Cyclic Creep Behavior Research Articles

Transition in Failure Mechanism Under Cyclic Creep in 316LN Austenitic Stainless Steel

Cyclic creep behavior of a type 316LN austenitic stainless steel was investigated in the temperature range from 823 K to 923 K (550 °C to 650 °C). A transition from fatigue-dominated to creep-dominated failure mode was observed with an increase in the mean stress. The threshold value of mean stress for the transition was seen to be a strong function of the test temperature. Occurrence of dynamic strain aging proved beneficial owing to a substantial reduction in the strain accumulation during cyclic loading.

Comparative Assessment for Static Creep and Tension-Tension Cyclic Creep Behaviors of Modified 9Cr-1Mo Steel

Cyclic creep (CC) tests were performed with the magnitudes of the stress range of constant stress ratio (R=0.1) under continuous tension-tension loading cycles at a hold time of 10minute at 600oC for normalized and tempered modified 9Cr-1Mo steel. Cyclic strain-time curves were obtained from the controlled cyclic creep tests, and the cyclic creep behavior was investigated and compared with the static creep (SC) behavior. In the present test conditions, the results of creep rupture time and creep rate showed retardation behaviour when compared with those of the static creep. Creep rupture ductility decreased with a decrease in stress, and the ductility was almost the same for the CC and SC. In the fracture micrographs, the CC showed typical ductile fracture preceded by many dimples which occurred in the equi-axed core. The dimple size and area decreased with a decrease in the stress ranges. The precipitates formed in the CC test specimens were more coarsened than those in CC ones.

Cyclic Creep and Recovery Behaviour of Glass Fibre Reinforced Epoxy Composite

This paper presents the results of an experimental and numerical investigation of the tensile behaviour of glass fibre reinforced epoxy composite for incremental and cyclic creep-recovery load conditions. The proposed constitutive model incorporates linear viscoelasticity coupled with damage and viscoplasticity for prediction of the creep-recovery responses. The material parameters are determined by fitting the experimental results. Scanning electron microscopy (SEM) of composite fracture surfaces is performed to explain the failure mechanisms. A computational algorithm for the integration of the proposed constitutive model at the material point level is derived and implemented into the finite element code ABAQUS. The model predictions are found to be in good agreement with the experimental data.

Fluorescence spectroscopy applied to study cyclic creep behaviour and internal stresses of semi-crystalline high-density polyethylene

Cyclic creep or ratcheting behaviour has been characterized on semi-crystalline high-density polyethylene (HDPE) at 293 K using in situ extrinsic fluorescence spectroscopy. Fluorescent probe inserted in polymer offers the opportunity to establish a relationship between fluorescence phenomena and mechanical behaviour and more specifically to internal stresses. The influence of mean stress (σm), and stress amplitude (σa), on ratcheting strain (εr) and strain amplitude (εa) was studied in term of internal stresses development. The physical base of internal stress state has been discussed.

Cyclic creep and recovery behavior of Nextel™720/alumina ceramic composite at 1200 °C

The cyclic creep and recovery behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200°C in laboratory air and in steam environments. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The main objective of this study was to assess the influence of various sustained creep and cyclic creep loading histories on the creep lifetime, creep strain rate, accumulated creep strain as well as on the recovery of creep strain at near zero stress. Cyclic creep-recovery tests were performed for maximum stress levels of 100 and 125MPa with creep and recovery periods ranging from 3min to 1h. In laboratory air, lifetimes produced in cyclic creep and recovery tests significantly exceeded those obtained in sustained creep tests. Introduction of intermittent periods of unloading and recovery at near zero stress into the creep loading history resulted in an appreciable improvement in creep lifetime. Presence of steam considerably degraded the material performance. In steam, lifetimes produced in cyclic creep and recovery tests were close to those obtained in sustained creep tests. Composite microstructure, as well as damage and failure mechanisms were investigated.

Test on cyclic creep behavior of mucky clay in Shanghai under step cyclic loading

Based on the cyclic triaxial test under step loading, the variation of cyclic creep and pore water pressure with increase in cyclic stress ratio (CSR) and load cycle of mucky clay in Shanghai were analyzed. The results show that there are two threshold values of stability cyclic stress ratio and failure cyclic stress ratio during the step loading which divide the development of cyclic creep strain into gradual stability stage, rapid growth stage and instantaneous failure stage. When the stress level is less than the failure cyclic stress ratio, the recoverable elastic strain is nearly linear with CSR. The cyclic creep properties of mucky clay are obviously influenced by the loading frequency under the same stress level.

Cyclic creep and fracture of a Cu–SiO 2 bicrystal at an intermediate temperature of 673 K

Cyclic creep and fracture behavior at 673 K of orientation-controlled Cu–SiO 2 bicrystals with [0 0 1] twist 20° grain boundaries was investigated. The cyclic creep and life depended on both the stress amplitude and the frequency of the cyclic load. Most bicrystals fractured intergranularly. The number of cycles to failure shortened drastically with decreasing the frequency and with increasing the stress amplitude, while the time to failure remained nearly the same irrespective of the frequency. Since the cyclic creep life was controlled by the occurrence of grain-boundary fracture, the above observations can be understood reasonably by considering stress concentration and void formation at grain-boundary SiO 2 particles. When grain-boundary sliding takes place, the particles impede the sliding and the stress concentration sites are created. This causes the intergranular fracture and controls the cyclic creep life.

Modeling of the transient responses of the vocal fold lamina propria

The human voice is produced by flow-induced self-sustained oscillation of the vocal fold lamina propria. The mechanical properties of vocal fold tissues are important for understanding phonation, including the time-dependent and transient changes in fundamental frequency ( F 0 ) . Cyclic uniaxial tensile tests were conducted on a group of specimens of the vocal fold lamina propria, including the superficial layer (vocal fold cover) (5 male, 5 female) and the deeper layers (vocal ligament) (6 male, 6 female). Results showed that the vocal fold lamina propria, like many other soft tissues, exhibits both elastic and viscous behavior. Specifically, the transient mechanical responses of cyclic stress relaxation and creep were observed. A three-network constitutive model composed of a hyperelastic equilibrium network in parallel with two viscoplastic time-dependent networks proves effective in characterizing the cyclic stress relaxation and creep behavior. For male vocal folds at a stretch of 1.4, significantly higher peak stress was found in the vocal ligament than in the vocal fold cover. Also, the male vocal ligament was significantly stiffer than the female vocal ligament. Our findings may help explain the mechanisms of some widely observed transient phenomena in F 0 regulation during phonation, such as the global declination in F 0 during the production of declarative sentences, and local F 0 changes such as overshoot and undershoot.

Experimental and Numerical Study of Cyclic Creep of Cast Magnesium Alloy at High Temperature

The cast magnesium alloys as AM50 offer a good strength, ductility and surface finish for automotive industry. But the poor creep resistance limited its application to power components such as engine and transmission cases at temperatures in excess of 100°C. In order to investigate the cyclic creep behavior of Magnesium Alloy at high temperature, creep tests of plate specimens AM50 were conducted in this work. Based on the analysis about the microstructure and defects of AM50 under the condition of cyclic creep, a cyclic creep constitutive model with isotropic and scalar damage parameter was developed. Furthermore, the proposed model was experimentally verified by analyzing the cyclic creep and recovery response of Cast Magnesium alloy under cyclic loading with dwell time. Comparisons between calculated results and experimental data showed good agreement.

Cyclic creep of copper due to axial cyclic and tensile mean stresses

The behaviour of materials under axial stress-controlled cycling with tensile mean stress within the plastic region is governed by a superposition of cyclic creep and fatigue processes. Cyclic creep may dominate the damage process and cause distortional failure before the onset of fatigue cracking. The aim of this work was to investigate cyclic creep in copper under such loading conditions at room temperatures, and to derive an empirical relationship for cyclic creep in terms of imposed stress amplitude and mean stress, via a previously proposed exponential mean-stress function that accounts for the effect of mean stress on cyclic behaviour. It is found that cyclic creep behaviour at room temperatures could be described by a power-law relation similar to that of static creep at high temperatures.

Influence of bonding on the tensile creep behavior of paper in a cyclic humidity environment

It has been shown, as paper structure is improved through increased bonding (by increasing relative bonded area or specific bond strength), a fully efficient loaded structure can be achieved. Once fully efficient, further improvements in bonding become redundant and have no effect on some paper deformation behaviors; deformation is dictated only by the characteristics of the fibers. Although previous work had shown this was true for elastic modulus and short time duration stress-strain behavior, it has only recently been shown to be true for constant humidity tensile creep behavior. In this study, the goal was to ascertain if cyclic humidity tensile creep behavior (known as accelerated creep) would follow the same trend. To accomplish this, sheets were made at differing levels of relative bonded area and specific bond strength. This was done by applying two different wet pressing levels (to alter relative bonded area) and using bonder and debonder (to change specific bond strength). It was found that accelerated creep behavior of paper sheets is no different than constant humidity creep behavior; changing bonding does not influence accelerated creep if the sheet has a fully efficient loaded structure. If the sheet structure is inefficiently loaded (there is no redundancy in bonding), accelerated creep will be affected by bonding. However, it is proposed that the only reason accelerated creep is influenced by bonding when inefficiently loaded is because constant humidity creep behavior determines the accelerated behavior and it is influenced, in this case, by bonding.

Influence of Lüders band formation on the cyclic creep behaviour of a low-carbon steel for piping applications

Abstract The paper presents a study on the influence of formation of Luders band on the cyclic creep or ratcheting behaviour in low carbon steel bound for pipeline applications. The Luders band formation has been characterized by carrying out stress-controlled monotonic tensile tests in terms of the stress required for it to propagate and the amount of strain accumulated during its propagation. Low-cycle fatigue at different strain ranges and cyclic stress-controlled tests at different stress ratios and stress rate combinations have been carried out. It has been observed that the hysteresis behaviour in conditions favouring Luders band formation leads to a significant enhancement in ratcheting and cyclic softening in the low-cycle fatigue tests. The evolution of the hysteresis loop behaviour has been characterized by the Bauschinger energy parameter. The accelerated strain softening has been attributed faster proliferation of dislocation sources and to greater dislocation annihilation rates as a consequen...

Cyclic creep behaviour described in terms of a one-parameter kinetic model

Abstract A previously proposed model for describing strain-controlled cyclic plastic deformation is employed to describe cyclic creep behaviour. The model, derived via the evolutionary behaviour of mobile dislocation density, is directly applicable to the case where flow is governed by the glide motion of dislocation through an obstacle field, such as forest dislocations. For the case of stress-controlled loading, the model is modified to account for kinematic hardening and mean-stress shift that occur during cyclic creep deformation. Cyclic creep is modeled as due to the coupled behaviour of the breakdown of dislocation cell structures on the one hand, and the formation and development of new cell structures on the other hand. These two processes oppose each other in that while the former softens the material and enhances flow, the latter hardens the material and tends to decrease flow. The competition between these processes and the relative rate of one to the other during loading determines the overall shape of the strain-cycle (or strain–time) curve. The predictive capability of the present creep model is compared with experimental cyclic creep data of polycrystalline copper subjected to combined cyclic and mean stresses, and agreement between the two is very good.

Cyclic creep and fracture behavior of Cu–SiO2 bicrystals with [011] twist boundaries

Abstract Orientation-controlled copper bicrystals with dispersed SiO 2 particles were cyclically deformed at 673 K in vacuum. Although the fatigue lives of all the bicrystals increased monotonically with decreasing stress amplitude at a higher stress amplitude region, some of them showed discontinuous and abrupt life shortening at a lower stress amplitude region. This transition of the cyclic creep behavior was brought about by the occurrence of brittle intergranular fracture caused by grain-boundary sliding (GBS) and particle dragging by GBS to form the elongated voids. At the lower stress region, such voids were easily formed by GBS. At the higher stress region, however, grain-boundary steps that densely formed by plastic deformation effectively suppressed GBS, resulting in the absence of brittle intergranular fracture. The mechanism of grain-boundary and stress-amplitude-dependent fracture behavior will be discussed in relation to GBS.

Multiaxial creep and cyclic plasticity in nickel-base superalloy C263

Physically-based constitutive equations for uniaxial creep deformation in nickel alloy C263 [Acta Mater. 50 (2002) 2917] have been generalised for multiaxial stress states using conventional von Mises type assumptions. A range of biaxial creep tests have been carried out on nickel alloy C263 in order to investigate the stress state sensitivity of creep damage evolution. The sensitivity has been quantified in C263 and embodied within the creep constitutive equations for this material. The equations have been implemented into finite element code. The resulting computed creep behaviour for a range of stress state compares well with experimental results. Creep tests have been carried out on double notched bar specimens over a range of nominal stress. The effect of the notches is to introduce multiaxial stress states local to the notches which influences creep damage evolution. Finite element models of the double notch bar specimens have been developed and used to test the ability of the model to predict correctly, or otherwise, the creep rupture lifetimes of components in which multiaxial stress states exist. Reasonable comparisons with experimental results are achieved. The γ ′ solvus temperature of C263 is about 925 °C, so that thermo-mechanical fatigue (TMF) loading in which the temperature exceeds the solvus leads to the dissolution of the γ ′ precipitate, and a resulting solution treated material. The cyclic plasticity and creep behaviour of the solution treated material is quite different to that of the material with standard heat treatment. A time-independent cyclic plasticity model with kinematic and isotropic hardening has been developed for solution treated and standard heat treated nickel-base superalloy C263. It has been combined with the physically-based creep model to provide constitutive equations for TMF in C263 over the temperature range 20–950 °C, capable of predicting deformation and life in creep cavitation-dominated TMF failure.

Dislocation mechanisms of mean stress effect on cyclic plasticity

Abstract The dislocation mechanisms behind sagging behavior of autosuspension spring steels are far from being understood due to their complicated microstructures. In this study, systematic cyclic loading tests were carried out on two model materials - industrial pure iron and spheroidized 1045 steel – to understand the mechanisms behind sagging. It is found that it is the different dislocation microstructures that control the cyclic creep behavior of different materials. With iron samples with low mean stress values, higher mean stresses do not trigger cell structure formation mainly due to the little requirement for high plastic strain amplitude and hence the lack of necessity for high dislocation density, and a large number of dislocation interactions through large mobile dislocations. In the case of the 1045 steel samples, the collapse of pre-existent substructures and the movement of newly generated mobile dislocations are the main reasons for the cyclic softening observed. The massive reorganization process of dislocation configurations and the competition between softening and hardening both carry on throughout the entire cycling process, but the softening process is found to be the predominant factor.

Experimental investigation of some parameters on isothermal cyclic fatigue creep behavior of AISI 3137 steel at low homologous regimes

Experimental investigation of some parameters on isothermal cyclic fatigue creep behavior of AISI 3137 steel at low homologous regimes

Temperature dependence of cyclic creep behavior of a Cu–SiO 2 bicrystal with a [0 1 1] 20° twist boundary

Temperature dependence of cyclic creep behavior has been studied using a Cu bicrystal containing dispersed SiO 2 particles and a [0 1 1] 20° twist boundary. Failure occurred at shorter times with increasing temperature and stress amplitude. The fracture-mode of the bicrystals changed sharply from transgranular to intergranular at a certain critical stress amplitude. The brittle intergranular fracture that occurred below the critical stress amplitude caused discontinuous life shortening. The observed fracture-mode change at the critical stress amplitude is discussed in relation to grain-boundary sliding and grain-boundary cracking.

Ratchetting process in the stainless steel AISI 316L at 300 K: an experimental investigation

The cyclic creep behaviour of austenitic stainless steel (316L) was studied at room temperature. Particular attention has been paid to the effect of peak stress (Σ max) and mean stress (Σ m) on the ratchet strain rate. Threshold stress (230 MPa) for the transition from retardation to acceleration in the cyclic creep has been observed and interpreted as a transition between planar slip to wavy slip. The effect of stacking fault energy on this threshold stress is also studied. The cyclic creep rate increases as a function of Σ max. The influence of maximum stress has been considered as an effect of tensile plastic strain history. This has been previously characterised in terms of back stress and effective stress. In particular, three domains have been identified—R 0, R I and R II—where the cyclic mechanisms are different. The slip mode and long-range internal stress state is at the origin of the three cyclic creep domains. Moreover, the evolution of internal stress states during cyclic creep test has been studied.

Time-dependent cyclic deformation and failure of 63Sn/37Pb solder alloy

The paper presents an investigation of fatigue characterization of 63Sn/37Pb solder material at varying loading rates and dwell times at room temperature both experimentally and analytically. The cyclic creep (ratchetting) behavior of the eutectic solder is found to be strongly dependent on the stress rate, cyclic mean stress and dwell time. The ratchetting failure of the solder material is therefore highly time-dependent. Based on the experimental results, the time-dependent deformation and damage models are established, and the cyclic failure criteria under different loading conditions were proposed. These models can be used to predict the ratchetting deformation and failure behavior, and the strain rate-dependent low cyclic fatigue life of the solder material. In addition, the effects of dwell time on low cycle fatigue of the materials can also be taken into account.