Damage characteristics in flex-beam like structures for hinge-less and bearing-less helicopter rotor blades:


An efficient and economic design of all composite structures for hinge-less and bearing-less helicopter rotor blades often requires carefull investigation of their damage characteristics. The rotor blades rotating at a high speed are the primary load-carrying members in a helicopter. The appearance of cracks or delaminations in composite rotorcraft flexbeams can lead to degradation in flapwise and lagwise performance of the rotor blade. Also, as the inflicted damage may propagate catastrophically during operation, online structural health monitoring is necessary to ensure safety. The main problem in helicopter tail rotor flexbeams, which are made of 0 deg predominantly, is longitudinal cracks. These cause very less stiffness reduction. Hence the use of laminated composites in rotorcraft flexbeams requires assessment of their damage such that even longitudinal cracks causing very small reduction in stiffness can be detected. This is a primary motivation for exploring nonlinear nonclassical effects in these flex-beam like structures, which are far more sensitive to stiffness changes.

1. Salunkhe, S., Singh, C. V. and Guruprasad, P. J., 2017. Effect of matrix cracks and delamination on extension-twist coupling of thin pretwisted composite strips. Composite Structures, 180: 234-250.

2. Thirupathi, M., Mitra, M. and Guruprasad, P. J., 2017. Spectrally formulated one-dimensional element for analysis of wave propagation in pretwisted anisotropic strips. Composite Structures, 162: 261-270.

Size effects in crystalline materials:

The goal of this research work is to further the understanding of crystal plasticity, particularly at reduced structural and material length scales. Fundamental understanding of plasticity is central to various challenges facing design and manufacturing of materials for structural and electronic device applications. The development of microstructurally tailored advanced metallic materials with enhanced mechanical properties that can withstand extremes in stress, strain, and temperature, will aid in increasing the efficiency of power generating systems by allowing them to work at higher temperatures and pressures. High specific strength materials can lead to low fuel consumption in transport vehicles. Experiments have shown that enhanced mechanical properties can be obtained in materials by constraining their size, microstructure (e.g. grain size), or both for various applications. For the successful design of these materials, it is necessary to have a thorough understanding of the influence of different length scales and evolving microstructure on the overall behavior. The objectives of this research work are: (1) to investigate scale and size effects due to dimensional constraints; (2) to investigate size effects due to microstructural constraints; and (3) to develop a size dependent hardening model through coarse graining of dislocation dynamics.

1. Kiener, D., Guruprasad, P. J., Dehm, G., and Benzerga, A. A, 2011. Work hardening in micropillar compression: In situ experiments and modeling. Acta Materialia, 59:3825-3840.

Fatigue in metals and metallic alloys:

The main objective of the research work is to develop a computational framework to investigate fatigue damage initiation in metals and metallic alloys. The framework is based on fundamental material physics at the mesoscale level, which captures the plasticity behavior of the material by directly accounting for nucleation, glide and interaction of dislocations. This framework naturally accounts for the development of a heterogeneous stress field within the material, unlike classical continuum plasticity models. It is proposed to use this framework to develop an understanding between the emergence of Persistent Slip Bands (PSBs) during fatigue and nucleation of cracks at the surface grains of the material.

The computational method used in the above two research topics is based on the coupling between discrete dislocation dynamics (DDD) and finite element method (FEM). The fields (displacements, strains and stresses) due to dislocations are obtained analytically by assuming the dislocations to be embedded in a homogeneous, isotropic, infinite elastic medium. The complimentary problem for the image fields, which correct for the applied boundary conditions, is solved using FEM. The total fields are then determined by the superposition of the fields obtained from DDD and FEM. In this framework, plasticity is a natural outcome of the glide of the dislocations, which are driven by the Peach-Koehler force. The glide of dislocations is governed by a viscous-drag relation. In addition to this, the framework accounts for short-range dislocation interactions, for example: formation of Lomer-Cottrell locks as shown below.

1. Load, A., Tak, T. N., Prakash, A., Guruprasad, P. J., Hutchinson, C. and Samajdar, I., 2017. Relating residual stress and substructural evolution during tensile deformation of an Aluminum-Manganese alloy. Metallurgical and Materials Transcations A, 48(11): 5317-5331.

Development of novel shape memory composite for morphing applications:

Traditionally, morphing in aviation has been discussed within the context of enabling an aircraft to achieve optimum flight conditions under conflicting mission requirements. For example, a common proposed mission requirement is to achieve optimum flight conditions under cruise and dash [1]. More recently, on the 29th April, 2015, NASA released a press circular claiming the successful flight tests on shape-changing wing for next generation aviation [2]. Initial test results and the inferences drawn from the test flight, using a new morphing wing technology, project fuel cost saving, airframe weight reduction and an overall decrease in aircraft noise during takeoffs and landings. Infact, a successful application of morphing technology has been in the design of reconfigurable engine nozzle fan chevron [3] in the Boeing 787 airplane. Shape memory alloy has been used as the material enabling this variable geometry configuration of the chevron (VGC). Boeing Inc. claims that the VGC design has helped them to achieve community noise reduction, shockcell noise reduction and improved cruise performance. The above two examples highlight the utility of adaptive structures in enabling morphing in airplanes for reasons other than just achieving optimum flight conditions. The examples show that morphing can also be effectively used to improve system’s performance even under single mission involving multiple operating conditions. In this project we focus on conceiving, designing and developing technology towards achieving hingeless control surfaces using shape memory composites

1. Saurav, A. K., Mujumdar, P. M. and Guruprasad, P. J., 2017. Numerical implementation of phase transition based constitutive model for 2D and 3D SMP structural elements. Proceedings of The seventh International Conference on Theoretical, Applied, Computational and Experimental Mechanics, Kharagpur, India.

2. Bhola, L., Mujumdar, P. M. and Guruprasad, P. J. 2017. Thermo-mechanical analysis of an SMP composite and sandwich structure. Proceedings of the nineetenth National Seminar on Aerospace Structures, Vellore, India.