This online course focuses on Rockwell and Brinell hardness testing and Vickers and Knoop microhardness testing. Participants will learn about how the tests are performed, test sample requirements, test parameter selection, and testing requirements. The course can be completed in 30 minutes.
Mr. Jennings describes a test now being considered for determining the point of optimum superheat for lifting iron from a static to a dynamic condition, with tensile strength of alloyed cast iron of 80,000 lb. per sq. in. and of heat-treated iron of 100,000 lb. per sq. in. When this field is entered, increased temperature becomes necessary for consistent results, and a series of tests is being run to discover approximately the temperature at which breakdown of the carbon nucleus occurs. The electric furnace, Mr. Jennings asserts, offers a non-oxidizing and non-contaminating method of melting iron at any desired temperature and allows iron to become high-brow and choosy.
Peculiarly complex in its cellular structure, wood is subject to a deformation that accompanies changes in its moisture content that is neither uniform nor isometric. Deformation is generally about 50 times as great in the radial direction of the log as longitudinally and about twice as great circumferentially as radially; so, when moisture changes occur due to changes in the degree of humidity of the surrounding air, the behavior of wood is very uncertain. Conditions are complicated further by the manner in which drying takes place. A description is given of how water is contained in wood, including details of wood structure, and the action of moisture in causing swelling and subsequent shrinking is discussed. The fiber-saturation point marks the limit of the amount of moisture that can enter between the fibrils, at which limit swelling ceases. It is determined by making endwise compression tests on a series of small blocks of the wood, as its drying proceeds.
The author limits his consideration of rear axles to that of the bevel-gear type and takes up the subject under the heads of load-carrying member, gearing, driving-shaft, brakes and materials. Several different forms of cast and pressed axle housings are briefly described. Mention is then made of the axle gearing and data given showing the end thrust for straight-tooth bevel pinions. The methods of supporting pinions and of attaching bevel gears to the differential are discussed. Forms of differentials are considered, several different conventional types being illustrated. The subject of driving shafts is briefly reviewed, as is also that of brakes and brake materials. The author concludes his paper by figures showing the tensile strength, elastic limit and elongation of the metals used in the various parts of the rear axle and also explains some oil-retaining and dust-protecting features of design.
It is the purpose of this paper to discuss in detail unusually precise structural model design, construction, and test procedure. A statement of the laws of similarity to which all structural models must conform if precise results are to be obtained in the simplest manner is given by Mr. Loudenslager. Scale selection, choice of materials, and construction methods are all considered by him. Three model members of designs which can represent a number of prototype properties are described in detail, as well as the use of each of these members, and the determination of bending, torsional, tensile, and compressive stresses by means of simple equipment. In addition, design formulas are set forth for a member which represents the axial, torsional, and bending (in two planes) properties. Finally, three test procedures, all applicable to a variety of simple or complicated indeterminate structures, and the methods used in evaluating the test results are presented by the author.
MATERIALS for mechanical, heavy, and upholstery springs are discussed in this paper. The best practice in making mechanical springs is to use material already hardened. Forming springs of hardened materials makes it easy to detect hardening cracks and nonuniform temper, and longer spring life can be expected. Heavy springs are those made of stock of large section - so large that the spring must be formed while the material is hot. SAE 1085 and 1090 are actually better heavy-spring materials than the popular SAE 1095 because they have greater endurance. The ideal material for upholstery springs would have high tensile strength and low elastic limit before fabrication. Afterwards, it would develop a high elastic limit from a low-temperature heating. Cold-drawn products come closest to the ideal.
THIS paper presents the results of investigations conducted at the University of Michigan under the sponsorship of the International Nickel Co., Inc., to determine the influence of carbon, silicon, and phosphorus contents and of section size on the mechanical properties of ductile cast iron. Carbon contents in the range of 2.75% to 4.10% had a minor controlling influence on the mechanical properties. Silicon, through its influence on the matrix structure and its solid solution hardening effect, exerts a major effect on the mechanical properties. Increasing the silicon content from 0.1% to 5.0% produces linear increases in tensile and yield stregnths and hardness and decreases in ductility. Increasing the phosphorus content from 0.04 to 0.40% increases hardness and decreases ductility. Ductile iron exhibits moderate section sensitivity, either in the as-cast or fully annealed conditions. With increasing section thickness the properties are all lowered slightly.
IN this paper the author explains a new method for the determination of the fatigue life of bevel and hypoid gears. This method is said to offer a means for comparing various tooth forms and gear mountings. Briefly, it consists of making a layout in the mean normal section of the tooth, and of calculating the tensile stress in the fillet when the maximum load is applied at its highest position on the tooth. Consideration is given to the fact that in cases where the contact ratio is sufficient to ensure at least two teeth in contact at all times the load will be divided between the teeth. Such factors as impact, inertia, and temperature are given consideration. A graph is plotted using this calculated stress and the number of cycles to failure resulting from extensive bending fatigue tests on both bevel and hypoid gears. With the aid of this graph the fatigue life of new gear designs may be estimated.
THE design and application of tractor bevel gears is covered in this paper. The authors discuss the problems involved, under the following headings: 1. Basic bevel-gear systems in use, based upon the method of cutting. 2. Method of calculation and selection of factors determining the static and maximum tensile stresses. 3. Summary of static and maximum tensile stresses, and fatigue life analysis. 4. Materials and heat-treatment.
STATIC fatigue of rubber is defined by the authors as a progressive breakdown under the influence of a static load, whereas dynamic fatigue is defined as the progressive loss of strength due to successive cycles of stress. The static fatigue life is the time required for rupture under a static load. Test data presented on the tension static fatigue of rubber indicate that the static fatigue lives of the samples are functions of the stresses acting on them; that the static fatigue lives fall off rapidly with increasing stresses; and that the dependents of static fatigue life on the stress is a function of the stock, among other things. Curves of reduction of tensile due to static fatigue show that the tensiles of samples under load actually decrease and that the decrease is greater, the greater the time under load.
THE control of residual stresses in automotive cylinder blocks is discussed in this paper. According to the authors, this study was started because a small percentage of castings was being scrapped because of cooling cracks in the valve compartment wall. They discovered that the greatest single factor in inducing trapped stresses in castings is too great a difference in the cooling rates for different portions between 1400 F and 600 F. They also found out that residual tensile stresses can be eliminated in a particular section by keeping its cooling rate at least as fast as that of other significant sections. Moreover, it is possible to move trapped stresses from one section to another by overcooling. They have also developed a method that gives quantitative results that serve as good index values of the trapped stresses present even in complex castings.
RESULTS of an investigation into the effect of shot-peening variables and the resulting residual stresses on fatigue life are reported in this paper. Leaf springs were the simple specimens heat-treated, cold worked, and tested in this study. Some of the conclusions reached are: 1. There is a minimum shot velocity for each shot size to obtain best fatigue life, and this value is much lower than that normally used. 2. Exposure time for this type of shot-peened specimen beyond some minimum value is wasteful and costly. 3. Shot size has little influence on fatigue life for these specimens. 4. Shot peening specimens while under tensile strain greatly increases fatigue life at 200,000 psi nominal stress over that of nonpeened or strain-free-peened specimens. 5. Shot peening these specimens gave residual compressive stresses 50% of yield strength, and these stresses can be increased to more than 50% by strain peening. 6.
IT is doubtful whether we are getting more net work from metals today in dynamically loaded parts than was obtainable 25 years ago, and no super-strength-alloy discoveries seem imminent; however, much can be done to increase the fatigue strength of many machine parts made from ordinary structural materials by merely extending processes already known to be satisfactory, and avoiding practices that reduce fatigue strength. We have today new concepts of fatigue failure: Fatigue failures result only from tension stresses, never from compressive stresses. Any surface, no matter how smoothly finished, is a stress-raiser. Structural materials are not rigid. Many fatigue failures can be traced to elastic deflection for which no allowance was made in design. From experience with practical machine parts, we can only conclude that stress calculations by textbook methods are wholly inadequate unless we generously temper our calculations with experience.
PRESENT-DAY efforts to produce wood aircraft in large quantities have uncovered many new problems, for wood has certain peculiarities that must be taken into consideration by the engineer, if he is to design structures that make full use of the benefits to be derived from wood. Attempts to take advantage of the high tensile strength of wood will lead to failures in shear, because loads theoretically in tension practically always have shear components that are great enough to overcome the low shear strength of wood. Moisture content also has a great effect on the strength of wood; and the moisture equilibrium of a piece of wood will vary with the relative humidity and temperature to which it is exposed. Mr. Peterson discusses some of the problems confronting the wood aircraft manufacturer under three headings: fabrication, static testing, and detail design. The fabrication of wood structures revolves around the production of strong glue joints.
BY the use of carefully considered designs, the authors contend, any transportation unit in use today can be made largely of aluminum and considerably lighter than the same unit made entirely of ferrous materials. In most cases, they declare, it is possible to make a weight reduction of approximately 50% from the weight of an iron or steel part when aluminum is used. Results of a test made on a 36-passenger aluminum-alloy bus are reported, indicating that calculated stresses do not correspond very closely with measured stresses. This finding is attributed to the fact that a bus body is a complex, statically indeterminate structure and the accuracy of design calculations is wholly dependent upon the accuracy of the assumptions upon which they are based. Strain gages were used to measure the stresses actually occurring in the parts.
An investigation was conducted to learn more about the interrelations between the various physical and mechanical properties of gray iron. Particular emphasis was placed on exploring the factors which control the fatigue properties of gray iron. Four basic strength parameters -- density, hardness, dynamic elastic modulus, and damping capacity --were evaluated and correlated with tensile strength and mean fatigue limit. The dynamic elastic modulus and damping capacity were determined from conventional resonant frequency equipment. Several quantitative relations were developed which permit estimation of the tensile strength and fatigue limit of gray iron without knowledge of the specific chemistry or solidification rate.