Creep behavior of pultruded GFRP elements – Part 2: Analytical study

This paper presents an analytical study about the viscoelastic time-dependent (creep) behavior of pultruded GFRP elements made of polyester and E-glass fibers. Experimental results reported in Part 1 are firstly used for material characterization by means of empirical and phenomenological formulations – a good general agreement is obtained using the following analytical models: (i) Findley’s power law, (ii) Bruger–Kelvin model and (iii) Prony–Dirichlet series. Based on accelerated characterization methodology – Time-Stress Superposition Principle (TSSP) coupled with Findley’s law, for a reference stress of 20% of the material ultimate stress, an elastic deformation increase of 30% is obtained after 50,000 h. The creep parameters and deformation estimated by using the Findley’s model derivations indicate a consistent prediction of time-dependent deformation and viscoelastic properties of the two types of elements analysed – laminates and beam. A straightforward formulation to predict the time-dependent elastic modulus is applied, showing that the flexural stiffness should be reduced by 25% of its initial value after 1-year and as much as 50% after 50-years. Similarly, the power law coupled to Euler’s classical beam theory suggests a reasonable adaptability to the creep phenomenon in the linear regime and proved to provide accurate predictions for deflections under flexural loading up to 40% of the ultimate strength. After 50 years, under normal service load level (1/3 of the failure load), the total creep deflection will attain almost twice the initial deflection. If taking into account the shear deformation (Timoshenko’s postulated) of the full-size element with “effective” stiffness properties such estimate is reduced nearly 25%.     

Study of geometric stability and structural integrity of self-healing glass seal system used in solid oxide fuel cells

A self-healing glass seal has the potential to restore its mechanical properties upon being reheated to the solid oxide fuel cell (SOFC) stack operating temperature. Such a self-healing feature is desirable for achieving high seal reliability during thermal cycling. Self-healing glass is also characterized by its low mechanical stiffness and high creep rate at SOFC operating temperatures. Therefore, the geometric stability and structural integrity of the glass seal system are critical to its successful application in SOFCs. This paper describes studies of the geometric stability and structural integrity of the self-healing glass seal system and the influence of various interfacial conditions during the operating and cooling-down processes using finite element analyses. For this purpose, the test cell used in the leakage tests for compliant glass seals, conducted at Pacific Northwest National Laboratory (PNNL), was taken as the initial modeling geometry. The effect of the ceramic stopper on the geometric stability of the self-healing glass sealants was studied first. Two interfacial conditions of the ceramic stopper and glass seals, i.e., bonded (strong) or unbonded (weak), were considered. Then the influences of interfacial strengths at various interfaces, i.e., stopper/glass, stopper/PEN, as well as stopper/IC plate, on the geometric stability and reliability of glass during the operating and cooling processes were examined.     

The consequences of recovery for the analysis of creep in steel structures

Creep strain and recovery data for ferritic steels are used to construct a simple model that separates permanent strain from visco-elastic, recoverable strain. The model is shown to be consistent with data from tests under gradually varying stress. The implications of this model are examined for the design analysis of representative structures. It is shown that the modelling of recovery is important in some circumstances and not in others.     

Experiments and prediction of in-cavity stress for injection molded part

Prediction of residual stress of the injection molded polymers is one of the most challenging issues in this process. To investigate the development of this residual stress, creep experiments were carried out and creep rule was found. In the light of the experimental results, a creep model for predicting in-cavity stress of the molding was built. The elastic module of material was obtained with Tait equation and its viscous factor obtained with inversion method. In-cavity stress was calculated with the model and finite element method for an injection molded plate made by ABS. The predicted results was verified by the experiments and compared with relaxation model. The results showed that the new model was more accurate than relaxation model. The solution of the problem will effectively prompt the numerical simulation of injection molding, and will be a valuable development for the quality control.     

The role of creep and fatigue in determining the high-temperature behaviour of AISI H11 tempered steel for aluminium extrusion dies

The present study was aimed to analyze the effect of loading cycles on the behaviour of the AISI H11 tool steel commonly used for aluminium extrusion dies working at high temperatures and under high, cyclic stresses. A technological test method in which the specimen geometry resembled the mandrel of a hollow extrusion die was developed. Finite element analyses were performed to aid in determining specimen geometry and dimensions as well as the levels of stress to be applied to the specimen so as to replicate the conditions typically encountered by industrial hollow extrusion dies. Tests were performed on a Gleeble thermomechanical simulator by heating the specimen using Joule's effect and by applying loading for up to 6.30 h or till specimen failure. Displacements during the tests at 380, 490, 540 and 580 °C and under the average stresses of 400, 600 and 800 MPa were determined. The specimens were tested under creep (with the load held at a fixed value), fatigue (cyclic loading) and creep–fatigue (cyclic loading with a 3 min dwell-time) loading, thereby allowing a direct comparison between different deforming mechanisms. The results showed that the test could physically simulate the cyclic loading on the hollow die during aluminium extrusion and that the creep condition represented the most severe working condition. In addition, the tests could reveal the interaction between creep and fatigue mechanisms.     

Creep-rupture strength prediction of an epoxy composite under tension

It is known that the durability of materials is related to the conditions under which they are exploited, including the loading that they undergo. The material “durability-constant load” relation is known as Creep-Rupture Strength (CRS). It takes a long time to determine CRS experimentally, and it proves to be expensive. In this paper we check a method for predicting CRS under tension that employs data from short term tests. The results for loading under tension are compared with results for the same material subjected to bending. An erratum to this article can be found at     

Creep and fracture in concrete: a fractional order rate approach

The paper analyses the interaction between strain-softening and time-dependent behaviour in the case of quasi-static fracture of concrete. A viscous element based on a fractional order rate law is coupled with a micromechanical model for the fracture process zone. This approach makes it possible to include a whole range of dissipative mechanisms in a single rheological element. Creep fracture in mode I conditions is analysed through the finite element method, the cohesive (or fictitious) crack model and a new space and time integration scheme. The comparison with creep tests executed on three-point bending conditions shows a good agreement.     

Creep behavior of cold-rolled nanocrystalline pure copper

The creep of cold rolled nanocrystalline Cu has been investigated at 20–50 °C. The most reasonable stress exponent is found to be 2, the corresponding activation energy is 0.82 eV. The results imply that the creep is possibly associated with grain boundary sliding. The increase of threshold stress after rolling is associated with the increase of microstrain.     

双层组合套管在定向井盐层段的应用

中原油田在卫２８１井定向井段首次下入双层组合套管，以预防盐层套管的损坏。本文以弹性力学有关理论为指导，对盐层段双层组合套管强度进行了理论推导计算。实践表明，双层组合套管在盐层段和其它特殊层段的应用，对预防套管损坏是切实可行的。     

Abnormal precipitation behavior of Ni3Al phase

Evolution of Ni₃Al phase in a nickel-base directionally solidified superalloy has been investigated during creep rupture testing (1253 K/150 MPa). Besides the splitting, rafting and Ostwald ripening cases, many finer Ni₃Al particles have been firstly observed to precipitate from the rafted Ni₃Al particles. A simple qualitative interpretation has been proposed.