Fatigue Crack Growth In Steel Research Articles

Corrosion Fatigue Crack Growth of Steels in Various Environments

Corrosion fatigue crack growth of three steels namely: AISI C1018, AISI 4340, and Stainless 17-7PH is studied in various environments. Testing medias include natural sea water, polluted sea water from an industrial site, drinking water produced from desalination process, and laboratory environments. Results obtained for the three steels are consistent in showing that polluted sea water is the most aggressive environment followed by low bicarbonate and alkalinity drinking water, then natural sea water. An attempt is made to explain this extraordinary aggressiveness of the polluted sea water and drinking water.

On the effect of crack face contact and friction due to fracture surface roughness in edge cracks subjected to external shear

An elastic-plastic model predicting the reduction of mode III stress intensity factors by fracture surface roughness is presented. Normal contact forces arise when shear displacements cause the crack faces to be wedged open due to mismatch of the fracture surface asperities. These normal forces, in turn, generate frictional forces which act in opposition to the applied shear stresses and reduce the effective stress intensity factor. The fracture surface asperities are proposed to have a maximum amplitude (crack opening) and an average wavelength. This leads to three regions of behavior: region 1, where the shear displacement at the crack mouth is less than the wavelength of the asperities; region 2, where the crack mouth shear displacement is greater than the asperity wavelength; and region 3, where the crack tip displacement is greater than the wavelength of the asperities thereby wedging the crack open to the maximum amplitude over the entire fracture surface. The magnitude of the normal stresses are proposed to be limited by the yield stress which implies that weak materials will offer less frictional resistance to shear crack growth. The dependence of the frictional stress intensity factor on crack length, applied stress intensity factor, asperity height, wavelength of the fracture surface roughness, and yield strength are discussed. The model will be used to analyse the results of two published experimental studies—monotonie mode III fracture toughness testing of Al 2O 3 and mode III fatigue crack growth in steel.

A 4-electrode analogue for estimating electrochemical reactions with bare metal surfaces at the crack tip

Summary A 4-electrode in situ fracture technique has been developed as an analogue for estimating, under potentiostatic control, the kinetics of electrochemical reactions with a bare metal surface which is coupled to an “oxidized” surface of the same material. The technique simulates the equilibration reactions that occur at the tip of a growing crack. The relevance and effectiveness of this technique have been demonstrated by the good correlation between independently measured charge transfer data and corrosion fatigue crack growth rate data obtained under an externally applied cathodic potential. The results suggest that the electrode potential at the crack tip does not deviate substantially from the free corrosion potential. With the aid of this technique, a consistent and quantifiable picture of the role of electrochemical reactions in corrosion fatigue crack growth is beginning to emerge. Further quantitative understanding of corrosion fatigue crack growth of steels in aqueous environments is now possible, and is being fully explored.

Some considerations on the occurrence of intergranular fracture during fatigue crack growth in steels

The occurrence of a maximum in the percentage of intergranular fracture on the fracture surface during the transition from intermediate to low fatigue crack growth rates has been observed for a high strength steel. It is suggested that transgranular planar slip leading to slip localization is essential in promoting intergranular fracture when the cyclic plastic zone size becomes equal to the prior austenite grain size.

Correlation between electrochemical reactions with bare surfaces and corrosion fatigue crack growth in steels

The results of this preliminary correlation demonstrate the relationship between electrochemical reactions with bare metal surfaces and CF crack growth for steels in aqueous environments, and is very encouraging. These findings provide further support for the hypothesis of surface/electrochemical react control, and indicate the effectiveness of the in-situ fracture technique in providing unambiguous measurements of bare-surface reaction kinetics. For quantitative modeling, efforts will be needed to provide measurements of current and charge densities, and to address the issue of actual surface areas of cracks. Additional research is in progress and will be reported.

Inclusion-induced anisotropy of fatigue crack growth in steel

Experiments on a free–machining steel have been undertaken to elucidate the role of non–metallic inclusions in fatigue crack growth. In the mid–to–high stress intensity range ∆K of the crack growth date da/dN versus ∆K curve inclusions are found to influence fatigue crack growth rates by plastic strain intensification. High crack growth rates are associated with testing orientations which exhibit low initiation values of fracture toughness. In the low growth rate regime, orientations which exhibit low values of fracture toughness give high threshold values of ∆K. Crack growth behaviour in this regime is strongly influenced by crack closure phenomena.MST/318

Inclusion-induced anisotropy of fatigue crack growth in steel

Inclusion-induced anisotropy of fatigue crack growth in steel

The Application of Hydrogen Permeation Measurements to the Study of Corrosion Fatigue Crack Growth in Steels

The Application of Hydrogen Permeation Measurements to the Study of Corrosion Fatigue Crack Growth in Steels

Fatigue Crack Growth of Steels in 3% Salt Water

Behavior of fatigue crack in 3% salt water is experimentally examined under the condition of controlled stress intensity factor as well as constant load amplitude. The emphasis is placed on the effects of crack opening load, frequency and test temperature on the crack growth rate.The results obtained are summarized as follows;(1) Since there is notable effect of wedge action due to the corrosion products on the crack closure, the influence of corrosion of the fatigue crack growth can be seen in a much better fashion with the diagram of the crack growth rate da/dN vs. the effective stress intensity factor range than that of da/dN vs. the stress intensity factor range.(2) As the stress ratio increases, the wedge action of the corrosion product decreases and the effective stress intensity factor range approaches to the nominal stress intensity factor range.(3) Performing tests under several combinations of load cycle frequencies and water temperatures, it is found that the growth rates in 3% salt water can be almost seven times as large as those in air for some cases. Making use of the above an accelerated test may become possible.

A quantitative analysis of structure sensitive fatigue crack growth in steels

A quantitative theory for structure sensitive fatigue crack growth is constructed. The theory relates fatigue crack growth rate to mean free path for dislocation motion, Young's modulus and yield strength. The theory agrees well with the measured growth rates in a number of steels.

Determining the maximum value of various forms of corrosion of thin-walled parts made of D16T alloy

1. J.M. Barsom, "The effect of cyclic stress form on corrosion fatigue crack propagation below K~scc in a high yield strength steel," in: Corrosion Fatigue, NACE-2 (1972), pp. 424-436. 2. O. Vosikovsky, "The growth of a fatigue crack in X-65 pipe steel in tests with a low frequency of cycles in salt and fresh water," Tr. Amer. Obshc. Inzh. Mekh. (Tr. ASME) [Russian translation], Ser. D, 97, 12-20 (1975). 3. Yu. M. Kostrykin (editor), Instructions for Analysis of Water Vapor and Sediments in the Thermal Power Industry [in Russian], Energiya, Moscow (1967). 4. O.N. Romaniv, Ya. N. Gladkii, and Yu. V. Zima, "The influence of structural factors on the kinetics of fatigue cracks in structural steels," Fiz.-Khim. Mekh. Mater., No. 2, 3-15 (1978). 5. O.N. Romaniv, "The rules of crack growth in corrosion fatigue of steels," Fiz.-Khim. Mekh. Mater., No. 3, 14-29 (1980). 6. A.K. Mindyuk, "The carbon-hydrogen influence on the strength and mechanical-corrosion properties of steel," Preprint No. 12, Fiz.-Mekh. Inst. Akad. Nauk UkrSSR, L'vov (1978). 7. J .C. Radon, C. M. Branco, and L. E. Culver, "Crack blunting and arrest in corrosion fatigue of mild steel," Int. J. Fract., 1__2, No. 3, 467-469 (1976). 8. L . K . L . Tu and B. B. Seth, "Threshold fatigue crack growth in steels," J. Testing Eval., 6, No. 1, 66-74 (1978). 9. B.F. Brown, "Stress corrosion cracks," in: The Theory of Stress Corrosion of Alloys, Atlanta, Ga. (1971), pp. 187-203. 10. H. P. Leckie and A. W. Loginow, "Stress corrosion behavior of high strength steel," Corrosion, 24, No. 9, 291-297 (1968). 11. R. Johnson, J. Bretherton, B. Tomkins, P. M. Scott, and D. R. V. Silvester, "The effects of sea water corrosion on fatigue crack propagation in structural steel," in: Proceedings of the European Offshore Steels Research Seminar, Cambridge (1978), pp. 387-414. 12. P. Doig and P. E. J. Elewitt, "Limitations of potentiostatic control in steel corrosion crack growth measurements," Met. Trans., A9, No. 3, 357-362 (1978).

Fatigue crack propagation work coefficient—a material constant giving degree of resistance to fatigue crack growth

Study on fatigue crack growth in steels was carried out from energetic point of view, i.e. taking account of plastic work around the fatigue crack. Based on the examination of the relation between fatigue crack growth rate (d a/d N) and the plastic work around the fatigue crack tip ( W 0.02 in SUS304, Fe-3Si and HT 60 steels, a material constant-fatigue crack propagation work coefficient- Q 0.02 is proposed. It is the ratio of W 0.02 to d a/d N and means the degree of the resistance to fatigue crack growth. Numerical expression of Q 0.02 by mechanical properties was derived, which is given by Q 0.02= 9.3x10 1(σ y+σ 0.2) σ y 1.3 Comparison of Q 0.02 of various steels showed that Q 0.02 of high strength steels is very small compared with that of low strength steels. Graphical representation of the relation between Q 0.02 and d a/d N at various values of ΔK/ σ y for steels revealed that d a/d N at given value of ΔK/ σ y increase with decreasing Q 0.02. It is shown that fatigue crack growth behaviour of a steel (d a/d N- ΔK relation) can be obtained from the Q 0.02-d a/d N diagram by knowing the mechanical properties. Discussion on design stress level of the steels is also given.

Acoustic emission analysis during fatigue crack growth in steel

Emissions from fatigue crack growth have been examined for three steels — A533B steel, low carbon steel and H1 weld metal — using ACEMAN, a system for acoustic emission source location and analysis. The specimen size was sufficiently large for crack propagation under plane strain conditions and for time-difference analysis using multiple acoustic sensors. Signal recognition discriminators and location resolution allowed detection of signals below background noise. Emissions, detected during 80% of the upward-going part of the load cycles, most probably arose from the region of intense plastic strain near the crack tip and from unsticking behind the tip where crack closure had occurred. Expansion of the elastic-plastic interface with crack growth was ruled out as the dominant source mechanism on several grounds. With increasing crack severity, as determined by the cyclic stress intensity factor, ΔK, the rate of emission activity increased. The event count during the creation of unit crack area was, however, independent of crack growth rate. Thus, a specific emission parameter, γ, was derived, relating emission count to new crack area. For the three steels measured, γ lay in the range 2 – 44 events per square millimetre of crack growth. Analysis of signal amplitudes in A533B steel showed that amplitude spectra varied with ΔK. At the highest ΔK (67 MN m −3 2 ) a high amplitude emission source was activated.