Browsing by Author "Ashraf, Farhan"

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    Analytical fatigue life formulation for notches informed by crystal plasticity
    (Elsevier, 2022-06-17) Ashraf, Farhan; Cini, Andrea; Castelluccio, Gustavo M.
    Damage from small manufacturing defects often go unnoticed until fatigue cracks have grown beyond repairability. These cracks initiate at defects with dimensions on par with the microstructure length scale (e.g., 5–200 µm deep), which affects fatigue variability and renders most engineering prognosis methods inapplicable. This work develops a novel microstructure-sensitive formulation that reduces computational efforts by decoupling geometric and microstructural contributions to fatigue cracking. Crystal plasticity finite element models with and without geometry induced strain gradients were considered to assess the role of defects independently from the microstructure. The analysis results in a fatigue life analytical formulation whose parameters depend on the microstructure and defect morphology.
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    History and temperature dependent cyclic crystal plasticity model with material-invariant parameters
    (Elsevier, 2022-12-06) Ashraf, Farhan; Castelluccio, Gustavo M.
    Cyclic deformation of metallic materials depends on the interaction of multiple mechanisms across different length scales. Solid solution atoms, vacancies, grain boundaries, and forest dislocations interfere with dislocation glide and increase the macroscopic strength. In single phase metallic materials under cyclic loading, the localization of dislocation densities in sessile substructures explains a significant fraction of the strain hardening. Upon cycling, these dislocation structures evolve across stable configurations, which depend on the strain accumulation. This work advances substructure-sensitive crystal plasticity models capable of quantifying the cyclic hardening history at various temperatures for single phase FCC materials. The framework predicts the cyclic evolution of dislocation substructure based on the activation of cross slip activation for Al, Cu, and Ni single- and poly-crystals up to 0.5 homologous temperature. The increase in cross slip with temperature and deformation induces a transformation in dislocation structures, which predicts secondary hardening without any additional provision. Moreover, the approach relies on material-invariant mesoscale parameters that are specific to dislocation substructures rather than a material system. Hence, we demonstrate that crystal plasticity predictive power can be augmented by parameterizing the model with single crystal experimental data from multiple materials with common substructures. As a result, the crystal plasticity model shares parameter information across materials without the need for additional single crystal experimental data for calibration.
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    On the similitude relation for dislocation wall thickness under cyclic deformation
    (Elsevier, 2022-03-10) Ashraf, Farhan; Castelluccio, Gustavo M.
    Dislocation substructures have been extensively characterized to explain the origin of strain hardening. Regions of high dislocation densities (walls) constrain the glide of mobile dislocations in regions of lower dislocation density (channels). Transmission electron microscopy (TEM) has shown that the distance in between walls is inversely proportional to flow stress, which is often referred as to the similitude principle. However, it still remains unclear whether a similitude scaling law exists for the dislocation wall thickness or wall fraction. The understanding of such a scaling law is instrumental to support substructure-based crystal plasticity, validate dislocation dynamics models, and explain substructure formation processes. Hence, this work surveys TEM images from various FCC metallic materials under cyclic deformation at various temperatures to assess the existence of the similitude principle for dislocation walls. The results demonstrate that the wall thickness does follow similitude, but the proportionality constant depends on the temperature for some structures.
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    Performance evaluation of Cu-Ni 90/10 alloyed structures exposed to various seawater compositions and their remaining service life estimation
    (Cranfield University, 08/11/2022) Sarfraz, Syed Ali; Abbas, Muntazir; Sarfraz, Shoaib; Ashraf, Farhan
    The Cu-Ni 90/10 alloy is extensively used in seawater applications mainly because of its excellent heat transferability, resistance toward corrosion and marine fouling. The corrosion resistance of Cu-Ni 90/10 has been found to be far superior in open natural seawater, however, several premature failures have often been reported during their exposure in the pollutant-rich seawater typically found near harbours, jetties and coastlines. This paper investigates the corrosion behaviour of Cu-Ni 90/10 alloyed coupons exposed to natural seawater, and pollutant-rich harbour seawater in a submerged position. Moreover, this research also investigates the corrosion mechanism on marine heat exchanger tubes of material that failed prematurely while operating in similar seawater compositions. The field experimental results for short-term corrosion results from coupons, and the long-term corrosion results from heat exchanger tubes have been evaluated, to formulate a relationship and corrosion modelling.
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    Physics-based modelling of cyclic deformation and microstructure-sensitive fatigue crack propagation from shallow scribes
    (Cranfield University, 2020-12) Ashraf, Farhan; Castelluccio, Gustavo M.; Khan, Muhammad Ali
    Face-centered cubic (FCC) metals with low to medium stacking fault energy (SFE) develop similar mesoscale substructures under cyclic loading. The formation of these substructures is controlled by dislocation interactions and loading conditions. For instance, cross slip facilitates cell formation and Hirth locks define the labyrinth structure. In the case of aluminium (high SFE metal), cross slip is easily activated and a cell structure is often observed. However, it is not always recognised that aluminium can also form PSBs at low temperatures. This highlights that the underlying mechanism controlling the cyclic response in aluminium is not different from other FCC metals. This work proposes the role of mesoscale substructure as a material-invariant among FCC metals to predict the cyclic response of aluminium. The effect of number of cycles on modelling dislocation substructures is explored, which is found to trigger a change in dislocation structures in aluminium at 298K. A crystal plasticity framework based on mesoscale substructures is developed to study the cyclic response of aluminium under different crystal orientations, strain amplitudes, number of cycles, and temperatures. Finally, this work implemented the crystal plasticity model to study the microstructure-sensitive crack propagation from shallow scribes in pure aluminium. The gradient of fatigue indicator parameters (FIPs) is estimated as crack extends inside a grain with explicit microstructure simulations, which followed the same decaying trend predicted by experiments. Thereby, an engineering solution is proposed to couple microstructural and geometric gradients at the crack tip independently. The model predicted the transgranular fatigue life with independently coupled gradients that agree well with experiments.
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    Revitalising metallic materials: a path towards a sustainable circular economy
    (MDPI, 2023-07-28) Ashraf, Farhan; Lodh, Arijit; Pagone, Emanuele; Castelluccio, Gustavo M.
    Improving materials’ productivity and reusability can advance circularity by reducing extraction and enabling efficient fully recyclable parts and systems. However, the pursuit of circular materials often focuses on the choice among reusing, repairing, or recycling materials, with limited consideration for techniques that can proactively revitalise materials. Consequently, the adoption of preventive material therapies remain relatively scarce and unexplored. This work discusses the potential for revitalising metallic materials with preventive maintenance prior to detectable damage and we identify techniques that can effectively prolong the structural lifespan of metallic components. By analysing the literature and considering the eco-footprint and implementation feasibility within the aerospace sector, this study ranks approaches based on their circularity impact and provides valuable insights to guide future research in the field of materials maintenance.
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    A robust approach to parameterize dislocation glide energy barriers in FCC metals and alloys
    (Springer, 2021-08-03) Ashraf, Farhan; Castelluccio, Gustavo M.
    The mechanical response of metallic materials is controlled by multiple deformation mechanisms that coexist across scales. Dislocation glide is one such process that occurs after bypassing obstacles. In macroscopic well-annealed single-phase metals, weak obstacles such as point defects, solid solution strengthening atoms, short-range dislocation interactions, and grain boundaries control dislocation glide by pinning the scarce dislocation density. This work investigates the dislocation glide energy barrier in face-centered cubic (FCC) metallic materials by considering a crystal plasticity model that computes the yield strength as a function of temperature. The dislocation glide energy barrier is parameterized by three different formulations that depend on two parameters. A Monte Carlo analysis randomly determines all other coefficients within uncertainty bounds identified from the literature, followed by fitting the two energy barrier parameters to experimental data. We consider ten FCC materials to demonstrate that the methodology characterizes robustly the dislocation glide energy barrier used by crystal plasticity models. Furthermore, we discovered a correlation between the glide barrier and the stacking fault energy that can be used as a basis to infer the glide activation energy.