Small Deformation Strain Research Articles

Fundamental Investigations of the Deformation Behavior of Single-Crystal Ni-Mn-Ga Alloys and Their Polymer Composites via the Introduction of Various Fields

To meet the great requirements of future technologies, such as robots, single-crystal (SC) Ni-Mn-Ga alloys and their composites were designed and investigated in this study. Ferromagnetic shape memory alloys (FSMAs) are promising materials for applications in high-speed actuators, which are core components of robots; however, there are some issues of embrittlement and small deformation strain. Therefore, in this work, we first prepared SC Ni-Mn-Ga alloys for fundamental investigations of the shape deformations under the application of different fields (e.g., compressive and magnetic fields). Second, the SC Ni-Mn-Ga alloys were integrated with polymers of epoxy resin or silicone rubber to solve the embrittlement problem. The obvious two-stage yielding and sudden intensifying of the magnetization both suggest martensite variant reorientation (MVR) under the compressive and magnetic fields, respectively. Micro-computed tomography (μCT) and an X-ray diffractometer were utilized for the observations of shape deformation brought about by the MVR of the SC Ni-Mn-Ga particles in the polymer matrix. Clear MVR and shape deformation could be found in the compressed composites.

Formation of residual stresses during quenching of Ti17 and Ti–6Al–4V alloys: Influence of phase transformations

The formation of internal stresses during quenching of titanium alloys from the β phase field are investigated both experimentally and by simulation, in order to show the effects of phase transformations. Two titanium alloys are considered: the β-metastable Ti17 alloy and the α+β Ti–6Al–4V alloy. During the quench into water of laboratory scale samples (40 mm diameter cylinders), no phase transformations occurred in the Ti17 alloy because of its β-metastable character and the fast cooling. However, β→α+β and β→α′ phase transformations occurred in the Ti–6Al–4V sample. Both alloys are compared in order to highlight the effects of the phase transformations. Residual stresses were determined by neutron diffraction, by the contour method and at the surface by the hole drilling method. A model for the coupled thermal, mechanical and metallurgical evolutions is established in order to simulate the quenching operations. The material model for the Ti17 alloy was established in a previous study. Regarding the Ti–6Al–4V alloy, modeling approaches and experimental data from literature are utilized to build the material model. From both experiment and simulation, it is found that the internal stress evolutions are governed by the thermal gradients. The phase transformations have a weak impact because of the small deformation strains induced by the phase change. Nevertheless, good prediction of the phase transformation kinetics is necessary for accurate simulations, because the α and α’ phases strengthen the alloy, thereby limiting the plastic straining at the origin of the residual stresses. As most plastic strains are cumulated at high temperature, the thermomechanical model should be established accurately over the temperature ranges in which there is a significant proportion of β phase.

Evolution of acoustic softening effect on ultrasonic-assisted micro/meso-compression behavior and microstructure

The application of ultrasonic vibration is an effective method to overcome the processing problems in micro/meso-forming. Previously it was observed that ultrasonic vibration could reduce flowing stress in the forming process, called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition leading to apparent average stress reduction, acoustic softening and ultrasonic impact leading to real stress reductin. However, the evolutional characteristics and the mechanism of acoustic softening on material deformation is still not clear. And in most previous studies only the average stress but not the oscillatory stress was measured due to the convenience of dynamic force sensing system, which confused the different ultrasonic volume effects, acoustic softening, stress superposition and ultrasonic impact. The purpose of this study is to investigate the effects of acoustic softening on micro/meso-compression behavior and microstructure evolution. An ultrasonic-assisted compression test system with dynamic force sensing technology was developed. And a series of ultrasonic-assisted micro/meso-compression tests at different amplitudes were carried out on pure copper C1100O combining the microstructure analysis by EBSD technique. By analyzing the waveform of the oscillatory stress in the process, acoustic softening was successfully separated from the stress superposition and it was found that the deformation strain plays an important role on the effect of acoustic softening. The stress reduction by acoustic softening increases with the flowing strain or ultrasonic amplitude increasing. Besides, there is an evolutionary transition of acoustic softening ratio between small strain and large strain. When acoustic softening occurs, the low-angle grain boundaries distribute randomly in grains, compared to the piled distribution without ultrasonic assistance, implying motions of the low-angle grain boundaries or dislocation is improved by acoustic softening, resulting in the real stress reduction. In addition, with small deformation strain, the elongated grain becomes equiaxed and dislocation density is significantly reduced, which may be the result of the increased dislocation annihilation due to ultrasonic-induced dynamic recovery. However, with the deformation strain increasing to some extent, acoustic hardening gradually becomes significant, leading to much less effectiveness of acoustic softening on dislocation density reduction. The findings of this study provide an instructive understanding of the underlying mechanisms of acoustic softening in ultrasonic-assisted micro/meso-forming.

The Small Deformation Strain Tensor as a Fundamental Metric Tensor

In the general theory of relativity, the fundamental metric tensor plays a special role, which has its physical basis in the peculiar aspects of gravitation. The fundamental property of gravitational fields provides the possibility of establishing an analogy between the motion in a gravitational field and the motion in any external field considered as a noninertial system of reference. Thus, the properties of the motion in a noninertial frame are the same as those in an inertial system in the presence of a gravitational field. In other words, a noninertial frame of reference is equivalent to a certain gravitational field. This is known as the principle of equivalence. From the mathematical viewpoint, the same special role can be played by the small deformation strain tensor, which describes the geometrical properties of any region deformed because of the effect of some external agent. It can be proved that, from that tensor, all the mathematical structures needed in the general theory of relativity can be constructed.

New insights to the effects of ausforming on the bainitic transformation

The effects of prior deformation conditions on the bainitic transformation are investigated using dilatometry and optical metallography. The influences of both deformation temperature and strain on bainite transformation kinetics and morphology are examined in superbainitic carbide-free steel. The transformation is retarded by deformation at high temperature due to the retardation of bainite growth by dislocation debris in deformed austenite. At low deformation temperatures, the initial transformation rate is accelerated due to the presence of prolific nucleation sites in deformed austenite. The growth rate is, again, retarded by deformation. The combined effect of accelerated nucleation and retarded growth results in a complex dependence of the phase transformation kinetics on applied strain. For small deformation strains at 300°C, the overall transformation kinetics is faster than that in the non-deformed material. From a practical point of view, this provides an important opportunity to reduce the processing times for carbide-free bainitic steels. Interestingly, both the final transformed bainite and retained austenite fraction under these conditions are much higher than the non-deformed material. The retardation of growth dominates the overall transformation kinetics at large strains and low temperature leading to a high volume fraction of martensite in the final microstructure.

Two-way shape memory effect of a TiNiHf high temperature shape memory alloy

The two-way shape memory effect in a Ti 36Ni 49Hf 15 high temperature shape memory alloy (SMA) has been systematically studied by bending tests. In the TiNiHf alloy, the martensite deformation is an effective method to get two-way shape memory effect even with a small deformation strain. When the TiNiHf alloy is deformed at a full martensite state, the deformation mechanism is the martensite orientation accompanied by the dislocation slip. The experimental results indicated that the internal stress field formed by the bending deformation is in the directions of the preferentially oriented martensite variants formed during the bending deformation. Upon cooling the preferentially oriented martensite variants form under such an oriented stress field, which should be responsible for the generation of the two-way shape memory effect. Proper training process and aging benefit the formation of the oriented stress field, resulting in the improvement of the two-way shape memory effect. When the training strain is 7.1% and the training temperature is room temperature, a two-way shape memory strain of 0.88% is obtained in the Ti 36Ni 49Hf 15 alloy. This value is the maximum two-way shape memory strain in the TiNi-based high temperature SMAs so far. In comparison with the TiNi alloy, the TiNiHf high temperature SMA shows the two-way shape memory effect with the relative poor stability due to the lower strength of martensite.

Two-way shape memory effect induced by martensite deformation and stabilization of martensite in Ti 36Ni 49Hf 15 high temperature shape memory alloy

The two-way shape memory effect (TWSME) induced by martensite deformation and the stabilization of martensite of a Ti 36Ni 49Hf 15 high temperature shape memory alloy (SMA) have been investigated. The experimental results indicated that the martensite deformation is an effective method to get TWSME in the Ti 36Ni 49Hf 15 alloy even with a small deformation strain. During martensite deformation the martensite reorientation and the dislocation slip occur simultaneously. Thus an internal oriented stress field is easily created to induce the TWSME. The TWSME of the Ti 36Ni 49Hf 15 alloy shows poor stability during the subsequent thermal cycling. In addition, the martensite stabilization is caused by the martensite deformation. The variation of the elastic and irreversible energies should be responsible for this.

New insight into agarose gel mechanical properties.

The current study focuses on the effects of the molecular weight on the mechanical behavior of agarose gels. The small strain rheology and large strain deformation/failure behavior of three different molecular weight agarose gels have been examined, with the results expressed in term of molar concentration. For small deformation strains, the gelation temperature at low concentrations and the critical concentration for gel formation are strongly affected by the molecular weight. In addition, the elasticity of the network is also very sensitive to this parameter. It has been demonstrated that the experimental gelation cure curves can be superimposed on a universal gelation master curve, independent of the cure time. This would indicate self-similarity of the network at different scales, irrespective of concentration. A relationship between the elastic modulus and the molecular weight has been extracted from these results, where the molecular weight dependence exhibits a power law exponent of 2.42. For large deformation strains, the Poisson ratio has been estimated to be 0.5 for each of the agarose types examined, which indicates that these gels are incompressible. The strain at failure is largely dependent on the molecular weight, and is essentially independent of the biopolymer concentration. This result highlights the fact that the strain at failure is sensitive to the connectivity distances in the gel network. However, the failure stress and Young's modulus of agarose gels show a dependence on both concentration and molecular weight. The observations regarding Young's modulus are in good agreement with those found for small deformation strain rheology for the shear modulus. One of the primary advantages of using the lowest molecular weight agarose is that higher molar concentrations can be reached (more molecules per unit volume). However, the mechanical response of agarose gels is very sensitive to the molecular weight at fixed molar concentration, and if the present results are extrapolated to very low molecular weight, it can be suggested that below a limiting molecular weight a percolating network will not be formed, as suggested by the Cascade model (Carbohydr. Polym. 1994, 23, 247-251). This speculation is based on the influence of the "connectivity" at long distances, which influences the strain at failure (when the strain at failure is zero, the system is not connective).

Semi-solid deformation of non-dendritic structures—I. Phenomenological behavior

Semi-solid deformation mechanisms are proposed to be four major mechanisms: liquid/flow (LF); flow of liquid incorporating solid particles (FLS); sliding between solid particles (SS); and plastic deformation of solid particles (PDS) mechanisms. The segregation phenomenon during semi-solid deformation, in which the solid particles segregate in the center portion, while the liquid segregates around the edge, is predicted by the phenomenological model. As the solid fraction and the deformation rate increase, the SS and PDS mechanisms become more dominant, as predicted by the phenomenological model. At small deformation strain, the center area dominated by the SS and PDS mechanisms is smaller for lower deformation rate than that for higher deformation rate; however, as the deformation strain increases, the situation is reversed, which is due to the composite deformation rate effects predicted by the phenomenological model. The effects of the deformation temperatures and deformation rates will be discussed in Part II, based on the present framework

Torsional Creep Buckling of Thin-Walled Open Tubes With a Cross Section Having an Axis of Symmetry

The variational theorem by Sanders, McComb, and Schlechte [1] is applied to find the critical collapse time of an open thin-walled tube with a cross section having an axis of symmetry subjected to torsional creep buckling. Large deformation strains are considered. It is shown that small deformation strains yield inaccurate results in predicting the critical time. A simplified stress distribution is introduced which gives a closed-form solution. More accurate stress patterns present considerable difficulties and a tedious numerical integration is needed. In examining most cases, however, the simplified stress configuration predicts the critical time very accurately.