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When adhesively bonded joints are subjected to a cyclic load, they will primarily behave viscoelastically. Under certain loading conditions, they can also experience permanent deformation, or ratcheting. This research considered loading conditions that result in ratcheting for a toughened and standard adhesive in scarf joint coupons. Adhesive strain was measured using edge mounted resistance strain gages that was verified with digital image correlation. At low stress levels both adhesives exhibited a purely viscoelastic response. At high stress levels, the standard adhesive exhibited a viscoelastic response in tension-tension and a ratcheting response under reversed loading. At high stress levels the toughened adhesive exhibited ratcheting during tension-tension and reversed loading with higher ratcheting strain in tests that involved reversed loading. Enhanced ratcheting during reversed loading is consistent with a kinematic hardening response, which these adhesives have been shown to follow.

The present paper aims to perform an experimental and theoretical study regarding the inelastic deformation progressive accumulation observed in a polyvinylidene fluoride (PVDF) specimen machined from flexible pipe internal pressure sheaths under cyclic load-unload tensile tests and in monotonic tensile tests. Experiments show that the elastic mechanical behaviour is not rate-dependent because the Young's modulus does not suffer a considerable variation with change in the strain rate. However, the strain hardening induced by the plastic deformation is strongly rate-dependent. Hence, the mechanical behaviour of this polymer is not elasto-plastic or viscoelastic, but elasto-viscoplastic. Also, the kinematic hardening strongly increases with growth of accumulated inelastic strain, making it much more significant than the isotropic hardening. This paper proposes an elasto-viscoplastic mechanical model capable of describing the cyclic and monotonic inelastic behaviour for an arbitrary loading history. The kinenatic hardening observed for this PVDF makes it possible to identify experimentally all material parameters that arise in the model by performing load-unload tests using flat specimens, eliminating buckling problems and the need to control eventual misalignment in the universal test machine grips. Good agreement was noted when comparing the theoretical results with experiments. [ABSTRACT FROM AUTHOR]

Uniaxial stress-controlled ratcheting behaviors of expanded PTFE (ePTFE) under cyclic compressive loads were tested. The effects of temperature, stress rate and mean stress on the ratcheting behaviors of ePTFE considering multiple load paths were discussed in detail. Results present that the steady ratcheting strain is rate-independent when the stress rate is less than about 0.1 MPa/s, while it approximately linearly decreases with increasing the stress rate for greater stress rate. Additionally, the steady ratcheting is temperature-independent when the temperature is greater than about 150 °C, but it nearly linearly increases with enhancing the temperature for lower temperature. Especially, the stress rate almost has little effect on the ratcheting strain of ePTFE at 200 °C. Moreover, the accumulated ratcheting strain enhances rapidly in about the first 80 cycles, and subsequently tends to shakedown in the subsequent cycles for each load path. Furthermore, if a higher stress is used in the prior cycling, the greater ratcheting strain may be produced, and a negative ratcheting strain rate can be obtained in the subsequent cycling with lower mean stress due to the greater strain hardening and deformation resistance produced by the previous higher stress. [ABSTRACT FROM AUTHOR]

Compressive creep-fatigue experiments on PTFE gaskets with small stress amplitude were tested. The effects of peak holding times and temperatures on creep-ratcheting behaviors were further discussed. Results showed that at room temperature, the accumulated creep-ratcheting strain corresponding to 400 cycles for peak holding 1min is nearly 1.06 times that for peak holding time 0.5min and 1.3 times that without peak holding. While, at 100 °C, 150 °C and 200 °C, they are almost 1.1 times and 1.6 times, 1.1 times and 1.7 times, 1.05 times and 1.9 times, respectively. This means although the accumulated creep-ratcheting strains for peak holding 0.5min and 1min are very close to each other, but short peak holding time still has obvious effect on the accumulated deformation comparing that without peak holding. Moreover, the creep-ratcheting deformation with short peak holding time and small stress amplitude for PTFE gaskets can be simply estimated by the corresponding compressive static creep strain at the same peak stress in practical engineering with relatively good accuracy. [ABSTRACT FROM AUTHOR]

The present paper is concerned with the experimental and theoretical investigation of the progressive accumulation of inelastic deformation observed in cyclic tension tests performed on a particular polyamide. The elastic properties are not strongly affected by the strain rate, but the strain hardening induced by the plastic deformation is rate-dependent. Thus, the material behaviour is elasto-viscoplastic rather than viscoelastic or elasto-plastic. For the polymer studied in this paper, the kinematic hardening is much more significant than the isotropic hardening, and a negative plastic strain rate may occur even with a positive stress. The kinematic hardening is strongly dependent, not only on the accumulated plastic strain, but also on the loading rate. An elasto-viscoplastic mechanical model able to describe the cyclic inelastic behaviour for an arbitrary loading history is proposed. All parameters that arise in the theory are identified experimentally. The preliminary theoretical results concerning the modelling of cyclic load-unload tests are in good agreement with the experiments. [ABSTRACT FROM AUTHOR]

Uniaxial stress-controlled ratcheting experiments on PTFE gaskets under cyclic compressive loads with small stress amplitude were performed. The effect of temperature on the deformation behavior was considered. Results showed that the compressive modulus decreases rapidly when the temperature increases from 100 °C to 200 °C. Compressive ratcheting deformation with cycles increase significantly with the increases of temperature. The ratcheting deformations at 100 °C, 150 °C and 200 °C are nearly two, three and five times that at room temperature, respectively. Most of ratcheting deformation mainly occurs during the first 20 cycles because the subsequent ratcheting rate and strain range are small and much less than those in the previous cycles. The accumulated deformation under cyclic loads with small stress amplitude is relatively approach to the static compressive creep with the same peak stress. Therefore, the accumulated deformation with time of PTFE gaskets obtained by cyclic compression with small stress amplitude can be estimated by the corresponding static creep deformation with good accuracy under the approximate stress rate and the same temperature, especially at room temperature. [ABSTRACT FROM AUTHOR]

A series of in-plane biaxial cyclic tests were conducted to investigate the biaxial ratcheting behavior of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes via cruciform shaped specimens. A tailored biaxial cyclic testing system and an integrated non-contact displacement detecting system were used. The effects of stress amplitude, mean stress, stress rate and constant stress in the Y-direction on ratcheting behavior of PVDF UF membranes in the X-direction were studied. It was found that the ratcheting strain increases more rapidly as stress amplitude and mean stress become larger. However, the ratcheting strain and its rate decreased with increasing the stress rate or the constant stress in the Y-direction. Furthermore, the biaxial ratcheting tests were carried out on PVDF UF membrane with water immersion. It was found that the water immersion environment significantly accelerated the accumulation of ratcheting strain and its rate. [ABSTRACT FROM AUTHOR]

The long-term performance of polymers under cyclic loading is important for safety assessments in engineering applications. The deformation process under the cyclic loading can be accelerated through use of temperature and stress. As for asymmetric cyclic loading, so called ratcheting, a time-temperature-stress (TTS) equivalence method in which all the parameters have clear physical meanings and can be determined experimentally, was proposed to predict the long-term cyclic loading behavior for polycarbonate using short-term data. Taking into consideration the effects of both the mean stress and the stress amplitude, the ratcheting compliance was defined and its evolution function was also provided. Next, the TTS equivalence method was validated using the long-term ratcheting test results for the polycarbonate. Time, temperature, and stress do show equivalent effects on long-term ratcheting of polycarbonate. Using the proposed method, time and cost can be dramatically saved for the assessment of the long-term cyclic loading performance of polycarbonate. [ABSTRACT FROM AUTHOR]

Abstract: The present paper is focused on study of the plastic behaviour and strain rate sensitivity of cylindrical polymer mortar specimens in monotonic and cyclic compression tests. The experimental results indicate elasto-viscoplastic behaviour: the load-carrying capacity of the specimen increases significantly with strain rate while the elastic properties remain practically constant. Under cyclic loading and unloading the material may exhibit hysteresis (a phase lag), which leads to a progressive accumulation of deformation. In this case, despite the high scatter of the experimental results, the number of cycles until failure tends to be smaller for lower loading frequencies. Preliminary ideas for a phenomenological model able to describe such nonlinear elasto-viscoplastic behaviour are also presented. [Copyright &y& Elsevier]

Abstract: The present paper is concerned with the experimental investigation of the progressive accumulation of deformation observed in cyclic tension tests performed on an epoxy polymer. The strain rate dependency is almost negligible and the material behaviour is elastic-plastic rather than elastic-viscoplastic or viscoelastic. Under cyclic loading and unloading, this polymer exhibits hysteresis (a phase lag), which leads to dissipation of mechanical energy and to the accumulation of cyclic plastic deformation. It is verified experimentally that this phenomenon is essentially due to kinematic hardening. Preliminary ideas for an elastic-plastic phenomenological model able to describe the plastic cyclic behaviour of this epoxy polymer are also presented. [Copyright &y& Elsevier]

Abstract: A series of uniaxial ratcheting experiments has been carried out on cold compaction polytetrafluoroethylene (PTFE) specimens. All the tests were performed under stress control at elevated temperature. The effects of mean stress, stress amplitude, applied temperature and their histories on the ratcheting behavior of PTFE were studied. It is shown that, as the applied temperature was raised, the elastic modulus of PTFE declined rapidly. The ratcheting strain increased as the mean stress, stress amplitude and temperature increased. Especially, when the temperature was over 100°C, the ratcheting strain accumulated rapidly. Furthermore, the loading histories also play an important role in the progress of ratcheting. Previous cycling with higher mean stress and stress amplitude greatly restrains ratcheting strain of subsequent cycling at lower ones. Such a phenomenon is due to the enhancement of the material deformation resistance caused by the previous loadings. As the applied temperature changes, the ratcheting strain still accumulates along the direction of mean stress. [Copyright &y& Elsevier]

Abstract: A series of multiaxial ratcheting experiments have been performed on polyteterafluoroethylene (PTFE) solid cylindrical specimens. All the tests were conducted under cyclic shear strain with a constant axial stress at room temperature. The effects of axial stress, shear strain range, shear strain rate and their histories on the ratcheting behavior of PTFE were studied. It is shown that the ratcheting strain depends on the constant axial stress, cyclic shear strain range and shear strain rate. The ratcheting strain increases more rapidly as the constant axial stress or shear strain range become larger, or the shear strain rate is reduced. Furthermore, the loading histories also play an important role in the progress of ratcheting. The prior cycling with higher axial stress, larger strain range or lower strain rate greatly restrains ratcheting strain of subsequent cycling at lower strains. Such phenomenon is due to the enhancement of the material deformation resistance caused by the prior loadings. [Copyright &y& Elsevier]

Abstract: In this paper, loading rate dependence of stress–strain response and ratcheting behavior under compression for PTFE were studied. The effects of loading rate, mean stress and stress amplitude on the ratcheting behavior of PTFE are discussed. It is shown that PTFE is a rate dependent material, but it is insensitive to loading rate when the rate is higher than 40N/s. Ratcheting strain increases as the loading rate decreased. When loading rate is higher than 40N/s, there is little effect of loading rate on ratcheting strain rate. At the same stress amplitude (mean stress), the ratcheting strain and ratcheting strain rate increase as mean stress (stress amplitude) increased. [Copyright &y& Elsevier]