An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking along the wavy toe of manual fillet welds between structural steel plates. Stress relieved and as-welded cruciform joints were tested under R = -1 and R = 0 loading at different stress amplitudes. The depth and the opening level of cracks as small as 10-20 pm were monitored using miniature strain gauges installed along the toe apex, in combination with beach marking. Most of the "initiation life" (25% to 50% of total life), conventionally defined by a crack depth of 0.5 mm, is consumed in short crack propagation. Three types of short crack development for different combinations of local mean stress and stress range are identified and analyzed. Growth rates in as-welded specimens are faster than in stress relieved specimens, which results in shorter "initiation lives". This is associated with a higher effective stress range, particularly under R = -1 loading where cracks are open over nearly the full stress range. The V-notch stress intensity factor is a promising parameter to rationalize the crack "initiation life". It takes into account the thickness effect experimentally observed. Under R = -1 loading of as-welded joints, using R = 0 data and taking the whole stress range gives a reasonably conservative approximation of the crack "initiation life".

The high cycle fatigue resistances of aluminum-silicium-magnesium 357 alloy prepared by semi-solid forming (SSM) and conventional permanent mold casting (PM) are compared under fully reversed loading. Results, reported in S-N diagrams, show that rheocasting improves the as-cast alloy mean fatigue strength, by 36% at 10 7 cycles. Part of this improvement is explained by the fact that more SSM specimens are defect free than PM specimens. Comparison of the S-N diagrams also reveals that precipitation hardening slightly increases the fatigue strengths of the PM and SSM alloys, and that eutectic modification has no effect on the fatigue performance of the SSM alloy. Observation of small cracks using replicas shows the existence of crack growth decelerations at grain boundaries. No similar decelerations are observed when the crack enters a new a-Al cell within a grain. According to these results, it is proposed that in the absence of defects, the fatigue strength of aluminum alloy 357 is a function of the grain size (D) rather than of the secondary dendrite arm spacing (SDAS) or the spherical diameter of the alpha phase globules (u sph ). Thus, it is concluded that the fatigue strength improvement of the SSM alloy is also related to the smaller grain of the rheocast specimens.

Fatigue failure of a gas-nitrided 4140 steel under axial cyclic loading results from a competition between surface crack initiation in the nitrided case and internal "fish-eye" cracking inside the core material. When nitriding is deep enough, the internal mechanism prevails in smooth specimens and fatigue strength improvement as compared to base metal is about 20%. In the present study, three V-notched specimens (blunt -medium -severe) are designed to be representative of the stress gradient (i) in a small rotary bending specimen, (ii) at the root of a gear tooth, and (iii) at the root of a very sharp notch. The cracking mechanism depends on the notch severity. The nitrided blunt notch fails from a fish-eye nucleated at the case/core boundary whereas the medium and sharp notches fail from surface cracks. The high-cycle fatigue strength improvement varies from 80% for the blunt notch and to more than 100% for the sharp notch. The notch fatigue behaviour of nitrided steel is discussed by comparing the evolutions of internal and surface fatigue strengths with relative stress gradient.

A B S T R A C TThe effects of advancing speed and rotational speed on the microstructure and the mechanical properties of friction stir welded 7075-T6 aluminium alloy sheets were studied. The fatigue strength of sound joints was measured and compared to tensile testing results. Macrographs and microhardness maps were carried out to reveal the microstructure transformations. Fractographic observations were made to identify the failure mechanisms. The effects of welding parameters on the fatigue strength are discussed in terms of welding pitch k (mm/rev) and heat input (J/mm). At a high welding pitch, crack initiation at the root of the circular grooves left by the tool on the weld surface is the most detrimental failure mechanism. As the size and the depth of the grooves are related to the welding pitch, the fatigue strength increases when the welding pitch is reduced. However, when the heat input is excessive, the failure is caused by sub-surface defects produced after abnormal stirring and/or by softening of the heat-affected zone. Lateral lips on the weld surface edges also have an effect on the fatigue strength for intermediate welding pitch values. A = Elongation after tensile test FSW = Friction stir welding F X = Longitudinal force applied by the tool during welding F Z = Vertical force applied by the tool during welding H = Heat input HAZ = Heat affected zone k = Welding pitch l 0 = Gauge length of tensile test specimen LOP = Lack of penetration N = Number of cycles at rupture P FORCE = Longitudinal force power P TORQUE = Torque power P TOT = Total power R = Fatigue stress ratio T = Torque TMAZ = Thermomechanically affected zone UTS = Ultimate tensile strength v = Advancing speed Correspondence: F. Gemme.

Fatigue testing under fully reversed axial loading (R=−1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.