Accession Number : ADA119099

Title :   Micromechanisms of Crack Extension in Alloys.

Descriptive Note : Doctoral thesis,


Personal Author(s) : Blind,John A

PDF Url : ADA119099

Report Date : Jan 1982

Pagination or Media Count : 293

Abstract : This work describes an investigation of the effect of manganese-bearing dispersoid particles on the fracture characteristics of peak aged Al-Mg-Si alloys. Current elastic and elastic-plastic fracture toughness parameters are reviewed. The applicability of these parameters as criteria for the onset of crack extension and their variation with alloy microstructure are examined based on experimental results for a series of alloys containing differing volume fractions of disperoid. High purity alloys were used to isolate the effect of dispersoids from that of coarse constituent particles. Also, for comparison, one alloy of commercial purity containing both disperoids and coarse iron-bearing inclusions was studied. The ductile fracture toughness, JIc, and the resistance to instability as measured by the tearing modulus, T, were found to increase with disperoid content. The values of these parameters were lower in the commercial purity alloy than for the equivalent high purity alloy. The effect of an increasingly triaxial state on fracture strain was measured using a series of notched tensile specimens. The alloy ductility was found to increase with disperoid content and to decrease with increased stress triaxiality. Crack tip plastic zone sizes on the midplane of compact tension specimens were measured using the SEM electron channelling pattern technique. Zone size was found to correlate with the square of the applied stress intensity factor. The constant of proportionality decreased with increasing disperoid content. The calculated work per unit area of new crack surface was found to correspond to the ductile fracture toughness measured for each alloy.

Descriptors :   *Alloys, *Crack propagation, *Fracture(Mechanics), Microstructure, Mechanical properties, Ductility, Stress concentration, Strain(Mechanics), Elastic properties, Plastic deformation, Notch toughness, Tensile properties, J integrals, Purity, High rate, Aluminum alloys, Manganese alloys, Silicon alloys, Particles, Stability, Equations, Theses

Subject Categories : Properties of Metals and Alloys

Distribution Statement : APPROVED FOR PUBLIC RELEASE