Creep Evolution Research Articles

Measurement of Micro Region Creep Deformation in the Multi-Layer Coating with Digital Speckle Correlation System

Abstract A micro region measuring system has been developed to achieve the time-dependent deformation of multi-layer coating at high temperature. The proposed system uses a long-distance microscope and digital speckle correlation method to evaluate the full creep strain fields, which is especially suited for the long-term measurement of the nonuniform deformation. The creep tension tests are performed to obtain the creep strain distribution and evolution inside the thermal barrier coating specimen. According to the deformation measured from different micro regions, we obtained some fundamental knowledge about the time-dependent behavior of the failure mechanism in multi-layer coatings. The accuracy of the new measuring system and the corresponding error analyses are also discussed in this paper.

Creep of a C-S-H gel: a micromechanical approach

Both clays and calcium silicate hydrates(the main hydration products of Portland cements) exhibit a microstructure made up of lamellar particles. The microscopic mechanism responsible for the macroscopic creep of such materials is often described as the relative sliding of the sheets. This paper proposes a micromechanical approach to estimate the macroscopic creep behavior rising from this microscopic mechanism. The asymptotic evolution of creep at both short- and long-term is especially investigated. More precisely, a non-vanishing initial elastic strain is retrieved. At long-term, a threshold on porosity appears. At lower porosities, the creep evolution admits an asymptotic strain. At higher porosities, it admits an asymptotic strain rate.

Asperity creep under constant force boundary conditions

This work presents an analysis of the transient creep deformation of a hemisphere in contact with a rigid flat, loaded by a constant force. The analysis is based on extensive finite element simulations, using a Garofalo creep law. Motivated by the simulations, an analytical framework is derived. Starting from the trivial case of a cylinder, the analytical framework can be generalized by exchanging a few functionals; this will describe the spherical geometry under analysis. The necessary functionals are derived by using a combination of analytical and empirical models. The resulting model accurately predicts the creep evolution of arbitrary asperities for a wide parameter range, requiring only the bulk material parameters. The results are interpreted in view of transient friction effects with creep as their possible cause.

Elastic response in wood under moisture content variations: analytic development

Creep evolution of timber structures results from the interaction between mechanical stresses due to different loads and hydric stresses due to moisture content variations. This paper deals with a thermodynamic approach in order to take into account a realistic elastic behavior under moisture content variations. In this context, memory effect, experimentally observed, is introduced employing a mechano-sorptive stress driven by a function dependent of the moisture content variations. This new internal thermodynamic variable enables to define an original separation of the free energy into an instantaneous recoverable part and a stored part during the last drying phase. This energy enables the modeling of the nonreversible strain process during the unloading phase. The locate state method is employed in order to define the thermodynamic function which traduces an indirect hereditary behavior between moisture content history and the stress state in the material.

Application structurale d'un modèle THCM pour le béton au jeune âge

ABSTRACT This paper presents a model predicting the early age cracking of structures. It is a promising tool to help industrials choose the materials and the construction techniques which reduce the cracking risk. The proposed model uses a multiphasic modelling of binder hydration predicting the hydration development of several anhydrous species (clinker, fly ash, silica fume…). The results of this first modelling are used as input data for a mechanical modelling of the non linear behaviour of concrete at early age (evolution of mechanical properties, creep, and damage).

Analytical studies on creep of sealed concrete under multiaxial stresses using a microplane model

The Poisson's ratio resulting from multiaxial creep of concrete, which has been reported by some experimental works, is controversial. When calculated from measured strains, Poisson's ratio is very sensitive to small experimental error. This sensitivity makes it difficult to determine whether the ratio varies with time or remains constant, and whether it has a different value with stress states. To understand precisely the properties of multiaxial creep of concrete, a new approach is needed. In this study, microplane model is applied to optimally fitting test data extracted from experimental studies on multiaxial creep of concrete. Double-power law is used as a model to represent volumetric and deviatoric creep evolutions on a microplane. Six parameters representing the volumetric and deviatoric compliance functions are determined from regression analysis, and the optimum fits accurately simulate the test data. Poisson's ratio is calculated from the optimum fits and its value varies with time. Regression analysis is also performed on the assumption that multiaxial creep of concrete is isotropic. Four parameters are determined for this condition, and the error between the optimum fits and the test data is slightly larger than that for the six-parameter regression results. The constant Poisson's ratio with time is obtained from the results of the four-parameter analysis, and the constant value can be used in practice without serious error.

미세평면 모델을 적용한 다축응력 상태의 콘크리트 크리프 분석

Poisson's ratio due to multiaxial creep of concrete reported by existing experimental works was controversial. Poisson's ratio calculated from measured strain is very sensitive to small experimental error. This sensitivity make it difficult to find out whether the Poisson's ratio varies with time or remain constant, and whether the Poisson's ratio has different value with stress states or not. A new approach method is needed to resolve the discrepancy and obtain reliable results. This paper presents analytical study on multiaxial creep test results. Microplane model as a new approach method is applied to optimally fitting the test data extracted from experimental studies on multiaxial creep of concrete. Double-power law is used as a model to present volumetric and deviatoric creep evolutions on a microplane. Six parameters representing the volumetric and deviatoric compliance functions are determined from regression analysis and the optimum fits accurately describe the test data. Poisson's ratio is calculated from the optimum fits and its value varies with time. Regression analysis is also performed assuming that Poisson's ratio remains constant with time. Four parameters are determined for this condition, and the error between the optimum fits and the test data is slightly larger than that for six parameter regression results. The constant Poisson's ratio with time is obtained from four parameter analysis results and the constant value can be used in practice without serious error.

Continuous monitoring of an active fault in a plate suture zone: a creepmeter study of the Chihshang Fault, eastern Taiwan

Data from continuously monitored creepmeters across the active Chihshang Fault in eastern Taiwan are presented. The Chihshang Fault is an active segment of the Longitudinal Valley Fault, the main suture between the converging Philippine and Eurasian plates in Taiwan. Since the 1951 earthquake (Mw=7.0), no earthquake larger than magnitude 6.0 occurred in the Chihshang area. At least during the last 20 years, the Chihshang Fault underwent a steady creep movement, resulting in numerous fractures at the surface. Five creepmeters were installed in 1998 at two sites, Tapo and Chinyuan, within the Chihshang active fault zone. One-year results (from August 1998 to July 1999) show a horizontal shortening of 19.4±0.3mm and 17.3±0.7mm, at Tapo and Chinyuan, respectively. These annual shortening rates are in a good agreement with other estimates of strain rate independently obtained from geodetic measurements and geological site investigation. The creepmeter measurements were made on a daily basis, providing accurate information on the previously unknown evolution of creep during the year. The records of fault creep at the Tapo site thus revealed close seasonal correlation with average rainfall: the period of high creep rate coincides with the wet season, whereas that of low creep rate coincides with the dry season. Also, in comparison with the Tapo site, the creep behaviour as a function of time is complex at the Chinyuan site. Possible factors of irregularity are under investigation (thermal effect acting on the concrete basement of the creepmeters, earth tide effect, water table variations in a nearby rice field, and rainfall). The comparison between GPS measurements across the Longitudinal Valley (31mm/year of horizontal displacement) and the creepmeter measurement across the Chihshang Fault zone (17–19mm/year of horizontal displacement) suggests that there exists other shortening deformation across the active fault zone in addition to those we have measured from the creepmeters.

Fictitious degree of hydration method for the basic creep of early age concrete

At the Magnel Laboratory for Concrete Research, University of Ghent, Belgium, an extensive experimental programme was set up in order to study the basic creep behaviour of early age concrete. Tests were carried out for a loading age ranging from 12 hours to 14 days, 2ith two different stress levels: 20% and 40% of the compressive strength at the age of loading. Three different cement types were considered: one Portland cement and two blast furnace slag cements. Based on the experimental results a new fundamental model is proposed. The basic creep evolution is completely related to the evolution of the degree of hydration. Time is no longer an explicit parameter. The stress-strain non-linearity of the basic creep is correlated with the stress-strain non-linearity of the instantaneous deformation at loading. Furthermore, based on the fundamental basic creep model and following the principles of the equivalent time method a new degree of hydration based formulation for the early age basic creep under varying stresses is developed: the fictitious degree of hydration method. Simulations of experimental results show that this new method provides a good alternative for the traditional superposition method.

Small punch test method assessment for the determination of the residual creep life of service exposed components

The small punch test, also known as miniature disk bend test, is a relatively new method for the creep test of specimens of small dimensions. Experiments have shown that the small punch test can be used to describe the time to failure by means of an equation of the Dorn type, in which stress is replaced by load. The activation energies calculated using such a modified equation are somewhat smaller than the activation energies calculated from the results obtained using conventional creep testing methods. However, the constant value of the activation energy and the almost constant value of the load exponent in the modified Dorn equation confirm the hypothesis about the almost iso-stress nature of the small punch creep test. A finite element analysis of the small punch test has been carried out taking into account the constitutive theta -projection concept of the law of creep of the experimental steel 14 MoV 6 3, obtained from conventional, constant load creep test data. The viscosity parameters were evaluated using the theta -projection concept, and numerical simulation was performed by prescribing the experimentally determined history of the displacement of the punch. An incremental timestepping procedure was used, which included updating of a state variable, which monitors the evolution of creep in variable load problems. There was found to be good agreement between the calculated and actual loads for the secondary stage of creep.

Solder joint lifetime assessment of electronic devices

The continuum mechanical and constitutive description of eutectic tin–lead(–silver) solder is discussed with respect to the expected mean lifetime of solder joints of electronic devices under thermal cycling conditions, especially, the creep damage evolution in the joints. Since the mechanical loads are introduced into the model by thermal loads and due to the thermal expansion mismatch of different components, the solder joints are modelled by fine meshes of non-linear triangular finite elements, whereas the remaining surrounding structure is modelled by linear beams, linear quadrilaterals and specific compatibility elements. The strain-rate and temperature-dependent creep evolution and stress–strain relations of tin–lead(–silver) are presented in tensorial form. A hypothesis of the creep damage evolution is also offered. Simulation results on the creep damage evolution in solder joints of quad flat packs (QFP) with gull wing leads are discussed and compared with thermal cycling experiments. © 1998 John Wiley & Sons, Ltd.

Towards a more fundamental non-linear basic creep model for early age concrete

Based on an extensive experimental programme a new model is proposed for the basic creep of early age concrete in compression. In this model the degree of hydration is an important parameter influencing the creep evolution as well as the final creep value. In this way the fundamental nature of the degree of hydration in the early age basic creep behaviour is illustrated. The non-linearity of the basic creep at early ages is taken into account.

A continuum damage mechanics model for cyclic creep fracture

A new continuum damage mechanics-based model is proposed to describe cyclic creep behavior of selected engineering materials under complicated loading conditions. The loading conditions involve both load holding time and load dwell time. Besides the damage parameter used by most continuum damage mechanics-based models, another state variable, the internal stress, is used to describe the microstructural changes of the materials due to creep deformation. The effect of load cycling process, the effect of load dwell time and the interaction between creep and low cycle fatigue are accounted for in evaluating the material deterioration by creep evolution and damage development. Finite element analysis results are compared with experimental results, and good correlations between them are found.

Modelling of combined high-temperature creep and cyclic plasticity in components using continuum damage mechanics

A computer-based finite-element viscoplastic damage solver has been developed for the analysis of structural components subject to combined cyclic thermal and mechanical loading. The solver, which is based on continuum damage mechanics, is able to predict the combined evolution of creep and cyclic plasticity damage by solution of the combined boundary-initial value problem. The computational difficulties which arise due to the different timescales, associated with material behaviour at the different temperatures within a thermal field, have been overcome by the use of a normalization technique. The high computer demands associated with the detailed numerical solution of combined thermo-mechanical problems, which have prevented their use in design, have been overcome by the development of a novel ‘cycle jumping’ method which avoids repetitive calculations over those cycles in which the damage fields change insignificantly. Between the ‘jumps’ in the solution technique, the damage and the strain rate fields are coupled and hence allow stress redistribution. The finite-element solver has been used to successfully predict the high temperature behaviour of a slag tap component subjected to cyclic thermal loading generated by infrared heaters and water cooling ducts. The initiation of damage and micro-cracking has been found to occur early in the lifetime at approximately 3000 cycles adjacent to the cooling duct; and the propagation of failure zones has been found to stabilize at 60 000 cycles after which no further damage evolution occurs. A further development of the technique, which requires even less computer resource, is the ‘cycle leaping’ method which neglects the effect of stress redistribution over large numbers of cycles, by leaping from the first few cycles to the final state. With this method good predictions have been made of the fields of damaged and micro-cracked material in the final state of the slag tap component. The technique has potential for use at the early or conceptual stages of design.

A Microstructural model for primary creep

The evolution of primary creep is modelled by the formation of sub-cells in which a pattern of internal stresses exists. The internal stresses have two components, one a general isotropic impediment to dislocation movement, the other an elastic back stress leading to kinematic hardening. The stresses are built up by the polarised accumulation of dislocations and they recover by diffusional relaxation within the sub-cells. The model differs from others in focussing on the sub-cell size and misorientation as the important state variables.

Les modelés hérités du Costa Rica (Inherited landforms of Costa Rica)

Abstract. - Over 3 100 m height, recent glacial and nival forms are present. The middle level is marked by increasing evolution of creep and solifluction under rain forest. Below 1 500 m height, stepped terraces and glacis, as big gullies, are the consequences of semi-arid crisis, which later took place between 23 000 and 5- 6 000 years B.P.