Papers by Keyword: Damage

Abstract: Macroscopic damage models can describe the toughness behavior and formability of metals in terms of limit strains. However, it requires time-, cost-, and material-intensive calibration. In this work, a simulation framework is proposed to derive macroscopic damage model parameters and related properties directly from the microstructure. For this purpose, statistically Representative Volume Elements of the investigated DP1000 steel were generated utilizing the Python framework DRAGen. This was based on quantitative characterization of EBSD measurements of the present microstructure. Mechanical properties were assigned to the geometrical microstructure model by calibrating a phenomenological Crystal Plasticity model for distinct phases. Martensite cracking was identified as the predominant damage mechanism. This behavior on the microscale was represented by an isotropic brittle damage model in DAMASK, using a fracture mechanical literature value as the critical energy release rate parameter. The presented modeling approach enables stress state-dependent prediction of macroscopic damage properties out of the present microstructure.

Abstract: Understanding the relationships between microstructure and (mechanical) properties is inevitable for the design of modern structural metallic materials. A crucial property for most high-strength steels is ductile damage tolerance, since ductile damage can accumulate during cold forming, which either leads to failure in the forming process or subsequently affects the performance. Structure-property relations are often investigated using numerical methods, e.g. crystal plasticity (CP) modeling with representative volume elements (RVE). In a previous study, CP-simulations on 3D-RVE were coupled with surrogate modeling techniques performing a variance-based sensitivity analysis. This analysis enables quantitative descriptions of the relationships between microstructure features with the damage tolerance, quantified by individual indicators for individual damage mechanisms. To investigate the effect of the material model and the corresponding phase properties, 500 sRVE simulations were carried out with different CPparameter sets and the damage tolerance is investigated. All sets stem from the same DP800 but were calibrated with different approaches. Surrogate models were trained on the simulative database to calculate Sobol Indices (SI), which are a measure of how strong damage tolerance is affected by a particular microstructure feature. The SI are compared for the individual material models and damage indicators. The structure-property quantification is heavily influenced by the different material models, resulting in different values for the SI and a different order for the individual microstructure features. The main factor for the pronounced differences is the differently evolving mechanical phase contrast between ferrite and martensite.

Abstract: The influence of the stress state on damage evolution, fracture behavior, and component performance is well established for proportional loading conditions. In contrast, many industrial sheet-forming processes involve non-proportional loading paths, which can significantly alter material hardening and fracture responses. Recent results have shown, that load direction changes affect damage evolution in the dual-phase steel DP800. This paper aims to investigate to what extend these results can be transferred to the aluminum alloy AA6082-T6. Therefore, specimens are first prestrained in uniaxial tension and subsequently reloaded either in the same direction or orthogonally, using additional tensile tests. Fracture strains during the subsequent tensile tests are determined by Aramis DIC. Orthogonal load direction changes lead to an increased fracture strain for DP800, but decreased fracture strain for AA6082. While the observed behavior of DP800 can be attributed to the void morphology, which is established during prestraining, the results of AA6082 indicate different damage mechanisms which cause this behavior.

Abstract: Open-die forging is an incremental bulk metal forming process for producing large, safety-relevant components such as turbine and generator shafts. Besides achieving the target geometry, the process improves mechanical properties through grain refinement and the elimination of casting-related defects. With the increasing use of high-alloy steels, precise process control is required to prevent surface and internal cracking caused by material damage. However, predictive models for damage evolution under the thermo-mechanical conditions of open-die forging remain limited, particularly with respect to high-temperature recrystallization and the incremental process character with inherent pause times. In this work, a recrystallization-sensitive damage model was developed and validated for open-die forging. The parameters of the Lemaitre damage formulation were determined for the cold work tool steel D2 (1.2379, X155CrVMo12-1) using hot tensile tests over the relevant forging temperature range. Dynamic recrystallization kinetics were characterized by hot compression tests and described using an Avrami-type JMAK formulation, while static recrystallization behavior was analyzed by stress relaxation experiments and also modeled with JMAK kinetics. These results enabled the quantification of recrystallized fractions as functions of strain, temperature, strain rate, and dwell time. To link microstructural evolution with damage development, tailored recrystallization states were generated in dilatometer experiments and examined metallographically with respect to void formation and healing. The extended model was implemented in a finite element framework and validated through open-die forging experiments on demonstrator geometries, showing its capability to predict damage initiation under industrially relevant conditions.

Abstract: Cracks significantly affect the structural integrity and functionality of mechanical components. While most existing studies focus on identifying straight cracks using dynamic response (DR) data, the characterisation of crack paths, especially curved ones, remains limited. This gap is critical, as the path of crack propagation plays a vital role in determining the severity of structural damage, particularly in critical regions of plate structures. The large number of possible crack paths has made systematic research in this area difficult. Therefore, this study proposes a novel methodology for modelling both straight and curved crack paths in plate structures to analyse their DR using the Finite element method (FEM). Straight cracks are represented by coordinate pairs, while curved cracks are defined using second-order polynomial equations. A combination-based approach is employed to generate feasible curved paths within a bounded region, allowing variation in crack shapes, lengths, and geometries. The results demonstrate that the proposed methodology effectively reduces the total number of crack path configurations from 7140, an impractically large set for detailed analysis, to a manageable subset of 288. This reduction facilitates more efficient implementation in both numerical simulations and experimental investigations without compromising the representational diversity of crack path geometries. They also show that the crack path has a greater influence on the dynamic response than crack length, offering a more comprehensive framework for crack path identification and evaluation.

Abstract: The construction, operating principles, and Li-ion battery thermal runaway mechanisms were analyzed. The external mechanical damage to a Li-ion battery with the uncontrolled thermal runaway development was investigated. The battery self-heating temperature regime was determined. A possible reactions set leading to intense materials self-heating and decomposition was considered. The battery self-heating stopping by immersing it in a container with a water excess relative to the stoichiometric amount for the lithium metal maximum mass that can accumulate was investigated. The change in resulting aqueous solution pH was measured, and the hydrogen release was also recorded. Reaction completion time dependences was established. The water required amount to absorb the heat that could be released during the reaction was calculated, which correlated with the experimental data. Possible measures to Li-ion batteries prevent and stop the burning were considered.

Abstract: Irradiation of materials in space or nuclear applications is unavoidable and it is well known that it modifies their properties (electronic, optical, thermal, mechanical, ..) due to the formation of point and complex defects (vacancies Vs, self-interstitial atoms SIAs, cavities, bubbles, dislocation loops, dislocation lines, precipitates). As this review shows, irradiation can also be very useful for intentionally optimizing material properties and, when performed under very well controlled conditions, for understanding defects properties and their impact on large-scale material properties. Knowledge of how damage is created and accumulated in materials is needed to better understand the behavior of materials under irradiation, in particular their radiation resistance for nuclear applications or to know the best irradiation conditions for optimizing their properties in electronic or optical applications. Experimental characterization of damage is an essential element in achieving this objective, and is very often coupled with simulation. This paper presents general information on the introduction of damage during irradiation of materials and various examples illustrating the typical advantages of the Positron Annihilation Spectroscopy (PAS) technique for the study of radiation damage.

Abstract: The paper presents statistics of navigation events occurring on the Romanian sector of the Danube, in general and focused on those whose consequences represented a potential danger of pollution of the Danube water.The information presented and analysed shall be based on publicly available information provided and made available by the relevant competent authorities.The large amount of cargo transported per transport unit (approximately 1000 tons) can cause pollution with the most adverse consequences for the marine environment, but also for live animals and vegetation in the event of a navigation accident. Improper handling of cargo in oil terminals can also lead to accidental pollution with major consequences for the environment, for live animals and vegetation.Thus, involuntary stranding, collision between two ships, fire on board are direct sources of pollution when the integrity of the hull is affected, and fuels, lubricants, greases on board, and or cargo end up in the marine environment.The actions carried out to eliminate the consequences of pollution on the Danube from ships have different aspects and methods of intervention depending on the area where the pollution occurred, the polluting.

Abstract: In this study, the low energy impact properties of flax/epoxy, glass/epoxy and hybrid flax-glass/epoxy laminates are evaluated for two different stacking sequences: a unidirectional [0]₈ and a cross-ply [0/90]_2s. For flax laminates, the base reinforcement is made of the combination of a unidirectional flax layer and a flax mat layer, where the mat phase consisted of short flax fibers used as a binder for the unidirectional phase. All laminates were tested under uniaxial tension both before and after impact and were molded at a fiber volume fraction of 40%. The results indicate that the specific stiffness of the flax fiber composite is approximately 7% higher than that of the glass fiber composite, regardless of the stacking sequence used. Concerning low-energy impact resistance, the cross-ply laminate demonstrates superior performance with higher impact resistance and less permanent deformation compared to the unidirectional laminate. The study also explores the hybridization of flax and glass fibers, suggesting a promising approach that leverages the synergistic effects of employing two different types of fibers in the composite. The comparison of energy absorption during impact shows that the hybrid fibers/epoxy composite has a higher energy absorption capacity than the glass fiber/epoxy composite. Additionally, hybridization helps mitigate the degradation of tensile properties caused by impact, representing an effective strategy to enhance the mechanical properties of the flax fiber composite post-impact.

Abstract: This work presents the finite element modelling of porosity in super alloys coatings, developed with cobalt-base/chromium/molybdenum/silicon metallic powders, which were thermally sprayed on oil & gas steel pipeline substrates, with the aim to protect the steel against H₂S and CO₂ corrosive environments. Therefore, in the developed finite element models, a small level of porosity, identified and analysed on the cross-section of the developed coatings, was incorporated in the developed models in order to perform a more realistic analysis of the structural response of the coating with some level of porosity by the local damage modelling technique. The porosity was incorporated in the developed finite element models with the micromechanical Gurson-Tvergaard-Needleman damage model, consequently the damage model parameters of Gurson-Tvergaard-Needleman model were calibrated against the true stress-strain material curve of the coating. The damage model was applied only on the finite elements subjected to higher bending loads. The values of and damage parameters are in the range of those published in the literature, for different type of steels, however value was lower, showing that for super alloy coatings, is quite lower than for steels. For the case of the initial and critical void volume fraction, the best calibrate values are higher compared to steels values reported in the literature. The relative density was similar compared to data published in the literature. Once the damage model parameters were properly calibrated, the modelling was employed to evaluate the stresses and strain states in the coating/substrate structure and in coating-substrates interface. The developed models were able to properly simulate the hardening material response of the coating with good agreement with material data. The results showed that Gurson-Tvergaard-Needleman damage modelling technique was able to model porosity damage in cobalt-base/chromium/molybdenum/silicon hard coatings, since numerical results agree well with true stress-strain material curve of coating material.