the heterogeneity. Approximately sixty indentations were made during a single indentation test on the aggregate part of AC. A small dwell time was applied to reduce the viscous effect of the mastic

Effect of Poisson’s Ratio on Material Properties Characterization by Nanoindentation with a Cylindrical Flat Tip Indenter

Nano indentation technology is commonly used to determine the mechanical properties of different kinds of engineering materials. The young’s modulus of the materials can be calculated with the load depth data obtained from an indentation test with a known Poisson’s ratio. In this investigation the NANOVEA micro/nanoindentation tester with a cylindrical flat-tip indenter will be used to find the elastic modulus, hardness and Pile up. Low carbon steel AISI1018, alloy steel AISI 4340 and aluminum alloy 6061 were selected for the case study. Finite element (FE) analysis using axisymmetric 3-D models used to establish the relationship between Poisson’s ratio and the deformation of indentation / materials strain hardening exponential index with a cylindrical flat tip indenter. The modeling was done by considering the Poisson ratio ranging from 0 to 0.48 in order to find the influences of Poisson ratio on the elasticplastic properties was verified by associated experimental results. From the modeling results, it was found the indentation depth has very little effect on calculating the elastic modulus of the sample material in the same Poisson ratio and the hardness slowly increases with the increase of maximum indentation depth as well as increase the Poisson ratio. The maximum pile up value for the three materials decreases with the decrease of Poisson ratio that was very sensitive

Instrumented Nanoindentation Studies Of Deformation In Shape Memory Alloys

Near equi-atomic nickel titanium (NiTi) shape memory alloys (SMAs) are a class of materials characterized by their unique deformation behavior. In these alloys, deformation mechanisms such as mechanical twinning and stress induced phase transformation between a high symmetry phase (austenite) and a low symmetry phase (martensite) additionally occur and influence mechanical behavior and thus their functionality. Consequently, applications of SMAs usually call for precise phase transformation temperatures, which depend on the thermomechanical history and the composition of the alloy. Instrumented indentation, inherently a mechanical characterization technique for small sampling volumes, offers a cost effective means of empirically testing SMAs in the form of centimeter scaled buttons prior to large-scale production. Additionally, it is an effective probe for intricate SMA geometries (e.g., in medical stents, valves etc.), not immediately amenable to conventional mechanical testing. The objective of this work was to study the deformation behavior of NiTi SMAs using instrumented indentation. This involved devising compliance calibration techniques to account for instrument deformation and designing spherical diamond indenters. Substantial quantitative information related to the deformation behavior of the shape memory and superelastic NiTi was obtained for the first time, as opposed to existing qualitative indentation studies. For the case of shape memory NiTi, the elastic modulus of the B19\u27 martensite prior to twinning was determined using spherical indentation to be about 101 GPa, which was comparable to the value from neutron diffraction and was substantially higher than typical values reported from extensometry (68 GPa in this case). Twinning at low stresses was observed from neutron diffraction measurements and was attributed to reducing the elastic modulus estimated by extensometry. The onset of predominantly elastic deformation of the twinned martensite was identified from the nanoindentation response and the elastic modulus of the twinned martensite was estimated to be about 17 GPa. Finite element modeling was used to validate the measurements. For the case of the superelastic NiTi, the elastic modulus of the parent austenite was estimated to be about 62 GPa. The onset of large-scale stress induced martensite transformation and its subsequent elastic deformation were identified from the nanoindentation response. The effect of cycling on the mechanical behavior of the NiTi specimen was studied by repeatedly indenting at the same location. An increase in the elastic modulus value for the austenite and a decrease in the associated hysteresis and residual depth after the initial few cycles followed by stabilization were observed. As for the case of shape memory NiTi, finite element modeling was used to validate the measurements. This work has initiated a methodology for the quantitative evaluation of shape memory and superelastic NiTi alloys with instrumented spherical indentation. The aforementioned results have immediate implications for optimizing thermomechanical processing parameters in prototype button melts and for the mechanical characterization of intricate SMA geometries (e.g., in medical stents, valves etc.) This work was made possible by grants from NASA (NAG3-2751) and NSF (CAREER DMR-0239512) to UCF

ELASTIC-PLASTIC INDENTATION DEFORMATION IN HOMOGENEOUS AND LAYERED MATERIALS: FINITE ELEMENT ANALYSIS

The complex phenomenon of indentation deformation is studied using finite element analysis for both homogeneous and layered materials. For the homogeneous materials, the elastic-plastic deformation at large indentation depth is studied. The variation of the load-displacement curves as well as the variation of the energy ratio with the applied indentation depth for different strain hardening indices is presented. The power law relation between the indentation load and depth for shallow indentation becomes invalid for deep indentation. The ratio of plastic energy to total mechanical work is a linear function of the ratio of residual indentation depth and maximum indentation depth. For the layered materials (film-substrate systems), the elastic deformation under an indenter is studied. Various material parameters are investigated, including film thickness and modulus. A generalized power law equation is presented for characterizing the indentation load-displacement responses of film-substrate structures

Nanoindentation testing of soft polymers : computation, experiments and parameters identification

Since nanoindentation technique is able to measure the mechanical properties of extremely thin layers and small volumes with high resolution, it also became one of the important testing techniques for thin polymer layers and coatings. This dissertation is focusing on the characterization of polymers using nanoindentation, which is dealt with numerical computation, experiments and parameters identification. An analysis procedure is developed with the FEM based inverse method to evaluate the hyperelasticity and time-dependent properties. This procedure is firstly verified with a parameters re-identification concept. An important issue in this dissertation is to take the error contributions in real nanoindentation experiments into account. Therefore, the effects of surface roughness, adhesion force, friction and the real shape of the tip are involved in the numerical model to minimize the systematic error between the experimental responses and the numerical predictions. The effects are quantified as functions or models with corresponding parameters to be identified. Finally, data from uniaxial or biaxial tensile tests and macroindentation tests are taken into account. The comparison of these different loading situations provides a validation of the proposed material model and a deep insight into nanoindentation of polymers.Da Nanoindentation die Messung der mechanischen Eigenschaften von dünnen Schichten und kleinen Volumen mit hoher Auflösung ermöglicht, hat sich diese Messmethode zu einer der wichtigsten Testmethoden für dünne Polymerschichten und -beschichtungen entwickelt. Diese Dissertation konzentriert sich auf die Charakterisierung von Polymeren mittels Nanoindentation, die in Form von numerischen Berechnungen, Experimenten und Parameteridentifikationen behandelt wird. Es wurde ein Auswertungsverfahren mit einer FEM basierten inversen Methode zur Berechnung der Hyperelastizität und der zeitabhängigen Eigenschaften entwickelt. Dieses Verfahren wird zunächst mit einem Konzept der Parameter Re-Identifikation verifiziert. Fehlerquellen wie Oberflächenrauheit, Adhäsionskräfte, Reibung und die tatsächlichen Form der Indenterspitze werden in das numerische Modell eingebunden, um die Abweichungen der numerischen Vorhersagen von den experimentellen Ergebnissen zu minimieren. Diese Einflüsse werden als Funktionen oder Modelle mit dazugehörigen, zu identifizierenden Parametern, quantifiziert. Abschließend werden Messwerte aus uni- oder biaxialen Zugversuchen und Makroindentationsversuchen betrachtet. Der Vergleich dieser verschiedenen Belastungszustände liefert eine Bestätigung des vorgeschlagenen Materialmodells und verschafft einen tieferen Einblick in die bei der Nanoindentation von Polymeren ablaufenden Mechanismen

Estudio experimental y simulación numérica de las medidas de microdureza en aleaciones Al-Fe en diferentes velocidades de barridos con rayo láser

In the Al–2.0 wt.%Fe alloy the laser surface remelting (LSR) treatment was executed to investigate the treated and untreated layers areas, at different laser beam scanning, among them, 80, 100 and 120 mm/s, to respect, was presented and discussed about microstructural characteristics using the FEG and EDS techniques, and numerical experiments of pyramidal indentations of the LSR-treated systems were conducted using the FEM method. In the sample-treated cross-sectional area, the microstructure presented a columnar growth characteristic, a lot of nano-porosities and large size of the molten pool geometry in low laser beam scanning, however, in high laser beam scanning, the microstructure consisted of a cellular arrangement or fine-grained microstructure, the nano-porosities concentration and the molten pool geometry are slightly decreased. Besides, the micro-hardness in the LSR-treated area increased slightly as a function of increase of the laser beam scanning, but, the micro-hardness was much higher than the untreated sample. Meanwhile, modeling of indentation on COMSOL of the LSR-treatment by finite element method of the micro-hardness was successfully calculated. Therefore, a good agreement was found between experimental and simulated data.En la aleación de Al-2,0 % Fe se realizó el tratamiento de refundición superficial con láser (RSL) para investigar las muestras con capas tratadas y no tratadas con diferentes velocidades de barrido con rayos láser, entre ellas, 80, 100 y 120 mm/s, respectivamente. En este artículo se presentaron y discutieron las características microestructurales utilizando las técnicas FEG y EDS. Además se llevaron a cabo experimentos y de simulación numérica por MEF de las indentaciones piramidales de la superficie tratados y no tratados con LSR. En la sección transversal de la muestra tratada con barrido lento de rayos láser y, específicamente en la geometría de la piscina fundida, la microestructura presentaba características de crecimiento columnar y también muestra muchas nanoporosidades. Sin embargo, con barrido de rayo láser alto, la microestructura muestra una disposición celular con grano fino, no obstante, la concentración de nanoporosidades y el tamaño de la geometría de la piscina fundida se redujeron ligeramente. Además, las medidas de la microdureza en la zona tratada con RSL aumentó ligeramente en función del aumento de la velocidad del rayo láser, pero la microdureza fue mucho mayor que en la muestra no tratada. También, se calculó con éxito el modelaje de la indentación de la microdureza con el software COMSOL de la muestra tratada por RSL y no tratada por el método de elementos finitos. Por lo tanto, se encontró un buen acuerdo entre los datos experimentales y los simulados