【正文】 however will deform a large amount prior to rupture． Excessive deformation， without fracture， may cause a machine to fail because the deformed part interferes with a moving second part． Therefore， a part fails(even if it has not physically broken)whenever it no longer fulfills its required function． Sometimes failure may be due to abnormal friction or vibration between two mating parts． Failure also may be due to a phenomenon called creep， which is the plastic flow of a material under load at elevated temperatures． In addition， the actual shape of a part may be responsible for failure． For example， stress concentrations due to sudden changes in contour must be taken into account． Evaluation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductile． In general， the design engineer must consider all possible modes of failure， which include the following． —— Stress —— Deformation —— Wear —— Corrosion —— Vibration —— Environmental damage —— Loosening of fastening devices The part sizes and shapes selected also must take into account many dimensional factors that produce external load effects， such as geometric discontinuities， residual stresses due to forming of desired contours， and the application of interference fit joints． Cams are among the most versatile mechanisms available． A cam is a simple twomember device． The input member is the cam itself， while the output member is called the follower． Through the use of cams， a simple input motion can be modified into almost any conceivable output motion that is desired． Some of the mon applications of cams are —— Camshaft and distributor shaft of automotive engine —— Production machine tools —— Automatic record players —— Printing machines —— Automatic washing machines —— Automatic dishwashers The contour of highspeed cams (cam speed in excess of 1000 rpm) must be determined mathematically． However， the vast majority of cams operate at low speeds(less than 500 rpm) or mediumspeed cams can be determined graphically using a largescale layout． In general，the greater the cam speed and output load， the greater must be the precision with which the cam contour is machined． DESIGN PROPERTIES OF MATERIALS The following design properties of materials are defined as they relate to the tensile test． Static Strength． The strength of a part is the maximum stress that the part can sustain without losing its ability to perform its required function． Thus the static strength may be considered to be approximately equal to the proportional limit， since no plastic deformation takes place and no damage theoretically is done to the material． Stiffness． Stiffness is the deformationresisting property of a material． The slope of the modulus line and， hence， the modulus of elasticity are measures of the stiffness of a material． Resilience． Resilience is the property of a material that permits it to absorb energy without permanent deformation． The amount of energy absorbed is represented by the area underneath the stressstrain diagram within the elastic region． Toughness． Resilience and toughness are similar properties． However， toughness is the ability to absorb energy without rupture． Thus toughness is represented by the total area underneath the stressstrain diagram， as depicted in Figure 2． 8b． Obviously， the toughness and resilience of brittle materials are very low and are approximately equal． Brittleness． A brittle material is one that ruptures before any appreciable plastic deformation takes place． Brittle materials are generally considered undesirable for machine ponents because they are unable to yield locally at locations of high stress because of geometric stress raisers such as shoulders， holes， notches， or keyways． Ductility． A ductility material exhibits a large amount of plastic deformation prior to rupture． Ductility is measured by the percent of area and percent elongation of a part loaded to rupture． A 5%elongation at rupture is considered to be the dividing line between ductile and brittle materials． Malleability． Malleability is essentially a measure of the pressive ductility of a material and， as such， is an important characteristic of metals that are to be rolled into sheets． Hardness． The hardness of a material is its ability to resist indentation or scratching． Generally speaking， the harder a material， the more brittle it is and， hence， the less resilient． Also， the ultimate strength of a material is roughly proportional to its hardness． Machinability． Machinability is a measure of the relative ease with which a material can be machined． In general， the harder the material， the more difficult it is to machine． COMPRESSION AND SHEAR STATIC STRENGTH In addition to the tensile tests， there are other types of static load testing that provide valuable information． Compression Testing． Most ductile materials have approximately