rai - nasa 2 abstract an understanding of the elevated temperature tensile creep, fatigue, rupture,...

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RAIRAI

The Materials & Manufacturing Directorate, Air Force Research Laboratory (AFRL/RXL), Wright-Patterson AFB sponsored this work under contracts F33615-01-C-5234 and F33615-03-D-2354-D004;

Approved for Public Distribution, Approval # WPAFB 07-0109, 10/17/2007

Tensile Creep and Fatigue of Sylramic-iBN Melt-Infiltrated SiC Matrix Composites: Retained Properties, Damage

Development, and Failure Mechanisms

32nd International Conference on Advanced Ceramics and CompositesJanuary 27 – February 1, 2008

Daytona Beach, FL

Robert Miller and Greg OjardPratt & Whitney, East Hartford, CT

Greg MorscherOhio Aerospace Institute, Cleveland, OH

Jalees Ahmad and Unni SanthoshResearch Applications, San Diego, CA

Yasser GowayedAuburn University, Auburn, AL

Reji JohnAir Force Research Laboratory, AFRL/RXLMN,Wright-

Patterson AFB, OH

https://ntrs.nasa.gov/search.jsp?R=20080006464 2018-06-04T11:59:08+00:00Z

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Abstract

An understanding of the elevated temperature tensile creep, fatigue, rupture, and retained properties of ceramic matrix composites (CMC) envisioned for use in gas turbine engine applications are essential for component design and life-prediction. In order to quantify the effect of stress, time, temperature, and oxidation for a state-of-the-art composite system, a wide variety of tensile creep, dwell fatigue, and cyclic fatigue experiments were performed in air at 1204oC for the SiC/SiC CMC system consisting of Sylramic-iBN SiC fibers, BN fiber interphase coating, and slurry-cast melt-infiltrated (MI) SiC-based matrix. Tests were either taken to failure or interrupted. Interrupted tests were then mechanically tested at room temperature to determine the residual properties. The retained properties of most of the composites subjected to tensile creep or fatigue were usually within 20% of the as-produced strength and 10% of the as-produced elastic modulus. It was observed that during creep, residual stresses in the composite are altered to some extent which results in an increased compressive stress in the matrix upon cooling and a subsequent increased stress required to form matrix cracks. Microscopy of polished sections and the fracture surfaces of specimens which failed during stressed-oxidation or after the room-temperature retained property test was performed on some of the specimens in order to quantify the nature and extent of damage accumulation that occurred during the test. It was discovered that the distribution of stress-dependent matrix cracking at 1204oC was similar to the as-produced composites at room temperature; however, matrix crack growth occurred over time and typically did not appear to propagate through thickness except at final failure crack. Failure of the composites was due to either oxidation-induced unbridged crack growth, which dominated the higher stress regime (>179 MPa) or controlled by degradation of the fibers, probably caused by intrinsic creep-induced flaw growth of the fibers or internal attack of the fibers via Si diffusion through the CVI SiC and/or microcracks at the lower stress regime (< 165 MPa).

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Outline:•Approach and methodology•Examination of standard specimens after air testing

–Residual properties: what’s left in the material

–Optical microscopy: extent of matrix cracking–Fracture surface microscopy: nature of failure–Life-degrading mechanisms

•Conclusions


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Damage Evolution

Approach and Methodology• Specimens were examined after creep or fatigue test if

failed, or after a post-test room temperature failure if the specimen survived creep/fatigue condition. A few specimens which had survived the creep/fatigue condition were polished without post-test failure.

• Post-test room temperature fast fracture tests were often performed with acoustic emission to assess damage evolution and history dependent σ/ε behavior

• One section of the failed specimen was cut and polished along the length (edge or face alignment) for optical microscopy in order to determine the extent and amount of matrix cracking

• One section of the failed specimen was cut in order to examine the fracture surface (for most tests, the furnace was shut off with failure of the specimen which minimized the amount of oxidation on the fracture surface post-failure resulting in a fairly pristine fracture surface crack surface) via FESEM


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Damage Evolution: Standard Specimens Examined

--YesYes758,073,478179/26HCF(1/30hz)018-p14

--YesYes151,672,011193/28HCF(1/30hz)018-p13

YesYesNo3894.2 E07165/24HCF(1/30hz)018-p12

--YesYes2178,493220/32HCF(1/30hz)018-p09

--YesYes1131,100220/32HCF(1/30hz)018-p06

--YesYes725,217220/32HCF(1hz)017-p09

----Yes1154,546220/32HCF(1/30hz)017-p07

--YesYes

518,457220/32HCF(1hz)-43ksi

overload017-p05

--YesYes

16--220/32Creep after RT HCF (1/30hz)016-p05

YesYesNo168134220/32DF007-p07

Yes--No100--165/24creep007-p04

Yes--No100--110/16creep007-p03

--YesYes1953--110/16creep006-p10

Yes--No250125193/28DF006-p08

Yes--No105193/28DF006-p07

----Yes1508--165/24creep006-p06

--YesYes478--165/24creep006-p05

YesYesNo2036--110/16Creep006-p02

YesYesNo1269--165/16Creep006-p01

----No105193/28DF005p06

----No105165/24DF002-p03

RT Ret TestFESEM

Failure at TemperatureTime, hrCycles

Stress, MPa/ksiTest

Specimen(1300-01-)


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Damage Evolution: Standard Specimens

Specimens subjected to a wide range of conditions were examined (arrows)

0

50

100

150

200

250

300

0.1 1 10 100 1000 10000Exposure Time (hrs)

Max

App

lied

Stre

ss (M

Pa)

Dwell Fatigue 1204ºC (did not fail)Creep 1204ºC (did not fail)Creep 1204ºC Failure30 Hz (failed)30 Hz (did not fail)1 Hz (failed)0.33 Hz (failed) - Ref 9


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Damage Evolution: RT Retained Properties

With creep/fatigue comes increases in residual compressive stress and increased stress to cause matrix cracking• For the case of 192 and 220 MPa, matrix cracks formed during

fatigue; however, further matrix cracking required much higher stresses than creep or fatigue condition due to matrix relaxation

0

50

100

150

200

250

300

350

400

450

500

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Strain, %

Stre

ss, M

Pa

residual compressive

stress

As-ProducedE = 259 GPa

110 MPa, 100h DFE = 250 GPa

165 MPa, 100h DFE = 230 GPa

192 MPa, 250h DFE = 230 GPa

220 MPa, 168h DFE = 221 GPa

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500Stress, MPa

Nor

m C

um A

EAs-Produced

110 MPa, 100h DF

165 MPa, 100h DF

192 MPa, 250h DF

220 MPa, 168h DF

AE onset stresses


Strain offset for clarity

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Effect of accumulated strain and residual stress on matrix cracking stress• Matrix cracking stress increases nearly proportionately with

increases in residual compressive stress

100120140160180200220240260280300

0 0.05 0.1 0.15 0.2Time-dependent LCF or Creep Strain, %

AE

Ons

et S

tres

s, M

Pa

193 MPa (28ksi)165 MPa (24ksi)110 MPa (16ksi)As-Produced

creep specimens

150

170

190

210

230

250

270

0 50 100 150 200Residual Compressive Stress, MPa

AE

Ons

et S

tres

s, M

Pa

As-produced110 MPa (16ksi)165 MPa (24ksi)192 MPa (28ksi)220 MPa (32ksi)

30 Hz

Creep


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Residual strength with time of test… note significant degradation did not occur until longest time/low stress conditions

0

100

200

300

400

500

600

1 10 100 1000 10000Time of test, hr

RT

Res

idua

l Str

engt

h, M

Pa

220 MPa (32ksi)193 MPa (28ksi)165 MPa (24ksi)110 MPa (16ksi)As-Produced

DF, HCF, or Creep Stress:


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Damage Evolution: Optical Microscopy

Surface 90 minicomposite (192 MPa, 10 hours, did not fail in

rupture)

Back-to-back 90 minicomposite cracks which extended to the

surface through a 0 minicomposite (165 MPa; 1508

hr creep rupture)


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Matrix cracks that extend at least two plies from the surface and in some cases have fractured fibers in the matrix crack wake. This

specimen had undergone 220 MPa 30 Hz fatigue and lasted approximately 1.2 hours at 1200C.


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With increasing stress and/or time, fiber-bridged (traverses a 0o minicomposite) matrix crack density increases

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 10 100 1000 10000Time, hr

Cra

ck d

ensi

ty, m

m-1

220 MPa (32 ksi) HCF220 MPa (32 ksi) DF193 MPa (28 ksi) HCF193 MPa (28 ksi) DF126 MPa (24 ksi) Creep

most cracks go through two plies or more, ~ 1/3 of the cracks are unbridged

most cracks go through one to two plies, a few unbridged

mostly surface cracks, a few through one ply 165 MPa (24 ksi) Creep

193 MPa (28 ksi) Dwell Fatigue

two plies, some unbridged


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Matrix crack density at 1200oC has similar dependence on stress as RT; however, cracks are not through-the-cross-section

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500

Stress, MPa

Cra

ck D

ensi

ty, m

m-1

RT Crack DensityBased on All AE

Events RT Crack DensityBased on only High AE

Energy Events

Tunnel/microcrack formation 30 HZ HCF

DF

1 HZ HCFCreep

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300

Stress, MPa

Cra

ck D

ensi

ty, m

m-1

RT Crack DensityBased on All AE

Events RT Crack Density

Based on only High AE

Energy Events

Tunnel-microcrack formation

30 HZ HCF

DF

1 HZ HCF

Creep


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Damage Evolution: Fracture Surface High Stress

30 hz fatigue, 179 MPa for 8.1x106 cycles (~ 75 hrs) – unbridged crack growth from an edge


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Damage Evolution: Fracture Surface High Stress

30 hz fatigue, 220 MPa for 178,493 cycles (~ 1.6 hrs) –unbridged crack growth from a face


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Damage Evolution: Fracture Surface Low Stress1300-01-006-p02, 110 MPa creep for 2036 hours followed by room

temperature residual strength test – No evidence of oxidation-assisted fiber failure

• Two specimens that did fail at 110 & 165 MPa showed a small triangular region of oxidized, unbridged crack emanating from one corner of the cross-section


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Life Degrading Mechanisms (> 179 MPa)

Effective notch-size effect considering the unbridgedcrack as a notch

0.500

0.550

0.600

0.650

0.700

0.750

0.800

0.850

0.900

0.950

1.000

0 1 2 3 4

Notch size, mm

Nor

mal

ized

Net

Sec

tion

Stre

ss RT Double-edged Notch [13-17]

RT Surface Notch [8]

DF Single Surface Crack(RT Residual Strength)

30 Hz F Single Edge Crack(2200oF)

30 Hz F Single Surface Crack (2200oF)

1Hz HCF Single Edge Crack (2200oF)


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Life Degrading Mechanisms (< 165 MPa)

For low stress, long-time conditions, strength-degradation not apparently due to oxidation

• Intrinsic creep-degradation of fibers

• Si attack of fibers from free Si

165 MPa creep specimen that failed after 1500 hours


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Conclusions• Creep at 1204oC of specimens resulted in matrix

relaxation and higher proportional limits• Matrix crack density at 1204oC similar to RT Good

basis for starting point of life-model– However, matrix cracks were not through-the-cross-section

• Degradation Mechanism: > 179 MPa (~ M.C. Stress), stressed-oxidation induced unbridged crack growth led to failure

• Degradation Mechanism < 165 MPa, reduction in retained strength at RT not usually due to oxidation fiber-degradation due to intrinsic creep-degradation or Si attack

– Note, two specimens that did fail showed unbridged crack at a corner of the cross-section


rai - nasa 2 abstract an understanding of the elevated temperature tensile creep, fatigue, rupture,...

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