Crack Propagation In Steel Research Articles

Acoustic emission monitoring of fatigue crack propagation

Abstract The monitoring of fatigue crack propagation in steel and welded steel compact tension and T-section girder test specimens, using an advanced acoustic emission system with accurate source location, is described. The compact tension test specimens were subjected to load ratios of 0.1, 0.3, 0.5 and 0.7 while the T-section girders were subjected to a load ratio of 0.3. Located acoustic emission events were filtered for a narrow band containing the fatigue crack, and separated for different regions of the applied load range. The test results indicate that acoustic emission count rates, for small percentages of the applied load range close to the peak load, show reasonable correlation with crack propagation rates. Based on these correlations it may be possible to predict the remaining service life of fatigue damaged structures from the results of short term acoustic emission monitoring.

Influence of a gaseous atmosphere on fatigue crack propagation

The paper presents a review of the current knowledge on the influence of gaseous atmospheres, and primarily ambient air, on fatigue crack propagation in metallic alloys. Experimental evidence of the effect of exposure to ambient air or any moist environment on fatigue crack propagation in steels is first proposed. The different interacting processes are analyzed so as to clearly uncouple the influence of the various factors on crack growth resistance. Two distinct mechanisms are identified : the adsorption of vapour molecules and hydrogen assisted fracture at crack tip.

The effects of hyperbolic heat conduction around a dynamically propagating crack tip

Using infrared detectors, Zehnder and Rosakis (1991, J. Mech. Phys. Solids 39(3), 385), Zehnder and Kallivayalil (1991, SPIE ISS4A, 48) and Mason and Rosakis (1992, SN Report 92-2), have recorded the temperature field around a dynamically propagating crack tip travelling at constant velocity in several metals. At the same time, Tzou (1990a, J. Heat Transfer 112, 21, 1990b, Inst. Heat Mass Transfer 33(5), 877) has suggested that the temperature field around a propagating crack tip might exhibit some of the characteristics of hyperbolic heat conduction. In this paper a corrected solution of the hyperbolic heat conduction equation for a travelling point source is derived. Then an experimental estimate of the active plastic zone (heat generating zone) at a crack tip is used for various experimental conditions to examine the possible effects of hyperbolic heat conduction around a propagating crack tip. Finally, using the actual experimental conditions of Zehnder and Rosakis (1991) Zehnder and Kallivayalil (1991) and Mason and Rosakis (1992) it is shown that no effects of hyperbolic heat conduction are observed around a propagating crack tip. It is seen that, due to adiabatic conditions at the crack tip during these experiments, the solution of the hyperbolic heat equation is indistinguishable from the solution of the parabolic heat conduction equation for crack propagation in steel.

Temperature effects on the mechanism of time independent hydrogen assisted fatigue crack propagation in steels: Marrow, T.J., King, J.E. and Cotterill, P.J. Acta Metall. Mater. Aug. 1992 40, (8), 2059–2068

An inverse heat conduction problem in a system is solved using a non-integer identified model as the direct model for the estimation procedure. This method is efficient when some governing parameters of the heat transfer equations, such as thermal conductivity or thermal resistance, are not known precisely. Reliability of the inversion depends on the precision of the identified model. From considerations on the analytical solutions in simple cases and on the definition of non-integer (or fractional) derivative, the non-integer model appears to be the most adapted. However, some experiments do need to be carried out on the physical thermal system before it can be identified. An application that consists in estimating the heat flux in a turning tool insert during machining is presented. First, identification is performed using a specific apparatus that permits a simultaneous measurement of temperature and heat flux in the insert. Then, during machining, heat flux can be estimated from temperature using this identified model.

Temperature effects on the mechanism of time independent hydrogen assisted fatigue crack propagation in steels

The effects of temperature on hydrogen assisted fatigue crack propagation are investigated in three steels in the low-to-medium strength range; a low alloy structural steel, a super duplex stainless steel, and a super ferritic stainless steel. Significant enhancement of crack growth rates is observed in hydrogen gas at atmospheric pressure in all three materials. Failure occurs via a mechanism of time independent, transgranular, cyclic cleavage over a frequency range of 0.1–5 Hz. Increasing the temperature in hydrogen up to 80°C markedly reduces the degree of embrittlement in the structural and super ferritic steels. No such effect is observed in the duplex stainless steel until the temperature exceeds 120°C. The temperature response may be understood by considering the interaction between absorbed hydrogen and micro-structural traps, which are generated in the zone of intense plastic deformation ahead of the fatigue crack tip.

Overload retardation during fatigue crack propagation in steels of different strengths: Blom, A.F. Scand. J. Metall. 1989 18, (4), 197–202

Fast Breeder Test Reactor (FBTR) is a 40MWt13MWe, mixed carbide fueled, sodium cooled, loop type reactor built at Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam. The Reactor Vessel (RV) is manufactured using modified AISI 316 austenitic stainless steel material as per FBTR specification. The acceptance criteria for non-destructive examination, quality of weld, test requirement, tolerances on various dimensions etc. specified in FBTR specification are very stringent compared to ASME Section III, Div. I, Class I components and other international codes applicable to pressure vessels and nuclear power plant components. During the manufacture and inspection of the Reactor Vessel, a systematic approach has been adopted towards the improvement of various procedures to achieve very high reliability of the Reactor Vessel. This paper explains the details of results achieved on fabrication tolerances, destructive and non-destructive testing on materials and welds and final tests on the reactor vessel.

Crack tip modeling of hydrogen environement embrittlement: Application to fracture mechanics life prediction

An integrated fracture mechanics life prediction prodedure has been advanced over the past 20 years to mitigate hydrogen-environment-enhanced subcritical crack propagation in steels. Extensive date define monotonic load threshold stress intensities and corrosion fatigue growth kinetics for ferritic and martensitic steels of a wide variety of strengths and exposed to gaseous and aqueous environments. Broadly predictive crack tip models are evolving based on advances in elastic-plastic fracture mechanics, micromechanics and occluded crack chemistry. Hydrogen environment sensors, in sity crack propagation monitors and prototype testing enable evaluations and modifications of fracture mechanisc predictions. While the approach provides major advances towards reliable material usage in energy systems, uncertainties remain; specific issues are identified for future research.

Practical strain extensometry for biaxial cruciform specimens—Part 2

Of a number of methods for testing materials under biaxial stress conditions, the cruciform specimen with tension/compression loading of the arms allows exploration of the whole in-plane failure envelope. This paper highlights the problems arising in the measuring, monitoring and control of strains in cruciform specimens subject to static and cyclic biaxial loading. Three extensometers are described which have been used respectively for testing high strain fatigue in steels, fatigue crack propagation in steels and fatigue and fracture of a glass reinforced polyester composite. Each extensometer uses a full bridge resistance strain gauge circuit; two are based on bending of a beam and the third on deformation of a thin ring. Examples are shown of stress/strain loops for equibiaxial and shear conditions from fatigue tests on a steel and g.r.p.

Practical strain extensometry for biaxial cruciform specimens—Part 1

Of a number of methods for testing materials under biaxial stress conditions, the cruciform specimen with tension/compression loading of the arms allows exploration of the whole in‐plane failure envelope. This paper highlights the problems arising in the measuring, monitoring and control of strains in cruciform specimens subject to static and cyclic biaxial loading. Three extensometers are described which have been used respectively for testing high strain fatigue in steels, fatigue crack propagation in steels and fatigue and fracture of a glass reinforced polyester composite. Each extensometer uses a full bridge resistance strain gauge circuit; two are based on bending of a beam and the third on deformation of a thin ring. Examples are shown of stress/strain loops for equibiaxial and shear conditions from fatigue tests on a steel and g.r.p.

Crack propagation of steels during low cycle thermal-mechanical and isothermal fatigue at elevated temperatures

The surface and through crack propagation of three high temperature steels in low cycle thermal-mechanical and isothermal fatigue at elevated temperatures was investigated. The rate of crack propagation obtained was correlated with the range of cyclicJ-integral, ΔJf. It was found that there is a linear relationship on alog-log plot regardless of materials, test conditions, and crack configurations. Furthermore, fatigue life predicted by integrating the equation of crack propagation was compared with experimental results.

A viscoplastic analysis of rapid crack propagation experiments in steel

I n order to investigate rapid Mode I crack propagation in steel under large scale yielding conditions a finite element program was developed in which the material is treated as elastic-viscoplastic, based on a theory by P. P erzyna. In this model, rate effects in the plastic deformation process are accounted for. The parameters of the constitutive model were determined from ordinary tensile tests on small specimens by varying the loading rate. It was found that the strain rate can satisfactorily be related to the increase of the inelastic stress-strain relation raised to some power n. The FEM-program was applied to a number of crack propagation experiments on highly loaded SEN specimens of a high strength steel. For comparison some experiments were also analysed assuming linear elastic and simulated elastic-plastic material behaviour with no rate sensitivity. It was found that combined plastic and viscous effects are of great importance for an adequate description of rapid crack growth when large scale yielding is at hand. The energy flow to the crack tip region in the viscoplastic case, which is used as a fracture parameter, seems to converge to finite and non-trivial values for sufficiently small values of the element size along the crack tip. This may be due to the asymptotic dominance of the elastic field at the crack-tip for the particular rate-dependent model used. Geometry effects, which are present in a linear elastic analysis, are shown to be negligible in the present elastic-viscoplastic analysis with respect to different specimen heights.

Features of crack propagation in steel during high-temperature low-cycle fatigue

Features of crack propagation in steel during high-temperature low-cycle fatigue

Plastic work of fatigue crack propagation in steels and aluminum alloys

The plastic work per unit area of fatigue crack propagation,U, is one of the parameters controlling the rate of fatigue crack propagation,dc/dN. The equation,dc/dN = A ΔK 4/(σfy 2μ U), was previously shown to fit the data for 7 iron and aluminum base alloys for the range of thedc/dN vs ΔK curve where the Paris relation is valid. Values ofU are now available for 6 additional alloys covering a much wider range of σy 42 to 868 MN/m2. For the total populationA = (2.8 ± 0.9) X 10-3 where 2.8 is the mean and 0.9 is the standard deviation. In this equation, σy is the 0.2 pct offset cyclic yield stress and μ is the shear modulus. The parameterU is related to microstructure and should be of interest to the metallurgist. Generally,U varies oppositely to σy due to decrease in the plastic zone size; however, the plastic strain amplitude and degree of localization of the plastic strain in the plastic zone are also important.

Fatigue Crack Propagation in Steels under Random Loading

This investigation was carried out in order to clarify experimentally fatigue crack propagation in steels under stationary random fully reversed tension-compression loading. Sine wave loads and random loads with different power spectra were applied to S20C-equivalent and S35C-equivalent steel specimens under displacement-controlled and load-controlled loading. The fundamental properties of fatigue crack propagation, the effect of power spectra, and the threshold for crack propagation in steels under random loading were clarified.

Fatigue crack propagation in steels with different yield strengths

1. Fatigue crack propagation in low-alloy and low-carbon steels can be described by the relationship d2a/dN=C(ΔK)n. Two sections — lower and upper — are observed on the curves of crack growth, differing from each other in their slope to the ΔK axis. 2. The length of the fatigue crack corresponding to the change from one section to the other increases with increasing yield strength of the steels. 3. On the lower section, with a steeper slope to the ΔK axis, corresponding to the initial stage of crack growth, the average rate of crack propagation is considerably smaller than in the upper section. 4. In the upper section the relationship between exponent n and coefficient C and the yield strength and ultimate strength of the steels is almost linear. 5. The average rate of fatigue crack propagation in low-alloy and low-carbon steels decreases with increasing yield strength, as the result of which the life of the steel increases. The largest values of vav were obtained for steel 15kp and the smallest for steel 10G2S1.

Environmentally assisted fatigue crack propagation in steel

The influence of various gasenous environments on fatigue crack propagation has been determined for three quenched and tempered steels with yield strength levels of 800 to 1400 MN/m2. The crack growth rate was increased by an order of magnitude in low pressure (13 KPa) hydrogen, and by a factor of two in most mildly aggressive environments relative to the growth rate in vacuum. The gases oxygen, acetylene, carbon monoxide, and nitrous oxide were dominant in a combined environment with hydrogen while methane and carbon dioxide had only a small effect on crack propagation when added to hydrogen. The crack propagation in acetylene was intermediate between that in hydrogen and the mildly aggressive environments. The increase in fatigue crack propagation rate in the hydrogen environment was dependent on the temperature and the cyclic stress intensity. The fracture mode was transgranular for all conditions except the hydrogen influenced HP-9-4-20 fractures. These results are discussed relative to various stages of the hydrogen embrittlement mechanisms. In pacticular, the results are discussed with respect to the adsorption-dissociation of the environment, transport of the gaseous specie within the plasticly deformed zone by mobile dislocations and interaction with segregated impurities within the metal.

Fatigue Crack Propagation in Steels under Random Loading

Fatigue Crack Propagation in Steels under Random Loading

Effect of Temperature on the Rate of Fatigue Crack Propagation in Steels during Low Cycle Fatigue

Effect of Temperature on the Rate of Fatigue Crack Propagation in Steels during Low Cycle Fatigue

Estimation of the impact strength of steels at cryogenic temperatures

1. The character of the stress state in the vicinity of notches in impact specimens, which is determined by the notch, has a substantial influence on the γ → α transformation taking place as a result of cooling and deformation of steels with metastable structures; this leads to changes in the ductility of steel. 2. Increasing the notch depth in impact specimens of metastable austenitic steels produces a decrease in their impact strength, especially in the temperature range of intense γ → α transformation. 3. The reduction produced in the impact strength of structurally stable steels by increasing the notch depth is much smaller than in the case of metastable steels, since it is associated only with a change in the stress state. 4. The energy of crack propagation in steels of various strength and with various structures can be determined by the Gulyaev and Drozdovskii methods in the entire range of cryogenic temperatures. 5. Electron fractography of fracture surfaces of impact specimens is quite an effective method of estimating the impact properties of cryogenic steels.

A mechanistic theory of hydrogen embrittlement of steels

AbstractConsideration of the relation between the cohesive energy and the surface free energy leads to the inference that dissolved hydrogen reduces the maximum cohesive resistive force of which the iron lattice is capable. This forms the basis of a mechanistic model for the velocity of hydrogen‐induced crack propagation in steels. The crack grows when the local tensile elastic stress normal to the plane of the crack equals the local maximum cohesive force per unit area as reduced by the large concentration of hydrogen drawn there by the effect of elastic stress on the chemical potential of hydrogen. The velocity of the growth is given by the solution of the relevant transport equation for the accumulation of hydrogen. Although quantifiable in principle the theory remains implicit because of current ignorance of the needed functional relationships, but some qualitative predictions and insights are possible, as well as the specification of important experiments.