Cyclic Tension Tests Research Articles

Implications of warm-mix asphalt on long-term oxidative ageing and fatigue performance of asphalt binders and mixtures

The use of warm mix asphalt (WMA) has been increasing in recent years due to its ability to reduce the production temperatures of asphalt concrete. The long-term implications of reduced production temperatures and, hence, reduced short-term ageing on long-term performance remain largely unknown. This study evaluates the effect of age hardening in WMA binders and mixtures compared with hot mix asphalt (HMA) binders and mixtures with respect to fatigue damage. Two WMA technologies are considered: foaming by water injection and Evotherm modification. For this study, the asphalt mixtures were subjected to laboratory conditioning in a forced air convection oven to simulate long-term field ageing according to AASHTO R30. The asphalt mixtures and extracted binders were subjected to linear viscoelastic and fatigue characterisation following ageing. Because oxidative ageing occurs within the asphalt binder phase of asphalt concrete, this paper focuses on the relative performance of WMA and HMA binders at various ageing levels and compares this binder performance to the respective mixture performance. Cyclic direct tension tests were used to measure the fatigue resistance of the asphalt mixtures, and the linear amplitude sweep (LAS) test was used to measure the fatigue resistance of the binders. Simplified viscoelastic continuum damage (S-VECD) analysis was performed to interpret the fatigue test results and predict the fatigue performance of the binders and mixtures using a pavement structural model. The results demonstrate that after substantial long-term ageing, differences between the fatigue performance of WMA and HMA become insignificant. The results also demonstrate good agreement between the binder and mixture results, indicating that the LAS test coupled with S-VECD analysis is able to capture the binder's contribution to mixture fatigue.

Performance-Based Moisture Susceptibility Evaluation of Warm-Mix Asphalt Concrete through Laboratory Tests

This study used the cyclic direct tension test, indirect tensile strength test, and Hamburg wheel tracking device (HWTD) test to evaluate the moisture susceptibility of warm-mix asphalt (WMA) mixtures. The stripped areas that were quantified by digital imaging analysis were derived from the cyclic direct tension, indirect tensile strength, and HWTD tests and were compared with the mechanical properties of the mixtures to identify sensitive moisture susceptibility indicators. These methods were applied to a Superpave® 9.5-mm hot-mix asphalt mixture and five corresponding WMA mixtures that used the following technologies: (a) Evotherm 3G that contained a chemical additive, (b) foaming, (c) WMA-A that contained a chemical additive and was under development, (d) WMA-B that contained an organic additive and was under development, and (e) WMA-C that also contained an organic additive (different from that used in WMA-B) and was under development. Fatigue life ratios obtained from the simplified viscoelastic continuum damage model combined with layered viscoelastic analyses were determined to be the most sensitive indicators for moisture susceptibility. The stripping infection points derived from the HWTD tests also showed good sensitivity to moisture conditioning; however, each stripping infection point was affected by the permanent deformation characteristics of a given mixture as well as its moisture susceptibility; therefore, the observation was inconclusive.

Evolution of lattice strain and phase transformation of β III Ti alloy during room temperature cyclic tension

An in situ high-energy X-ray diffraction cyclic tension test was carried out on a β III Ti alloy to study its micromechanical behavior and the stress-induced phase transformation. Pre-strained material showed a microscopic multi-stage re-loading behavior following the sequence of elastic deformation, stress-induced martensite (SIM) transformation, a second stage of elastic deformation followed by a final stage of SIM transformation. Based on the relationship of internal strains and diffraction intensities between the β phase and the SIM, it is concluded that after a small strain deformation, the austenite is divided into two different sets of grains with different properties. Those that previously experienced phase transformation have a lower critical stress for the SIM transformation due to residual martensite and dislocations, while the rest have a higher trigger stress and only transform to martensite after the stress is back to levels comparable to where transformation was seen in the previous cycle. The different properties within the same austenite grain family cause the multistage re-loading behavior. The reverse phase transformation during unloading was impeded by the combination of increased dislocation density in the austenite and the increased tensile strain in the martensite prior to unloading.

Cyclic mechanical behavior of 316L: Uniaxial LCF and strain-controlled ratcheting tests

With the purpose of analyzing the fatigue behavior under loading conditions relevant for the primary cooling circuit of a light water nuclear reactor, a set of uniaxial low cycle fatigue and strain-controlled ratcheting tests (also named ‘cyclic tension tests’) has been performed at room temperature and at 200°C on specimens manufactured from two different batches of stainless steel grade 316L. The experiments have been repeated varying strain amplitude, cyclic ratcheting rate and ratcheting direction in order to investigate the influence on the cyclic deformation behavior. In strain-controlled ratcheting tests, the stress response is found to be a superposition of two hardening mechanisms: the first one due to the zero mean strain cycling and the second one linked with the monotonic drifting of mean plastic strain. An approach is proposed to distinguish the effect of each mechanism and the influence of the test parameters on the hardening mechanisms is discussed.

Use of Cyclic Direct Tension Tests and Digital Imaging Analysis to Evaluate Moisture Susceptibility of Warm-Mix Asphalt Concrete

This paper presents a simplified viscoelastic continuum damage material model for the evaluation of moisture susceptibility of asphalt concrete. The model is based on cyclic direct tension testing and layered viscoelastic analysis. The visual stripping inspection afforded by digital imaging analysis is also proposed as an intuitive and straightforward method for moisture susceptibility evaluation. These methods were applied to a Superpave® 19-mm hot-mix asphalt mixture and corresponding warm-mix asphalt mixtures modified by a polyethylene wax-type additive with and without an antistripping agent. The fatigue life predicted by the simplified viscoelastic continuum damage and layered viscoelastic analysis models had a strong correlation with the percentage of stripping determined from specimen surfaces that were fractured during cyclic direct tension testing of the hot-mix and warm-mix asphalt mixtures with various asphalt contents. In addition, a polyethylene wax-type additive combined with an antistripping agent was found to provide a longer fatigue life and less stripping than a pure polyethylene wax-type additive. The findings from this paper should provide guidance to agencies and material engineers in developing asphalt binder modifiers that lengthen the fatigue life of pavements and reduce moisture susceptibility.

Effects of Deformation Behavior and Processing Temperature on the Fatigue Performance of Deep-Rolled Medium Carbon Bar Steels

The effects of processing temperature on the deep-rolling response of three medium carbon bar steels, a quenched and tempered 4140 alloy, a 0.34C, 1.21Mn, 0.66Si nontraditional bainitic alloy, and a 0.36C, 1.37Mn V-microalloyed ferrite plus pearlite steel, was assessed through bending fatigue. The significantly different deformation behaviors of the three alloys were characterized through standard and nonstandard quasi-static and cyclic uniaxial tension and compression tests at room temperature (RT) and in situ at temperatures up to 634 K. Deep rolling, performed at RT and at elevated temperature (HT) in the dynamic strain-aging (DSA) regime, increased measured endurance limits by 51-62 pct (RT) and 96-117 pct (HT) as compared with the baseline condition. The enhanced fatigue performance by RT deep rolling primarily reflected the effects of the introduction of favorable residual stresses. The improved fatigue performance from HT deep rolling was attributed to the enhanced resistance to strain reversal of the material deformed during deep rolling, due to a change in deformation mechanism from dislocation-interstitial interactions in the DSA regime during processing, which inhibited mechanically induced relaxation of residual stress during cyclic loading.

Plastic behaviour of an epoxy polymer under cyclic tension

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.

Analysis of unified continuum damage mechanics model of gas turbine rotor steel: Life assessment

In this research, the unified creep continuum damage mechanics model of Lemaitre is used to model the stress–strain behavior of a gas turbine rotor. The unified damage law can model the creep and low cycle fatigue damage. It is based on the increment of the accumulated inelastic strain, which may be due to either creep damage or low cycle fatigue damage. The parameters of the unified continuum damage mechanics model, including Q, Norton constants, and constants of the nonlinear kinematic hardening model are determined and optimized by cyclic tension and relaxation tests. A careful three-dimensional thermal analysis is carried out and the temperature gradient is obtained. This analysis is used to determine stresses of the gas turbine rotor. Creep and low cycle damage are also modeled and stress–strain evolution in the gas turbine rotor is obtained. The replica tests are performed on four points of the rotor surface and formation of the creep voids is verified under a scanning electron microscope. Based on the continuum damage mechanics analysis, the creep–fatigue interaction is considered in modeling and the remaining life of the rotor is estimated.

Measurement of Nonlinear Mechanical Properties of Surfactant-Added Poly(dimethylsiloxane)

This paper presents a measurement of the nonlinear mechanical property of the surfactant-added poly(dimethylsiloxane) (PDMS) elastomer. The tension test samples with the different concentrations of the surfactant were prepared using a simple mixing, coating, curing, and cutting process. A 10 times cyclic tension test with a fixed magnitude of applied strain and a single-pull-to-failure tension test were sequentially performed with a micro-tensile tester at room temperature. Compared with the pure PDMS, the surfactant-added PDMS was more flexible and its yield strain also greatly increased. From the measured stress–strain relationship, the stress softening and the residual strain was observed in the first cyclic tension tests. Such a hysteresis was highly related with the concentration of the surfactant in preparing PDMS film. As the concentration of the surfactant increased, the hysteresis increased remarkably. From second cyclic tension test, a hysteresis dramatically reduced and the strain–stress curves were almost converged in 10 times cyclic test. Three nonlinear mechanical models like a Neo–Hookean, Mooney–Rivlin, and Ogden were computed from the experimentally measured stress–strain data. The Ogden model for the surfactant-added PDMS showed a good agreement with the experimental data. In the case of the Neo–Hookean and Mooney–Rivlin models, they could be preferable for the structural analysis of the micro device with the surfactant-added PDMS in the small strain region. This could provide the useful and reasonable nonlinear mechanical models of the surfactant-added PDMS for rapid computational prototyping from the hydrophilic PDMS.

A Mullins softening criterion for general loading conditions

Samples of carbon-black filled styrene butadiene rubbers (SBRs) were submitted to successive nonproportional loadings in order to define a general criterion for the Mullins softening. For this purpose, each sample was initially submitted to uniaxial or biaxial preloadings followed by a cyclic uniaxial tension test. An original experimental analysis aimed at defining the activation threshold for the Mullins softening during cyclic uniaxial loadings. The experimental data provide substantial evidences establishing the surface of the maximum directional stretch undergone by the material as a relevant Mullins softening criterion. The latter was used to successfully predict the Mullins softening surfaces for additional loading cases.

Uniaxial, load-controlled cyclic deformation of recrystallized molybdenum sheet

► Cyclic deformation of Mo was studied using R = 0.1 in the range 25–500 °C. ► At 25 °C, unrecrystallized Mo shows more ratcheting than recrystallized Mo. ► Ratcheting rate increases, then decreases and then increases prior to failure. ► Ratcheting strain at 25 °C increases with increases in mean stress. ► Ratcheting strain is higher at higher temperatures for given loading conditions. Monotonic tensile tests and uniaxial load-controlled tension–tension cyclic tests were performed on commercial purity (99.99%) molybdenum in the recrystallized state in the temperature interval spanning RT (25 °C) to 500 °C range. Cyclic deformation behavior was characterized by analyzing the ratcheting strain, cyclic hardening response, and fatigue life. The response of unrecrystallized and recrystallized Mo was compared at room temperature; the influence of test variables including mean stress, stress amplitude and R ratio on ratcheting strain in recrystallized Mo at room temperature was also examined. The magnitude and rate of ratcheting strain evolution with cycles appear to depend on both the mean stress and the stress amplitude. Microstructural evolution during cyclic loading was characterized by conducting interrupted tests at room temperature as well as at 400 °C. Observations confirm dislocations cell structure formation during cyclic plastic deformation at room temperature and at 400 °C, the process occurring more rapidly at the higher temperature; in addition, at the higher temperature some of the cell walls appeared to have transformed into subgrain boundaries, suggesting partial recovery.

Constitutive framework for the modeling of damage in collagenous soft tissues with application to arterial walls

In this paper a new material model is proposed for the description of stress-softening observed in cyclic tension tests of collagenous soft tissues such as arterial walls, for applied loads beyond the physiological level. The modeling framework makes use of terms known from continuum damage mechanics and the concept of internal variables introducing a scalar-valued variable for the representation of fiber damage. A principle is given for the construction of damage models able to reflect remanent strains as a result of microscopic damage in the reinforcing collagen fiber families. Particular internal variables are defined able to capture the nature of arterial tissues that no damage occurs in the physiological loading domain. By application of this principle, specific models are derived and fitted to experimental data. Finally, their applicability in numerical simulations is shown by some representative examples where the damage distribution in arterial cross-sections is analyzed.

Experimental characterization and modelling of the cyclic softening of carbon-black filled rubbers

Several carbon-black filled styrene-butadiene rubbers (SBRs) were submitted to uniaxial tension cyclic tests at large strain for investigating filled rubbers cyclic softening. In order to study the effect of the material composition (amount of fillers and crosslink density) and of the stretch intensity, an original method for cyclic softening characterization is proposed. The softening is seen as an amplification of the stretch in the rubber gum, and a stretch amplification factor is then introduced as the softening parameter. The latter is evaluated for various materials and several levels of cyclic stretch intensity. Material softening is shown to increase with the loading intensity, the amount of carbon-black fillers and the decrease of the crosslink density. In terms of modelling, the introduced softening parameter provides access to the entire stress-stretch responses of the softened material for any cycle. Therefore, a model is written for the description and the prediction of the stress-stretch responses of filled rubbers during proportional cyclic tests.

Sensitivity of the Carbon Fiber Interface in Epoxy

In order to improve the gauge factor of polymer-matrix carbon fiber reinforced composites (CFRP) for strain sensing, the carbon fiber sensitive interface was constructed in epoxy by effectively overlapping the CFRP laminates in local area. The strain sensitivities of the carbon fiber interface were proved by the cyclic tension test on the base specimens covered by the carbon fiber composites with the sensitive interface, and their much bigger gauge factors were revealed by the comparison with the polymer-matrix continuous carbon fiber composite. The sensitivity of the carbon fiber interface is originated from the change of the interfacial points due to the interfacial stress.

Uniaxial Constitutive Law for structural concrete members under monotonic and cyclic loads

In this paper, a new constitutive law for concrete is proposed to predict the non-linear behavior of reinforced concrete members. The proposed model is intended to provide improvements on modeling the non-linear hysteretic behavior of concrete structures in the finite element coding. The independent damage and fracture parameters in compression and in tension have been introduced to the constitutive law of concrete degradation, due to increasing loads. In the sample of non-linear monotonic compressive loading, the law has been derived from a basic mathematical model obtained previously. In addition, in the sample of monotonic tension loading, the same law is adopted based on the latter basic mathematical model. The main novelty of the proposed Uniaxial Constitutive Law (UCL) lies in the fact that all the required input data can be obtained through conventional monotonic and cyclic compression and tension tests.

Experimental study and constitutive modelling of the passive mechanical properties of the porcine carotid artery and its relation to histological analysis: Implications in animal cardiovascular device trials

The present study focusses on the determination, comparison and constitutive modelling of the passive mechanical properties of the swine carotid artery over very long stretches in both proximal and distal regions. Special attention is paid to the histological and mechanical variations of these properties depending on the proximity to the heart. The results can have clinical relevance, especially in the research field of intravascular device design. Before the final clinical trials on humans, research in the vascular area is conducted on animal models, swine being the most common due to the similarities between the human and swine cardiovascular systems as well as the fact that the swine size is suitable for testing devices, in this case endovascular carotid systems. The design of devices usually involves numerical techniques, and an important feature is the appropriate modelling of the mechanical properties of the vessel. Fourteen carotid swine arteries were harvested just after sacrifice and cyclic uniaxial tension tests in longitudinal and circumferential directions were performed for distal and proximal samples. The stress–stretch curves obtained were fitted with a hyperelastic anisotropic model. Stress-free configuration states were also analyzed. Finally, human and swine samples were processed in a histological laboratory and images were used to quantify their microconstituents. The statistical analysis revealed significant differences between the mechanical behavior of proximal and distal locations in the circumferential but not in the longitudinal direction. Circumferential direction samples show clear differences both in residual stretches and tensile curves between the two locations, while the features of longitudinal specimens are independent of the axial position. The statistical analysis provides significant evidence of changes depending on the position of the sample, mainly in elastin and SMC quantification.

Measurement of nonlinear mechanical properties of PDMS elastomer

This paper presents the measurement of the nonlinear mechanical properties of polydimethylsiloxane (PDMS) elastomer based on the mixing ratio of base polymer to curing agent. Strip-type PDMS samples with different mixing ratios were prepared using a simple coating, curing, and cutting process. A cyclic uniaxial tension test with a fixed magnitude of applied strain and a single-pull-to-failure tension test were performed with a micro-tensile tester at room temperature.Our new finding is that when the PDMS is mixed with excessive curing agent, stress softening occurs and residual strain exists in cyclic tension tests when the magnitude of the applied strain increases. For the PDMS-05 samples, in which the mixing ratio of base polymer to curing agent was 5 to 1, there were large differences in the stresses for the same strain level under loading and unloading during the first cycle with a 100% fixed strain amplitude, but the softening effect of the stress in the PDMS dropped rapidly starting from the second cycle.Nonlinear mechanical Neo-Hookean, third-order Mooney, and second-order Ogden models of three different PDMS films were computed from the stress–strain data. The results showed that all models were preferable for the small strain region of PDMS compared with other models. In the nonlinear, large strain region, only the second-order Ogden model properly described the mechanical behavior of the PDMS, while the Neo-Hookean and third-order Mooney-Rivlin models were too stiff or flexible in the measurement range. The bulk modulus of PDMS increased with the amount of curing agent in it. Therefore, the second-order Ogden model is preferable for analyzing the PDMS structure over the entire measurement range. This could provide reasonable mechanical models of PDMS for rapid computational prototyping and for designing active and passive components from PDMS.

Ratchetting of porcine skin under uniaxial cyclic loading

Skin soft tissue (e.g., porcine skin) was tested in vitro under uniaxial cyclic loading, and its biomechanical responses were investigated to realize some basic properties which are very significant in assessing the fatigue life of skin soft tissue. The results show that a cyclic accumulation of peak and valley strain, which can be terminologically called as ratchetting in terms of material science of metals, occurs in the porcine skin during cyclic tension–unloading, tension–tension and compression–unloading tests. Observed ratchetting of porcine skin depends on load level and loading orientation greatly and also presents remarkable rate dependence due to the viscosity of skin soft tissue. The ratchetting is much more remarkable during the test at lower loading rate than that at higher loading rate. Moreover, some basic properties of porcine skin were also investigated by monotonic tension, compression and creep tests in order to address the ratchetting more comprehensively. Finally, collagen fiber bundles in the porcine skin and their variation during monotonic and cyclic tension tests were observed microscopically in term of standard iron-hematoxylin staining method. The observations are useful to realize the micro-mechanism of ratchetting deformation.

Finite element implementation of a microstructure-based model for filled elastomers

To describe the inelastic mechanical behavior of filled elastomers a microstructure-based material model for uniaxial loadings has been developed. The generalization of this one-dimensional material description to a fully three-dimensional constitutive model has been accomplished by using the concept of representative directions. The generalized model shows a very good agreement with cyclic uniaxial tension and compression tests as well as simple shear measurements for several rubber compounds. The FE-implementation enables finite element simulations of technical components though the original input model predicts the material behavior for uniaxial loadings only.

Strength of CFRP open hole laminates made from NCF, TFP and braided preforms under cyclic tensile loading

Tensile tests and tension–tension cyclic tests were performed on carbon fibre reinforced plastics open-hole laminates. Specimens made from non-crimp fabric preforms, tailored fibre placement preforms and braided preforms in three different lay-up configurations ([+45]8 s, [+45/0/−45]6 s, [0/90]8 s) were investigated and compared. The laminates were manufactured using a vacuum assisted processing technology and Hexcel RTM6 epoxy resin. Tensile strength and residual tensile strength values were measured and compared with unnotched specimens in order to evaluate the notch sensitivity. To evaluate deterioration during cyclic testing a two-dimensional digital image correlation system was used to capture deformation images of the specimen surface in the open-hole area. Observed similarities and differences in deformation and in load–elongation graphs of the tested specimens are discussed.