K. S. R. K. Murthy’s research while affiliated with Indian Institute of Technology Guwahati and other places

Low-velocity impact on fiber-reinforced polymer (FRP) composites may cause subsurface interface delaminations. In the case of such multiple delaminations, neighboring delaminations may grow and coalesce into larger delamination under post-impact loading. The resistance to such growth is substantially influenced by the strength and stiffness of epoxy resins used in FRP composite laminates. Therefore, the current study attempts to determine how adding carbon nanotubes (CNTs) to epoxy might potentially enhance the resistance to such delaminations. A three-dimensional (3D) finite element analysis has been carried out for laminates having single embedded delamination as well as two neighboring embedded delaminations to determine the interlaminar stresses and strain energy release rate using virtual crack closure integral. Results from the current analysis show that while parameters like shape, size, and relative spacing of delamination influence the growth, however in all the cases, adding CNTs to epoxy significantly improves the resistance to the growth of both single and interacting delaminations. The effect is observed to be more pronounced when the delaminations are closely spaced and tend to coalesce into a large delamination.

The present work aims to investigate the effect of adding carbon nanotubes (CNTs) to epoxy on enhancing the resistance to growth of an existing embedded delamination in post-impact loading in a carbon/CNT + epoxy laminate. 3D finite element analyses are performed to determine the interlaminar stresses and strain energy release rate components at the elliptical delamination front which is elliptical using Irwin’s crack closure integral. Results show that a significant increase in resistance to growth of embedded elliptical delamination could be achieved by mixing CNTs to epoxy limited to a specific weight percentage of CNTs only.

A sharp V-notched plate under plane stress loading conditions is always accompanied by a strong three-dimensional (3D) stress–strain region close to the notch tip, followed by a 3D-2D transition state and a region dominated by the 2D stress–strain field. Experimental studies on planar V-notched bodies constitute an important part of notch fracture mechanics and therefore it is important to know the extents of the above fields for accurate measurement of field variables. In the present work, experimental verification of the extent of the 3D stress–strain field, 3D-2D transition zone and plane stress zone ahead of the tip of a sharp V-notch has been carried out in order to suggest a minimum radius required for identification of the plane stress dominant zone. Knowing this value apriori is vital for accurate sampling of the 2D field variables in any experimental study on the sharp V-notched configurations. In order to achieve this, strain gage experiments on the single-edge notched tensile (SENT) specimens have been conducted. The above zones have been identified by studying the error in the measured mode I NSIF of the SENT specimen. The experimental results of the current study agree well with the results of published numerical works. The results clearly show that the minimum radius of the plane stress dominant zone has been found to be 1.25 times the thickness of the plate. Results also show that 1.5 times the thickness is the most conservative and dependable value for this radius. The results of the present investigation also indicate that not only the identification of the plane stress region, but also the locations of the sampling points within the plane stress region are important for achieving the required accuracy.

The singularity of the sharp V-notch problems changes with the change in notch angle. It is not possible to employ quarter-point elements (QPEs) for the analysis of sharp V-notches, as they provide only the inverse square root singularity. If QPEs are also used for notches, then a unified analysis of notches and cracks can be performed with less computational cost. In the present work, for the very first time, an attempt is made to use the QPEs to compute the notch stress intensity factors (NSIFs) of sharp V-notch problems. To this end, a point substitution technique is proposed for the calculation of mixed-mode (I/II) NSIFs using the QPEs at the notch tip. Certain critical points are identified on the QPEs along the notch flanks using the proposed technique. Utilizing finite element displacements at these critical points, the mixed-mode NSIFs are computed on coarse meshes. The results for several mode I, mode II and mixed-mode (I/II) benchmark example problems clearly show excellent agreement with the published solutions and provide a way to employ the QPEs in sharp V-notch problems.

In the present work, mixed mode (I/III) fracture studies have been investigated using a new specimen and an out- of-plane loading fixture that can be used with the conventional uniaxial universal testing machines. The proposed specimen is a single edge cracked circular (SECC) specimen which can be employed on both metallic and non- metallic materials under static and fatigue loads. A large number of three-dimensional finite element analyses have been performed on the proposed specimen setup to demonstrate the efficacy of the proposed setup. Experimental fracture studies have also been performed under mixed mode (I/III) loading conditions. The pre- sent experimental results are in very good agreement with the available mixed mode (I/III) fracture criteria. Experimental results indicate that the pure mode III fracture toughness is nearly 1.46 times greater than that of the mode I. This has been found due to an increase in the inelastic region around the crack tip in pure mode III and is in accordance with the published results. Detailed analyses of the fractured surfaces have also been carried out. It has been demonstrated that the fractured surfaces become more rough as the mode III component in- creases. Preliminary results of mixed mode (I/III) fatigue crack growth studies on Al 7075-T6 using the metallic SECC specimen have also been presented. The results of the present study confirm that the proposed specimen setup can be used to study the mixed mode fracture behavior of linear elastic materials under static and fatigue loading.

In fibre reinforced polymer (FRP) composite laminates subjected to low velocity impact, one of the common mode of failure is ply breakage followed by interfacial delamination at the site of ply break. Improving the delamination resistance at the interface is therefore important and the present work examines quantitative understanding of how the addition of CNTs to epoxy might enhance the resistance to such delamination at the interface. A laminate made of plies with CNT-epoxy as the matrix and carbon fibre as the reinforcement has been considered to be broken through the full width. A full three dimensional FE analysis was carried out for such a carbon/CNT-epoxy laminate for different wt% of CNTs. Resistance to delamination has been assessed by computing critical strain energy release rate ( G c ) using quadratic stress criterion (QSC) and virtual crack closure integral (VCCI). From the results, by adding CNTs to epoxy, a significant improvement in the delamination resistance could be observed. However, the improvement is till a certain specific CNT wt% and adding CNTs further leads to a significant reduction in delamination resistance. In addition, the effect of various factors such as fiber orientation of the adjacent continuous ply and resin layer thickness on critical strain energy release rate ( G c ) have been studied. The results show that G c also depends upon the fiber orientation of the adjacent ply and on the resin layer thickness. As the resin layer thickness increases, a significant increase in G c is observed.