MICROMECHANISMS OF CLEAVAGE FRACTURE IN POLYCRYSTALLINE IRON

The initiation and propagation of cleavage microcracks in coarse-grained vacuum-melted ferrite, containing 0.035 and 0.007 per cent carbon, were studied by means of tensile tests carried out between room temperature and -196 C, and by special metallographic procedures. Cleavage microcracks develop in ferrite during the strain-hardening portion of the stress-strain curve at low temperatures, and are initiated mainly by cracks which form in the carbides. Twinning does not play an important role in crack initiation over the entire temperature range studied. Carbide cracks during plastic deformation at all temperatures investigated, but they lead to microcracks in the ferrite only when the applied stress is high enough to permit the carbide cracks to act as Griffith cracks. Carbide cracks also lead to the formation of large voids during the necking of specimens tested in the ductile and transition temperature regions. Pre-existing twins provide strong barriers to microcrack propagation. Twinning also causes the disappearance of the discontinuous-yield phenomenon at low temperatures. A model for microcrack initiation by carbide cracking is proposed, and the conditions leading to brittle fracture are discussed. (Author)

• Supplemental Notes:
  • Report on Metallurgical Structure.
• Corporate Authors:

  Massachusetts Institute of Technology

  Center for Transportation Studies, Room 1121
  Cambridge, MA  United States  02139
• Authors:
  • McMahon Jr, C J
• Publication Date: 1964-5

Media Info

• Pagination: 59 p.

Subject/Index Terms

• TRT Terms: Brittleness; Carbides; Cleavage fracture; Cracking; Deformation curve; Ductility; Ferrites; Fracture mechanics; Iron; Metallography; Strain hardening; Tensile properties; Tension tests
• Identifier Terms: Ship Structure Committee
• Uncontrolled Terms: Crack initiation; Metal structure
• Old TRIS Terms: Phase studies; Tensile test
• Subject Areas: Marine Transportation; Materials;

Filing Info

• Accession Number: 00327722
• Record Type: Publication
• Source Agency: National Technical Information Service
• Report/Paper Numbers: SSC-161 Prog Rpt.
• Contract Numbers: NObs88279
• Files: TRIS
• Created Date: Feb 18 1981 12:00AM