Variation in the physical properties during fatigue and variation in the fatigue strength of boronized St. 20 steel

Changes in both the magnetic permeability and the electrical resistance were used to study the kinetics of fatigue fracture of boronized low-carbon steel.     

Effect of liquid low-melting metals on the fatigue strength of carbon steel in relation to the stress frequency

It was established that the increase in the endurance of steel under the influence of molten metals is larger in the case of specimens with stress raisers. On the other hand, increasing the stress frequency produces a reduction in the endurance of steel in molten metals.     

Cavitation peening to improve the fatigue strength of nitrocarburized steel

Shot peening is a commonly employed technique used to improve the fatigue strength of nitrocarburized components. However, the compound layer of the component can be broken by this technique. Cavitation peening (CP) is an alternative shotless technique, which can increase the fatigue strength of the component without separation of the compound layer. To evaluate the potential of CP as a means for improving fatigue strength, nitrocarburized carbon steel (JIS S50C) has been analyzed in the non‐peened and CP conditions. The fatigue strength of CP specimens was increased by 15% in comparison with that of non‐peened specimens. This increase in the fatigue strength of CP specimen was achieved by the increase in the maximum hardness and compressive residual stress within the diffusion zone.     

Control of austenite steel fatigue strength

The physical mechanisms of structure-phase states and dislocation substructures evolution and partial restoration of resource durability in conditions of pulse electric current stimulation at the intermediate stage of fatigue curve are established in austenite steels by method of transmission electron microscopy.     

Effect of cyclic stress frequency on the fatigue strength of steel in relation to temperature and presence of stress raisers

It was shown that the endurance of steel St. 45 at 20–250° C is not dependent on the stress reversal frequency; at higher temperatures, the endurance increases with increasing frequency, this effect being more pronounced in the case of notched specimens.     

Tests to determine the fatigue strength of steel castings containing shrinkage

Little information is available on the relationships between defects and the fatigue strength of steel casting. Some fatigue tests were therefore carried out on low strength steel castings containing deliberately introduced shrinkage defects. Failure in most tests originated at defects which could be identified on radiographs, but on the basis of the radiographs, it would not have been possible to predict either the site of the failure or the fatigue strength of the individual specimens. Even gross centre-line defects had little effect on the fatigue strength of specimens tested in four point bending, although substantially decreasing the strength of specimen tested in tension. A fracture mechanics analysis was attempted but was not satisfactory due to the difficulty in estimating the stress intensity factors for the irregular flaws concerned and because of excessive yielding in many specimens.     

Corrosion fatigue behavior of friction stir processed interstitial free steel

In this study, interstitial free (IF) steel plates were subjected to double-sided friction stir processing (FSP). The fine-grained structure with an average grain size of about 12 μm was obtained in the processed zone (PZ) with a thickness of about 2.5 mm. The yield strength (325 MPa) and ultimate tensile strength (451 MPa) of FSP IF steel were significantly higher than those of base material (BM) (192 and 314 MPa), while the elongation (67.5%) almost remained unchanged compared with the BM (66.2%). The average microhardness value of the PZ was about 130 HV, 1.3 times higher than that of the BM. In addition, the FSP IF steel showed a more positive corrosion potential and lower corrosion current density than the BM, exhibiting lower corrosion tendency and corrosion rates in a 3.5 wt% NaCl solution. Furthermore, FSP IF steel exhibited higher fatigue life than the BM both in air and NaCl solution. Corrosion fatigue fracture surfaces of FSP IF steel mainly exhibited a typical transgranular fracture with fatigue striations, while the BM predominantly presented an intergranular fracture. Enhanced corrosion fatigue performance was mainly attributed to the increased resistance of nucleation and growth of fatigue cracks. The corrosion fatigue mechanism was primarily controlled by anodic dissolution under the combined effect of cyclic stress and corrosive solution.     

Thickness effects on the fatigue strength of welded steel cruciforms

Over 100 fatigue tests were conducted on high strength welded steel (HSLA-80) cruciforms of different thickness. Tests were conducted under both constant and random amplitude axial loads to characterize thickness effects on fatigue strength. Specimens were similar in size, except for the thickness which was varied between four nominal values. Examination of both experimental and analytical results (obtained using linear cumulative damage and Rayleigh approximation) indicates thicker specimens exhibit lower fatigue lives under both constant and random amplitude loadings. These results, when compared with the commonly used ‘fourth root rule' thickness correction formula, indicate the latter to be generally conservative, particularly at low stress levels.     

Influence of corrosive media on the fatigue strength of duralumin in relation to the loading frequency

Electrode potentials of D16T duralumin specimens during fatigue tests in a 3% NaCl solution at loading frequencies of 200 and 3000 cpm were measured and the fatigue strength of this material tested in air and water at loading frequencies of 200 and 6000 cpm was determined. It was established that the effect of corrosive media on the fatigue limit and endurance of duralumin depends on the loading frequency.     

Mean stress effect on fatigue strength of stainless steel

Influence of mean stress on fatigue life and fatigue limit was investigated for Type 316 stainless steel. The results for prestrained specimens revealed that fatigue life was almost the same in the same strain range regardless of stress amplitude, maximum peak stress and mean strain. The fatigue life was shortened when applying the mean stress for the same strain range, whereas it was increased for the same stress amplitude. It was shown that the reduction in fatigue life was brought about by the change in the effective strain range, which was caused by the increase in minimum peak stress and the ratcheting strain. The fatigue life could be predicted conservatively even if the mean strain was applied by assuming the effective strain range to be equal to the total strain range (by assuming the crack mouth to be never closed). It was concluded that the mean stress correction was not necessary for the load-controlled cyclic loading and for the region where the ratcheting strain was constrained.     

Influence of rest periods on the fatigue strength of structural steel

The effect of rest periods on fatigue life was investigated. Small size specimen and constant load amplitudes were used with a new developed fatigue testing machine in alternating bending. Rest periods of 1.5 min were introduced after each half cycle. The cycling rate was 0.25 cpm. Comparative tests on specimen tested without rest periods showed a substantial increase in fatigue life due to rest periods.