Ferdinand Dobeš

On the Monkman–Grant relation for small punch test data

Creep behavior of chromium steel (X20CrMoV 12 1) and of the low-alloy steel (14MoV 6 3) was studied at elevated temperatures. The tests were performed (i) in small punch arrangement at constant force and (ii) in conventional uniaxial mode at constant tensile stress. Time to fracture and the minimum deflection rate in small punch tests are related in a similar manner as the corresponding quantities in conventional creep tests. A formula is established for re-calculating the minimum creep rate from the minimum deflection rate.

Small Punch Testing in Creep Conditions

The creep behavior of low-alloy steel was studied using small punch tests under the action of constant force. Disks with a diameter of 8 mm and a thickness of 0.5 mm were pushed against a die with a bore of 4 mm diameter using a ball of 2.5 mm diameter. The time dependence of the central deflection was registered. The resulting dependence has the sigmoidal shape typical of conventional creep tests but with a very significant primary stage. The conventional constant-stress creep tests were performed for comparison. A simple method for the approximate correlation of the force in punch tests with the stress in conventional creep tests of the same duration is suggested.

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 evaluat...

Choice of evolution equation for internal stress in creep

Abstract The internal stress values in the course of the primary and the steady-state stages of creep in a Fe3wt.%Si alloy were measured by a technique analogical to the strain transient dip test technique. Two types of evolution equation, i.e. phenomenological and derived from dislocation kinetics, were examined by correlating them with the experimental data. In the application of the second group of equations, characteristics of the real dislocation structure were taken into account. The results of structure investigations are discussed in relation to the concept of internal stress in both homogeneous and heterogeneous dislocation structures, in the latter case using the composite model of the dislocation structure.

Interpretation of steady-state creef rate in mechanically alloyed AlCO alloys

Abstract The steady-state creep rate of pure aluminium and of four mechanically alloyed aluminium alloys was obtained within more extensive study of the creep behaviour of these materials. Creep data can be rationalized by introducing a threshold stress below which the creep rate is negligible. The estimated threshold stress grows with the increasing content of both carbon and oxygen. Within the threshold stress concept it can be shown that the data are in accord with the natural creep law. he steady-state creep rate can alternatively be described by the equation of thermally activated detachment of dislocations from dispersed particles. A new procedure for evaluation of the relaxation parameter characterizing this process is presented.

Influence of Friction on Stress and Strain Distributions in Small Punch Creep Test Models

The FEM modeling of small punch tests on miniaturized thin discs (SPT) of two heat resistant steels was performed. The FE models did represent the creep SPT, i.e. tests with constant acting force. It was shown that different values of the surface friction coefficient used in the calculations have significant impact on the calculated stress and strain state and consequently on the deformed shape of the disc. Thus, the surface friction coefficient should be considered one of the key factors for any correct correlation of SPT and uniaxial creep test results. Proper attention must be paid to the friction conditions during the long term creep deformation. An attempt to define simple approach how to relate the SPT with uniaxial tests is suggested. Some of the calculated results are compared with experiment.

Internal stress and heterogeneous dislocation structure in creep

The present work deals with the problem of microstructural interpretation of internal stress measured in high-temperature creep by the strain-transient dip-test technique. The development of microstructure in the course of creep is considered a transition from uniformly distributed dislocations to a well-developed substructure. The substructure is supposed to have a composite character that consists of cell or subgrain boundaries with few dislocations in cell or subgrain interiors. Model equations for the relation of internal stress to the parameters of the dislocation structure are discussed and examined with reference to experimental data. The evolution of internal stress in creep, evaluated using different formulae, is compared with the evolution of macroscopically measured internal stress. The use of applied-stress dependences of microstructure parameters permits quite realistic estimates of the values of internal stress in steady-state creep.

Relation between Uniaxial and Equi-Biaxial Creep and Creep Fracture Behaviour in P91 Steel

Conventional uniaxial creep tests on massive specimens and small punch creep tests on thin discs were performed on P91 steel at temperatures from 823 to 873 K. The shapes of corresponding time dependences, i. e., creep curves and time dependence of the disc deflection, are very similar. A simple linear relationship between the applied stress in creep tests and the acting force in small punch tests leading to identical times to rupture was empirically derived. The proportionality factor reaches a value close to values observed in other creep-resistant ferritic steels.

Tertiary creep and creep fracture in mechanically alloyed Al–C–O alloys

The tertiary creep and creep fracture of a set of four mechanically alloyed aluminium alloys with different contents of oxygen and carbon were studied at 623 and 723 K. At low applied stress, the strain to fracture is low, the time to fracture is power-law dependent upon the applied stress, and the specimens fail by intergranular fracture. At high applied stress, the development of a pronounced tertiary stage is observed, the elongation to fracture is increased, the time to fracture is exponentially dependent upon the applied stress, and the fracture appearance is transgranular. The transition stress decreases with increasing temperature and volume fraction of the secondary phases. Analysis of the tertiary creep in the high-stress region leads to the conclusion that the necessary condition for fracture is given by the achievement of a critical fraction of damaged area.

Interpretation of the creep behaviour of dispersion-strengthened Al-FeNi alloys in terms of the detachment mechanism

Abstract Creep data of three powder-metallurgy-processed dispersion-strengthened Al-FeNi alloys are re-analysed in terms of the detachment model. It is shown that the data can be described by means of this model. However, the calculated stress dependence of the steady state creep rate at the highest temperature and the calculated radii of particles are smaller than the observed values.