K. Milička

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.

Threshold creep behaviour of aluminium dispersion strengthened by fine alumina particles

Abstract Results of an investigation of creep in aluminium strengthened by 2.6 vol.% fine alumina particles—ODS–2.6Al alloy—are presented with an emphasis on threshold creep behaviour. At temperatures ranging from 623 to 723 K the measured minimum creep strain rates covered six orders of magnitude. The true threshold stress σ TH decreases with increasing temperature approximately linearly and the same holds for σ TH / G ratio, where G is the shear modulus of the matrix metal—aluminium. The threshold stress is suggested to originate from attractive dislocation/particle interaction although the detachment stress σ d scales the temperature dependence of the shear modulus. The minimum creep strain rate ϵ m is controlled by matrix lattice diffusion and the true stress exponent n of the minimum creep strain rate is close to 5. The results for ODS–2.6Al are compared with similar ones for ODS–Al–30SiC p composite. From the comparison it follows that the load transfer effect is absent in this latter composite with heavily dispersion strengthened matrix and that the threshold stress effect plays the dominant role, similar to that in ODS–2.6Al alloy.

Creep behaviour of ODS aluminium reinforced by silicon carbide particulates : ODS Al-30SiCp composite

Abstract Results of an investigation of creep behaviour in ODS aluminium reinforced by silicon carbide particulates—an ODS Al–30SiC p composite—are reported. The minimum tensile creep strain rates were measured at temperatures of 623, 673 and 723 K; applied stresses ranged from 2.77×10 −3 to 7.74×10 −3 G, where G is the shear modulus of aluminium. The creep in the composite is associated with a relatively high true threshold stress which decreases with increasing temperature more strongly than the shear modulus. The true threshold stress is suggested to originate predominantly from an attractive dislocation–fine alumina particle interaction; the presence of SiC particulates does not seem to contribute to it significantly. The minimum creep strain rate is matrix lattice diffusion controlled and the true stress exponent of this strain rate is close to 5. In this respect, the creep behaviour of the ODS Al–30SiC p composite is similar to that of an Al–30SiC p composite. Depending on the conditions of applied stress and temperature, the minimum creep strain rate in the ODS Al–30SiC p composite is up to 8 orders of magnitude lower than that in the Al–30SiC p composite. This effect of strengthening of aluminium matrix by fine alumina particles is largely, but not entirely, due to the higher threshold stress in the ODS Al–30SiC p composite. Some other possible contributions to this effect are discussed, but the load transfer is not considered to play any significant role in the heavily alumina particle strengthened Al–30SiC p composite.

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...

Creep behaviour of leaded brass

Abstract The creep behaviour of Cu–36wt.% Zn alloys containing 2.5 wt.% of lead has been investigated. Constant tensile stress creep experiments were carried out in the temperature range from 523 to 823 K and under constant stresses from 5 to 250 MPa. Scanning electron microscopy and energy dispersive X-ray analysis were performed on crept as well as uncrept parts of the specimens in order to examine the mechanisms of creep deformation in leaded brass. Attention has been paid to the role of lead on the creep behaviour of brass, the influence of temperature on creep curves, the stress sensitivity parameter and apparent activation energy of creep. The creep data obtained are examined by several methods of conventional analysis of steady-state creep.

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.

An analysis of creep data for two Fe-13Cr-1.5 Mo-Ti alloys strengthened by TiO2 and TiO2+Y2O3 particles

In the present paper, the {dot {var_epsilon}}{sub m} (T, {sigma}) creep data published by Zakine et al. for two Fe-13Cr-1.5Mo-Ti alloys strengthened by particles of TiO{sub 2} (alloy DT) and TiO{sub 2} + Y{sub 2}O{sub 3} (alloy DY) are analyzed. From the analysis the following conclusions can be drawn. Creep behavior is associated with the true threshold stress {sigma}{sub TH} decreasing with increasing temperature more quickly than the Young modulus E. The minimum creep strain rate {dot {var_epsilon}}{sub m} is matrix lattice diffusion controlled, the true applied stress exponent of minimum creep strain rate n is equal to or slightly lower than 7.5 for the alloy DY and equal to or slightly higher than 6.5 for the alloy DT. Hence the creep strain rate o the alloys DY and DT can be described by eqn. with n = 7.5 and 6.5, respectively. Since the threshold stress is not proportional to the Young modulus while the Orowan bowing stress is, the threshold stress cannot be identified with the detachment stress unless the relaxation factor k{sub R} depends on temperature.

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.