Gruppe Physikalische Metallkunde Abt. Füge- und ... · Chemical analysis of materials . ......

$: Gruppe Physikalische Metallkunde Abt. Füge- und ... · Chemical analysis of materials . ... Premature fracture of a shot peened connecting . rod during fatigue testing . ... Cracked$
Materials Sci ence & Technolog y

Aircraft Failures M. Roth

EMPA Duebendorf

1. Chemical analysis of materials 2. Nondestructive testing methods 3. Mechanical testing (1 example) 4. Fractography (3 examples) 5. Metallography: Microstructural investigations (3 examples) 6. Micro- and surface analysis (2 examples) 7. Conclusions

Lecture «Advanced Materials and Structures» Institute of Metals Research, Shenyang, October 21-23, 2013


Aircraft Failures

Materials investigated:

1. Fe-base alloys 2. Ni-base superalloys 3. Al-base alloys 4. Mg-base alloy 5. Coating systems: NiP-coating, Cadmium-coating


Aircraft Failures

Components investigated:

1. From aircraft-structures 2. Gas turbines (aircraft engine) Comment: Most of the case studies are based on failures in aircrafts: Orders

from the Swiss Aircraft Accident Investigation Bureau, Berne (Switzerland). The results are for public use!


1. Fractography 2. Metallography 3. Micro- and surface analysis Collect relevant materials data: a) Chemical investigations - composition of the materials (+XRD, RFA) - analysis of oil, fuel, etc. - H-analysis b) Mechanical testing - mechanical data: yield strength, UTS, elongation to fracture, notch impact test - fatigue properties (HCF, LCF) + corrosion - stress corrosion cracking - creep - crack growth measurements - residual stresses Apply nondestructive testing methods: - ultrasonics, eddy current testing,

radiography, visual inspection, magnetic powder inspection Example: Premature fracture of a shot peened connecting rod during fatigue testing

Principles of Failure Analysis


Premature fracture of a shot-peened connecting rod during fatigue testing

Shot-peened connecting rod



Shot-peened connecting rod Residual stress

measurement



Shot-peening of turbine-blades



Shot-peened connecting rod Residual stress distribution Compressive residual stresses Increase of fatigue life

Why premature fracture? Metallography


Fractography

Methods: - visual, pocket lens, stereomicroscope - scanning electron microscope (SEM) Macro- und microscopical investigations: - fracture characteristics, fracture path - fracture surface marks - site of fracture initiation - surface condition at fracture initiation site Example 1: Fracture of the main landing gear of a passenger jet Example 2: Fracture of a pin in a military jet → microanalysis Example 3: Fracture of a turbine blade in a jet engine → microstructural investigations


Scanning Electron Microscope


Fractography



Fracture surface marks

Fractured main landing gear: Chevron pattern



Plain stress state

Plain strain state



Crack propagation


Fractography



Site of fracture initiation

Wheel of main landing gear of a military aircraft Material: Mg-base alloy

Beach marks indicate crack initiation site


Example 1: Fracture of the main landing gear of a passenger jet

- Aircraft: Fokker 100, KLM, landing at Geneva airport - Fracture of the main landing gear during landing - No injuries of passengers, aircraft heavily damaged


- Aircraft: Fokker 100, KLM, landing at Geneva airport - Fracture of the main landing gear during landing - No injuries of passengers, aircraft heavily damaged




Housing of main landing gear Material: wrought Al-alloy

Crack initiation site

Arrows indicate crack propagation of overload fracture



Designation of fractured pieces Main crack initiation site



Conclusions Overload fracture: worst case! Material ok: mechanical data, microstructure, chemical analysis Add. investigations:

Misfunction of the aircrafts ABS-breaking system caused very high stresses in the main landing gear


Example 2: Fracture of a pin in a military jet

Fractured pin



Fastening ring (attached later)



Arrows indicate crack initiation site: multiple cracking

Beach marks

Overload fracture



Pin from another jet: no „bending“ of the fatigue crack



SEM-investigation:

Arrows indicate striations



SEM-investigation:

Mixed structure of striations and micro-dimples



SEM-investigation:

Microductile overload fracture with shear dimples



Increased crack growth at the circumference of the pin



Crack along grinding tracks

Add. investigations: microanalysis of pin coating



Why „bending“ of the fatigue crack?

60o


Example 3: Fracture of a turbine blade in a jet engine

Secondary damage



SEM-investigation: Intergranular fracture



SEM-investigation: Inter- and transgranular fracture: Creep and (locally) fatigue

pores

beach marks

Add. investigations: microstructure


Microstructural investigations

- Analysis of material ↔ Microstructure !

- Microstructural defects • due to manufacture • due to operation conditions

- Microstructure: primary + secondary damage

- Metallographical investigation: • microsection (destructive) • orientation of the microsection plane Example 1: Fracture of a turbine blade in a jet engine Example 2: Verification of overheating in a turboprop-engine Example 3: Premature fracture of a shot peened connecting rod during fatigue testing Example 4: Cracked lugs from a generator



Microsection

Intergranular cracks



Optical microscopy: Pores on grain boundaries: creep damage



Optical microscopy: Cracks at surface are oxidised: Check by NDT is difficult to observe



Conclusions: Heavily creep damaged material Neighbouring blades show no creep damage Last overhauling of turbine: the blade was most probably not sorted out!


Example 2: Verification of overheating in a turbo-prop engine

1st stage turbine blades Nickel base superalloy IN 738 service temperature: ca. 760 oC

Short or long term overheating? Maximum temperature?



Blade root: Gamma prime distribution



Blade middle part: Gamma prime distribution Coarsening of Gamma prime



Test piece from blade root: Gamma prime distribution Heat treatment: 950 Grad C, 15 minutes



Test piece from blade root: Gamma prime distribution Heat treatment: 950 Grad C, 36 hours



Test piece from blade root: Gamma prime distribution Heat treatment: 950 Grad C, 15 minutes

No short time overheating, Long term overheating ! Turbine was pre-damaged

Conclusion:


Example 3: Shot peened connecting rod mechanical testing

Failure during fatigue test

Residual stress distribution ok!


Example 3: Shot peened connecting rod

SEM of shot-peened surface Numerous cracks



OM: microsection Numerous cracks starting from shot-peened surface

Fracture surface

Shot-peened surface



OM: microsection Microcracks due to extensive plastic deformation after shot-peening

Conclusion: Too „strong“ shot-peening“


Micro- and Surface Analysis

Methods: - Scanning Electron Microscope with EDX - Elektronprobe Analysis with WDX - Surface Analysis: Auger, ESCA, SIMS Example 1: Fracture of a pin in a jet engine Example 2: Crack in the pin of the engine suspension of a passenger jet



- Scanning Electron Microscope with EDX - Elektronprobe Analysis with WDX - Surface Analysis: Auger, ESCA, SIMS



Scanning Electron Microscope with EDX Elektronprobe Analysis with WDX Surface Analysis: Auger, ESCA, SIMS

lateral resolution

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Microprobe Analysis with WDX (EPMA)


Example 1: Fracture of a pin in a jet engine

NiP-coating against corrosion

Multiple cracking


Fracture of a pin in a jet engine

Microprobe analysis: element mapping and line-scans


Fracture of a pin in a jet engine

NiP-coating: 7 – 9% P

Tensile residual stresses


Example 2: Crack in the pin of the engine suspension of a passenger jet

4.10.1992-Amsterdam: El Al 747 Freighter crashed in appartment block

Official reports: corrosion fatigue of the pins



Preparation of the pin in UHV

Pin is coated with Cadmium against corrosion Cadmium embrittlement of the steel pins?



SEM-image in Auger apparatus

Original fracture surface

Fracture made in UHV (Auger-app.)



Auger map of element Cadmium

Cadmium Embrittlement !



Conclusions: Auger analysis: Cadmium in the bulk material Pin was pre-damaged by Cadmium embrittlement Cadmium diffuses from the coating into the steel


Aircraft Failures

Each investigation method has advantages, but also limits!

- Fractography - Microstructural investigations - Micro- and surface analysis - Mechanical testing → Optimal coordination of the different methods → Clear conclusions in terms of the cause of the failure Experts!

Conclusions:

Gruppe Physikalische Metallkunde Abt. Füge- und ... · Chemical analysis of materials . ......

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