Fatigue in Materials: Fundamentals, Multiscale Modeling and Prevention : Fatigue Characterization Using Advanced Experimental Methods in 2D and 3D
Sponsored by: TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Ashley Spear, University of Utah; Jean-Briac le Graverend, Texas A&M University; Antonios Kontsos, Drexel University; Tongguang Zhai, University of Kentucky

Tuesday 2:00 PM
February 28, 2017
Room: 23C
Location: San Diego Convention Ctr

Session Chair: Antonios Kontsos, Drexel University

2:00 PM  
Miniaturised Ultrasonic Fatigue Testing: Jicheng Gong1; Arutyun Arutyunyan1; Isaac Cabrera1; Angus Wilkinson1; 1University of Oxford
     We have developed a methodology for testing small volumes of materials to failure in the (very) high cycle fatigue regime. At the micro-scale (300 nm to ~5 Ám wide) cantilevers are cut by focused ion beam into the surface a bulk sample block, while at the meso-scale (50-200 Ám wide) we have used laser micro machining to cut cantilever structures into thin metallic foils. In both cases cyclic deflections are excited in the cantilevers through vibration of the sample block/foil using a high power ~20 kHz ultrasonic generator. The samples are monitored in situ using optical microscopy, while intermittent SEM and EBSD analysis can be undertaken across the entire sample. Results from testing campaigns on Ti alloys (commercially pure, and Ti-6Al-4V), Ni-based superalloy IN718, and stainless steel 304L will be presented.

2:20 PM  Invited
Crack Initiation and Propagation in Nickel-based Superalloy Microcrystals during In Situ Scanning Electron Microscopy High Cycle Fatigue Testing: Steven Lavenstein1; Gi-Dong Sim1; Bryan Crawford1; Paul Shade2; Michael Uchic2; Christopher Woodward2; Jaafar El-Awady1; 1Johns Hopkins University; 2AFRL
    Although the cyclic response of materials are of great importance, in situ micro-scale fatigue tests to-date have been limited in the number of loading cycles that can be completed in a practical amount of time. Here, we present a novel in situ, high-cycle fatigue testing methodology using a combination of Focused Ion Beam (FIB) fabrication and nanoindentation. The cyclic loading is imposed by using high frequency actuator dynamics. The amplitude and frequency of the oscillating force can be optimized to the desired values. Utilizing this methodology, we conduct a systematic study on the effect of crystal size on the fracture and fatigue life of microcantilever nickel-based superalloys under different loading conditions and crystallographic orientations. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam’s dynamic stiffness and SEM imaging.

2:40 PM  Invited
Investigating Very High Cycle Fatigue Behavior of Ti-6242S Using In-situ Ultrasonic Fatigue in an E-SEM: Jason Geathers1; Christopher Torbet2; J Wayne Jones1; Samantha Daly2; 1University of Michigan; 2University of California, Santa Barbara
    The effects of microstructure and environment on fatigue damage accumulation processes in the very high cycle fatigue (VHCF) regime were investigated in a Ti-6242S titanium alloy. Although strains are nominally elastic in VHCF, local cyclic plastic strain accumulation occurs at the micro-scale and drives crack initiation and early growth behavior. An experimental methodology is employed which combines ultrasonic fatigue at 20 kHz, scanning electron microscopy, and Digital Image Correlation (DIC) to examine the evolution of full-field strains at the micro-scale. Crack initiation as a function of microstructural neighborhoods and specific microstructural features was correlated with cyclic strain accumulation as measured by DIC. The influence of environment on short crack growth behavior from focused ion beam machined micro-notches located at critical microstructural sites was also examined. The role of microstructure and environment on strain localization, crack initiation, and short crack growth will be described.

3:00 PM  
Novel High-throughput Experiments for Early Damage Evolution in FCC Materials in the High and Very Cycle Fatigue Regime: Thomas Straub1; Michael Buck1; Chris Eberl2; 1University of Freiburg; 2Fraunhofer Institute for Mechanics of Materials IWM
    A material’s fatigue lifetime is determined by the crack formation process: damage accumulation in individual grains, micro crack initiation, short crack formation. While short crack evolution has been under investigation for years, crack initiation is still difficult to observe and characterize. Therefore, the focus of this work lies on the implementation of a novel high-throughput methodology for damage evolution investigation. This can be achieved by sample size reduction and implementation of a sensitive resonant measurement method. HCF tests with fcc materials show a highly reproducible resonant frequency decrease correlating to damage initiation. The subsequent short crack formation can be observed optically or analyzed ex situ in an SEM. This methodology gives in-situ insights of the damage evolution distribution in individual grains and neighbors, depending on the number of cycles, the local stress amplitude, and the grain orientation. This contribution will present fatigue results of pure Ni and potentially pure Cu.

3:20 PM  
Characterization of Crack Propagation in Ni-based Superalloys Using High Energy X-ray Techniques: Diwakar Naragani1; Michael Sangid1; Paul Shade2; Peter Kenesei3; Hemant Sharma3; 1Purdue University; 2Air Force Research Laboratory; 3Advanced Photon Source
    In this study we employ a suite of techniques, based on x-ray synchrotron experiments that allow us to track a naturally nucleated crack, which is propagating through the bulk of a Ni-based superalloy specimen under cyclic loading. Absorption contrast tomography is performed to determine the complex 3d crack morphology and track the crack tip position. Near-field high energy diffraction microscopy (HEDM) is conducted in the beginning to characterize the microstructure, producing micron resolution grain maps with intragranular orientation spread. The cyclic loading is sequentially interrupted to conduct far field HEDM to characterize the microstructure around the crack. Grain centroid position, averaged orientation and strains are determined for each grain in the sample especially at the crack tip. The reconstructions elucidate temporal and spatial micromechanical evolution of grains as the crack proceeds to grow. Findings are used to study and predict crack growth path and time to failure in Ni superalloys.

3:40 PM Break

4:00 PM  
CPFE Simulations and In-situ Laue Micro-diffraction to Reveal the Geometry of a Forming Vein during Fatigue: Ainara Irastorza-Landa1; Nicolo Grilli1; Helena Van Swygenhoven2; 1Paul Scherrer Institute & EPFL; 2Paul Scherrer Institut
    The formation of a vein during cyclic shearing of a single Cu crystal can be followed in Laue diffraction transmission by following the lattice rotation spatially resolved [Acta Mat, 112(2016)184]. The evolving dislocation microstructure was analyzed in terms of lattice rotation, lattice curvature and apparent geometrically necessary dislocation densities. The evolution of the GND traces showed a clear redistribution of the pre-existing GNDs and the appearance of regions surrounded by GND walls with no GNDs inside. Such regions were recognized as developing vein structures. Because Laue transmission integrates over the thickness of the sample, no experimental information can be obtained about the structure of the vein in the beam direction. Here, we show that the vein geometry can be obtained by comparing lattice curvature tensor components form CPFE simulations with those experimentally derived. These simulations are performed using the newly developed crystal plasticity model [J.Mech. Phys. Solids 84(2005)424].

4:20 PM  Invited
Fatigue Crack Growth and Fracture of Flexible Metallic Sheets: Wade Lanning1; Syed Javaid1; James Collins1; Christopher Muhlstein1; 1Georgia Institute of Technology
    In our previous work we demonstrated that transitions from inter- to transgranular fatigue crack growth modes in some face-centered cubic, nanograined metals are associated with cyclic grain coarsening. However, both sheet thickness and grain morphology contribute to the apparent loss of fatigue crack growth and fracture resistance. In this presentation we will explore how cyclic plastic deformation evolves ahead of cracks in very thin, flexible, wrought Au and Al sheets. A series of experiments on wrought, centimeter-scale specimens will be compared and contrasted with the performance of micrometer-scale, vapor deposited films to establish which crack tip parameters best describe the driving forces for crack advance.

4:40 PM  
Short Crack Growth in Ni-base Superalloys during Micro-bending Fatigue: Gi-Dong Sim1; Zafir Alam1; Gyuseok Kim2; Paul Shade3; Chris Woodward3; Kevin Hemker1; 1Johns Hopkins University; 2University of Pennsylvania; 3Air Force Research Laboratory
    In this presentation, we will discuss experimental methods developed to study statistics of short-crack growth in 500 Ám thick Ni-base superalloy (single crystal and polycrystalline) foils. The goal is to identify intermittent, avalanche dynamics of short-crack growth and possible connections to the “avalanche oscillator” mechanism for plasticity. Using a novel resonant-based micro-bending fatigue setup, we monitor the resonance frequency (RF) change of the foil while applying cyclic loading and correlate discrete RF drops to short-crack growth. Furthermore, we employ microfabrication techniques to deposit multiple micron-scale thin metal lines on top of the foil (dielectric layer is deposited in between). These channels are designed to result in open circuits during crack propagation, and the electrical resistance of the channels is recorded using four-point resistance method. From resistance change, intermittent crack growth can be quantified and analyzed for studying self-organized criticality of short-crack growth.

5:00 PM  
The Role of Particle Fracture in Early Fatigue of Aluminum Alloys: Brian Wisner1; Konstantinos Baxevanakis1; Antonios Kontsos1; 1Drexel University
    A microstructrure-sensitive experimental approach coupled with a multiscale computational method are presented to investigate the role of particle fracture in early fatigue damage initiation in Aluminum alloys. In precipitate-hardened Aluminum alloys, iron rich particles fracture well before significant failure occurs at the specimen level. Particle fractures are investigated at the time and scale they occur by coupling in situ testing in a scanning electron microscope with nondestructive evaluation including Digital Image Correlation, for surface strains, and Acoustic Emission monitoring from inside the microscope chamber. In addition, a Crystal Plasticity Finite Elements method is used to compute deformation patterns near a particle, related to strains experimentally measured. These results are subsequently used in a plasticity model in which damage initiation criteria are implemented to activate particle-specific failure, as well as to study the stress wave propagation from such events related to experimentally detected Acoustic Emission.

5:20 PM  
In Situ Microstructural Fatigue Investigation of Magnesium Alloys: Chengyang Mo1; Brian Wisner1; Antonios Kontsos1; 1Drexel University
    This research focuses on understanding the relationship between microstructural changes and fatigue behavior of Magnesium alloys. Due to its hexagonal close-packed structure, basal, prismatic and pyramidal slip as well as deformation twinning are activated during mechanical loading. These crystallographic deformation mechanisms result to a particular macroscopic fatigue behavior characterized by an unusual shape of the obtained hysteresis loops showing anisotropic hardening and asymmetric tension/compression yield stresses. In this talk, results obtained by performing in situ Digital Image Correlation measurements combined with Electron Backscattered Diffraction measurements are used to investigate microstructural changes and strain localizations at the grain level during fatigue loading. The result provides insights into the effects of deformation twinning on microplasticity and crack initiation.