Load Relaxation Tests Research Articles

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.

High Temperature Deformation Behavior of a Ti-45.5Al-2Cr-4Nb-0.4B Intermetallic Compound

High temperature deformation behavior of a Ti-Al intermetallic compound has been investigated in this study. Specimens with a near gamma and a lamella structure were obtained by performing heat treatment at temperatures from 1200 to 1330°C for 24 hr, respectively, and stabilized at 900°C for 4 hr followed by air cooling. A series of load relaxation tests has been conducted on these samples at temperatures ranging from 850 to 950°C to construct flow curves in the strain rate range from 10-6/s to 10-3/s. Strain hardening was observed even at the temperature of 950°C in both the near gamma and the lamella structures. Further aging treatment for 12 hr at the test temperatures has been found to cause negligible softening in both microstructures, providing the strong applicability of this alloy system in the temperature range.

Anomalous temperature dependence and tension–compression asymmetry of Fe 3Al intermetallics—an internal variable approach

Tension/compression asymmetry in the high-temperature flow and anomalous yield/flow stress of Fe–28 at% Al–5 at% Cr alloy has been investigated. Flow curves were obtained from a series of tensile and compressive load relaxation tests. Constitutive relations at each deformation conditions were formulated using the internal variable theory based on dislocation dynamics. In this study, high-temperature flow stress of selected material was to be safely described as the sum of internal stress and frictional stress. The anomaly and peak temperature of yield strength seem to be controlled by the anomaly of internal stress and the relative portion of internal and frictional stress in total flow stress, respectively. Asymmetric flow stress was observed in analogy with yield strength. In terms of constitutive parameters, critical stress for frictional flow ( Σ 0) exhibited considerably higher value in compression, which physically postulates increased critical resolved shear stress (CRSS). Considering the crystallography and microstructure, it is presumably due to the ‘extrinsic’ effect of non-deviatoric stress tensor, as proposed in other studies.

High-temperature deformation behavior of a gamma TiAl alloy—Microstructural evolution and mechanisms

The present investigation was carried out in the context of the internal-variable theory of inelastic deformation and the dynamic-materials model (DMM), to shed light on the high-temperature deformation mechanisms in TiAl. A series of load-relaxation tests and tensile tests were conducted on a fine-grained duplex gamma TiAl alloy at temperatures ranging from 800 °C to 1050 °C. Results of the load-relaxation tests, in which the deformation took place at an infinitesimal level (e ≅ 0.05), showed that the deformation behavior of the alloy was well described by the sum of dislocation-glide and dislocation-climb processes. To investigate the deformation behavior of the fine-grained duplex gamma TiAl alloy at a finite strain level, processing maps were constructed on the basis of a DMM. For this purpose, compression tests were carried out at temperatures ranging from 800 °C to 1250 °C using strain rates ranging from 10 to 10−4/s. Two domains were identified and characterized in the processing maps obtained at finite strain levels (0.2 and 0.6). One domain was found in the region of 980 °C and 10−3/s with a peak efficiency (maximum efficiency of power dissipation) of 48 pct and was identified as a domain of dynamic recrystallization (DRx) from microstructural observations. Another domain with a peak efficiency of 64 pct was located in the region of 1250 °C and 10−4/s and was considered to be a domain of superplasticity.

Constitutive analysis of the high-temperature deformation of Ti–6Al–4V with a transformed microstructure

The high-temperature deformation mechanisms of Ti–6Al–4V with a transformed microstructure were determined within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on samples with a lamellar structure containing different alpha-platelet thicknesses at temperatures of 715–900 °C. The flow stress-versus-strain rate curves for all the microstructures were well fit with an inelastic-deformation equation describing grain-matrix deformation (GMD) (dislocation glide+dislocation climb). However, for heavily pre-deformed specimens, grain-boundary sliding (GBS) as well as GMD was evident. The GBS rate was found to be most rapid for the microstructure with the thinnest alpha laths/platelets. Softening of heavily deformed material was attributed to a decrease in the internal-strength variable σ ∗ associated with reduced alpha–beta interface strength and to the occurrence of GBS.

동적재료모델 및 연화모델을 응용한 SAF 2507 강의 열간단조 유한요소해석

High temperature deformation and softening behavior of SAF 2507 super duplex stainless steel (SDSS) has been investigated in connection with an FEM analysis of hot forging process. Flow curves at various strain rates and temperatures were determined first from compression tests, and the kinetics of dynamic recrystallization were also formulated through the analysis of load relaxation test results. Using the dynamic materials theory proposed by Prasad, the deformation behavior was effectively determined for various conditions. Constitutive relations and recrystallization kinetics formulated from the test results were then implemented in a commercial FEM code. The forming load as well as the distribution of recrystallized volume fraction after forging was successfully predicted by means of the flow stress compensation formulated upon the volume fraction of recrystallization and adiabatic heating.

Viscoelastic Behaviour of Wood Fibres during the Hot Pressing of Medium Density Fibreboard

Summary Sufficient stress relaxation within a medium density fibreboard (MDF) mat during hot pressing is critical for producing a panel with acceptable mechanical properties and dimensional stability. This paper describes a series of load relaxation tests of wood fibres conducted at temperatures and moisture contents similar to those within an MDF mat during the hot pressing process (temperatures ranging from 80°C to 100°C with relative humidity levels ranging from 9% to 68%). The results clearly show an increase in relaxation when either the temperature or relative humidity is increased. The relaxation behaviour of MDF is also measured and the trends observed are comparable to those of the wood fibres. Taguchi experimental design is employed to analyse the effects of temperature, moisture content and initial stress on the viscoelastic behaviour of MDF.

Temperature and grain size dependence of superplasticity in a Zn−0.3wt.%Al alloy

The superplastic deformation behavior of quasi-single phase Zn-0.3 wt. %Al was investigated. A series of load relaxation and tensile tests was conducted at various temperatures ranging from RT (20 °C) to 200 °C. The recently proposed internal variable theory of structural superplasticity was applied. The flow curves obtained from load relaxation tests were shown to consist of contributions from interface sliding (IS) and accommodating plastic deformation. In the case of quasi-single phase Zn-0.3 wt.% Al alloy with an average agrain size of 1 μm, the IS behavior could be described as a viscous flow process characterized by a power index of Mg=0.5. A large elongation of about 1400% was obtained at room temperature and the strain rate sensitivity parameter was about 0.4. Although relatively large-grained (10 μm) single phase alloy showed a high value of strain rate sensitivity comparable to that of fine-grained alloy at very low strain rate range, IS was not expected from the analysis based on the internal variable theory of structural superplasticity at room temperature. As the temperature increased above 100 °C, however, the contribution from IS was observed at a very low strain rate range. A high elongation of ∼400% was obtained in a specimen of 10-μm-grain-size at 200 °C under a strain rate of 2×10−4/sec.

Internal variable approach to microstructural change in 7475 aluminium alloy during superplastic deformation

Flow behaviour and microstructural changes in fine grained 7475 Al during superplastic deformation have been investigated. A series of mechanical tests and transmission electron micrography has been conducted at various temperatures ranging from 430 to 516°C. Quantitative constitutive parameters have been determined from load relaxation tests by applying the internal variable theory of structural superplasticity proposed recently. The validity of the mechanical analysis is proved by the evolution of the microstructure (precipitate free zones PFZs) during the accommodation process. Additional information about the mechanism of PFZ formation was determined by microchemical analysis.

Fabrication of spray-formed hypereutectic Al–25Si alloy and its deformation behavior

This article reports the fabrication of hypereutectic Al–25Si alloy, which is expected to be applied to the cylinder liner part of the engine block of an automobile due to its excellent wear resistance, low density and low thermal expansion coefficient, through a spray casting process and the characterization of the microstructural and the mechanical properties of this alloy. The OSPREY process used in this study is one of spray forming techniques, which can produce semi-finished products such as billet, tube, and plate with rapid solidification (RS) structure and density in a single operation from molten metal. The obtained microstructure of the hypereutectic Al–25Si alloy appeared to consist of Al matrix and equiaxed Si particles with average diameter of 5–7 μm. To characterize the deformation behavior of this alloy, a series of load relaxation and compression tests have been conducted at temperatures ranging from RT to 500 °C. The strain rate sensitivity parameter ( m) of this alloy has been found to be very low (≤0.1) below 300 °C and reached maximum value of about 0.2 at 500 °C. During the deformation above 300 °C in compression, remarkable strain softening has been observed. The extrusion has been successfully conducted at the temperatures of 300 °C and above with the ratio of area reduction of 28 and 40 in this study.

Gamma TiAI 합금의 고온 변형거동 - 미세조직의 변화 및 변형기구

A series of load-relaxation tests and tensile tests were conducted to study the high temperature deformation mechanism of fine duplex gamma TiAl alloy at temperatures ranging from 800 to 105<TEX>$0^{\circ}C$</TEX>. Results of load relaxation test showed that deformation behavior at a small imposed strain (<TEX>$\varepsilon$</TEX>≒0.05) was dominated by dislocation glide and dislocation climb. To investigate the deformation behavior at a large amount of strain, the processing map was constructed using a dynamic materials model. Two domains were characterized in the processing map obtained at a strain level of 0.6. One domain was found at the region of 98<TEX>$0^{\circ}C$</TEX> and <TEX>$10^{-3}/sec$</TEX> with a peak efficiency of 48%, which was identified as a domain of dynamic recrystallization from the microstructural observation. The order was observed at the region of 125<TEX>$0^{\circ}C$</TEX> and <TEX>$10^{-4}/sec$</TEX> with a peak efficiency of 64%. The strain rate sensitivity measured indicates that the material was deformed by the superplasticity in the region.

High temperature deformation behavior of a γ TiAl alloy determined using the load-relaxation test

The high temperature deformation behavior of a two-phase γ TiAl alloy was investigated as a function of temperature and γ/α2 volume fraction. For this purpose, load-relaxation and tension tests were conducted at temperatures ranging from 800 to 1050°C. When a small amount of deformation was imposed (ε≈0.04), the experimental stress-versus-strain-rate data for the fine-grained TiAl alloy were well fit by predictions based on grain-matrix deformation and dislocation climb derived from the inelastic deformation theory. Evidence of grain-boundary sliding was not observed in this instance. However, after a large amount of pre-deformation (ε≈0.8), the constitutive behavior changed significantly, indicating that grain-boundary sliding was also operating, an effect attributable to the occurrence of dynamic recrystallization during the deformation. With an increase in the volume fraction of the α2 phase, the flow stress for grain-matrix deformation was not varied, but that of dislocation climb decreased significantly. It was also found that the γ/α2 boundaries were more susceptible to cavity initiation than the γ/γ boundaries.

2 상 Ti3Al-xNb 계 금속간 화합물들의 초소성 특성에 미치는 상분율의 영향

A study has been made to investigate the boundary sliding and its accommodation mode with respect to the variation of <TEX>$\alpha$</TEX><TEX>$_2$</TEX>/<TEX>$ eta$</TEX> volume fraction during superplastic deformation of two-phase Ti<TEX>$_3$</TEX>Al-xNb intermetallics. Step strain rate and load relaxation tests have been performed at 950, 970 and 99<TEX>$0^{\circ}C$</TEX> to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic deformation. The results show that the grain matrix deformation and boundary sliding of the three intermetallics containing 21, 50 and 77% in <TEX>$ eta$</TEX> volume fractions are well described by the plastic deformation and viscous flow equations. Due to the equal accommodation of both <TEX>$a^2$</TEX> and <TEX>$\beta$</TEX> phases, the accommodation modes for fine-grained materials are in good agreement with the iso-strain rate models. The sliding resistance analyzed for the different boundaries is the lowest in the <TEX>$\alpha$</TEX><TEX>$_2$</TEX>/<TEX>$\alpha$</TEX><TEX>$_2$</TEX> boundary, and increases in the order of <TEX>$\alpha$</TEX><TEX>$_2$</TEX>/<TEX>$\alpha$</TEX><TEX>$_2$</TEX><< <TEX>$\alpha$</TEX><TEX>$_2$</TEX>/<TEX>$\beta$</TEX> = <TEX>$\beta$</TEX>/<TEX>$\beta$</TEX>, which plays an important role in controlling the superplasticity of the alloys with the various <TEX>$\alpha$</TEX><TEX>$_2$</TEX>/<TEX>$\beta$</TEX> phase ratio.

Durability study of a polymeric composite material for structural applications

AbstractAn experimental study was carried out to investigate the mechanical behavior of a structural carbon based composite for infrastructure industry. Creep and load relaxation experiments were conducted to investigate the rate sensitivity behavior for a temperature range of 20–60°C. The results show that the mechanical properties were degraded under elevated temperature conditions. A threshold stress was measured, at any given temperature, below which no observable strain rate is detected. Results show that load relaxation experiment can be used as an effective tool to study durability and long‐time creep behavior. The load relaxation test methodology for the prediction of model parameters was found to be more time and cost efficient than traditional long‐time creep tests.

A unified model for inelastic deformation of polycrystalline materials — application to transient behavior in cyclic loading and relaxation

Abstract Hart proposed a unified state variable model which is capable of predicting the nonelastic deformation for low and high temperatures over a very large strain rate range. He developed these relations from experimental evidence and a heuristic consideration of micro-mechanism of deformation in polycrystalline materials. Some of the important parameters can be uniquely determined using load relaxation tests in the fully loaded regime. The model was based on the realization that the dislocation motion is limited by the dislocation glide friction and by the resistance to strong barriers to dislocation motion represented by the hardness parameter, σ ∗ . A transient model based on the non-steady state condition for the frictional glide process is introduced here using a microhardness parameter as a new state variable. The microhardness parameter represents the strength or the average lifetime of the mobile dislocations relative to the frictional glide viscous drag process. The evolution of the microhardness parameter is very similar to that proposed for the hardness parameter except that the recovery plays a major role. The power law relationship for the frictional glide process is shown to be a steady state condition for the transient behavior in this model. The results show that the model can predict the transient behavior for both cyclic loading and the reloading phenomena during inelastic deformation and load relaxation.

Relaxation behavior and modeling

Abstract Load relaxation tests deliver several orders of magnitude of inelastic strain rate data while elastic strains are converted into inelastic strains [see Lemaitre and Chaboche, 1994. (Mechanics of Solid Materials, Oxford University Press, Cambridge p. 264)]. Hart used this test for providing information on the inelastic deformation behavior for modeling purposes. Characteristic relaxation curves were obtained with ductile metals and alloys at room and high temperature showing a scaling relation derived from Hart's theory. Subsequent testing with servo-controlled testing machines and strain measurement on the gage length showed that an increase of prior strain rate also increased the average relaxation rate. For relaxation tests starting in the flow stress region, the relaxation curves can be independent of the stress and strain at the start of the relaxation tests. For the modeling of these newly found relaxation behaviors and other phenomena the viscoplasticity theory based on overstress (VBO) has been introduced. It is shown that VBO admits a long-term (asymptotic) solution that can be used with sufficient accuracy for the flow stress region of the stress–strain diagram. The long-term solution predicts the observed relaxation behaviors and that the relaxation curves coincide when shifted along the stress axis. This behavior is observed for the recently obtained data and is confirmed by two sets of the Hart-type data when they are plotted according to the new method.

Effects of microstructural evolution on superplastic deformation characteristics of a rapidly solidified Al-Li alloy

This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10−2/s to 10−1/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy.

Superplastic deformation of a fine-grained Zn–0.3wt.%Al alloy at room temperature

The aims of this study are firstly to obtain a fine and stable microstructure in a quasi-single phase Zn–Al alloy and secondly to characterize the superplastic deformation behavior of this alloy performing load relaxation and tensile tests, as well as a microstructural study using a scanning and a transmission electron microscopy. A very fine and stable microstructure with an average grain size of about 1 μm was produced by modifying the conventional thermomechanical treatment process for commercial Al alloys to obtain a remarkable elongation of 1400% at room temperature, which is the largest ever reported for this alloy. The flow curves of log σ versus log ε ̇ constructed from load relaxation test and a series of tensile tests conducted under various initial strain rates appeared to have a sigmoidal shape typical for superplastic materials. Even after the specimen was elongated up to 100% at room temperature, no evidence for grain growth was noticed. From the observations of microstructural changes and the scratch offsets on the surface of a deformed specimen, the dominant deformation mechanism could be identified as the grain boundary sliding (GBS), possibly accommodated by plastic flow due to dislocation glide near grain boundaries. At a still higher strain rate, the accommodation mechanism of GBS appears to change from dislocation activity near grain boundaries to deformation twinning at stress concentrations such as triple junctions and grain boundary ledges. When the grain size was increased by an order, predominant deformation mechanism appeared to be dislocation activity evidenced by the cell structure reducing the tensile elongation substantially to around 100%.

Effects of Strain Accumulation on the Superplastic Deformation Behavior of 7075 AL Alloy

The superplastic deformation behavior of a fine-grained 7075 Al alloy has been investigated within the framework of an internal variable theory for inelastic deformation. The theory takes the dislocation glide process within and across the grain boundaries (grain matrix deformation (GMD)) as the major accommodation mechanism for the grain boundary sliding (GBS). The flow curves were obtained by performing a series of load relaxation tests at the various prestrain values to examine the effects of accumulated strain on the superplastic deformation behavior. The most significant result obtained in this study is that the grain boundary characteristics change gradually with the strain accumulation from an initially Newtonian viscous flow signified with the power index value of M g =1 to a non-Newtonian flow with the value of M g =0.5 commonly observed in the various microduplex alloys such as Ti-6Al-4V. The variation of GBS characteristics with the prestrain is then examined by observing the microstructural evolution with the strain through the use of transmission electron microscopy (TEM).

Effects of Temperature and Microstructure on the Superplasticity in Microduplex Pb-Sn Alloys

Superplastic deformation behavior of microduplex Pb-Sn alloys has been investigated in this study. The effects of test temperature and microstructure were examined within the framework of the internal variable theory of structural superplasticity. A series of load relaxation and tensile tests were conducted at room temperature for the alloys with various microstructures such as as-cast eutectic, equiaxed eutectic, hypoeutectic and hypereutectic alloys. For eutectic alloys, load relaxation tests were carried out from room temperature to 140 °C. The flow curves obtained from load relaxation tests on the superplastic Pb-Sn alloys were shown to consist of the contributions from interface sliding (IS) and the accommodating plastic deformation. The IS behavior could be described as a viscous flow process characterized by the power index value of Mg = 0.5, suggesting the onset of intense phase boundary sliding (PBS). Superplasticity of hypoeutectic alloy was comparable to that of superplastic eutectic alloys and superior to that of hypereutectic alloy. As the test temperature increased, contribution from IS appeared to be exhibited in lower stress and faster strain rate region.