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The aim of this study is to experimentally investigate the fatigue behavior of additively manufactured (AM) NiTi (i.e. Nitinol) specimens and compare the results to the wrought material. Additive manufacturing is a technique in which components are fabricated in a layer-by-layer additive process using a sliced CAD model based on the desired geometry. NiTi rods were fabricated in this study using Laser Engineered Net Shaping (LENS), a Direct Laser Deposition (DLD) AM technique. Due to the high plateau stress of the as-fabricated NiTi, all the AM specimens were heat-treated to reduce their plateau stress, close to the one for the wrought material. Two different heat treatment processes, resulting in different stress plateaus, were employed to be able to compare the results in stress- and strain-based fatigue analysis. Straincontrolled constant amplitude pulsating fatigue experiments were conducted on heat-treated AM NiTi specimens at room temperature (~24°C) to investigate their cyclic deformation and fatigue behavior. Fatigue lives of AM NiTi specimens were observed to be shorter than wrought material specifically in the high cycle fatigue regime. Fractography of the fracture surface of fatigue specimens using Scanning Electron Microscopy (SEM) revealed the presence of microstructural defects such as voids, resulting from entrapped gas or lack of fusion and serving as crack initiation sites, to be the main reason for the shorter fatigue lives of AM NiTi specimens. However, the maximum stress level found to be the most influential factor in the fatigue behavior of superelastic NiTi.
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Fatigue Criticality Assessment of Volumetric Defects in Notched Specimens: A Non‐Destructive Approach
ABSTRACT This study utilizes linear elastic fracture mechanics to assess the fatigue criticality of volumetric defects in notched specimens with varying geometries. Contrasting to the existing literature, this study assesses the fatigue criticality of defects, prior to fracture, via a non‐destructive inspection technique, that is, X‐ray computed tomography (XCT). Treating volumetric defects as cracks, based on Murakami's definition, the approach calculates their Mode‐I stress intensity factor (SIF) with their local stresses obtained via linear elastic finite element analysis and utilizes the SIF to represent their criticality. For validation, cylindrical and flat specimens with notch root radii of 5 and 50 mm of AlSi10Mg and 17‐4 precipitation hardened stainless steel were fabricated, XCT scanned, and tested under fatigue loading. All crack initiating defects, observed from fractography, fell within the 99.3 percentile of the defects with the highest stress intensity factor in the respective specimens.
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- Award ID(s):
- 2319690
- PAR ID:
- 10600689
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Fatigue & Fracture of Engineering Materials & Structures
- ISSN:
- 8756-758X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1111/ffe.14698
