Key Engineering Materials Vols. 353-358

Abstract: Stress corrosion cracking (SCC) behaviors of pipeline steel X70 in various near-neutral pH soil environments with characteristics of eastern China have been studied through electrochemical measurements and slow strain rate tests (SSRT) at various electrode potentials in four solutions containing different typical soils in eastern China as well as in NS-4 solution. The SCC susceptibility in the four soil solutions was generally higher than that in NS-4 solution. There was a general trend that SCC susceptibility increased with decreasing the potential, suggesting that hydrogen induced cracking probably plays a key role in the cracking at least at low potentials and the parameters of cathodic protection in engineering should be carefully optimized to avoid SCC.

Abstract: In order to investigate the influence of loading frequency on the fatigue behaviors of the high strength steel, ultrasonic fatigue tests were carried out for a high-carbon-chromium steel and the results were compared with those of fatigue tests using conventional rotary bending fatigue test machine with a frequency of 52.5Hz. The different of fatigue strength at ultrasonic frequency level and conventional frequency level is very small and the S-N curve obtained from 20 kHz or 52.5 Hz shows the step-wise shape. The fatigue crack occurred from inclusions on the subsurface site in the long life regime and the typical surface fracture occurred in the short life one though the loading frequency level is different. It is indicated that ultrasonic fatigue method is an effective method to investigate the fatigue properties in super-long life region.

Abstract: A crack on the conductive interface between two dissimilar piezoelectric ceramics under electromechanical loading is investigated. The closed form of the singular crack tip fields for the interface crack is derived here using an analysis based on analytic functions. It is shown that the interfacial crack-tip field consists of a pair of oscillatory singularities. A closed form of the solution for a finite crack on the conductive interface between dissimilar piezoelectric media is also derived.

Abstract: Very high cycle fatigue (VHCF) properties of high-pressure die cast Magnesium alloy AZ91HP have been investigated. Ultrasonic fatigue tests up to 109 cycles were conducted at the loading frequency of 20 kHz, under R=-1 condition and in ambient air. The experimental results show that specimens fail even after 107 cycles although the scatter seems to be large probably due to the presence of materials defects. However, there seems to be a fatigue limit at about 109 cycles. The fractures contain typical brittle features, with the fatigue cracks seen to initiate from the porosity in the material, either from the surface or beneath.

Abstract: This paper investigated the thermo-mechanical bending failure characteristic of air plasma-sprayed (APS) thermal barrier coating (TBC) system at 1000 oC by three-point bending test. A through-width surface pre-crack on the centre surface of the top ceramic coating (TC) in TBC sample was firstly designed and made by air plasma spraying technology. The fracture characteristic and spallation phenomena of the TBC were investigated under bending test in air at 1000 oC. The effects of constant displacement rate, span width of bending equipment and thermal aging time on the thermo-mechanical bending failure of the APS TBC system were slight at 1000 oC. Using SEM observations and EDX analysis, the interface cracks initiation, propagation and coalescence were discussed in detail. Furthermore, the fracture surface located within the TBC close to the TBC/TGO interface for all APS TBC samples under bending at 1000 oC.

Abstract: The effect of stress concentration factor on the fatigue properties of typical austenitic stainless steel SUS304 have been investigated using the circumferentially notched specimens. The notch of specimens has six kinds of radii, i.e. ρ = ∞ (i.e. plain specimen), 2.0, 1.0, 0.6, 0.3, and 0.1 mm with constant notch depth (t=0.2mm). Though the fatigue cracks in the specimens with a blunt notch initiate at one point, those in the specimens with a sharp notch initiate at several points. There exist the slip bands in the surface of the specimen under the stress amplitude of fatigue limit by 1×107 cycles, and do not exist the non-propagating micro-cracks in all kinds of the specimens. Furthermore, it has been found that notch sensitivity of austenitic stainless steels is higher than that of a typical plain carbon steels under the higher stress concentration factor region.

Abstract: Static ductile crack propagation tests were conducted using two pressurised thin carbon-manganese steel tube specimens, and the crack tip opening angle (CTOA) was measured using digital images captured on specimen surface. The critical values of CTOA for static crack propagation decreased with crack extension while propagating crack was short. After the crack grew up to the distance of the tube specimen thickness, the critical CTOA exhibited nearly constant. These tendencies were the same as those of referential data in the thin single edge notched tension (SENT) specimens of the same tubes. Using the data of both tube specimens and SENT specimens, it was noted that the higher crack velocities slightly reduced the constant CTOA. A global constraint factor was calculated for each tube specimen, and the constant CTOA decreased with the increase of the global constraint factor. In addition, a dynamic ductile crack propagation test of a tube specimen was carried out, and a crack velocity was measured. The estimation of the constant CTOA for dynamic crack propagation in the tube specimen was performed by using the crack velocity.

Abstract: Rolling contact fatigue (RCF) cracks initiated from surface and subsurface defects are typical failure modes of bearing systems. In this paper, the effects of surface defects on RCF behavior of M50NiL and M50 steels were studied experimentally. Artificial dents were introduced on the rolling surface by using Rockwell hardness tester. The influences of dent shape and dent shoulders were examined by thrust-type RCF tests. Surface cracks initiation, propagation and spalling were monitored by scanning electron microscope (SEM) observation. The results showed that artificial dents reduce RCF lives of M50NiL and M50 steels with mineral oil lubrication. The fatigue failure initiates at the surface defect with the effects of dent shape and dent shoulder. M50NiL steel has higher contact fatigue resistance than M50. The features of surface and sub-surface cracks propagation during RCF tests were also observed.

Abstract: In this paper, the non-local theory of elasticity was applied to obtain the dynamic behavior of a Griffith crack in functionally graded piezoelectric materials under the harmonic anti-plane shear stress waves. The problem can be solved with the help of a pair of dual integral equations. Unlike the classical elasticity solutions, it is found that no stress and electric displacement singularities are present at the crack tips, thus allows us to use the maximum stress as a fracture criterion.