Fazeel Khan

Rate (time)-dependent deformation behavior: an overview of some properties of metals and solid polymers

Abstract The inelastic deformation behaviors of metals and polymers are discussed with the aim of finding a common base that would simplify academic and engineering analyses. Only monotonic loading conditions at room temperature are considered. For loading at different rates, nonlinear relations between loading rate and stress level, creep stress level and creep strain, and relaxation rate and stress were common to both type of materials. There are, of course, significant differences in elastic properties, strength levels and the strains involved. Special properties such as relaxation behaviors and creep anomalies can be qualitatively and quantitatively reproduced by the state variable model VBO (viscoplasticity theory based on overstress). Since experimental investigations typically concentrate on one particular aspect of inelastic deformation behavior such as creep or strain-rate dependence, it is often difficult to gather a comprehensive data set for a given material. In spite of this, considerable similitude in the deformation behavior of metals and polymers in various test conditions has nevertheless been established.

Amorphous and Semicrystalline Solid Polymers: Experimental and Modeling Studies of Their Inelastic Deformation Behaviors

The study of the inelastic deformation behavior of six amorphous and semicrystalline polymers was performed to develop and verify the capabilities of a constitutive material model. The test conditions consisted of piecewise constant strain rates for loading and unloading. Immediate control mode switching capability permitted using load control for creep and recovery tests. Positive, nonlinear rate sensitivity was observed in all cases for monotonic loading and the prior loading rate was found to have a strong influence on creep, relaxation and strain recovery (emulating creep at zero stress) tests. In particular, a fast prior rate engenders a larger change in the output variable: strain in conditions of creep and stress drop in relaxation. Based on the absence of any distinctive deformation traits, the preponderance of data collected in the experimentation program suggests that both categories of polymers can be modeled using the same phenomenological approach. Modeling of the experimental data is introduced with a uniaxial form of the Viscoplasticity Theory Based on Overstress for Polymers (VBOP). Simulations and model predictions are provided for various loading histories. Additional modifications necessary to extend the theory to finite deformation and inelastic compressibility are then presented. An objective formulation is obtained in the Eulerian framework together with the recently proposed logarithmic spin by Xiao [Xiao, H., Bruhns, O., and Meyers, A., 1997, “Hypoelesticity Model Based Upon the Logarithmic Stress Rate ,” J. Elast., 47, pp. 51–68].

Dynamic Characterization of Magneto-Rheological Elastomers in Shear Mode

The paper presents dynamic shear properties of magneto-rheological elastomers (MREs) under various loading conditions. It particularly focuses on characterization of MREs under compression-shear type combined loading, as it represents realistic loading conditions in various engineering systems and structures. In this study, MRE samples were fabricated by curing a two component elastomer resin with 30% content of 10 mum sized iron particles by volume. In order to vary the magnetic field during shear testing, a test fixture was designed and fabricated in which two permanent magnets could be variably positioned on either side of the specimen. By changing the distance between the magnets, the fixture allowed for varying the magnetic field that passes uniformly through the sample. Using this test setup and a dynamic test frame, a series of shear tests of MRE samples was performed by varying the magnetic field and frequency of loading. The results show the MR effect (percent increase in the materials ldquostiffnessrdquo) increases as the magnetic field increases and loading frequency increases within the range of the magnetic field and input frequency considered in this study. The results further show that the elastic modulus of the precompressed MREs increases as compared with that of MREs without precompression.

Characterization of the mechanical properties of a new grade of ultra high molecular weight polyethylene and modeling with the viscoplasticity based on overstress

Enhancements to the service life and performance of orthopedic implants used in total knee and hip replacement procedures can be achieved through optimization of design and the development of superior biocompatible polymeric materials. The introduction of a new or modified polymer must, naturally, be preceded by a rigorous testing program. This paper presents the assessment of the mechanical properties of a new filled grade of ultra high molecular weight polyethylene (UHMWPE) designated AOX(TM) and developed by DePuy Orthopaedics Inc. The deformation behavior was investigated through a series of tensile and compressive tests including strain rate sensitivity, creep, relaxation, and recovery. The polymer was found to exhibit rate-reversal behavior for certain loading histories: strain rate during creep with a compressive stress can be negative, positive, or change between the two during a test. Analogous behavior occurs during relaxation as well. This behavior lies beyond the realm of most numerical models used to computationally investigate and improve part geometry through finite element analysis of components. To address this shortcoming, the viscoplasticity theory based on overstress (VBO) has been suitably modified to capture these trends. VBO is a state variable based model in a differential formulation. Numerical simulation and prediction of all of the aforementioned tests, including good reproduction of the rate reversal behavior, is presented in this study.

Revitalizing the Engineering Curriculum Through Studio Based Instruction

Engineering curricula, regardless of the specific discipline, need to evolve. Realizing also that the pedagogical value of any educational artifact is closely linked to the methods of instruction used to interact with students, it is imperative that the development of new learning materials be accompanied by the implementation of innovative techniques with demonstrated success in knowledge transfer. This paper presents details of the development of studio styled modules associated with groups of courses within the mechanical and manufacturing engineering curriculum. Within each studio, newly developed activities engage students through experiential learning techniques. These activities, or learning exercises, represent a fusion of hands-on experimentation and computational simulation/analysis in key areas of engineering, such as dynamical systems, thermal sciences and materials. This endeavor is also intended to promote STEM education through enhancements in the quality of technical content, methods of instruction, training of student as effective educators, and the establishment of outreach activities expected to have an enduring effect on the preparation and recruitment of young talent into the sciences.Copyright

Development of educational techniques for computational-experimental analysis

A curriculum wide initiative to enrich course content and increase student engagement in experiential learning through the adoption of new learning modalities is underway in the Department of Mechanical and Manufacturing Engineering, Miami University, OH. The projects entail the development of online learning modules which interweave experimental and computational analysis. The modules incorporate multimedia content which has been prepared with undergraduate and graduate student participation. The ComEx website has been designed to enable easy uploading/updating of material. The distinctive feature of the ComEx studios is the thematic linking of the content which allows them to be used for multiple classes with a progressive advancement in technical content. This paper presents details of the studio model: motivation, methodology, implementation and assessment. The learning modules can utilized by faculty to introduce new lab derived content, which may be related to their research, into a traditional class only course format. Additional benefits of the modules include the ability to independently review specific topics in preparation of advanced courses or for a research project. Assessment of the efficacy of the modules is being performed by students surveys completed online, and by four external (faculty) reviewers. Continuous improvement of the modules is underway.

Integrating Experimental Data Analysis Through Online Modules

In various universities, including Miami University (MU), an undergraduate course in vibrations may be offered in a lecture-only format. However, several concepts in vibrations, such as natural frequencies, damping, mode shapes etc., may be improved immensely from experimental demonstrations and hands-on activities for students to fully grasp the concept and its application. In recent years, several online experiments and resources have been developed in the area of dynamical systems and controls in order to provide an experiential learning environment. With the support of the National Science Foundation, a series of Computational-Experimental (ComEx) learning modules are being developed for integrating experimental, computational and validation studies in the mechanical and manufacturing engineering curriculum at MU. These learning modules are web based and are intended for dissemination to a wide audience extending beyond the students at MU. In this paper, salient features of these online learning modules, which integrate experimental data analysis for mechanical vibration course, are presented. Three different group activities associated with these modules are presented with specific details of the activities, assessment plans, and student perceptions of the modules. The content of these modules is evolving based on feedback from students and external, expert evaluators. It is anticipated that such learning studios can be used by instructors who teach lecture based vibration and control courses, and this resource will yield more insight into the theory, computation and practical applications of essential concepts in this area.Copyright

Development of Experimental Techniques for Thermoelectric Properties Characterization of Low-Dimensional Structures

This work reports current efforts in developing experimental techniques applicable for thermoelectric properties characterization at micro and nanoscale. A one-dimensional transport model was used to asses the effects of heat leakage, non-symmetric boundary conditions, and electrical contact resistance, on thermoelectric properties measurements performed by transient Harman method. If the above effects are important, the thermoelectric figure of merit cannot be extracted directly from the ratio between the Seebeck voltage and the resistive voltage drop across the sample. On the other hand, measurements of both thermal conductivity and Seebeck coefficient can be performed if the temperature drop across the sample is acquired simultaneously with the voltage drop. The theoretical model and the experimental technique are validated by measurements performed on bulk calibration samples. Furthermore, this work shows that the spatial resolution of thermoelectric properties characterization methods can be enhanced by using scanning probe based techniques. Preliminary results are presented for Seebeck coefficient measurements of p-type or n-type calibration samples performed using an AFM probe instrumented with a temperature sensor.

Student perception and knowledge: Assessment of online Computational-Experimental (ComEx) learning modules

In order to enhance the existing course materials and to imbed experimental data derived from real-life engineering case studies, several online Computational-Experimental (ComEx) learning modules have been developed in recent years by the investigators. Several assessment instruments were developed to evaluate the effectiveness of these modules. In particular, the assessment surveys were developed to evaluate student perceptions of the effectiveness, usefulness and quality of the exercise/activities of these modules. In order to evaluate student learning and knowledge associated with a given concept, assignments and pre/post assessment quizzes were designed by mapping the learning outcomes of a given module. In this paper, the design of the assessment instruments and the ensuing data from learning studios on Dynamics, Vibration and Control and a specialized learning module is presented. This studio consists of learning modules in the form of exercises, case studies, tutorials and interactive tools. Assessment instruments were developed for each of these activities. The design of survey instruments, questionnaire, results and statistical analysis of the survey for two years (2012-2013) of students perception of these modules are summarized here. Assessment results of the student learning through pre- and post-quizzes for the year 2013 are also summarized and presented in this paper.

Upgrading the engineering curriculum through thematic learning modules

A National Science Foundation (NSF) sponsored project was initiated in 2011 at the Department of Mechanical and Manufacturing Engineering (MME), Miami University, with the aim of creating online learning modules to increase the efficacy of the coursework. The modules were designed to instil proficiency in computational and experimental analysis, and were grouped along five thematic lines in the form of subject studios. The modules contain multimedia content and are designed for use as supplementary course material, or independently by students in preparation for research tasks or review of specific concepts from prior courses. When used to augment existing courses, a salient feature of the modules is the ability to introduce lab/experiment derived content to lecture-only course formats. This has been found to be particularly effective in engaging students and enabling the introduction of new topics with minimal disruption to the existing daily syllabus of a course. The projects development and implementation phase will end mid-year. Since assessment and continuous improvement were integral parts of the project, this paper presents data from the two year assessment process and describes the changes to the project. The paper also includes a discussion of plans for expansion of modules and practices for keeping content current.