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.     

管道蠕变测量中温度测定的重要性

根据电力行业标准ＤＬ４４１－９１《火力发电厂高温高压蒸汽管道蠕变监督导则》中关于蠕变标记测量方法的要求，通过推导得到了测量温度的差值与换算到０℃时周长差值的关系式，并列举了北仑电厂实测得到的一些数据来进一上说明温度测量的重要性。     

Stress field near interface edge of elastic-creep bi-material

Stress fields on elastic-creep bi-material interfaces with different geometry of the interface edge are analyzed by finite element method. The results reveal that the stress highly concentrates near the interface edge at the loading instant and it gradually decreases as the creep-dominated zone expands from the small-scale creep to the large-scale creep. The stress singularity due to creep which resembles the HRR stress singularity appears near the interface edge in all cases. The stress intensity near the interface edge time-dependently decreases and becomes constant when the transition reaches the steady state. The magnitude is scarcely influenced by the edge shape of elastic material, though it depends on the edge shape of creep material. The stress intensity during the transition can be approximately predicted by the J-integral at the loading instant.