【正文】 or the design must be pleted so rapidly that there is not enough time for testing.4) The part has already been designed manufatufacured, and tested and found to be unsatisfactory. Analysis is required to understand why the part is unsatisfactory and what to do to improve it.It is with the last three categories that we shall be mostly concerned. This means that the designer will usually have only published values of yield strength, ultimate strength, and percentage elongation. With this meager information the engineer is expected to design against static and dynamic loads, biaxial and trysail stress states, high and low temperatures, and large and small parts! The data usually available for design have been obtained from the simple tension test, where the load was applied gradual and the strain given time to develop. Yet these same data must be used in designing parts with plicated dynamic loads applied thousands of times per minute. No wonder machine pets sometimes fail.To sum up, the fundamental problem of the designer is to use the simple tensiontest data and relate them to the strength of the part, regardless of the stress state or the loading situation.It is possible for two metals to have exactly the same strength and hardness, yet one of these metals may have a superior ability to absorb overloads, because of the property called ductility. Ductility is measured by the percentage elongation which occurs in the material at fracture. The usual dividing line between ductility and brittleness is 5 percent elongation. A material having less than 5 percent elongation at fracture is said to be brittle, while one having more is said to be ductile.The elongation of a material is usually measured over 50mm gauge length. Since this is not a measure of the actual strain, another method of determining ductility is sometimes used. After the specimen has been fractured, measurements are made of the area of the cross section at the fracture. Ductility can then be expressed as the percentage reduction in crosssectional area.The characteristic of a ductile material which permits it to absorb large overloads is an additional safety factor in design. Ductility is also important because it is a measure of that property of a material which permits it to be coldworked. Such operations as bending and drawing are metalprocessing operations which require ductile materials.When a material is to be selected to resist wear, erosion, or plastic deformation, hardness is generally the most important property. Several methods of hardness testing are available, depending upon which particular property is most desired. The four hardness numbers in greatest use are the Brine, Rockwell, Vickers, and Koop.Most hardnesstesting systems employ a standard load which is applied to a ball or pyramid in contact with the material to be tested. The hardness is then expressed as a function of the size of the resulting indentation. This means that hardness is an easy property to measure, because the test is nondestructive and test specimens are not required. Usually the test can be conducted directly on an actual machine elementFundamentals of Mechanical DesignMechanical design means the design of things and systems of a mechanical naturemachines, products, structures , devices , and instruments .For the most part mechanical design utilizes mathematics ,the materials sciences ,and the engineeringmechanics sciences.The total design process is of interest to us .How does it begin? Does the engineer simply sit down at his desk with a blank sheet of paper? And, as he jobs down some ideas, what happens next? What factors influence or control the decisions which have to be made? Finally, then, how does this design process end?Sometimes , but not always ,design begins when an engineer recognizes a need and phrasing it in so many words often constitute a highly creative act because the need may be only a vague discontent ,a feeling of uneasiness , or a sensing that something is not right .The need is usually not evident an all. For example, the need to do something about a foodpackaging machine may be indicated by the noise level, by the variation in package weight, and by slight but perceptible variations in the quality of the packaging or right.There is a distinct difference between the statement of the need and the identification of the problem which follows this statement. The problem is more specific .If the need is for cleaner air, the problem might be that of reducing the dust discharge form powerplant stacks, or reducing the quantity of irritants from automotive exhausts.Definition of the problem must include all the specifications for the thing that is to be designed. The specifications are the input and output quantities, the characteristics and dimensions of the space the thing must occupy and all the limitations on these quantities. We can regard the thing to be designed as something in a black box .In this case we must specify the inputs and outputs of the box together with their characteristics and limitations .The specifications define the cost , the number to be manufactured , the expected life ,the range ,the operating temperature ,and the reliability .There are many implied specifications which result ether from the designers particular environment or from the nature of the problem itself. The manufacturing processes which are available, together with the facilities of a certain plant, constitute restrictions on a designer freedom , and hence are a part of the implied specifications . A small plant ,for instance ,may not own coldworking machinery .Knowing this ,the designer selects other metalprocessing methods which can be performed in the plant .The labor skills available and the petitive situation also constitute implied specifications .After the problem has been defined and a set of written and implied specification has been obtaine