Zbigniew L. Kowalewski

Mechanisms-based creep constitutive equations for an aluminium alloy

Abstract A number of mechanisms-based constitutive equations were assessed in an effort to describe the creep behaviour of an aluminium alloy at 150°C. It was found that a sinh function of stress, rather than the usually used power law, is best able to describe the strain rate and rupture behaviour over the narrow stress range analysed. A single state variable theory which represents a dominant damage mechanism is not capable of predicting the shape of the tertiary curve; however, a two state variable theory which represents two mechanisms provides a good description. The two relevant mechanisms identified are creep-constrained cavitation and ageing of the particulate microstructure. The non-linear equations which describe both these physical mechanisms are complex and strongly coupled. This makes it difficult to determine the constants in the constitutive equations from experimental data. The paper reports the development of automated numerical optimization techniques which overcome these difficulties.

Effect of cyclic loading on the yield surface evolution of 18G2A low-alloy steel

Experimental analysis is presented of the plastic properties of 18G2A steel (notation according to Polish Standards) in the as-received state, and of the same material subjected to cyclic predeformation in different directions of the two-dimensional stress space (σxx, τxy). The analysis was made by studying the position in stress space and by determination of typical dimensions of the yield surfaces. The initial yield surface has been determined using a number of specimens which were loaded up to the plastic range along different stress directions, and this surface was used as the starting point for comparative studies of yield surfaces of the cyclic prestrained material. Cyclic predeformations were induced by loading at ambient temperature. After predeformation, yield surfaces were determined by the technique of sequential probes of the single specimen. The anisotropic yield condition due to Szczepinski was shown to model the experimental results well. Prior cyclic loading induced the softening effect observed during subsequent monotonic loading of the steel in the plastic strain range considered.

Experimental and theoretical evaluation of a high-accuracy uni-axial creep testpiece with slit extensometer ridges

The paper evaluates a previous uni-axial creep testpiece design which has slitted extensometer ridges to relieve the constraint to axial deformation provided by conventional, or unslitted, ridges. The results of experiments are reported on slitted and unslitted ridged testpieces; and they are used together with theoretical studies to evaluate the effectiveness of the slitted ridged testpiece design for different specimen gauge lengths. Firstly, the test results are used to derive true constitutive equations (i.e. those equations for testpieces without ridges), these are then used to predict the response of the majority of the testpieces, using the creep Continuum Damage Mechanics (CDM) Finite Element solver DAMAGE XX. The creep rupture mechanisms, lifetimes and accuracy of the measured creep strains have been determined theoretically for testpieces with unslitted and slitted extensometer ridges and gauge lengths of 50, 30 and 10 mm. Close agreement has been obtained between the experimental and theoretical results for the creep failure mode and lifetimes. It has been shown how the accuracy of the creep behaviour measured using short gauge length testpieces (10 mm), of the type used in tension-compression testing, may be greatly improved by using slitted extensometer ridges.

Comparison of Properties of Magnetoacoustic Emission and Mechanical Barkhausen Effects for P91 Steel After Plastic Flow and Creep

The correlation between magnetoacoustic emission intensity and mechanical Barkhausen effect intensity has been tested in the case of P91 steel having different microstructure states due to either plastic flow at room temperature or speeded-up creep, performed under stress 290 MPa at temperature 773 K. The plastic strain range for the first set of samples has been from 2.0% up to 10.5% and for the second one from 0.85% up to 10.0%. Both effects revealed analogous monotonous decrease of their intensities as a function of resulting plastic strain. The decrease was higher for plastic flow than for creep. Those results are well consistent with results of tensile tests of the as damaged samples.

Mechanical Properties of A359/SiCp Metal Matrix Composites at Wide Range of Strain Rates

The paper presents constitutive model of the aluminium metal matrix composite reinforced by a silicon carbide. Developed equation includes an empirically estimated term which takes into account softening effects of the composite due to reinforcement damages at a large strain. Experimental investigation of the aluminium based MMCs reinforced by silicon carbide of volume fraction equal to 0%, 10%, 20% and 30% were carried out. Tests were conducted at wide range of strain rates and large magnitudes of strains. Comparison between experimental and predicted data shows that the elaborated model may be applied for composite materials in computer simulations of large deformations.

Experimental investigation of an anisotropy in copper subjected to predeformation due to constant and monotonic loadings

Abstract Experimental analysis of the plastic properties for both an aged pure copper and the same material subjected to various types of predeformation is presented. The analysis was made by studying; the position in stress space and by determination of dimensions of the yield surfaces. The initial yield surface has been determined using the probing technique of a single specimen and this surface was used as the starting point for comparative studies of yield surfaces of the prestrained material. The material predeformations of the same magnitude were induced either by a creep process at elevated temperature or by monotonic loading at ambient temperature. After required values of plastic prestrain were attained, yield surfaces were determined by the same method as for the non-prestrained material. The anisotropic yield condition was used to create a least squares fit of the experimental data.

Multiparameter analysis of the Barkhausen noise signal and its application for the assessment of plastic deformation level in 13HMF grade steel

The paper presents the results of multiparameter analysis of Barkhausen noise (BN) signal properties. In addition to the commonly used quantifiers of the BN signal, such as amplitude, integral of the BN envelope or results of pulse count analysis, we propose an additional analysis based on the change in magnetizing current amplitude. As it turns out the character of the change of the BN signal (as a function of the plastic deformation level) measured for various magnetizing currents differs significantly. Being so, a comparison of the results obtained for at least two magnetizing intensities gives a much better description of a plastic deformation level. In addition to that we observe two monotonic changes in the BN signal properties—a systematic shift of the BN signal peak position and the increase in the frequency for which the maximum in the BN FFT spectra occurs.

Assessment of cyclic properties of 18G2A low-alloy steel at biaxial stress state

SummaryAn evolution of plastic properties of the 18G2A low-alloy steel due to cyclic predeformation in different directions of the two-dimensional stress space (σxx,τxy) is assessed on the basis of cyclic curves. The initial material properties have been experimentally evaluated by the analysis of preliminary yield surface. It was made by studying the position in stress space and typical dimensions of the yield surface. The initial yield locus has been determined using a number of specimens which were loaded up to the plastic range along different stress directions. This surface was used as the starting point for comparative studies of plastic properties evolution due to the cyclic prestraining. Cyclic predeformations were induced by monotonic loading at ambient temperature under constant and gradually decreasing amplitude. It is shown that prior cyclic loading induced a softening effect observed during subsequent monotonic loading of the steel. A new concept of the assessment of plastic properties of the cyclically prestrained material is proposed. It deals with the “cyclic yield surface” determination.

Possibility of Application of Magnetoacoustic Emission for the Assessment of Plastic Deformation Level in Ferrous Materials

The paper presents the results of an attempt to use the magnetoacoustic emission (MAE) signal for the nondestructive evaluation of plastic deformation levels in samples subjected to tensile loading. Four varied materials have been investigated: 13HMF, CSN12021 and P91 steels and commercially available Armco Iron. As it turned out various magnetic and magnetoacoustic properties of materials subjected to plastic tensile deformation behave in different ways for various materials, yet it is universally true that the steepness of the magnetic hysteresis loops decreases, thus leading to the increase in the separation of the “knee” portions of those loops. The MAE signal, being directly connected with the processes taking place for the magnetic field intensity at which those “knees” are observed (namely the 90° domain wall creation/annihilation and movement) can be used for the detection of that increased separation. As a suitable parameter we propose either the MAE peak separation (if two maxima are visible) or the separation between the peaks obtained for increasing and decreasing magnetizing field. An important feature of those parameters is that in all the investigated materials we observe a monotonous increase of such separation.

Creep behavior of copper under plane stress state

Abstract The aim of this paper is to present results of creep experiments carried out on thin-walled tubular specimens of pure copper under combined tension and torsion at 573 K. The experimental program consists of creep tests for the material in the virgin state, and for the same material prestrained plastically at the room temperature by uniaxial tension or by pure torsion. The modified Nortons law, based on experimental observations, was proposed to describe the rate of secondary creep. This modification is based on the assumption that the material parameters may be expressed as the functions depending on the direction of the loading stress vector. Results are discussed in terms of a creep surface concept. The changes in size and shape of the creep surfaces caused by plastic prestrains express the evolution of the initial creep surface for virgin material.