EFFECTS OF HEAT TREATMENT AND ENVIRONMENT ON CRACK PROPAGATION RATE IN STEEL GC-4 UNDER CORROSION FATIGUE AND STRESS CORROSION

Comparative investigations were carried out of the effect of heat treatment regimes Jor steel GC-4(40CrMnSiMoVA)on its crack propagating rates,from corrosion fatigue, (da/dN)_(CF),or stress corrosion cracking,(da/dt)_(SCC),in media with various constituents and pH values.Both(da/dN)_(CF) and(da/dt)_(SCC) accelerate with the increase of yield stress of the steel,yet the former is far less than the later.In comparison with media,the (da/dt)_(SCC) in distilled water is slightly greater than that in 3.5% NaCl solution,and the (da/dN)_(CF) in distilled water is far less than that in 3.5% NaCl solution.With the pH value increasing in 3.5% NaCl solution,the(da/dN)_(CF) lowers down and the(da/dt)_(SCC) speeds up.An explanation was also proposed with concept of the cyclic hardening and softening at crack tip,as well as the crack closure and occluded cell effect.

Compliance-based testing method for fatigue crack propagation rates of mixed-modeⅠ-Ⅱcracks

Mixed-mode I-II crack-based fatigue crack propagation(FCPⅠ-Ⅱ)usually occurs in engineering structures;however,no theoretical formula or effective compliance test methods have been established for FCPI-IIto date.For mixed-mode I-II flawed components,based on the principle of mean-value energy equivalence,we propose a theoretical method to describe the relationship between material elastic parameters,geometrical dimensions,load(or displacement),and energy.Based on the maximum circumferential stress criterion,we propose a uniform compliance model for compact tensile shear(CTS)specimens with horizontal cracks deflecting and propagating(flat-folding propagation)under different loading angles,geometries,and materials.Along with an innovative design of the fixture of CTS specimens used for FCPI-IItests,we develop a new compliancebased testing method for FCPⅠ-Ⅱ.For the 30Cr2Ni4MoV rotor steel,the FCP rates of modeⅠ,modeⅡ,and mixed-modeⅠ-Ⅱcracks were obtained via FCP tests using compact tension,Arcan,and CTS specimens,respectively.The obtained da/d N versusΔJ curves of the FCP rates are close.The loading angleαand dimensionless initial crack length a0/W demonstrated negligible effects on the FCP rates.Hence,the FCP rates of mode I crack can be used to predict the residual life of structural crack propagation.

As-extruded AZ31B Magnesium Alloy Fatigue Crack Propagation Behavior

The fatigue crack propagation rate of as-extruded AZ31B magnesium alloy was studied. Compact tension [C（T）] of the notch direction parallel （T-L）, vertical （L-T）, and inclined at 45o to the extrusion direction was investigated. The experimental results indicate that the crack propagation direction is parallel to the extrusion direction for T-L and L-T specimens, whereas the specimen inclined at 45o has an angular deflection of 9° to 11° toward the extrusion direction. The T-L specimen has the fastest fatigue crack propagation rate, and the L-T specimen has the slowest rate, the fatigue crack propagation rate of the specimen inclined at 45o is between the two directions. The crack tip propagates by both transgranular and intergranular fractures. Fatigue fractures consist of cleavage plane or quasi-cleavage and are brittle fractures. The fatigue striation occurs for specimens inclined at 45o and its size is 3-15 μm.

Effect of electric boundary conditions on crack propagation in ferroelectric ceramics

In this paper, the effect of electric boundary conditions on Mode I crack propagation in ferroelectric ceramics is studied by using both linear and nonlinear piezoelectric fracture mechanics. In linear analysis, impermeable cracks under open circuit and short circuit are analyzed using the Stroh formalism and a rescaling method. It is shown that the energy release rate in short circuit is larger than that in open circuit. In nonlinear analysis, permeable crack conditions are used and the nonlinear effect of domain switching near a crack tip is considered using an energy-based switching criterion proposed by Hwang et al.（Acta Metal. Mater.,1995）. In open circuit, a large depolarization field induced by domain switching makes switching much more diffcult than that in short circuit. Analysis shows that the energy release rate in short circuit is still larger than that in open circuit, and is also larger than the linear result. Consequently,whether using linear or nonlinear fracture analysis, a crack is found easier to propagate in short circuit than in open circuit, which is consistent with the experimental observations of Kounga Njiwa et al.（Eng. Fract. Mech., 2006）.

Analysis Approach to Durability Based on Material Initial Fatigue Quality and S-N Curve

Based on probabilistic fracture mechanics approach, a new concept of material initial fatigue quality （MIFQ） is developed. Then, the relation between S-N curve and crack propagation curve is studied. From the study, a new durability analysis method is presented. In this method, S-N curve is used to determine crack growth rate under constant amplitude loading and evaluate the effects of different factors on durability and then the structural durability is analyzed. The tests and analyses indicate that this method has lower dependence on testing, and higher accuracy, reliability and generality and is convenient for application.

EFFECT OF CATHODIC PROTECTION ON FRACTURE BEHAVIOUR OF LOWALLOY STEELS IN SEAWATER

Fracture behaviour of low-alloy steels AIST4340,HY100,Welten60,AISIA537 and A131 in artificial seawater under static,cyclic loading and at cathodic protection potential has been investigated by using the techniques of fracture mechanics, electrochemistry and electronfractography.The results reveal that at hydrogen evolution potentials(cathodic pro- tection potential)the critical yield strength required for the occurrance of SCC decreases from 985 at corrosion potential(E_c)to 872 MPa.The effect of cathodic protection on crack propagation of corrosion fatigue(CF)is not simple,it is closely related to the yield strength of steels and their SCC behaviour.For the steels with high yield strength,cathodic protection promotes(da/dN)_Ⅱ evidently,and reduces △K_(th) value.The(da/dN)-△K curves dis- play a plateau at the third stage of CF for steels with medium or low yield strength.It is men- tioned that the cathodic protection potential for oceaneering constructures should be control- led at top level of the protective range.It seems reasonable that the strength of steel for oceaneeing use might be increased by 200 or 300 MPa.

FATIGUE FRACTURE BEHAVIOR OF LOW CARBON MARTENSITE AND MARTENSITE/BAINITE DUPLEX STRUCTURE IN STEEL 20CrMnMo

Fatigue crack propagation rate,da/dN,and threshold stress intensity range,ΔK_(th),of steel 20CrMnMo containing low carbon martensite or low carbon martensite/bainite(LCM/B) duplex structure,obtained by oil quenching and austempered at 360℃,have been measured using specimens under four-point bending.Observation was also made of the crack path and fracture morphology with relation to microstructure.The formation of LCM/B duplex structure,which caused by 20% lower bainite after short-time isothermal treatment,may re- markably increase ΔK_(th)and considerably decrease da/dN.The effect of microstructure on da/dN and ΔK_(th)was discussed with the emphasis on the crack propagation path.

FATIGUE BEHAVIOUR OF M/B DUPLEX STRUCTURE IN A HIGH CARBON Cr-Mo-W STEEL

Determination was carried out of fatigue crack propagation rate of steel 65Cr5Mo3W2VSiTi (LM2)with various bainite contents and M/B duplex structure tempered at various tempera- tures.The threshold value of fatigue for each processing was estimated by experimental data. The mechanism for fatigue crack propagation of M/B duplex structure in steel LM2 seems to be non-continuous.The calculated fatigue crack micro-propagation rates are found to agree with the experimental data.

Effect ofαphase on fatigue crack growth of Ti-6242 alloy

Fatigue crack growth as a function ofαphase volume fraction in Ti-6Al-2Sn-4Zr-2Mo（Ti-6242）alloy was investigated using fatigue testing,optical microscopy,scanning electron microscopy,and transmission electron microscopy.Theα＋βannealing treatments with different solid solution temperatures and cooling rates were conducted in order to tailor microstructure with differentαphase features in the Ti-6242 alloy,and fatigue crack growth mechanism was discussed after detailed microstructure characterization.The results showed that fatigue crack growth rate of Ti-6242 alloy decreased with the decrease in volume fraction of the primaryαphase（αp）.Samples with a large-sizedαgrain microstructure treated at high solid solution temperature and slow cooling rate have lower fatigue crack growth rate.The appearance of secondaryαphase（αs）with the increase of solid solution temperature led to crack deflection.Moreover,a fatigue crack growth transition phenomenon was observed in the Paris regime of Ti-6242 alloy with 29.8% αp（typical bi-modal microstructure）and large-sizedαgrain microstructure,owing to the change of fatigue crack growth mechanism.