STRAIN RATE–DEPENDENT BEHAVIOR OF UNCURED RUBBER: EXPERIMENTAL INVESTIGATION AND CONSTITUTIVE MODELING

ABSTRACT The strain rate dependence of uncured rubber is investigated through a series of tensile tests (monotonic, multistep relaxation, cyclic creep tests) at different strain rates. In addition, loading/unloading tests in which the strain rate is varied every cycle are carried out to observe their dependence on the deformation history. A strain rate–dependent viscoelastic–viscoplastic constitutive model is proposed with the nonlinear viscosity and process-dependent recovery properties observed in the test results. Those properties are implemented by introducing evolution equations for additional internal variables. The identified material parameters capture the experiments qualitatively well. The proposed model is also evaluated by finite element simulations of the building process of a tire, followed by the in-molding.

Nickel-Based Slag-Remelted Superalloy for Ti-Alloys Isothermal Forging Die-Tool

The work purpose is to evaluate the application of nickel-based CrNi73CuBeTeAl superalloy (tungsten-free), made by electroslag remelting as a die tool material for isothermal forming. Isothermal deformation was simulated by tests for high-temperature cyclic creep. The specimens with high and × in plan dimensions (both parallel and perpendicular to the ingots central axis) were cut from the bottom, top and central zone, near the lateral surface and in the ingots central axis area. The tests were performed at 900, 940 and 980 °C constant temperature with 100, 200 and 300 MPa cyclic pressure in 0.02, 0.2 and 2.0 mm/s velocities range corresponded to 10-3-10-1 1/s strain rates. Such tools application field is compressor blades Ti-forgings die forging and sizing with ε = 1.0-3.0% residual upsetting deformation of dies during operation. It is shown that the die life can be extended to 400-700 loading cycles at 850-950 °C operating temperature and pressure up to 150 MPa.

Study on Cyclic Cumulative Deformation Characteristics and the Equivalent Cyclic Creep Model of Soft Clay

Suction caisson foundations can be used to anchor tension leg platforms. The soil at the bottom of the caisson undergoes both unloading and cyclic loading under wind and wave loads. However, the problem of cyclic cumulative deformation of soft clay under unloading has rarely been addressed. So, the strain cumulative deformation and strain softening characteristics of soft clay are studied by cyclic triaxial tests. The test results show that under low static deviator stress ratios and dynamic deviator stress ratios, the soil has a low level of strain accumulation and softening. As the dynamic deviator stress ratios increase, the cumulative cyclic deformation gradually increases, which rapidly develops in the early stage and tends to stabilize in the later stage. Moreover, the softening index gradually increases and is linearly related to the logarithm of the number of cycles. The cyclic cumulative deformation of the soil increases with increases in unloading stress and dynamic deviator stress, showing a creep characteristic of attenuation and then stabilization. Based on the tests, an equivalent cyclic creep model is established to describe the strain accumulation and softening of soil and verified through comparison with the test results. Then, the model is extended to a three-dimension model, and a finite element subroutine is developed for studying the strain cumulative deformation and strain softening characteristics of soft clay.

Cyclic Creep Behavior and Modified Life Prediction of Bainite 2.25Cr-1Mo Steel at 455 °C

Uniaxial static and cyclic creep tests were carried out on bainite 2.25Cr-1Mo steel at 455 °C. Effects of the unloading rate from 0.6 to 39 MPa/s and valley stress duration from 0 to 30 min on the cyclic creep deformation behavior were discussed. The results indicated that the fracture behavior under static and cyclic creep conditions showed a consistent ductile mode. The strain accumulation rate under cyclic creep was significantly retarded as compared with static creep due to the presence of anelastic recovery which was apparently influenced by the unloading conditions. For cyclic creep tests, the unrecoverable strain component determined by a systematic classification of the stress–strain curve was the true damage. A modified life prediction method proposed based on the unrecoverable strain component presented a good life prediction for cyclic creep.

Biomechanical Evaluation Comparing Pulvertaft Weave and Side-to-Side Tenorrhaphy Using Porcine Tendons

Background: Pulvertaft Weave (PTW) is an established method of tenorrhapy in tendon transfers. Previous studies have suggested that a Side-to-Side (STS) tenorrhapy is easier to perform has the same advantages and has greater load to failure, ultimate load and stiffness compared to PTW. However, there is insufficient data comparing behaviour of STS and PTW during cyclical loading. The aim of this study is to compare these two methods in terms of creep after cyclic landing. Methods: Fresh porcine flexor digitorum tendons were used. Ten tendon PTW and ten STS repairs were performed. Cross sectional diameter was measured. The tendon repairs were tested by applying tension up to 25N for 100 cycles, followed by tension up to 75N for 100 cycles, followed by loading to failure. Force-displacement data was used to determine the creep of the repaired tendon. Results: All tendons survived 100 cycles of loading at 25N. After 1 cycle of loading, the mean cyclic creep in the PTW group was almost 3 mm larger than in the STS group (p = 0.046). After 100 cycles, the mean cyclic creep in the PTW group was 4.4 mm larger (p = 0.008). The cyclic creep rate was significantly larger in the PTW group (p < 0.001). All STS but only four PTW repairs survived after cyclic loading at 75N (p = 0.01). After 1 cycle and 100 cycles, mean creep of the surviving PTW samples was almost 7 mm (p = 0.006) and almost 9 mm (p = 0.004) larger than the STS group. The mean load to failure was four times larger in the STS group than the PTW group (p = 0.004). Conclusions: STS repairs have a significantly smaller permanent elongation after cyclic loading at 25N and 75N, a significantly smaller cyclic creep rate, require a significantly larger load to fail. This implies that STS repairs are less likely to elongate after cyclic loading and can withstand greater loads. These properties can be valuable in allowing patients to commence mobilisation immediately after surgery.

Thermostability of Ionic Nitriding of Aluminium Alloys at Isothermal and Thermal-cycle Influence

Тhe study of the main parameter that limits the use of pistons made of aluminum alloys in boosting engines - heat resistance and its dependence on diffusion ion-nitrided layers. In the analysis of recent studies and publications, the most common methods of heating samples during tests, their advantages and disadvantages are identified. The task of the study is to identify the relationship of factors that determine the heat resistance of ion-nitrided aluminum alloys under isothermal and thermocyclic effects for effective analysis of the mechanism of phenomena and control of the technological process of strengthening the pistons. The availability of this information will increase the strength and reliability of both the coatings themselves and the parts with coatings at the stage of their design. A set of laboratory equipment based on the Nutcracker installation was used for the study, which provides cyclic change of temperature and load of the sample, control and recording of temperature values, loading and deformation and operation in automatic mode. Flat samples of aluminum alloy AL2I with heat-protective diffusion ion-nitrided layers applied in the VIPA-1 installation and samples without treatment were used. Characteristics of the thermal cycle: 500 ° C ↔ 720 ˚C. heating time - 10 sec., cooling 15 sec., which corresponds to the real operating conditions of the pistons. The fixed value of the load was 0.8σВ according to DSTU 2637-94 and the criteria for determining the allowable stress in the Standards of ASME (80% of the average value of stresses that lead to a total deformation of 1%). The curves of isothermal and thermocyclic creep having three clearly expressed sites of creep are defined, depending on temperature their duration is different. Isothermal and thermocyclic creep curves of aluminum alloy AL21 with heat-protective diffusion ion-nitrided layers have three distinct creep areas, depending on the temperature their duration is different. This is due to the presence in the surface layer of the reinforcing phase AlNi; uniformity of distribution of components of a covering on a surface and degree of their refractoryness. This is due to the complexity of the movement of dislocations, which on their way face the need to overcome large obstacles in their movement, caused by the presence of a reinforced layer. Conclusions. Based on the obtained curves of isothermal and thermo cyclic creep, it is established that the type of creep curves depends on the temperature and conditions of its influence on the composition "base-coating"; the time to failure in the thermo cyclic test mode is less than in the isothermal; in the process of creep of aluminum alloy AL21 with heat-protective diffusion ion-nitrided layers, two competing processes are observed: hardening due to plastic deformation and removal of hardening under the influence of elevated temperature; despite the different composition of heat-protective diffusion ion-nitrided layers, the resistance to isothermal and thermo cyclic creep of the reinforced aluminum alloy AL21 increased by 1.8 ... 2.2 times.