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Failure of castings in the mining industry: applications of fracture mechanics

Dr.Richard Clegg

Director, Explicom Pty Ltd

Editor-in-Chief, Engineering Failure Analysis

Adjunct Professor, Queensland University of Technology

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Outline

• Engineering Failure Analysis

• Failure analysis and fracture mechanics

• Case studies

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Engineering Failure Analysis

• Publisher: Elsevier

• Published since 1993

• Current submissions – around 1130 so far in 2018

• Acceptance rate ~ 30%

• Rankings

• Impact Factor (2017 - 2.148)

• 53 out of 130 Mechanical Engineering journals (Q2)

• 11 out of 33 Materials Science, characterisation and testing (Q2)

• CiteScore (2017 - 2.41)

• 37 out of 265 journals (Q1)

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Australia’s Top Exports 2017

1 Iron ores, concentrates $49.3 billion

2 Coal, solid fuels made from coal $40.6 billion

3 Petroleum gases $20.5 billion

4 Gold (unwrought) $13.1 billion

5 Aluminum oxide/hydroxide $5.8 billion

6 Wheat $4.7 billion

7 Crude oil $4 billion

8 Copper ores, concentrates $3.6 billion

9 Frozen beef $3.5 billion

10 Wool (uncarded, uncombed) $2.9 billion

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Costs of failures

• Failures occur in industry and can lead to significant consequential loss. Major incidents can be costly

• Varanus Island (June 2008)• Deepwater Horizon (2010)

• Typical Costs of Failure• Replacement components• Loss of production• Loss of confidence in

products• Injury or death• Damage to reputation and

social licence to operate• Consequential damage

(environmental, societal costs)

Varanus Island

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• Very few incidents are “acts of God” or an “unexpected combination of circumstances”.

• Many incidents are the result of a process of safety degradation.

• Past experience has shown that major incidents rarely “just happen”, but are typically pre-dated by a culture that tolerates small failures.

• Risk management and safety have become an important part of corporate culture in Australian mining.

• The investigation of unexplained failures, even minor, has become an important part of the culture of companies.

Incident investigations and failure analysis

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Framework for viewing failures

Design Manufacture

Use

Design – Everything that was the

responsibility of the design team - OEM

Manufacture – Everything that was the

responsibility of the OEM (including

subcontracted manufacturers) in ensuring

that the design was correctly realised in a

manufactured form

Use - Everything that was the

responsibility of the end user, including

method of operation, repairs and

unauthorised modifications

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Good Failure Analysis

Observation

Keen observation

Reliable background

Hypothesis

Good experimental work

Knowledge and understanding of failures

Synthesis

Communicate with stakeholders

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Understanding failures

Knowledge and

understanding of

failures

Basic Engineering

Science

Past Experience

Experienced

mentors

Communities

of practice

Manufacturers

and suppliers

Personal

Experience

Published Case

Studies

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Where can metallurgical examination help!

• Useful Triage - A metallurgical examination needs to be part of a wider failure analysis

• Main strengths are in:• Help determine the “failure story” – HOW did the failure

happen?• Identification of the mechanisms of failure: Creep,

fatigue, stress corrosion cracking, surface fatigue etc.• Generally, this is descriptive and not numerical (fracture

mechanics)• Confirming the grade of material used• Identifying any metallurgical defects present (quality of the

manufacture).

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What is a Failure Analysis - Opinion or Fact?

• The result of a failure analysis is an OPINION – Not a FACT.

• Scientific and engineering forensic analysis are used to develop and support an OPINION.

• Experimental work can be used to either support or discount theories.

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Fracture Mechanics and Failure Analysis

• Where does fracture mechanics fit into failure analysis?

• Help improve the “failure story” - Sanity check.

• Answer specific questions.

• Characteristics of mining equipment

• Large components, high strength.

• High costs

• Reliability is important

• General problems

• Fatigue, corrosion, wear

• Some specific problems (mercury embrittlement in gas plants, caustic cracking in alumina refineries)

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Fatigue failure

• Large corner crack in the I beam section

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Fracture Mechanics

• Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. (Wikipedia).

• Applications• Fundamental understanding of crack behaviour• Material property determination (quality assurance)• Fitness for Service assessments (BS7910, API579,

SINTAP etc)• Failure analysis

• Application in Failure Analysis is somewhat limited, despite the potential to provide quantitative assessments of crack-related problems.

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Fitness for service applications

• What do I do if I find a crack or defect?• Ignore?• Repair immediately?• Schedule a repair in the near future?

• What is the largest defect that I can tolerate in my structure?

• How fast will a defect grow?

• Structures typically assessed to standards such as AS3788 (Pressure Equipment - In-Service inspection) with reference to design codes such as AS1210 – Pressure Vessel Code

• Tools for structural integrity• API579 – Fitness for Service• BS7910 – Guide to methods for assessing the acceptability of

flaws in metallic structures• SINTAP – European guide

• Naturally need to be conservative – design and FFS assessments.

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Failure analysis (forensic analysis) compared with FFSForensic Analysis Design Analysis

The aim is to provide information to

enable the investigation to proceed

The aim is to provide safe design

We know the position and orientation

of the failure

Engineering judgement required to

predict possible failure modes

Operating conditions not necessarily

well-known

Best estimates of stress and loads are

available

Material can be tested Material data dependent on data

sheets and predictions from

manufacturing (CTOD Testing)

Engineering analysis can be as

“exact” as the data can allow and the

client requires

Design codes provide conservative

analysis (e.g. BS7910/API579)

Documentation required along with

retention of evidence

Documentation required

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Failure Analysis Diagram Approach

• In assessing a failed component, several standards are a useful starting point

• API579 – Fitness for Service

• BS7910 – Guide to methods for assessing the acceptability of flaws in metallic structures

• SINTAP – European guide

• Generally used for Fitness for Service assessments, but contain useful data on stress intensities and toughness estimates.

• Can be used as a basis for forensic analysis of fractured components WTH CARE.

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18

Fracture of structures

• failure of load bearing structures generally by yield or fracture

Yeilding dominant

General plasticity

Defects are (sub)microscopic

Fracture - dominant

Highly locallised plasticity

Defects are macroscopic

Fracture of structures

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19

Domains of Fracture Mechanics

Linear elastic

fracture mechanics

Elastic plastic

fracture mechanics Plastic collapse

Ductility

Defect size

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20

Brittle Fracture - Stress intensity

• In brittle materials, the magnitudes of the stresses ahead of the crack tip can be fully characterised by a single figure, the stress intensity, KI.

• Solution assumes that almost all of the material behaves elastically (small scale yielding only)

• Deviations from the geometry described can be incorporated in the stress analysis.

• Usual to incorporate them as a factor, Y in the stress intensity equation

K aI =

K Y aI =

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Crack like flaw assessment – API579 –Section 9

• Level 1 – Screening assessment. Consists of plots of allowable flaw length versus temperature

• Level 2 – Basic fracture mechanics analysis. Uses the failure assessment diagram (FAD) approach

• Level 3 – Advanced fracture mechanics analysis. May involve finite element analysis of a component with a crack

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Level 2 Assessment

• Use of fracture mechanics to assess criticality of flaws

• Data required• Defect geometry• Material properties

• Yield and tensile strength• Fracture toughness• Crack growth model (?)

• Loads/stresses• Primary stresses (membrane and

bending)• Secondary stresses and Residual

stress

Loading

Material Crack

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Failure Analysis Diagram

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Kr

Lpr

Acceptable

Not Acceptable𝐾𝑟 =

𝐾𝐼𝑃 +Φ0𝐾𝐼

𝑆𝑅

𝐾𝑚𝑎𝑡

𝐿𝑟𝑃 =

𝜎𝑟𝑒𝑓𝑃

𝜎𝑦

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Prestressing wire failure

• A client installed prestressing cables to fix a temporary foundation system in a construction.

• In the hours and days after the cables were fitted, the cables began to fail and eventually the temporary foundation walls collapsed into the construction site.

• I was asked to look into why the cables failed.

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Metallurgical Failure Analysis

• Many side cracks could be seen

Property Value

Tensile strength (MPa) 1780

Proof stress (0.1%) (MPa) 1550

Elongation 5%

C Mn P S Si Cr N

Failed 0.81 0.73 0.014 0.010 0.22 0.29 0.005

New 0.86 0.65 0.014 0.006 0.17 0.34 0.003

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Nature of the factures

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Microstructural analysis

• Cold drawn high carbon steel

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Cause of failure

• Wires consisted of high strength, cold drawn, high carbon steel.

• Stress corrosion cracking had led to multiple side cracks

• SCC cracks were up to 300 microns deep on the sections examined.

• What is the critical size for the SCC crack for failure?

• Why do I want to know this?

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Example – High Strength Wire

• What is the critical size for defects in prestressing wire?

• Treat wire as a rod with circular cross section and a surface flaw

• Critical Data• Defect geometry – unknown

• Diameter of wire, 5 mm• Material properties

• Yield strength, 1550MPa• Fracture toughness, 40MPam1

• Loads/stresses• 1000MPa (membrane only)

Note 1. Estimated from literature

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Determination of Ratios (API579)

• Reference Stress

• 𝐿𝑟𝑃 =

𝜎𝑟𝑒𝑓𝑃

𝜎𝑦

• Stress Intensity

• 𝐾𝑟 =𝐾𝐼𝑃+Φ0𝐾𝐼

𝑆𝑅

𝐾𝑚𝑎𝑡

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Evaluation using FAD

a = 0.05 mm

a = 0.25 mm

a = 0.45 mm

a = 0.70 mm

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Higher stress (1250 MPa)

a = 0.05 mm

a = 0.25 mm

a = 0.45 mm

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Lower Stress (750 MPa)

a = 0.05 mm

a = 0.25 mm

a = 0.45 mm

a = 0.75 mm

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Estimation of fracture toughness

• Fracture toughness is a critical parameter and not commonly measured.

• Estimates often need to be done on limited material and at low cost.

• Methods for estimating toughness• Direct measurement – ASTM E399/E1820/E1921

• Costly• May need more material than is available.

• Handbook/literature data• Not necessarily relevant to application• Embrittled material may not be well characterised

• Indirect measurement• Charpy• Instrumented Charpy (notched vs precracked)• Theory of Critical Distances

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Indirect measurements – Charpy Data

- Several Attempts at correlating Charpy data to toughness summarised in API 579 and BS 7910.

- Usually correlations are dependent on position on transition curve

- Lower Shelf- Transition- Upper Shelf

- Often rely on the establishment of the Reference temperature, T0.

- Methodologies generally determine Lower Bound toughness values whereas for failure analysis purposes, median values are probably more important

𝐾𝐽𝑐 = 30 + 70𝑒𝑥𝑝 0.019 𝑇 − 𝑇0

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Instrumented Charpy Data

• Instrumented Charpy machines can provide greater information than Charpy (load vs displacement data).

• Some authors propose methods for calculating KjC from notched-only instrumented data (Schindler)

• ISO Standard concerning instrumented Charpy (ISO 26843). Requires fatigue precracked samples.

Testing

Authority

Test

temperature

Average

Charpy

Estimate KIC

(average)

Minimum

Charpy

Estimate KIC

(minimum)

(J) (MPa√m) (J) (MPa√m)

Manufacturer Ambient 47.7 82.9 47.3 82.5

Testing Auth1 -15°C 33.3 69.3 20 53.6

Testing Auth2 -15°C 15 46.5 12 41.6

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Instrumented Charpy data

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EDM notched samples at 21 C.

-5

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Lo

ad

(N

), C

um

ula

tiv

e A

bso

rbed

En

erg

y (

J)

Displacement (mm)

Force Sample A

Force Sample T

Force Sample G

Force Sample M

Force Sample 11

Energy Sample A

Energy Sample T

Energy Sample G

Energy Sample M

Energy Sample 11

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EDM Notched at -20C.

-5

0

5

10

15

20

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Lo

ad

(N

), C

um

ula

tiv

e A

bso

rbed

En

erg

y (

J)

Displacement (mm)

Force Sample 2

Force Sample 5

Force Sample 7

Force Sample L

Force Sample 11

Energy sample 2

Energy Sample 5

Energy Sample 7

Energy Sample L

Energy Sample 11

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EDM Notched samplesTest

Temperatur

e

Sample No KIN

(See Note

1)

dKIN/dt

(See Note

2)

Rpd

(See Note

3)

Jd

(See Note

4)

KJd

(See Note

5)

Type of

Force

Diagram

Validity

Criteria

(See Note

6)

(°C) (MPa√m) (MPa√m/s) (MPa) (J/m2) (MPa√m)

-20 2 103.9 793 1015.8 47,000 103.9 I Yes

-20 5 108.7 702 1063.5 52,000 108.7 I Yes

-20 7 108.1 851 1056.9 51,000 108.1 I Yes

-20 L 106.5 977 1041.4 50,000 106.5 I Yes

-20 11 107.1 1060 1047.4 50,000 107.1 I Yes

Average 106.9 876.6 1,045.0 50,000 106.9

0 1 105.7 846 1033.7 52,000 108.7 I Yes

0 2 104.7 879 1023.6 47,000 103.4 I Yes

0 3 105.5 738 1032.0 49,000 105.6 I Yes

0 7 102.1 784 996.2 49,000 105.8 I Yes

0 J 104.1 723 1018.2 52,000 104.1 I Yes

Average 104.4 794.0 1,020.7 49,800 105.5

dJd/dt

(J/m2/s)

21 T 101.0 1.58e6 742.7 2.8e5 252.1 II No

21 A 100.6 1.51e6 732.0 2.5e5 240.6 II No

21 G 100.3 1.49e6 708.2 2.5e5 236.1 II No

21 M 102.1 1.63e6 764.7 3.1e5 266.3 II No

21 11 92.2 1.43e6 720.1 2.1e5 219.0 II No

Average 99.2 1.53e6 733.5 2.6e6 242.8

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Fatigue Pre-cracked at -15C

-2

-1

0

1

2

3

4

5

6

7

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Lo

ad

(N

)

Time (s)

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Fatigue pre-cracked Instrumented Charpy

• Pros

• Relatively small amount of material

• Easy to test at different temperatures

• Standard equipment (?)

• Cons

• Must fatigue pre-crack

• Small size

• Standard is a little bit complex and is limited in scope

Temperature (°C) KJc (MPa√m) KJc (MPa√m) (1T)

-15 73.2 62.2

0 100.4 86.6

ambient 223 157.6

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Pump crankshaft

• Failure of several large cast crankshafts (7 tonne)

• Failure by fatigue at journal radii.

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Fracture surface

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Casting porosity

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Crack initiation

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Fatigue curve

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1 10 100

da

/dN

(m

m/c

yc

le)

Stress Intensity Range (MPam)

R=0.1 Lower Upper R=0.5

• At R=0.1, Kth = 7 MPam

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Stress analysis of crankshaft

• FEA Analysis showed that maximum cyclic stresses were around 230 MPa.

• Approximate stress intensity range for 8 mm deep crack

• K= 40 MPam

• What is the defect tolerance?

• For K = 7 MPam, a<0.25 mm

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Summary

• Failure analysis continues to be an important process for assuring quality and safety

• Fracture mechanics can play a part, but needs to be:

• Reasonably accurate

• Streamline and efficient

• Cheap to implement

• Fracture mechanics can be used to improve opinions on the causes of failures.