Bohumír Strnadel

Cyclic stress-strain characteristics of TiNi and TiNiCu shape memory alloys

The authors have examined the responses of three types of TiNi and three types of TiNiCu shape memory alloys in a pseudoelastic state to mechanical cycling in hard cycles with a constant emax and in soft cycles with a constant σmax. It was found that the transformation stress of the B2 parent phase into martensite and the hysteresis (or the amount of energy dissipated during one cycle) diminish while the residual deformation increases as the number of cycles grows. Although the maximum deformation is greater in a soft than in a hard cycle, the critical stress for inducing martensite, at least over the first 10 cycles, declines more slowly in soft than in hard loading cycles. Ternary TiNiCu alloys displayed lower transformation deformations and transformation stresses than binary TiNi alloys. In both TiNi and TiNiCu alloys, higher nickel contents were found to increase the critical stress for slip, thereby suppressing the residual plastic deformation after the specimens were unloaded and helping to stabilize the cyclic stress-strain curves.

Effect of mechanical cycling on the pseudoelasticity characteristics of TiNi and TiNiCu alloys

Abstract This paper presents the findings of an experimental study of how mechanical cycling of TiNi and TiNiCu shape memory alloys in the pseudoelastic (PE) state affects their residual elongation after unloading, their critical stress for martensite formation and their hysteresis or amount of energy dissipated during one cycle. Specimens were cycled in two basic modes: hard loading cycles at a constant ϱ max and soft ones at a constant σ ms . In the hard cycling the authors further investigated how the PE characteristics respond to various strain rates and how the strain rate changes. Each of the examined alloys was cycled in the PE deformation mode at a temperature where each specimen can be deformed at the same constant critical stress for martensite formation in the first cycle of the test. As the number of cycles increases, the residual strain ϱ o grows, while both the stress σ ms for martensite transformation and the hysteresis W decrease. The rate at which ϱ o grows depends on σ s , σ ms during cycling and the type of cycling mode. By considering the two factors σ s and σ ms , the rather complicated effect of cyclic deformation on the PE characteristics was explained. Cycling at higher strain rates has been found to increase the residual elongation left after the specimen is unloaded and to cause a more raped decline of the critical stress for martensite formation as cycling continues. After changes in the elongation rate the stability of the cyclic stress-elongation diagram depends on the amount of residual elongation present and on the stability of that diagram during the first cycling at the original elongation rate.

Statistical analysis of ice fracture characteristics

This paper addresses the fracture toughness and strength characteristics of ice taken from Notoro Lagoon in the Okhotsk Sea close to Hokkaido. Experimental values of tensile and bending strengths, and fracture toughness of sea ice conformed to Weibull statistical distribution. The proposed model predicts variation in fracture toughness as a function of the statistical distribution of ice grain sizes, effective surface energy, and elastic constants of ice. A very good agreement between experimental cumulative probability of fracture toughness and predicted distribution of fracture toughness of sea ice has been found. Computing the Weibull stress of sea ice, the dependence of fracture probability on stress intensity factor has been established. This result is in very good agreement with the presented method for the prediction of fracture toughness of sea ice.

Failure of steels caused by hydrogen induced microcracking

A model of hydrogen induced microcracking in pressure vessel steels has been developed. It is assumed that the initiation of microcracks is caused by a localized increase in hydrogen concentration at inclusions. The propagation at these site initiated microcracks results in fisheye formation and is controlled by a local stress intensity factor and fracture resistance of the matrix. Stability of the hydrogen induced brittle microcrack is dictated by the relative increase of both these quantities. This criterion has been used for the estimation of fisheye crack size at the fracture surface as a function of temperature. The introduced dimensionless coefficient of fracture resistance increase rate grows as the temperature increases. Very good agreement of predicted fisheye crack size with experimental fractographic observations has been found.

Distribution of dimple sizes on the fracture surface of spheroidized steel in the transition region

Abstract Fractography on fractured K IC specimens of spheroidized steel has revealed a statistical distribution of dimple sizes (in the ductile failure region) dependent on the temperature and distance from the pre-crack tip. The statistical distributions of carbide sizes and dimple sizes conform closely to a Weibull distribution. The size parameter of the statistical distribution of dimples increases with the distance from the pre-crack tip, but diminishes as the temperature rises. The shape parameter declines as the distance from the pre-crack tip increases, but grows with the test temperature. The experimental findings have confirmed a probabilistic theory of void nucleation ahead of the pre-crack tip. The dimple density on the fracture surface at a certain distance from the pre-crack tip corresponds to the density per unit area of carbides multiplied by the probability of void nucleation.

Low temperature transcrystalline failure in spheroidized steels

Abstract This paper presents a comprehensive statistical theory of cleavage initiation in spheroidized steels, based on assessments of the microstructural conditions that govern cleavage in each of three characteristics temperature domains which differ by their controlling micromechanisms. The microscopic cleavage criteria, used in previous work on low-temperature transcrystalline cleavage, are reformulated so as to take into account possible misalignment between the stress direction and the perpendicular to a potential cleavage plane.

Statistical model of cleavage crack propagation in spheroidized steels

Abstract This contribution presents a model of cleavage crack propagation in spheroidized steels. Microcrack initiation is considered in precipitated carbide particles when the local stress exceed cleavage strength σf of the carbide given by the Griffith criterion. A major crack is formed by microcrack linking leading to the final fracture instability. Using crack transition probabilities the probability of crack arrest at any given point within the active volume was calculated. The model suggests that the susceptibility of a microstructure to cleavage failure can be assessed by the least value of ( x s σ 2 y K 2 IC ) parameter at which the probability of crack arrest Pii is equal to unity.

Prediction of fracture toughness for a pressure vessel steel with mixed bainite-ferritic microstructure

Abstract This paper presents a model for fracture toughness prediction of a 10MnNi2Mo steel having mixed bainite-ferritic microstructure as used for the pressure components of a nuclear reactor primary circuit. In addition to cleavage microcrack nucleation in carbide particles, consideration is given to a dislocation mechanism for nucleation of microcracks in ferrite as proposed by Cottrell. The presented model showed good agreement between the predicted and experimentally determined values of stress intensity factor.

Critical characteristic ligament for cleavage fracture in spheroidized steels

Abstract A method is proposed to estimate the critical characteristic ligament corresponding to the onset of cleavage fracture in spheroidized steels. This parameter depends on the size of microstructural elements and orientation of the cleavage plane with reference to the local stresses as governed by the magnitude and direction of the applied load. Assuming that the probability of cleavage fracture initiation is the largest in region ahead of a pre-existing crack, the critical ligament size is determined by invoking a cleavage strength and position of the cleavage plane. Results are obtained for a mild steel microalloyed with niobium. Appropriate heat treatment were applied to produce a microstructure with very low ferrite content and predominantly spherically-shaped carbides. The size of the critical ligament is found to increase with decreasing temperature.

The characteristic distance of spheroidized steels

This paper presents a statistical method for calculation of the characteristic distance of spheroidized steels. The hitherto accepted cleavage stress criterion has been reformulated so as to take into account possible misalignment between the stress direction and the perpendicular to a cleavage plane. The new cleavage stress criterion implies a certain limit for the angular misalignment; when the angle exceeds this limit, the initiated microcrack can no longer spread across the carbide-ferrite interface. The elementary probability of cleavage initiation, determined using the stress criterion from probability densities of carbide particle sizes and misalignment angles, has been processed to establish the conditions governing calculations of the characteristic distance. This characteristic distance has been proved to grow with the temperature.