Roop Lal (Asst. Prof.)

Transcription

1 Fracture of metal and alloys, brittle and ductile, fracture, FRACTURE It is defined as the separation of a specimen into two or more parts by an applied stress. Fracture can be classified as either brittle or ductile. The former occurs after little or no plastic deformation, whereas the latter occurs after extensive plastic deformation. A comparison of these are made in Table 7. Table 7. Fracture is caused by physical and chemical forces and takes place in two stages: (i) crack initiation, i.e. initial formation of a crack and (ii) (ii) crack propagation, i.e. spreading of crack. Metals exhibit different types of fractures, which depend upon (i) type of material (ii) rate of stressing, i.e. loading (iii) state of stress and (iv) temperature. The main types of fractures shown by metals are: (i) brittle fracture (ii) shear fracture (iii) cleavage fracture and (iv) ductile fracture (Fig. 36). Fig. 36 Types of fractures observed in metals subjected to uniaxial tension (a) brittle fracture (b) shear fracture (c) ductile fracture (d) completely ductile fracture Brittle Fracture: The word brittle is associated with a minimum of plastic deformation, i.e. with a brittle fracture the material fractures with very rapid propagation of crack with very little or no plastic deformation like a china cup. The salient features of brittle fracture are:

2 (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) Brittle fracture occurs when a small crack in a material grows and the movement of crack involves very little plastic deformation of the metal adjacent to the crack. Growth continues until fracture occurs (crack propagation). At the surface of a material, the atoms do not have as many neighbours as those in the interior of a solid and therefore they form fewer bonds. Obviously, surface atoms are at a higher energy than a plane of interior atoms. Brittle fracture contains in destroying the interatomic bonds by normal stresses. Brittle fracture in metals is characterised by a rapid rate of crack propagation with minimum energy of absorption, with no gross deformation and very little microdeformation. Adjacent parts of the metal are separated by stresses normal to the fracture surface. This does not produce plastic deformation and therefore requires less energy than a ductile failure where energy is introduced in the process of forming dislocations and other imperfections within the crystal. Brittle fracture occurs along crystal planes with fewer atomic bonds, i.e. characteristic crystallographic planes called as cleavage planes. The fracture is termed as cleavage fracture. Brittle fracture occurs at or below the elastic limit of a material. Normally brittle fracture follows the grain boundaries which can be identified by their granular and shiny look. In some instances this type of fracture can be caused by grain-boundary films of hard brittle second phase, like that formed by bismuth in copper. The tendency for brittle fracture increases with decreasing temperature, increasing strain rate and stress concentration conditions usually produced by a notch. Brittle fracture is to be avoided at all cost, because it is very dangerous and occurs without warning and usually produces disastrous consequences. Brittle fractures are of practical importance due to the failures of pressure vessels, bridges, pipe lines, hulls of ships, etc. Ductile Fracture This signifies large plastic deformation, and occurs after extensive plastic deformation prior to and during the propagation of the crack. This requires considerable energy which is absorbed in forming dislocations and other imperfections (defects) in metals. In a ductile fracture, there are three successive events involved: (i) Test sample or specimen begins necking and minute cavities form in the necked region. The plastic deformation is concentrated in this region and indicates that the formation of cavities is closely linked to plastic deformation, hence to the dislocation movement, thereby taking the longest time in the fracture process, (ii) The cavities coalesce and form minute crack at the centre of the test specimen, and The crack propagates outwardly to the surface of the specimen by a shear separation in a direction 45 to the tensile axis, resulting in a familiar cup and cone type fracture.

3 The various stages involved in ductile fracture are shown in Fig. 38. Necking begins at the points of plastic instability where the increase in strength due to strain-hardening fails to compensate for the decrease in cross-sectional area. This occurs at maximum load. The formation of a neck introduces a triaxial state of stress in the region. A hydrostatic component of tension acts along the axis of the specimen at the centre of the necked region. Many fine cavities form in this region (Fig. 38(c)), and under continued straining these grow and coalesce into a central crack. This crack grows in a direction perpendicular to the axis of the specimen until it approaches the surface of the specimen. It then propagates to the surface of the specimen in a direction roughly 45 to the tensile axis to form the cone part of the fracture (Fig. 38(e)). One can also explain the fracture of ductile materials in terms of work-hardening coupled with crack nucleation and growth. The initial cavities are often observed to form at foreign inclusions where gliding dislocations can pile up and produce sufficient stress so that void or micro crack is formed. Let us consider a test specimen which is subjected to a slowly increasing load. Material begins to work harden, when the elastic limit is exceeded. The permanent elongation increases with increasing the load and the cross sectional area decreases simultaneously. The associated decrease in area leads to the formation of a neck in the test specimen as stated earlier. Due to high dislocation density of the necked region and the material being subjected to a complex stress, i.e., no longer a simple tensile stress, the dislocations are separated from each other because due to repulsive inter atomic forces. The dislocations come closer together with the increase of the resolved shear stress on the slip plane. The cracks are formed due to high shear stress and the presence of low angle grain boundaries. Once a crack is formed then it can grow or elongate by means of dislocations which slip. We may note that crack propagation for this mechanism is along the slip plane and these cracks coalesce. Obviously, one crack grows at the expense of others and finally crack growth results in failure. Fig. 38 Various successive stages in the ductile fracture of a specimen (tensile test) Ductile to Brittle Transition This is commonly observed in BCC metals and almost missing in most of the FCC metals. This transition is observed at low temperatures, extremely high rates of strain or notching the

4 material. This is very important when selecting materials for engineering purposes. The notched bar impact test for metals can be used to determine the temperature over which the transition from ductile to brittle takes place. Such a temperature is termed as transition temperature. One can explain the ductile to brittle transition with the help of Fig. 42. Figure shows the plot of brittle fracture stress (σf) and the yield stress (σy) as a function of temperature or strain rate. We note that the curve Fig. 42 Ductile to brittle transitions as a function of grain size and temperature for brittle fracture stress rises slightly to the left because the surface energy increases as temperature decreases. We note a strong temperature dependence in the yield stress curve as in BCC metals and metal oxide ceramics. From figure it is clear that the two curves intersect and a vertical line is drawn at the point of intersection, which is called the ductile-brittle transition temperature. Now, if a material is stressed at a temperature or strain rate which is to the right side of line CD, it will reach its yield point prior it reaches the brittle fracture stress and will undergo some plastic deformation prior to fracture. However, applying a stress under conditions which lie left of the line CD will result in brittle fracture. Obviously, at all temperatures, below the transition temperature, the fracture stress is smaller than that of the yield stress. This reveals that fracture stress may be controlled by the yield stress. As the applied stress reaches a value equal to the yield stress, the crack is nucleated at the intersection of the slip planes and propagates rapidly. The temperature range over which the rapid changes takes place is termed as the transition region. For mild steel, the consumption of energy in an impact test as a function of temperature is shown in Fig. 43. By fast loading, one can achieve a high strain rate in impact testing machines. We know that increasing the strain rate is equivalent to lowering the temperature. This means the materials which are ductile when strained slowly at a given temperature will behave in a brittle manner when subjected to a high strain rate. We may note that the ductile to brittle transition is quite dangerous from a design point of view.

5 Fig. 43 For mild steel, the variation of impact energy as a function of temperature T Fatigue Failure Fatigue: It is the failure of a material by fracture when subjected to a cyclic stress. Fatigue can occur at a stress whose amplitude is much smaller than the static load required to produce fracture. The maximum stress that a material can withstand without failure for a specific large number of cycles of stress is termed its fatigue or endurance limit. Fatigue is distinguished by three main features: (i) loss of strength (ii) loss of ductility and (iii) increased uncertainty in strength and service life. Engineering materials are often subjected to fluctuating loads while in service. Few examples of components which are subjected to fluctuating loads or alternate stresses are: (i) aircraft wings subjected to turbulent air (ii) leaf springs bent to and fro (iii) connecting rods pushed and pulled in piston engines and (iv) some parts of compressors, pumps and turbines, etc. subjected to repeated loading and vibration. If a metal wire is bent to and fro several times, it ultimately breaks. Rotating and vibrating parts of machines as in aeroplanes are liable to undergo fatigue and cause accidents. Fatigue fractures occur without any warning. They result in brittle fracture. About 80% of failure in engineering components takes place due to fatigue failure. Steel have generally a fatigue limit which is normally 0.4 to 0.5 times the tensile strength of the material. Under following conditions, the fatigue fracture progresses rapidly: (i) maximum tensile stress of sufficiently high value (ii) large vibrations or fluctuations in applied stress (iii) large number of cycles of applied stress and

6 (iv) other variables which may change the conditions of failure such as stress concentration, over loading, corrosion, residual stresses, etc. Due to difficulty of recognizing fatigue conditions, fatigue failures comprise percentage of failures occurring in engineering. The point at which the curve flattens out is termed as the fatigue limit and is well below the normal yield stress. The fatigue strength is usually defined as the stress that produces failure in a given number of cycles usually To avoid stress concentrations, rough surfaces and tensile residual stresses, fatigue specimens must be carefully prepared. Fatigue properties of a material are affected by several factors, e.g.: (i) Corrosion: The effect of corrosion is to reduce the number of cycles required to reach for every stress amplitude. We can protect the steel against salt corrosion by chromium or zinc-plating. (ii) Surface Finish: Scratches, dents, identification marks can act as stress raisers and so reduce the fatigue properties. It is reported that shot peening a surface produces surface compressive residual stresses and improves the fatigue performance. Electro-plating produces tensile residual stresses and have a detrimental effect on the fatigue properties. (iii) Temperature: As a consequence of oxidation or corrosion of the metal surface increasing, increase in temperature can lead to a reduction in fatigue properties. (iv) Micro Structure of an Alloy: Composition of an alloy and its grain size can affect its fatigue properties. In comparison to coarse grained steel, finer grain size steels have higher strength. In addition to higher strength, fine grain size also results in better resistance to cracking, better machine finish and improved plastic deformation. In comparison to fine grains, coarse grained steels are less tough and have a greater tendency for distortion. Addition of lead and sulphur in steel increases its machinability and can act as nuclei for fatigue and so reduce fatigue properties. (v) Residual Stresses: Residual stresses are produced by fabrication and finishing processes. Case hardening of steels by carburising results in compressive residual stresses, on surface it improves the fatigue. Several machining processes produce tensile residual stresses, which impair the properties. (vi) Heat Treatment: This reduces residual stresses within a metal. By producing compressive residual stresses in surfaces, case hardening improves fatigue properties. However hardening and tempering treatments reduce surface compressive stresses and so adversely affect fatigue properties. (vii) Stress Concentrations: These are caused by sudden changes in cross-section, keyways, holes, or sharp corners can more easily lead to a fatigue failure. Even a small hole lowers fatigue-limit by 30%. To study the effect of stress raisers on fatigue, a specimen containing V-notch or circular notch is prepared. When specimen is loaded, the notch has the following effects: (i) a taxial state of stress is produced (ii) stress gradient is set up from the root of the notch to the center of the specimen (iii) there is stress concentration at the notch. A crack is developed due to stress concentration at the root. Fatigue Fracture This results from the presence of fatigue cracks, usually initiated by cyclic stresses, at surface imperfections, e.g. machine markings and slip steps. Although the initial stress concentration associated with these cracks are too low to cause brittle fracture, however they may be sufficient to cause slow growth of the cracks into the interior. Eventually the cracks may become sufficiently deep and therefore the stress concentration exceeds the fracture strength and sudden failure occurs. It is reported that the extent of the crack propagation process depends upon the brittleness of the material under test. In brittle materials the crack grows to

7 a critical size from which it propagates right through the structure rapidly, whereas with ductile materials the crack keeps growing until the area left cannot support the load and an almost ductile fracture suddenly occurs. Fatigue Failure: One can recognize fatigue failures by the appearance of fracture. Fatigue failure has a number of specific features compared with failure under static loads: (i) It occurs at lower stresses than the failure at static loads, i.e., lower than the yield strength or ultimate strength. Fig.12 Stress cycles for testing of fatigue (ii) (iii) (iv) Failure starts on the surface (or near it) locally, in places of stress (strain) concentration Local stress concentrators are formed by surface defects appearing on cyclic loading or notches as traces of surface treatment or the effect of the surrounding medium. Failure occurs in a number of stages; accumulation of defects in the material; nucleation of fatigue cracks; gradual propagation and joining of some cracks into single main crack; and rapid final destruction. Failure has the typical structure which reflects the sequence of fatigue processes. A failure usually has the initial zone of destruction (the zone of nucleation of micro cracks), the fatigue zone, and the final failure zone. The initial zone of failure is usually near the surface and has small size and smooth surface. The fatigue zone is the zone where a fatigue crack gradually develops. It has typical concentric ripple lines which are an evidence of jumpwise propagation of fatigue cracks. The fatigue zone develops until the increasing stresses in the gradually diminishing actual section attain a level at which

8 instantaneous destruction takes place and forms the zone of final failure. The main basic reasons for taking place of fatigue failures are: (i) Surface imperfections like machining marks and surface irregularities. (ii) Stress concentrations like notches, keyways, screw threads and matching under-cuts. (iii) At low temperature the fatigue strength is high and decreases gradually with rise in temperature. (iv) Fatigue strength reduces by corroding environments. Following surface treatments like polishing, coatings, carburizing, nitriding, etc., their effect can be reduced. Mechanism of Fatigue Failure This is associated with the development and accumulation of micro plastic deformations in the surface layer and is based on dislocation movement. The possibility of dislocation movement at stresses below the yield limit is due to random orientation of crystals in the metal structure. At rather low mean stresses, the stresses appearing in some crystals may be sufficient for movement of weakly locked dislocations. Besides thin surface layers (to a depth of 1-2 grains) have a typically low stress for operation of Frank-Read dislocations sources. For this reasons, microplastic deformations and damage of thin surface layers can be observed in mild (annealed) metals already at an early stage of loading. Microplastic deformations manifest themselves in the form of slip lines in the surface, with the density of these lines increasing as the number of cycles increases. As dislocations emerge to the surface, their damage is enhanced and appears as steps. Slip lines become wider, change to slip bands, and gradually degenerate into extrusions and intrusions (Fig. 8.14). An extrusion is a bulge and intrusion, a depression of slip band. Extrusions and intrusions (i.e. ridges and recesses) form a rough relief of the surface. Recesses are places of concentration of strain, and therefore, of vacancies and dislocations. A high local density of vacancies and dislocations can result in the formation of voids and loose places which merge together to form sub micro cracks. The development and joining of submicrocracks in turn leads to the formation of microcracks. Fatigue are often referred to as progressive fractures. We have already remarked that fatigue is a result of cumulative process involving slip. High temperature increases the mobility of atoms, facilitating greater slip and deformation before fracture. Highly localized stress is also developed at abrupt changes in cross-section, at the base of surface scratches, at the root of a screw thread, at the edge of small inclusions of foreign substances, and at a minute blow hole or similar internal defects. We may see that these are typical conditions which give rise to the susceptibility to failure by fatigue.

9 Effect of Alloying Elements, A homogeneous mixture of two or more metals or a metal and a non-metal when fused together at a certain temperature forms a new metal after solidification, termed as an alloy. Metals in actual commercial use are almost exclusively alloys, and not pure metals, since it is possible for the design engineer to have an infinite variety of physical properties in the product by varying the metallic composition of the alloy. Alloys are normally harder than their components, less ductile and may have a much lower conductivity, whereas the highly purified single crystal of a metal is very soft and malleable, with high electrical conductivity. This is why pure metals are used only for specific applications. The alloy is usually more corrosion resistant and less affected by atmospheric conditions. The conductivity of an alloy varies with the degree of order of the alloy and the hardness varies with the particular heat treatment. Alloys are classified as binary alloys, composed of two components; as ternary alloys, composed of three components; or as multicomponent alloys. Most commercial alloys are multi component. The composition of an alloy is described by giving the percentage (either by weight or by atoms) of each element in it. Metal alloys by virtue of composition, are often grouped into two classes: (i) ferrous and non-ferrous. Ferrous alloys are those in which iron is the principal constituent, include steels and cast irons. The nonferrous alloys are all alloys that are not iron based. Alloys are widely used in industry because their physical and chemical properties can be easily varied to suit the exact individual requirement. One can achieve this by preparing alloys of different metals. The alloying elements are added to improve one or more of the following properties: (a) tensile strength, hardness and toughness (c) machinability, (e) hardenability (g) fatigue resistance, etc. (b) corrosive and oxidation resistance, (d) elasticity (f) creep strength and ALLOY SYSTEMS The improvement in the properties of an alloy system depends upon the following factors: (i) Manner in which the two or more metals are mixed with each other. (ii) The percentage of different alloying metals/or elements. (iii) Temperature at which these are cooled, etc. It is possible that two or more metals may be soluble in each other in liquid state but may or may not be soluble in each other in solid state. It is possible that they may retain their identity even if they are soluble in liquid state, e.g. cadmium and bismuth are soluble in each other in liquid state but insoluble in each other in the solid state. It is also possible that the two or more metals may be soluble in each other in liquid as well as solid state, e.g. copper and nickel are soluble in each other in the liquid as well as in solid state. Obviously, one cannot distinguish copper from nickel.

10 SOLID SOLUTIONS A solid solution forms when, as the solute atoms are added to the host material, the crystal structure is maintained, and no new structures are formed. In other words, when elements completely dissolve in each other in liquid and or solid state the resulting phase is called solid solution. Solid solutions form readily when solvent and solute atoms have similar sizes and electron structures, so that it is compositionally or chemically homogeneous and the component atoms of the elements cannot be distinguished physically or separated mechanically. There is a homogeneous distribution of the constituents in the solid state so as to form a single phase or solid solution. Basically, solid solutions are of two types: (a) Substitutional Solid Solution When the two metals in solid solution form a single face centred cubic lattice, i.e., in general solute or impurity atoms replace or substitute for the host atoms, is called as substitutional solid solution. (i) Random substitutional solid solutions and (ii) Ordered substitutional solid solutions. Fig. 1 (a) Solid solutions (b) substitutional solid solutions and (c) interstitial solid solution (b) Interstitial Solid Solutions These can form, for instance, on melting together transition metals and non-metals with a small atomic radius (H, N, C or B). The possibility of obtaining an interstitial solution is mainly determined by the size factor; i.e., the size of a solute atom must be equal to or slightly smaller than the size of an interstitial void. Alloy Steel An alloy steel may be defined as a steel to which elements other than carbon are added in sufficient amount to produce an improvement in properties. The alloying is done for specific purposes to increase wearing resistance, corrosion resistance and to improve electrical and magnetic properties, which cannot be obtained in plain carbon steels. The chief alloying elements used in steel are nickel, chromium, molybdenum, cobalt, vanadium, manganese, silicon and tungsten. Each of these elements confer certain qualities upon the steel to which it is added. These elements may be used separately or in combination to produce the desired characteristic in steel.

11 Following are the effects of alloying elements on steel: 1. Nickel. It increases the strength and toughness of the steel. These steels contain 2 to 5% nickel and from 0.1 to 0.5% carbon. In this range, nickel contributes great strength and hardness with high elastic limit, good ductility and good resistance to corrosion. An alloy containing 25% nickel possesses maximum toughness and offers the greatest resistance to rusting, corrosion and burning at high temperature. It has proved to be of advantage in the manufacture of boiler tubes, valves for use with superheated steam, valves for I.C. engines and spark plugs for petrol engines. A nickel steel alloy containing 36% of nickel is known as invar. It has nearly zero coefficient of expansion. So it is in great demand for measuring instruments and standards of lengths for everyday use. 2. Chromium. It is used in steels as an alloying element to combine hardness with high strength and high elastic limit. It also imparts corrosion-resisting properties to steel. The most common chrome steels contains from 0.5 to 2% chromium and 0.1 to 1.5% carbon. The chrome steel is used for balls, rollers and races for bearings. A nickel chrome steel containing 3.25% nickel, 1.5% chromium and 0.25% carbon is much used for armour plates. Chrome nickel steel is extensively used for motor car crankshafts, axles and gears requiring great strength and hardness. 3. Tungsten. It prohibits grain growth, increases the depth of hardening of quenched steel and confers the property of remaining hard even when heated to red colour. It is usually used in conjuction with other elements. Steel containing 3 to 18% tungsten and 0.2 to 1.5% carbon is used for cutting tools. The principal uses of tungsten steels are for cutting tools, dies, valves, taps and permanent magnets. 4. Vanadium. It aids in obtaining a fine grain structure in tool steel. The addition of a very small amount of vanadium (less than 0.2%) produces a marked increase in tensile strength and elastic limit in low and medium carbon steels without a loss of ductility. The chrome-vanadium steel containing about 0.5 to 1.5% chromium, 0.15 to 0.3% vanadium and 0.13 to 1.1% carbon have extremely good tensile strength, elastic limit, endurance limit and ductility. These steels are frequently used for parts such as springs, shafts, gears, pins and many drop forged parts. 5. Manganese. It improves the strength of the steel in both the hot rolled and heat treated condition. The manganese alloy steels containing over 1.5% manganese with a carbon range of 0.40 to 0.55% are used extensively in gears, axles, shafts and other parts where high strength combined with fair ductility is required. The principal uses of manganese steel is in machinery parts subjected to severe wear. These steels are all cast and ground to finish. 6. Silicon. The silicon steels behave like nickel steels. These steels have a high elastic limit as compared to ordinary carbon steel. Silicon steels containing from 1 to 2% silicon and 0.1 to 0.4% carbon and other alloying elements are used for electrical machinery, valves in I.C. engines, springs and corrosion resisting materials.

12 7. Cobalt. It gives red hardness by retention of hard carbides at high temperatures. It tends to decarburize steel during heat-treatment. It increases hardness and strength and also residual magnetism and coercive magnetic force in steel for magnets. 8. Molybdenum. A very small quantity (0.15 to 0.30%) of molybdenum is generally used with chromium and manganese (0.5 to 0.8%) to make molybdenum steel. These steels possess extra tensile strength and are used for air-plane fuselage and automobile parts. It can replace tungsten in high speed steel Design considerations. Following are the general considerations in designing a machine component : 1. Type of load and stresses caused by the load. The load, on a machine component, may act in several ways due to which the internal stresses are set up. 2. Motion of the parts or kinematics of the machine. The successful operation of any machine depends largely upon the simplest arrangement of the parts which will give the motion required. The motion of the parts may be : (a) Rectilinear motion which includes unidirectional and reciprocating motions. (b) Curvilinear motion which includes rotary, oscillatory and simple harmonic. (c) Constant velocity. (d) Constant or variable acceleration. 3. Selection of materials. It is essential that a designer should have a thorough knowledge of the properties of the materials and their behaviour under working conditions. Some of the important characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or hardened, machinability, electrical conductivity, etc. 4. Form and size of the parts. The form and size are based on judgment. The smallest practicable cross-section may be used, but it may be checked that the stresses induced in the designed cross-section are reasonably safe. In order to design any machine part for form and size, it is necessary to know the forces which the part must sustain. It is also important to anticipate any suddenly applied or impact load which may cause failure. 5. Frictional resistance and lubrication. There is always a loss of power due to frictional resistance and it should be noted that the friction of starting is higher than that of running friction. It is, therefore, essential that a careful attention must be given to the matter of lubrication of all surfaces which move in contact with others, whether in rotating, sliding, or rolling bearings.

13 6. Convenient and economical features. In designing, the operating features of the machine should be carefully studied. The starting, controlling and stopping levers should be located on the basis of convenient handling. The adjustment for wear must be provided employing the various take up devices and arranging them so that the alignment of parts is preserved. If parts are to be changed for different products or replaced on account of wear or breakage, easy access should be provided and the necessity of removing other parts to accomplish this should be avoided if possible. The economical operation of a machine which is to be used for production, or for the processing of material should be studied, in order to learn whether it has the maximum capacity consistent with the production of good work. 7. Use of standard parts. The use of standard parts is closely related to cost, because the cost of standard or stock parts is only a fraction of the cost of similar parts made to order. The standard or stock parts should be used whenever possible ; parts for which patterns are already in existence such as gears, pulleys and bearings and parts which may be selected from regular shop stock such as screws, nuts and pins. Bolts and studs should be as few as possible to avoid the delay caused by changing drills, reamers and taps and also to decrease the number of wrenches required. 8. Safety of operation. Some machines are dangerous to operate, especially those which are speeded up to insure production at a maximum rate. Therefore, any moving part of a machine which is within the zone of a worker is considered an accident hazard and may be the cause of an injury. It is, therefore, necessary that a designer should always provide safety devices for the safety of the operator. The safety appliances should in no way interfere with operation of the machine. 9. Workshop facilities. A design engineer should be familiar with the limitations of his employer s workshop, in order to avoid the necessity of having work done in some other workshop. It is sometimes necessary to plan and supervise the workshop operations and to draft methods for casting, handling and machining special parts. 10. Number of machines to be manufactured. The number of articles or machines to be manufactured affects the design in a number of ways. The engineering and shop costs which are called fixed charges or overhead expenses are distributed over the number of articles to be manufactured. If only a few articles are to be made, extra expenses are not justified unless the machine is large or of some special design. An order calling for small number of the product will not permit any undue expense in the workshop processes, so that the designer should restrict his specification to standard parts as much as possible. 11. Cost of construction. The cost of construction of an article is the most important consideration involved in design. In some cases, it is quite possible that the high cost of an article may immediately bar it from further considerations. If an article has been invented and tests of hand made samples have shown that it has commercial value, it is then possible to justify the expenditure of a considerable sum of money in the design and development of automatic machines to produce the article, especially if it can be sold in large numbers. The aim

14 of design engineer under all conditions, should be to reduce the manufacturing cost to the minimum. 12. Assembling. Every machine or structure must be assembled as a unit before it can function. Large units must often be assembled in the shop, tested and then taken to be transported to their place of service. The final location of any machine is important and the design engineer must anticipate the exact location and the local facilities for erection. Creep Materials are often placed in service at elevated temperatures and exposed to static mechanical stresses (e.g., turbine rotors in jet engines and steam generators that experience centrifugal stresses, and high-pressure steam lines). Deformation under such circumstances is termed creep. Defined as the time-dependent and permanent deformation of materials when subjected to a constant load or stress, creep is normally an undesirable phenomenon and is often the limiting factor in the lifetime of a part. It is observed in all materials types; for metals it becomes important only for temperatures greater than about 0.4 Tm (Tm absolute melting temperature). GENERALIZED CREEP BEHAVIOR A typical creep test consists of subjecting a specimen to a constant load or stress while maintaining the temperature constant; deformation or strain is measured and plotted as a function of elapsed time. Figure 28 is a schematic representation of the typical constant load creep behavior of metals. Upon application of the load there is an instantaneous deformation, as indicated in the figure, which is mostly elastic. The resulting creep curve consists of three regions, each of which has its own distinctive strain time feature. Primary or transient creep occurs first, typified by a continuously decreasing creep rate; that is, the slope of the curve diminishes with time. This suggests that the material is experiencing an increase in creep resistance or strain hardening, deformation becomes more difficult as the material is strained. For secondary creep, sometimes termed steady-state creep, the rate is constant; that is, the plot becomes linear. This is often the stage of creep that is of the longest duration. The constancy of creep rate is explained on the basis of a balance between the competing processes of strain hardening and recovery, recovery being the process whereby a material becomes softer and retains its ability to experience deformation. Finally, for tertiary creep, there is an acceleration of the rate and ultimate failure. This failure is frequently termed rupture

15 Figure 28 Typical creep curve of strain versus time at constant stress and constant elevated temperature. The minimum creep rate ε/ t is the slope of the linear segment in the secondary region. Rupture lifetime tr is the total time to rupture. Uni axial compression tests are more appropriate for brittle materials; these provide a better measure of the intrinsic creep properties in as much as there is no stress amplification and crack propagation, as with tensile loads. Compressive test specimens are usually right cylinders or parallelepipeds having length-to-diameter ratios ranging from about 2 to 4. For most materials creep properties are virtually independent of loading direction. Possibly the most important parameter from a creep test is the slope of the secondaryportion of the creep curve ( ε t in Figure 28); this is often called the minimum or steady-state creep rate εs. It is the engineering design parameter that is considered for long-life applications, such as a nuclear power plant component that is scheduled to operate for several decades, and when failure or too much strain are not options. On the other hand, for many relatively shortlife creep situations (e.g., turbine blades in military aircraft and rocket motor nozzles), time to rupture, or the rupture lifetime, is the dominant design consideration; it is also indicated in Figure 28. Of course, for its determination, creep tests must be conducted to the point of failure; these are termed creep rupture tests. Thus, a knowledge of these creep characteristics of a material allows the design engineer to ascertain its suitability for a specific application. STRESS AND TEMPERATURE EFFECTS Both temperature and the level of the applied stress influence the creep characteristics (Figure 29). At a temperature substantially below 0.4Tm and after the initial deformation, the strain is virtually independent of time.with either increasing stress or temperature, the following will be noted: (1) The instantaneous strain at the time of stress application increases, (2) The steady-state creep rate is increased, and (3) The rupture lifetime is diminished.

16 Figure 29 Influence of stress σ and temperature T on creep behavior. ALLOYS FOR HIGH-TEMPERATURE USE There are several factors that affect the creep characteristics of metals. These include melting temperature, elastic modulus, and grain size. In general, the higher the melting temperature, the greater the elastic modulus, and the larger the grain size, the better is a material s resistance to creep. Relative to grain size, smaller grains permit more grain-boundary sliding, which results in higher creep rates. This effect may be contrasted to the influence of grain size on the mechanical behavior at low temperatures [i.e., increase in both strength and toughness. Stainless steels, the refractory metals, and the super alloys are especially resilient to creep and are commonly employed in high temperature service applications. The creep resistance of the cobalt and nickel super alloys is enhanced by solid-solution alloying, and also by the addition of a dispersed phase that is virtually insoluble in the matrix. In addition, advanced processing techniques have been utilized; one such technique is directional solidification, which produces either highly elongated grains or single-crystal components. Another is the controlled unidirectional solidification of alloys having specially designed compositions wherein two-phase composites result.