PSI - Issue 13

Taketo Kaida et al. / Procedia Structural Integrity 13 (2018) 1076–1081 Taketo Kaida et al. / Structural Integrity Procedia 00 (2018) 000–000

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2

1. Introduction Fractography has been a key technique for identifying the cause of fracture in post-mortem specimens/structural parts e.g., aircraft parts, engine, and axle. In terms of metal fatigue, striation spacing on the fracture surface has been utilized, because it quantitatively corresponds to the fatigue crack propagation rate (Forsyth and Ryder, (1960)). This fact helps to estimate the fatigue crack propagation rate and the associated loading history, which is crucial for preventing future accidents through structural design. However, the striations are not always observed. Therefore, the capacity of the current fractography methods for quantitative analysis is limited to a high stress intensity factor in specific microstructures and crystallographic orientations. Towards generic estimation of fatigue crack propagation rate through fractography, we focus on the microstructural traces of fatigue crack propagation underneath the fracture surface. In other words, characterization of microstructure underneath the fracture surface with different crack lengths, ∆ K , is expected to provide information useful for the analysis of the fatigue crack propagation process. In this study, we used two microstructural characterization techniques: electron backscatter diffraction (EBSD) analysis and electron channeling contrast imaging (ECCI). Scanning electron microscopy (SEM)-based EBSD measurements were carried out to analyze the evolution of the plastic zone underneath the fracture surface from the viewpoint of the crystallographic orientation gradient (Nishikawa et al., (2011); Onishi et al., (2016); Koyama et al., (2017)). ECCI is also a SEM-based technique, which can even resolve a single dislocation (Zaefferer and Elhami, (2014)). ECCI also enables crack/damage-specific microstructural characterization in bulk specimens (Koyama et al., (2013); Kaneko et al., (2016); Habib et al., (2018)). Therefore, we can easily show the dislocation patterns at a specific mechanical condition (∆ K ) in a post-mortem bulk specimen. As a first challenge in the advanced fractography, this paper demonstrates the fracture surface of a single crystalline bcc Fe-3Al alloy, coupled with the corresponding EBSD and A Fe-3Al single crystal sample was used for the fractographic analyses. The alloy was fully ferritic. Figure 1(a) shows the specimen geometry. The specimen was fatigue-tested until fracture at a stress ratio of R = 0, a frequency of 50 Hz, and a stress amplitude of 75.1 MPa. Figure 1(b) shows the overview of the fatigue-fractured specimen. For the crystallographic analyses, the tensile axis was set parallel to the [110] direction. A notch was introduced perpendicularly to the tensile axis, with its tip oriented along the [-110] direction, using electric discharge machining. The fatigue life was 1.1 × 10 6 cycles. The fracture surface was then studied under SEM. The SEM used for the fractographic analysis was JSM-IT300 manufactured by JEOL Ltd. Afterwards, half of the specimen thickness was removed from the side by mechanical grinding and polishing. In order to minimize the mechanical damage, the specimen surface was ground using emery paper, diamond pastes of 9 and 3 μm, and colloidal silica with a particle size of 60 nm. The polished surface underneath the fracture surface was investigated along the crack propagation direction using EBSD and ECCI. The SEM machine used for the EBSD measurements was SU6600, manufactured by Hitachi Ltd. The EBSD measurements were carried out at the accelerating voltage of 20 kV and the probe current of 47 μA, with the beam step size being 0.5 μm. The SEM used for the ECCI observations was a MERLIN machine, manufactured by Carl Zeiss Microscopy GmbH. The ECCI observations were conducted at the accelerating voltage of 30 kV and the probe current of 10 nA. We calculated ∆ K at each observation region, using the following equation (Murakami, (1987)). ECCI microstructural analysis. 2. Experimental procedure

a W

(1)

) ( a F     ,

 

K

2

3

4

(2)

( F 

1.12 0.231 10.55 21.72 )      







3

0.39

where, σ is loading stress, a is fatigue crack length, and W is the plate width.