Ongoing Research

Triaxiality Dependent Cohesive Zone Model for Ductile Fracture in Structural Metals

The dominant mechanism of fracture in ductile metals is the growth and coalescence of microvoids nucleating at second phase particles. Central to the growth of these voids is the triaxiality of the state of stress. In the ductile fracture of a structure, triaxiality of stress at fracture depends on whether the structure is un-notched, notched or pre-cracked. Only a model which can account for the variations in stress-state in prediction of initiation and propagation of macroscopic crack like defects would facilitate the transfer of fracture model parameters from a simple laboratory test to real complex structures.

Related papers:

• Rashid, F. M., Banerjee, A., 2017. Simulation of fracture in a low ductility aluminum alloy using a triaxiality dependent cohesive model. Engineering Fracture Mechanics, 179, pp. 1-12.
• Kanhurkar, N., Rashid, F. M., Banerjee, A., 2014. On the role of triaxiality in mode-I resistance curves. International Journal of Fracture, Vol. 188 (1), pp. 59-70.
• Rashid F. M., Banerjee A., 2013. Implementation and validation of a triaxiality dependent cohesive model: experiments and simulations. International Journal of Fracture. Vol. 181 (2), pp. 227-239.
• Banerjee A., Manivasagam R., 2009. Triaxiality dependent cohesive zone model. Engineering Fracture Mechanics Vol 76 (12), pp. 1761-1770.

Structure-Stress-Property Relation in Cortical Bone

Cortical or compact bone, found in the mid-shaft of load bearing bones like femur and tibia, is a brittle, porous biomaterial. Being a living tissue, the local microstructure and porosity network of the bone evolves in response to the mechanical stresses that the bone is subjected to, and this in turn modifies the local mechanical properties. Understanding the relationship between microstructure and mechanical properties is crucial for applications such as extraction of bone grafts, in design of mechanically compatible implants and porous scaffolds for bone tissue engineering, to interpret loading history, to evaluate the effectiveness of chemical and physical therapeutical measures for bone healing etc. An important aspect of this understanding is the development and testing of models that incorporate microstructural features and predict material properties such as failure strength, elastic modulus, fracture paths, etc. Such models, if general enough, would also be of use in understanding failure behavior of a wider class of brittle materials with a well defined porosity network, such as wood, rock etc.

Related papers:

• Mayya, A., Banerjee A., and Rajesh, R., 2017. Role of matrix behaviour on compressive fracture of bovine cortical bone. Physical Review E, 96(5).
• Mayya, A., Banerjee A., and Rajesh, R., 2016. Splitting fracture in bovine bone using a porosity based spring network model. Journal of the Royal Society Interface, 13(124) 20160809.
• Mayya, A., Banerjee A., and Rajesh, R., 2016. Haversian microstructure in bovine femoral cortices: An adaptation for improved compressive strength. Material Science and Engineering: C, 59, pp. 454-463.
• Mayya, A., Banerjee A., and Rajesh, R., 2013. Mammalian cortical bone in tension is non-Haversian. Scientific Reports 3: 2533.

Translaminar Fracture of Fiber-Reinforced Composite Laminates

We characterize translaminar fracture in plain-weave, fabric-reinforced composite for mode-I and loads with low mixity. From fractography the effect of mixity on failure mechanisms and complex crack paths is established. Fracture experiments performed on modified CT specimen are analysed using the modified compliance method assuming quasi-brittle behavior of the composite. Higher mixity is shown to encourage fiber-matrix debonding which while lowers the resistance to initiation of an effective crack, results in significantly higher energy dissipation for continued stable growth of the effective crack.

The mechanical as well as fracture properties of complex brittle materials, such as composite materials, are known to be statistically distributed. The fracture process in them involves formation of microcracks at multiple sites, their interaction and growth leading to final failure which is hard to model using classical fracture mechanics theory or using standard finite element method. We develop a random spring-network model to simulate the characteristic features of the fracture process and its dependence on the mode-mixity as observed in the experimental data.

Related papers:

• Boyina, D., Kirubakaran, T., Banerjee, A., Velmurugan, R., 2015. Mixed-mode translaminar fracture of woven composites using a heterogeneous spring network. Mechanics of Materials, 91, pp. 64-75.
• Boyina, D., Banerjee, A., Velmurugan, R., 2014. Mixed-mode translaminar fracture of plain-weave composites. Composites Part B: Engineering Vol. 60, pp. 21-28.

Fatigue Crack Growth Modeling using Cohesive Zone Model

Micro-structural damage due to repetitive sub-critical loads leading to fatigue failure is a key issue in design and assessment of structural integrity of components such as rotating parts of automobiles, fuselage and other components of aircrafts, compressors, pumps, and turbines. Classical approaches to design against fatigue failure of a specific material involve characterisation of total fatigue life to failure in terms of cyclic stress range (S–N curve). For defect tolerant design of the same material, the region of steady crack growth under cyclic load of an initially cracked specimen is represented by the Paris law. Both, stress-life approach and Paris law, are empirical in nature and though they describe the fatigue behavior of the same material, they use an independent set of parameters. As a result, the prediction of fatigue life at different stress states requires determination of a large set of parameters that can be used for prediction only for a limited range of conditions.

Related papers:

• Nijin I.S., Kumar R.S., Banerjee A., 2018. Role of stress-state on initiation and growth of a fatigue crack. International Journal of Fatigue.
• Kumar R. S., Nijin I. S., Bharadwaj, M. V., Rajkumar, G., Banerjee A., 2016. Stress-state dependent model for fatigue crack growth. Frattura ed Integrità Strutturale. 38:p19.
• Jha D., Banerjee A., 2012. A cohesive model for fatigue failure in complex stress-states. International Journal of Fatigue Vol 36 (1), pp. 155-162.

Indentation Fracture of MEMs Materials

Polysilicon is a brittle material whose strength statistics is strongly affected by the presence of defects that are inherent to the manufacturing processes and are typically characterized by Weibull type distributions. The fracture strength of polysilicon is an important indicator of the mechanical performance of any device and thus, it being scattered compromises overall reliability. For safe operation and life assessment of these devices it is of importance to incorporate the statistical nature of the fracture properties of MEMS materials in the design process. Here, 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. 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.

Related papers:

• Satheesh, S. M., Banerjee, A., Bhattacharya, E., 2017. Determination of polysilicon Weibull parameters from indentation fracture. Thin Solid Films, 642, pp. 76-81.

Compressive Response of Brittle Cellular Solids

Cellular solids, being light weight and efficient energy absorbers, find wide applications in packaging, structural, thermal insulation, crash worthiness etc. Under compression, cellular solids, owing to their geometric construction, are able to withstand large strains during which crushing of cellular solid occurs at a nearly constant load; even though the parent material is of brittle behaviour. For damage tolerant design of these cellular solids it is of great importance to develop predictive models that are able to incorporate local fracture processes to reproduce their effective macroscopic response to large compressive loads.

Fracture of Adhesive Bonded Joints

Adhesive bonding has been recognized as a potential replacement for the traditional joining methods (welding and riveting) in many structural applications. To ensure the structural integrity of adhesive bonded joints it is of importance to characterize their fracture properties and develop predictive models with easily transferable model parameters.

In the fracture behavior of adhesive bonded joints, the adhesive layer thickness is known to play a significant role. The thinner adhesive layer develops higher constraint compared to relatively thicker adhesive layers and in order to capture these effects for different constraint conditions it is shown that, cohesive model that incorporates the stress-state effects in traction-separation law is better suited than a conventional fixed cohesive law.