Ali Ghahremaninezhad

Ductile failure behavior of polycrystalline Al 6061-T6

Ductile failure in polycrystalline aluminum alloys is explored through uniaxial tension and notched tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through microscopy to discern the mechanisms of failure and to evaluate the local strain evolution quantitatively. Fractographic observations are used to identify the onset and evolution of damage processes during deformation and failure of these aluminum alloys. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter. Lower bound estimates of the failure strain at moderate triaxiliaties are obtained from the experiments.

The sandia fracture challenge: Blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.

Ductile failure behavior of polycrystalline Al 6061-T6 under shear dominant loading

Ductile failure in polycrystalline aluminum alloys under pure shear as well as with superposed tension and compression loading is explored through the modified Arcan shear experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through quantitative microscopy to discern the mechanisms of failure and to evaluate the local strain evolution quantitatively. Fractographic observations are used to identify the onset and evolution of damage processes during deformation and failure of these aluminum alloys. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter. Lower bound estimates of the failure strain in low triaxiality conditions are obtained from the experiments. It is shown that strain-to-failure decreases monotonically with stress triaxiality in stark contrast with recent works where a reverse behavior in low stress triaxiality levels was reported. Eventual failure that occurs through void growth and coalescence is shown to be restricted to a very small region within the localized deformation band.

The Sandia Fracture Challenge: blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.

Crack nucleation from a notch in a ductile material under shear dominant loading

We examine the nucleation of a crack from a notch under a dominant shear loading in Al 6061-T6. The specimen is loaded in nominally pure shear over the gage section in an Arcan specimen configuration. The evolution of deformation is monitored using optical and scanning electron microscopy. Quantitative measurements of strain are made using the 2nd phase particles as Lagrangian markers which enable identification of the true (logarithmic) strains to levels in the range of two. Electron microscopy reveals further that the 2nd phase particles do not act as nucleation sites for damage in the regions of pure shear deformation. The initial notch is shown to “straighten out”, forming a new, sharper notch and triggering failure at the newly formed notch. Numerical simulations of the experiment, using the conventional Johnson–Cook model and a modified version based on grain level calibration of the failure strains, reveal that it is necessary to account for large local strain levels prior to the nucleation of a crack in order to capture the large deformations observed in the experiment.

The Effect of Curing Temperature on the Properties of Cement Pastes Modified with TiO2 Nanoparticles

This paper investigates the effect of curing temperature on the hydration, microstructure, compressive strength, and transport of cement pastes modified with TiO2 nanoparticles. These characteristics of cement pastes were studied using non-evaporable water content measurement, X-ray diffraction (XRD), compressive strength test, electrical resistivity and porosity measurements, and scanning electron microscopy (SEM). It was shown that temperature enhanced the early hydration. The cement pastes cured at elevated temperatures generally showed an increase in compressive strength at an early age compared to the cement paste cured at room temperature, but the strength gain decreased at later ages. The electrical resistivity of the cement pastes cured at elevated temperatures was found to decrease more noticeably at late ages compared to that of the room temperature cured cement paste. SEM examination indicated that hydration product was more uniformly distributed in the microstructure of the cement paste cured at room temperature compared to the cement pastes cured at elevated temperatures. It was observed that high temperature curing decreased the compressive strength and electrical resistivity of the cement pastes at late ages in a more pronounced manner when higher levels of TiO2 nanoparticles were added.

An Investigation into the Properties and Microstructure of Cement Mixtures Modified with Cellulose Nanocrystal

This paper aims to examine the effect of cellulose nanocrystals (CNC) on the hydration, transport behavior, and microstructure of cement mixtures. The addition of CNC delayed hydration at an early age but improved hydration at later ages. A small increase in the electrical resistivity of the cement mixtures with CNC was observed. Statistical nanoindentation showed a small tendency to a larger volume fraction of high density calcium-silicate-hydrate (C-S-H) and a smaller volume fraction of low-density C-S-H in the mixture with CNC.

Effect of Biomolecules on the Nanostructure and Nanomechanical Property of Calcium-Silicate-Hydrate

Inspired by nature, this paper investigates the effect of biomolecules, such as amino acids and proteins, on the nanostructure and mechanical stiffness of calcium-silicate-hydrate (C-S-H). Amino acids with distinct functional groups, and proteins with different structures and compositions were used in the synthesis of the C-S-H nanocomposite. The atomic structure was examined using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The morphology was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). AFM nanoindentation was used to evaluate the Young’s modulus of the modified C-S-H. Positively charged, H-bond forming and hydrophobic amino acids were shown to influence the atomic structure of C-S-H. The effect of negatively charged amino acid on atomic structure was more pronounced at higher C/S ratio. A noticeable increase in silicate polymerization of C-S-H modified with proteins at high C/S ratio was observed. The microscopic examination demonstrated a globular morphology for all samples except for C-S-H modified with hemoglobin, which showed a platelet morphology. The Young’s modulus of C-S-H with amino acids and proteins showed a general reduction compared to that of the control C-S-H.

A Study of Calcium-Silicate-Hydrate/Polymer Nanocomposites Fabricated Using the Layer-By-Layer Method

Calcium-silicate-hydrate (CSH)/polymer nanocomposites were synthesized with the layer-by-layer (LBL) method, and their morphology and mechanical properties were investigated using atomic force microscopy (AFM) imaging and AFM nanoindentation. Different sets of polymers were used to produce CSH/polymer nanocomposites. The effect of different factors including dipping time, calcium to silicate ratios (C/S ratios) and pH on morphology was investigated. CSH/polymer nanocomposites made with different sets of polymers showed variation in morphologies. However, the Young’s modulus did not seem to reveal significant differences between the nanocomposites studied here. In nanocomposites containing graphene oxide (GO) nanosheet, an increase in the density of CSH particles was observed on the GO nanosheet compared to areas away from the GO nanosheet, providing evidence for improved nucleation of CSH in the presence of GO nanosheets. An increase in roughness and a reduction in the packing density in nanocomposites containing GO nanosheets was observed.

The Effect of Glucose on the Properties of Cement Paste

This study presents the results of an investigation of the effect of glucose on the hydration, microstructure, and properties of cement pastes. The hydration was studied using non-evaporable water content measurement and thermogravimetric analysis (TGA). Electrical resistivity as a measure of the transport property of cement pastes was evaluated using electrochemical impedance spectroscopy (EIS). It was found that all cement pastes showed a similar degree of hydration at late ages. The cement paste with a low concentration of glucose (0.05%) showed a slightly higher compressive strength and electrical resistivity compared to the control cement paste. This was most likely due to improved workability and dispersion of cement particles in this cement paste. However, the cement paste with 0.25% glucose showed a lower electrical resistivity compared to the control cement paste.