Anuradha Banerjee

We examine the specific role of the structure of the network of pores in plexiform bone in its fracture behaviour under compression. Computed tomography scan images of the sample pre- and post-compressive failure show the existence of weak planes formed by aligned thin long pores extending through the length. We show that the physics of the fracture process is captured by a two-dimensional random spring network model that reproduces well the macroscopic response and qualitative features of fracture paths obtained experimentally, as well as avalanche statistics seen in recent experiments on porcine bone.

Microstructural variations in bovine femoral cortices and its possible implications for the bone's mechanical behavior are characterized for a mature and a young bovine femur. Histological examination at several locations shows the presence of Haversian systems to be largely confined to the posterior region of any cross-section. Haversian bone is shown to have higher compressive strength than the non-Haversian primary bone present in the corresponding anterior regions. The anatomical variation in the compressive strength along diaphysis is found to correlate strongly with the Haversian density. Based on the differences in the failure surfaces observed from compressive failure, it is argued that the presence of Haversian systems plays a role in deflection of crack path, leading to non-prismatic failure surfaces. As biomaterials, such as bone cement and implants, closely interact with bone material, the structure-property relation established here can provide a basis for better design of future biomaterials.

In the cohesive framework, a stress-state dependent cohesive model, combined with an irreversible damage parameter has been used in simulation of fatigue crack growth initiation and continued growth. The model is implemented as interface elements and plane strain simulations of crack initiation and growth under cyclic loading are performed. The stressstate of neighboring continuum elements is used in the traction-separation behavior of the cohesive elements. The model is shown to be able to reproduce the typical initiation life as well as fatigue crack growth curves. Further, the effect of the cohesive fatigue parameter on the initiation life and crack growth rates is established.

Insight into the role of triaxiality in mode-I, plane strain resistance curves of a representative ductile metal has been gained. Growth of a macroscopic crack is simulated as per modified boundary layer formulation for a range of constraint parameter with the fracture process represented by a triaxiality dependent cohesive model. In contrast to the predictions by a fixed cohesive law, the study shows that by including the effect of triaxiality on the work of separation, the stick-slip nature or the non-uniformity in the rate of the crack growth and its manifestations on the plastic wake and fracture surface can be predicted that are closer to trends observed in experimental literature. © 2014 Springer Science+Business Media Dordrecht.

In the simulation of the ductile fracture process in a low ductility aluminum alloy, the limitations of the current implementation of a stress-state dependent cohesive model are identified. Ductile fracture data was generated at moderate triaxiality with experiments on a range of notched bars while at high triaxiality in growth of a pre-existing mode-I crack in compact test specimens. In the corresponding finite element analysis, cohesive elements obeying a stress-state dependent cohesive law were introduced in the plane where material separation was expected to occur. By recognizing that the effect of model parameters is decoupled in fracture at moderate triaxiality, a procedure is outlined to determine the unique combination of model parameters that is shown to reproduce the experimental data for the entire range of triaxiality well. It is argued that the necessity of a plane strain core and its thickness is largely driven by the extent to which plastic deformation spreads during the growth of crack.

A local approach is taken to characterize statistical distribution in fracture behavior of polysilicon thin film using indentation fracture data. Berkovich indentation tests were performed on a three micron thick polysilicon film and the average fracture toughness was evaluated using conventional approach, that ignores the effect of other layers, to be 0.7 MPam. Weakest link theory when applied to an equivalent model of center loaded penny crack that is wedged open with uniform indent pressure is able to estimate the Weibull strength and modulus for the polysilicon to be well within the range reported in literature. Further, using the Weibull parameters, thus determined, on model of standard fracture test specimen, fracture toughness of polysilicon is estimated to be 1.5 MPam. A discussion is developed emphasizing the importance of local approach that accounts for the differences in elastic behavior of the layers while characterizing fracture toughness.

This study focuses on crack identification in beams from vibration measurements using principles of dynamic state estimation. The FEM is used to model the beam with cracked-beam elements that account for the presence of an edge crack under near-tip elasto-plastic conditions. The crack size and its location are treated as the variables that are identified using a particle filter algorithm. A parametric study is first carried out with synthetic measurements to numerically analyze the performance of the algorithm. Subsequently, using measurements acquired from physical experiments involving a cantilever beam subjected to arbitrary excitations, the proposed algorithm is used to identify the size and location of crack-like defects. The proposed method does not require measurements of the undamaged beam, hence, can be used for crack identification in beams for which no earlier measurements are available.

The present study characterizes translaminar fracture in plain-weave, fabric-reinforced composite under mixed-mode loads. From fractography of the fracture specimen, the effect of mixity on the failure mechanisms and crack paths is established. Finite element analysis is performed using effective elastic properties of the composite material obtained from meso-scopic analysis. The fracture properties are characterized based on modified compliance method assuming quasi-brittle behavior of the composite. Higher mixity is shown to encourage fiber-matrix debonding which while lowers the fracture toughness for initiation of an effective crack, results in significantly higher energy dissipation for continued stable growth of the effective crack. © 2014 Published by Elsevier Ltd.

In the present work, the role of structure of porosity network in fracture behavior of plexiform bone under compression is examined. Using a 2D spring network model, the fracture paths and macroscopic response are determined for a range of elastic and fracture behavior of the springs. The differences in response for samples with similar overall porosity are attributed to the differences in the porosity network and distribution. Further, simulations with homogeneous distribution of porosity show that it is desirable to have short and diffused porosities, similar to remodeled bone.

Uni-axial compressive failure of silica-epoxy based heterogeneous honeycombs is investigated in detail for a range of volume fractions. Introduction of heterogeneity in compression of staggered-square honeycomb is seen to result in damage initiation at multiple locations and subsequent damage growth to be more stable compared to pure epoxy in which damage was observed to be localized until peak load when catastrophic failure of the honeycomb specimen occurs. The increase in stiffness and comparative stability of the response is accompanied with reduction in strength, however, between 0 and 5% the total work of compressive failure is comparable. From the elastic-plastic analysis it is evident that the non-linearity in the response of pure honeycombs, prior to peak load, is largely due to formation of plastic hinges near corners of cells, whereas in case of heterogeneous honeycomb the non-linearity is mostly due to debonding of hard filler particles and matrix cracking leading to damage growth in cell walls.