Plenary Lectures

3rd Generation (3G) Steels with strengths well above 1GPa has opened up new opportunities for vehicle lightweighting. For many applications high strength must be coupled with sufficient ductility to withstand impact during a crash or forming operations. For stretch forming applications ductility can be adequately characterized by the tensile elongation. However, for forming operations involving bending or out of plane deformation it is the true ductility, i.e. the true strain at fracture, that represents the critical parameter. For some 3G steels true ductility can be remarkable, with fracture strains up to 0.8. This appears to be due to a combination of factors that provide damage tolerant microstructures in these materials. Two primary mechanisms involve grain refinement and TRIP effects, while the mechanical homogeneity of the phases also plays a significant role. With regard to the latter, Figure 1 illustrates the effect in a DP1300 steel to which V has been added. In the V-modified steel the strength of the martensite has been lowered while the ferrite is stronger. This reduction in micromechanical heterogeneity reduces the strain gradient across M-F interfaces making damage nucleation more difficult. The result is a factor of two increase in true ductility.
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In spite of considerable experimental study, the mechanisms and understanding of Hydrogen embrittlement in metals remain open. No approaches are able to predict embrittlement conditions in austenitic steels without fitted inputs. New experiments on fcc high entropy alloys, such as CoCrFeMnNi, present an additional paradox, absorbing more H than Ni or austenitic 304 stainless steel (SS304) but being more-resistant to embrittlement. Here, a new theory of embrittlement in fcc metals is presented based on the role of H in driving an intrinsic ductile-to-brittle transition at a sharp crack tip. Hydrogen at the crack tip reduces the decohesion energy and prevents dislocation emission/blunting, and both are needed for embrittlement. The theory quantitatively predicts a critical room-temperature H concentration above which an alloy is embrittled. Using first-principles DFT to compute the relevant alloy properties including H absorption, good agreement with available experiments for the transition concentration is found for the alloys SS304, SS316L, CoCrNi, CoNiV, CoCrFeNi and CoCrFeMnNi. The theory rationalizes why CoNiV is the most-resistant alloy and why SS316L is more resistant than the HEAs CoCrFeNi and CoCrFeMnNi. The theory thus opens a path toward computationally-guided discovery of embrittlement-resistant alloys, although limitations and challenges are discussed.
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Interfaces are the most inconspicuous and yet the most profound influencers of material behavior by far. They are capable of imparting large enhancements in fatigue-resistance and increase in failure strength, authoritatively dictating performance in several domains spanning biomedical devices, energy-harvesting, aeronautics, and space exploration. The enhancements in safety and performance fundamentally rests on a foundation of materials where metal fatigue is alleviated by materials design. A gaping scientific void debarring such a development is the lack of understanding of the two most fundamental interfaces in functional materials: Twin Boundaries (TBs) and Habit Planes (HPs). The current talk will address this void, explaining nanostructures and evolutionary mechanisms of these interfaces under external stimuli at multiple scales. This understanding will further be catapulted by the development of a suite of novel, ab-initio, fully-predictive microstructural models from the nano- to the meso-scale, explaining the key interface characteristics responsible for fatigue. Such an approach will advance knowledge on TBs, topological nanostructure of the HP, reveal their interplay in fatigue-damage mechanisms and establish tailorable design targets to alleviate fatigue. Such an approach dealing with large number of TBs and materials with varying compositions in the thousands require research that is devoid of empiricism and transcend
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Adhesive joints are increasing used in engineering components, particularly in the bonding of dissimilar materials. This talk focuses on deformation and failure mechanisms of a sandwich joint: brittle fracture in elastic layers, ductile failure by cohesive zone modelling, diffusion-controlled attack of an interface and toughening of an adhesive layer by the presence of micro-architectured reinforcement.
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This presentation first briefly introduces the concept of materials/mechanics informatics, which integrates machine learning with materials/mechanics science and engineering to accelerate materials/mechanics, products and manufacturing innovations. Then, this presentation reports a domain knowledge-guided machine learning strategy and demonstrates it by studying the oxidation behaviours of ferritic-martensitic steels in supercritical water and oxidation behaviours of FeCrAlCoNi based high entropy alloys (HEAs) at high temperatures. This strategy leads to the development of formulas with high generalization and accurate prediction power, which are most desirable to science, technology, and engineering.
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Studies of the strength deterioration of blood vessel were conducted under in vitro pulsatile pressure conditions. Furthermore, as an application of this results to the non destructive inspection, an algorithm of noninvasive diagnosing blood vessel diseases was established by detecting the acceleration response of blood vessel wall under pulsatile conditions, which estimates viscoelasticity of blood vessel wall characterized by our proposed parameter of I*. This method and theory were shown to be used to predict coronary artery disease by the clinical research. Furthermore, this method was applied to detect non-invasively the existence of aneurysm based on chaos theory. In this research, by dividing frequencies that compose the frequency of the pulsatile velocity of blood vessel wall into low and high frequency regions and conducting attractor analyses of the trajectory of pulsatile blood vessel wall, the possibility of accurate selective detection of blood vessel diseases such as mechanical deterioration of blood vessel wall ( low frequency region) and morphological change of blood vessel wall that are aneurysm (high frequency region) was indicated. In this lecture, close link of mechanical behavior of blood vessel wall with clinical disease of blood vessel is conducted.
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The presentation will provide an overview of recent progress and challenges in fracture and fatigue assessments of corrosion resistant alloy (CRA) pipes having dissimilar weldments, with a particular focus on marine risers and subsea infrastructure. Despite advances in existing technology, there are still restrictions imposed by the use of new materials, more hostile environment and extreme loading conditions to obtain lighter and more cost-effective structures without compromising operation safety and environment protection. Clearly, there is a need of developing improved technological capabilities in connection with innovative and advanced procedures for fracture and fatigue assessments of critical components for subsea applications to ensure more reliable and fail-safe operations of the infrastructure for production and transportation of oil and gas in deep water offshore hydrocarbon reservoirs.
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Predicting fretting-fatigue life under multiaxial and variable loading conditions requires taking into account the effects of stress gradients and the fact that stress concentration areasn where cracks may intiate, are mobile and move, along with the contact front, when the normal load varies. This paper proposes a model, analogous to linear fracture mechanics approaches, to take into account the gradient effects around the contact front through intensity factors and to describe the displacement of the contact front and the non-linear behaviour of the contact partial slip region with an incremental model based on these intensity factors.
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In this paper, an overview of recent experiments and some simulations aimed at understanding the
fracture behavior of magnesium is presented. The effects of crystallographic orientation, notch acuity,
temperature and strain rate are examined. The results show that tensile twins critically influence the
fracture mechanism operative near a crack or notch tip. On the other hand, they contribute significantly to
plastic dissipation and toughening. Also, they impart hardening which can retard micro-void growth.
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