Numerical models and experimental simulation of irradiation hardening and damage effects on the fracture toughness of 316L stainless steel

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Abstract

In nuclear environments, irradiation hardening and damage have a detrimental effect on materials performance. Among others, fracture toughness of austenitic stainless steels decreases under neutron irradiation. Helium arising from transmutation reactions is one source of embrittlement leading to that decrement and it is here assumed as a case study, austenitic steel 316L being the material under investigation. The experimental reproduction of irradiation hardening effect on yield stress is attempted here by pre-strain under tensile loading at room temperature. The experimental production of porosity is attempted by inducing ductile damage, creep damage or a combination of them. Damage at the microstructural level is analyzed by metallography, fractography, X-ray tomography and quantified by image processing.After calibrating the elastic, the plastic and the porous plastic constitutive equations by the means of tensile tests on smooth and notched specimens, results from damaging experiments are validated by finite element analysis using the Gurson-Tvergaard-Needleman model. The numerical models obtained represent different levels of damage into the material, as induced by the experiments.Material presenting different levels of damage is then machined for fracture toughness evaluation in the shape of sharp-notched round bars. Fracture toughness initiation is inferred from the load vs. displacement plots applying an opportune fracture criterion. In order to test the suitability of the Gurson-Tvergaard-Needleman model, the load vs. displacement results are validated by retrofitting opportune constitutive laws for each “damaged” state. Retrofitting is discussed in relation to the type of damage produced.Results show that the reproduction of the macroscopic effect of irradiation hardening on yield stress may be attempted for 316L by a pre-strain tensile loading at room temperature for levels up to 1.5 dpa or slightly more. These interrupted tensile tests did not give evidence of void volume fraction production. Creep tests at 650 °C showed sensitization at the grain boundaries but not porosity into the matrix. Creep tests at 1000 °C created 1.2% to 1.8% void volume fraction from grain boundary sliding. Finally, one 7% pre-strained specimen was subjected to creep test at 900 °C and stopped at 5% creep strain, without evidence of porosity into the matrix.Fracture toughness tests on the “damaged” states obtained before showed a decrement of fracture toughness initiation when compared with “undamaged” 316L. Specimens with 30% and 40% eng. strain presented a sensible decrement and exhibited a brittle-like behaviour. The differences in porosity size and physical processes involved suggest not stating that a correlation exists with the helium embrittlement effect on the same property. The Gurson-Tvergaard-Needleman model worked for the “undamaged” material. It proved to be not suited for the brittle-like 30% and 40% eng. strain “damaged” materials because it did not capture the experimental progression of damage.In the end, fracture toughness numerical predictions were made using different values of initial void volume fraction. It was argued that, starting from a threshold value, the brittle-like 30% and 40% eng. strain “damaged” materials revert to a ductile behaviour.

Keyword(s)

fracture toughness; stainless steel

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