Cassandra M. Degen

Impact and Lap Shear Properties of Ultrasonically Spot Welded Composite Lap Joints

Ultrasonic spot welding (USSW) is a widely used technique for joining thermoplastics where high frequency, low amplitude vibrations are applied through an ultrasonic horn resting on the polymer surface to create frictional heat, producing a solid state joint between polymer sheets. Advantages such as short weld cycle time, fewer moving components and reproducibility make this technique attractive for automation and industrial use. The goal of this work was to evaluate the feasibility and analyze the lap shear and impact strength of a composite material joint created using ultrasonic spot welding. The base material used for the joints was a composite consisting of a polycarbonate matrix with chopped glass fibers. The strength of the lap joints was determined through experimental lap shear and impact testing. A finite element analysis was conducted for more thorough insight into the stress patterns in the lap joints. Experiments showed that the ultrasonically spot welded joints tested in tensile lap shear loading carried a load 2.3 times higher than adhesive joints and the impact tested joints had an impact strength 3.5 times higher than adhesive joints. The results of this work suggest ultrasonic spot welding as a viable joining method for thermoplastic composite materials.

Characterization of Thermoplastic Matrix Composite Joints for the Development of a Computational Framework

Many industries, notably the automotive and aerospace industries, are now utilizing thermoplastic matrix composites (TPMCs) for their improved strength and stiffness properties compared to pure thermoplastic polymers, as well as their manufacturability compared to traditional thermoset matrix composites. The increase in the utilization of TPMCs ushers in the need for the development and characterization of joining methods for these materials. A widely used technique for joining thermoplastics is ultrasonic spot welding (USSW). During USSW, high frequency, low amplitude vibrations are applied through an ultrasonic horn resting on the polymer surface. The vibrations induce frictional heat, producing a solid state joint between polymer sheets. Advantages such as short weld cycle time, fewer moving components and reproducibility make this technique attractive for automation and industrial use. Prior work showed USSW as a feasible, repeatable joining method for a polycarbonate matrix filled with chopped glass fibers. The mechanical properties required for full characterization of the TPMC used in this work were not provided by the manufacturer. As such, the constitutive behavior of both as-received and USSW thermoplastic composite material (polypropylene matrix filled with 30 wt% chopped glass fibers) was characterized. The fiber orientation and distribution in TPMCs has a direct impact on constitutive behavior. To characterize these qualities, optical techniques such as scanning electron microscopy (SEM) and micro computed tomography (micro-CT) were employed. Digital image correlation (DIC) was used to acquire full field strain measurements from the composite material under different loading scenarios. Because the constitutive behavior of polymers is greatly dependent on temperature, temperature measurements during the USSW process and measurement of mechanical properties as a function of temperature will be conducted through infrared (IR) imaging and dynamic mechanical analysis (DMA), respectively. Following the calibration of the constitutive model for the polypropylene matrix TPMC, the mechanical and thermal properties will be used to develop a computational framework for the purpose of predicting the structural response of a composite joint under various loadings.

Fabrication of 3D Thermoplastic Sandwich Structures Utilizing Ultrasonic Spot Welding

The ultimate goal of this work was to create 3D polycarbonate sandwich structures fabricated utilizing ultrasonic spot welding (USSW). First, a study of weld strength vs. weld time, frequency, amplitude of vibration and depth of penetration was carried out. Lap shear and peel tests were used to quantitatively characterize weld strength, and from these results, a set of parameters for USSW of polycarbonate was determined. To create the sandwich structures, three layers of 1.59 mm polycarbonate sheet were stacked together, and using an alternating pattern of USSW, the middle layer was joined to each of the outer layers. Post-weld processing of the stacked layers involving heat and pressurization of the structure with air generated a 3D open cell geometry sandwich structure of polycarbonate. 3-point bend and drop impact tests were conducted to study the stiffness and the impact properties of the sandwich structures, and the results suggest that the fabricated polycarbonate sandwich structures might be suitable for applications requiring high strength and energy absorption while maintaining low weight. Additionally, the ability to easily incorporate different geometries in the structure allows the possibility to design structures with tailored mechanical properties.

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