Kali Babu Katnam

The University of Manchester · Department of Mechanical, Aerospace and Civil Engineering

A reliable numerical damage model has been developed for adhesively bonded joints under fatigue loading that is only dependant on the adhesive system and not on joint configuration. A bi-linear traction–separation description of a cohesive zone model was employed to simulate progressive damage in the adhesively bonded joints. Furthermore, a strain-...

Advanced composite materials have gained popularity in high-performance structural designs such as aerospace applications that require lightweight components with superior mechanical properties in order to perform in demanding service conditions as well as provide energy efficiency. However, one of the major challenges that the aerospace industry f...

Three-dimensional and two-dimensional Digital Image Correlation (DIC) have been used to evaluate the evolution of deformation and strain in composite single lap bonded joints prior to failure. In general, composite components are increasingly being joined using structural adhesives for aerospace and other safety critical applications. Reliable desi...

Adhesively bonded composite single lap joints were experimentally investigated to analyze the bondline stress concentrations and characterize the influence of adhesive ductility on the joint strength. Two epoxy paste adhesives—one with high tensile strength and low ductility, and the other with relatively low tensile strength and high ductility—wer...

Structural adhesives are being widely used in the aerospace and automobile industries. However, in many applications, aggressive environments damage the adhesive systems and degrade the structural performance of bonded members. Cohesive zone models are often employed in the numerical analysis of adhesively bonded structural joints. To accurately mo...

Traditional thermosetting composites have high specific strength and stiffness but exhibit poor damage tolerance. To address this, fibre hybridisation emerges as a cost-effective strategy. This study investigates the transverse shear failure of fibre-hybrid unidirectional composite laminae, specifically primary E-glass fibres with secondary fibres...

In this paper, the effect of intra-yarn fibre hybridisation on the homogenised elastic properties and micro- and meso-scale matrix stress fields in 2D woven composite laminae (i.e. plain, 2/2 basket, 2/2 twill and 5-harness satin) is studied with a two-scale homogenisation scheme—employing a representative volume element model at micro-scale and a...

This paper investigates the effect of intra-laminar fibre hybridisation, i.e., primary and secondary fibres within a matrix, on the homogenised properties and micro-stress fields in uni-directional polymer composite laminae. The study is focused on S-glass/epoxy laminae which are hybridised with secondary fibres (e.g., polypropylene). Two-dimension...

In this work, unidirectional composite laminae with intra-laminar fibre hybridisation (i.e. two fibre types within a matrix) are studied to understand the influence of solid and hollow glass fibre content on the homogenised specific lamina properties of and the matrix micro-stress fields in carbon/solid-E-glass/epoxy and carbon/hollow-E-glass/epoxy...

This study investigates the effect of fiber‐hybrid non‐woven veils on the interlaminar fracture of out‐of‐autoclave carbon/epoxy laminates. The Mode‐I and Mode‐II interlaminar toughening performance and R‐curves behavior of out‐of‐autoclave composite laminates with carbon and thermoplastic fiber‐hybrid veils are investigated. Pure carbon veil (i.e....

This study investigates the effect of intra-laminar fibre hybridisation on the homogenised lamina properties and micro-stress fields, with an emphasis on natural and synthetic fibre combinations. A representative volume element (RVE) approach, considering random fibre distributions, is employed to study unidirectional flax/E-glass/epoxy composite l...

Voids in composite laminates can influence damage initiation, propagation and lead to catastrophic failure. It is therefore important to characterize the morphology and content of voids reliably in order to decide deployment or disposal of a composite component. In this work, thin carbon/epoxy non-crimp-fabric laminates with varying void contents/p...

Core-shell rubber (CSR) particles and thermoplastic polyphenylene sulphide (PPS) veils are used to achieve hybrid interlaminar toughening of carbon/epoxy laminates by using vacuum assisted resin infusion and out-of-autoclave curing. Double cantilever beam (DCB) tests are carried out to investigate the effect of hydrothermal conditioning on the mode...

The effect of short micro-fibre-based veils on the mode-I and mode-II interlaminar fracture energies and R-curves of composite laminates is investigated in this study—with an emphasis on the role of veil binders. Non-woven polyphenylene sulfide veils with 10 g/m² areal weight and two different binders (i.e. cross-linked styrene and cross-linked pol...

Conventional thermosetting composites laminates have high specific in-plane properties but low damage tolerance. An approach to enhance damage tolerance in composite laminates is to use towlevel fibre hybridization with thermoplastic fibres. This paper aims to investigate post-cure thermally-induced residual micro-stress fields within the matrix an...

In this paper, the effect of intra-yarn hybridisation on the macroscopic homogenised properties of natural fibre-based hybrid composites (NFHCs) was computationally investigated. Many researchers have experimentally investigated hybrid effects on their mechanical properties. Only a limited number of computational studies on NFHCs have been reported...

This study aimed to develop a rigorous method for reproducing the microstructure of UD lamina with yarn-level hybridization. In the computational model, two different fibres are combined in an epoxy matrix. The RVE's von Mises stress distribution is investigated under transverse loadings. The effect of the fibre type on the matrix varies. In this c...

This paper investigates the effect of non-hybrid and hybrid toughening, via core-shell rubber (CSR) nanoparticles and non-woven micro-fibre veils, on the delamination resistance and crack migration in carbon fibre/epoxy laminates under mode-I and mode-II conditions—with an emphasis on the effect of veil fibre properties on toughening mechanisms and...

This study investigates the influence of hybrid toughening—via core-shell-rubber (CSR) particles and non-woven thermoplastic veils—on the delamination resistance, crack migration and R-curve behaviour in carbon fibre/epoxy laminates under mode-I and mode-II conditions. Core-shell rubber particles, varying in size from 100 nm to 3 μm, with 0–10 wt%...

View Video Presentation: https://doi.org/10.2514/6.2022-1422.vid The objectives of this research are to study the role of weave architecture (i.e. binder tow size and binder tension) and post-cure cooling rate on thermally-induced macro-scale residual stress distribution in orthogonal 3D woven composites. Multi-scale representative volume element m...

Fibre-reinforced polymer (FRP) composites generally have a layered architecture and are commonly manufactured with thermosetting resins—making them susceptible to interlaminar fracture (i.e. delamination), which is often a major concern in structurally critical applications. As a result, various approaches have been explored to enhance interlaminar...

This is the accepted version of the paper. This version of the publication may differ from the final published version. Permanent repository link: https://openaccess.city.ac.uk/id/eprint/24143/ Link to published version: http://dx. Abstract Accelerated curing of high performance fibre-reinforced polymer (FRP) composites via microwave heating or rad...

This is the accepted version of the paper. This version of the publication may differ from the final published version. Permanent repository link: https://openaccess.city.ac.uk/id/eprint/24143/ Link to published version: http://dx. Abstract Accelerated curing of high performance fibre-reinforced polymer (FRP) composites via microwave heating or rad...

Advanced composite materials are widely used in civil and military applications due to their superior specific strength and stiffness. However, they are susceptible to damage during impact loading. In order to improve the impact performance, yarn hybridisation and fibre architecture have been investigated in this study in order to evaluate their in...

This study investigates the influence of thermoplastic non-woven veils on the interlaminar fracture energy and toughening mechanisms under mode-I dominant loading conditions in out-of-autoclave resin-infused carbon fibre-epoxy laminates. Two different non-woven micro-fibre veils, Polyetherimide (PEI) and Polyphenylene Sulfide (PPS), of low areal we...

An experimental study is conducted with the aim of balancing in-plane mechanical properties and impact damage tolerance in composite woven laminates using a quasi-unidirectional (quasi-UD) woven fabric (i.e. low crimp architecture) and yarn-level fibre hybridisation. In this work, composite laminates are manufactured with and without yarn-level hyb...

Advanced composites are widely used in primary and secondary structural applications, for example, aerospace, automotive, marine, and renewable energy sectors. But it is well recognized that the impact resistance and damage tolerance of composite laminates are in general poor, which is a major challenge for optimizing structural designs. In this re...

Accelerated curing of high performance fibre-reinforced polymer (FRP) composites via microwave heating or radiation, which can significantly reduce cure time and increase energy efficiency, has several major challenges (e.g. uneven depth of radiation penetration, reinforcing fibre shielding, uneven curing, introduction of hot spots etc). This artic...

An approach for predicting and extracting through-thickness residual stresses in tailored composite laminates (i.e. laminates with local variations in lay-up sequence and/or thickness) is presented. Tailored composite laminate configurations can be explored in some novel structural applications (e.g. morphing laminates) by incorporating unsymmetric...

A soft-patch scarf repair approach, i.e. in situ vacuum assisted resin infusion of repair patches, is investigated for repairing damaged composite laminates. A polyetherimide (PEI) veil was introduced at the parent-patch interface (i.e. resin-rich region or bondline) in resin infused scarf patches to examine the role of interface toughening mechani...

Renewable energy sources such as wind energy—together with energy efficient technologies—are essential to meet global energy demands and address climate change. Fibre-reinforced polymer composites, with their superior structural properties (e.g., high stiffness-to-weight) that allow lightweight and robust designs, play a significant part in the des...

Some unsymmetric polymer–matrix composite laminates show multi-stable behaviour, which can provide opportunities for morphing composite structures. It is known that through-thickness residual stresses are introduced in unsymmetric laminates during the manufacturing process. In this research work, a combined experimental–numerical analysis is propos...

Structural bonding—using toughened epoxy adhesives—is an attractive alternative to conventional mechanical joining techniques for composite structural applications in the aircraft industry, e.g. wings and fuselages. However, an important design aspect is the fracture behavior in bonded joints under mode-I stress conditions (i.e. peel dominated frac...

Toughened epoxy adhesives are frequently used to bond metals and polymer-matrix composite materials in many structural applications. The mechanical properties of adhesives are often characterised by testing either bulk adhesive specimens or bonded joints (i.e. in-situ form). In this paper, cylindrical bulk specimens with circumferential notches wer...

In this paper, the fatigue response of adhesively bonded joints under variable amplitude (VA) cyclic loading was predicted using a numerical model. The adhesive layer was modelled using the cohesive zone model with a bi-linear traction-separation response. A damage model, incorporating fatigue load ratio effects, was utilised in conjunction with th...

Advanced structural adhesives are now an important joining technique in automobile and aerospace applications. The perceived uncertainty in the long-term structural performance of bonded members when subjected to static/fatigue loads in aggressive environments is probably restricting an even more widespread use of this joining technology. In this a...

Two-part epoxy paste adhesives are frequently used to bond metals and composite materials in many structural applications. After mixing two reactive parts (by weight or volume ratio), adhesive paste is applied to the substrate surfaces and cured at elevated temperatures. Air-entrapment during mixing and/or application process often produces micro-v...

Pre-preg and non-crimp-fabric composite single lap bonded joints were manufactured and investigated to characterise the bond quality and static failure behaviour. A two-part epoxy adhesive was employed to bond composite laminates. The composite panels, which were treated with low pressure oxygen plasma, were bonded in a hot drape former and then cu...

Experimental and numerical studies have been undertaken on hybrid fibre–metal (aluminium–Glare) laminate (FML) doubler joints to investigate their static response under tension loading. The specimens either have the fibres parallel to the loading direction (spanwise) or perpendicular to the loading direction (chordwise). Inevitably sheets in these...

Adhesively bonded metal laminates are used in aerospace applications to achieve low cost, light weight structures in the aerospace industry. Advanced structural adhesives are used to bond metal laminae to manufacture laminates, and to bond stringers to metal laminate skins. Understanding the failure behaviour of such bonded structures is important...

Structural bonding—using toughened epoxy adhesives—is an attractive alternative to conventional mechanical joining techniques for composite structural applications in the aircraft industry, e.g. wings and fuselages. However, an important design aspect is the fracture behaviour in bonded joints under mode-I stress conditions (i.e. peel dominated fra...

The fatigue resistance of adhesively bonded joints is an important aspect of reliable structural design in many sectors. In this paper, the effect of load ratio on the fatigue behaviour of adhesively bonded joints was investigated using both experimental and numerical approaches. Single lap joints were tested under cyclic loading at different load...

Structural adhesives are used widely in aerospace and automotive applications. However, fatigue damage in these adhesives is an important factor to be considered in the design of adhesively bonded structural members that are subjected to cyclic loading conditions during their service life. Fatigue life of adhesively bonded joints depends mainly on...

In the design of cold-formed steel purlins based on Eurocode 3, the lateral bending moment in the free flange of the purlin, when in tension, is assumed to be zero due to flange curling and second order effects. To investigate the validity of this design assumption, non-linear analytical and finite element models for analyzing how the gravity load...

Cold-formed steel purlins are commonly used in the construction of industrial buildings. The roof sheeting increases the strength of the attached purlin to a considerable extent by providing lateral and rotational restraints. Further, the rotational restraint plays an important role on the buckling behaviour of the attached purlin. There is a need...

Sandwich panels are attached to cold-formed steel purlins in roofs of industrial buildings to provide insulation. As the strength of the attached purlins is considerably increased due to the lateral and rotational restraints provided by the sandwich panels, estimating these restraints is important in the design of purlins. The rotational restraint...

This paper deals with stringer stiffened cylinders on local supports. For shell structures, the imperfections of the constructions tend to have a major influence on the structural behaviour. In this contribution, the results of a number of numerical simulations of discretely supported cylinders with different imperfection shapes are presented.

The contraction of the heart muscle cell is investigated using a Finite Element Model (FEM). When a prestretched muscle is activated in a laboratory set-up, it can contract isometrically (constant length) until the afterload is reached. From that moment on the muscle can contract isotonically (constant stress). Both phases of the contraction are de...

Some mechanical aspects of the free expansion of a stainless steel stent are investigated by means of a Finite Element (FE) model. Using aspects of symmetry the model was reduced to a 30 degrees segment of half of the stent, having a positive effect on the needed CPU time. As we focused on the free expansion of the uniform, internally loaded stent,...

Cold-formed steel Z-purlins are commonly used in the construction of industrial building roofs. As the restraining effect due to roof sheeting increases the strength of the purlin to a considerable extent, its design is largely influenced by the amount of rotational restraint provided by roof sheeting to purlins. Though it has been an area of resea...

This paper deals with the problem of finding the ultimate load of thin-walled, coldformed steel purlins. A finite element model has been built with the ABAQUS package and numerical computations are carried out by using the modified Riks algorithm that is implemented in the package. Due to numerical problems it is not easy to find the maximum load....

Cold-formed steel purlins are restrained both laterally and rotationally because of the attached roof sheeting. In most cases, the amount of the lateral restraint provided by the roof sheeting is very large when compared to the rotational restraint - which is always flexible. Studying the failure behaviour of continuous purlins by including the res...