Hitesh Kapoor

Optimal Design of Unitized Panels with Curvilinear Stiffeners

This paper summarizes some numerical results of optimal design study of curvilinear stiffened panels where the reference axes of the stiffeners can be curvilinear. An integrated approach to use different capabilities in relevant fields, such as NURBS, DistMesh, and MSC.NASTRAN, etc., are adopted in the MATLAB environment. Numerical studies on optimal stiffened panel designs of buckling dominant problems are made to look into the effects of certain factors, such as, orientation, spacing, location, and curvature, etc., on the optimal designs. It is observed that curvilinear placement of stiffeners plays the role of orientation, spacing, location and intersection placement of infinitesimal straight stiffeners, and provides an enhanced design space. This enhanced design space may lead to better designs than using straight stiffeners though it is not always so. This work reveals the necessity to use global optimization techniques to perform topology/placement/shape optimization of curvilinear stiffened panels along with size optimization.

Nonlinear Response of Highly Flexible Structures to Air Blast Loads: Application Shelters

The dynamic response of flexible membrane structures to blast loads is described, with application toward shelter systems. Shelters such as the Tent Extendable Modular Personnel tents fall into the category of highly flexible membrane structures due to their large deformation behavior. Material testing was conducted to determine the material properties of the liner and the canvas material. A finite element model for the shelter structure was developed, and the dynamic response to blast loading conditions was obtained. Transient analysis was performed based on an implicit approach in which the time integration was done using the Newmark method, and the Newton-Raphson method was used for the nonlinear analysis. Dynamic responses consisting of displacement time histories and dynamic stresses were computed for the membrane model. A parametric study was performed, and design recommendations are presented.

Experimental Structural Health Monitoring of Z-Fibre Reinforced, Co-Cured Composite Pi-Joints using Lamb wave Propagation

The goal of this research is to establish a methodology for damage detection in unreinforced and z-fiber reinforced cocured composite pi- joints using lamb wave based structural health monitoring technique. Because of the lack of natural reinforcement in the thickness direction, delamination has been a predominant failure mode besides other failure modes in laminated composites. Z-fiber reinforcement is one of the ways of controlling or delaying delamination and thus, delaying the failure. Here, DCB (Double Cantilever Beam) and Pi-Joint specimens, with and without z-fiber reinforcement, are considered for experimental analysis. Damage is experimentally induced in the specimen under static loading. Lamb wave propagation based structural health monitoring is performed using PZT sensors in a pitch-catch arrangement. Amplitude vs. time and amplitude vs. frequency response are plotted for various excitation frequencies. At lower frequencies (particularly at 20 KHz), pure A0 mode is generated, which is confirmed by out of phase response of simultaneous PZT sensors. From the response data analysis, presence of damage in unreinforced, z-fiber reinforced DCB and pi-joint specimens is confirmed.

Lamb wave propagation in Z-pin reinforced co-cured composite pi-joints

This paper presents an initial study on Lamb wave propagation characteristics in z-pin reinforced, co-cured composite pi-joints for the purposes of structural health monitoring (SHM). Pi-joint test articles were designed and created to replicate a co-cured, all composite skin-spar joint found within a typical aircraft wing structure. Because pi-joints exhibit various complex damage modes, formal studies are required if SHM systems are to be developed to monitor these types of joints for potential damage. Experiments were conducted on a undamaged (healthy) and damaged test articles where Lamb waves were excited using one lead zirconate titanate (PZT) transducer. A three-dimensional (3D) scanning laser Doppler vibrometer (LDV) was used to collect high-density scans of both the in-plane and out-of-plane velocity measurements. In the damaged test article, where delamination, matrix cracking, and fiber breakage can clearly be seen, changes in both the fundamental antisymmetric A0 and symmetric S0 Lamb wave modes are apparent. In both test articles, the effects of narrow geometry, discontinuity due to the attachment of the web, and thickness has detectable effects on Lamb wave propagation. From the comparisons between Lamb waves propagating through the undamaged and damaged test articles, it is clear that damage can be detected using Lamb waves in z-pin reinforced, co-cured composite pi-joints for this case of extensive damage.

Nonlinear Fluid-Structure Interaction of Flexible Shelters under Blast Loading

The present work describes methods for defining and estimating blast loading over a tent and the resulting pressure loading inside the tent due to transmitted blast waves and proposes a new methodology for estimating such loadings on flexible structures. For this purpose, loading conditions and structural responses of a flexible tent with an interior Collective Protection System (CPS) that was subjected to an explosion were analyzed. This CPS tent protects personnel from biological and chemical agents with a pressurized liner inside the tent as an environmental barrier. Field tests for this structure showed unexpected damage to the liner, and most of the damage occurred on the tents leeward side. To solve this problem, various tests and analyses using a commercial finite element method, ANSYS, to model the structural dynamics and a Computational Fluid Dynamics code, Fluent, to model the flow field were performed involving: material characteristics of the liner, canvas, and zip seals; modeling of the blast loading over the tent and inside the tent; and structural response of the tent to the blast loading. The results were compared with the field test data obtained by the Air Force Research Laboratory. The experimental pressure data were gathered from pressure gauges attached to the tent surfaces at different locations. The comparison showed that the proposed methods can provide a good design tool to analyze the loading conditions for rigid or flexible structures under explosive loads. Also, it was found that the blast loading and the structural response cannot be analyzed separately, due to the interaction between the flexible structure and the dynamic pressure loading. Therefore, the effect of fluid-structure interaction should be considered in the pressure load calculation on the structure when the structural deflection rate can influence the solution of the flow field surrounding the structure.

Effects of Z-pins on Lamb waves in composite plates

This experimental research investigates the effects of adding z-pins to a carbon fiber reinforced plate (CFRP) on Lamb wave propagation, such as mode conversion and reflections. The motivation for this study is derived from the current and expected future use of z-pins in aircraft structures coupled with the requirement to design structural health monitoring (SHM) systems for detecting damage in regions of composite structures with z-pins. This experimental study is conducted on two 4.8 mm thick CFRP test articles, where one plate has a 20 by 279 mm2 band of z-pins and the other does not. The z-pins have an average diameter of 0.28 mm and are inserted through the thickness of the panel with an area density of 4% before curing. A three-dimensional (3D) laser Doppler vibrometer (LDV) was employed to collect velocity measurements over a 1 mm uniformly-spaced grid of 17,899 scan points. Time-sequenced 3D LDV scans are presented to show that adding this relatively small amount of z-pins to a 4.8 mm thick CFRP has few measureable effects on Lamb wave propagation.

Analysis of Composite and Sandwich Beams using Adaptive, Variable-Order NURBS Element-Free Galerkin Formulation

The present paper utilizes the non-oscillatory nature of higher-order NURBS basis as shape functions and their derivatives to compute interlaminar normal stress in composite and sandwich beams, accurately and eciently. This avoids the extra step in postprocessing generally required in regular finite element formulation. Element-Free Galerkin formulation is derived for the first-order shear-deformable composite and sandwich beams and is adaptive in nature. The displacement and its higher derivatives are computed and compared with the analytical-exact solution. It is seen that the NURBS a shear-deformable beam is more ecient than the higher order B-Spline beam (HOBS). The interlaminar normal stress play an important role in through-the-thickness failure in composite and thus, require its accurate calculation, which necessitates the accurate calculation of second order derivative of displacement. Interlaminar shear and normal stresses are computed directly without the extra step required in post-processing. Various numerical examples are tested and validated with the analytical-exact solution.

Locking-free and Stabilized Geometrically Nonlinear NURBS Isogeometric Finite Element Analysis of Laminated Composite Plate

This research present the development of geometrically nonlinear Nurbs isogeometric nite element analysis of laminated composite plates. First-order, shear-deformable laminate composite plate theory is utilized in deriving the governing equations using a variational formulation. Geometric nonlinearity is accounted for in Von-Karman sense. Nurbs quadratic and higher-order elements are constructed where k-re nement has no analogous in regular nite element. Isotropic, orthotropic and laminated composite plates are studied for various boundary conditions, length to thickness ratios and ply-angles. Computed center de ection is found to be in an excellent agreement with the literature and required fewer degrees of freedom/control point when compared with regular nite element analysis. For thin plate analysis, k-re ned, quadratic Nurbs element is found to remedy the shear locking problem, i.e. it eliminated the need for reduced integration of transverse shear sti ness terms. k-re ned quadratic Nurbs element also provided stable nonlinear de ection (hourglass instability) for distorted quadrilateral meshes. This research presents the development of Nurbs Isogeometric nite element where k-re ned Nurbs element eliminates the need for extra step required in developing locking-free and stable shear-deformable plate elements. Graduate Research Assistant(Ph.D student), Aerospace & Ocean Engineering Department, 215 Randolph Hall, Blacksburg, VA-24060, Student Member,AIAA yMitchell Professor, Aerospace & Ocean Engineering Department, 213E Randolph Hall, Blacksburg, VA-24060, Associate Fellow, AIAA