In recent years, the Finite Fracture Mechanics approach, originally proposed by Leguillon in 2002, has been applied successfully to several material and geometrical configurations. However, up to now, most of the applications were restricted to two-dimensional geometries. In the present paper, we provide an insight to a simple yet challenging three-dimensional case, namely the flat elliptical crack. Results are provided in analytical form.
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Keywords: Keynote
ON THE DIFFICULTY OF IMPLEMENTING THE COUPLED CRITERION TO PREDICT GLASS FRACTURE [Keynote]
Glass is an extremely brittle material that behaves almost perfectly linear elastic until it fractures. The linear-elastic fracture mechanics (LEFM) approach described by Griffith’s energy criterion is typically used to explain failure from a pre-existing crack like defect. However, LEFM reaches its limits in explaining failure processes at general stress concentration points and implementing the Coupled Criterion (CC) to take over is a tricky task. This mainly because it requires the knowledge of the tensile strength of the material which is a parameter not easy to characterize in glass. It is in general defined through a statistical law and relies strongly with surface flaws. The general aim of this work is to give an overview of the current understanding of glass tensile strength.
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RELATING NANOSCALE STRUCTURE AND PROPERTIES TO MACROSCALE FRACTURE TOUGHNESS FOR BULK METALLIC GLASSES [Keynote]
Bulk metallic glasses (BMGs) can range from exceptionally tough to brittle depending on their structural state; however, quantifying their structure-property relationships has been an unresolved challenge. Our findings revealed that local hardness variations within the BMG microstructure strongly affect the fracture behavior. Moreover, the hardness heterogeneities are controlled by the size and volume fraction of FCC-like medium-range order (MRO) clusters. We have proposed a model of ductile phase softening whereby relatively soft FCC-like MRO clusters sit in a matrix of harder icosahedral dominated ordering, while micropillar compression testing has revealed how the activation of these clusters into shear transformation zones can be negatively affected by oxygen impurities which in turn lower the fracture toughness.
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INSIGHTS INTO VOID NUCLEATION AND GROWTH IN A DUAL PHASE STEEL BY SMALL SCALE MECHANICAL TESTING [Keynote]
Dual phase (DP) steels are comprised of a soft ferrite matrix and hard martensite islands. They are often used in automotive applications due to their advantageous combination of high strength and good ductility. During forming, DP steels can suffer from ductile damage, i.e. the formation and growth of voids, which typically occur by interface decohesion and martensite fracture [1]. As of now, the void content of a deformed part cannot precisely be predicted and, therefore, safety factors are used to assure the required mechanical properties and component lifetime. These safety factors are opposing sustainability and light-weight design. Consequently, the DFG-funded collaborative research center TRR188 aims at a quantitative characterization, prediction and control of ductile damage during forming.
In the talk, micromechanical experiments on the plasticity and fracture of single ferrite grains and martensite islands of two nominal identical steel grades will be presented. While one steel grade exhibits a low ferrite and a high martensite strength, the other shows a significantly stronger ferrite and lower strength martensite compared to the first steel grade [2]. This results in huge differences in the void nucleation and growth characteristics of the two steel grades.
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IMPACT OF GRAIN BOUNDARY MODIFICATIONS ON FRACTURE TOUGHNESS OF TUNGSTEN BASED NANOMATERIALS [Keynote]
Nanostructured materials commonly excel with respect to their strength, but their ductility and toughness remain limiting factors for deployment in safety related applications. In this work, using grain boundary engineering concepts in conjunction with severe plastic deformation for microstructural refinement, we aim to develop nanostructured and nanocomposite materials that overcome these limitations. Since material volumes are limited, we utilize small scale testing approaches to examine the respective material properties such as strength, ductility and fracture toughness. We detail on the one hand challenges and recent advancements in small scale fracture experiments, and on the other hand the effectiveness of the mentioned grain boundary engineering approaches to design outstanding nanomaterials overcoming strength-ductility-toughness limitations.
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HIGH-CYCLE FATIGUE IN THE TEM: NANOCRYSTALLINE METALS [Keynote]
In-situ TEM high-cycle fatigue experiments on electron transparent thin films of nanocrystalline Pt and Cu have revealed not only microstructural-sensitive crack propagation, but also unexpected microstructural-scale crack healing. Based on the experimental observations, atomistic modeling, and continuum-scale microstructural modeling, the mechanism appears to be crack flank cold welding facilitated by local compressive microstructural stresses and/or grain boundary migration. While these observations are specific to pure nanocrystalline metal thin films under a high-vacuum environment, there are potentially much broader ramifications. The existing observations can be used to help rationalize suppressed fatigue crack propagation rates in vacuum, subsurface, or under contact-inducing mixed-mode stresses; and even the precipitous decline in propagation rates near the fatigue threshold.
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EFFECT OF ELECTRIC CURRENT ON PRE-CRACKED THIN METALLIC SHEETS: FROM CRACK PROPAGATION TO CRACK HEALING [Keynote]
Both scientifically and technologically, it is important to study the effects of electric current pulses on the structural integrity of metallic components. As an electric current reverses its direction across a crack, massive current crowding occurs at the crack tip, thereby generating a non-uniform temperature field sourcing away from the crack tip, and considerable electromagnetic forces are generated on the crack faces that open the crack in Mode I. Recent studies have shown that due to the synergistic effects of the above two stimuli, a pre-existing flaw may grow as well as heal upon application of an electric current pulse of high density. While one is a bane for structural integrity, the other one is a boon to in-service components. Here, we will discuss the reasons behind crack propagation upon application of an electric current and then explore the attributes responsible for a transition from flaw propagation to flaw healing upon passage of an electric current pulse. Furthermore, the synergetic role of mechanical load and magnetic field in the propagation of a pre-existing flaw will be discussed. We will establish the complementary roles of electric current, magnetic field and mechanical load in the failure of pre-cracked metallic sheets.
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RESISTANCE TO FRACTURE AND FATIGUE IN ADDITIVELY MANUFACTURED ALLOYS [Keynote]
Ti-6Al-4V fabricated by the laser powder bed-fusion (LPBF) process consists of metastable α’ microstructure and columnar prior β grain (PBGs) mesostructures. These micro-and mesostructures adversely affect fracture toughness (KIc) in as-built conditions. After an optimized post-processing heat-treatment, the KIc of LPBF Ti-6Al-4V improves by ⁓104%; however, the anisotropy in KIc persists due to preferential crack growth along the columnar PBGs. In another LPBF fabricated β Ti-alloy, Ti41Nb, the crack tortuosity from the mesostructures formed by compositional segregation improves the KIc by ⁓80%. These results demonstrate extrinsic toughening in AM alloys. While such toughening from mesostructures enhances AM alloys’ reliability, the processing-induced defects present in them, i.e., porosity, significantly reduce their high cycle fatigue (HCF) resistance. Therefore, in the second part of the present study, the HCF life of 316L and 17-4 PH steels produced by the binder jet printing process was investigated. The hot isostatic pressing (HIP) was employed on these steels to improve their HCF life. The HCF life of HIPed 17-4 PH steel is comparable to their conventionally manufactured counterparts; however, in 316L, HIP fails to improve fatigue life. Based on these findings, the microstructural origin for fracture and fatigue resistance in AM alloys are discussed.
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SIMILITUDE A BASIC CORNERSTONE FOR THE ANALYSES OF CRACK PROPAGATION [Keynote]
The similitude concept in fracture mechanics is the base for the understanding the size and loading condition dependent or independent crack propagation resistance. In the present overview few selected examples for monotonic and cyclic loading will be presented and underlying mechanisms are discussed.
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INVESTIGATION OF HIERARCHICAL POROUS STRUCTURES USING PHASE-FIELD FRACTURE MODELING INFORMED BY MOLECULAR DYNAMICS SIMULATION [Keynote]
The mechanical integrity of hierarchical porous structures depends on their pore morphology. To investigate the role of pore morphology on the mechanical and fracture behaviors of these complex systems, a multi-scale approach has been proposed. This paper shows how molecular dynamics simulations provide the means to extract material properties at the atomistic scale to further inform phase-field fracture technique at the continuum scale in an attempt to understand the mechanical response of these porous materials.
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