RCFA – Mechanical Components Failure Analysis – Part 1

Presented by Sandeep Gupta, AGM - AO

Mechanical Component Failure Analysis

• It is first level of Root Cause Failure Analysis

• It concentrates on understanding & identifying fracture mechanism of the failed components.

• Provides us the right direction to look for human / latent roots of failure that has set in fracture mechanism.

• It is based on theories of fracture mechanism.

• It requires a good prior understanding of material properties and different stress systems induced in mechanical components by different types of applied forces.

Root Cause Failure Analysis – Different levels

Physical Roots

Physical roots are mechanisms that caused the part to break / fracture

-Corrosion-Wear-Overload-Fatigue

Human Roots

Inappropriate human intervention (acts of commission as well as omission) that result in the mechanisms responsible for failures

-Design-Manufacturing-Maintenance-Installation-Operating errors-Situation blindness

Latent Roots (System weakness)

Practices / policies / actions that allow inappropriate human actions to occur

-Lack of training-Lack of skill-Corporate policies-Pressures to carry out jobs in hurry-Lack of supervisory system-Lack of system for design validation / review

Level -1 Level -2 Level -3

Determining root causes ( human roots) of fatigue failures of mechanical components

General approach for finding root cause of mechanical component failures

Material fracture mechanism theories

Material properties

ActivityTool Reqd.

Step 2

Determine failure / fracture mechanism by examining failed component

Determine type of forces / loads which generated the stresses required to trigger the identified failure mechanism

Mechanics of solids – Various forces / loads and consequent stress systems

Step 1 Step 3

Determine the source of such loads arising out of interaction between the component and its environment / other elements of machine)

Dynamics of machines – Free body diagram and force analysis +Machine specific knowledge

An example - Belt conveyor roller shaft failure

Step 1 Step 3Step 2

Failure / fracture mechanism is found to be rotating bending fatigue failure by examining failed shaft cross section.

Bending force / load is required on part of the shaft beyond drive side bearing to trigger the observed failure mechanism

Sources of bending load on the part of shaft beyond drive side bearing could be (1) Weight of shaft mounted geared motor – normal condition(2) Vertical downward force exerted by geared motor holding down bolts in case of mismatch of distance between shaft CL & geared motor bottom and shaft CL and mounting plate top.

Step 1 : Determine failure / fracture mechanism by examining failed component

( Determining the physical roots )

Physical Roots of Mechanical Component FailuresStudies have revealed that there are only four major physical mechanisms responsible for failure of mechanical components

Physical roots of mechanical component failures - % contribution

Type of physical failure %

Corrosion 23

Fatigue 57

Wear 15

Corrosion fatigue 17

Overload 19

As per study conducted by M/s Sachs, Salvaterra & Associates Inc.

Determining failure mechanism of mechanical components

Step 2 : Determine type of forces / loads which generated the stresses required to trigger the identified failure mechanism

Fatigue Failure - An introduction• In 1800s several investigators in Europe observed that bridge and railroad

components were cracking when subjected to repeated / variable loading.

• Fatigue as term for metal failure under influence of variable loading is first used by Poncelet of France in 1839.

• It is a phenomenon associated with variable loading. Just as we human beings get fatigue when a specific task is repeatedly performed, in a similar manner metallic components subjected to variable loading get fatigue, which leads to their premature failure under specific conditions

1. Without stress fluctuations fatigue cannot happen.

2. Fatigue happens at stress levels well below the tensile strength of the material.

3. Where corrosion is present, the fatigue strength of metals further decreases.

4. The crack takes measurable time to progress across the fracture face.

5. Crack starts at local stress concentration locations in material.

6. Fatigue failure can not occur from pure compressive stress.

Variable Loading – What and how ?

Static / Constant Loading

Stre

ss /

Loa

d

Time t in sec

Stre

ss /

Loa

d

Time t in sec

Variable Loading

Variable Loading : Applied load OR the induced stresses on a component changes with timeExamples1. Change in magnitude of applied load – Punching or shearing operations2. Change in direction of load application – Connecting rod3. Change in point of load application – Rotating shaft

Types of Variable Loading

Variable Loading – Fully Reversed

Variable Loading – Repeated

Variable Loading – Fluctuating

Fatigue Fracture Mechanism

Crack Inititiation ( Region –I) : A fatigue failure begins with a small crack; the initial crack may be so minute and can not be detected. The crack usually develops at a point of localized stress concentration like discontinuity in the material, such as a change in cross section, a keyway or a hole.

Crack Propagation ( Region –II) : Once a crack is initiated, the stress concentration effect become greater and the crack propagates. Consequently the stressed area decreases in size, the stress increase in magnitude and the crack propagates more rapidly.

Sudden Fracture ( Region –III) :Until finally, the remaining area is unable to sustain the load and the component fails suddenly

Ultimate tensile Strength vs Fatigue Strength (Endurance limit)

In general for steels,

Se’ = 0.5 Sut For Sut <=1400 MPa

= 700 MPa For Sut >1400 MPa

Different materials behave differently with repeated loading

Strength – No. of cycles curve for ferrous materials

Different approaches for design of mechanical components to prevent fatigue failure

Stress Concentration factors

• Most common locations for stress concentration1. Steps or grooves in shafts2. Welds in the stressed area of a component3. Holes in components4. Keyways and key seats5. On a bolt body , the transition to the threaded section6. Shrink fitted components with sharp corners7. Rough surface perpendicular to the stress field

Fracture surface features of a typical fatigue failure

Fatigue failure - features of fracture surfaceOrigin : This is the point where the cracking actually started and is the oldest and smoothest part of fracture surface

Fatigue Zone (FZ) : This is zone of progress of crack with each cycle of stress. Smoothness of this zone gives clue that crack growth was slow.

Fatigue Striations : These are crack growth experienced by part with each stress cycle. These are normally visible under high magnification.

Beach / Progression / Conchoidal marks : If the cyclical load on the part is not constant while the crack is growing , the growth rate and surface appearance will change, and the result of these load changes are the progression marks.

Instantaneous Zone : When the load on the component becomes greater than the remaining strength, the piece suddenly fractures across the instantaneous zone.

This final fracture may be ductile or brittle but mostly it is brittle fracture. Surface is rough and crystalline in appearance.

Size of instantaneous zone (IZ) is in indication of the stress on the part at the time of final fracture.

Fatigue Fracture Surface

FATIGE PROPAGATION ZONE

FATIGE INITIATION – CRACK ORIGIN

BEACH / PROGRESSION/ CONCHORDIAL MARKS

FINAL INSTANTANEOUS FRACTURE ZONE

Fatigue failure - fractured surface of shaft

Fatigue failure - fractured surface of shaft – closer view

Crack origin - initiation

Fatigue failure - fractured surface of shaft – progression marks and striations

This fatigue failure started at the keyway . The fracture face is relatively smooth up until the ends of the two arrows . From there to the final fracture progression marks are readily visible.

Progression marks are readily visible features on the fracture surface while striations generally visible under magnification

Fatigue failure - fractured surface of shaft –striations

Fatigue striations of low carbon alloy steel: This scanning electron microscope fractograph shows the roughly horizontal ridges (striations) which are advance of the crack front with each stress cycle.

Fatigue striations showing the result of spectrum loading in a laboratory test of aluminum alloy 7075-T6. In this test, the specimen was loaded 10 cycles at a high stress, then 10 at a lower stress. This produced 10 large striations and then 10 small striations

Cell house 1 – CBTS belt conveyor roller shaft fracture surface

Cell house 2 – Electrolyte circulation pump shaft fracture surface

Fatigue zone 1

Fatigue zone 2

Ratchet mark

Instantaneous fracture zone

A small Instantaneous fracture zone in comparison to Fatigue zone indicates that shaft was lightly loaded but have gone through a considerable no. of cycles before final fracture

Leaching 2 – CN reactor motor shaft fracture surface

Fatigue zone

Ratchet marks are shown by green arrows

Instantaneous zone

The probable loads which caused the fatigue failure may be

(1) Excessive radial thrust force exerted on motor shaft pinion by 1st stage gear as involute profile / pressure angle / root – addendum dia. of pinion and gear are not matching

(2) Radial thrust due to loosening of holding down bolts along with oscillation of lantern.

Primary origin

Leaching 2 – Lime jumbo bag hoist drum supporting shaft fracture surface

Fatigue zone

Instantaneous zone

Fatigue failure of supporting shaft was result of eccentricity between drum outer bearing housing support and its supporting shaft .

Leaching 2 – Lime jumbo bag hoist drum supporting shaft fracture surface

Fatigue failure of supporting shaft was result of eccentricity between drum outer bearing housing and its supporting shaft .

Fatigue failure - Plane bending vs Rotating bendingPlane Bending : Ex. bending of

leaf spring.

Plane Bending :• Bisector of IZ points to crack origin

Rotating bending : Ex. a rotating shaft subjected to bending load of belt drive

Rotating Bending :• Bisector of IZ DOES NOT points to crack origin as

the crack grows unequally with shaft rotation.• It is possible to determine the direction of

rotation of shaft by looking at fractured surface.

Which direction shaft was rotating ?

Fatigue failure – Ratchet marks• The term ratchet marks is used to

describe features that are very useful in identification of fatigue fractures and in locating and counting the number of fatigue origins.

• These marks are perpendicular to the surface from which fatigue fracture originate.

• If a part were more heavily stressed , the fatigue cracks would start at several places simultaneously as the effective stress increases the fatigue strength at more places. When adjacent crack front which are in same plane overlap, the metal in the overlap area will fracture and give rise to ratchet mark.

• When two ratchet marks grow in different directions , the primary origin lies between them.

Fracture surface of a shaft

Formation of ratchet marks in fillet of stepped shaft under uniform rotating bending failure

Fatigue failure – Plane bending• Fracture surface of a 3.6 inch diameter axle

housing tube showing four major fatigue fracture origins at the bottom.

• This part was subjected to unidirectional bending stresses.

• The metal was a medium carbon steel with a hardness of 217-229 HB.

• From the origin areas at the bottom , the fatigue cracks progressed up both sides of the tube and joined at the small final rupture area at the top

• Coarseness of fracture surface increased from bottom to top as the crack propagation speed increased.

Fatigue failure –Rotating bending

• No progression marks showing that the load did not vary during the life of the crack.

• The IZ is small indicating that the load was relatively light.

• If load was light and unvarying, why did the piece crack ? An inspection of the part shows a sharp radius at a step in shaft causing a high stress concentration .

• With the actual stress being the load stress multiplied by the stress concentration, the effective stress in the corner radius was enough to cause many crack origins.

Distinction between effects of from high stress and high stress concentrations

Motor shaft 900 r/ min. failed in 24 hours•IZ is very small showing there was a light load at the time of failure.•No progression marks, showing loading was constant.•Huge no. of tiny ratchet marks with an origin between them•Many fracture origins with a relatively light load indicates there is very high stress concentration.

Motor shaft 900 r/ min. failed in 12 hours•IZ is much larger indicating that load was much larger at the time of failure.•No progression marks, showing loading was constant.•Few ratchet marks and therefore few fracture origins.•Many fracture origins with a relatively light load indicates there is very high stress concentration.

Determining type of loads which caused the fatigue failure

Fatigue Failure – Plane (Unidirectional) Bending

Low stress with low stress concentration

High stress with low stress concentration

Low stress with high stress concentration

1. The ratchet marks on either side of the primary origin grow in slightly different directions, indicating the fracture began at this origin

2. Note in case of low stress, high stress concentration, the corners of the progression marks turn downward because the high stress concentration accelerates crack growth

Fatigue Failure – Plane (Unidirectional) BendingRo

und

X se

ction

Rect

angu

lar X

sec

tion

eg. P

late

or b

ar st

ock

Fatigue Failure –Reverse ( two way) bending

Low stress with low stress concentration

High stress with low stress concentration

Low stress with high stress concentration

The fact that the fatigue zone on one side is larger than the fatigue zone on the other side does not necessarily means the stress on other side is also higher.

Fatigue Failure –Reverse ( two way) bendingRo

und

X se

ction

Rect

angu

lar X

sec

tion

eg. P

late

or b

ar st

ock

Fatigue Failure –Rotating Bending

Low stress with low stress concentration

High stress with low stress concentration

Low stress with high stress concentration

The high total stress at the exterior of the shaft causes failures to begin at many locations . This high total stress may just be the result of the load on the part or it may be caused by a high stress concentration acting on a moderate or low load.

Fatigue Failure –Rotating Bending

Fatigue failures - Tension / Compression load

Fatigue failures - Torsional Load

Have we understood ? Which failure is it ?

Failure of blades of MnO2 slurry tank agitator blades in Cell house -1

Have we understood ? Which failure is it ?

Failure of a universal joint

Have we understood ? Which failure is it ?

Failure of a paper mill refiner shaft

Have we understood ? Which failure is it ?

Failure of stem for a 2-m wide gate valve of sewage treatment plant . It failed 3 months after installation

Have we understood ? Which failure is it ?

Failure of forged connecting rod of AISI 8640 steel.

Have we understood ? Which failure is it ?

Failure of 1045 carbon steel motor shaft . The point of failure was at the shoulder of the customer takeoff end

Have we understood ? Which failure is it ?

Failure of motor shaft

Step 3 : Determine sources of such loads - arising out of interaction between the component and its environment / other elements of machine)

Typical force analysis of rotor of a horizontal end suction single stage volute centrifugal pump

• Various forces on a pump rotor could be divided in two groups depending on direction

• 1. Axial loads : • (a) Hydrostatic force acting on the impeller front and

back shrouds• (b) The momentum force due to the change in

direction of the fluid flow through the impeller • (c) The hydrostatic force due to the hydraulic pressure

acting on the impeller (suction) opening and shaft • 2. Radial loads

Force analysis of a single stage overhung impeller horizontal centrifugal pump – Normal operation

Wr = weight of rotor ( impeller, shaft and coupling)

Force analysis of a single stage overhung impeller horizontal centrifugal pump – Abnormal Operation / installation

Wr = weight of rotor ( impeller, shaft and coupling)

Bending load due to misalignment with motor shaft or bent shaft or coupling unbalance

Additional hydraulic load on impeller due to1. Off duty operation ( far away opern from BEP)2. Pockets in casing due to air ingression3. Cavitation4. Impeller unbalance

Force analysis of a belt conveyor head pulley driven by shaft mounted geared

Force analysis of a belt conveyor head pulley driven by shaft mounted geared motor – Normal operation

T1

T2

Weight of pulley

Bearing reactions

Bearing reactions

Applied torque by geared motor

Weight of geared motor

Force analysis of a belt conveyor head pulley driven by shaft mounted geared motor – Abnormal operation / misalignment

Downward bending load of holding down bolts ( due to misalignment )

Additional load on pulley shaft due to 1. Non co-

linearity of two stub shafts

2. Excessive tensioning of belt

T1

T2

Weight of pulley

Bearing reactions

Bearing reactions

Applied torque by geared motor

Weight of geared motor

Typical Motor shaft loading pattern

Motor shaft – areas of stress concentration

Force analysis of a key mounted in key – Normal fitting

Force analysis of a key mounted in key – abnormal fitting

Excessive impact crushing load on keyway and key if key is loose fit in shaft. This may also result in fatigue failure of shaft itself. Loose fitting keys effect would be very prominent in machines having dynamic load or load reversal like agitator, chain conveyor

Key loose fitting contributing to shaft failure

Input shaft of a gear reducerDrive end of a large pump

In both above cases, inappropriate assembly practices contributed substantially to the failures. Loose key and coupling fitting results in fretting, which continually reduces the fatigue strength of the shaft material. Eventually, the strength drops to the point at which a fracture begins.

Fillet Radii and key Chamfers

Fatigue failure prevention by design improvement

Aluminum alloy 7075-T73 landing gear torque arm assembly redesigned to eliminate fatigue fracture at a lubrication hole

A : Arm configuration , original and improved design

B : Fracture surface where arrows indicate multiple crack origin