Shaft failures

8/13/2019 Shaft failures

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FAILURES OF SHAFTS

Dept of Mechanical Engineering, SJCE, Mysore Page 1

1. INTRODUCTION

A SHAFT is a metal barusually cylindrical in shape and solid, but sometimes

hollowthat is used to support rotating components or to transmit power or motion by rotary or

axial movement. Even fasteners, such as bolts or studs, can be considered to be stationary shafts,

usually with tensile forces, but sometimes combined with bending and/or torsional forces. In

addition to failures in shafts, this article will discuss failures in connecting rods, which translate

rotary motion to linear motion (and conversely) and in piston rods, which translate the action of

fluid power to linear motion. Shafts operate under a broad range of service conditions, including

dust-laden or corrosive atmospheres and temperatures that vary from extremely low, as in arctic

or cryogenic environments, to extremely high, as in gas turbines. In addition, shafts may be

subjected to a variety of loadsin general, tension, torsion, compression, bending, or

combinations of these. Shafts are also sometimes subjected to vibratory stresses.

Apart from wear by bearings, which can be a major contributor to shaft failure (see the

section Wear in this article), the most common cause of shaft failure is metal fatigue. Fatigue

is a weakest link phenomenon; hence, failures start at the most vulnerable point in a dynamically

stressed areatypically a stress raiser, which may be mechanical, metallurgical, or sometimes a

combination of the two. Mechanical stress raisers include such features as small fillets, sharp

corners, grooves, splines, keyways, nicks, and press or shrink fits. Shafts often break at edges of

press-fitted or shrink-fitted members, where high degrees of stress concentration exist. Such

stress concentration effectively reduces fatigue resistance, especially when coupled with fretting.

Metallurgical stress raisers may be quench cracks, corrosion pits, gross non-metallic inclusions,

brittle second-phase particles, weld defects, or arc strikes.

Occasionally, brittle fractures are encountered, particularly in low-temperature

environments or as a result of impact or a rapidly applied overload. Brittle fracture may thus be

attributable to inappropriate choice of material because of incomplete knowledge of operating

conditions and environment or failure to recognize their significance, but it may also be the result

of abuse or misuse of the product under service conditions for which it was not intended. Surface

treatments can cause hydrogen to be dissolved in high-strength steels and may cause shafts to

become embrittled even at room temperature. Electroplating, for instance, has caused failures of

high-strength steel shafts. Baking treatments applied immediately after plating are used to ensure

removal of hydrogen. Ductile fracture of shafts is usually caused by accidental overload and is

relatively rare in normal operation. Creep, a form of distortion at elevated temperatures, can lead

to stress rupture and can also cause shafts having close tolerances to fail because of excessive

changes in critical dimensions.

To a lesser degree, shafts can fracture from misapplication of material. Such fractures

result from use of materials having high ductile-to-brittle transition temperatures, low resistance

to hydrogen embrittlement, temper embrittlement, or caustic embrittlement, or chemical

compositions or mechanical properties other than those specified. In some instances, fractures

may originate in regions of partial or total decarburization or excessive carburization, where

mechanical properties are different because of variations in chemical composition.


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2. FATIGUE FAILURES

Fatigue in shafts can generally be classified into three basic subdivisions:

Bending fatigue, Torsional fatigue, and Axial fatigue. Bending fatigue can result from

these types of bending loads: unidirectional (one-way), reversed (two-way), and rotating. In

unidirectional bending, the stress at any point fluctuates. Fluctuating stress refers to a change in

magnitude without changing algebraic sign. In reversed bending and rotating bending, the stress

at any point alternates. Alternating stress refers to cycling between two stresses of opposite

algebraic sign, that is, tension (+) to compression (-) or compression to tension. Torsional fatigue

can result from application of a fluctuating or an alternating twisting moment (torque).

The axial location of the origin of a fatigue crack in a stationary cylindrical bar or shaft

subjected to a fluctuating unidirectional-bending moment evenly distributed along the length will

be determined by some minor stress raiser, such as a surface discontinuity. Beach marks (also

called clamshell, conchoidal, and crack-arrest marks) of the form shown in Fig. 1(a) and (b) are

indicative of a fatigue crack having a single origin at the point indicated by the arrow. The crack

front, which formed the beach marks, is symmetrical relative to the origin and retains a concave

form throughout. Both the single origin and the smallness of the final-fracture zone in Fig. 1(a)

suggest that the nominal stress was low. The larger final-fracture zone in Fig. 1(b) suggests a

higher nominal stress.

Fig.1 Fatigue marks produced from single origins at low and high nominal stresses and

from multiple origins at high nominal stresses. Fatigue marks are typical for a uniformly

loaded shaft subjected to unidirectional bending. Arrows indicate crack origins; final

fracture zones are shaded.


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Figure 1(c) shows a typical fatigue crack originating as several individual cracks thatultimately merged to form a single crack front. Such multiple origins are usually

indicative of high nominal stress. Radial steps (ratchet marks) are present between crack

origins.

Figures 1(d), (e), and (f) show typical fatigue beach marks that result when a change insection in a uniformly loaded shaft provides a moderate stress concentration. With a low

nominal stress, the crack front changes from concave to convex before rapture (Fig. 1d).

At higher nominal stresses, the crack front flattens and may not become convex before

final fracture (Fig. 1e and f).

A change in section in a uniformly loaded shaft that produces a severe stressconcentration will lead to a pattern of beach marks such as that shown in Fig. 1(g), (h), or

(j). An example of a severe stress concentration is a small-radius fillet at the junction of a

shoulder and a smaller-diameter portion of a shaft or at the bottom of a keyway. Such a

fillet usually results in the contour of the fracture surface being convex with respect to the

smaller-section side.

The crack-front pattern shown in Fig. 1(g) was produced by a low nominal stress. Thecrack front in Fig. 1(h) developed more rapidly because of a higher stress in the

peripheral zone. Multiple crack origins, high nominal stress, and unidirectional bending

usually produce the beach-mark pattern shown in Fig. 1(j).

3. CONTACT FATIGUE

Contact fatigue occurs when components roll, or roll and slide, against each other under

high contact pressure and cyclic loading. Pitting occurs after many repetitions of loading and is

the result of metal fatigue from the imposed cyclic contact stresses. Factors that govern contact

fatigue are contact stress, relative rolling/sliding, material properties, and metallurgical, physical,

and chemical characteristics of the contacting surfaces, including the oil film that lubricates the

surfaces.

The significant stress in rolling-contact fatigue is the maximum alternating shear stress

that undergoes a reversal in direction during rolling. In pure rolling, as in antifriction bearings,

this stress occurs slightly below the surface and can lead to the initiation of subsurface fatigue

cracks. As these cracks propagate under the repeated loads, they reach the surface and producecavities, or pits.

When sliding is superimposed on rolling, as in gear teeth, the tangential forces and

thermal gradient caused by friction alter the magnitude and distribution of stresses in and below

the contact area. The alternating shear stress is increased in magnitude and is moved nearer to the

surface by friction resulting from the sliding action.


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4. WEAR

Wear of metal parts is commonly classified into either of two categories:

a) Abrasive wearb) Adhesive wear

Abrasive wear, the undesired removal of material by a cutting mechanism, can reduce

the size and destroy the proper shape of a shaft. The shaft may then fail by another means, such

as by fracture, or may cease to perform its designed function. Foreign particles, such as sand, dirt

and other debris, in the lubricant can cause wear of a shaft.

Adhesive wearhas a characteristic torn appearance because the surfaces actually weld

together, then are torn apart by continued motion, creating a series of fractures on both surfaces.

This indicates that metal-to-metal contact took place between clean, uncontaminated mating

surfaces. Because excessive frictional heat is generated, adhesive wear often can be identified by

a change in the microstructure of the metal. For example, steel may be tempered or rehardened

locally by the frictional heat generated.

5. BRITTLE FRACTURE OF SHAFTS

Brittle fractures are associated with the inability of certain materials to deform plastically

in the presence of stress at the root of a sharp notch, particularly at low temperatures. Brittle

fractures are characterized by sudden fracturing at extremely high rates of crack propagation,

perhaps 1830 m/s (6000 ft/s) or more, with little evidence of distortion in the region of fracture

initiation. EXAMPLE: A fractured input shaft used in NASCAR racing shown in Fig.2 was

received for analysis to investigate the cause of failure. Results indicate the shaft fractured due

to fatigue progression from an intergranular stress crack, initiated at a dot peen identification

marking on the shaft.

Fig.2 A close-up view of the fracture surface. The blue arrow points to the fracture origin

at a brittle intergranular zone. Two fatigue zones are observed propagating over

approximately 33% of the fracture surface prior to final torsional overload. Fatigue arrest

marks and oxidation are noted in fatigue zone 2.


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6. DUCTILE FRACTURE OF SHAFTS

Ductile fractures, which result from micro void coalescence, exhibit evidence of

distortion (plastic flow) at the fracture surface similar to that observed in ordinary tensile-test or

torsion-test specimens. When a shaft is fractured by a single application of a load greater than the

strength of the shaft, there is usually considerable plastic deformation before fracture. This

deformation is often readily apparent upon visual inspection of a shaft that fractured in tension,

but is often not obvious when the shaft fractured in torsion.

This ability of a material to deform plastically (permanently) is a property known as

ductility. The appearance of the fracture surface of a shaft that failed in a ductile manner is also a

function of shaft shape, the type of stress to which the shaft was subjected, rate of loading, and,

for many alloys, temperature. In general, ductility is decreased by increasing the strength of the

metal by cold work or heat treatment, by the presence of notches, fillets, holes, scratches,

inclusions, and porosity in a notch-sensitive material, by increasing the rate of loading, and for

many alloys, by decreasing the temperature.

Ductile fracture of shafts occurs infrequently in normal service. However, ductile

fractures may occur if service requirements are underestimated, if the materials used are not as

strong as had been assumed, or if the shaft is subjected to a massive single overload, such as in

an accident. Fabricating errors, such as using the wrong material or using material in the wrong

heat treated condition (for example, annealed instead of quenched and tempered), can result in

ductile fractures.

Fig. 3. The macro photographs of the tensile tested and the ductile fracture surface

of tensile tested specimen.

7. DISTORTION OF SHAFTS

Distortion of a shaft can render the shaft incapable of serving its intended function.

Permanent distortion simply means that the applied stress has exceeded the yield strength (but

not the tensile strength) of the material. If it is not feasible to modify the design of the shaft, the

yield strength of the shaft material must be increased to withstand the applied stress. Yield

strength may be increased either by using a stronger material or by heat treating the original

material to a higher strength. Creep, by definition, is time-dependent strain (distortion) occurring

under stress imposed at elevated temperature, provided the operational load does not exceed the

yield strength of the metal. If creep continues until fracture occurs, the part is said to have failed

by stress rupture.


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Creep can result from any type of loading (tensile, torsion, compression, bending, and so

on). Some high-temperature applications, such as gas turbines and jet aircraft engines, require

materials to operate under extreme conditions of temperature and stress with only a limited

amount of deformation by creep. In other high-temperature applications, the permissibledeformation is high and may not even be limited as long as rupture does not occur during the

intended life of the part. For this type of service, stress-rupture data, rather than long-term creep

data, are used for design. Buckling, a third type of distortion failure, results from compressive

instability. It can occur if a long slender rod or shaft collapses from compressive axial forces.

The load required to cause buckling can be changed only by design changes, not by metallurgical

changes, such as heat treatment, in a given type of metal.

8. CORROSION OF SHAFTS

Most shafts are not subjected to severe reduction in life from general corrosion or

chemical attack. Corrosion may occur as general surface pitting, may uniformly remove metal

from the surface, or may uniformly cover the surface with scale or other corrosion products.

Corrosion pits have a relatively minor effect on the load carrying capacity of a shaft, but they do

act as points of stress concentration at which fatigue cracks can originate. A corrosive

environment will greatly accelerate metal fatigue; even exposure of a metal to air results in a

shorter fatigue life than that obtained under vacuum. Steel shafts exposed to salt water may fail

prematurely by fatigue despite periodic, thorough cleaning.

Aerated salt solutions usually attack metal surfaces at the weakest points, such as

scratches, cut edges, and points of high strain. To minimize corrosion fatigue, it is necessary to

select a material that is resistant to corrosion in the service environment or to provide the shaft

with a protective coating. Most large shafts and piston rods are not subject to corrosion attack.

However, because ship-propeller shafts are exposed to salt water, they are pressure rolled, which

produces residual surface-compressive stresses and inhibits origination of fatigue cracks at

corrosion pits. Also, rotating parts, such as centrifugal compressor impellers and gas-turbine

disks and blades, often corrode.

Centrifugal compressors frequently handle gases that contain moisture and small amounts

of a corrosive gas or liquid. If corrosion attack occurs, a scale is often formed that may be left

intact and increased by more corrosion, eroded off by entrained liquids (or solids), or thrown off

from the rotating shaft. Stress-corrosion cracking occurs as a resuit of corrosion and stress at the

tip of a growing crack. Stress corrosion cracking is often accompanied or preceded by surface

pitting; however, general corrosion is often absent, and rapid, overall corrosion does not

accompany stress-corrosion cracking. The tensile-stress level necessary for stress-corrosion

cracking is below the stress level required for fracture without corrosion.

The critical stress may be well below the yield strength of the material, depending on the

material and the corrosive conditions. Evidence of corrosion, although not always easy to find,

should be present on the surface of a stress-corrosion-cracking fracture up to the start of final

rupture. All of the common materials used in shafts may undergo stress-corrosion cracking under

certain specific conditions. Factors that influence stress-corrosion cracking, either directly or

indirectly, include microstructure, yield strength, hardness, corrodent(s),


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concentration of corrodent(s), amounts and nature of water, pH, and applied and residual

stresses, degree of cold working, and chemical composition of the base metal.

Additional information is available in the article Stress-Corrosion Cracking in this

Volume. Corrosion fatigue results when corrosion and an alternating stressneither of which issevere enough to cause failure by itselfoccur simultaneously; this can cause failure. Once such

a condition exists, shaft life will probably be greatly reduced. Corrosion-fatigue cracking is

usually transgranular; branching of the main cracks occurs, although usually not as much as in

stress-corrosion cracking. Corrosion products are generally present in the cracks, both at the tips

and in regions nearer the origins.

FIG. 4 CORROSION OF SHAFT

Causes:

Corrosion on the shaft in the area of the lip contact will interfere with lips ability to seal

against the shaft surface properly. The increased surface roughness may provide leakage paths

and lip wear may increase from higher roughness.

Action or Countermeasures

1. Apply corrosion-resistant shaft material2. Use replaceable corrosion-resistant shaft sleeve3. Change assembly design to limit access of corrosive contaminates4. Change to seal design that will protect shaft from corrosion so lip can function normally5. If corrosion from inventory storage before assembly - change inventory system

9. INFLUENCE OF FABRICATING PRACTICES

Surface discontinuities produced during manufacture, repair, or assembly (into a

machine) of a shaft can become points of stress concentration and thus contribute to shaft failure.

Operations or conditions that produce this type of stress raiser include

Manufacturing operations that introduce stress raisers, such as tool marks and scratches Manufacturing operations that introduce high tensile stresses in the surface, such as

improper grinding, repair welding, electro machining, and arc burns

Processes that introduce metal weakening, such as forging flow lines that are not parallelwith the surface, hydrogen embrittlement from plating, or decarburization from heat

treatment


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Fatigue strength may be increased by imparting high compressive residual stresses to the surface

of the shaft. This can be accomplished with such processes as surface rolling or burnishing, shot

peening, tumbling, coining, or induction hardening.

Improper Machining: There are many ways in which improper machining can lead to shaftfailures, and unless they are recognized, correction of service-failure problems can be difficult.

The metal on the surface of a machined part can be cold worked and highly stressed to an

appreciable depth (approximately 0.5 to 0.8 mm, or 0.020 to 0.030 in.). Occasionally, the heat

generated in machining, particularly in grinding, is sufficient to heat a thin layer of the steel

above the transformation temperature and thus cause martensitic hardening at the surface upon

cooling. Stresses resulting from thermal expansion and contraction of the locally heated metal

may even be great enough to cause cracking of the hardened surface layer (grinding cracks).

Rough-machining operations can produce surface cracks and sharp corners, which concentrate

stresses.

Example 7:Fatigue Fracture of a 4140 Steel Forged Crankshaft Resulting From Stress Raisers

Created During Hot Trimming. Textile-machine crankshafts like that shown in Fig. 5 were

usually forged from 1035 steel, but because of service conditions, the material was changed to

4140 steel. The forgings were made from 5.5-cm (2.15625-in.) diameter bar stock by cutting to

length, hot bending, upsetting, hot-trimming flash, hot pressing, and visually inspecting before

shipping.

The crankshafts were failing by transverse fracture of one cheek after one to three years of

service. The expected life was 20 years of continuous service. One complete forging that had

fractured (No. 1 in this Example) and a section containing the fractured cheek on the shorter

shaft of another forging (No. 2) were sent to the laboratory so that the cause of failure could be

determined.

Investigation: Visual examination of the fracture surfaces of both crankshafts revealed

indications of fatigue failure; however, the origins were not readily visible (Fig. 5b). The

surfaces had a clean fine-grain structure, but the edges were peenedevidently the result of

damage after fracture.

The surfaces of the cheeks at the parting line contained rough grooves from hot trimming

of the flash and from snag grinding (Fig. 5c and d). Forging 2 contained the most severe of such

markings.

Longitudinal and transverse sections were prepared and etched with hydrochloric acid at

a temperature of 71 to 77 C (160 to 170 F). The steel was of good quality and contained the

normal amount of nonmetallic inclusions but no segregation or pipe.

Examination of an as-polished specimen revealed no intergranular oxidation. Etching the

specimen with a 10% sulphuric acid and 10% nitric acid solution revealed no evidence of

burning or overheating of the steel.

An area containing shallow surface folds was found on the outer face of one cheek of the

throw on forging (Fig. 5e). As shown in Fig. 5(f), the metal around one of the folds contained

some ferrite, and the forged surface was slightly decarburized. Also, a fatigue crack had initiated

in the fold and was propagating across the cheek. Examination of a section through the fracture

surface disclosed cold working of the surface, which could have been the result of a rather


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extended period of fatigue cracking. Chemical analysis of the two forgings found the metal to be

4140 steel, as specified. The hardness of the two forgings at the subsurface, midradius, and core

ranged from 19 to 22 HRC. The specified hardness of the machined forging was unknown.

Tensile strength of the forgings ranged from 790 to 817 MPa (114.5 to 118.5 ksi), andyield strength at 0.2% offset was 580 to 620 MPa (84 to 90 ksi). Elongation in 3.8 cm (1.5 in.)

was 20.6 to 22%, and reduction of area was 56.2 to 59.4%. These properties were representative

of 4140 steel quenched and tempered to a hardness of 20 to 22 HRC. The general microstructure

of the forgings was tempered bainite; the grain size was ASTM 6 to 8.

Conclusions:Fatigue cracking resulted in the transverse fracture of one cheek in each of the two

crankshafts submitted for examination. In crankshaft 1, fatigue cracks were initiated at a shallow

hot-work defect. A rough surface resulting from hot trimming or snag grinding of the forging

flash was the point of initiation of fatigue cracks in crankshaft .

Corrective measures: Before being machined, the forgings were normalized, hardened and

tempered to 28 to 32 HRC to increase fatigue strength. The quenching procedure was changed to

produce a more complete martensite transformation and to increase the ratio of yield strength to

tensile strength. The surfaces were inspected by the magnetic-particle method, and shallow folds,

notches, or extremely rough surfaces were removed by careful grinding.

Identification Marks:Excessive stresses may be introduced in shafts by stamped identification

marks that indicate manufacturing date or lot number, steel heat number, size, or part number.

The location of such a mark and the method by which it is made can be important. Identification

marks should not be placed in areas of high bending or torsional stresses. For shafts, this often

requires that they be located either on the end face or on an adjoining collar, but surface-finish

requirements on the end of the shaft cannot be ignored if thrust loads are taken at that location.

Stamping of marks with straight-line portions is the most likely to cause cracks, although

characters with rounded contours also can cause cracking (Fig. 5). Stamping of metal shaft

surfaces should be avoided because it is impossible to predict on which surface stamp marks will

cause cracking in service.


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Fig. 5 Forged 4140 steel textile-machine crankshaft that fractured in fatigue originating at

machining marks and forging defects. (a) Configuration and dimensions (given in inches).

(b) Fracture surface. (c) Hot trim marks. (d) Snag grinding marks. (e) Hot folds. (f) Section

through a hot fold.

10. INFLUENCE OF METALLURGICAL FACTORS

The fatigue properties of a material depend primarily on microstructure, inclusion

content, hardness, tensile strength, distribution of residual stresses, and severity of the stress

concentrators that are present.

Internal discontinuities, such as porosity, large inclusions, laminations, forging bursts,

flakes, and centreline pipe, will act as stress concentrators under certain conditions and may

originate fatigue fracture. To understand the effect of discontinuities, it is necessary to realize

that fracture can originate at any locationsurface or interiorwhere the stress first exceeds

material strength. The stress gradient must be considered in torsion and bending because the

stress is maximum at the surface, but is zero at the center or neutral axis. In tension, however, the

stress is essentially uniform across the section.

If discontinuities such as those noted above occur in a region highly stressed in tension

by bending or torsional loading, fatigue cracking may be initiated. However, if the

discontinuities are in a low-stress region, such as near a neutral axis, they will be less harmful.

Similarly, a shaft stressed by repeated high tensile loading must be free from serious


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imperfections, for there is no neutral axis; any imperfection can be a stress concentrator and can

be the origin of fatigue cracking if the stress is high with respect to the strength.

Example: Fatigue Cracking of a Forged 4337 Steel Master Connecting RodBecause Of Non-metallic Inclusions: Routine inspection of a reciprocating aircraft engine

revealed cracks in the master connecting rod. Cracks were observed in the channel-shaped

section consisting of the knuckle-pin flanges and the bearing-bore wall. The rods were forged

from 4337 (AMS 6412) steel and heat treated to a specified hardness of 36 to 40 HRC.

Investigation: Visual examination revealed H-shaped cracks in the wall between the

knuckle-pin flanges (Fig. 6a). The cracks originated as circumferential cracks, then propagatedtransversely into the bearing-bore wall. Magnetic-particle and x-ray inspection before sectioning

did not detect any inclusions in the master rod.

Fig. 6 Forged 4337 steel master connecting rod for a reciprocating aircraft engine that

failed by fatigue cracking in the bore section between the flanges.

(a) Configuration and dimensions (given in inches).

(b) Fractograph showing inclusions (arrows) and fatigue beach marks

Macroscopic examination of one of the fracture surfaces revealed three large inclusions

lying approximately parallel to the grain direction and fatigue beach marks around two of the

inclusions. The inclusions and beach marks are shown in Fig.6 (b). Microscopic examination of a

section through the fracture origin showed large non-metallic inclusions that consisted of heavy

concentrations of aluminium oxide (Al2O3). These inclusions were of the type generally

associated with ingot segregation patterns. The hardness of the rods, 36 to 40 HRC, and the

microstructure of the heat-treated alloy steel were satisfactory for the application. A preliminary

stress analysis indicated that the stresses in the area of cracking, under normal operating

conditions, were relatively low compared with other areas of the rod, such as in the shank andthe knuckle-pin straps.


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Conclusions: The rod failed in fatigue in the bore wall between the knuckle-pin flanges.

Fatigue was initiated by the stress-raising effect of large non-metallic inclusions. The

non-metallic inclusions were not detected by routine magnetic particle or x-ray inspectionbecause of their orientation.

Recommendations: The forging vendors were notified that non-metallic inclusions of a

size in excess of that expected in aircraft-quality steel were found in the master connecting rods.

Forging techniques that provided increased working of the material between the knuckle-pin

flanges to break up the large non-metallic inclusions were not successful. A

non-destructive-testing procedure for detection of large non-metallic inclusions was established.

Shaft failures

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