Continuous and discontinuous modelling of ductile fracture

In many metal forming processes (e.g. blanking, trimming, clipping, machining, cutting) fracture is triggered in order to induce material separation along with a desired product geometry. This type of fracture is preceded by a certain amount of plastic deformation and requires additional energy to be supplied in order for the crack to propagate. It is known as ductile fracture, as opposed to brittle fracture (as in ceramics, concrete, etc). Ductile fracture originates at a microscopic level, as the result of voids initiated at inclusions in the material matrix. These microscopic degradation processes lead to the degradation of the macroscopic mechanical properties, causing softening, strain localisation and finally the formation of macroscopic cracks. The initiation and propagation of cracks has traditionally been studied by fracture mechanics. Yet, the application of this theory to ductile fracture, where highly nonlinear degradation processes (material and geometrical) take place in the fracture process zone, raises many questions. To model these processes, continuum models can be used, either in the form of softening plasticity or continuum damage mechanics. Yet, continuous models can not be applied to model crack propagation, because displacements are no longer continuous across the crack. Hence, a proper model for ductile fracture requires a different approach, one that combines a continuous softening model with a strategy to represent cracks, i.e. displacement discontinuities. This has been the main goal of the present work. In a combined approach, the direction of crack propagation is automatically determined by the localisation pattern, and its rate strongly depends on the evolution of damage (or other internal variables responsible for the strain softening). This contrasts with fracture mechanics, where the material behaviour is not directly linked to the crack propagation criteria. Softening materials have to be supplied with an internal length, which acts as a localisation limiter, thereby ensuring the well-posedness of the governing partial differential equations and mesh independent results. For this purpose, a nonlocal gradient enhancement has been used in this work, which gives similar results to nonlocal models of an integral form. A number of numerical methods are available to model displacement discontinuities in a continuum. In the present context, we have used a remeshing strategy, since it has additional advantages when used with large strain localising material models: it prevents excessive element distortions and allows to optimise the element distriviii bution through mesh adaptivity. As a first step towards a continuum-discontinuum approach, an uncoupled damage model is used first, in which damage merely acts as a crack initiation-propagation indicator, without causing material softening. Since uncoupled models do not lead to material localisation, no regularisation is needed. Yet, uncoupled approaches can not capture the actual failure mechanisms and therefore, in general, can give reliable results only when the size of the fracture process zone is so small that its effect can be neglected. When the size of the fracture process zone is large enough, a truly combined model must be used, which is developed in the second part of this study. Due to softening, the transition from the continuous damage material to the discrete crack occurs gradually, with little stress redistribution, in contrast with the previous uncoupled approach. The gradient regularised softening behaviour is introduced in the yield behaviour of an elastoplastic material. The combined model has been applied satisfactorily to the prediction of ductile failure under shear loading conditions. Third, to be able to apply the model to more general loading conditions, the material description has been improved by introducing the influence of stress triaxiality in the damage evolution of a gradient regularised elastoplastic damage model. The model has been obtained using the continuum damage mechanics concept of effective stress. Results show how compressive (tensile) states of triaxiality may increase (decrease) the material ductility. Finally, the combined approach is applied to the modelling of actual metal forming processes, e.g. blanking, fine blanking, score forming. The gradient regularisation has been implemented in an operator-split manner, which can be very appealing for engineering purposes. To capture the large strain gradients in the localisation zones, a new mesh adaptivity criterion has been proposed. The results of the simulations are in good agreement with experimental data from literature

Dynamic behaviour of AA 2024 under blast loading : experiments and simulations

The dynamic behaviour of AA2024-T3 is investigated. Dynamic tensile tests using a servo-hydraulic and a light weight shock testing machine (LSM) have been performed. The servo-hydraulic test machine proves to be more reliable and reaches higher strain rates. Neither test revealed any strain rate effect of AA2024-T3. Two types of fracture tests were carried out to determine the dynamic crack propagation behaviour of this alloy, using prestressed plates and pressurized barrels, both with the help of explosives. The prestressed plates proved to be not suitable, whereas the barrel tests were quite reliable, allowing to measure the crack speeds. Computer simulations with a user defined, rate dependent cohesive zone model were in agreement with experiments, capturing the rate toughening effect

Physics of IED Blast Shock Tube Simulations for mTBI Research

Shock tube experiments and simulations are conducted with a spherical gelatin filled skull–brain surrogate, in order to study the mechanisms leading to blast induced mild traumatic brain injury. A shock tube including sensor system is optimized to simulate realistic improvised explosive device blast profiles obtained from full scale field tests. The response of the skull–brain surrogate is monitored using pressure and strain measurements. Fluid–structure interaction is modeled using a combination of computational fluid dynamics (CFD) simulations for the air blast, and a finite element model for the structural response. The results help to understand the physics of wave propagation, from air blast into the skull–brain. The presence of openings on the skull and its orientation does have a strong effect on the internal pressure. A parameter study reveals that when there is an opening in the skull, the skull gives little protection and the internal pressure is fairly independent on the skull stiffness; the gelatin shear stiffness has little effect on the internal pressure. Simulations show that the presence of pressure sensors in the gelatin hardly disturbs the pressure field

Mechanisms of Hearing Loss after Blast Injury to the Ear

Given the frequent use of improvised explosive devices (IEDs) around the world, the study of traumatic blast injuries is of increasing interest. The ear is the most common organ affected by blast injury because it is the bodyﾒs most sensitive pressure transducer. We fabricated a blast chamber to re-create blast profiles similar to that of IEDs and used it to develop a reproducible mouse model to study blast-induced hearing loss. The tympanic membrane was perforated in all mice after blast exposure and found to heal spontaneously. Micro-computed tomography demonstrated no evidence for middle ear or otic capsule injuries; however, the healed tympanic membrane was thickened. Auditory brainstem response and distortion product otoacoustic emission threshold shifts were found to be correlated with blast intensity. As well, these threshold shifts were larger than those found in control mice that underwent surgical perforation of their tympanic membranes, indicating cochlear trauma. Histological studies one week and three months after the blast demonstrated no disruption or damage to the intra-cochlear membranes. However, there was loss of outer hair cells (OHCs) within the basal turn of the cochlea and decreased spiral ganglion neurons (SGNs) and afferent nerve synapses. Using our mouse model that recapitulates human IED exposure, our results identify that the mechanisms underlying blast-induced hearing loss does not include gross membranous rupture as is commonly believed. Instead, there is both OHC and SGN loss that produce auditory dysfunction

Physics of shock tube simulated IED blast for mTBI research

The objective of this research is to understand the blast propagation into the human skull and brain causing mTBI and use this knowledge for enabling design of effective protection measures against them. A shock tube including sensor system was optimized to simulate realistic IED blast profiles obtained from full scale field tests. Shock tube experiments and numerical simulations are combined in order to gain confidence in the measure devices (i.e. pressure, strain gages, etc) and understand the mechanisms of wave propagation in air and simplified ‘head’ surrogate. A cube and a sphere containing gelatin (brain simulant) are used for this purpose. The shock tube experiments are used as a reference to validate the numerical simulations combining a CFD (computations fluid dynamics) model of the air and a finite element model of the simplified ‘head’ surrogate. An accurate IED shock tube pressure-time profile depends on the dimensions of tube and driving unit and is highly sensitive to the exact location in the tube. The validity of the results depends on an accurate material characterization in the highly dynamic blast loading rate. Parameters necessary to characterize an IED blast wave will be explained as well as effect of material characteristics, skull openings etc. on the blast wave propagation into the simplified head surrogate

Blast resistance behaviour of steel frame structrures

The effect of a blast explosion on a typical steel frame building is investigated by means of computer simulations. The simulations help to identify possible hot spots that may lead to local or global failure. Since the blast energy is transferred to the structure by means of the façade, it is crucial to proper model the façade using an adequate failure criterion

Fracture and strain rate behavior of airplane fuselage materials under blast loading

The dynamic behavior of three commonly used airplane fuselage materials is investigated, namely of Al2024-T3, Glare-3 and CFRP. Dynamic tensile tests using a servo-hydraulic and a light weight shock testing machine (LSM) have been performed. The results showed no strain rate effect on Al2024-T3 and an increase in the failure strain and failure strength of Glare-3, but no stiffening. The LSM results on CFRP were inconclusive. Two types of fracture tests were carried out to determine the dynamic crack propagation behavior of these materials, using prestressed plates and pressurized barrels, both with the help of explosives. The prestressed plates proved to be not suitable, whereas the barrel tests were quite reliable, allowing to measure the crack speeds. The tougher, more ductile materials, Al2024-T3 and Glare-3, showed lower crack speeds than CFRP, which failed in a brittle manner

Fracture and strain rate behavior of airplane fuselage materials under blast loading

The dynamic behavior of three commonly used airplane fuselage materials is investigated, namely of Al2024-T3, Glare-3 and CFRP. Dynamic tensile tests using a servo-hydraulic and a light weight shock testing machine (LSM) have been performed. The results showed no strain rate effect on Al2024-T3 and an increase in the failure strain and failure strength of Glare-3, but no stiffening. The LSM results on CFRP were inconclusive. Two types of fracture tests were carried out to determine the dynamic crack propagation behavior of these materials, using prestressed plates and pressurized barrels, both with the help of explosives. The prestressed plates proved to be not suitable, whereas the barrel tests were quite reliable, allowing to measure the crack speeds. The tougher, more ductile materials, Al2024-T3 and Glare-3, showed lower crack speeds than CFRP, which failed in a brittle manner