Subsequent Creep Deformation Research Articles

The effects of internal stresses on the creep deformation investigated using in-situ synchrotron diffraction and crystal plasticity modelling

In this study we investigated the evolution of meso-scale internal stresses and those effects on local deformation behaviour during incremental plastic and creep deformation in type 316H stainless steel at 550 °C, using in-situ X-ray synchrotron diffraction and crystal plasticity modelling. Owing to the fast data accusation rate of synchrotron diffraction technique, for the first time, the transient behaviour of different grain families was captured during initial fast stress relaxation period of the displacement-controlled creep dwells. Significantly it is found that the evolution of internal stresses during time independent plastic deformation is distinct from that during time dependent creep deformation. During plastic deformation, lattice strains in the {311} grain family exhibit linear behaviour whereas during creep deformation it exhibits non-linear behaviour, instead the {111} grain family exhibit linear behaviour. A novel unified constitutive law was devised within crystal plasticity framework based on the theoretical physics; the model successfully predicts the macroscopic deformation behaviour as well as the distinction between the evolution of meso-scale internal stresses during plastic and creep deformation, therefore, correctly accounting for the effect of internal stresses generated during plastic deformation on the subsequent creep deformation. The validated model has elucidated the grain-neighbouring effects on individual grain deformations.

Vertebroplasty reduces progressive ׳creep’ deformity of fractured vertebrae

Elderly vertebrae frequently develop an “anterior wedge” deformity as a result of fracture and creep mechanisms. Injecting cement into a damaged vertebral body (vertebroplasty) is known to help restore its shape and stiffness. We now hypothesise that vertebroplasty is also effective in reducing subsequent creep deformations. Twenty-eight spine specimens, comprising three complete vertebrae and the intervening discs, were obtained from cadavers aged 67–92 years. Each specimen was subjected to increasingly-severe compressive loading until one of its vertebrae was fractured, and the damaged vertebral body was then treated by vertebroplasty. Before and after fracture, and again after vertebroplasty, each specimen was subjected to a static compressive force of 1kN for 1h while elastic and creep deformations were measured in the anterior, middle and posterior regions of each adjacent vertebral body cortex, using a 2D MacReflex optical tracking system. After fracture, creep in the anterior and central regions of the vertebral body cortex increased from an average 4513 and 885 microstrains, respectively, to 54,107 and 34,378 microstrains (both increases: P<0.001). Elastic strains increased by a comparable amount. Vertebroplasty reduced creep in the anterior and central cortex by 61% (P=0.006) and 66% (P=0.017) respectively. Elastic strains were reduced by less than half this amount. Results suggest that the beneficial effects of vertebroplasty on the vertebral body continue long after the post-operative radiographs. Injected cement not only helps to restore vertebral shape and elastic properties, but also reduces subsequent creep deformation of the damaged vertebra.

非比例繰返し負荷によるマイクロ・メゾ構造変化と後続クリープ変形解析(OS11h 材料の組織・強度に関するマルチスケールアナリシス)

非比例繰返し負荷によるマイクロ・メゾ構造変化と後続クリープ変形解析(OS11h 材料の組織・強度に関するマルチスケールアナリシス)

Grain Boundary Morphology and Its Effect on Creep of TiAl Alloys

Three kinds of microstructures with different grain boundary morphologies and their creep properties of a Ti-47Al-2Nb-2Mn+0.8 vol%TiB 2 alloy are investigated. Tensile creep tests and microstructural examinations indicatethat a stabilized fine-grained fully lamellar (FGFL) microstructure with relatively smooth grain boundaries shows inferior creep resistance. A stabilized fully lamellar (FL) microstructure with well-interlocked grain boundaries and wider lamellar spacing yields reduced minimum strain rate and extended creep rupture life. Furthermore, a nearly lamellar microstructure (NL) with well-interlocked grain boundaries exhibits better creep resistance than the stabilized FGFL microstructure though it has four times wider lamellar spacing and 15 vol% equiaxed y grains at the grain boundaries, but worse creep resistance than the stabilized FL microstructure. Examinations to the deformed microstructures show that grain boundary instability involving spheroidization of the lamellae is a major microstructural degradation process, resulting in fine globular regions at the grain boundaries. Voids develop along the grain boundaries, particularly in the fine globular regions, leading to intergranual fracture. It is suggested that grain boundary sliding (GBS) is operating in the stabilized FGFL microstructure, and promotes mutually with the grain boundary instability during subsequent creep deformation, resulting in increased minimum strain rate and shortened tertiary stage. The well-interlocked grain boundary inhibits the onset of GBS and enhances the grain boundary stability effectively. These results demonstrated that the grain boundary stability has a great effect on creep behavior of TiAl Alloys.

Densification Behavior of Titanium Alloy Powder Under Hot Pressing

Densification behavior of titanium alloy powder was investigated under hot pressing at various pressures and temperatures. Experimental date were obtained for densification of titanium alloy powder under an instantaneous loading and subsequent creep deformation during hot pressing. The constitutive models of Fleck et al. and the modified Gurson were employed for thermo-phastic deformation under the instantaneous loading and that f Abouaf and co-workers for creep deformation of titanium alloy powder during hot pressing. By implementing these constitutive equations into a finite element program(ABAQUS), finite element results were compared with experimental data during hot pressing. To investigate the effect of friction between the power and die wall, density distributions of power compacts were measured and compared with finite element calculations. Finite element results from the models of Fleck et al. and the modified Gurson agreed reasonably good with experimental data for densification and density distribution of titanium alloy powder under the instantaneous loading during hot pressing. Finite element results from the model of Abouaf and co-workers, however, somewhat overestimate experimental data for creep deformation of power compacts during hot pressing.

Effect of uniaxiai ratcheting on subsequent creep deformation

Experimental observation and numerical simulation are conducted to clarify the effect of viscosity on deformation of materials. First, creep tests after the primary uniaxial ratcheting are carded out using Type 304 stainless steel. Specimens are subjected to cyclic tension-unloading with step and triangular waves during 20000 seconds as the primary ratcheting. After the ratcheting the subsequent creep test at the maximum stress is conducted during 20000 seconds. The experimental results show that the values of the subsequent creep strain after the ratcheting depend on the wave type and the time period of the ratcheting. The shorter time period of the ratcheting leads to the larger subsequent creep strain. Numerical simulations are also carried out using the constitutive model based on dislocation density [Estrinet al.]. As a result, the equivalent value of the ratcheting strain to creep strain is found out from the simulations and experiments.

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.

Substructure in a creep deformed lamellar TiAl alloy

It has recently been demonstrated that the primary creep of a fully-lamellar (FL) TiAl alloy can be significantly reduced by a prestraining technique. Specifically, when a FL-TiAl specimen was first crept to a few percent of strain at high stresses (>240 MPa), the specimen became more resistant to a subsequent creep deformation at lower stresses (<240 MPa) even after a prolonged loading. Although changing interaction between strain hardening and recovery processes which strongly affect the creep rate at a given temperature and stress. Both strain hardening and recovery processes are intimately related to the mobilization and immobilization of dislocations. In the present study, the substructure of crept alloys is examined in order to understand how and to what extent a prestraining can affect the creep behavior of the FL-TiAl alloys. The influence of lamellar orientation (with respect to the stress axis) in the deformation substructure is also investigated.

An Experimental Study on the Effect of Prior Plastic Straining on Creep Behavior of 304 Stainless Steel

Constant strain-rate plastic straining followed by creep tests were conducted to investigate the effect of prior plastic straining on the subsequent creep behavior of 304 stainless steel at room temperature. The effects of plastic strain and plastic strain-rate were delineated by a specially designed test procedure, and it is found that both factors have a strong influence on the subsequent creep deformation. A creep model combining the two factors is then developed. The predictions of the model are in good agreement with the test results.

A Rate-Dependent Inelastic Constitutive Model—Part II: Creep Deformation Including Prior Plastic Strain Effects

In this part, a constitutive model for creep deformation is presented, which includes the effect of prior plastic straining. The model is capable of predicting the influence of plastic or creep prestrain and step loading on the subsequent creep deformation as well as cross effect between prestrain and creep strain directions in a multiaxial stress state. The predictions of the model are compared with the experimental data and the agreement is found to be very good. This model, in conjunction with that presented in Part I, would enable one to predict material response to complex loading sequences in the creep-plasticity range.

The effect of thermal pretreatment on the creep of hardened cement paste

Miniature, saturated, hardened cement paste specimens were pretreated for 2 days at various temperatures in the range 20–92°C. Specimens were then loaded for 6 days and unloaded; immediate, creep and recovery strains were monitored, as were changes in the microstructure -- polysilicate and nitrogen sorption analyses were used for this purpose. For both young (28d) and mature (2.5y) specimens, thermal pretreatment resulted in significant changes in the microstructure; polysilicate content and average pore size increased while surface area decreased. In young specimens, the pretreatment resulted in a reduction in subsequent creep deformation; good correlation was found between creep and the polysilicate content at the time of load. The results for mature specimens showed a different trend; in these, pretreatment resulted in an increased creep response. Polysilicate measurements made before and after loading showed significant differences. In young specimens polysilicate content increased during loading. In contrast, depolymerisation appeared to result from loading mature specimens.