Load Relaxation Data Research Articles

A new long-term indentation relaxation method to measure creep properties at the micro-scale with application to fused silica and PMMA

A new procedure is developed to measure strain rate sensitivity m and apparent activation volume V* using nanoindentation relaxation experiments. This procedure is based on the control of the dynamic contact stiffness as measured by the continuous stiffness measurement module (CSM). Load decrease is monitored while maintaining the contact stiffness constant. It allows for very stable measurements of load relaxation (up to 10 h) since the contact area between the tip and the sample surface is kept constant. This improvement is significant as it increases by two orders of magnitude the measurement time compared to classical constant displacement experiments. The load relaxation data are interpreted using an analogy to uniaxial tests in order to extract representative material’s parameters. An excellent agreement with literature data is found for fused silica and PMMA. Nanoindentation relaxation experiment is proved to be an accurate procedure to extract viscoplastic parameters of materials under very low strain rates.

Glass Fiber-Reinforced PTFE Gasketed-Joint Under a Retorque

Joints gasketed with viscoelastic seals often receive an application of a secondary torque, i.e., retorque, in order to ensure joint tightness and proper sealing. The importance of understanding gasketed joint behavior under various loading conditions and test parameters is paramount to a successful design. The motivation of this study is to characterize and analytically model the initial and retorque load relaxation response of a single 25% glass-fiber reinforced polytetrafluorethylene (PTFE) gasket-bolted joint with serrated flange detail by a single set of experimentally determined modeling constants. The Burger-type viscoelastic modeling constants of the material are obtained through optimization from a baseline load relaxation data and compared to a variety of test cases for both initial and reloadings. Determination of a retorque parameter, α, allowing modeling constants identified from an initial loading to predict the retorque relaxation showed the retarded elasticity or K2 term to be most influential in predicting retorque response. Finally, the validity of the viscoelastic model with the retorque parameter is shown to reasonably predict retorque relaxation responses of all test cases investigated.

Development of ocular viscosity characterization method.

Glaucoma is the second leading cause for blindness. Irreversible and progressive optic nerve damage results when the intraocular pressure (IOP) exceeds 21 mmHg. The elevated IOP is attributed to blocked fluid drainage from the eye. Methods to measure the IOP are widely available, but methods to measure the viscous response to blocked drainage has yet been developed. An indentation method to characterize the ocular flow is developed in this study. Analysis of the load-relaxation data from indentation tests on drainage-controlled porcine eyes showed that the blocked drainage is correlated with increases in ocular viscosity. Successful correlation of the ocular viscosity with drainage suggests that ocular viscosity maybe further developed as a new diagnostic parameter for assessment of normal tension glaucoma where nerve damage occurs without noticeable IOP elevation; and as a diagnostic parameter complimentary to conventional IOP in conventional diagnosis.

Spherical indentation load-relaxation of soft biological tissues

Elastic-viscoelastic correspondence was used to generate displacement–time solutions for spherical indentation testing of soft biological materials with time-dependent mechanical behavior. Boltzmann hereditary integral operators were used to determine solutions for indentation load-relaxation following a constant displacement rate ramp. A “ramp correction factor” approach was used for routine analysis of experimental load-relaxation data. Experimental load-relaxation tests were performed on rubber, as well as kidney tissue and costal cartilage, two hydrated soft biological tissues with vastly different mechanical responses. The experimental data were fit to the spherical indentation ramp-relaxation solutions to obtain values of short- and long-time shear modulus and of material time constants. The method is used to demonstrate linearly viscoelastic responses in rubber, level-independent indentation results for costal cartilage, and age-independent indentation results for kidney parenchymal tissue.

Similarity and scaling in creep and load relaxation of single‐crystal halite (NaCl)

This work explores the physical basis for Hart's mechanical equation of state in high‐temperature plasticity. The experiments seek to identify a possible microstructural basis for the “hardness” parameters associated with load relaxation curves. The experiments also seek to examine the microstructural basis for scaling in load relaxation data and to explore the relationship between creep and load relaxation. Constant stress creep and load relaxation tests were conducted on [100] oriented single crystals of halite at 700°C and stresses between 0.6 and 3 MPa. Load relaxation tests were performed at 400°C up to a stress level of 13 MPa. After testing, specimens were sectioned, and dislocation densities and subgrain size distributions were measured. Results at 700°C reveal that distributions of subgrain size in crystals crept at different stress levels are similar to each other; that is, they have the same shape but different average subgrain sizes depending on stress level. Hardness curves obtained from load relaxation experiments at different levels of work hardening were found to correspond to different average subgrain size. Load relaxation data from 700°C and 400°C belong to a single‐parameter family of curves, with hardness curves translating onto each other with a scaling slope m = 0.33 ± 0.05. Subgrain size distributions generated in creep are statistically identical to those from load relaxation. The hardness parameter, σ*, specified as the (apparent) high‐strain rate limit of stress in the load relaxation data, is approximately 50Gb/DI, where G is the shear modulus, b is the Burgers vector, and DI is the mean intercept subgrain diameter. During creep under constant stress the subgrain size evolves until a steady value is approached. The experimental data lend credence to Hart's interpretation that load relaxation data represent (nearly) constant “structure” with subgrain size playing the role of the structural variable.

Running exercises improve the strength of a partially ruptured Achilles tendon

Objectives: To examine the effects of running and swimming exercises on the functional performance and mechanical strength of a recovering Achilles tendon. Methods: 30 Sprague-Dawley rats had surgical transection of...

Comparison of single and multiple applications of GaAlAs laser on rat medial collateral ligament repair.

To examine single versus multiple applications of the gallium aluminum arsenide (GaAlAs) laser on the healing of surgically injured medial collateral ligaments (MCLs) in rats. Sixteen rats were studied, with 12 receiving surgical transection to their right MCL and 4 receiving a sham injury. Group 1 (n = 4) received a single dose of GaAlAs laser therapy (wavelength 660 nm, average power 8.8 mW, pulse 10 kHz, dosage 31.6 J/cm(2)) directly to their MCL during surgery. Group 2 (n = 4) received 9 doses of GaAlAs laser therapy applied transcutaneously on alternate days (wavelength 660 nm, average power 8.8 mW, pulse 10 kHz, dosage 3.5 J/cm(2)). The controls (Group 3, n = 4) received one session of placebo laser at the time of surgery, with the laser equipment shut down, while the sham injured Group 4 (n = 4) received no treatment. Biomechanical tests for structural stiffness, ultimate tensile strength (UTS), and load-relaxation were done at 3 weeks after injury. The stiffness and UTS data were normalized by expressing as a percentage of the left side of each animal before statistical analysis. The load-relaxation data did not show any differences between the groups (P = 0.18). The normalized stiffness levels of Groups 2 (81.08+/-11.28%) and 4 (92.66+/-13.19%) were significantly higher (P = 0.025) than that of the control Group 3 (58.99+/-15.91%). The normalized UTS of Groups 2 (81.38+/-5.68%) and 4 (90.18+/-8.82%) were also significantly higher (P = 0.012) than that of the control (64.49+/-9.26%). Although, Group 1 had higher mean stiffness and UTS values than the control, no statistically significant difference was found between these two groups. Multiple laser therapy improves the normalized strength and stiffness of repairing rat MCLs at 3 weeks after injury. The multiple treatments seem to be superior to a single treatment when the cumulative dosages are comparable between the two modes of application.

Fracture toughness and load relaxation of dentin bonding resin systems

Objectives. The fracture toughness (K IC) and load relaxation of four dentin bonding resins were determined to characterize some of the mechanical properties of these materials after polymerization. Methods. A total of 40 single-edge bar specimens were fabricated, 10 each of four commercially available brands, and subjected to three-point bending until fracture, as described in ASTM Standard E399-83 (1991a). The critical stress intensification factor, K IC, was derived for each specimen and compared by analysis of variance and Scheffé's multiple comparisons test (p < 0.01. To study the load relaxation characteristics, five rectangular specimens (without notches) of each brand were subjected to three-point loading until a predetermined limiting load value was reached.The test load was allowed to relax for 4 min, after which the specimen was unloaded to the zero load condition, and the load was allowed to build up on its own accord for 3 min. Load relaxation values were measured from the chart, and the mean percent load drop was calculated. The load relaxation data were compared using analysis of variance and Scheffé's multiple comparisons test (p < 0.05). Results. The fracture toughness (K IC) values of the four adhesive resins studied in this investigation ranged from 0.37 – 0.94 MPa.m 0.5 and were statistically different from each other (p < 0.001). The load relaxation values were found to be greatest within the first 0.5 min, with the total load relaxation of the four bonding agents ranging from 16% – 30%. Two of the materials studied showed significantly different short-term load relaxation behavior than the other two resins (p < 0.05). Significance. Bonding agents can be implicated as one of the factors that weaken the interface between the dentin and the composite restorative material. These materials are capable of a rapid short-term response, demonstrating significant load relaxation in the first 0.5 min after loading.

Effect of Time-Dependent Matrix Behavior on the Evolution of Processing-Induced Residual Stresses in Metal Matrix Composites

The usefulness of load-relaxation experiments in investigating the need for rate-dependent matrix material models for predicting the processing-induced residual stresses in metal matrix composites (MMCs) is examined in this study. The experimental data is used in the selection of the appropriate constitutive equations for modeling the cooling process after composite consolidation. Boron/Al-2024 MMC is chosen as the model material system. Bodner-Partom unified model and a classical creep-plasticity model based on the relaxation experiments are used to represent the rate-dependent behavior of Al-2024. The results show that conventional B-P model based on purely monotonic strain-rate tests is not able to capture the inelastic strain variations under practical cooling conditions. The classical creep-plasticity model has been shown to be an effective alternative for this purpose as it considers load-relaxation data while deriving the material constants for the constitutive equations.

Indentation load relaxation experiments with indentation depth in the submicron range

Indentation load relaxation (ILR) experiments with indentation depths in the submicron range are described. Under appropriate conditions, the ILR data are found to yield flow curves of the same shape as those based on conventional load relaxation data. Variations in flow properties as a function of depth in submicron metal films deposited on a hard substrate are detected by the experiments described.

The high strain rate superplastic deformation mechanisms of mechanically alloyed aluminum IN90211

The tensile behavior of mechanically alloyed (dispersion strengthened) IN90211 was investigated at strain rates between 0.0001 and 340 s −1 at temperatures between 425 and 475 °C. At strain rates above 0.1 s −1, superplastic elongations were obtained (maximum elongation of 525% at 475 °C and 2.5 s −1). Superplastic elongations were found to result from grain boundary sliding. The data were analyzed assuming that a threshold stress resists dislocation motion. The threshold stresses were obtained assuming n = 2 (grain boundary sliding) or n = 3 (solute drag) for the stress exponent. Both assumptions provided equally credible values for a temperature-dependent threshold stress between 1% and 20% of the Orowan looping stress. The n = 3 threshold stresses agreed with load relaxation data, but the n = 2 values corresponded to the lower limit of the superplastic deformation regime, as indicated by creep tests at lower strain rates. Based upon the threshold stress theories in the literature, the threshold stresses are suggested to arise from a combination of local and general climb of lattice dislocations over particles. Consideration of the activation energies, details of flow behavior, and stress relaxation experiments provided strong evidence for the n = 3 solute drag mechanism to be the rate-limiting process in the superplastic deformation regime.

A comparison of load relaxation, tensile, and creep data of AlMg alloys in the range of alloy-class creep behavior

A comparison of load relaxation, tensile, and creep data of AlMg alloys in the range of alloy-class creep behavior

Load relaxation of cold-worked and stress-relieved Zircaloy-4 near 673 K

Abstract Load-relaxation data obtained for both cold-worked and stress-relieved Zircaloy-4 are presented. The stress-relaxation curves, plotted as the logarithm of the applied stress against time, are interpreted in terms of an anelastic plastic constitutive equation, whose plastic component involves jog-drag, cell-formation and dynamic strain-ageing. Finally the parameters obtained by fitting the experimental curves to the model, the influence of dynamic strain-ageing and the possible mechanism involved in the anelastic contribution are analysed in detail.

Deformation Properties of a Lead-Tin (95–5) Solder

AbstractConsecutive load relaxation tests are performed on a 95 Pb-5 Sn specimen near room temperature, then at 69 ° C and 1°C. The initial relaxation runs at room temperature show evidence of static recovery, which diminishes after a number of load relaxations have been performed. Evidence of grain boundary sliding is also found. The specimen hardens slowly at 69°C, as manifested by successive load relaxation data falling at lower strain rates. At 1°C, the material slowly softens. These temperature effects are believed to result from a dislocation-solute interaction involving tin atoms in the lead matrix.

Load-relaxation testing at elevated strain rates

The feasibility of a new mechanical testing technique is described. This technique allows the collection of load-relaxation type data at higher strain rates than is possible in a conventional load-relaxation test. This technique was used on commercial purity aluminium and 304 stainless steel at strain rates up to 5 sec−1. Good agreement with Hart's state variable model for plastic deformation was found.

Load relaxation of stainless steel Type AISI 304 near 563 K

Load relaxation data reported by Anciaux, near 563 K, on mill-annealed Type AlSl 304 stainless steel are interpreted in terms of a general expression which describes stress relaxation as thermally activated dislocation motion over precipitates. Several physical parameters are obtained from the stress-strain rate curves and it is shown that for some stress relaxation curves the internal stress does not remain constant during the relaxation.

On the analysis of stress relaxation data

The various equations, used in the literature to describe the stress relaxation behaviour of metals and alloys, are shown in normalized stress versus strain rate plots, in such a way that the predictions of the different theoretical models can be compared directly. Finally, it is shown how to fit the experimental stress relaxation curves to a particular theoretical model and an example is given for load relaxation data taken in Zry-4 at 673 K.

Load relaxation studies of zircaloy-4

Tensile and load relaxation tests as a function of temperature were performed on zircaloy-4 tubing with applied stress in the direction of the tube axis. The phenomenological model, based on Hart's plastic equation of state, was capable of describing a portion of the load relaxation data. The effects of strain aging and deformation twinning are discussed, based on the same data.

Load relaxation in aluminum: I. Theory of plastic deformation. II. Plastic equation of state

According to the theory of thermally-activated deformation, the plastic strain rate equality\((\dot \in _p )_t = _{ - dt} = (\dot \in _p )_t = _{dt} \) will hold in a load relaxation experiment, wheret = 0 is de-fined as the time at which the crosshead stops. In this theory, plastic flow is intrinsically time dependent and its rate is controlled by interaction of glide dislocations with thermal obstacles(e.g. forest dislocations). The strain rate equation is of the form\(\dot \in _p = \dot \in _p (\sigma ,S,T)\) and att = 0 none of these variables changes instantaneously. Measurements reported here for [111] aluminum single crystals indicate that this prediction is wrong. The ratio\((\dot \in _p )_t = _{dt} /(\dot \in _p )_t = _{ - dt} \) is near zero at low stress and approaches unity only at high stress. This result is predicted if plastic strain itself is time-independent (athermal), as in the author’s recent theory. Time-dependent strain is then the result of thermal changes in structure, namely loss (recovery) and rearrangement of obstacle dislocations. Experi-ments were also done to test further the essential hypothesis of Hart’s recent formula-tion of an equation of state for plastic deformation-namely that each distinct σ-@#@ \(\sigma - \dot \in \) curve derived from load relaxation data corresponds to a unique “hardness” state and that re-covery does not occur. Significant differences were observed in the 77 K strsss-strain curves for 295 K relaxed and unrelaxed samples which indicate that substantial loss and some rearrangement of dislocations has occurred during the relaxation. It is concluded from both experiments that load relaxation in aluminum is a manifestation of recovery creep and cannot be taken as evidence for a plastic equation of state.

Comparison of load relaxation data of type 316 austenitic stainless steel with Hart's deformation model

Load relaxation experiments have been performed on Type 316 stainless steel at temperatures up to 650°C. The resulting stress-plastic strain rate relationships below 500°C show the behavior of a plastic equation of state. The experimental results suggest that at lower temperatures the deformation is controlled by dislocation glide and at higher temperatures dislocation glide becomes less important. Hart's phenomenological model based on a plastic equation of state is used to account for the experimental data.