The instrumented Charpy impact test, the dynamic fracture toughness test and the static fracture toughness test using the DC electrical potential method were carried out on Ti6Al4V alloys. Ti6Al4V alloys with various microstructures were used to investigate the relation between microstructural factors, including fractographic measurements, and various toughness criteria, i.e. Charpy absorbed energies (Et, total absorbed energy; Ei, apparent crack initiation energy; Ep, apparent crack propagation energy); dynamic fracture toughness Jd; static fracture toughness (JIc (EPM), by DC electrical potential method or KQ); static fracture propagation resistance curve (JR curve); and static tearing modulus Tmat. The toughness of the Ti6Al4V alloy as evaluated by the Charpy test was different from that given by the dynamic toughness test mainly because the microstructural unit which controlled the fracture was different in the different tests. The toughness evaluated by means of the JR curve or Tmat does not always coincide with that given by JIc (EPM). Therefore it is necessary to use an appropriate toughness criterion that corresponds to the method of design. It was found that the microstructure had a greater influence on Tmat than on JIc (EPM). In addition, the relationships between microstructural factors (i.e. the areal fraction of the primary α phase, the mean free path in the primary α, the depth of dimples, the prior β grain size) and the various toughness criteria mentioned above must be clearly defined.
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