Karim H. Muci-Küchler

Simulation of a Refill Friction Stir Spot Welding Process Using a Fully Coupled Thermo-Mechanical FEM Model

Friction stir spot welding (FSSW) is a solid state joining technology that has the potential to be a replacement for processes like resistance spot welding and rivet technology in certain applications. To optimize the process parameters and to develop FSSW tools, it is important to understand the physics of this complex process that involves frictional contact, high temperature gradients, and large deformations. This paper presents a fully coupled thermo-mechanical finite element model (FEM) model of the plunge phase of a modified refill FSSW. The model was developed in Abaqus/Explicit and the simulation results included the temperature, deformation, stress, and strain distributions in the plates being joined. An experimental study was also conducted to validate the temperatures predicted by the model. The simulation results were in good agreement with the temperatures measured in the experiment. Also, the model was able to predict in a reasonable fashion the stresses and plastic strains in the plates.

Distribution of Bacteria in Simplified Surrogate Extremities Shot With Small Caliber Projectiles

Experiments were conducted to determine bacteria distribution trends in wound cavities of simplified surrogate extremities shot using small caliber projectiles. Two different shapes of targets, cylindrical and square, were used in this study. Cylindrical targets are more representative of an extremity but create difficulties while conducting tests due to inconsistent cavity lengths and optical distortions. Square targets, which are not as susceptible to the problems mentioned above, could be used in place of cylindrical ones if their shape does not significantly affect the distribution of bacteria within the wound cavity. Surface contamination of the targets in the experiments was represented using a circular piece of filter paper moistened with a solution with a known amount of Escherichia coli strain K-12. The projectiles used were 11.43-mm (0.45-in) caliber round nose projectiles shot from a commercially available air rifle. The permanent cavities were extracted from the targets and sliced into small, evenly spaced segments and the area surrounding the permanent cavities was removed with a biopsy punch. The radial tears that were made by the formation of the temporary cavity and surround the permanent cavity were removed using a scalpel. The permanent cavity and radial tears for each section were processed and plated on agar plates. Commercial software was used to count the number of colony forming units on each plate and the percentage of the total bacterial colony count per segment was determined. High speed video and motion analysis software was used to qualitatively and quantitatively compare the temporary cavities in the cylindrical and square targets. The data from the experiments showed that the bacteria distribution trends for the cylindrical and square targets were similar even though the maximum openings of the temporary cavity at the entrance and exit locations were higher for the cylindrical ones. For both target shapes, the bacterium was evenly distributed between the permanent cavity and the radial tears in the middle sections of the “wound tracks.” In addition, significantly higher amounts of bacterium were found in the entrance and exit segments compared with the rest of the segments in the “wound tracks”.Copyright

Influence of Projectile Momentum and Kinetic Energy on Bacterial Distribution in an Extremity Surrogate “Wound Track”

This paper presents an experimental study that examines the relationship between the initial momentum and the initial kinetic energy of a projectile and the distribution of bacterial contamination along a “wound track” created in an extremity surrogate representative of the superior (upper) portion of the lower leg (i.e., the calf region) of an average adult human male. Initial surface contamination was represented using circular filter paper moistened with a solution containing 5 × 106 colony forming units per milliliter (CFU/ml) of Escherichia coli strain K-12 that was previously transformed to express green fluorescent protein (GFP) and be resistant to ampicillin. The contaminated filter paper and extremity surrogate were perforated with 11.43-mm (0.45-in) caliber round nose lead projectiles shot from commercially available air rifles. To match the initial momentum and/or kinetic energy between experiments, 11.0 g (170 grain) and 14.9 g (230 grain) projectiles were shot at velocities ranging from 145 m/s to 195 m/s. The “wound track” was extracted from the extremity surrogate and sliced into small, evenly spaced segments and the permanent cavity was removed from each segment using a biopsy punch, liquefied, and grown on selective agar containing ampicillin. Examination of the bacterial colony count and area covered by bacteria colonies per segment allowed comparison of differences between trends in the bacteria distribution along the “wound track”. The results obtained showed that, for the cases considered, the bacterial distribution trends were similar for the experimental groups with like initial kinetic energies.Copyright

Increasing Student Awareness of Non-Traditional Career Paths in Mechanical Engineering

This paper reports the effect of the curriculum changes in the Mechanical Engineering (ME) department at the South Dakota School of Mines and Technology (SDSM&T) in two freshman courses. Besides introducing fundamental concepts and principles for mechanical engineering, these courses utilize guest speaker sections to introduce career opportunities, and integrate marketing and corporate policy into engineering design projects.An engineering motivation survey and a career awareness questionnaire were developed and employed to better understand the impact of the new curriculum on students’ attitudes toward and desire to persist in Mechanical Engineering. Freshmen students’ intrinsic and extrinsic motivations and career awareness were assessed in pre- and post-tests at the beginning and the end of a semester.The results obtained showed that the introduction of the non-traditional career paths into the two courses students’ perceptions of the career options that ME graduates can pursue and showcased alternatives that proved to be more attractive to under-represented (female) students. Although the students spent less time in traditional engineering topics, their engineering self-efficacy was not diminished, and in fact, the students’ intrinsic motivation was significantly improved. Additionally, students perceived stronger inclusion in the ME program.Copyright

Using Practical Examples to Motivate the Study of Product Development and Systems Engineering Topics

Most undergraduate mechanical engineering curricula contain one or more courses that provide an introduction to the product design and development process. These courses include some topics that, without the proper motivation, may be perceived by students as being of low relevance. In addition, they also cover topics that may seem to be somewhat abstract and difficult to apply unless they are preceded by examples that clearly illustrate their practical value.The tasks of identifying customer needs and setting target specifications are typical examples of the first scenario described above. In general, engineering students have the notion that the activities of the detailed design phase are the ones that really matter and that those activities are the ones that determine the ultimate success of a product. They are so concerned with designing the physical components of the product correctly that they spend little time and effort in other steps that are necessary to make sure that they are designing the right product.The tasks of concept generation and defining the architecture of a product are good examples of the second scenario mentioned in the first paragraph. Most students quickly proceed to pick a concept that they think is viable without carefully exploring the entire solution space. In addition, when considering relatively complex products, many students don’t spend enough time considering aspects such as defining the interfaces between different components. As a result, student teams end up with a collection of components that are individually well-designed but integrate poorly, and the end product suffers accordingly.Short, introductory examples demonstrating the importance of tasks like the ones mentioned above were created in order to get the attention of students and spark their interest in learning about such topics. These presentations were also created with the intent that they would motivate students to apply what they had learned when designing their own product or system.Through the examples, which corresponded to real-world product development efforts, students were exposed to not just well-designed and well-made products or systems that turned out to be successful, but also to products or systems that failed in the marketplace or experienced significant problems because the designers failed to adequately perform a task such as identifying customer requirements. The latter clearly showcased the importance of such tasks and conveyed the fact that good technical design work can be rendered moot by failing to put the required effort into the early stages of the development of a product or system.This paper presents the general criteria used and the approach followed to select and develop short introductory examples for the topics of identifying customer needs, setting target specifications, concept generation, and systems architecture. It briefly describes the examples selected and presents the results of a pilot assessment that was conducted to evaluate the effectiveness of one of those examples.Copyright

Incorporating Basic Systems Thinking and Systems Engineering Concepts in a Sophomore-Level Product Design and Development Course

Although many US undergraduate mechanical engineering programs formally expose students to the basic concepts, methodologies, and tools used for the design and development of new products, the scope is usually limited to products of low complexity. There is a need to include activities in the undergraduate curriculum that allow students to learn basic systems engineering concepts, that promote the development of their systems thinking skills, and that allow them to practice these skills. This paper describes an initial effort at integrating systems engineering concepts in the curriculum focusing on a sophomore-level product development course. The paper discusses the approach that was used to identify topics related to systems thinking and systems engineering, provides the list of topics that were selected, and outlines the approach that will be used to incorporate those topics in the course. In addition, it provides the results of a pilot self-efficacy survey focusing on some of the topics selected that was delivered to junior students who had already taken a formal product development course. Although a specific course was considered, the same approach could be used in the context of the entire mechanical engineering undergraduate curriculum. Also, the results presented in the paper could be easily adapted to similar courses at other institutions.Copyright

Accurate Prediction of Strains and Stresses in 2-D Elasticity Using Adini’s Finite Element

In the interest of obtaining accurate stress predictions in linear elastic problems while keeping the computational cost low, a finite element solution approach using cubic elements that include not only the displacement but also the spatial derivatives of the displacement as nodal degrees of freedom (DOFs) is explored in this paper.The proposed approach has the advantage that the nodal values of the strains, and hence the stresses, can be directly computed from the finite element solution and, as shown in this paper, it is capable of converging faster to the analytical solution than the commonly used reduced integration Serendipity quadratic element.Because the proposed approach is capable of achieving high accuracy using less DOFs, it is possible to use coarser meshes than with conventional elements. This is of particular importance in dynamic problems in which explicit techniques are used and the size of the time step is tied to the element size. Moreover, the proposed approach can be beneficial in non-linear problems in which stepping techniques are used to solve a linearized problem and the strains or stresses of the current step are used as input for the following step.Copyright

Higher Order Finite Elements for the Accurate Prediction of Temperature Gradients in Heat Conduction Problems

When the Finite Element Method (FEM) is used to solve heat conduction problems in solids, the domain is typically discretized using elements that only include the nodal values of the temperature as Degrees of Freedom (DOFs). If the values of the spatial temperature gradients are needed, they are typically computed by differentiating the functional representation for the temperature inside the elements. Unfortunately, this differentiation process usually leads to less accurate results for the temperature gradients as compared to the temperature values.For elliptic problems, like steady state heat conduction, with Neumann Boundary Conditions (BCs), recent research related to Adini’s element suggests that higher order elements that include spatial derivatives of the primary field variable as nodal DOFs are promising for obtaining accurate values for those quantities as well as providing a higher order of convergence than conventional elements.In this paper, steady state and transient heat conduction problems which involve Dirichlet BCs or both Dirichlet and Neumann BCs are studied and a new auxiliary BC is proposed to increase the accuracy of the FE solution when Dirichlet BCs are present. Examples are used to illustrate that Adini’s elements converge faster and are more computationally economical than the conventional Lagrange linear elements and Serendipity quadratic elements when auxiliary BCs are used.Copyright

Experimental and Numerical Study of Soft Tissue Surrogate Behavior Under Ballistic Loading

Studying the high strain rate behavior of soft tissues and soft tissue surrogates is of interest to improve the understanding of injury mechanisms during blast and impact events. Tests such as the split Hopkinson pressure bar have been successfully used to characterize material behavior at high strain rates under simple loading conditions. However, experiments involving more complex stress states are needed for the validation of constitutive models and numerical simulation techniques for fast transient events. In particular, for the case of ballistic injuries, controlled tests that can better reflect the effects induced by a penetrating projectile are of interest. This paper presents an experiment that tries to achieve that goal. The experimental setup involves a cylindrical test sample made of a translucent soft tissue surrogate that has a small pre-made cylindrical channel along its axis. A small caliber projectile is fired through the pre-made channel at representative speeds using an air rifle. High speed video is used in conjunction with specialized software to generate data for model validation.A Lagrangian Finite Element Method (FEM) model was prepared in ABAQUS/Explicit to simulate the experiments. Different hyperelastic constitutive models were explored to represent the behavior of the soft tissue surrogate and the required material properties were obtained from high strain rate test data reported in the open literature. The simulation results corresponding to each constitutive model considered were qualitatively compared against the experimental data for a single projectile speed. The constitutive model that provided the closest match was then used to perform an additional simulation at a different projectile velocity and quantitative comparisons between numerical and experimental results were made. The comparisons showed that the Marlow hyperelastic model available in ABAQUS/Explicit was able to produce a good representation of the soft tissue surrogate behavior observed experimentally at the two projectile speeds considered.Copyright

Comparison of Numerical and Experimental Results of Small Scale Compressed Gas Blast Experiments Involving a Surrogate Head Form

Exposure to a shock wave from an explosive blast can result in injury to the human body even if external signs of trauma are not present. Gaining a better understanding of the mechanisms contributing to those injuries can result in the design of better personal protective equipment (PPE). Compressed gas blast experiments can be conveniently used to explore the mechanical response of PPE systems and instrumented surrogate head forms to blast loading scenarios in a laboratory environment. Likewise, numerical simulations can be used to study relevant field variables related to the compressed gas blast and its effects on the target. In this regard, experimental data is needed to validate simulation results.This paper presents an experiment that uses a small scale compressed gas blast generator to explore the pressure distribution around a surrogate head form due to blast loading. The compressed gas blast generator is an open-end shock tube which creates a shock wave when the diaphragm that separates the high pressure and low pressure (ambient air) regions ruptures. The overpressures on selected locations of the surrogate head form were measured with piezoelectric pressure sensors and the data was processed to obtain positive phase durations and positive phase impulses. The surrogate head form was positioned off-axis from the exit of the compressed gas blast generator to preclude the discharge flow from affecting the overpressure measurements. A three-dimensional Coupled Eulerian-Lagrangian (CEL) model of the experiment described above was prepared in Abaqus/Explicit. Selected numerical and experimental results were compared and there was good agreement between them.Copyright