Load Relaxation Behavior Research Articles

Load-Relaxation Characteristics of Chemical and Physical Hydrogels as Soft Tissue Mimics

Load-relaxation under a constant state of deformation is a common characteristic of hydrated materials, including hydrogels and biological tissues. Overall, mechanical response in such materials is a strong function of underlying structure, which in hydrogels depends on whether the gel is formed through physical or chemical cross-linking. In order to use hydrogels in biomedical applications where their properties are matched to those of native tissues, it is critical to understand these underlying structure-properties relationships. The objective of current work is to quantitatively characterize the load-relaxation behavior of physical and chemical gels and perform a comparative analysis with several biological tissues. Microindentation-based load-relaxation experiments were performed on three physical (agar, alginate, and gelatin) gels and one chemical (polyacrylamide) gel with a range of experimental time frames. All three physical gels exhibit strong time-dependent load-relaxation behavior where faster indentation leads to pronounced load-relaxation over short time-scales. The polyacrylamide gel is largely time-independent and exhibits negligible relaxation within short time-scales. The material property intrinsic permeability, which relates to underlying pore structure, was time-independent for both physical and chemical gels. A comparative analysis reveals that different aspects of the time-dependent properties of biological tissues are captured by physical and chemical hydrogels, with implications for tissue engineering applications.

Penetration and relaxation behavior of dry lunar regolith simulants

A return to the lunar surface has been identified as an important near-term goal for NASA and other international space agencies. Along with recent and upcoming missions to asteroid bodies and other extraterrestrial surfaces, this demands a comprehensive understanding of the mechanical properties of the regolith we are likely to encounter. Historically, penetrometers have been used to garner information about the near-surface, but in situ measurements give correlational data at best. To extend the usefulness of penetrometer measurements and provide more information about the regolith properties, we conducted laboratory penetration and subsequent relaxation measurements of two regolith simulants, one angular and cohesive and the other rounded and noncohesive, using a mechanical conical probe specially-designed to allow bulk sample relaxation. We measured penetration resistance and load-relaxation behavior for both simulants under constant displacement rate penetration at pressures ranging from 1E−5 Torr to 700 Torr and at relative densities of ~20% and ~80%. Penetration resistance was sensitive to density and insensitive to test pressure and simulant type. Relaxation was sensitive to all three conditions, and certain elastic parameters of a best-fit mechanical relaxation model can be used to differentiate between simulants, sample density, and test pressure. A combination of penetration and relaxation data may therefore be used to explore these properties in a field setting, and the relaxation behavior can also be tentatively correlated to more fundamental soil mechanics properties such as shear moduli. Furthermore, penetration/relaxation tests into sieved samples (coarse- and fine-grained versions of the simulants) show that penetration resistance can be used to determine relative particle size with respect to conical probe diameter and relaxation behavior indicates that a combination of fine- and coarse-grain particles produces a more cohesive behavior (in a mildly cohesive simulant) than either the fine- or coarse-grained counterparts. Additional testing in varying simulants and at more extreme ranges of temperature, pressure, density, and water ice content is suggested to refine these measurements, along with field testing to determine the viability of the use of this technology terrestrially as well as on future planetary missions.

Ramp-hold relaxation solutions for the KVFD model applied to soft viscoelastic media

The standard step-hold load-relaxation profile can yield variable estimates of mechanical properties due to the difficulty in achieving a step strain experimentally. A ramp-hold profile overcomes this limitation if appropriate model functions can be derived. Utilizing Boltzmann hereditary integral operators for two indentation geometries, analytical ramp solutions for load-relaxation were developed based on the Kelvin–Voigt fractional derivative (KVFD) model. The results identify three model parameters for characterizing viscoelastic behavior from a single model curve fit to the data: the elastic modulus E0, fractional-order parameter α, and relaxation time constant . The quantitative nature of the analysis was validated through measurements on gelatin emulsion samples exhibiting viscoelastic behavior. KVFD-model-based solutions provide mathematically simple and experimentally flexible descriptions of load-relaxation behavior for a range of viscoelastic properties and experimental conditions; e.g. one closed-form solution can fit the ramp and the hold phases of the relaxation time series. Experiments show that the solution for a spherical indenter and plate compressor each fit well to the corresponding experimental relaxation curves with a coefficient of determination R2 > 0.98. Parameters obtained from the spherical-tip indentation and plate-compression geometries agree within one standard deviation, confirming that the ramp solution based KVFD model yields consistent measurements for characterizing viscoelastic materials.

Superplasticity and load relaxation behavior of extruded Mg–8Sn–3Al–1Zn alloy at 250 °C

The tensile properties and load relaxation behavior of extruded Mg–8Sn–3Al–1Zn alloy at 250°C were investigated. Superplasticity was observed in the alloy, which exhibited tensile elongations of 450–1050% at strain rates in the range of 5×10-4–5×10−3s−1. Load relaxation results at 250°C indicated that grain boundary sliding is responsible for the superplastic properties of the alloy, even though it does not act as a primarily operating deformation mechanism.

Force decay of orthodontic elastomeric chains by using three different mechanisms simulating canine retraction

The ideal force-delivery system must: provide optimal tooth moving forces that elicit the desired effects, be comfortable and hygienic for the patient, require minimal operator manipulation and patient cooperation and provide rapid tooth movement with minimal mobility during orthodontic therapy, the elastomeric chains have the greatest potential to fulfill these requirements. Materials and Methods: This in vitro study was designed to determine the effect of three different mechanisms for canine retraction : (6-3 , 6-5-3 and chain loop ) on the load relaxation behavior of three types of elastomeric chains : (maximum clear , maximum silver and extreme silver) from the same company (Ortho Technology company) with two different brand configurations: closed loop and open (short filament) chains under effect of time at (zero time, 24hr., 7, 14 , 21 and 28 days) in artificial saliva. Results: Statistical analysis showed that there was a highly significant difference in the mean percentage force decay for the three different mechanisms (P≤ 0.001).For all the three types, the 6-3 mechanism had the smallest mean percentage force decay. There was a highly significant difference in the mean percentage force decay for the different types (P≤ 0.001). For all three mechanisms, extreme silver elastomeric chains had the smallest percentage force decay while maximum silver elastomeric chains had the highest percentage force decay. Conclusion: This study illustrated that for all the three types of elastomeric chains, the (6-3) mechanism had the smallest mean percentage force decay. This finding suggests that it may be most efficient to retract a canine utilizing elastomeric chain directly from the molar hook to the canine bracket. The chain loop mechanism may not be indicated for space closure in vivo due to the excessive physiological force values involved with this mechanism.

Comparison of mechanical behaviors of enamel rod and interrod regions in enamel

Abstract

An internal variable approach of orientation dependent deformation mechanism of rolled AZ31 magnesium alloy

Load relaxation behavior of an AZ31 Mg alloy has been studied in relation to temperature and orientation dependence. The rolled plate with 50 mm thickness was first homogenized at 400 °C for 4 h before preparing test specimens in the directions of 0° (RD), 45°, and 90° (ND) from the rolling direction. A series of tensile tests was consequently carried out under a strain rate of 10−2/s at room temperature (RT), 100 °C, and 200 °C. The RD specimens were found to deform mainly by dislocation slips without twinning. The 45° and 90° specimens were, on the other hand, found to deform in a combined mode of twinning and dislocation slips. Load relaxation tests were also performed to obtain flow curves in terms of stress and strain rate at the three different temperatures. The flow curves in terms of stress vs. strain rate were also found to consist of a plasticity curve due to dislocation glides in the lower strain rate region and a twin curve in the high strain rate range for the 45° and 90° specimens as prescribed by an internal variable theory. These results were consistent with the tensile test results and were further confirmed by microstructure observations.

Load Relaxation Behavior of Metastable Austenitic Stainless Steels at Quasi-Static Strain Rates

Instantaneous strain rate sensitivity of metastable austenitic stainless steel EN 1.4318 was studied at quasi-static strain rates (2 × 10−4 ... 10−1 s−1) using load relaxation tests carried out at room temperature. The results indicate that at the studied deformation conditions the instantaneous strain rate sensitivity of the test material decreases with decreasing strain rate but increases with increasing plastic strain. Rapid strain aging was also observed to occur at the studied deformation conditions.

Load relaxation behavior of a Zr 41.2Ti 13.8Cu 12.5Ni 10 Be 22.5 bulk metallic glass

The load relaxation behavior within the supercooled liquid region of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 bulk metallic glass has been investigated. To explain the relationship between normalized stress and relaxation time, two different stress relaxation modes such as a Kohlrausch–Williams–Watts (KWW) behavior and a simple power law were applied to the short and long relaxation time regimes, respectively. The apparent activation energy for stress relaxation is 126 ± 10 kJ/mol. Flow curves were obtained by converting load–displacement data into a flow stress–strain rate relation, resulting in three different deformation characteristics through a wide strain rate region interpreted in terms of strain rate sensitivity. A prediction of hot workability has also been attempted by constructing a power dissipation map based on a dynamic materials model.

Thermal Fatigue Characteristics of Heat-Resisting Stainless Steel for Automotive Exhaust

Thermal fatigue is a complex phenomenon encountered in materials exposed to cyclically varying temperatures in the presence or absence of external load. Continually increasing working temperature and growing need for greater efficiency and reliability of automotive exhaust require immediate investigation into the thermal fatigue properties especially of high temperature stainless steels. In this study, thermal fatigue properties of 304 and 429EM stainless steels have been evaluated in the temperature ranges of 200-800oC and 200-900oC. Systematic methods for control of temperatures within the predetermined range and measurement of load applied to specimens as a function of temperature during thermal cycles have been established. Thermal fatigue tests were conducted under fully constrained condition, where both ends of specimens were completely fixed. Thermal fatigue property of STS 304 was superior to that of STS 429EM. Load relaxation behavior at the temperatures of thermal cycle was closely related with the thermal fatigue property.

Load relaxation of a flat rigid circular indenter on a gel half space

When a saturated elastic gel is suddenly indented to a fixed displacement, δ, by a flat circular rigid indenter, the solvent cannot escape immediately, this gives rise to a pressure gradient in the solvent and the solvent flows until the pressure in it goes to zero everywhere, and all the stresses are transferred to the elastic network. This load transfer process causes, the indenter load, F, to decrease with time t, with a characteristic time constant, τ, which is given by a 2/ D c, where D c is the cooperative diffusion coefficient and a is the radius of the circular punch. This load relaxation behavior can be used to extract the elastic moduli and the cooperative diffusion coefficient of the gel. A detailed finite element analysis shows the load relaxation as a function of normalized time ( t/ τ) and how it depends on the shear modulus G and the Poisson’s ratio ν of the drained gel. Using this result, we found that the energy release rate available for detaching the punch in a pull-off test can be estimated. It is found that the energy release rate decreases with time for a fixed displacement.

Development of creep resistant die cast Mg–Sn–Al–Si alloy

A study has been made on the tensile properties and high temperature properties of die cast Mg–Sn–Al–Si (TAS831) alloy. The microstructure of TAS831 alloy is characterized by the presence of thermally stable Mg 2Sn particles within matrix and along grain boundaries. It also contains a small volume fraction of thermally stable Mg 2Si particles. It has been shown that TAS831 alloy has better combinations of tensile properties at room and elevated temperatures than die cast AZ91 alloy. Creep properties of TAS831 alloy are also superior to those of AZ91 alloy. Analyses of creep behavior and load-relaxation behavior at elevated temperatures in the context of internal variable theory indicate that the presence of thermally stable dispersoids in TAS831 alloy increases the resistance to dislocation movement, thereby improving creep properties over those of AZ91 alloy.

Load Relaxation of Helical Extension Springs in Transient Thermal Environments

The load relaxation behavior of small Elgiloy helical extension springs has been evaluated by a combined experimental and modeling approach. Isothermal, continuous heating, and interrupted heating relaxation tests of a specific spring design were conducted. Spring constants also were measured and compared with predictions using common spring formulas. For the constant heating rate relaxation tests, it was found that the springs retained their strength to higher temperatures at higher heating rates. A model, which describes the relaxation behavior, was developed and calibrated with the isothermal load relaxation tests. The model incorporates both time-independent deformation mechanisms, such as thermal expansion and shear modulus changes, as well as time-dependent mechanisms such as primary and steady state creep. The model was shown to accurately predict the load relaxation behavior for the continuous heating tests, as well as for a complex stepwise heating thermal cycle. The model can be used to determine the relaxation behavior for any arbitrary thermal cycle. An extension of the model to other spring designs is discussed.

Mechanical properties of the scapholunate ligament correlate with bone mineral density measurements of the hand

The mechanical properties of the scapholunate ligament have been previously examined in small sample sizes, with ultimate load and occasionally stiffness reported. The present study examined 16 scapholunate ligaments in uniaxial extension at two rates and determined stiffness, ultimate load, and stress relaxation properties. Mean stiffness values of 66.4 ± 28.6 N/mm at an elongation rate of 50 mm/min and 94.5 ± 44.4 N/mm at an elongation rate of 100 mm/min were found. Relaxation behavior, determined by the percent load remaining after 100 s, was found to be 68.1 ± 12%. Mean ligament ultimate loads were 357 ± 110 N ( n=8). In eight specimens, failure occurred in bone. Positive correlations were observed between bone mineral density of the hand and ligament stiffness, ligament ultimate load, and bone ultimate load. No correlation was observed between bone mineral density and ligament load relaxation behavior. The results provide a comprehensive understanding of scapholunate ligament biomechanics and demonstrate a relationship between bone and ligament properties.

The viscoelastic bending stiffness of fiber-reinforced composite Ilizarov C-rings

In order to optimize the design of unidirectional fiber-reinforced composite (Ilizarov) C-rings, the viscoelastic load relaxation behavior was analyzed under a point load. Initially, the deflection and bending stiffness were calculated from the Castigliano theorem and the Euler–Bernoulli bending theory for the elastic solution. The viscoelastic relaxation and creep behavior were then derived from the elastic solution by using the correspondence theorem. Besides the orthotropic mechanical properties of the composite, the asymmetric mechanical properties due to different tensile and compressive properties were also considered. With the exception of the deviation, which was affected by a relatively large thickness ratio to the radius of the C-ring, the calculated relaxation showed good agreement with the experimental result.

High Temperature Load Relaxation Behavior of Superplastic Iron Aluminides

Superplastic deformation and high temperature load relaxation behavior of coarse-grained iron aluminides have been investigated in this study. Iron aluminides with compositions of Fe-28 at.% Al and Fe-28 at.% Al-5 at.% Cr were prepared and thermomechanically treated to obtain an average grain size of about 500 μm. A series of load relaxation tests was conducted at temperatures ranging from 600 to 850°C. The flow curves obtained from load relaxation tests were found to have a sigmoidal shape and to exhibit stress vs. strain rate data in a very wide strain rate range from 10 -7 /s to 10 -2 /s. Tensile tests have been conducted at 850°C and at various initial strain rates ranging from 3×10 -5 /s to 1×10 -2 /s to investigate the superplastic deformation behavior. Maximum elongation of ∼500 % was obtained at the initial strain rate of 3×10 -5 /s and the maximum strain rate sensitivity was found to be 0.68. Microstructural observation through the optical microscopy has been carried out on the deformed specimens and it has revealed the evidences for grain boundary migration and grain refinement to occur during deformation, suggesting the dynamic recrystallization mechanism as reported earlier. The activation energy was evaluated as 387 kJ/mol in the strain rate range from 10 -4 /s to 10 -2 /s, which is very close to that for creep deformation in Fe 3 Al alloys.

Effect of toluene diisocyanate index on morphology and physical properties of flexible slabstock polyurethane foams

The effect of toluene diisocyanate (TDI) index on the physical properties, structure, and morphology of flexible slabstock polyurethane foams was investigated. Foams based on a 2700 molecular weight triol, 6 pph water, and varying amounts of an 80/20 mixture of 2,4- and 2,6-TDIs were characterized using a number of physical property and morphological measurements. Extraction experiments using dimethyl formamide (DMF) showed that increasing the index increased the level of covalent crosslinking with perhaps a maximum being reached at an index ca. 100. Viscoelastic measurements also supported the claim of increased crosslinking with TDI index. The initial load in load relaxation experiments at 65% strain systematically increased with increasing TDI while the percent decay in a 3-h period decreased. Temperature and/or humidity “plasticized” the load relaxation behavior in all the foams studied, indicating that the hard segment domain physical “crosslinks” play a significant role in the properties of these materials. Interestingly, compression set measurements appeared to be independent of the index, likely due to some level of hard segment continuity, but the induced recovery of the compression set at elevated temperatures was indeed sensitive to the index. The amount of recovery systematically increased with increasing TDI index due to the more enhanced “recoverable” covalent network. Scanning electron microscopy (SEM) studies of the foams showed that the cellular structure was not significantly affected by the index. However, SEM also showed that the structure of the high index foam was not greatly altered by the extraction process while the lowest index foam's cellular structure was severely disfigured. The fine structure of the foams was found to be influenced by the TDI index. Small angle X-ray scattering, differential scanning calorimetry, and dynamic mechanical analysis all provided evidence that an increase in the TDI index promoted some phase mixing of the soft and hard segments. FTIR showed that the short-range ordering within the hard segment domains displayed a maximum at an index of 100. This was attributed to the concentration of hard segment domains being lower at a lower index and their ordering being disrupted at higher indexes due to more extensive covalent crosslinking prior to completion of phase separation. Wide angle X-ray scattering results also confirmed that for the highest index level, the short-range ordering of the TDI moieties was decreased. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2395–2408, 1997

Mechanical properties of Al-Li-Cu icosahedral quasicrystals

Poly-quasicrystalline samples of an icosahedral Al-Li-Cu alloy have been deformed in compression at a constant engineering strain rate, at temperatures ranging from 300 to 773 K ( T m ≅ 896 K). The samples are very brittle below about 500 K but rather ductile above this temperature, with an upper yield stress decreasing linearly with increasing temperature, from 225 MPa at about 473 K to 22 MPa at 773 K. Load relaxation behavior and activation parameter values suggest that, in this material, plastic deformation at high-temperature is controlled by a thermally activated process that involves atomic diffusion.

Influence of diethanolamine on hard segment ordering in flexible polyurethane foams

In this study the microphase separation and structural organization of the hard segment domains were studied as a function of diethanolamine (DEOA) content for two series of model foams based on toluene diisocyanate (TDI) and glycerol initiated ethylene-oxide-capped propylene-oxide or glycerol initiated propylene-oxide. It was found that as the DEOA content was increased, the more disordered the hard segment domains became. Extractions in dimethyl formamide (DMF) showed that increasing the DEOA content increased the gel fraction, confirming that DEOA further crosslinked the system. More importantly, it was also found that the hard segment domains were also influenced by the DEOA content where the foams lacking DEOA had a much higher level of short range ordering. This was first confirmed by wide-angle x-ray scattering (WAXS) patterns where the amorphous character was much more pronounced as the DEOA content was increased. In addition, Fourier transform infrared (FTIR) data from the carbonyl region showed that the level of bidentate urea or strong hydrogen bonding also decreased as the DEOA content was increased, confirming that the short range ordering does decrease with increased DEOA content. This observation is believed to help explain the greater ease of “plasticization” by temperature and moisture observed of foams that included DEOA in their formulation. This was exemplified by comparing the load relaxation behavior at a low temperature (35°C) and a high temperature (100°C) between two foams varying in DEOA content. At ambient conditions (40°C, 35%RH), the foam with DEOA exhibited slightly higher loads at any given loading time, while at elevated conditions (100°C, 98%RH), there was very little difference between the two foams. Although the observed load relaxation behavior was mostly a result of the increased crosslinking, the increased closed cell content induced by the greater crosslinking also contributed to the load values. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:525–537, 1997

Influence of diethanolamine on hard segment ordering in flexible polyurethane foams

In this study the microphase separation and structural organization of the hard segment domains were studied as a function of diethanolamine (DEOA) content for two series of model foams based on toluene diisocyanate (TDI) and glycerol initiated ethylene-oxide-capped propylene-oxide or glycerol initiated propylene-oxide. It was found that as the DEOA content was increased, the more disordered the hard segment domains became. Extractions in dimethyl formamide (DMF) showed that increasing the DEOA content increased the gel fraction, confirming that DEOA further crosslinked the system. More importantly, it was also found that the hard segment domains were also influenced by the DEOA content where the foams lacking DEOA had a much higher level of short range ordering. This was first confirmed by wide-angle x-ray scattering (WAXS) patterns where the amorphous character was much more pronounced as the DEOA content was increased. In addition, Fourier transform infrared (FTIR) data from the carbonyl region showed that the level of bidentate urea or strong hydrogen bonding also decreased as the DEOA content was increased, confirming that the short range ordering does decrease with increased DEOA content. This observation is believed to help explain the greater ease of “plasticization” by temperature and moisture observed of foams that included DEOA in their formulation. This was exemplified by comparing the load relaxation behavior at a low temperature (35°C) and a high temperature (100°C) between two foams varying in DEOA content. At ambient conditions (40°C, 35%RH), the foam with DEOA exhibited slightly higher loads at any given loading time, while at elevated conditions (100°C, 98%RH), there was very little difference between the two foams. Although the observed load relaxation behavior was mostly a result of the increased crosslinking, the increased closed cell content induced by the greater crosslinking also contributed to the load values. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:525–537, 1997