Ductility takes into account the material capability to plastically deform. This parameter is not only modified by temperature but it is strongly affected by the stress triaxiality that, in the case ...of positive hydrostatic stress, reduces the material strain to failure. Due to the importance of this parameter in engineering design many attempts to predict the evolution of ductility with stress triaxiality have been done. Here, a nonlinear continuum damage model, as proposed by the author, is used to obtain the evolution of material ductility with stress triaxiality. The expression found relates the strain to failure in multi-axial state of stress regime only to the uniaxial strain to failure, to the damage strain threshold, to the material Poisson's ratio, and, of course, to stress triaxiality. The proposed model was successfully verified comparing the predicted evolution of material ductility with the experimental data relative to several metals. The procedure for the damage parameters identification is also discussed in details.
Fracture behavior of additively manufactured (AM) Ti‐6Al‐4V has been investigated under quasistatic and impact loading. Taylor cylinder impact tests, on material printed along different directions, ...have been performed at various velocity to determine high‐rate material deformation and impact velocity for damage initiation. Test results revealed that, although the AM material under quasistatic loading condition shows better characteristics than the corresponding wrought material grade, under impact condition, fracture in AM material occurred at an impact velocity almost half of that of wrought grade and at a strain 10 time less of the quasistatic uniaxial fracture strain. Microscopy investigation seems to indicate that pre‐existing microvoids produced by the AM process promote shear band development under impact loading causing fracture at much lower strain.
Understanding the mechanisms of traumatic ocular injury is helpful to make accurate diagnoses before the symptoms emerge and to develop specific eye protection. The comprehension of the dynamics of ...primary blast injury mechanisms is a challenging issue. The question is whether the pressure wave propagation and reflection alone could cause ocular damage. To date, there are dissenting opinions and no conclusive evidence thereupon. A previous numerical investigation of blast trauma highlighted the dynamic effect of pressure propagation and its amplification by the geometry of the bony orbit, inducing a resonance cavity effect and a standing wave hazardous for eye tissues. The objective of the current work is to find experimental evidence of the numerically identified phenomenon. Therefore, tests aimed at evaluating the response of porcine eyes to blast overpressure generated by firecrackers explosion were performed. The orbital cavity effect was considered mounting the enucleated eyes inside a dummy orbit. The experimental measurements obtained during the explosion tests presented in this paper corroborate the numerical evidence of a high-frequency pressure amplification, enhancing the loading on the ocular tissues, attributable to the orbital bony walls surrounding the eye.
The market for ductile cast iron as a substitute material increases every year. The material structural performance is strongly dependent on the microstructure. Micromechanics can help in ...understanding the role played by the microstructure constituents as well as the effects associated to the shape of the spheroids, their density, surface roughness, etc. The potential of application for micromechanics modeling can be further increased if features such damage mechanics and residual stresses are incorporated. In this paper, a micromechanics modeling approach based on the unit cell development has been developed paying particular attention to the role and the behavior of the constituents. Residual stresses, resulting from the cooling down to room temperature, have been demonstrated to be critical for an accurate prediction of the non-linear behavior of the DCI in the early deformation range. As far as damage mechanics is concerned, it has been demonstrated that voids nucleating from debonded spheroids are not sufficient to explain catastrophic failure at the macroscale while the occurrence of additional ductile damage in the matrix material is the driving process for rupture in ferritic DCI.
Although diffusional flow creep is often considered out of practical engineering applications, the need for a model capable to account for the resulting action of both diffusional and dislocation ...type creep is justified by the increasing demands of reliable creep design for very long lives (exceeding lOO.OOOh), high stress-low temperatures and high temperature-low stress regimes. In this paper, a creep model formulation, in which the change of the creep mechanism has been accounted for through an explicit dependence of the creep exponent n on stress and temperature, has been proposed. An application example of the proposed approach to high purity aluminum is given.
One of the most important features of micromechanical models, with respect to other global approaches to fracture, is that progressive damage and failure can be described only by parameters ...characteristic of the material and not of the geometry. In spite of the large number of papers on micromechanical modeling presented in literature, a detailed assessment the geometry transferability of model parameters has been addressed in a limited number of works. In most of the cases, the model parameters transferability is analyzed only for one geometrical configuration other than that – usually a uniaxial tensile bar – on which the parameters set has been identified. In this paper, the continuum damage mechanics approach, as proposed by Bonora, N., 1997. A non-linear CDM model for ductile failure. Engineering Fracture Mechanics 58, 11–28 is used to model ductile damage processes in ferritic steels. The geometry transferability of the damage parameters is demonstrated both in the range of low and high stress triaxiality. The possibility to accurately predict constraint effects on material crack resistance curve is demonstrated.
An advanced creep modeling, based on dislocation mechanics and incorporating damage effects, is developed at continuum scale. In the proposed formulation, creep damage does not depend on time ...(time-independent damage formulation) but on the accumulated creep strain. Thus, the tertiary creep stage can be predicted as the evolution of the secondary stage in which the current stress is increased by damage effects, and possible other microstructural instability processes, in addition to geometry modifications. The proposed formulation extends the initial continuum damage mechanics approach proposed by Kachanov in order to have a more explicit correlation between material creep response, damage mechanics and material microstructure. The possibility to account for possible microstructure modifications that may occur as a result of solid-solution kinetics, by means of the identification of the evolution law of damage parameters is discussed. An example of the applicability of the proposed model to IMI834 titanium alloy is given.
•Ductile crack growth is predicted with no need to calibrate model parameters as for similar works.•A curve for the maximum ductile crack growth prior to cleavage is determined and validated.•The ...effect of ductile crack growth (DCG) prior to cleavage on the Weibull stress was determined.•The effect of DCG on Q constraint was shown and quantified for selected specimen geometries.•The importance of DCG on cleavage fracture probability was shown analyzing near tip stress.
There are compelling experimental evidences that demonstrate a significant effect of specimen size, a/W ratio and ductile tearing on cleavage fracture toughness values (KJc) measured in the ductile-to-brittle transition region of materials such as ferritic steels. In this work, the influence of ductile tearing and constraint loss on Weibull stress and failure probability in ductile to brittle transition (DBT) region is investigated. The study was carried out using a modeling approach that combines the modified Beremin model (MBM) for cleavage fracture and the Bonora damage model (BDM) for ductile tearing. Here, CT and SEB, with deep and shallow crack, specimen geometries, which are characterized by different crack tip constraint, have been analyzed. Results show that the occurrence of ductile crack growth in the mid-to-upper transition region affects the nature of the stress field in the region surrounding the crack tip in terms of maximum principal stress peak, its spatial gradient – which has a direct consequence on the calculated Weibull stress – and stress triaxiality, which affects the constraint loss. This combination of effects leads to much lower fracture toughness values that those predicted by not considering ductile crack growth.
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