Papers by Ashraf Bastawros
Precision Engineering-journal of The International Societies for Precision Engineering and Nanotechnology, Oct 1, 2018
In this study, both finite element analysis (FEA) and experimental observations were used to inve... more In this study, both finite element analysis (FEA) and experimental observations were used to investigate the single CBN grain wear in high-speed grinding of Inconel 718 superalloy. The wear characteristics for each grinding pass were numerically assessed utilizing the tensile and compressive strength limits of the cutting grain. Additionally, stress distribution within the grain, chip formation and grinding force evolution during multiple passes were investigated. The combined experimental and numerical results show that the CBN grain wear has two major modes: the macro fracture on the grain top surface propagating from the rake surface, and the micro fracture near the cutting edges. The resultant tensile stress is the main factor inducing grain wear. The cutting edges will be under self-sharpening due to the grain wear. With multiple micro cutting edges engaged in grinding process, the limited material removal region was divided into different sliding, ploughing and cutting dominant regions. Overall, the ratio of material elements removed by a cutting process ranges from 80% to 20%, and continue to decrease during the grinding process. With a stronger effect of the cutting process, larger fluctuation of the grinding force will commence, however its average value remains below that with stronger sliding and ploughing process characteristics.
ECS Meeting Abstracts, 2017
Stress corrosion cracking (SCC) is a primary degradation mode in steel oil and gas pipelines (1).... more Stress corrosion cracking (SCC) is a primary degradation mode in steel oil and gas pipelines (1). Pre-emptive detection and control of SCC of pipeline steel in high-pH soils is difficult owing to a prolonged incubation stage of precursory intergranular corrosion (IGC). In high-pH environments, pipeline steel exhibits greatest susceptibility to SCC within a narrow potential range between the active dissolution peak potential and the passivation potential, which may suggest an important role for oxide formation (2,3). In the present work, IGC is characterized by electrochemical and microscopic measurements together with in situ stress monitoring. The latter method is well-suited to detect stress generated by internal grain boundary oxidation (4). Low-carbon ferritic steel samples (X70 type) were cut from a pipeline specimen. After polishing, the specimens were mounted in electrochemical cells with the front surface exposed to solution (aqueous 1 M NaHCO3at pH 8.0-8.2) and the back sur...
Journal of Materials Research, Mar 1, 2009
The interfacial fracture toughness and the adhesion strength of two dissimilar materials are gove... more The interfacial fracture toughness and the adhesion strength of two dissimilar materials are governed by the diffusion interfacial thickness and its mechanical characteristics. A new testing methodology is implemented here to estimate the actual interfacial thickness from a series of nanoindentations across the interface, under the same applied load, with tip radius and indentation depth many times larger than the interface thickness. The bimaterial system used is a semicrystalline polymer interface of isotactic polypropylene and linear low-density polyethylene. The laminate is prepared under a range of diffusion temperature to yield diffusion interfaces of 0 to 50 nm. A numerical relationship is developed using two-dimensional (2D) finite element simulation to correlate the true interfacial thickness, measured by transmission electron microscopy, with the experimentally estimated apparent interfacial thickness, derived from the transition domain of a series of indents across the interface. A range of material-pairs property combinations are examined for Young’s modulus ratio E1/E2 = 1 to 3, yield strength ratio σY1/σY2 = 1 to 2.5, and interfacial thickness of 0 to 100 nm. The proposed methodology and the numerically calibrated relationship are in good agreement with the true interfacial thickness.
Soft Matter, 2014
By combining the field-stiffening effect of magnetorheological (MR) elastomers and the Euler buck... more By combining the field-stiffening effect of magnetorheological (MR) elastomers and the Euler buckling mechanism, we developed a brush-like magneto-active structure with highly tuneable stiffness. When the applied mechanical load is within a certain range, the effective stiffness of the structure can be tuned by several orders of magnitude with the applied magnetic field. The performance of the structure and its dependence on various synthesis parameters, such as the curing field and filler concentration, were investigated experimentally. It is found that the increase in the critical load for buckling is more than the contribution from the stiffening of the MR elastomer. To unravel the relation between the stiffness increase and the applied field, a theoretical model with coupled mechanical deformation and magnetic field is established. The prediction of the model agrees well with experimental results. The theory may also be used to model the behaviour of other similar materials, such as MR gels. The MR brush developed in this research hold promise for potential applications in smart structures or devices that require mechanical stiffness to be tuneable in a relatively large range. As the amplification mechanism is independent of the base material, it could be used in conjunction with emerging MR materials for further enhanced performance.
Proceedings of SPIE, Apr 26, 2012
A new design for a multi-layer dielectric elastomer actuator, reinforced with periodic stiffeners... more A new design for a multi-layer dielectric elastomer actuator, reinforced with periodic stiffeners is presented. The resulting actuator enables complex out-of-plane motion without the need of the elastomer membrane prestretch. An in situ optical imaging system is used to capture the complex deformation pattern and track the non-planar displacement and curvature under the applied voltage. The role of the stiffeners periodicity, , on the macroscopic actuator response is analyzed numerically utilizing ABAQUS finite element software. A user-material subroutine is developed to represent the elastomer deformation under the applied electric field. It is found that the actuator force-stroke characteristics can be greatly changed by varying , while maintaining the same overall actuator stiffness. The numerical results showed a band of localized deformation around the stiffeners. The refinement of the stiffener, , increases the total actuated volume within the span of the actuator, and thereby the macroscopic actuator stroke. The stored elastic strain energy within the actuator is also increased. might be further refined down to the actuator sheet thickness, wherein the localized deformation bands overlaps This is the practical limit of the stiffeners spacing to achieve the largest macroscopic actuator strokeThe developed experimental and modeling framework would enable the exploitation and optimization of different actuator designs to achieve a preset load-stroke characteristic.
The microstructure of oil pipeline steels is polycrystalline due to the presence of a complex gra... more The microstructure of oil pipeline steels is polycrystalline due to the presence of a complex grain boundary (GB) network in them. The performance of them when exposed to stresses in corrosive environments is largely dictated by the interplay between the hydrogen (H) diffusion, the dislocation slip, and their interactions with the GBs. However, the slip-GB interaction in the presence of H and the subsequent cracking along the GBs remains not fully understood up to date. It is challenging to simultaneously resolve the collective dislocation motions away from the GBs together with the H diffusion near the atomically structured GBs using single-scale techniques. To address this challenge, taking bcc iron as a model material, here we present a a concurrent atomistic-continuum (CAC) computational analysis of the interaction between dislocation slip and a H-charged GB. With the large number of dislocations-mediated plastic flow away from the GB together with the atomistic structure evolution at the GB being simultaneously retained, several main findings from our simulations are: (i) the large number of dislocation-mediated pileup-induced internal stress nearby the H-charged GB can be as high as 3GPa; (ii) the more dislocations accumulated nearby the GB, the slower H diffusion ahead of the slip-GB intersection; (iii) H atoms diffuse fast behind the pileup tip, get trapped within the GB, and diffuse slow ahead of the pileup tip; (iv) the local stresses nearby the pileup tip exhibits a strong heterogeneity along the dislocation line direction. This differs from the common wisdom in many existing 2D theories/models. The buildup of such a high local stress heterogeneity leads to inhomogeneous H diffusion within the GB plane. The CAC simulation-predicted local H diffusivity, Dpileup−tip, and local stresses, σ, nearby the GB are then correlated with each other and consolidated into a mechanics model by considering the dislocation pileup as an Eshelby inclusion. These findings will provide researchers with opportunities to: (a) characterize the coupled dynamics between plasticity, H diffusion, and crack initiation underlying the hydrogen-induced cracking (HIC); (b) develop mechanism-based constitutive rules to be used in diffusion-plasticity coupling models for understanding the interplay between mechanical and mass transport in materials at the continuum level; and (c) connect the atomistic deformation physics of polycrystalline materials with their overall performance in aqueous environments, which is currently difficult to achieve in laboratory experiments.
Pipeline and Hazardous Materials Safety Administration2020PDFTech ReportBastawros, AshrafIowa Sta... more Pipeline and Hazardous Materials Safety Administration2020PDFTech ReportBastawros, AshrafIowa State UniversityUnited States. Department of Transportation. Pipeline and Hazardous Materials Safety AdministrationUnited States. Department of Transportation. Pipeline and Hazardous Materials Safety Administration. Competitive Academic Agreement Program (CAAP)United States. Dept. of Transportation. Pipeline and Hazardous Materials Safety AdministrationUnited StatesCorrosionCrackingElectrical impedanceElectrochemistryNondestructive testsPetroleum pipelinesPipeline safetyResistance (Electricity)SteelSteel pipeStress corrosionPromoting SafetyDTPH5616HCAP01Final ReportCorrosion of underground natural gas and liquid petroleum pipelines occurs by a variety of forms and requires specialized methods to detect and control. Stress corrosion cracking (SCC) is a form of corrosion that results in clusters or colonies of cracks on the surface of the affected pipelines. While the three conditions necessary for SCC (metal susceptibility, critical service stress and potent environment) are well understood, SCC remains as a leading operational safety concern. During SCC, the majority of the life of a pipe remains within the nucleation stage, wherein micro-damage is percolating very slowly, and below detection thresholds for commonly deployable nondestructive evaluation techniques. Utilizing our integrated understanding, from current CAAP project, for electrochemical transport current, stress, and morphology evolution during SCC early stage damage percolation in pipeline steel, the university plans to develop and characterize two complementary methods that are expected to provide quantitative measure of the damage level during the early stage of SCC, and thereby render a safe operational condition of the pipeline. This framework relies on a model based prediction of the onset and progression of SCC subsurface damage to enable the development of testing methodologies for accurate measurements of corrosion depth and extent of initial shallow cracks. The methodologies utilize: (i) electrochemical impedance spectroscopy (EIS). Intergranular corrosion penetration and stress corrosion crack length are manifested by particular features of the impedance spectra. Aided by the modeling framework, geometrical descriptors of corrosion can be determined directly from impedance spectra. And (ii) tailored 4-point probe (4PB) resistance/impedance measurements. We have observed extensive lattice damage in the near surface layer. The preliminary measurement of 4PB showed subtle changes in the measured electrical resistance. A tailored 4PB with optimized configuration and electronic circuit will be developed to measure the changes in electrical resistance of near surface layer as a function of the corrosion environment and history. Aided by geometric modeling of corroded cross section, geometrical descriptors of corrosion can be determined from the measured changes of electrical resistance. The complementary techniques would provide quantitative measure of the extent of subsurface damage, grain boundary grooving and early stage percolation of shallow cracks to enable further development of NDE corrosion detection techniques. The team spans several disciplines with diverse expertise in fracture and fatigue damage evolution (Bastawros), electrochemistry (Hebert) micro- and nano-measurements (Shrotriya), NDE for pipelines in-line inspection (Bond), and leverages the support and ongoing work with BP and others on pipe monitoring. This work leverages the provided sample set in a previous CAAP project by Kiefner and Associates. The model guided phenomenological understanding of the SCC mechanochemistry, and the quantifiable laboratory measurements of such degradation at different stages, will assist in the development of future deployable NDE methodology for the detection and monitoring of the early stage of SCC. These insights will enhance the operator ability to monitor changes in parameters germane to the corrosion prevention, while mitigating the corrosion impact on the pipeline sector.111
MRS Proceedings, 2003
A novel experimental configuration is devised to measure the evolution of the deformation field a... more A novel experimental configuration is devised to measure the evolution of the deformation field and the corresponding hardening evolution within soft metallic films constrained by hard layers. The experimental configuration provides pure shear state within the constrained film. The material system utilized comprised ductile layer of tin based solder, encapsulated within relatively hard copper shoulders. Different tin-lead compositions are tested with grain size approaching the film thickness. The in-plane strain distribution within the film layer is measured by a microscopic digital image correlation system. The hardening evolution within such highly gradient deformation field is monitored qualitatively through a 2D surface scan with a nanoindentor. The measurements showed a highly inhomogeneous deformation field within the film with discreet shear bands of concentrated strain. The localized shear bands showed long-range correlations of the order of 3-4 the grain size. A size-dependent macroscopic response on the layer thickness is observed. However, the corresponding film thickness is approximately 100-1000 times larger than those predicted by non-local continuum theories and discrete dislocation.
Conference proceedings of the Society for Experimental Mechanics, Sep 21, 2016
This study explores a probable correlation between the degradation of bond line toughness in adhe... more This study explores a probable correlation between the degradation of bond line toughness in adhesively bonded joints and the degradation/changes in measurable mechanical properties of the adhesive layer by indentation hardness due to contamination. The proposed framework utilizes the large scale bridging of interfacial fracture proposed by Tvergaard and Hutchinson (Philos Mag A 70:641–656, 1994). A typical adhesive/adherend material system (EA9394/Hexcel IM7-G/8552) exposed to different contaminants at the same concentration was examined. Nano indentation technique, considered as a non-destructive testing methodology compared to the bond line thickness, was utilized to measure the adhesive mechanical properties. In addition, macroscopic mode-I fracture toughness was independently measured by double cantilever beam test. Finite element method employing cohesive zone method was used to rationalize the experimental results and the prospective scaling-laws. The combined experimental results of macroscopic properties and the numerical results of the interfacial properties suggest a scaling between the interfacial cohesive fracture toughness and the measurable flow stress. While the proposed scaling is verified to a common adhesive-adherend system in aerospace industry, with additional examination of other systems, the proposed scaling law facilitates the utilization of the non-destructively evaluated indentation hardness to serve as an indicator for the bond line macroscopic fracture toughness.
Inspired by bat wings which exhibit compliant response at lower stretch level to expand and a dra... more Inspired by bat wings which exhibit compliant response at lower stretch level to expand and a drastic stiffness change at higher stretch level to enable fl apping. The bat-wing skin is highly anisotropic. The skin is more stretchable on spanwise direction and its stiffness becomes more than two orders of magnitude higher after stretched on chordwise direction. On the other hand, the skin on chordwise direction itself is less stretchable and with one order of magnitude of stiffness increase. A new composite dielectric elastomer actuator is developed to mimic such complex mechanical response of the bat through sets of variable stiffness reinforcement ligaments. The composite actuator is capable of initial large predetermined stretch, then exhibiting an order of magnitude increase in the actuator structural stiffness. The stiffners dimensions and geometric layout controls both the initial stretch and the fi nal actuator stiffness. Details of the design, fabrication, and structural performance will be given in this study. Design domain limitations will also be explored.
Adhesively bonded joints are commonly used in airframe structures due to their superior strength ... more Adhesively bonded joints are commonly used in airframe structures due to their superior strength to weight ratio, lower maintenance cost, and longer service life. The bonded joint reliability is affected by the environmental conditions, such as moisture, temperature, and service environment. In this study, the role of contamination during the bonding process on the joint mechanical integrity is examined for a set of common in-service hydraulic oils. An adhesive (Hysol EA 9394)/ adherend carbon fi ber composite (Hexply IM7/8552) were examined for different contaminants exposure level. The fracture characteristics are evaluated from Double Cantilever Beam (DCB) test and Single Shear Lap. The fractal surface is analyzed by surface topographic analysis. An FEM study employing cohesive zone method was implemented. The cohesive zone properties for Mode-I and Mode-II were calibrated from a reference DCB and End-Notched Flexure tests, respectively. The experimental and modeling study showed a major effect of the contamination to lower the joint fracture toughness through both a reduction in the adhesion strength as well as shielding of plasticity within the adhesive layer. In addition, it is found that a trace level of hydraulic fl uid, which is accepted as clean surface according to military standards, still degrades the bond-line strength to 70% of its rated level.
Journal of Materials Research, Jun 16, 2021
Alluaudite-type sodium iron sulfate, Na 2.56 Fe 1.72 (SO 4) 3 , has the highest redox potential i... more Alluaudite-type sodium iron sulfate, Na 2.56 Fe 1.72 (SO 4) 3 , has the highest redox potential in any Fe 3+ /Fe 2+ environment; here, its electronic structure is investigated by combining X-ray absorption spectroscopy (XAS), semi-empirical ligand field multiplet (LFM) simulations, and ab initio calculations. The LFM simulations for the Fe L 2,3-edge XAS reveals the exceedingly ionic character of the Fe−O bond due to the strong inductive effect of the sulfate group; this is one of the main factors, which enhance the electrode potential of Na 2.56 Fe 1.72 (SO 4) 3. Both experimental XAS measurements and ab initio calculations indicate electrochemical charging (desodiation) gives more hybridized iron 3d and oxygen 2p states. The degree of covalency between iron and oxygen changes reversibly along with the change of the iron valence state.
A heat sink comprising an Al alloy open cell material within an insulated channel is described. I... more A heat sink comprising an Al alloy open cell material within an insulated channel is described. It is attached to a silicon substrate and cooled with circulating air. The influence of foam morphology and heat sink dimensions on both the heat transfer coefficient and the pressure drop are measured. Trends in the cell wall diameter, d, the heat sink thickness, b, and the air velocity, v, are established. The findings are compared with a model based on the cross flow of a fluid over a bank of cylinders. There is qualitative consistency with the measured effects of d and v, but significant differences in details. Moreover, measured trends with b oppose those in the model. The implication is that the fluid flows are influenced by the heat sink thickness. Further studies are needed to assess these effects.
The aim of the work is to provide measurable precursor signals associated with the initiation sta... more The aim of the work is to provide measurable precursor signals associated with the initiation stage of near-surface damage and cracking, as depicted in Fig. 1.1. We have identified many salient features during the early stage of the SCC process (Stages 1, 2 on Fig. 1.1), including residual stress build-up, near-surface (within few microns) defect percolation, and changes of dislocation dynamics and measurable changes of the surface osmic resistance. We developed a model-based prediction of the onset and progression of SCC subsurface damage and assessed the electrochemical impedance spectroscopy (EIS) to measure the extent of surface damage. Such a framework would enable the development of appropriate field-deployable NDE technology with the needed spatial and temporal resolutions.
Acta Materialia, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Uploads
Papers by Ashraf Bastawros