Mechanical Durability Assessment of an Energy-Harvesting Piezoelectric Inverted Flag

This paper presents results from a practical assessment of the endurance of an inverted flag energy harvester, tested over multiple days in a wind tunnel to provide first insights into flapping fatigue and failure. The inverted flag is a composite bimorph, composed of PVDF (polyvinylidene difluoride) strips combined with a passive metallic core to provide sufficient stiffness. The flag, derived from an earlier, more extensive study, flaps with a typical amplitude of ~120 degrees and a frequency of ~2 Hz, generating a constant power of ~0.09 mW in a wind velocity of 6 m/s. The flag was observed to complete ~5×105 cycles before failure, corresponding to ~70 h of operation. The energy generated over this lifespan is estimated to be sufficient to power a standard low-power temperature sensor for several months at a sampling rate of one sample/minute, which would be adequate for applications such as wildfire detection, environmental monitoring, and agriculture management. This study indicates that structural fatigue may present a practical obstacle to the wider development of this technology, particularly in the context of their usual justification as a ‘deploy and forget’ alternative to battery power. Further work is required to improve the fatigue resistance of the flag material.

TOWARDS A QUALITATIVE-QUANTITATIVE DECISION-MAKING AID FOR FATIGUE LIFE EVALUATION OF NAVAL HIGH SPEED LIGHT CRAFT

The Life of Type (LOT) of a naval High Speed Light Craft (HSLC) can be limited by its structural fatigue life. The fatigue life of a ship is influenced by many factors, such as geometry, fabrication quality, the long-term load distribution, and analytical techniques. The complex dependencies between these factors and the fatigue life cause uncertainty in predictions. A lack of understanding of uncertainty can adversely affect the management of LOT risks, resulting in the reduction of availability of the ship and costly repairs. Therefore, improved understanding of the benefits and limitations of different fatigue life evaluation approaches informs the management of risks relating to the ship’s LOT. This paper presents the first phase of work in a comparative analysis of different fatigue life evaluation approaches for naval HSLC. The present work involves a holistic data review, codification of the data to reveal key themes, and individual expert comparative analysis of the different approaches. The next phase of the study is also described.

Structural fatigue and fracture of shape memory alloy actuators: Current status and perspectives

Shape Memory Alloy (SMA)-actuators are efficient, simple, and robust alternatives to conventional actuators when a small volume and/or large force and stroke are required. The analysis of their failure response is critical for their design in order to achieve optimum functionality and performance. Here, (i) the existing knowledge base on the fatigue and overload fracture response of SMAs under actuation loading is reviewed regarding the failure micromechanisms, empirical relations for actuation fatigue life prediction, experimental measurements of fracture toughness and fatigue crack growth rates, and numerical investigations of toughness properties and (ii) future developments required to expand the acquired knowledge, enhance the current understanding, and ultimately enable commercial applications of SMA-actuators are discussed.

Fatigue Characterization on a Cast Aluminum Beam of a High-Speed Train Through Numerical Simulation and Experiments

The cast aluminum beam is a key structure for carrying the body-hung traction motor of a high-speed train; its fatigue property is fundamental for predicting the residual life and service mileage of the structure. To characterize the structural fatigue property, a finite element-based method is developed to compute the stress concentration factor, which is used to obtain the structural fatigue strength reduction factors. A full-scale fatigue test on the cast aluminum beam is designed and implemented for up to ten million cycles, and the corresponding finite element model of the beam is validated using the measured data of the gauges. The results show that the maximum stress concentration occurs at the fillet of the supporting seat, where the structural fatigue strength reduction factor is 2.45 and the calculated fatigue limit is 35.4 MPa. Moreover, no surface cracks are detected using the liquid penetrant test. Both the experimental and simulation results indicate that the cast aluminum beam can satisfy the service life requirements under the designed loading conditions.

Initial Experience with Subchondral Stabilization for Bone Marrow Lesions of the Midfoot and Forefoot

Disabling foot pain is often accompanied by MRI evidence of bone marrow edema which may represent early structural fatigue. Emerging evidence suggests subchondral stabilization with injectable calcium phosphate can alleviate pain associated with bone marrow edema in the hindfoot, ankle and knee; however, there is no data supporting its use or safety for midfoot or forefoot lesions. We identified 54 patients who underwent SS of various midfoot/forefoot osseous structures in our practice over a four-year period. All patients proved recalcitrant to standard conservative measures, and all had advanced imaging appreciating BME. VAS for pain at 1, 3, 6, and 12 months postoperatively served as the primary outcome measure. 41 patients were included with a mean age of 54.3 ± 14.9 years and mean follow up of 14.1 ± 6.9 months. Patients saw a significant decrease in VAS pain as early as 1 month postoperatively (p<0.05). Mean postoperative VAS at 12 months was 2.11 ± 2.50, and the mean reduction in VAS pain from preop to 12 months postop was -5.00 (95% CI -3.44 to -6.56, p<0.05). Fourteen patients (34%, 14/41) were pain free at 12 months. Treatment of more than one bone (unadjusted OR 6.23 [95% CI 1.39 to 27.8], P=0.017) was associated with a greater likelihood of not achieving a pain free status at 12 months. Initial experience suggests that SS was both safe and effective in our patient population. Simultaneous treatment of multiple bones should be entered into with caution, and further research on the subject is necessary. Level of Evidence: IV (Retrospective Case Series)

Numerical Investigations of Shape Memory Alloy Fatigue

This work deals with numerical investigations of the functional and structural fatigue on shape memory alloys (SMAs). A thermodynamically consistent, three-dimensional constitutive model is employed, adopting a continuum damage perspective. Fatigue life is predicted by considering a macroscopic model. Numerical simulations are compared with experimental data taken from the literature to demonstrate the model’s ability to capture the general thermomechanical behavior of SMAs subjected to different loading conditions. Uniaxial and torsion tests are discussed; thermal loads are also analyzed considering the influence of the maximum temperature on the fatigue life of SMAs. Cyclic degradation of the shape memory effect is investigated in the sequence. Results show that numerical simulations are in good agreement with the experimental data, including the fatigue life estimation.