Himayat Ullah - Academia.edu (original) (raw)

Papers by Himayat Ullah

Structural Engineering and Mechanics, 2017

2022 19th International Bhurban Conference on Applied Sciences and Technology (IBCAST), Aug 16, 2022

A constitutive model based on the concept of continuum damage mechanics has been proposed to stud... more A constitutive model based on the concept of continuum damage mechanics has been proposed to study the progressive damage behavior of composite laminates under ballistic impact. The proposed model is investigated in five steps: First, the quadratic form of damage initiation criteria are presented to predict the initiation of failure in different modes. Second, an exponential form damage evolution law combined with characteristic length based fracture energy approach has been presented. Stiffness degradation is characterized by a variable determined by the equivalent displacement for each failure mode. Third, an experimentally verified strain rate model that considers the rate dependency of the strength and modulus of the composite laminate is considered. Fourth, cohesive elements are inserted at every inter-layer for modeling the delamination evolution. Fifth, the constitutive model has been combined with an element erosion algorithm for the removal of highly distorted elements. Simulations have been performed using reduced integration hexahedra elements (RIHE) and full integration hexahedra elements (FIHE). Implementation of cohesive elements exhibited better delamination progression. Experimentally verified strain rate model enhanced the efficiency of the model showing good correlation between the present simulations and experimental observations, in terms of damage patterns, residual velocity, kinetic energy and ballistic limit.

Composites Science and Technology, Jul 1, 2013

Fabric-reinforced polymer (FRP) composites are increasingly employed in aerospace structures and ... more Fabric-reinforced polymer (FRP) composites are increasingly employed in aerospace structures and sports products. In these applications, they are usually subjected to large-deflection quasi-static and dynamic bending deformations. Such loading conditions induce damage within the material at various scale levels affecting their strength, stiffness and energy-absorbing capability. For this purpose, mechanical behaviour of woven carbon and glass fabric-reinforced polymers (C/GFRPs) composites in onand off-axis orientations is first quantified by carrying out large-deflection quasi-static bending tests followed by dynamic ones employing an Izod type impact tester. The obtained stress-strain and force-deflection plots showed CFRP laminates fractured due to brittle carbon fibres. The off-axis CFRP and both the onand off-axis GFRP samples showed damage and nonlinearity at low impact energy with residual load carrying capability. Further, off-axis laminates of both materials and on-axis GFR...

Woven composites such as carbon and glass fabric-reinforced polymers (C/GFRPs) laminates are incr... more Woven composites such as carbon and glass fabric-reinforced polymers (C/GFRPs) laminates are increasingly employed in aerospace structures and sports products. In these applications, they are usually subjected to large-deflection quasi-static and dynamic bending deformations during service. Such loading conditions induce damage within the material at various scale levels affecting their strength, stiffness and energy-absorbing capability. Further, FRPs especially made of carbon fibres are inherently brittle, proven to sudden and catastrophic failure without yielding like ductile materials. For this purpose, mechanical behaviour of woven CFRP and GFRP composites in onand off-axis orientations is first quantified by carrying out large-deflection quasi-static bending tests followed by dynamic ones employing an Izod type impact tester. Both types of laminates of various orientations were tested at loads increasing up to failure to determine their energy-absorbing capability. The obtaine...

In this research paper, effect of geometric imperfection on buckling of cylindrical shell subject... more In this research paper, effect of geometric imperfection on buckling of cylindrical shell subjected to different types of loadings has been investigated using Finite element analysis and compared with analytical model by Donnell and semi Empirical model based on experimentation [1,2] of a perfect shell. In finite element analysis Newton Raphson and Arc length methods are used. Based on the presented results conclusions can be drawn concerning the shell behavior and its sensitivity to different loadings.

Light weight thin walled cylindrical shells subjected to external loads are prone to buckling rat... more Light weight thin walled cylindrical shells subjected to external loads are prone to buckling rather than strength failure. In this paper, buckling investigation of thin walled cylindrical shells under axial compression is presented. Buckling failure is studied using analytical, numerical and semi empirical models. Analytical model is developed using Classical Shell small deflection theory. A Semi empirical model is obtained by employing experimental correction factors based on the available test data to the theoretical model. A finite elements model is built using ANSYS FEA Code for the same shell. Finally, the different results obtained using the three analysis methods are compared. The comparison reveals that analytical and numerical linear model results match closely with each other but are higher than the empirical values. To investigate this discrepancy, non linear buckling analysis with large deflection effect, is carried out. The effect of geometric imperfection is also stud...

Journal of Engineering and Applied Sciences, 2017

The use of Titanium and its alloys in modern era can never be discarded. Continuous developments ... more The use of Titanium and its alloys in modern era can never be discarded. Continuous developments in many industries like aviation and power generation increased its use in many applications, not only because of its chemical properties but also because of its excellent properties including, high strength to density ratio, withstand with high stresses, better fatigue and corrosion properties and ability to resist high temperatures. However, all these properties of this alloy results in poor machinability, hence, make the automatic machining of these alloys nearly difficult. The main limitation of titanium and its alloys is that it produces long chips during machining which tangled around tool, hence; reduce tool life and sometime leads to fracture during conventional turning (CT). In recent days new two alloys are developed named Ti-6Al-7Zr-3Nb-4Mo-0.9Nd and Ti-5Al-7Zr-7Nb-0.7Nd with better performance in machining. These two developed alloys contain 0.9% and 0.7% neodymium by weigh...

Scientia Iranica, Aug 29, 2017

The enhanced strength, fatigue life, and corrosion resistance properties of Tialloys have attract... more The enhanced strength, fatigue life, and corrosion resistance properties of Tialloys have attracted many industries for their utilization in various components exposed to extreme operating conditions. The machining of these alloys using conventional machining techniques is one of the main challenges in its wide application in many components, and there is an obvious demand to analyse the materials' response to these alloys in machining processes by developing simulation-based models. The materials' behaviour used in simulation of machining processes is usually determined by means of Split-Hopkinson-Pressure-Bar (SHPB) setup. A 3D thermo-mechanically coupled Finite-Element (FE) model of SHPB is developed in the current work to analyse materials response of-Ti-15333 at the selected temperature, strain rate, and strain. The obtained materials' response to the tested alloys is used in 3D thermo-mechanically coupled FE model of ultrasonically assisted turning and conventional turning at various tested cutting conditions. The developed FE model is used for parametric analysis of-Ti-15333 machining, and the obtained FE results are in good agreement with experimental results.

Journal of Physics: Conference Series, 2011

Carbon fibre-reinforced polymer (CFRP) textile composites are widely used in aerospace, automotiv... more Carbon fibre-reinforced polymer (CFRP) textile composites are widely used in aerospace, automotive and construction components and structures thanks to their relatively low production costs, higher delamination and impact strength. They can also be used in various products in sports industry. These products are usually exposed to different in-service conditions such as large bending deformation and multiple impacts. Composite materials usually demonstrate multiple modes of damage and fracture due to their heterogeneity and microstructure, in contrast to more traditional homogeneous structural materials like metals and alloys. Damage evolution affects both their in-service properties and performance that can deteriorate with time. These damage modes need adequate means of analysis and investigation, the major approaches being experimental characterisation, numerical simulations and microtomography analysis. This research deals with a deformation behaviour and damage in composite laminates linked to their quasi-static bending. Experimental tests are carried out to characterise the behaviour of woven CFRP material under large-deflection bending. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit to study the deformation behaviour and damage in woven CFRP laminates. Multiple layers of bilinear cohesive-zone elements are employed to model the onset and progression of inter-ply delamination process. X-ray Micro-Computed Tomography (MicroCT) analysis is carried out to investigate internal damage mechanisms such as cracking and delaminations. The obtained results of simulations are in agreement with experimental data and MicroCT scans.

Engineering Failure Analysis

Composite laminates employed in various sports products are usually subjected to large-deflection... more Composite laminates employed in various sports products are usually subjected to large-deflection dynamic bending during their service. The chapter first describes the experimental characterisation of composites as well as investigation of various damage modes ensued under these loading conditions replicated by means of Izod-type dynamic tests. It then describes the development of three-dimensional finite-element models employing a cohesive-zone method to study the onset, progression and interaction of some damage modes observed experimentally. The developed numerical models are capable of simulating damage mechanisms in laminates and their interaction observed in the tests.

Carbon Fibre Reinforced Polymer (CFRP) composites are widely used in several high performance str... more Carbon Fibre Reinforced Polymer (CFRP) composites are widely used in several high performance structures such as aeroplanes, automobiles and wind turbines. In these applications holes are made for access to view, cut-outs for weight reduction as well as joining of composite members. However, these hole possess a threat to the strength and damage tolerance of composites in service. The strength of composites varies with hole diameter known as hole size effect. The hole size effect, becomes more complex once associated with specimen size effect (strength variation with specimen size) as well as anisotropy and heterogeneity of composite material. The leading influencing factors linked with the hole size effect are in-plane and transverse plane thickness, stacking sequence and hole diameter of the specimen. Extensive experimental, analytical and Finite Element (FE) based studies have been done by the researchers in past on this account. Despite these studies, differences still persist a...

This paper describes structural analysis of a large composite wind turbine blade under extreme lo... more This paper describes structural analysis of a large composite wind turbine blade under extreme loading. These extreme loading can cause damage and significant drop in the load-carrying ability of large composite wind turbine blades. Therefore, it is vital to investigate the structural strength of the blades for their safe operation during its life time. In this study first, a three-dimensional (3D) surface model of the blade is generated using NuMAD (Numerical Manufacturing and Design), which greatly simplifies the 3D model generation of a wind turbine blade, and effectively handles all information of blade airfoils, material properties and their geometrical configuration. The blade model is then evaluated for its ability to withstand the extreme loading conditions as specified in the relevant standards using finite element analysis (FEA) software ANSYS. Results show that the stresses and deformation in all parts except shear webs are within the design limits specified by standards....

An extensive study of mechanical behaviour of fibre-reinforced polymer (FRP) composite laminates ... more An extensive study of mechanical behaviour of fibre-reinforced polymer (FRP) composite laminates was carried out in order to understand complex mechanisms of damage initiation and propagation under impact loads within a broad range of load rates-from low to high. The cases studied included (i) low load-rate, low-energy impacts on woven carbon-and glassfibre fabric-reinforced laminates, typical for sports or automotive applications, (ii) ballistic impact on woven-fabric FRP laminates, (iii) blast-related impact on CFRP-based laminates and (iv) hypervelocity impact.

The International Journal of Advanced Manufacturing Technology

A hybrid turning is presented for turning of Ti-15V-3 Al-3 Cr-3 Sn alloy. In this technique, cutt... more A hybrid turning is presented for turning of Ti-15V-3 Al-3 Cr-3 Sn alloy. In this technique, cutting insert is vibrated in velocity direction with the help of ultrasonic transducer and external heat is provided to the machined workpiece to gain collective benefits of both arrangements in cutting of hard-to-cut alloys. The studied alloy was investigated numerically and experimentally using hybrid turning process to determine its rewards in decline of measured cutting forces and enhancement in quality of machined surface. The assessment for thermal evolution in the cutting process was carried out both numerically and experimentally, and an accurate prediction of process zone temperatures is achieved. A significant improvement in dry turning of the studied alloy was achieved in terms of substantial decline in cutting forces and no substantial alterations in the metallurgy of the tested material. An elastoplastic thermo-mechanically coupled finite-element model for oblique-turning process is established to investigate the effect of heat and vibration on output parameters numerically. The developed model was used to explore the influence of selected machining parameters (depth-of-cut, feed rate, cutting speed, and tool nose radius) on three components of forces, stresses, and process zone temperature. Comparative case studies were executed with the developed models of conventional-turning, hot-conventional-turning, and hybrid turning and were confirmed by the outputs from tests carried out on the in house prototype available at Loughborough University, United Kingdom. The model was used for two-dimensional ultrasonic vibration in all three axis and resulted no significant drop in the cutting forces when compared to the studied hybrid turning process.

Journal of the Brazilian Society of Mechanical Sciences and Engineering

Scientia Iranica

Shot peening is a surface treatment processes usually used for the improvement of fatigue strengt... more Shot peening is a surface treatment processes usually used for the improvement of fatigue strength of metallic parts by inducing residual stress field in them. The evaluation of shot peening parameters experimentally is not only very complex but costly as well. An attractive alternative is the explicit dynamics finite element (FE) analysis having the capability of accurately envisaging the shot peening process parameters using a suitable material's constitutive model and numerical technique. In this study, ANSYS/LS-DYNA software was used to simulate the impact of steel shots of various sizes on 2618-T61 aluminium alloy plate described with strain rate dependent elasto-plastic material model. The impacts were carried out at various incident velocities. The effect of shot velocity and size on the induced compressive residual stress and plastic deformation were investigated. The results demonstrated that increasing the shot velocity and size yielded in an increase in plastic deformation of the target. 2 Results obtained were close to the published ones, and the numerical models were capable to capture the pattern of residual stress and plastic deformation observed experimentally in aluminium alloys. The study is quite helpful in determining and selecting optimal shot peening parameters for the surface treatment of aluminium alloy parts.