mixed mode crack propagation during needle penetration for surgical interventions

Abstract An accurate description of the penetration mechanics of flexible needles into target soft tissues is a complex task, including friction at the needle-tissue interface, large strains, non-predetermined penetration trajectories, fracture under mixed-mode loading and so on. In the present work, a finite element algorithm is employed to simulate the two-dimensional deep penetration of a flexible needle in a soft elastic material. The fracture process of the target material during penetration is described by means of a cohesive zone model, with a suitable mixed-mode criterion for determining the propagation direction of the crack. To illustrate the potential of the numerical algorithm, we have performed some simulations of the insertion of a flexible needle with an asymmetric tip, and the results are presented in terms of force-penetration curves as well as of the obtained penetration paths in the target tissue

Mode II fracture toughness for non-planar frictional cracks

Abstract Traction-free and planar cracks represent a rather idealized picture of the physical reality, commonly used in fracture mechanics problems. In the present paper, the influence of roughness and friction of crack surfaces is examined in relation to both the resulting near-tip stress field and the fracture resistance under monotonic mixed-mode loading. A two-dimensional model is presented where an elastic-plastic-like constitutive interface law is adopted to describe the Mode I/II coupling between displacements and tractions along the crack surfaces. The solution is obtained using the Distributed Dislocation Technique (DDT). By considering a linear piecewise periodic profile of the crack, the present model is employed to quantify the mode II fracture toughness of different types of natural stones under varying mode I compressive load

LASER-BASED ADDITIVELY MANUFACTURED POLYMERS: A REVIEW ON PROCESSES AND MECHANICAL MODELS

Additive manufacturing (AM) is a broad definition of various techniques to produce layer-by-layer objects made of different materials. In this paper, a comprehensive review of laser-based technologies for polymers, including powder bed fusion processes (e.g. selective laser sintering, SLS) and vat photopolymerisation (e.g. stereolithography, SLA), is presented, where both the techniques employ a laser source either to melt or cure a raw polymeric material. The aim of the review is twofold: (i) to present the principal theoretical models adopted in the literature to simulate the complex physical phenomena involved in the transformation of the raw material into AM objects; (ii) to discuss the influence of process parameters on the physical final properties of the printed objects, and in turn on their mechanical performance. The models being presented simulate: the thermal problem along with the thermally activated bonding through sintering of the polymeric powder in SLS; the binding induced by the curing mechanisms of light-induced polymerisation of the liquid material in SLA. Key physical variables in AM objects, like porosity and degree of cure in SLS and SLA respectively, are discussed in relation to the manufacturing process parameters, as well as to the mechanical resistance and deformability of the objects themselves

Size effect on the fracture resistance of rough and frictional cracks

Elastic fracture mechanics commonly defines the fracture resistance of brittle materials within an idealised picture of planar and traction free cracks. An efficient approach to describe the interface conditions in realcracks, such as those occurring in concrete, ceramics or stones, is to include the effect of both roughness and friction by means of a constitutive relationship between opposite points on the interface. In the present paper,we use a numerical technique, based on the solution of singular integral equations, to derive the near-tip stress field with various interface conditions. Then, the technique is applied to investigate the size effect of the interface roughness, where such an effect is related to the ratio between the characteristic length of the roughness and the nominal length of the crack. It is found that the resulting near-tip stresses can be profoundly influenced by the crack path, particularly for short cracks

size effect on the fracture resistance of rough and frictional cracks

Elastic fracture mechanics commonly defines the fracture resistance of brittle materials within an idealized picture of planar and traction-free cracks. An efficient approach to describe the interface conditions in real cracks, such as those occurring in concrete, ceramics or stones, is to include the effect of both roughness and friction by means of a constitutive relationship between opposite points on the interface. In the present paper, we use a numerical technique, based on the solution of singular integral equations, to derive the near-tip stress field with various interface conditions. Then, the technique is applied to investigate the size effect of the interface roughness, where such an effect is related to the ratio between the characteristic length of the roughness and the nominal length of the crack. It is found that the resulting near-tip stresses can be profoundly influenced by the crack path, particularly for short crack

Fracture toughness of rough and frictional cracks emanating from a re-entrant corner

In mixed-mode conditions, the competing contribution of the different stress intensity factors predicts fracture initiation load as well as crack propagation direction. Commonly, mixed-mode fracture resistance is based on the assumption of smooth and frictionless cracks. However, the effect of friction and roughness cannot be neglected when mixed mode loading occurs, as in the case of a crack emanating from a re-entrant corner. In this paper, the effect of friction and roughness is evaluated through a simple saw-tooth model in a three-quarter-infinite plane (corresponding to a 90 degree re-entrant corner). The crack surfaces are assumed to be globally smooth, and roughness and friction are incorporated through a constitutive law between opposite crack surfaces. The solution is found using the distributed dislocation method, and an iterative algorithm is needed due to the non-linearity of the model. The effect of friction and roughness angle is discussed for a simple case

Nutritional characterization and shelf-life of packaged microgreens

Comprehensive nutritional profile of six microgreens, including proximate composition and bioactive compounds

Poro-viscoelastic material parameter identification of brain tissue-mimicking hydrogels

Understanding and characterizing the mechanical and structural properties of brain tissue is essential for developing and calibrating reliable material models. Based on the Theory of Porous Media, a novel nonlinear poro-viscoelastic computational model was recently proposed to describe the mechanical response of the tissue under different loading conditions. The model contains parameters related to the time-dependent behavior arising from both the viscoelastic relaxation of the solid matrix and its interaction with the fluid phase. This study focuses on the characterization of these parameters through indentation experiments on a tailor-made polyvinyl alcohol-based hydrogel mimicking brain tissue. The material behavior is adjusted to ex vivo porcine brain tissue. An inverse parameter identification scheme using a trust region reflective algorithm is introduced and applied to match experimental data obtained from the indentation with the proposed computational model. By minimizing the error between experimental values and finite element simulation results, the optimal constitutive model parameters of the brain tissue-mimicking hydrogel are extracted. Finally, the model is validated using the derived material parameters in a finite element simulation

In Situ Formation of Zwitterionic Ligands: Changing the Passivation Paradigms of CsPbBr3 Nanocrystals

CsPbBr3 nanocrystals (NCs) passivated by conventional lipophilic capping ligands suffer from colloidal and optical instability under ambient conditions, commonly due to the surface rearrangements induced by the polar solvents used for the NC purification steps. To avoid onerous postsynthetic approaches, ascertained as the only viable stability-improvement strategy, the surface passivation paradigms of as-prepared CsPbBr3 NCs should be revisited. In this work, the addition of an extra halide source (8-bromooctanoic acid) to the typical CsPbBr3 synthesis precursors and surfactants leads to the in situ formation of a zwitterionic ligand already before cesium injection. As a result, CsPbBr3 NCs become insoluble in nonpolar hexane, with which they can be washed and purified, and form stable colloidal solutions in a relatively polar medium (dichloromethane), even when longly exposed to ambient conditions. The improved NC stability stems from the effective bidentate adsorption of the zwitterionic ligand on the perovskite surfaces, as supported by theoretical investigations. Furthermore, the bidentate functionalization of the zwitterionic ligand enables the obtainment of blue-emitting perovskite NCs with high PLQYs by UV-irradiation in dichloromethane, functioning as the photoinduced chlorine source.publishedVersionPeer reviewe