Fatigue Crack Propagation Kinetics Research Articles

Fatigue fracture of steel with ferrite-martensite composite structure

The article presents results of the fatigue tests of natural steel composite material for cyclic bending in a zero loading cycle. Natural ferritemartensite composite (NFMC) has a structure of alternating layers of viscous ferrite and strong martensite, which determines special mechanism of crack deceleration under loading. Zero loading cycle presumes presence of tensile forces directed only in one side, which makes it possible to avoid work hardening of crack edges during its growth. A diagram of fatigue fracture was constructed using data obtained on kinetics of fatigue crack propagation and its growth rate, depending on the number of vibration cycles. Comparison of test results for the samples made of steel of the same chemical composition was carried out. In one case, the secondary sorbite structure ran through the traditional heat treatment. In the other, quenching of the initial row ferrite-pearlite structure in intercritical temperature range, led to obtaining ferrite-martensite composite layered structure. These materials had the same hardness, but the difference in structure organization caused the NFMC structure steel advantage in terms of resistance to fracture under cyclic loading. When crack approaches the martensite-ferrite interface, delamination occurs in ferrite due to tensile stresses parallel to the crack plane. Growth of a crack stops before additional energy is supplied for a new crack generation under conditions close to the uniaxial stress state. Method for determining characteristics of kinetics of crack growth under fatigue loading is presented and recommended for testing steels and alloys under conditions of cyclic load changes.

Effect of corrosive media on the cracking resistance of pipe steel under load

The kinetics of fatigue crack propagation has been studied in full-thickness specimens cut from a main pipeline under the simultaneous action of a cyclic load and a hydrogen sulfide-containing corrosive medium. The degradation mechanisms of the material are discussed using the results of optical microscopy.

Kinetics of Fatigue Crack Propagation at the Tooth Root of a Cylindrical Gear Wheel

The present paper proposes an analytical method for determining the course (path) of crack propagation. A fatigue crack arises at the tooth root of a cylindrical geared wheel due to loading connected with gear work conditions. The cracking model was built using the adequate NASGRO2/3 formula; moreover, for comparison purposes, Forman and Paris-Erdogan’s laws were utilized. The length of an increase (propagation) in the crack was directly calculated by means of the boundary element method.

Fatigue crack growth of bainitic rail steel welds

The objective of the present study is to develop and evaluate novel weld repairs of bainitic rail steel defects, such as detail fracture, induced by in service loading. Slots were machined in the bainitic rail steel to simulate the removal of service defects. Multipass gas metal arc welding was used to fill and repair the slots. Three-point bend tests of the parent and welded steels revealed that the flexural weld efficiency was 75%. Fatigue crack propagation (FCP) tests were performed on specimens from the welded joints and compared with the parent materials to determine the mechanical integrity of the slot repairs. The fatigue lifetime and FCP kinetics were similar for the parent and slot welded bainitic steels, indicating similar resistance to FCP. The fatigue fracture surface morphology of the parent and welded bainitic rail steels both revealed mostly a ductile fracture mechanism in all the three FCP stages.

Fatigue Damage Tolerance of Bainitic and Pearlitic Rail Steels

The microstructure-properties relationships, fatigue crack growth, and fracture surface morphology of J6 bainitic and premium pearlitic rail steels are studied. Specimens are cut from the middle of each railhead along the longitudinal direction of the rail using electrical discharge machining. Fatigue crack propagation tests are conducted under load control conditions using a servo hydraulic material testing system. A simple form of the fatigue power law is used to rank the fatigue crack growth kinetics of the two materials. The results show that bainitic steel has superior fatigue damage tolerance as represented by the fatigue lifetime, fatigue crack propagation kinetics, and fatigue fracture surface morphological features. Fracture surface analysis of fatigue failed specimens revealed the various mechanisms by which bainitic steel acquired its superior resistance to fatigue crack growth. Bainitic rail steel displayed more ductile fracture features such as tearing, and extensive ridge formation during the stable crack propagation process than pearlitic steel. These features are responsible for the crack deceleration and indicate a considerably high energy consuming process associated with the crack propagation of bainitic steel. Pulled-up pearlitic lamella, limited microcracks, and microvoid coalescence are found on the fracture surface of pearlitic rail steel during the stable crack process. The unstable crack propagation region of bainitic steel exhibits both large and small dimples indicative of high resistance to material separation. On the other hand, cleavage and intergranular separation are associated with the unstable crack region in pearlitic steel.

Stepwise fatigue crack propagation in polyethylene resins of different molecular structure

Stepwise fatigue crack propagation in a range of polyethylene resins, some of which are candidates for use in pipes for natural gas distribution, was studied. Examination of the effect of molding conditions on fatigue crack propagation in a pipe resin indicated that fast cooling under pressure produced specimens with the same crack resistance as specimens taken from a pipe extruded from this resin. The mechanism of stepwise crack propagation in fatigue was the same as reported previously for creep loading. Observations of the region ahead of the arrested crack revealed a complex damage zone that consisted of a thick membrane at the crack tip followed by a main craze with subsidiary shear crazes that emerged from the crack tip at an angle to the main craze. The effects of molecular parameters, such as molecular weight, comonomer content, and branch distribution, on the kinetics of fatigue crack propagation were examined. Correlation of creep and fatigue crack resistance made it possible to relate fatigue fracture toughness to molecular parameters by invoking concepts of craze fibril stability developed for creep. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2355–2369, 1998

Detection, identification and control of fatigue crack propagation kinetics on the basis of AE signals generated by crack edge contact pp. 21–26 (1991)

Detection, identification and control of fatigue crack propagation kinetics on the basis of AE signals generated by crack edge contact pp. 21–26 (1991)

Elevated temperature cracking of RSP aluminum alloy 8009: Characterization of the environmental influence

Degradation of the material properties of the Al alloy is examined to determine the effects of moist air and predissolved hydrogen on elevated-temperature fatigue and fracture resistance. Experiments are conducted at 175 C in both moist air and high vacuum with as-processed specimens and specimens that are vacuum-heat-treated. Fracture mechanics characterizations are made for initiation and propagation fracture toughnesses during rising load, fatigue-crack propagation kinetics, and sustained-load crack-growth rates. Time-dependent embrittlement at intermediate temperatures is identified in both plate and extrusion samples of the Al-Fe-Si-V alloy 8009. Intermediate temperature cracking is found to be the same for each case in both vacuum and moist air, and the vacuum heat treatment does not significantly affect the results.

The influence of water vapour on the fatigue crack propagation kinetics in pure aluminium single crystals: Tong, Z.-X., Lin, S. and Hsiao, C.-M. Metall. Trans. A May 1989 20A, (5), 925–933

The Fuyang River system (FRS) in north China, for a long time, is seriously polluted with organic compounds and heavy metals due to industrialization. However, the information on heavy metal pollution in this area is still limited, and health risks raised by trace elements are neglected up to now. To characterize the heavy metal pollutants and assess their potential ecological risks scientifically in FRS, surface sediments were collected from 66 sampling sites selected according to the hydrological and anthropogenic conditions along the river. A total of twelve metal pollutants (e.g., Cr, As, and Hg) in the sediments were detected among the distributaries. A combining application of geoaccumulation index (Igeo), ratio of secondary phase and primary phase (RSP), and the ecological risk factor (Eri) in this study gave systematic assessment results of single or combined pollution status raised by heavy metals in this area. The results show that Cr, Ni, Cu, As, Cd, Co, and Sn are mainly dispersed in the river reaches of Xingtai City and pose potential health risks in midstream, as per the geoaccumulation index and Pearson’s correlation analyses. In particular, Cd accumulates strongly in sediments of Ming River and Aixinzhuang dam from Xingtai City. In upstream and downstream of FRS, the potential ecological risk is low, except in Yongnian County where high ecological risk was caused by Cd and Hg. These findings provide new insights into the pollution characteristics and assessment of the potential ecological risks induced by heavy metals along FRS, which suggest new directions should strategically tend to typical pollutants control by policy formulation and taking effective measures to prevent and manage heavy metal pollution in North China.

The influence of water vapor on the fatigue crack propagation kinetics in pure aluminum single crystals

In general, water vapor accelerates the fatigue crack propagation rate in pure aluminum (99.999 pct) single crystals under Mode II or Mode I–II loading. This effect can be explained by the fact that hydrogen generated during corrosion fatigue enters into the specimen and is transported by dislocation sweeping during fatigue deformation from the surface to the interior of the crystal for a much longer distance than by diffusion. It is hydrogen which accelerates the fatigue crack propagation rate. However, for certain orientations, the reverse effect termedunusual retardation was observed, which can be explained by the blunting effect due to the presence of numerous microcracks along the secondary PSB.

Effect of the loading cycle asymmetry on the cyclic cracking resistance characteristics of heat-resisting steels

1. In the representation of the experimental data on the relationships of fatigue crack growth in the da/dN-KI max coordinates, the increase of the stress ratio from 0.1 to 0.75 greatly reduces the rate of fatigue crack propagation in medium- and high-amplitude sections for all the steels examined and may exert an ambiguous effect on the relationships of fatigue crack propagation in the near-threshold region. 2. The Paris dependence satisfactorily describes the kinetics of fatigue crack propagation at various values of the stress ratio only in the medium-amplitude section of the da/dN-ΔKI diagram. Use of this dependence at high values of SIF is restricted for 15Kh2MFA (I) steel by the presence of the section of the insensitivity of the rate of fatigue crack propagation at the variation of SIF, whereas for 08Kh18N10T steel it is limited by the development of nonconstricted plastic strains at the tip of the fatigue crack preventing application of the parameters of linear fracture mechanics. 3. An increase in R from 0.1 to 0.75 for the 15Kh2MFA (I) and 15Kh2MFA (II) steels has no effect on the critical stress intensity factor in cyclic loading, but it greatly reduces the critical rate of fatigue crack propagation (da/dN)c. The quantity da/dNc can be accepted as the characteristic of the transition from fatigue to brittle failure and is sensitive to the load cycle asymmetry. 4. The dependence of the rate of fatigue crack propagation on the range of the opening displacement of the crack tip for all the specimens examined is invariant to the load cycle asymmetry both in the elastic and elastoplastic region.

Effect of particles of the insoluble phase Al9FeNi on the kinetics of fatigue crack propagation in alloy AK4-1

Effect of particles of the insoluble phase Al9FeNi on the kinetics of fatigue crack propagation in alloy AK4-1

Automated high-frequency test apparatus for study of the kinetics of fatigue-crack propagation

Automated high-frequency test apparatus for study of the kinetics of fatigue-crack propagation

Kinetics of fatigue-crack propagation in steels and in titanium and nickel alloys

Kinetics of fatigue-crack propagation in steels and in titanium and nickel alloys