【正文】 s． 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 the same properties in pression as in tension． The ultimate strength， however， can not be evaluated for pression． As a ductile specimen flows plastically in pression， the material bulges out， but there is no physical rupture as is the case in tension． Therefore， a ductile material fails in pression as a result of deformation， not stress． Shear Testing． Shafts， bolts， rivets， and welds are located in such a way that shear stresses are produced． A plot of the tensile test． The ultimate shearing strength is defined as the stress at which failure occurs． The ultimate strength in shear， however， does not equal the ultimate strength in tension． For example， in the case of steel， the ultimate shear strength is approximately 75% of the ultimate strength in tension． This difference must be taken into account when shear stresses are encountered in machine ponents． DYNAMIC LOADS An applied force that does not vary in any manner is called a static or steady load． It is also mon practice to consider applied forces that seldom vary to be static loads． The force that is gradually applied during a tensile test is therefore a static load． On the other hand， forces that vary frequently in magnitude and direction are called dynamic loads． Dynamic loads can be subdivided to the following three categories． Varying Load． With varying loads， the magnitude changes， but the direction does not． For example， the load may produce high and low tensile stresses but no pressive stresses． Reversing Load． In this case， both the magnitude and direction change． These load reversals produce alternately varying tensile and pressive stresses that are monly referred to as stress reversals． Shock Load． This type of load is due to impact． One example is an elevator dropping on a nest of springs at the bottom of a chute． The resulting maximum spring force can be many times greater than the weight of the elevator， The same type of shock load occurs in automobile springs when a tire hits a bump or hole in the road． FATIGUE FAILURETHE ENDURANCE LIMIT DIAGRAM The test specimen in Figure ．， after a given number of stress reversals will experience a crack at the outer surface where the stress is greatest． The initial crack starts where the stress exceeds the strength of the grain on which it acts． This is usually where there is a small surface defect， such as a material flaw or a tiny scratch． As the number of cycles increases，the initial crack begins to propagate into a continuous series of cracks all around the periphery of the shaft． The conception of the initial crack is itself a stress concentration that accelerates the crack propagation phenomenon． Once the entire periphery bees cracked， the cracks start to move toward the center of the shaft． Finally， when the remaining solid inner area bees small enough， the stress exceeds the ultimate strength and the shaft suddenly breaks． Inspection of the break reveals a very interesting pattern， as shown in Figure ． The outer annular area is relatively smooth because mating cracked surfaces had rubbed against each other． However， the center portion is rough， indicating a sudden rupture similar to that experienced with the fracture of brittle