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1. Ultra Low-Cycle Fatigue Behavior Comparison between Additively Manufactured and Rolled 17-4 PH (AISI 630) Stainless Steels

   https://doi.org/10.3390/met11111726  

   Gonzalez-Nino, David; Strasser, Timothy; Prinz, Gary S. (November 2021, Metals)

   This study investigates the mechanical behavior of additively manufactured (AM) 17-4 PH (AISI 630) stainless steels and compares their behavior to traditionally produced wrought counterparts. The goal of this study is to understand the key parameters influencing AM 17-4 PH steel fatigue life under ULCF conditions and to develop simple predictive models for fatigue-life estimation in AM 17-4 steel components. In this study, both AM and traditionally produced (wrought) material samples are fatigue tested under fully reversed (R = −1) strain controlled (2–4% strain) loading and characterized using micro-hardness, x-ray diffraction, and fractography methods. Results indicate decreased fatigue life for AM specimens as compared to wrought 17-4 PH specimens due to fabrication porosity and un-melted particle defect regions which provide a mechanism for internal fracture initiation. Heat treatment processes performed in this work, to both the AM and wrought specimens, had no observable effect on ULCF behavior. Result comparisons with an existing fatigue prediction model (the Coffin–Manson universal slopes equation) demonstrated consistent over-prediction of fatigue life at applied strain amplitudes greater than 3%, likely due to inherent AM fabrication defects. An alternative empirical ULCF capacity equation is proposed herein to aid future fatigue estimations in AM 17-4 PH stainless steel components. 

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2. A data-driven geometry-specific surrogate model for forecasting the load–displacement behavior until ductile fracture

   https://doi.org/10.1007/s10704-025-00839-1  

   Dey, Surajit; Kiran, Ravi (May 2025, International Journal of Fracture)

   Krishnaswamy, RaviChandar (Ed.)

   The present study aims to configure and train a data-driven geometry-specific surrogate model (DD GSM) to simulate the load–displacement behavior until fracture in cylindrical notched specimens subjected to uniaxial monotonic tension tests. Plastic strain hardening that governs the load–displacement behavior and ductile fracture in metals are history-dependent phenomena. With this, the load–displacement response until ductile fracture in metals is hypothesized as time sequence data. To test our hypothesis, a long short-term memory (LSTM) based deep neural network was configured and trained. LSTM is a type of neural network that takes sequential data as input and forecasts the future based on the learned past sequential trend. In this study, the trained LSTM network is referred to as DD GSM as it is used to forecast the load–displacement behavior until ductile fracture for the cylindrical notched specimens. The DD GSM is trained using the load–displacement data until fracture, extracted from the finite element analyses of notched cylindrical test specimens made of ASTM A992 steel. The damage leading to fracture was captured using the Gurson–Tvergaard–Needleman (GTN) model. Finally, the trained DD GSM is validated by predicting the overall load–displacement behavior, fracture displacement, and peak load-carrying capacity of cylindrical notched ASTM A992 structural steel specimens available in the literature that are not used for training purposes. The DD GSM was able to forecast some portions of the load–displacement curve and predict the fracture displacement and peak load-carrying capacity of the notched specimens. Furthermore, the geometric sensitivity of the trained DD GSM was demonstrated by simulating the load–displacement response of an ASTM A992 steel bar with a central hole. 

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3. Hydrogen-Assisted Cracking Fracture Analysis using High-Speed Camera and Delayed Hydrogen Cracking Test

   https://doi.org/10.1007/s11668-021-01308-2  

   Bourgeois, D.; Alexandrov, B. (February 2022, Journal of Failure Analysis and Prevention)

   Dissimilar metal welds (DMWs) are commonly used when a high strength steel is overlaid with a corrosion resistant alloy (CRA) for petrochemical applications. There have been reported failures of these DMWs during subsea service while under cathodic protection (CP). These failures are caused by local hydrogen embrittlement of susceptible microstructures that form at the weld fusion boundary. Hydrogen-assisted cracking (HAC) occurs as a result of the local embrittlement and is influenced by base/filler metal combinations, and welding and post-weld heat treatment (PWHT) procedures. A delayed hydrogen cracking test was used to simulate tensile load and hydrogen charging on 8630-FM 625 weld. The failure of this sample was recorded using a high-speed camera to capture the crack initiation and propagation during failure. Fractography was performed using a scanning electron microscope (SEM) along with energy dispersive spectroscopy (EDS). The fracture surfaces, EDS measurement and video timestamps revealed brittle fracture nucleation in the planar growth and CGHAZ regions of the weld. The cracking continued to propagate through the same regions of the weld leading to final ductile failure (microvoid coalescence) in the cellular dendritic region of the weld. 

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4. Fatigue Behavior and Cyclic Deformation of Additive Manufactured NiTi

   https://doi.org/10.1016/j.jmatprotec.2017.10.006  

   Bagheri, A.; Mahtabi, M.J.; Shamsaei, N. (October 2017, Journal of materials processing technology)

   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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5. COMPUTATIONAL SIMULATION OF DUCTILE FRACTURE IN BUCKLING RESTRAINED BRACES

   Terashima, M. Deierlein (July 2018, Proceedings of the U.S. National Conference on Earthquake Engineering)

   Since their first use in Japan about thirty years ago, Buckling Restrained Braces (BRBs) have been widely implemented in steel-framed buildings throughout the world. To date, most of the development and validation of BRB ductility has relied extensively on testing of full-scale braces under cyclic loading since no fracture evaluation method based on underlying micromechanics is currently available. Therefore, research is currently being undertaken to develop, validate and apply detailed finite element models to computationally simulate ductile fracture initiation and propagation in BRBs. As a part of this research, this paper presents an evaluation methodology of ductile fracture initiation using an Ultra-Low Cyclic Fatigue criterion, referred to as the Stress Weighted Damage Model (SWDM), along with detailed finite element analysis of BRBs. 

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