L. Patriarca

• University of Illinois Urbana-Champaign

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 surf...

Several factors must be considered within the assessment of parts produced by Additive Manufacturing (AM) such as, for example, heterogeneous microstructure, process-induced defects, surface quality, residual stresses, and dependence on the material’s properties with the building orientation. All these factors severely affect the resistance to stat...

The possibility to obtain optimized components with a reduced weight is the main driver of space and aeronautic industries in seriously considering the metal additive manufacturing (AM) technology for production. Despite the incontrovertible advantages offered by this manufacturing technique, the material produced is usually affected by the presenc...

One of the challenges associated with additive manufacturing (AM) is the definition of an assessment route which considers the main process signatures of the AM process. To this end, this work presents a complete benchmark activity for the assessment of an AlSi10Mg component produced by a laser powder bed fusion process, aimed at advancing the unde...

The cyclic R-curve describes the resistance of a material to fatigue crack propagation from the short to the long crack regimes and it is therefore an essential ingredient in any fracture mechanics-based fatigue assessment procedure. This work presents different testing procedures employed in the experimental determination of the cyclic R-curve, es...

Wire arc deposition is an interesting manufacturing technique for producing metallic parts made of Ni-based alloys, for cladding and for repairs in turbine components. This technique, when compared to power bed fusion processes, allows to obtain parts with a limited porosity. In this paper, a wide experimental campaign on the Alloy625 wire arc depo...

Probabilistic routines are necessary to determine the permissible design stress when the operational conditions of traditional gas turbines are further challenged to reach higher performances in terms of flexibility. This paper focuses on the definition of a probabilistic tool aimed to simulate the damage development under combined high and low cyc...

Micro-lattice materials represent nowadays a great opportunity for developing new ultra-lightweight materials. Design flexibility and multi-functional properties make them attractive for several applications in automotive, medical, space, aerospace and process industries. Predict fatigue resistance of these micro-structures is a key issue. In this...

The present work aims to compare the local strain fields on the microscopic scale of an austenitic stainless steel 316L sample during tension loading, obtained by experimental measurements and simulations. Experimentally, Digital Image Correlation (DIC) technique is used to track the strain maps during the specimen loading. Microstructure-based Fin...

The present work aims to compare the residual strain fields of the austenitic stainless steel 316L samples under specific loading conditions, obtained by different techniques. Experimentally, Digital Image Correlation (DIC) technique is used to track the (residual) strain maps during the specimen loading. Microstructure-based Finite Element (FE) mo...

Crack closure is one of the most important phenomenon for the comprehension of fatigue behavior of metallic alloys. The effect of mean stress, overloads and variable amplitude loadings can be predicted modeling the crack closure in terms of opening and closing stress levels. Experimentally, the characterization of the crack closure in high and low...

The full potential of additive manufacturing (AM) components is today yet to be reached. Space and aerospace industries are still conservative in the use of AM parts for safety–critical applications, mostly because of the uncertainties and low reproducibility that are often associated with the process. One of the most challenging issues is the fati...

Compressor blades can be subjected to Combined Cycle Fatigue (CCF) conditions considering the superposition of the service cycles characterized by a low cycle fatigue cycle controlled by the centrifugal, pressure and thermal load and the blade vibrations. Simple assessment concepts are available in the literature based on the mean minus three sigma...

Nowadays, several components for aerospace and energy applications (i.e., turbine disks, offshore pipelines, and combustor chambers) are designed following a safe-life approach in the low cycle fatigue (LCF) regime. As also in the LCF regime it is worth adopting a damage tolerance design, fatigue life is assessed considering a crack propagating fro...

The effect of residual stresses induced by shot-peening in a high-strength AISI 4340 steel has been studied with the purpose of deriving a consistent fatigue model incorporating the results of fatigue crack growth experiments in the threshold region for a broad range of load ratio (R-ratio ranging from −2.5 to 0.7), and the effect of short cracks b...

In the last two decades, visual image techniques such as Digital Image Correlation (DIC) enabled to experimentally determine the crack tip displacement and strain fields at small scales. The displacements are tracked during loading, and parameters as the Stress Intensity Factor (SIF), opening and closing loads, T-stress can be readily measured. In...

Numerical techniques have been widely applied in many recent works to investigate micro-scale behavior of materials. This work focuses on the analysis of strain localizations in a Nickel-based alloy, Haynes 230. Numerical models and experiments concern the study of the strain field generated around the crack tip inside a polycrystalline medium when...

The NiTiHf high-temperature shape memory alloys represent a significant advancement in extending the functionality of binary NiTi to elevated temperatures above 100 °C. Despite this potential, the previous results in the literature point to a disappointingly low shape memory strains with addition of Hf. On the other hand, based on theoretical analy...

The functionality of NiTiHf (25%Hf) shape memory alloy (SMA) was studied under isobaric temperature cycling experiments. Both the local and overall strains, determined with digital image correlation (DIC), displayed excellent stability and no measurable unrecovered elongation. The external stress levels were higher than 250 MPa and transformation t...

Compressor blades are generally subjected to Combined Cycle Fatigue (CCF) conditions, where the service cycles are characterized by a low cycle fatigue (LCF) cycle controlled by the centrifugal, pressure and thermal loads superimposed to high frequency loadings at positive load ratios R > 0.5 as generated, for example, by the blade vibrations. The...

The micro-lattices produced by additive manufacturing process (AM) represent a recent important advancement for engineering structural applications, in particular for weight-saving purposes. The design of the components manufactured with these meta-materials generally refers to the idealized structures. In reality, the geometry obtained by the AM p...

This paper discusses the experimental results obtained for a duplex γ-TiAl alloy deformed at room temperature. High resolution digital image correlation was used to measure local strain fields. The characterization of the microstructure was performed using optical microscopy and a precise correlation between the strain fields and the microstructure...

We report on unprecedented transformation strains exceeding 20% in tension for Ni50.5Ti36.2Hf13.3 shape memory alloy (SMA). The strain measurements were made at multiscales utilizing advanced digital image correlation. The display of excellent strain reversibility in shape memory (isothermal deformation between Mf and Af), isobaric thermal cycling...

In this work we present the high temperature functional behavior of the new Ni50.3Ti25Hf24.7 shape memory alloy (SMA). Very high transformation strains were measured during isobaric experiments at temperatures up to Af = 420 °C. For the [111]B2 orientation in tension, we measured with digital image correlation (DIC) averaged transformation strain o...

In this work the functional behavior of the new Ni51.2Ti23.4Hf25.4 high temperature shape memory alloy is investigated along three crystal orientations in compression and compared with the polycrystal behavior. Transformation temperatures were measured following specific aging treatments with the aim to optimize the shape memory and pseudoelastic b...

The presence of nano-sized coherent precipitates is well known to have crucial impact on the mechanical behaviors of a broad class of superelastic alloys. As a representative material, the pseudoelasticity of austenitic NiTi alloy in presence of a lenticular coherent Ni4Ti3 precipitate is investigated using atomic scale simulations. We predict the...

The transformation behavior of NiTiHf alloys is intriguing. In NiTiHf alloys, the experimental transformation strains have been reported to be considerably lower than theoretical transformation strains. In this study, the transformation strain is established with very careful strain measurements at small scales in isobaric and isothermal experiment...

We present an experimental and theoretical study of slip nucleation in high entropy FeNiCoCrMn alloy which possesses superb mechanical properties. Uniaxial compression experiments were conducted on the oriented single crystals. The single crystals permit evaluation of critical resolved shear stress (CRSS), and using Digital Image Correlation strain...

The Fe3Al alloy with D03 structure exhibits large recoverable strains due to reversible slips. Tension and compression experiments were conducted on single crystals of Fe3Al, and the onset of slip in forward and reverse directions were obtained utilizing high-resolution digital image correlation technique. The back stress provides the driving force...

The Fe3Ga base alloy is known to exhibit pseudoelasticity and the solute hardened Fe3GaB holds considerable promise as well. The present work aims at developing a theoretical model to establish the critical twinning stress in Fe3Ga with varying boron concentration. The theoretical model is based on the atomistic-micromechanical approach where we ut...

Changes in transformation temperatures and high-temperature mechanical behavior of the new Ni50.6Ti34.4Hf25.0 alloy were determined after selected aging treatments. Differential scanning calorimetry was used to compare the homogenized, solution-treated and aged specimens for a wide combination of aging temperatures and times. Isothermal deformation...

In this study, based on a regression of the crack tip displacements, the stress intensity range in fatigue is quantitatively determined for the shape memory alloy Ni2FeGa. The results are compared to the calculated stress intensity ranges with a micro-mechanical analysis accounting for the transformation-induced tractions. The effective stress inte...

In this work we examine a Ti-48Al-2Cr-2Nb alloy obtained with an additive manufacturing technique by Electron Beam Melting (EBM) by conducting monotonic and cyclic loading experiments both on tension and compression samples for investigating the influence of the microstructure in strain accumulation process by fatigue loading. The residual strain m...

Novel manufacturing process are more and more used to produce advanced structural materials such as the gamma titanium aluminide alloys (γ-TiAl). In this work we examine a Ti-48Al-2Cr-2Nb alloy obtained with an additive manufacturing technique by Electron Beam Melting (EBM) by conducting monotonic and cyclic loading experiments both on tension and...

We develop an analytical expression for twin nucleation stress in bcc metal and alloys considering generalized planar fault energy and the dislocations bounding the twin nucleus. We minimize the total energy to predict the twinning stress relying only on parameters that are obtained through atomistic calculations, thus excluding the need for any em...

In order to design efficient and light components for the aircraft industry preserving the safety of the design, more sophisticated design criteria are required for the application of new materials. In particular, the usage of novel manufacturing processes to produce advanced materials as the gamma titanium aluminide alloys (γ-TiAl) requires the in...

We establish an overall energy expression to determine the twin migration stress in bcc metals. Twin migration succeeds twin nucleation often after a load drop, and a model to establish twin migration stress is of paramount importance. We compute the planar fault energy barriers and determine the elastic energies of twinning dislocations including...

In order to design efficient and light components for the gas turbine engine industry, new design criteria are needed for the application of gamma titanium aluminide alloys to safety critical components. In particular, novel manufacturing processes to produce y-TiAl alloys require investigating the influence of the microstructure in the local damag...

Grain boundaries induce heterogeneities in the deformation response of polycrystals. Studying these local variations in response, measured through high resolution strain measurement techniques, is important and can improve our understanding of fatigue damage initiation in the vicinity of grain boundaries and material hardening. In this work, strain...

In this work, a Ti-48Al-2Cr-2Nb alloy obtained with a additive manufacturing technique by electron beam melting (EBM) has been examined by conducting high cycle fatigue tests both with plain specimens and with specimens with artificially introduced defects with the objective of studying the growth behavior of small cracks. A consistent model for pr...

Tension and compression experiments were conducted on body-centered cubic Fe -47.8 at pct. Cr single crystals. The critical resolved shear stress (CRSS) magnitudes for slip nucleation, twin nucleation and twin migration were established. We show that the nucleation of slip occurs at a CRSS of about 88 MPa, while twinning nucleates at a CRSS of abou...

The fatigue properties of a Ti-48Al-2Cr-2Nb alloy obtained by electron beam melting (EBM) has been examined by conducting high cycle fatigue tests performed at different R ratios both at room temperature and at high temperatures, comparable to those experienced by the components during service. Additionally, a fairly large set of specimens with art...

The fatigue properties of a Ti-48Al-2Cr-2Nb alloy obtained by electron-beam melting (EBM) with a patented process has been examined by conducting high cycle fatigue tests performed at different R ratios at room temperature. Fatigue-crack propagation tests have been performed for the purpose of characterizing the fatigue-crack growth rate and thresh...

The life assessment for a component subjected to high strain concentration in critical regions can be treated as a crack growth estimation starting from the first cycle of the component life. This approach assumes that the presence of same small surface defects or slip bands causes a quickly crack nucleation in critical regions where Low Cycle Fati...

The fatigue properties of a Ti-48Al-2Cr-2Nb alloy obtained by electron beam melting (EBM) with a patented process has been examined by conducting high cycle fatigue tests performed at different loading ratios at room temperature. Fatigue crack tests have been performed for the purpose of characterizing the fatigue crack growth rate and threshold of...

The fatigue properties of a Ti-48Al-2Cr-2Nb alloy obtained by electron beam melting (EBM) with a patented process has been examined by conducting high cycle fatigue tests performed at different loading ratios both at room temperature and at high temperatures, comparable to those experienced by the components during service. Some tests have been con...

The traditional experimental procedures used to generate thresholds (known as "AKdecreasing" and "constant Kmax") have been challenged because It seems they affect the experimental results, sometimes in a non conservative way. In order to fix this problem, different experimental procedures ("compression pre-cracklng constant amplitude" and "compres...