A shakedown analysis of high cycle fatigue of shape memory alloys

The paper presents a high-cycle fatigue criterion for shape memory alloys (SMAs) based on shakedown analysis. The analysis accounts for phase transformation as well as reorientation of martensite variants as possible sources of fatigue damage. In the case of high-cycle fatigue, once the structure has reached an asymptotic state, damage is assumed to become confined at the mesoscopic scale, or the scale of the grain, with no discernable inelasticity at the macroscopic scale. Using a multiscale approach, a high-cycle fatigue criterion analogous to the Dang Van model (Dang Van 1973) for elastoplastic metals is derived for SMAs obeying the Zaki–Moumni model for SMAs (Zaki and Moumni 2007a). For these alloys, a safe domain is established in stress deviator space, consisting of a hypercylinder with axis parallel to the direction of martensite orientation at the mesoscopic scale. Safety with regard to high-cycle fatigue, upon elastic shakedown, is conditioned by the persistence of the macroscopic stress path at every material point within the hypercylinder, whose size depends on the volume fraction of martensite. The proposed criterion computes a fatigue factor at each material point, indicating its degree of safeness with respect to high cycle fatigue.

Shape memory alloys (SMA) produce the largest anomalous reversible strains of all crystalline materials: eSM < 10% in comparison to eFM < 1.0% and eFE < 0.1% (FM magnetostrictive, FE electrostrictive). They are considered as active materials for applications in which large scale motions are required or because of their high damping capacity for mechanical vibrations. Their use is however limited if a large number of cycles is applied. A survey is given on fatigue phenomena in SMA: 1. classical mechanical fatigue—accumulation of defects, formation and growth of cracks, 2. shape memory fatigue—change of transformation temperatures, reduction or loss of memory, loss of pseudo elasticity or damping capacity. Finally, it is shown how a modification of microstructure by thermo-mechanical treatments (TMT) affects fatigue resistance. In this context primary defects (as introduced by TMT) and secondary defects (introduced by mechanical or thermal cycling) have to be distinguished. In case of training for the two-way effect secondary defects are introduced voluntarily.

Macroscopic modeling of functional fatigue in shape memory alloys

A universal symmetry criterion for the design of high performance ferroic materials

Effect of the amplitude of the training stress on the fatigue lifetime of NiTi shape memory alloys

Shape memory alloys (SMAs) have the potential to be used for a wide variety of microelectromechanical systems (MEMS) applications, providing a unique combination of large deflections and high work output. A major drawback for SMAs in many applications has been the low frequency response, which is typically on the order of 100 Hz or lower, even in microscale SMA actuators. In MEMS applications, the higher surface-to-volume ratios have enabled responses to be improved by an order or magnitude or more. By further shrinking the SMA film/device dimensions, the frequency response may be improved even further. In this paper, we present a new, simplified process for fabricating sputtered, thin film SMA MEMS actuators based on nickel-titanium alloy (NiTi or, aka, NITINOL) that consisted of only one photo step to pattern the actuators using SU8. When heated through its solid–solid phase transition from low-temperature martensite to high-temperature austenite, the NiTi alloy undergoes changes in associated physical properties, such as Young’s modulus, resistivity, and surface roughness, that are critical to controlling MEMS performance. For example, these material property changes allow for the design of active or passive microscale sensors and actuators. In the new process, we are able to fabricate ultrathin films of NiTi with nanoscale thickness, which can be thermally cycled through two stable positions very rapidly, making it an intriguing thermal sensor and actuator material for high frequency applications. Additionally, NiTi can be used as an active thermal switch through resistive (i.e. joule) heating. We demonstrated a greatly improved frequency response of up to 3000 Hz with turn on voltages as low as 0.5 V (corresponding to only 1 mW power consumption) for devices exhibiting microns of cantilever tip deflection over millions of cycles, indicating these new SMA MEMS actuators have potential application for low voltage switching, modulation and tuning.

In this thesis, the role of location-specific microstructural features in the fatigue performance of the safety-critical aerospace components made of Nickel (Ni)-base superalloys and linear friction welded (LFW) Titanium (Ti) alloys has been studied using crystal plasticity finite element (CPFE) simulations, energy dispersive X-ray diffraction (EDD), backscatter electron (BSE) images and digital image correlation (DIC).<br>In order to develop a microstructure-sensitive fatigue life prediction framework, first, it is essential to build trust in the quantitative prediction from CPFE analysis by quantifying uncertainties in the mechanical response from CPFE simulations. Second, it is necessary to construct a unified fatigue life prediction metric, applicable to multiple material systems; and a calibration strategy of the unified fatigue life model parameter accounting for uncertainties originating from CPFE simulations and inherent in the experimental calibration dataset. To achieve the first task, a genetic algorithm framework is used to obtain the statistical distributions of the crystal plasticity (CP) parameters. Subsequently, these distributions are used in a first-order, second-moment method to compute the mean and the standard deviation for the stress along the loading direction (σ_load), plastic strain accumulation (PSA), and stored plastic strain energy density (SPSED). The results suggest that an ~10% variability in σ_load and 20%-25% variability in the PSA and SPSED values may exist due to the uncertainty in the CP parameter estimation. Further, the contribution of a specific CP parameter to the overall uncertainty is path-dependent and varies based on the load step under consideration. To accomplish the second goal, in this thesis, it is postulated that a critical value of the SPSED is associated with fatigue failure in metals and independent of the applied load. Unlike the classical approach of estimating the (homogenized) SPSED as the cumulative area enclosed within the macroscopic stress-strain hysteresis loops, CPFE simulations are used to compute the (local) SPSED at each material point within polycrystalline aggregates of 718Plus, an additively manufactured Ni-base superalloy. A Bayesian inference method is utilized to calibrate the critical SPSED, which is subsequently used to predict fatigue lives at nine different strain ranges, including strain ratios of 0.05 and -1, using nine statistically equivalent microstructures. For each strain range, the predicted lives from all simulated microstructures follow a log-normal distribution; for a given strain ratio, the predicted scatter is seen to be increasing with decreasing strain amplitude and are indicative of the scatter observed in the fatigue experiments. Further, the log-normal mean lives at each strain range are in good agreement with the experimental evidence. Since the critical SPSED captures the experimental data with reasonable accuracy across various loading regimes, it is hypothesized to be a material property and sufficient to predict the fatigue life.<br>Inclusions are unavoidable in Ni-base superalloys, which lead to two competing failure modes, namely inclusion- and matrix-driven failures. Each factor related to the inclusion, which may contribute to crack initiation, is isolated and systematically investigated within RR1000, a powder metallurgy produced Ni-base superalloy, using CPFE simulations. Specifically, the role of the inclusion stiffness, loading regime, loading direction, a debonded region in the inclusion-matrix interface, microstructural variability around the inclusion, inclusion size, dissimilar coefficient of thermal expansion (CTE), temperature, residual stress, and distance of the inclusion from the free surface are studied in the emergence of two failure modes. The CPFE analysis indicates that the emergence of a failure mode is an outcome of the complex interaction between the aforementioned factors. However, the possibility of a higher probability of failure due to inclusions is observed with increasing temperature, if the CTE of the inclusion is higher than the matrix, and vice versa. Any overall correlation between the inclusion size and its propensity for damage is not found, based on inclusion that is of the order of the mean grain size. Further, the CPFE simulations indicate that the surface inclusions are more damaging than the interior inclusions for similar surrounding microstructures. These observations are utilized to instantiate twenty realistic statistically equivalent microstructures of RR1000 – ten containing inclusions and remaining ten without inclusions. Using CPFE simulations with these microstructures at four different temperatures and three strain ranges for each temperature, the critical SPSED is calibrated as a function of temperature for RR1000. The results suggest that critical SPSED decreases almost linearly with increasing temperature and is appropriate to predict the realistic emergence of the competing failure modes as a function of applied strain range and temperature.<br>LFW process leads to the development of significant residual stress in the components, and the role of residual stress in the fatigue performance of materials cannot be overstated. Hence, to ensure fatigue performance of the LFW Ti alloys, residual strains in LFW of similar (Ti-6Al-4V welded to Ti-6Al-4V or Ti64-Ti64) and dissimilar (Ti-6Al-4V welded to Ti-5Al-5V-5Mo-3Cr or Ti64-Ti5553) Ti alloys have been characterized using EDD. For each type of LFW, one sample is chosen in the as-welded (AW) condition and another sample is selected after a post-weld heat treatment (HT). Residual strains have been separately studied in the alpha and beta phases of the material, and five components (three axial and two shear) have been reported in each case. In-plane axial components of the residual strains show a smooth and symmetric behavior about the weld center for the Ti64-Ti64 LFW samples in the AW condition, whereas these components in the Ti64-Ti5553 LFW sample show a symmetric trend with jump discontinuities. Such jump discontinuities, observed in both the AW and HT conditions of the Ti64-Ti5553 samples, suggest different strain-free lattice parameters in the weld region and the parent material. In contrast, the results from the Ti64-Ti64 LFW samples in both AW and HT conditions suggest nearly uniform strain-free lattice parameters throughout the weld region. The observed trends in the in-plane axial residual strain components have been rationalized by the corresponding microstructural changes and variations across the weld region via BSE images. <br>In the literature, fatigue crack initiation in the LFW Ti-6Al-4V specimens does not usually take place in the seemingly weakest location, i.e., the weld region. From the BSE images, Ti-6Al-4V microstructure, at a distance from the weld-center, which is typically associated with crack initiation in the literature, are identified in both AW and HT samples and found to be identical, specifically, equiaxed alpha grains with beta phases present at the alpha grain boundaries and triple points. Hence, subsequent fatigue performance in LFW Ti-6Al-4V is analyzed considering the equiaxed alpha microstructure.<br>The LFW components made of Ti-6Al-4V are often designed for high cycle fatigue performance under high mean stress or high R ratios. In engineering practice, mean stress corrections are employed to assess the fatigue performance of a material or structure; albeit this is problematic for Ti-6Al-4V, which experiences anomalous behavior at high R ratios. To address this problem, high cycle fatigue analyses are performed on two Ti-6Al-4V specimens with equiaxed alpha microstructures at a high R ratio. In one specimen, two micro-textured regions (MTRs) having their c-axes near-parallel and perpendicular to the loading direction are identified. High-resolution DIC is performed in the MTRs to study grain-level strain localization. In the other specimen, DIC is performed on a larger area, and crack initiation is observed in a random-textured region. To accompany the experiments, CPFE simulations are performed to investigate the mechanistic aspects of crack initiation, and the relative activity of different families of slip systems as a function of R ratio. A critical soft-hard-soft grain combination is associated with crack initiation indicating possible dwell effect at high R ratios, which could be attributed to the high-applied mean stress and high creep sensitivity of Ti-6Al-4V at room temperature. Further, simulations indicated more heterogeneous deformation, specifically the activation of multiple families of slip systems with fewer grains being plasticized, at higher R ratios. Such behavior is exacerbated within MTRs, especially the MTR composed of grains with their c-axes near parallel to the loading direction. These features of micro-plasticity make the high R ratio regime more vulnerable to fatigue damage accumulation and justify the anomalous mean stress behavior experienced by Ti-6Al-4V at high R ratios.<br>

Introduction: The fracture resistance of interim restorative materials is important for success of dental restorations. Various fabrication methods have been employed to create provisional restorative materials, but impact of these methods on fracture resistance remains unclear. Understanding the influence of different fabrication techniques of interim restorative materials on fracture resistance is essential for optimising clinical outcomes, patient satisfaction, and overall dental treatment success. Aim: To compare the effect of different fabrication techniques on fracture resistance of interim restorations. Materials and Methods: A systematic search was conducted in electronic databases, which included PubMed, Scopus, and Corrections: Web of Science (WoS), to identify relevant studies published. Studies were included if they met the following criteria: Investigate the fracture resistance of provisional restorative materials, compare different fabrication techniques, published in English and peer-reviewed articles. Studies were excluded if they do not meet these criteria or are not available in full text. The quality of included studies was assessed using a validated tool modified Consolidated Standards Of Reporting Trials (CONSORT) checklist. A meta-analysis was performed to quantitatively analyse the effect of different fabrication techniques on the fracture resistance of provisional restorative materials. Subgroup analyses and sensitivity analyses was conducted if necessary. Results: The initial search yielded 254 potential articles. After removing duplicates and conducting a thorough screening of titles, abstracts, and full texts, 17 articles met the inclusion criteria for the study which were included for qualitative analysis. Data were extracted regarding the authors' details, the journals and years of publication, different fabrication methods and materials and evaluation of fracture resistance. The result revealed that there was no statistically significant difference for fracture resistance between Computer Aided Design and Computer Aided Manufacturing (CAD-CAM) and 3D-printed interim restorations {Mean difference=50.72 (95% CI=143.55, 244.99)} whereas there was statistically significant difference between CAD-CAM and conventional restorations {mean difference=282.58 (-411.59, -153.47)} and 3D printed and conventional interim restorations {Mean difference=169.75 (95% CI= 353.68, 14.17)}. The p-value is not significant but there is difference seen. Conclusion: Polymethyl Methacrylate (PMMA) CAD/CAM milled interim restorations demonstrated the highest fracture resistance among the groups compared in the study. This finding highlights the superior durability and strength of CAD/ CAM milling technology, particularly when using PMMA as the material for fabricating provisional restorations. The precise and controlled nature of the milling process ensures a strong, reliable restoration with high fracture resistance, making it the ideal choice for many clinical situations. A 3D-printing was shown to be a stronger and more reliable manufacturing method for interim restorations when compared to traditional conventional techniques (such as indirect methods). Although 3D-printed restorations exhibited lower fracture resistance than CAD/CAM milled restorations, they still offer a significant advantage over conventional techniques in terms of strength and reliability. This suggests that 3D-printing can be a viable alternative, especially when CAD/CAM milling is not available or feasible.

Thermomechanical modeling of polycrystalline SMAs under cyclic loading, Part III: evolution of plastic strains and two-way shape memory effect

Shape memory alloys (SMA) are used in many technological applications, thanks to their unique properties: superelasticity and shape memory (SM) effect. Many efforts have been made to improve performance of SMA wires to utilize them as thermal actuators also for many thousands of cycles. Near-equiatomic nickel-titanium compound is the most used materials for SM actuators because of its high recoverable values of strain and good cycling stability, if compared to the other known SMA. In this study, the functional properties of NiTi thin wires (80 μm) thermally cycled under a constant load (200 MPa) were investigated. In particular, the effect of two heating conditions, carried out by a step and a ramp current pulses, on functional fatigue of SMA has been studied. By means of an experimental apparatus, thermomechanical cycling, thermal loop under constant load and actuation time (AT) tests were carried out to investigate the alteration and the trend of recovered strain, irreversible strain, characteristic temperatures, and ATs of the thin SM wires.

&lt;p&gt;The main goal of this research is to develop equations which can be used for predicting the cyclic response of steel beam-column connections with superelastic Shape Memory Alloy (SMA) bolts and steel angles. Finite-element (FE) models are developed to simulate the cyclic behavior of SMA connections. The results are then validated against seven specimens from a previous experimental study. The validated models are used to perform sensitivity and response surface studies. Beam web slenderness ratio, SMA bolt diameter, and angle thickness are identified as the most significant factors affecting the cyclic response of SMA connections. Angle yielding is also found to be the dominant limit state in all connections, confirming their robust self-centering response. It is recommended to use beams of lower depth to minimize residual deformations and avoid early bolt fracture in these connections. The developed surrogate models can be used to accurately predict the SMA connections’ cyclic response.&lt;/p&gt;

The use of Shape Memory Alloy (SMA) materials for actuation represents a technological opportunity for the development of innovative applications. The issue of using the SMA actuators is an enticing solution that has attracted the interest of specialists in recent years, due to their numerous advantages (i.e. reliability, compactness and flexibility, reduced cost, linear or angular movement etc.). In addition, their unmatched energy density, easy integration into various structures and the simplicity in terms of design, have all motivated the authors to develop a barrier structure model, actuated by three SMA helical springs. The actuator used in this structure works as a linear actuator, contracting with great strength and speed, thus exerting the necessary force to lift the barrier arm in the SMA springs heat-activated, austenitic state. The first part of this paper provides a description of the accomplished model and of its functioning principle. The second part of the study presented in this paper focuses on the authors' investigation on how the performances of the actuator inserted in the barrier structure—namely the actuator's heating time, cooling time, actuation time and stroke—are affected by the SMA spring activating direct current values. The tests were conducted while keeping the geometrical parameters of SMA springs and the weight of the barrier arm constant.

This paper presents the methods and approach employed for certification of the F-22 air vehicle for high cycle and sonic fatigue design. To this end the approach employed on the F22 utilizes a four step process. This paper presents and discusses each step of the certification process in detail, beginning with Step 1, the development of design-to criteria for vibration and acoustics. In this step, the details of the derivation of the F-22 design specification and usage are presented. The various ground, wind tunnel, and prototype flight tests conducted to establish a baseline environmental definition are also discussed. With the establishment of a baseline design-to criterion, Step 2 follows with the development of a detailed design-to criterion. In this Step, the ground and laboratory tests conducted to develop design allowables and establish design margins are discussed. The analysis approach for establishing vibration and acoustic environments for design and qualification test are also presented. Step 3 in the certification process is verification of the vibroacoustic environments for design. Verification of the vibroacoustic environments is accomplished through engineering and manufacturing development (EMD) flight test. In Step 3, the requirements, execution and results of the F-22 EMD flight test program for vibroacoustics are discussed. Finally Step 4 presents the processes employed to validate the aircraft structure and air vehicle systems using the verified vibroacoustic environments determined in Step 3. The vibration and acoustic data reduction process is discussed and high cycle and sonic fatigue analyses methods are presented.

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;The Indian automotive industry has migrated from BS IV (Bharat stage IV) to BS VI (Bharat Stage VI) emission norms from 1&lt;sup&gt;st&lt;/sup&gt; April 2020. This two-step migration of the emission regulations from BS IV to BS VI demands significant engineering efforts to design and integrate highly complex exhaust after-treatment system (EATS). In the present work, the methodology used to evaluate the EATS of a high power-density 1.5-liter diesel engine is discussed in detail. The EATS assembly of the engine consists of a diesel oxidation catalyst (DOC), a diesel particulate filter with selective catalytic reduction coating (sDPF), urea dosing module and urea mixer. Typically, all these components that are needed for emission control are integrated into a single canning of shell thickness ~1.5mm. Moreover, the complete EATS is directly mounted onto the engine with suitable mounting brackets on the cylinder block and cylinder head. The mounting brackets of the after-treatment system should be stiff enough to withstand the high cycle fatigue vibration loads coming from the engine. On the other hand, the brackets should allow the system to expand freely under high temperatures to avoid any low cycle fatigue issues created due to thermal loads at 800 deg.C. Hence, a robust methodology is followed to ensure the robustness of the entire system. The methodology included the prediction of metal temperature using a conjugate heat transfer analysis, high-temperature modal analysis, stress-strain analysis and high-cycle vibration fatigue analysis. In order to evaluate the system from the thermal compliance point of view, a high-temperature stress-strain analysis is carried out in a thermal shock test cycle. The simulation results have shown that the brackets positioned at the near-zero thermal expansion zone could have a much lesser plastic strain due to reduced thermal stress. Taking advantage of this, the stiffness of the brackets could be increased to improve the modal frequency values and robustness against high-cycle vibration fatigue. The paper gives a detailed insight on optimizing the bracket design of the EATS to ensure that a robust design solution is finalized.&lt;/div&gt;&lt;/div&gt;

The intense development of adhesive restorative materials and parents' preferences for esthetic restorations prompt clinicians to use alternative restorative materials for primary molars. Amalgam, however, is the choice of material when it comes to occlusal stress bearing areas, either in primary or permanent molars. To overcome the drawbacks of amalgam and restorative adhesive materials, the bonded amalgam technique is employed. To evaluate microleakage and fracture resistance of bonded amalgam in primary molars, and compare it with the microleakage and fracture resistance of high-copper amalgam and composite resin materials. An in vitro study and 60 caries-free primary molars were used. A total of 60 samples were randomly divided into two equal groups for the evaluation of microleakage and fracture resistance. Class V cavities for microleakage study prepared on 30 samples and Class II mesio-occluso-distal cavities for fracture resistance study on other 30 samples were prepared and randomly divided into three equal groups. Group I received amalgam, Group II received bonded amalgam, and Group III received composite resins. The microleakage was viewed under a stereomicroscope. The fracture resistance was evaluated using a universal testing machine. Bonded amalgam exhibited minimum microleakage, when compared to amalgam and composite resin and was found to be statistically insignificant (P = 0.203), while amalgam showed better fracture resistance compared to bonded amalgam and composite resin. It was found to be statistically insignificant (P = 0.144). Bonded amalgam appears to be comparable to amalgam when microleakage is considered and to composite resin when fracture resistance is considered; hence, bonded amalgam can also be an alternative material to amalgam in primary molars.