Digital image correlation measurement of near-tip fatigue crack displacement fields

constant amplitude loading and load history effects

D Nowell, M E Kartal, P F P De Matos

Research output: Contribution to journalArticle

24 Citations (Scopus)

Abstract

Recent work by de Matos and colleagues employed digital image correlation to measure near tip displacement fields for fatigue cracks in 6082 T6 aluminium alloy. The main focus of this work was to directly measure fatigue crack closure, but the measurements can also be used to examine conditions at and ahead of the crack tip. In this paper, the results are re-analysed and compared to two crack-tip deformation models. The first assumes simple elastic deformation (according the Westergaard solution). This allows the history of crack-tip stress intensity to be examined. Reasonable agreement with the elastic model is obtained, although there is a residual stress intensity caused by the plastic wake, which gives rise to crack closure. The second model examined is a simple elastic–plastic assumption, proposed by Pommier and colleagues. This can be applied to constant amplitude loading, although the results obtained here are very similar to the elastic case. A slightly more complex load case (a single overload in an otherwise constant amplitude variation of load) gives a much more complicated crack-tip history. Here, the importance of crack-tip plastic displacement, represented by the second term in Pommier's model becomes much clearer. Load history effects are captured by the residual value of this term and its associated displacement fields as well as by stress intensity factor. The implications for further modelling and experimental work are discussed.
Original languageEnglish
Pages (from-to)3-13
Number of pages11
JournalFatigue & Fracture of Engineering Materials & Structures
Volume36
Issue number1
Early online date22 Jun 2012
DOIs
Publication statusPublished - Jan 2013

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Crack tips
Crack closure
Plastics
Elastic deformation
Stress intensity factors
Aluminum alloys
Residual stresses
Fatigue cracks

Keywords

  • crack-tip fields
  • digital image correlation
  • displacement measurement
  • elastic-plastic fatigue crack

Cite this

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title = "Digital image correlation measurement of near-tip fatigue crack displacement fields: constant amplitude loading and load history effects",
abstract = "Recent work by de Matos and colleagues employed digital image correlation to measure near tip displacement fields for fatigue cracks in 6082 T6 aluminium alloy. The main focus of this work was to directly measure fatigue crack closure, but the measurements can also be used to examine conditions at and ahead of the crack tip. In this paper, the results are re-analysed and compared to two crack-tip deformation models. The first assumes simple elastic deformation (according the Westergaard solution). This allows the history of crack-tip stress intensity to be examined. Reasonable agreement with the elastic model is obtained, although there is a residual stress intensity caused by the plastic wake, which gives rise to crack closure. The second model examined is a simple elastic–plastic assumption, proposed by Pommier and colleagues. This can be applied to constant amplitude loading, although the results obtained here are very similar to the elastic case. A slightly more complex load case (a single overload in an otherwise constant amplitude variation of load) gives a much more complicated crack-tip history. Here, the importance of crack-tip plastic displacement, represented by the second term in Pommier's model becomes much clearer. Load history effects are captured by the residual value of this term and its associated displacement fields as well as by stress intensity factor. The implications for further modelling and experimental work are discussed.",
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AB - Recent work by de Matos and colleagues employed digital image correlation to measure near tip displacement fields for fatigue cracks in 6082 T6 aluminium alloy. The main focus of this work was to directly measure fatigue crack closure, but the measurements can also be used to examine conditions at and ahead of the crack tip. In this paper, the results are re-analysed and compared to two crack-tip deformation models. The first assumes simple elastic deformation (according the Westergaard solution). This allows the history of crack-tip stress intensity to be examined. Reasonable agreement with the elastic model is obtained, although there is a residual stress intensity caused by the plastic wake, which gives rise to crack closure. The second model examined is a simple elastic–plastic assumption, proposed by Pommier and colleagues. This can be applied to constant amplitude loading, although the results obtained here are very similar to the elastic case. A slightly more complex load case (a single overload in an otherwise constant amplitude variation of load) gives a much more complicated crack-tip history. Here, the importance of crack-tip plastic displacement, represented by the second term in Pommier's model becomes much clearer. Load history effects are captured by the residual value of this term and its associated displacement fields as well as by stress intensity factor. The implications for further modelling and experimental work are discussed.

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