Mohammad Javad MahtabiUniversity of Tennessee at Chattanooga | Chatt · Department of Mechanical Engineering
Mohammad Javad Mahtabi
Doctor of Philosophy
About
38
Publications
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Introduction
I am an assistant professor of Mechanical Engineering at the University of Tennessee at Chattanooga. My research area is in Materials Engineering, Structural Engineering and Mechanical Engineering. Currently, our projects include additive manufacturing of smart and functional materials, medical devices, and fatigue and fracture behavior of additively manufactured metals and alloys, especially shape memory alloys. Our works involve various aspects of research from experimental, computational, and analytical works at different length-scales.
Additional affiliations
August 2018 - August 2024
September 2017 - August 2018
January 2014 - August 2017
Publications
Publications (38)
A cumulative energy-based damage model, called total fatigue toughness, is proposed for fatigue life prediction of superelastic NiTi alloys with various deformation responses (i.e., transformation stresses), which also accounts for the effects of mean strain and stress. Mechanical response of superelastic NiTi is highly sensitive to chemical compos...
In this paper, the effects of process-induced voids and surface roughness on the fatigue life of an additively manufactured material are investigated using a crack closure-based fatigue crack growth model. Among different sources of damage under cyclic loadings, fatigue because of cracks originated from voids and surface discontinuities is the most...
Fatigue behavior of superelastic NiTi alloys under variable amplitude loading is experimentally studied and two energy-based fatigue models were used to analyze the fatigue behavior. Cyclic tests were conducted in strain-controlled condition. Load sequence effects are studied by performing fatigue testing under load paths consisting of two steps of...
Precipitation hardening is considered the most feasible method for strengthening NiTiHf alloys. In order to design the optimum aging treatment to form precipitates, it is crucial to understand the effect of precipitates on the thermomechanical behavior of these alloys. In this research, the effect of H-phase precipitates was studied on the martensi...
In realistic applications, shape memory alloys are mostly under cyclic loading and, thus, fatigue failure is the major mode of failure in these components. Fatigue mainly starts from a nano- or micro-defect studying which is not feasible using experiments. Thus, Molecular Dynamics (MD) simulations are useful for obtaining understanding of the under...
The additive manufacturing process involves layer-by-layer fabrication of parts, where each layer potentially experiences unique thermal histories, including cooling rates and thermal gradients. These variations can impact the microstructure and, subsequently, mechanical properties of the final product, especially as the height of the build increas...
NiTi-based high temperature shape memory alloys (HTSMAs) can operate in high temperature applications, where binary NiTi cannot function. NiTiHf is among the most popular HTSMAs due to its lower preparation cost and good thermal stability. Various aspects of material properties of NiTiHf and the effect of different factors, such as composition, hea...
![a Grain morphology of the experimental study [11], b grain morphology...](publication/368156673/figure/fig2/AS:11431281125967093@1678503172465/a-Grain-morphology-of-the-experimental-study-11-b-grain-morphology-of-the-simulated_Q320.jpg)
Identifying the suitable process parameters is one of the necessities to control the microstructure, and consequently various properties of additively manufactured (AM) parts. To obtain suitable process parameters, it is crucial to understand the effects of each process parameter on various aspects of microstructure of the printed material. In this...
Thanks to its high strength-to-weight ratio and corrosion resistance, Ti-6Al-4V has gained a lot of attention in additive manufacturing (AM) of complex parts with aerospace and medical applications. The realistic loading condition in these applications is mostly cyclic, thus fatigue failure is the main mode of failure. On the other hand, due to pre...
A microstructure-based multistage fatigue (MSF) model was employed to study the process–structure–property relations for cyclic damage and fatigue life of NiTi alloy fabricated via an additive manufacturing (AM) technique. Various defect characteristics (i.e., level of porosity, pore size, and their spacing) and microstructural features (i.e., grai...
This study systematically evaluates the effects of laser powder bed fusion additive manufacturing (L-PBF-AM) parameters (hatch spacing and laser power) on the thermomechanical behavior and microstructure of Ni50.8Ti49.2 shape memory alloy. The samples were fabricated with hatch spacings from 40 to 240 µm and laser powers of 50 and 100 W at a consta...
Molecular dynamics simulations have been carried out in this study to investigate the stress-induced and temperature-induced phase transformations in NiTi shape memory alloy in the presence of Ni4Ti3 precipitate. NiTi alloys form the most popular class of shape memory alloys that have many applications in various industries. Therefore, computationa...
Shape memory alloys (SMAs) are able to recover large inelastic strains due to their thermal-/ stress-induced phase transformation between austenite and martensite. Stress raisers can either initially exist in SMA components as the manufacturing-induced micro-defects, or may nucleate upon monotonic/cyclic loading, for instance, due to decohesion of...
One of the most important Shape Memory Alloys (SMAs) is NiTi (an almost equiatomic alloy of nickel and titanium) which has many applications in different areas like aerospace and biomedical industries. This popularity arises from its Shape Memory Effect (recovering pre-deformed shape when heated) and Superelasticity (reversible elastic behavior cau...
By advancing additive manufacturing (AM) techniques for nitinol (NiTi), a shape memory alloy, more researchers investigate this tool for a wide range of applications. AM-fabricated NiTi parts may contain micropore/voids and have a rough surface due to the nature of the fabrication. As a solution, we proposed a modification to the NiTi-selective las...
NiTi (Nitinol) is a shape memory alloy with distinctive properties, such as shape memory, superelasticity, biocompatibility, and low density. All these unique properties make NiTi a great candidate in different applications. However, the conventional fabrication of NiTi encounters many challenges that significantly limits the practical applications...
Build orientation is one of the important parameters in the selective laser melting (SLM) process that could alter the mechanical and fatigue properties of the fabricated parts. In the first section of this paper, the monotonic tensile and low-cycle fatigue behavior of the SLM NiTi dog-bone samples fabricated in three different orientations are inv...
Effect of size and shape of the additively manufactured Nickel-Titanium (NiTi) alloys on their thermomechanical properties is experimentally studied. Although additive manufacturing is shown as a solution to the difficulties of machining of NiTi shape memory alloys (SMAs), our understanding of the effect of various processing (e.g., laser power, sc...
Nitinol has significant potential for biomedical and actuating-sensing devices, thanks to its functional properties. The use of selective laser melting (SLM) with Nitinol powder can promote novel applications aimed to produce 3D complex parts with integrated functional performances. As the final step of the production route, finishing processing ne...
This special issue is dedicated to the simulation as well as experimental studies of biomechanical behavior of biomaterials, especially those that are used for bone implant applications [...]
In this work, the effects of process parameters on the fabrication of NiTiHf alloys using selective laser melting are studied. Specimens were printed using bidirectional scanning pattern and with various sets of process parameters of laser power (100–250 W), hatch spacing (60–140 µm), and scanning speed (200–1000 mm/s). Cracking and delamination fo...
Process optimization of Laser-Based Additive Manufacturing (LBAM) systems is often complicated by the tradeoff between different mechanical properties as well as the relative density window. For instance, parts with similar relative densities can have noticeably different tensile mechanical properties (e.g., elongation-to-failure, yield strength, u...
Additive manufacturing (AM or three-dimensional (3D) printing) is a relatively new method that enables fabrication of metallic and nonmetallic parts directly from the CAD (computer-aided design) file. In general, in this method, the CAD file is sliced vertically into thin layers and each layer is fabricated on top of the previous layer until the fi...
The relationship between the microhardness and the loading transformation stress of superelastic NiTi is investigated at room temperature. Superelastic NiTi shows a loading stress plateau under mechanical loading due to austenite-to-martensite microstructural phase transformation. The stress level at which the transformation from austenite to marte...
The aim of this study is to experimentally investigate the fatigue behavior of additively manufactured (AM) NiTi (i.e. Nitinol) specimens and compare the results to the wrought material. Additive manufacturing is a technique in which components are fabricated in a layer-by-layer additive process using a sliced CAD model based on the desired geometr...
Two-dimensional elastic-plastic finite-element fracture simulations with the critical crack-tip-opening-angle (CTOA) fracture criterion have been used to evaluate the Two-Parameter Fracture Criterion (TPFC). The TPFC is a very simple approach derived from the elastic stresses around a sharp notch (crack) and Neuber's equation. Three different crack...
The solubility of Ar in α-Ti under temperature and pressure conditions typical to hot isostatic pressing (HIP) of pore-laden laser additive manufactured parts is investigated. The equilibrium pressure and radius of an Ar-containing pore during HIP, as a function of initial pore radius, are evaluated from a thermodynamics perspective. It is found th...
![Figure 4. Experimental data [5,13,14] and predicted fatigue curves for...](profile/Aref-Yadollahi/publication/322405891/figure/fig5/AS:668815100768260@1536469354345/Experimental-data-5-13-14-and-predicted-fatigue-curves-for-different-builds-of-LPBF_Q320.jpg)
The realistic loading condition for many components is multiaxial arising from multidirectional loading or geometry complexities. In this study, some multiaxial stress-based classical and critical plane fatigue models are briefly reviewed and their application for martensitic Nitinol under torsion and in-phase axial-torsion loading is evaluated. Th...
Two-dimensional elastic–plastic finite-element fracture simulations with the critical crack-tip-opening-angle fracture criterion were used to evaluate the two-parameter fracture criterion (TPFC). Three different crack configurations under tension and bending loads made of thin-sheet 2219-T87 aluminium alloy were analysed. A very wide range of width...
Experimental fatigue study has been conducted in this paper to investigate the effects of mean strain on the fatigue behavior of superelastic Nitinol in short life regime. Nitinol alloys are used in many applications such as aerospace and bioengineering in which these alloys may be subjected to cyclic loading with mean strains/stresses. For traditi...
Fatigue is generally defined as the gradual deterioration of materials under cyclic loading.
This type of failure is very important because of its unexpected nature. It can cause serious
damage to mechanical and biomedical systems without any warning. In the literature, fatigue
has been found to be responsible for 50–90% of all mechanical failur...
Finite-element fracture simulations using ZIP3D and critical CTOA fracture criterion on 7249 aluminum alloy have been made. Notch-strength analysis and fracture mechanics is explained. Comparisons have been made on the finite element simulation results and Two-Parameter Fracture Criterion (TPFC). Finite-element fracture simulations using ZIP2D (pla...
![Table 2 : Calculated seismicity parameters using Kijko method [28]](profile/Mohammad-Javad-Mahtabi/publication/268383761/figure/tbl1/AS:669056667492355@1536526948483/Calculated-seismicity-parameters-using-Kijko-method-28_Q320.jpg)
Estimation of earthquake parameters using probabilistic method has become very common in recent years. In Iran and especially for Tehran several studies have been performed to analyze the seismic hazard. Regarding the rapid development of lifeline facilities and the construction of few tall buildings in Tehran as well as the good correlation that e...
Questions
Questions (3)
Hello,
I am looking for stress-strain (or force-displacement and cross-section data) data for cyclic loading of a set of fatigue experiments for different materials, as well as the corresponding fatigue lives. I need this data for as many as recorded cycles for each experiment. I would really appreciate it if anyone can let me know where I can find such dataset.
Thanks in advance,
Mohammad
Hi everybody,
I have a problem in obtaining EBSD images and determining an accurate martensite area fraction for superelastic NiTi.
I have the lattice parameters for my NiTi specimens and have modified the EBSD files for NiTi in the austenite and martensite phases. However, since the XRD scans did not clearly show the lattice parameters for the martensitic phase, I used the reported values in the literature, where the austenite lattice parameters were exactly equal to my specimens.
Even with these modifications, the EBSD scans did not identify the amount of martensitic phase in my specimens. I know that there is a large amount of martensite in the specimen since the optical microscope image showed that.
I was wondering if anyone might know whether there is a specific setting or input that I may be missing in my EBSD scans that I am not obtaining the correct martensitic area fraction or if my situation is a common issue for NiTi.
Thank you,
Hello everyone,
Can anybody please explain what are the influential microstructural/processing features that control the shape of a superelastic NiTi alloy? I am looking for some quantitative relationship between some variables (e.g., grain size or texture) and features of the stress-strain response of this alloy, such as start and finish strains/stresses during loading and unloading, as well as the size of the loop formed during one cycle of loading. In other words, is it possible to make an acceptable estimation of those values before mechanically testing a real NiTi specimen?
Thanks,
Mohammad
