materials and is important in analysis of manufacturing operations involving deformation(see, for example, Forming and Forging, Volume 14, ASM Handbook). The amount of distortion that can occur at high rates of loading is difficult to analyze or predict precisely because The variation, or scatter, among replicate tests of mechanical properties is greater when strain rates are high than it is when strain rates are low Impulse or impact loading involves the propagation of high-velocity stress waves through the structure, which may not be well quantified, with the most extreme example being ballistic impact with strain rates on the order of 10/min Impulse loading creates an adiabatic condition that causes a local increase in temperature Effect of Temperature. Distortion failures caused by overload can occur at any temperature at which the flow strength of the material is less than the fracture strength. In this discussion, flow strength is defined as the average true stress required to produce detectable plastic deformation caused by a relatively slow, continuously increasing application of load fracture strength is the average true stress at fracture caused by a relatively slow, continuously increasing application of load. The flow strength and fracture strength of a material are temperature dependent, as is the elastic modulus(Youngs modulus, bulk modulus, or shear modulus) Figure 5 illustrates this temperature dependence schematically for polycrystalline materials that do not undergo a solid- state transformation. Two flow strengths are shown: one for a material that does not have a ductile-to-brittle transition in fracture behavior, such as metal with a face-centered cubic(fcc)crystal structure, and one for a body-centered cubic(bcc) material that exhibits a ductile-to-brittle transition modulus Fracture strength Ow a strength(bcc materials) F strength(fcc materials) Homologous temperature Fig 5 Diagram of the temperature dependence of elastic plastic, and fracture behavior of polycrystalline materials that do not exhibit a solid-state transformation. bcc, body- centered cubic; fcc, face-centered cubic; T, instantaneous absolute temperature; TM, absolute melting temperature of the material As shown in Fig. 5, the flow strength, fracture strength, and elastic modulus of a material generally decrease as temperature increases. If a structure can carry a certain load at 20C(70F), it can carry the same load without deforming at lower temperatures. Stressed members made of materials having a ductile-to-brittle fracture transition will sometimes fracture spontaneously if the temperature is lowered to a value below the transition temperature(e.g, see the article Overload Failures"in this volume) If the temperature is increased so that the flow strength becomes lower than the applied stress, a structure may deform pontaneously with no increase in load. a change in temperature may also cause an elastic-distortion failure because of a change in modulus, as might occur in a control device where accuracy depends on a predictable elastic deflection of a ontrol element or a sensing element. For most structural materials, the curve defining the temperature dependence of elastic and plastic properties is relatively flat at temperatures near 20C(70F). For steels, the modulus is slightly decreasing until temperatures of approximately 320 to 370C(600 to 700F), are reached, at which point modulus starts to decrease more rapidly
materials and is important in analysis of manufacturing operations involving deformation (see, for example, Forming and Forging, Volume 14, ASM Handbook). The amount of distortion that can occur at high rates of loading is difficult to analyze or predict precisely because: · The variation, or scatter, among replicate tests of mechanical properties is greater when strain rates are high than it is when strain rates are low. · Impulse or impact loading involves the propagation of high-velocity stress waves through the structure, which may not be well quantified, with the most extreme example being ballistic impact with strain rates on the order of 104 /min. · Impulse loading creates an adiabatic condition that causes a local increase in temperature. Effect of Temperature. Distortion failures caused by overload can occur at any temperature at which the flow strength of the material is less than the fracture strength. In this discussion, flow strength is defined as the average true stress required to produce detectable plastic deformation caused by a relatively slow, continuously increasing application of load; fracture strength is the average true stress at fracture caused by a relatively slow, continuously increasing application of load. The flow strength and fracture strength of a material are temperature dependent, as is the elastic modulus (Young's modulus, bulk modulus, or shear modulus). Figure 5 illustrates this temperature dependence schematically for polycrystalline materials that do not undergo a solidstate transformation. Two flow strengths are shown: one for a material that does not have a ductile-to-brittle transition in fracture behavior, such as metal with a face-centered cubic (fcc) crystal structure, and one for a body-centered cubic (bcc) material that exhibits a ductile-to-brittle transition. Fig. 5 Diagram of the temperature dependence of elastic, plastic, and fracture behavior of polycrystalline materials that do not exhibit a solid-state transformation. bcc, bodycentered cubic; fcc, face-centered cubic; T, instantaneous absolute temperature; TM, absolute melting temperature of the material As shown in Fig. 5, the flow strength, fracture strength, and elastic modulus of a material generally decrease as temperature increases. If a structure can carry a certain load at 20 °C (70 °F), it can carry the same load without deforming at lower temperatures. Stressed members made of materials having a ductile-to-brittle fracture transition will sometimes fracture spontaneously if the temperature is lowered to a value below the transition temperature (e.g., see the article “Overload Failures” in this Volume). If the temperature is increased so that the flow strength becomes lower than the applied stress, a structure may deform spontaneously with no increase in load. A change in temperature may also cause an elastic-distortion failure because of a change in modulus, as might occur in a control device where accuracy depends on a predictable elastic deflection of a control element or a sensing element. For most structural materials, the curve defining the temperature dependence of elastic and plastic properties is relatively flat at temperatures near 20 °C (70 °F). For steels, the modulus is slightly decreasing until temperatures of approximately 320 to 370 °C (600 to 700 °F), are reached, at which point modulus starts to decrease more rapidly
In fcc materials, and in bcc materials at temperatures above the transition temperature but at lower homologous temperatures(<0.3 TM), distortion(net section yielding) always accompanies overload fracture in sections that do not contain a severe stress raiser. At higher homologous temperature, an increase in temperature may cause a change from transgranular(TG)to intergranular (IG) fracture with a concurrent decrease in ductility. That is, there is a minimum ductility at elevated temperature(within the"creep"range)where the fracture mechanism changes from TG to IG fracture with a concurrent loss in ductility Creep, or time-dependent strain, is a relatively long-term phenomenon and can be distinguished from overload distortion by relating the length of time at temperature to the amount of distortion, as discussed in the article " Creep and Stress Rupture Failures"in this Volume. The specific mechanisms and associated kinetics of creep are temperature dependent While creep is sometimes considered to be limited to temperatures above one- half of the absolute melting point and is usually associated with an intergranular mechanism at those temperatures, it is important for the failure analyst to know that long-term creep deformation and even fracture can also occur at lower temperatures and via other mechanisms Changes in operating temperature can affect the properties of a structure in other ways. At temperatures higher than about 30 to 40% of TM depending on the alloy, microstructural changes may occur over time and degrade properties, allowing distortion and even fracture to occur. For example, if a martensitic steel is tempered at a given temperature and then encounters a higher temperature in service, yield strength and tensile strength will decrease because of overtempering Long-time exposure to moderately elevated temperatures may cause overaging in a precipitation-hardening alloy, with a corresponding loss in strength. It is well known to metallurgists that exposure to cryogenic temperatures may cause cracking in a martensitic steel due to the volume change accompany ing the transformation of retained austenite. What may not be as well appreciated is that even if cracking does not occur, the transformation may create a distortion failure due to dimensional growth or warpage in a close-tolerance assembly, such as a precision bearing When the temperature is changed, different coefficients of thermal expansion for different materials in a heterogeneous structure can cause interference between structural members(or can produce permanent distortion because of thermally induced stresses if the members are joined together). The failure analyst must understand the effect of temperature on properties of the specific materials involved when analyzing failures that have occurred at temperatures substantially above or below the design or fabrication temperature Effect of Stress Raisers and Complex Stress States. In sections that do contain stress raisers, net section yielding can still occur if the state of stress is plane stress; that is, one normal stress component is 0. If the stress raiser results in sufficient constraint to produce plane strain, then gross yielding and distortion will not be observed. However, localized distortion may accompany crack extension as the inherent ductility of the material manifests itself. At the microscale, the fracture may or may not show evidence of ductility. That is, the material may be microscale ductile or brittle. If it is microscale ductile, there may still be no evidence of ductility at the macroscale. In inherently brittle materials, where the fracture stress is equal to the flow stress, no gross or localized distortion accompanies fracture, as discussed further in the article Fractures Appearance and Mechanisms of Deformation and Fracture "in this Volume The yield stress is generally taken as the critical value at which plastic deformation initiates. In uniaxial tension, it is clear when the applied stress reaches the yield point. However, for more complex multiaxial stress states, the point at which yield is anticipated may not be as clear. Theories such as maximum shear stress, maximum distortion energy, and others are detailed in Ref 4 and may be applied by the failure analyst if there is uncertainty as to whether the stresses known to be applied were sufficient to cause the distortion observed References cited in this section J W. Jones, Limit Analysis, Mach. Des., Vol 45(No. 23 ), 20 Sept 1973, p 146-151 2. D. Goldner, Plastic Bending in Tubular Beams, Mach. Des., Vol 45(No. 24), 4 Oct 1973, p 152-155 3. D.J. Ulpi, Understanding How Components Fail, 2nd ed, ASM International, 1999, p 16-19 4. J.A. Collins, Failure of Materials in Mechanical Design, 2nd ed, wiley and Sons, 1993 Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Inappropriate Specifications Thefileisdownloadedfromwww.bzfxw.com
In fcc materials, and in bcc materials at temperatures above the transition temperature but at lower homologous temperatures (<0.3 TM), distortion (net section yielding) always accompanies overload fracture in sections that do not contain a severe stress raiser. At higher homologous temperature, an increase in temperature may cause a change from transgranular (TG) to intergranular (IG) fracture with a concurrent decrease in ductility. That is, there is a minimum ductility at elevated temperature (within the “creep” range) where the fracture mechanism changes from TG to IG fracture with a concurrent loss in ductility. Creep, or time-dependent strain, is a relatively long-term phenomenon and can be distinguished from overload distortion by relating the length of time at temperature to the amount of distortion, as discussed in the article “Creep and Stress Rupture Failures” in this Volume. The specific mechanisms and associated kinetics of creep are temperature dependent. While creep is sometimes considered to be limited to temperatures above one-half of the absolute melting point and is usually associated with an intergranular mechanism at those temperatures, it is important for the failure analyst to know that long-term creep deformation and even fracture can also occur at lower temperatures and via other mechanisms. Changes in operating temperature can affect the properties of a structure in other ways. At temperatures higher than about 30 to 40% of TM depending on the alloy, microstructural changes may occur over time and degrade properties, allowing distortion and even fracture to occur. For example, if a martensitic steel is tempered at a given temperature and then encounters a higher temperature in service, yield strength and tensile strength will decrease because of overtempering. Long-time exposure to moderately elevated temperatures may cause overaging in a precipitation-hardening alloy, with a corresponding loss in strength. It is well known to metallurgists that exposure to cryogenic temperatures may cause cracking in a martensitic steel due to the volume change accompanying the transformation of retained austenite. What may not be as well appreciated is that even if cracking does not occur, the transformation may create a distortion failure due to dimensional growth or warpage in a close-tolerance assembly, such as a precision bearing. When the temperature is changed, different coefficients of thermal expansion for different materials in a heterogeneous structure can cause interference between structural members (or can produce permanent distortion because of thermally induced stresses if the members are joined together). The failure analyst must understand the effect of temperature on properties of the specific materials involved when analyzing failures that have occurred at temperatures substantially above or below the design or fabrication temperature. Effect of Stress Raisers and Complex Stress States. In sections that do contain stress raisers, net section yielding can still occur if the state of stress is plane stress; that is, one normal stress component is 0. If the stress raiser results in sufficient constraint to produce plane strain, then gross yielding and distortion will not be observed. However, localized distortion may accompany crack extension as the inherent ductility of the material manifests itself. At the microscale, the fracture may or may not show evidence of ductility. That is, the material may be microscale ductile or brittle. If it is microscale ductile, there may still be no evidence of ductility at the macroscale. In inherently brittle materials, where the fracture stress is equal to the flow stress, no gross or localized distortion accompanies fracture, as discussed further in the article “Fractures Appearance and Mechanisms of Deformation and Fracture” in this Volume. The yield stress is generally taken as the critical value at which plastic deformation initiates. In uniaxial tension, it is clear when the applied stress reaches the yield point. However, for more complex multiaxial stress states, the point at which yield is anticipated may not be as clear. Theories such as maximum shear stress, maximum distortion energy, and others are detailed in Ref 4 and may be applied by the failure analyst if there is uncertainty as to whether the stresses known to be applied were sufficient to cause the distortion observed. References cited in this section 1. J.W. Jones, Limit Analysis, Mach. Des., Vol 45 (No. 23), 20 Sept 1973, p 146–151 2. D. Goldner, Plastic Bending in Tubular Beams, Mach. Des., Vol 45 (No. 24), 4 Oct 1973, p 152–155 3. D.J. Wulpi, Understanding How Components Fail, 2nd ed., ASM International, 1999, p 16–19 4. J.A. Collins, Failure of Materials in Mechanical Design, 2nd ed., Wiley and Sons, 1993 Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Inappropriate Specifications The file is downloaded from www.bzfxw.com
Errors in specification of material or method of processing for a part can lead to distortion failures. These errors are often the result of faulty or incomplete information being available to the designer. In such instances, the designer has to make assumptions concerning the conditions of service Example 2: Distortion Failure of an Automotive Valve Spring. The engine of an automobile lost power and compression and emitted an uneven exhaust sound after several thousand miles of operation. When the engine was dismantled, it was found that the outer spring on one of the exhaust valves was too short to function properly. The short steel spring and an outer spring taken from another cylinder in the same engine(both shown in Fig. 6) were examined in the laboratory to determine why one had distorted and the other had not Fig. 6 Valve springs made from patented and drawn high-carbon steel wire. Distorted outer spring (left)exhibited about 25% set because of proeutectoid ferrite in the microstructure and high operating temperature Outer spring(right) is satisfactory. Investigation. The failed outer spring(at left, Fig. 6) had decreased in length to about the same free length as that of its companion inner spring. Most of the distortion had occurred in the first active coil (at top, Fig. 6), and a surface residue of baked-on oil present on this end of the spring indicated that a temperature of 175 to 205C (350 to 400F) had bec reached. Temperatures lower than 120C (250F)usually do not cause relaxation(or set)in high-carbon steel springs The load required to compress each outer spring to a length of 2.5 cm(1 in. ) was measured. The distorted spring needed only 30 kg(67 Ib), whereas the longer spring needed 41 kg(90 Ib). The distorted spring had suffered 25% set, which was the immediate cause of the engine malfunction The microstructure of both springs was primarily heavily cold-drawn fine pearlite, but the microstructure of the distorted spring contained small amounts of proeutectoid ferrite. Although the composition of the spring alloy was unknown, the microstructure indicated that the material was patented and cold-drawn high-carbon steel wire. The distorted spring had a hardness of 43 HRC, and the longer spring had a hardness of 46 HrC. both hardness and microstructure indicated that the material in the deformed spring had 10% lower yield strength than material in the undeformed spring. The estimates of yield strength were considered valid because of two factors: the accuracy of the hardness testing and characteristically consistent ratios of yield strength to tensile strength for the grades of steel commonly used in spring wire Conclusions. The engine malfunctioned because one of the exhaust-valve springs had taken a 25% set in service Relaxation in the spring material occurred because of the combined effect of improper microstructure(proeutectoid ferrite) plus a relatively high operating temperature. The undeformed spring exhibited little or no set because the tensile trength and corresponding yield strength of the material(estimated from hardness measurements) were about 10% higher than those of the material in the deformed spring Recommendations. a higher yield strength and a higher ratio of yield strength to tensile strength can be achieved in the springs by using quenched-and-tempered steel instead of patented and cold-drawn steel. An alternative would be to use a more expensive chromium-vanadium alloy steel instead of plain carbon steel; the chromium-vanadium steel should be quenched and tempered. Regardless of material or processing specifications, if springs are stressed close to the yield point of the material, close control of material and processing plus stringent inspection are needed to ensure satisfactory rformance Service conditions are sometimes changed, invalidating certain assumptions that were made when the part was originally designed. Such changes include an increase in operating temperature to one at which the material no longer has the required strength, an increase in the load rating of an associated component, which the user may interpret as an increase in the allowable load on the structure as a whole, and an arbitrary increase in applied load by the user on the assumption that the component has a high enough safety factor to accommodate the added load
Errors in specification of material or method of processing for a part can lead to distortion failures. These errors are often the result of faulty or incomplete information being available to the designer. In such instances, the designer has to make assumptions concerning the conditions of service. Example 2: Distortion Failure of an Automotive Valve Spring. The engine of an automobile lost power and compression and emitted an uneven exhaust sound after several thousand miles of operation. When the engine was dismantled, it was found that the outer spring on one of the exhaust valves was too short to function properly. The short steel spring and an outer spring taken from another cylinder in the same engine (both shown in Fig. 6) were examined in the laboratory to determine why one had distorted and the other had not. Fig. 6 Valve springs made from patented and drawn high-carbon steel wire. Distorted outer spring (left) exhibited about 25% set because of proeutectoid ferrite in the microstructure and high operating temperature. Outer spring (right) is satisfactory. Investigation. The failed outer spring (at left, Fig. 6) had decreased in length to about the same free length as that of its companion inner spring. Most of the distortion had occurred in the first active coil (at top, Fig. 6), and a surface residue of baked-on oil present on this end of the spring indicated that a temperature of 175 to 205 °C (350 to 400 °F) had been reached. Temperatures lower than 120 °C (250 °F) usually do not cause relaxation (or set) in high-carbon steel springs. The load required to compress each outer spring to a length of 2.5 cm (1 in.) was measured. The distorted spring needed only 30 kg (67 lb), whereas the longer spring needed 41 kg (90 lb). The distorted spring had suffered 25% set, which was the immediate cause of the engine malfunction. The microstructure of both springs was primarily heavily cold-drawn fine pearlite, but the microstructure of the distorted spring contained small amounts of proeutectoid ferrite. Although the composition of the spring alloy was unknown, the microstructure indicated that the material was patented and cold-drawn high-carbon steel wire. The distorted spring had a hardness of 43 HRC, and the longer spring had a hardness of 46 HRC. Both hardness and microstructure indicated that the material in the deformed spring had 10% lower yield strength than material in the undeformed spring. The estimates of yield strength were considered valid because of two factors: the accuracy of the hardness testing and characteristically consistent ratios of yield strength to tensile strength for the grades of steel commonly used in spring wire. Conclusions. The engine malfunctioned because one of the exhaust-valve springs had taken a 25% set in service. Relaxation in the spring material occurred because of the combined effect of improper microstructure (proeutectoid ferrite) plus a relatively high operating temperature. The undeformed spring exhibited little or no set because the tensile strength and corresponding yield strength of the material (estimated from hardness measurements) were about 10% higher than those of the material in the deformed spring. Recommendations. A higher yield strength and a higher ratio of yield strength to tensile strength can be achieved in the springs by using quenched-and-tempered steel instead of patented and cold-drawn steel. An alternative would be to use a more expensive chromium-vanadium alloy steel instead of plain carbon steel; the chromium-vanadium steel should be quenched and tempered. Regardless of material or processing specifications, if springs are stressed close to the yield point of the material, close control of material and processing plus stringent inspection are needed to ensure satisfactory performance. Service conditions are sometimes changed, invalidating certain assumptions that were made when the part was originally designed. Such changes include an increase in operating temperature to one at which the material no longer has the required strength, an increase in the load rating of an associated component, which the user may interpret as an increase in the allowable load on the structure as a whole, and an arbitrary increase in applied load by the user on the assumption that the component has a high enough safety factor to accommodate the added load
Example 3: Bulging of a Shotgun Barrel Caused by a Change from Lead Shot to Iron Shot. A standard commercial shotgun barrel fabricated from 1138 steel deformed during a test that was made with a new type of ammunition. Use of the new ammunition, which contained soft iron shot with a hardness of about 72 HB, was intended to reduce toxicity; the old ammunition had contained traditional lead shot with a hardness of 30 to 40 HB Investigation. The shotgun barrel was of uniform inside diameter from the breech to a point 7.5 cm(3 in. from the muzzle; at this point, the inside diameter began to decrease("Before test"curve, Fig. 7a). This taper, or integral choke which is intended to concentrate the shot pattern, ended about 3. 8 cm(1-in. )from the muzzle, and the final portion of the barrel had a relatively uniform inside diameter 0.720 0.7|5 Afte O.71o test s s.705 0.700 0695 Before 0690 0685 00.51.015202.53.03.54.0 Distance from muzzle, in (a) 0850 0845 0840 After 0.835 90830 B825 Before test 0820 0.8|5 08|o 0.5|.01.52.02.53.03.54.0 Distance from muzzle in Fig. 7 Comparison of longitudinal profiles of an 1138 steel shotgun barrel before and after testing. 1000 rounds of a new type of ammunition were fired in the test(a) Inside diameter. (b) Outside diameter After a test in which 1000 rounds of ammunition containing soft iron shot were fired. the shotgun barrel had a longitudinal profile of inside diameter as shown in the"After test "curve in Fig. 7(a). Comparison of this curve with the Thefileisdownloadedfromwww.bzfxw.com
Example 3: Bulging of a Shotgun Barrel Caused by a Change from Lead Shot to Iron Shot. A standard commercial shotgun barrel fabricated from 1138 steel deformed during a test that was made with a new type of ammunition. Use of the new ammunition, which contained soft iron shot with a hardness of about 72 HB, was intended to reduce toxicity; the old ammunition had contained traditional lead shot with a hardness of 30 to 40 HB. Investigation. The shotgun barrel was of uniform inside diameter from the breech to a point 7.5 cm (3 in.) from the muzzle; at this point, the inside diameter began to decrease (“Before test” curve, Fig. 7a). This taper, or integral choke, which is intended to concentrate the shot pattern, ended about 3.8 cm (1 1 2 in.) from the muzzle, and the final portion of the barrel had a relatively uniform inside diameter. Fig. 7 Comparison of longitudinal profiles of an 1138 steel shotgun barrel before and after testing. 1000 rounds of a new type of ammunition were fired in the test. (a) Inside diameter. (b) Outside diameter After a test in which 1000 rounds of ammunition containing soft iron shot were fired, the shotgun barrel had a longitudinal profile of inside diameter as shown in the “After test” curve in Fig. 7(a). Comparison of this curve with the The file is downloaded from www.bzfxw.com
profile before the test shows that the effect of firing soft iron shot was to deform the gun barrel so that the choke taper ras shifted toward the muzzle. After the test, there was a bulge on the outside surface of the barrel, shown in comparisor to the longitudinal profile of the outside diameter before the test in Fig. 7(b). Deformation of the barrel had been detected after the first 100 rounds of iron-shot ammunition had been fired, and the bulge grew progressively larger as the test ontinued Apparently, the bore of the failed barrel was not concentric with the outside surface, because the wall thickness at a given distance from the breech varied widely among different points around the circumference. For example, at a distance of 5 mm(0. 2 in. )from the muzzle, the wall thickness varied from 1. 3 to 2 mm(0.051 to 0.080 in The microstructure of the barrel material was a mixture of ferrite and coarse pearlite. The alloy had a hardness of 163 to 198 HB(converted from Vickers hardness measurements) Based on previous tests, in which the hoop stress in shotgun barrels had been measured when lead-shot ammunition was fired, the safety factor had been estimated at 2.0. In this instance, it was concluded that wall thickness variations reduced the safety factor to about 1.3 for lead-shot ammunition. Previous tests had also shown that lead shot deformed extensively by impact with the bore in the choke zone of this type of gun barrel Analysis. The major stresses in the choke zone are produced by impact of shot pellets against the bore. When lead shot is used, the lead absorbs a considerable amount of the impact energy as it deforms. Soft iron shot, on the other hand, is much harder than lead and does not deform significantly. More of the impact energy is absorbed by the barrel when iron shot is used, producing higher stresses In this instance, had the gun barrel been of more uniform wall thickness around its circumference, it might not have deformed. However. it was believed that conversion to iron-shot ammunition would increase stresses in the barrel enough to warrant an increase in the strength of this type of barrel Conclusions. The shotgun barrel deformed because a change to iron-shot ammunition increased stresses in the choke zone of the barrel. Bulging was enhanced by a lack of uniformity in wall thickness Recommendations. Three alternative solutions to this problem were proposed, all involving changes in specifications The barrel could be made of steel with a higher yield strength The barrel could be made with a greater and more uniform wall thickne An alternative nontoxic metal shot with a hardness of about 30 to 40 HB could be developed for use in the ammunition Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A Moore, Packer Engineering and william Dobson, Binary Engineering Associates, Inc. Failure to Meet specifications Parts sometimes do not perform to expectations because the material or processing does not conform to requirements, leaving the part with insufficient strength. For instance, a part can be damaged by decarburization, as discussed here for a spiral power spring Figure 8 shows two spiral power springs that were designed to counterbalance a textile- machine beam. The spring at left in Fig. 8 was satisfactory and took a normal set when loaded to solid deflection in a presetting operation. The spring at right in Fig. 8, after having been intentionally overstressed in the same manner as the satisfactory spring, exhibited 15% less reaction force than was required at 180 angular deflection because it had taken a set that was 30 in excess of the normal set
profile before the test shows that the effect of firing soft iron shot was to deform the gun barrel so that the choke taper was shifted toward the muzzle. After the test, there was a bulge on the outside surface of the barrel, shown in comparison to the longitudinal profile of the outside diameter before the test in Fig. 7(b). Deformation of the barrel had been detected after the first 100 rounds of iron-shot ammunition had been fired, and the bulge grew progressively larger as the test continued. Apparently, the bore of the failed barrel was not concentric with the outside surface, because the wall thickness at a given distance from the breech varied widely among different points around the circumference. For example, at a distance of 5 mm (0.2 in.) from the muzzle, the wall thickness varied from 1.3 to 2 mm (0.051 to 0.080 in.). The microstructure of the barrel material was a mixture of ferrite and coarse pearlite. The alloy had a hardness of 163 to 198 HB (converted from Vickers hardness measurements). Based on previous tests, in which the hoop stress in shotgun barrels had been measured when lead-shot ammunition was fired, the safety factor had been estimated at 2.0. In this instance, it was concluded that wall thickness variations had reduced the safety factor to about 1.3 for lead-shot ammunition. Previous tests had also shown that lead shot was deformed extensively by impact with the bore in the choke zone of this type of gun barrel. Analysis. The major stresses in the choke zone are produced by impact of shot pellets against the bore. When lead shot is used, the lead absorbs a considerable amount of the impact energy as it deforms. Soft iron shot, on the other hand, is much harder than lead and does not deform significantly. More of the impact energy is absorbed by the barrel when iron shot is used, producing higher stresses. In this instance, had the gun barrel been of more uniform wall thickness around its circumference, it might not have deformed. However, it was believed that conversion to iron-shot ammunition would increase stresses in the barrel enough to warrant an increase in the strength of this type of barrel. Conclusions. The shotgun barrel deformed because a change to iron-shot ammunition increased stresses in the choke zone of the barrel. Bulging was enhanced by a lack of uniformity in wall thickness. Recommendations. Three alternative solutions to this problem were proposed, all involving changes in specifications: · The barrel could be made of steel with a higher yield strength. · The barrel could be made with a greater and more uniform wall thickness. · An alternative nontoxic metal shot with a hardness of about 30 to 40 HB could be developed for use in the ammunition. Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Failure to Meet Specifications Parts sometimes do not perform to expectations because the material or processing does not conform to requirements, leaving the part with insufficient strength. For instance, a part can be damaged by decarburization, as discussed here for a spiral power spring. Figure 8 shows two spiral power springs that were designed to counterbalance a textile-machine beam. The spring at left in Fig. 8 was satisfactory and took a normal set when loaded to solid deflection in a presetting operation. The spring at right in Fig. 8, after having been intentionally overstressed in the same manner as the satisfactory spring, exhibited 15% less reaction force than was required at 180° angular deflection because it had taken a set that was 30° in excess of the normal set