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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