Abstract
Fracture mechanics has evolved during the last 50 years and become a powerful, everyday tool for analysis of structural integrity of safety critical components. Despite the progress, the developed experimental characterization methods for fracture toughness are mainly applicable for macroscopically homogeneous materials. Therefore, the focus of this thesis is on investigating fracture toughness characterization for dissimilar metal welds (DMW), particularly the interface region between hard and soft materials. The objective is to investigate the effect of crack path and strength mismatch on fracture toughness of an Alloy 52 DMW. The thesis is based on three articles published in established journals in the field of fracture mechanics. The author of the thesis is the main contributor for these articles.
The strength mismatch affects the plastic η-factor used for determining the fracture toughness. The results show that the plastic η-factors determined at an interface between a hard and a soft material are equal to the solutions developed for homogeneous specimens. Variations in η-factor occur when distance between the crack and the soft/hard interface increases. The biggest change in η occurs as the outer part of the equivalent plastic strain zone ahead of the crack extends to the soft/hard interface. Secondly, for cracks that deviate towards the same weak region, the farther the crack is from the weakest region, the larger is the fracture toughness. This applies both in the brittle and ductile region. Noticeably, the cracks in strength mismatch zone continue the growth along a weak region, even if there is a softer zone adjacent to the weak zone. Thirdly, a model for predicting the effect of crack location on fracture toughness was derived and validated. The model is applicable as long as the initiation of brittle fracture occurs in a specific zone, the weak zone.
The observations made in this thesis contribute to more cost-efficient fracture toughness characterization of DMWs, and the results show that the current standardized fracture mechanical characterization methods can be applied for DMWs. The results can also be utilized for developing better characterization tools for welds. Future work should focus on investigating various DMWs to understand the fracture behavior under different conditions
The strength mismatch affects the plastic η-factor used for determining the fracture toughness. The results show that the plastic η-factors determined at an interface between a hard and a soft material are equal to the solutions developed for homogeneous specimens. Variations in η-factor occur when distance between the crack and the soft/hard interface increases. The biggest change in η occurs as the outer part of the equivalent plastic strain zone ahead of the crack extends to the soft/hard interface. Secondly, for cracks that deviate towards the same weak region, the farther the crack is from the weakest region, the larger is the fracture toughness. This applies both in the brittle and ductile region. Noticeably, the cracks in strength mismatch zone continue the growth along a weak region, even if there is a softer zone adjacent to the weak zone. Thirdly, a model for predicting the effect of crack location on fracture toughness was derived and validated. The model is applicable as long as the initiation of brittle fracture occurs in a specific zone, the weak zone.
The observations made in this thesis contribute to more cost-efficient fracture toughness characterization of DMWs, and the results show that the current standardized fracture mechanical characterization methods can be applied for DMWs. The results can also be utilized for developing better characterization tools for welds. Future work should focus on investigating various DMWs to understand the fracture behavior under different conditions
| Original language | English |
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| Qualification | Doctor Degree |
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| Award date | 25 Sept 2020 |
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| Print ISBNs | 978-952-64-0000-6 |
| Electronic ISBNs | 978-952-64-0001-3 |
| Publication status | Published - 1 Sept 2020 |
| MoE publication type | G5 Doctoral dissertation (article) |