Volume 4 No.2, Spring 2001
Purdue University School of Technology at New Albany
Thirty years of teaching Mechanical Engineering Technology has exposed this writer to a wide variety of issues and questions related to this subject matter. These miscellaneous thoughts are presented here as a way to informally organize some of these questions and explore possible answers. It is hoped that this brief presentation of potential answers to these mechanical engineering questions will serve as a stimulus to the reader to look beyond the traditional answers found in textbooks and presented in lectures. The following material is presented in no specific order. Each issue is a stand-alone subject and should be considered independent of the other topics addressed.
Dealing with introductory topics of engineering materials inevitably leads to questions about reporting the impact resistance of plastics. The data is commonly listed as so many in-lb/in, or in-lb/in of notch, or J/cm as the result of an IZOD impact test. This leads to questions about which dimension of the notch is being referenced. The confusion lies in whether it is the depth of the notch or its length which should be referenced. To my knowledge, none of the textbooks on materials clarify this matter for questioning students. ASTM standard D256-97 governing such tests states that the results are to be reported as energy absorbed per unit of specimen width or per unit of cross-sectional area under the notch. While the standard deals in depth with the factors pertaining to the notch, it is the width of the specimen, perpendicular to the plane of the swinging weight, that is reported.
Turning to the field of machine design, most textbooks on the subject refer to the Raimondi and Boyd charts when discussing the topic of plain journal bearings. From the standpoint of teaching, these charts are an excellent means of introducing student to this area of design. The charts expose students to the terminology used and the factors that should be considered when doing machine design. The Raimondi and Boyd charts are the result of the pioneering work of their creators back in the 1950ís. However, in many instances they have been replaced by more contemporary solutions that have been developed by various companies and universities. My concern as an instructor is that, unless told otherwise, students will think that these charts represent current analysis methods for plain bearings. Three major designers of plain bearings were contacted about this matter and, while all noted the groundbreaking nature of these charts, they all reported using other methods for their design work.
While on the subject of machine design let me turn to the analysis of bolted joints, specifically to the topic of sizing the bolt and nut for a particular load. Here again, the equations in the commonly used texts are good for exposing students to the terminology and some of the factors to consider in selecting a bolted joint. However, the unwary student may try to apply this knowledge to an actual joint thinking that these equations answer all the questions. It then becomes the task of the instructor to point out the limitations of these equations in such variables as joint tightening and relaxation, the determination of the proper torque to apply when tightening the joint, and the state of lubrication of the threads.
Any number of equations used in machine design and the analysis of material strength are derived based on the assumption that the proportional limit of the material being considered is not exceeded. Unfortunately, it is rare to find that assumption stated explicitly. The term elastic limit is used when any statement is made at all. The student must be made aware that the terms elastic limit and proportional limit are frequently used interchangeably when in fact the correct term is proportional limit. For a student who has been properly exposed to the terminology of the stress- strain diagram this inappropriate use of terminology should become apparent immediately.
Once a former student came in to my facility to use the heat-treating furnace to process a part he was making for a go-cart. Initially, he wanted it hardened based on the TTT diagram for the material used to make the part. This much he had learned in a course dealing with engineering materials. What he had not been told was that heating the part sufficiently in an ordinary heat-treating furnace would result in a change in the composition of the surface material and leave a pronounced scale. Furthermore, he had not been told of the difference between the TTT diagram and the continuous cooling diagram. Unfortunately, this individual had not learned that the best way to determine the heat treating procedure for a given metal is to obtain the suppliers recommendations, which are commonly supplied when a metal is purchased.
The effects on steel resulting from the hardening process came up during a recent classroom discussion. Information on this subject was obtained through a class/lab exercise in which the stress-strain diagrams of two samples of the same high carbon steel where developed. One sample was full annealed while the second was quenched and tempered. Upon obtaining the diagrams it became apparent that the tensile strength of the hardened steel had increased significantly while the slope of the linear portion of the two curves was identical. In other words, hardening had no effect on the stiffness of the samples. This same result can be found in typical reference materials on the properties of steel that has been heat-treated.
In the course of teaching introductory manufacturing processes it quickly became apparent that many students thought that it was not possible to weld cast iron. Sometime during their life experiences, perhaps either on the job or in an earlier class, they erroneously formed this idea. In fact, there are some very good publications that discuss the details of welding cast iron. These references state the precautions and limitations of doing this procedure. In general, welding cast iron is possible. The limiting variables are the type of iron and the configuration of the part. The students are assigned to write a paper that discusses the details of such welding to help clarify this issue.
An ideal topic to use to give students experience in the use of the Internet and the library is an investigation of the process used to extrude brittle materials such as marble, tungsten and molybdenum. Some older texts allude to doing this process but detailed information is limited. A thorough examination of the literature both in the library and through the Internet should find that it can be done using hydrostatic extrusion. This process involves surrounding the billet to be extruded by a liquid before forcing it through the die. The discussion of hydrostatic extrusion should produce many questions. Many of those questions will undoubtedly deal with the effects of very high pressures on liquids. Under sufficiently high pressures it has been reported that some liquids will solidify. However a search of the literature to date has not verified this phenomenon. During the lectures this process and its unique properties can be presented and used as a means for improving literature survey skills of the students
An introductory materials course is required for all first year students. This course requires only acceptance into the Mechanical Engineering Technology program and draws students of varied backgrounds. Some are straight out of high school, some have experience on the job with metals and plastics. Occasionally, a purchasing agent or a practicing engineer will enroll to broaden his or her background. Two common misperceptions occur year after year. The first is that a metal failed by crystallizing and the second is that a metal failed because it lost its temper. Students are introduced to the formation of the crystal structure of metals as they cool from the molten state. At that point in the curriculum they should learn that, with the exception of metallic glasses, all metals are in a crystalline state when they are at normal atmospheric temperatures. The idea of failure by crystallization may have arisen from observations of the appearance of the broken ends of a failed metal part. In many cases the facets of the crystals are visible.
Failure due to the lose of temper requires reviewing what the tempering process does to a metal. Tempering steel ,after it is quench hardened, increases the toughness of the steel while reducing its hardness. The term temper is also used to indicate the degree of cold rolling to which a metal is subjected. It is assumed that the temper referred to by students is that which is obtained after quenching. For this metal to lose its temper would require that it be subjected to a heating and cooling cycle to return its hardness to a level in the range of that obtained after quenching. It is also possible that the tempered structure was initially in an unstable state which, over time, returned to a harder level. Though this event had not been verified as possible students are exposed to this concept as motivation for further study.
One area of materials terminology which has not been clarified to this author's satisfaction is the use of the terms carbon-carbon and carbon-graphite when referring to a common type composite. It may be that both terms are correct but refer to different composites. However, nothing I have found to date indicates anything other than that the terms are used interchangeably. Students are made aware of this matter of terminology so that they will be better prepared to deal with such difficulties on the job.
Readers of this paper will probably have similar issues they have encountered in their own education, in their own classroom, or on the job site. I would be glad to hear from you. The purpose for everything that we do as instructors is to better educate our students.
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