Sanjeev K. Khanna

Sample preparation techniques for nano-mechanical characterization of glass fiber reinforced polyester matrix composites

Abstract Glass fiber reinforced plastics are widely used in a host of engineering applications. In these two component materials, the region around the boundary where the components meet is known as the ‘interphase’. The size of the interphase is typically of the order of few microns and understanding the mechanical properties of this microscopic region is central to understanding the mechanical behavior of the composite. Such a small region is amenable to nanomechanical investigation using nanoindentation techniques. For nano length scale investigation it is imperative to have a surface with a roughness of the order of 10s of nanometers. In this investigation techniques have been developed to obtain very low surface roughness in both monolithic polyester and glass fiber reinforced polyester composite samples using a combination of abrasion and etching.

Investigation of Nanomechanical Properties of the Interphase in a Glass Fiber Reinforced Polyester Composite Using Nanoindentation

Glass fiber reinforced polymer composites are widely used as structural materials. These two-component materials can be tailored to suit a large variety of applications. A better understanding of the properties of the fiber-matrix ‘‘interphase’’ can facilitate optimum design of the composite structure. The interphase is a microscopic region around the fiber and hence nano-scale investigation using nano-indentation techniques is appropriate to determine mechanical property variations within this region. In this study the atomic force microscope adapted with a commercial nanoindenter has been used to determine the variation of the elastic modulus across the interphase for different silane coated glass fiber reinforced polyester matrix composites. A comparative study of the elastic modulus variation in the various interphases is reported. The results are discussed in the light of the current limitations of the instrumentation and analysis.@DOI: 10.1115/1.1543966#

Numerical Simulation of Residual Stresses in a Spot Welded Joint

An incremental and thermal-electro-mechanical coupled finite element model has been presented in this study for predicting residual stress distribution in a spot welded steel joint. Approximate temperature dependent material properties, including physical and mechanical properties, have been considered. The Spot nugget shape and the residual stress distribution were obtained by simulation. The results obtained have been compared with experimental measurements, and good agreement is observed. The highest tensile residual stress occurs at the center of the nugget and the residual stress decreases towards the edge of the nugget.

Residual Stress Measurement in Spot Welds and the Effect of Fatigue Loading on Redistribution of Stresses Using High Sensitivity Moiré Interferometry

Residual stress distribution has been determined in spot welds, which are generally used to join mild steel sheets. Various spot weld configurations were investigated using the full-field, experimental, optical technique of high sensitivity moire interferometry. These stresses were found to be in the range 250-300 MPa (tensile) in the center and decreased to 40-100 MPa (tensile) at the edge of the weld nugget. Low cycle fatigue loading of the spot weld caused the residual stress to drop in the weld center by about 30 percent and increase at the edges by as much as 100 percent.

I Know This Is Supposed to Be More Like the Real World, but . . .: Student Perceptions of a PBL Implementation in an Undergraduate Materials Science Course.

This qualitative case study examines the initial implementation of a problem-based version of an undergraduate course in materials science for the purpose of identifying areas of improvement to the curriculum prior to a planned second implementation. The course was designed around problems that students work in small teams to solve under the guidance of facilitators, with early sequence problems designed to foster the problem-solving skills required to succeed in the course. This report describes students’ impressions of and experiences in the course as they worked to solve the final problem at the end of the semester and compares those impressions, where applicable, to impressions gathered after they had completed the first problem near the beginning of the semester. Using grounded theory techniques to analyze the data, six central themes emerged from the implementation: course structure, facilitation roles, student roles, group processes, coconstruction, and resources. Implications for practice and potential instructional design solutions that may aid in future implementations are discussed.

Flow and Heat Transfer in Micro Pin Fin Heat Sinks With Nano-Encapsulated Phase Change Materials

In this paper, a 3D conjugated heat transfer model for Nano-Encapsulated Phase Change Materials (NEPCMs) cooled Micro Pin Fin Heat Sink (MPFHS) is presented. The governing equations of flow and heat transfer are solved using a finite volume method based on collocated grid and the results are validated with the available data reported in the literature. The effect of nanoparticles volume fraction (C = 0.1, 0.2, 0.3), inlet velocity (Vin = 0.015, 0.030, 0.045 m/s), and bottom wall temperature (Twall = 299.15, 303.15, 315.15, 350.15 K) are studied on Nusselt and Euler numbers as well as temperature contours in the system. The results indicate that significant heat transfer enhancement is achieved when using NEPCM slurry as an advanced coolant. The maximum Nusselt number when NEPCM slurry (C = 0.3) with Vin = 0.015, 0.030, 0.045 (m/s) is employed, are 2.27, 1.81, 1.56 times higher than the ones with base fluid, respectively. However, with increasing bottom wall temperature, the Nusselt number first increases then decreases. The former is due to higher heat transfer capability of coolant at temperatures over the melting range of PCM particles due to partial melting of nanoparticles in this range. While, the latter phenomena is due to the lower capability of NEPCM particles and consequently coolant in absorbing heat at coolant temperatures higher than the temperature correspond to fully melted NEPCM. It was observed that NEPCM slurry has a drastic effect on Euler number, and with increasing volume fraction and decreasing inlet velocity, the Euler number increases accordingly.

Experimental Investigation of Static Interfacial Fracture in Orthotropic Polymer Composite Bimaterials Using Photoelasticity

The effects of static Mode I (opening mode) loading on orthotropic-orthotropic bimaterial interface cracks have been investigated using the experimental technique of transmission photoelasticity. For successful implementation of this experimental technique transparent and birefringent glass fiber reinforced polyester composite materials were developed, enabling the direct observation and recording of photoelastic fringes in the vicinity of the interface crack in the orthotropic bimaterial. It is our belief that this is the first of such experimental investigation. Opening and shearing mode stress-intensity factors and the strain energy release rates have been calculated for various combinations of the bimaterial halves. Results have been verified by mathematically regenerating the observed isochromatic fringe patterns. This study has made it feasible to investigate interfacial fracture in orthotropic-orthotropic bimaterials and orthotropic-isotropic bimaterials using photoelasticity. DOI: 10.1115/1.4000223

A Novel Transparent Glass Fiber-Reinforced Polymer Composite Interlayer for Blast-Resistant Windows

An optically transparent woven glass fiber-reinforced polyester composite has been fabricated. This composite has been used as an interlayer in the fabrication of a laminated glass-composite window panel for application in blast resistant windows. The transparency of the glass fiber-reinforced composite was achieved by matching the refractive index of the polyester matrix with that of glass fibers. Various chemical additives have been investigated for their effectiveness in modifying the refractive index of the polyester matrix. The composite interlayers mechanical properties under both quasi-static and dynamic loading conditions have been characterized in this study. The window panels were tested under various blast loading conditions. The panels perform well under U.S. General Services Administration (GSA) specified C, D and E blast loading levels.

Effectiveness of Introducing Problem Based Learning in the Undergraduate Engineering Curriculum

Problem solving is the primary intellectual activity of engineers. Therefore, enhancing problem-solving skills is essential for preparing engineering students for practice in the profession. A powerful approach for enhancing problem-solving skills is the problem-based learning (PBL) method. This paper presents the design and construction of a PBL-based course in materials science at the junior level in a mechanical & aerospace engineering (MAE) department. We assess the ability of a PBL course based on longer complex problems to enable students to learn both fundamental knowledge of the subject matter and also problem solving skills and contrast it with outcomes in a traditional lecture based course. The issues and challenges faced in assessing and implementing PBL are discussed.Copyright

A Case Study of Using Capstone Design as Basis for Curriculum-Wide Project-Based Learning

This paper describes an NSF funded project in the Mechanical and Aerospace Engineering (MAE) Department at the University of Missouri. A primary goal of this project is to systematically increase project-based learning (PBL) experiences throughout the MAE curriculum. To accomplish this goal, recent capstone design projects that need further refinements serve as the basis for PBL activities throughout the MAE curriculum. A major tool for facilitating these refinement efforts is a new senior/graduate Design Management course in which each student in this course learns how to plan and manage design projects. These students then implement their learning by serving as project team managers in the courses in which the refinement activities are being conducted. This paper provides a detailed case study of five refinements to one capstone design that took place in four different MAE courses during the Spring 2011 semester. The paper describes a Fall 2009 capstone project that consisted of designing a portable wood chipper. The student design was very promising, leading to a chipper with significantly greater chipping capacity than commercially available chippers of the same size and weight. However, several faculty members reviewed the results and identified additional opportunities for refining the design. This paper describes activities during Spring 2011 when students in four different MAE courses developed refinements to the original design. The roles of the Design Management students in these activities are discussed. The paper also includes a discussion of the methods and findings of the formative assessment process, including interviews with, and surveys of, faculty and students.Copyright