Magnetic Signatures and Magnetization Mechanisms for Grinding Burns Detection and Evaluation.

Grinding thermal damages, commonly called grinding burns occur when the grinding energy generates too much heat. Grinding burns modify the local hardness and can be a source of internal stress. Grinding burns will shorten the fatigue life of steel components and lead to severe failures. A typical way to detect grinding burns is the so-called nital etching method. This chemical technique is efficient but polluting. Methods based on the magnetization mechanisms are the alternative studied in this work. For this, two sets of structural steel specimens (18NiCr5-4 and X38Cr-Mo16-Tr) were metallurgically treated to induce increasing grinding burn levels. Hardness and surface stress pre-characterizations provided the study with mechanical data. Then, multiple magnetic responses (magnetic incremental permeability, magnetic Barkhausen noise, magnetic needle probe, etc.) were measured to establish the correlations between the magnetization mechanisms, the mechanical properties, and the grinding burn level. Owing to the experimental conditions and ratios between standard deviation and average values, mechanisms linked to the domain wall motions appear to be the most reliable. Coercivity obtained from the Barkhausen noise, or magnetic incremental permeability measurements, was revealed as the most correlated indicator (especially when the very strongly burned specimens were removed from the tested specimens list). Grinding burns, surface stress, and hardness were found to be weakly correlated. Thus, microstructural properties (dislocations, etc.) are suspected to be preponderant in the correlation with the magnetization mechanisms.

Coarse-Grained Lattice Modeling and Monte Carlo Simulations of Stress Relaxation in Strain-Induced Crystallization of Rubbers.

Two-dimensional triangulated surface models for membranes and their three-dimensional (3D) extensions are proposed and studied to understand the strain-induced crystallization (SIC) of rubbers. It is well known that SIC is an origin of stress relaxation, which appears as a plateau in the intermediate strain region of stress-strain curves. However, this SIC is very hard to implement in models because SIC is directly connected to a solid state, which is mechanically very different from the amorphous state. In this paper, we show that the crystalline state can be quite simply implemented in the Gaussian elastic bond model, which is a straightforward extension of the Gaussian chain model for polymers, by replacing bonds with rigid bodies or eliminating bonds. We find that the results of Monte Carlo simulations for stress-strain curves are in good agreement with the reported experimental data of large strains of up to 1200%. This approach allows us to intuitively understand the stress relaxation caused by SIC.

A Unique Fractional Derivative Operator to Simulate All Dynamic Piezoceramic Dielectric Manifestations: From Aging to Frequency-Dependent Hysteresis.

Ferroelectric materials are utilized in many applications ranging from capacitors to data storage. The hysteresis frequency dependence of ferroelectric materials has been well studied. However, the long-term dynamic behavior including aging has not been well documented due to the long time frame required to gather experimental data, but it is critical for understanding the lifespan of these materials in application. Previous work has shown that the hysteresis frequency dependence of dielectric properties can be accurately modeled in the time domain using fractional derivative operators applied on a large frequency bandwidth. Currently, the lowest frequencies tested have been restrained to the hysteresis cycle quasi-static threshold. Below this threshold, the hysteresis shape remains unchanged. This research expands the current knowledge by validating the use of fractional derivative operators in long-term aging models. The model data are experimentally validated using aged piezoelectric samples up to 107 s. These results confirm that the low and high dynamic dielectric material behaviors are linked and can be consequently modeled using fractional derivative operators.

Energy conversion in magneto-rheological elastomers.

Magneto-rheological (MR) elastomers contain micro-/nano-sized ferromagnetic particles dispersed in a soft elastomer matrix, and their rheological properties (storage and loss moduli) exhibit a significant dependence on the application of a magnetic field (namely MR effect). Conversely, it is reported in this work that this multiphysics coupling is associated with an inverse effect (i.e. the dependence of the magnetic properties on mechanical strain), denoted as the pseudo-Villari effect. MR elastomers based on soft and hard silicone rubber matrices and carbonyl iron particles were fabricated and characterized. The pseudo-Villari effect was experimentally quantified: a shear strain of 50 % induces magnetic induction field variations up to 10 mT on anisotropic MR elastomer samples, when placed in a 0.2 T applied field, which might theoretically lead to potential energy conversion density in the mJ cm-3 order of magnitude. In case of anisotropic MR elastomers, the absolute variation of stiffness as a function of applied magnetic field is rather independent of matrix properties. Similarly, the pseudo-Villari effect is found to be independent to the stiffness, thus broadening the adaptability of the materials to sensing and energy harvesting target applications. The potential of the pseudo-Villari effect for energy harvesting applications is finally briefly discussed.

Fatigue effect of elastocaloric properties in natural rubber.

In the framework of elastocaloric (eC) refrigeration, the fatigue effect on the eC effect of natural rubber (NR) is investigated. Repetitive deformation cycles at engineering strain regime from 1 to 6 results in a rapid rupture (approx. 800 cycles). Degradation of properties and fatigue life are then investigated at three different strain regimes with the same strain amplitude: before onset strain of strain-induced crystallization (SIC) (strain regime of 0-3), onset strain of melting (strain regime of 2-5) and high strain of SIC (strain regime of 4-7). Strain of 0-3 leads to a low eC effect and cracking after 2000 cycles. Strain of 2-5 and 4-7 results in an excellent crack growth resistance and much higher eC effect with adiabatic temperature changes of 3.5 K and 4.2 K, respectively, thanks to the effect of SIC. The eC stress coefficient index γ (ratio between eC temperature change and applied stress) for strains of 2-5 and 4-7 are γ2-5=4.4 K MPa(-1) and γ4-7=1.6 K MPa(-1), respectively, demonstrating the advantage of the strain regime 2-5. Finally, a high-cycle test up to 1.7×10(5) cycles is successfully applied to the NR sample with very little degradation of eC properties, constituting an important step towards cooling applications.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Analysis of ac-dc conversion for energy harvesting using an electrostrictive polymer P(VDF-TrFE-CFE).

Harvesting systems capable of transforming unused environmental energy into useful electrical energy have been extensively studied for the last two decades. The recent development of electrostrictive polymers has generated new opportunities for harvesting energy. The contribution of this study lies in the design and validation of electrostrictive polymer- based harvesters able to deliver dc output voltage to the load terminal, making the practical application of such material for self-powered devices much more realistic. Theoretical analysis supported by experimental investigations showed that an energy harvesting module with ac-to-dc conversion allows scavenging power up to 7 µW using a bias electric field of 10 V/µm and a transverse strain of 0.2%. This represents a power density of 280 µW/cm(3) at 100 Hz, which is much higher than the corresponding values of most piezo-based harvesters.

Fractional derivative operators for modeling the dynamic polarization behavior as a function of frequency and electric field amplitude.

Polarization phenomena in ferroelectric materials are frequency-dependent, and the present article describes the use of a fractional derivative for the understanding of these phenomena as well as modeling them as functions of frequency and electric field amplitude. The focus was first directed toward the definition and validation of the proposed model through comparisons between simulations and measurements for high electrical field excitation amplitudes on a large frequency bandwidth (major hysteresis loops, measured over 4 decades). Subsequently, the same comparisons were made under ultra-weak as well as weak electric fields. Large frequency bandwidths were tested in each case, and it was shown that the fractional term provided a very accurate modeling of the dynamic behavior of the ferroelectrics. The dielectric permittivity coefficient along the polarization direction epsilon(33)is a major parameter in ferroelectrics, and the frequency dependence of epsilon(33) is correctly reproduced by the model. The time-dependence of the polarization reversal/variation was accurately simulated by a fractional derivation (a 0.5 order derivative), however, the use of a first-order derivation term (i.e., viscous losses) was in poor agreement with experimental results. It was found that the model was valid for large excitation field amplitudes as well as for large frequency bandwidths.

Nonlinear pyroelectric energy harvesting from relaxor single crystals.

Energy harvesting from temperature variations in a Pb(Zn(1/3)Nb(2/3))(0.955)Ti(0.045)O(3) single crystal was studied and evaluated using the Ericsson thermodynamic cycle. The efficiency of this cycle related to Carnot cycle is 100 times higher than direct pyroelectric energy harvesting, and it can be as high as 5.5% for a 10 degrees C temperature variation and 2 kV/mm electric field. The amount of harvested energy for a 60 degrees C temperature variation and 2 kV/mm electric field is 242.7 mJ x cm(-3). The influence of ferroelectric phase transitions on the energy harvesting performance is discussed and illustrated with experimental results.

Pyroelectric energy conversion: optimization principles.

In the framework of microgenerators, we present in this paper the key points for energy harvesting from temperature using ferroelectric materials. Thermoelectric devices profit from temperature spatial gradients, whereas ferroelectric materials require temporal fluctuation of temperature, thus leading to different applications targets. Ferroelectric materials may harvest perfectly the available thermal energy whatever the materials properties (limited by Carnot conversion efficiency) whereas thermoelectric material's efficiency is limited by materials properties (ZT figure of merit). However, it is shown that the necessary electric fields for Carnot cycles are far beyond the breakdown limit of bulk ferroelectric materials. Thin films may be an excellent solution for rising up to ultra-high electric fields and outstanding efficiency. Different thermodynamic cycles are presented in the paper: principles, advantages, and drawbacks. Using the Carnot cycle, the harvested energy would be independent of materials properties. However, using more realistic cycles, the energy conversion effectiveness remains dependent on the materials properties as discussed in the paper. A particular coupling factor is defined to quantify and check the effectiveness of pyroelectric energy harvesting. It is defined similarly to an electromechanical coupling factor as k2=p2theta0/(epsilontheta33cE), where p, theta0, epsilontheta33, cE are pyroelectric coefficient, maximum working temperature, dielectric permittivity, and specific heat, respectively. The importance of the electrothermal coupling factor is shown and discussed as an energy harvesting figure of merit. It gives the effectiveness of all techniques of energy harvesting (except the Carnot cycle). It is finally shown that we could reach very high efficiency using 1110.75Pb(Mg1/3Nb2/3)-0.25PbTiO3 single crystals and synchronized switch harvesting on inductor (almost 50% of Carnot efficiency). Finally, practical implementation key points of pyroelectric energy harvesting are presented showing that the different thermodynamic cycles are feasible and potentially effective, even compared to thermoelectric devices.

Energy harvesting based on FE-FE transition in ferroelectric single crystals.

The pyroelectric properties of Pb(Zn(1/3)Nb(2/3))(0955)Ti(0.045)O(3) single crystals versus an electric field have been studied for energy harvesting in this paper. Two thermodynamic cycles (Stirling and Ericsson) were used for this purpose. By applying an electric field, a FE-FE transition was induced, abruptly increasing the polarization. This transition minimized the supplied energy and improved the harvested energy. By discharging the single crystal at a higher temperature, a gain of 1100% was obtained with the Stirling cycle at 1 kV/mm (gain is defined as harvested energy divided by supplied energy). The study revealed that Stirling cycles are more interesting for low electric fields. Based on experimental results, simulations were carried out to estimate energy harvesting in high electric fields to evaluate the performances of thin samples (single crystals or oriented thin films). At high electric fields, both cycles gave almost the same energy harvesting, but Ericsson cycles were more appropriate to control the voltage on the sample. The simulation led to a harvested energy of 500 mJ/g for an applied electric field equal to 50 kV/mm. The efficiency with respect to Carnot was raised 20%.

Modeling of elastic nonlinearities in ferroelectric materials including nonlinear losses: application to nonlinear resonance mode of relaxors single crystals.

(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) and (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) single crystals are considered to behave like soft Pb(Zr,Ti)O3 (PZT) ceramics because of their small mechanical quality factor Qm and poor stability under external disturbances (Qm > 500-1000 for hard PZT ceramic, and Qm < 100 for soft PZT and PMN-PT and PZN-PT single crystals). At weak signal excitation of the first resonance mode, the displacement at the end of a lateral bar is proportional to the Q31d31 figure of merit that is very close to that found for hard PZT. Indeed the very large piezoelectric coefficient compensates the low Qm. But increasing alternating current (AC) field results in the appearance of strong non-linearities through a shift of the resonance frequency and jumps phenomenon observed on increasing and decreasing frequency sweep. It is shown in this paper that these nonlinearities are due to the nonlinear elastic compliance that can be modeled by a third order development of the constitutive piezoelectric equations. Experiments on PMN-PT and PZN-PT single crystals are used for comparison with the model to show the viability of the approach. Both the frequency shift and jumps phenomenon are simulated with a very good agreement with experimental results. The importance is also shown of losses associated with the third order term responsible for the large decrease of the mechanical quality factor for high strain levels. Thus, the nonlinear losses are related to the hysteresis of domain wall motion when subjected to large displacements.

Stability of morphotropic (110) oriented 0.65PMN-0.35PT single crystals.

Electromechanical properties of (1-x)Pb (Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystals with x = 0.35 were investigated as a function of different external disturbances. The polarization dependence on the electromechanical properties was first studied in order to determine the best polarization path. The correlation with X-ray measured phase ratio is presented and shows that the maximum of electromechanical properties may be correlated with a minimum rhombohedral/tetragonal phase ratio. Temperature, stress, electric field, and time (aging) stability was studied in order to determine performance-limiting factors of these materials. The rhombohedral/tetragonal phase transition is observed on temperature (80 degrees C), inducing a decrease of the electromechanical coupling factor (from 85% to 50%); but the whole properties are recovered while returning to room temperature. Stress measurement shows a large depoling of sample for stresses above 30 MPa. The PMN-PT single crystals were found to be surprisingly stable during aging, except for mechanical and dielectric losses. The same tendency was found on alternating current (AC) electric field dependence.