Can Hydrogen Embrittlement of High Strength Steels Even Occur at Low Temperature? New published paper gave an answer!
New paper published at Materials Science & Engineering A.
Our recent results show that hydrogen embrittlement is present at sub-zero temperatures, causing an increase in fracture toughness reference temperature T0 and a small decrease in deformation capability. The relationship between the T0 and the impact toughness transition temperature T28J, which, in the case of ultra-high-strength steel, deviates from that observed for lower strength steels, is proposed to be affected by the hydrogen content.
What are the Common Room Temperature Characteristics of Low Ice Adhesion Surfaces?
New paper published in Scientific Reports.
Our results show that low ice adhesion strength does not correlate well with water contact angle and its variants, surface roughness and hardness. Low elastic modulus does not guarantee low ice adhesion, however, surfaces with low ice adhesion always show low elastic modulus. Low ice adhesion (below 60 kPa) of commercial surfaces uniquely associates with small water adhesion force.
New article in Journal of Applied Physics by Dr. Sigurd Pettersen
Investigation of thermal transport in polymer composites with percolating networks of silver thin films by the flash diffusivity method
by Sigurd R. Pettersen, Shijo Nagao, Helge Kristiansen, Susanne Helland, John Njagi, Katsuaki Suganuma, Zhiliang Zhang, and Jianying He*
The flash diffusivity method/laser flash analysis (LFA) is one of the most popular methods for finding the thermal conductivity of a large range of materials, including polymer composites for thermal and electronic interconnects. With standardized, commercial instruments available, it has become common practice even in peer-reviewed journal publications to only state the instrument model and manufacturer, and then give the estimated thermal conductivity as an absolute value without discussing the intermediate factors. In this paper, we show that both the absolute values and temperature-dependent behavior of the specific heat capacity of polymer composite materials varies significantly with the three most common methods used to estimate this input factor for the LFA method, and that this further has a significant impact on the estimated thermal conductivity. We also give a systematic theoretical overview of the methods used in the manuscript, as this to our best knowledge has not before been published in one single paper. We expect that this paper can be of large value for researchers interested in investigating thermal properties of polymer composites, and as a general starting point for researchers interested in using the LFA method.
Sigurd Rolland Pettersen successfully defended his PhD thesis: Conduction mechanisms in conductive adhesives with metal-coated polymer spheres
The first opponent: Prof. James Morris, Department of Electrical & Computer Engineering, Portland State University, Oregon, USA
The second opponent: Chief Scientist Maaike Margrete Visser Taklo, Department of Instrumentation, SINTEF ICT
The third opponent: Prof. Kjell Magne Mathisen, Dep. of Structural Engineering, NTNU
Zhiwei He successfully defended his PhD thesis: Nanotechnology for Anti-icing Application: From Superhydrophobic Surfaces to Super-low Ice Adhesion Surfaces
The first opponent: Associate Professor Shijo Nagao, Osaka University, Japan
The second opponent: Associate Professor Caroline Laforte, Université du Québec á Chicoutimi, Canada
The third opponent: Associate Professor Hilde Lea Lein, Department of Materials Science and Engineering, NTNU
Joint publication with China University of Petroleum (East China) in Physical Chemistry Chemical Physics
The reverse micelles (RMs) in supercritical CO2 (scCO2) are promising alternatives for organic solvents, especially for both polar and non-polar components are involved. Fluorinated surfactants, particularly the double-chain fluorocarbon surfactants, are appropriate to form well-structured RMs in scCO2. The mechanisms inherent to the self-assembly of the surfactants in scCO2 are still subject to discussion. In this study, molecular dynamics simulations were performed to investigate the self-aggregation behavior of di-CF4 based RM in scCO2 and a stable and spherical RM is formed. The dynamics process and the self-assembly structure in the RM reveal a three-step mechanism to form the RM, that is, small RMs, rod-like RMs and the fusion of rod-like RMs. The Hydrogen-bonds between headgroups and water molecules, and the salt-bridges linking Na+, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The result shows the di-CF4 molecule has the high surfactant coverage at the RM interface, implying the high CO2-philicity. Ths mainly results from the bend of the short chain (C-COO-CH2-(CF2)3-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful to understand the surfactant self-assembly phenomena and design new CO2-philic surfactants.
New article in RSC Advances by PhD candidate Yi Gong
The behavior of hydrophobic-core/hydrophilic-shell structured microgels at an interface: from Mickering emulsion to colloidosomes with dual-level controlled permeability
Colloidosomes have attracted great attention due to its special structure and broad applications, and the permeability is one of the key parameter of colloidosomes. In the manuscript, an effective and straightforward approach for fabricating novel colloidosomes from pH-responsive core-shell microgels is presented. One-pot surfactant-free synthesis of the microgels with hydrophobic core and hydrophilic shell is developed. The Model drug release results show that the permeability of colloidosomes can be coarsely controlled by pH and fine-tuned by the ratio of shell to core in microgels.
Compare to the works reported previously, the synthesis of microgels in one-pot process could avoid the preparation, isolation and purification of raw products, while microgels with well-defined core-shell structure are still obtained. Moreover, the core and shell of microgels can be tailor-designed by choosing different monomers; thus the method can be commonly used to integrate various functional materials into colloidosomes. The methodology revealed in the study not only provides a unique technique to control the permeability of colloidosomes but also opens a platform pathway to integrate multi-functionalities to the colloidosomes.
Delegation from Delft University of Technology to NTNU Nanomechanical Lab
NML Group seminar at Lofoten
New article in Computational Materials Science by PhD candidate Kai Zhao
The void growth in monocrystalline Cu and Fe are investigated by molecular dynamics simulations to reveal the ductile mechanisms based on dislocation emission and propagation. The results show that the void growth in Cu is governed by the collective interaction of stacking faults along four (111) planes. Three dominant mechanisms of void growth in Fe are identified: (i) for small voids, nucleation of twinning boundaries; (ii) for intermediate voids, emission of shear loops; (iii) for large voids, stacking faults nucleate at the void surface and then degenerate into shear loops. The slip-twinning transition rate of Fe at room temperature calculated according to Zerrili-Armstrong model is in the range measured by our atomistic simulations. Vacancy generation which promotes void growth results from the intersection of more than two stacking faults in Cu, while in Fe it is attributed to the jog dragging of screw dislocations. An analytical model based on nudged elastic band calculation is developed to include the strain rate dependence of the nanovoid-incorporated incipient yielding. This new model demonstrates that the critical radius of shear loop in Cu under a strain rate of 108 s-1 is on the order of Burgers vector. For both metals, the dislocation density has been calculated to elucidate the plastic hardening coupled with void growth. This work sheds new lights in exploring the atomistic origins of the void size and strain rate dependent mechanisms associated with dislocation activities close to void surface
New article in Engineering Fracture Mechanics by PhD candidate Haiyang Yu
Viscous regularization for cohesive zone modelling under constant displacement: an application to hydrogen embrittlement simulation
The convergence problem during the cohesive zone modelling of hydrogen embrittlement in constant displacement scenario is attributed to the numerical instability which is studied analytically in the present work. The property of numerical stability is directly associated with the number of solutions for the controlling equations from the failure initiation point. It is shown that all the cases with a non-unique solution are numerically unstable thereby having convergence problem. Linear elastic and elasto-plastic material models are considered in the derivation, and the convergence properties for both models are proved essentially the same. The viscous regularization proposed by Gao and Bower proves effective in solving the convergence problem with good accuracy under constant displacement, provided that the viscosity is small enough. This is further supported by a pipeline engineering case study where the viscosity regularized cohesive zone approach is applied to the hydrogen embrittlement simulation. The stabilizing mechanism of the viscous regularization is attributed to its capacity to enforce a single solution by modifying the controlling equations. The influence of viscous regularization on symmetry modelling is also discussed.
New article in Applied Physics Letters: Electrical four-point probing of spherical metallic thin films
New article published in Applied Physics Letters. In this manuscript, we are presenting a novel method for performing four-point electrical measurements on spherical thin films with micron-scale diameters. Such measurements have not been reported previously, as four-point measurements are normally performed on flat surfaces and with complete symmetry in probe positions. The method takes advantage of recent advances in commercially available micro-robots, which allows the positioning of four separately controlled electrical probes on very fine structures. Using finite element models to obtain geometric corrections factors yields the opportunity to estimate the resistivity of materials and structures where symmetric probe positioning is difficult to achieve. By this method, we show that the intrinsic resistivities of spherical thin films are higher than that of bulk metal. The findings are of large significance to the electronic packaging industry, where cost-efficiency and getting large gain from the consumed amount of precious metals are of large importance.
New article in Nanoscale: Nanoscale Deicing by Molecular Dynamics Simulation
Excessive icing is a general problem to human activities in low temperature environment. The aim of creating anti-icing materials, surfaces and applications rely on understanding the fundamental nanoscale ice adhesion mechanics. As increasing experimental trials on manufacturing anti-icing coatings have been carried out, theoretical knowledge on the atomistic determinants of ice adhesion is in urgent need. In this study, we employ all-atom modeling and molecular dynamics simulations to study ice adhesion, detaching and shearing on smooth silicon and graphene surfaces, aiming to decipher the basis of ice adhesion strength. We also study the mechanical effects of an aqueous water layer that sandwiched between ice and substrate, giving results to support previous experiments. Our results for the first time provide atomistic view on the key events of nanoscale deicing processes, and supply strong theoretical references for further anti-icing studies.
Verner Håkonsen winner of NTNU NanoLab Image contest 2016
PhD candidate Verner Håkonsen has won the NTNU NanoLab Image contest 2016. We congratulate Verner!
Image description: Self-assembled magnetic nanocubes into superstructured tubes, which again have self-assembled into "leaf-like" micropatterns. Instrument used: Hitachi S5500 S(T)EM.
Molly Bazilchuk, Best Student Talk Award
PhD candidate Molly Bazilchuk received the Best Student Talk Award from the 7th annual workshop of The Norwegian PhD Network on Nanotechnology for Microsystems. We congratulate Molly!
Electromechanical characterization of individual micron-sized metal coated polymer particles
New paper published in Journal of Applied Physics. In the paper we present a method of simultaneous electrical resistance and compression measurements of single micron-size metal coated polymer particles. The method allows fundamental physical insight into the mechanisms of electrical resistance in the interconnect where such particles are applied.
Contact resistance and metallurgical connections between silver coated polymer particles in isotropic conductive adhesives
Recently, there has been an increasing interest in silver thin film coated polymer spheres as conductive fillers in isotropic conductive adhesives (ICAs). Such ICAs yield resistivities similar to conventional silver flake based ICAs while requiring only a fraction of the silver content. In this work, effects of the nanostructure of silver thin films on inter-particle contact resistance were investigated. The electrical resistivity of ICAs with similar particle content was shown to decrease with increasing coating thickness. Scanning electron micrographs of ion milled cross-sections revealed that the silver coatings formed continuous metallurgical connections at the contacts between the filler particles after adhesive curing at 150 °C. The electrical resistivity decreased for all samples after environmental treatment for three weeks at 85 °C /85 % relative humidity. It was concluded that after the metallurgical connections formed, the bulk resistance of these ICAs were no longer dominated by the contact resistance, but by the geometry and nanostructure of the silver coatings. A figure of merit (FoM) was defined based on the ratio between bulk silver resistivity and the ICA resistivity, and this showed that although the resistivity was lowest in the ICAs containing most silver, the volume of silver was more effectively utilized in the ICAs with intermediate silver contents. This was attributed to a size effect due to smaller grains in the thickest coating.
CuO/Cu based superhydrophobic and self-cleaning surfaces
CuO/Cu based superhydrophobic surfaces with ordered micro/nanostructures have been prepared via a solution-immersion process combining with photolithography and argon ion beam etching. CuO nanoneedles grow only inside microholes on copper substrate due to delaying effect of both residual photoresist and carbon layer produced during Ar etching. The hierarchical structures and surfaces show a water contact angle of 152⁰, a contact angle hysteresis of 3⁰, and a low water adhesion force of 15 μN, indicating a good superhydrophobicity. The obtained surfaces also keep itself clean from carbon black, chalk dust and water, enabling a great potential in self-cleaning and anti-fouling applications.
PhD position in Arctic Icephobic Materials (IVT-51/16)
NTNU Nanomechanical Lab at the Department of Structural Engineering is looking for 1 new PhD candidate to further strengthen our group in the field of nanotechnology for anti-icing. The position is for 3 years without duties. Applications with CV, possible publications and other scientific works, certified copies of transcripts and reference letters must be submitted electronically to www.jobbnorge.no.
Project details can be found http://www.ntnu.edu/nml/aim
Position info can be found http://www.ntnu.edu/nml/open-positions
Joint Work with Brno University of Technology on DFT calculations of multiaxial stress-strain response of NiTi alloy
Petr Sestak, Miroslav Cerny, Jianying He, Zhiliang Zhang, Jaroslav Pokluda
Abstract: Present ab initio study was focussed on a response of NiTi martensite to a superposition of shear and tensile or compressive stresses acting normally to the shear planes. The theoretically predicted base-centered orthorhombic (BCO) ground-state structure was found unstable under uniaxial compression and two transformations, one from orthorhombic to a monoclinic symmetry and the other back from monoclinic to orthorhombic symmetry, were observed in the computational model. The former transformation shows that the uniaxial compressive stress of about 4GPa destabilizes the BCO structure by reducing its symmetry to the experimentally observed monoclinic one. However, superposition of small shear stresses remarkably lowers the compressive stress necessary for this destabilization. The latter transformation then draws the crystal lattice to the B19 structure. The theoretical shear strength of NiTi martensite was subsequently computed as a function of the normal stress. The results obtained show that the e_ect of the normal stress is surprisingly opposite to that calculated for NiTi austenite and other cubic metals, i.e., that the shear strength is lowered by the compressive normal stress and vice versa.
A Uniform Hydrogen Degradation Law for High Strength Steels Is Proposed
Abstract: The degrading effect of hydrogen on high strength steels is well recognized. The hydrogen degradation is dependent not only on hydrogen content, but also on geometric constraints or equivalently, level of stress triaxiality, which means the hydrogen degradation locus is not likely to be a unique material property. Experimental data on notched tensile tests reported by Wang et al. are analyzed via cohesive zone modelling, and a cohesive strength based uniform hydrogen degradation law is proposed upon normalization of hydrogen degradation loci with different specimen geometries. Since the e ects of hydrogen content and geometric constraints are decoupled during normalization, the proposed law is applicable to all the specimen geometries as a material property. This law is subsequently applied to simulate the constant loading tests performed on the same material. Excellent agreement is observed between the simulation and test results in terms of incubation time for fracture initiation and highest permissible initial hydrogen content. The inconsistency observed in one of the cases is discussed, suggesting that the e ects of strain rate and stress relaxation need to be taken into account in order to improve the transferability of the degradation law calibrated from tensile tests to constant loading situations.
Keywords: hydrogen embrittlement; high strength steel; cohesive zone modelling; hydrogen degradation law; constant loading test
Two FRINATEK projects signed with Research Council of Norway
Colleagues from NTNU Gjøvik visited Nanomechanical Lab to discuss collaboration
Fracture Mechanics and Nanomechanics
Tel: +47 73592530
Tel: +47 73594686
Adjunct Associate professor
Fracture Mechanics and Materials Technology
Fracture and Fatigue-Measurement Method