The fatigue behaviour of as-built parts produced by means of Laser-Powder Bed Fusion process (L-PBF) is primarily influenced by the presence of stress raisers on the surface, whose morphology strongly depends on the relative orientation between the surface and the build direction. This study aims to shed light into the factors representing the surface morphology that correlate with the fatigue performance of L-PBF Ti6Al4V specimens manufactured in different orientations. A fracture mechanics model based on measurable roughness parameters was employed for the prediction of the fatigue properties in both the finite life and endurance limit regions.
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Themes: JoDean Morrow & Paul Paris Memorial Symposium on Fatigue & Fracture
FUNCTIONAL FATIGUE PROPERTIES OF TINIZRSN BIOCOMPATIBLE SHAPE MEMORY ALLOY
Functional fatigue degrades the superelastic properties of shape memory alloys under cyclic loading. In the presence of geometric stress concentrations, the local stress fields are amplified resulting in local accumulation of irrecoverable strains and consequently loss of functionality. For the biocompatible TiNbZrSn system, grain size and solution treatment temperature play a major role in affecting the level of pseudoelastic strains and their evolution upon cycling. These aspects are quantitatively investigated in this work. Dogbone tensile specimens and samples with drilled circular holes are considered in this work and full field strain measurements are employed to quantitatively evaluate the localization in response leading to loss of functionality.
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NUMERICAL ANALYSIS OF ROLLING CONTACT FATIGUE CRACK GROWTH ON CURVED RAILWAY TRACKS
In this study, numerical analyses were conducted to investigate the non-proportional mixed-mode RCF crack growth behaviour in the presence of severe longitudinal, lateral and spin creepages. The whole procedure combined multi-body dynamic simulation (MBDS) and the extended finite element method (XFEM) in an indirectly coupled way. Attempts were also made to modify the FaStrip theory to obtain traction distributions based on elastoplastic contact pressures which were then applied in an XFEM model to predict surface crack growth directions. Parametric studies were also conducted to further quantify the influence of different creepage combinations on both crack growth directions at rail surface and crack growth rate at crack tips. It is concluded that the increase of either of the three creepages can significantly influence the phase and magnitude of stress intensity factor histories, albeit to different extents.
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FATIGUE TESTING FOR COATINGS: A SYSTEMATIC APPROACH USING MICRO-IMPACT TESTING ON TIN
A practical and systemic method to quantify coatings’ fatigue strength has been outlined and tested on titanium nitride (TIN) coating using multi-cycle and multi-load micro-impact testing with a spherical indent.
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CRACK TIP ENHANCED CRYSTAL PLASTICITY PHASE FIELD MODEL FOR CRACK PROPAGATION IN TI64 ALLOYS
This work introduces a computational fracture model for Ti64 alloy based on coupled Crystal Plasticity Phase Field model for fracture but also considers the atomistic mechanisms of plasticity at the crack tip. Atomistic simulations are conducted to identify the crack-tip mechanisms of plasticity and the continuum scale phase field model is augmented to account for this. Using the data generated using atomic scale Molecular Dynamic simulations, a functional form describing the evolution of dislocation density nucleating from the crack tip is obtained using Bayesian Inference and Genetic Programming based Symbolic Regression. The effect of nucleated dislocations in crack path and rate of crack propagation is evaluated. The additional plastic strain at the crack tip is also validated with results from Molecular Dynamics.
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A GENERALIZED TWO-PARAMETER DRIVING FORCE MODEL FOR SHORT AND LONG FATIGUE CRACK PROPAGATION
Numerous different crack growth modeling approaches have been developed to consider the short crack and long crack behaviors by accounting for the stress intensity range-based crack driving forces or the crack closure concept. However, those methods lacked a proper systematic approach to accurately account for the behavior of short cracks. Based on the recent systematic study performed in the authors’ group, a new generalized two-parameter driving force model is proposed to account for crack growth driving forces and corresponding crack growth thresholds to predict both short crack and long crack propagation behaviors. The model predicted crack growth rates are compared with crack growth data set of Ti-6Al-4V titanium and 2024-T3 aluminum alloys. Predicted results show good agreement with experimental crack growth data for these materials.
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INFRARED TEMPERATURE MEASUREMENT AND X-RAY TOMOGRAPHY FOR INTERNAL FATIGUE CRACK MONITORING DURING ULTRASONIC FATIGUE TESTS [Keynote]
The observation of fatigue cracks in the gigacycle fatigue regime is very difficult because they are very often initiating and propagating in the core of the specimens. This paper presents a methodology for detecting and monitoring internal fatigue cracks during ultrasonic fatigue tests. Using both the heat source located in the reverse cyclic plastic zone at the crack tip and the 3D geometry of the crack (from X-Ray tomography), finite element analysis is done to solve the heat transfert problem. This allow us to related the internal crack growth rate and the temperature field evolution versus time at the surface of the specimen. This proposed method has been successfully applied on specimens in cast aluminum alloy.
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MULTIAXIAL FATIGUE BEHAVIOR OF SLM TI6AL4V ALLOY: X-RAY COMPUTED Μ-TOMOGRAPHY ANALYSIS [Keynote]
Crack formation and propagation phenomena in selective laser melting (SLM) Ti-6Al-4V alloy samples were analyzed under combined axial and torsional fatigue loads. In fact, SLM defects lead to a lower fatigue strength and a larger fatigue life variation with respect to to conventionally manufactured parts. Internal defects were captured by X-ray computed μ-tomography (μ-CT) and their evolution was monitored by interrupted fatigue tests. Critical defects were analyzed by the strain intensity factor (SIF) using two differ- ent models based on the Murakami’s method: a modified Smith-Watson and Topper (MSWT) criterion and a virtual strain energy (VSE) criterion. The trend of the crack growth rate was analyzed by the effective defect area at different number of fatigue cycles. The μ-CT data were also used to build finite element models (FEM) of cracked samples to analyze the whole stress-strain distribution in the near crack tip region.
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BIAXIAL LOADING IMPACT ON FATIGUE CRACK PROPAGATION IN METALLIC MATERIALS
Multiaxial fatigue testing generates curved cracks that are extremely difficult to characterize using standard
compliance based and potential drop-based methods. Therefore, an automated online system was
developed to monitor the crack tips positions in plate cruciform specimens. The system periodically
evaluates the deformation fields at small areas around the crack tips by performing digital image correlation
(DIC) on images obtained by a moving camera triggered at desired phases of the loading cycle. The
displacement field obtained by DIC is fitted by a simple model that specifies the crack propagation direction
and enables it to iteratively find new crack tip positions. Moreover, the model is capable of computing the
crack opening displacements near the crack tip and thus characterizes the local loading. This approach
enables fully automated multiaxial testing with controlled crack length and crack tip loading. The method
was successfully tested on an AA5754 aluminium alloy sheet..Multiaxial fatigue testing generates curved cracks that are extremely difficult to characterize using standard
compliance based and potential drop-based methods. Therefore, an automated online system was
developed to monitor the crack tips positions in plate cruciform specimens. The system periodically
evaluates the deformation fields at small areas around the crack tips by performing digital
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BACK TO BASICS FOR THE FATIGUE CRACK GROWTH RATE IN METALLIC ALLOYS
The field of fracture mechanics started with Griffith’s energy concept for brittle fracture in 1920. In 1963, Paris et al. used a fracture mechanics’ parameter to introduce an equation for the fatigue crack growth rate in ductile materials and this equation is now commonly known as the ‘Paris law’. However, the Paris law and the semi-empirical models that followed ever since do not fully account for the main intrinsic and extrinsic properties involved with fatigue crack growth in metallic alloys. In contrast, here we introduce a dimensionally correct fatigue crack growth rate equation that is based on the original crack driving force as introduced by Griffith and the presence of plasticity in a metal to withstand crack propagation.
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