Minimum fatigue striation spacing and its stress amplitude dependence in a commercially pure titanium

We report the fatigue striation spacing of commercially pure titanium with respect to stress amplitude. Rotating bending fatigue tests were performed at 150, 175, 200 and 227 MPa in which samples did not fail at 150 MPa. No fatigue striation up to a certain crack length is observed for the studied conditions. For each condition, a minimum striation spacing value is found that decreases from 0.45 to 0.36 to 0.24 μm with increasing stress from 175 to 200 to 227 MPa, respectively. The minimum striation spacing remains constant up to a certain crack length for each condition. Thereafter, it starts increasing with crack length. It is quite understandable that the constant striation spacing over a certain crack length may not represent the macroscopic fatigue crack growth rate. The fatigue crack growth mechanism during the constant striation spacing region has been discussed in detail.     

A detailed study of the relationship between fatigue crack growth rate and striation spacing in a range of low alloy ferritic steels

It was shown that the measured average fatigue striation spacings predicted the fatigue crack growth rates for low alloy ferritic steels to within a range of ±2% to 35% with an overall average error band value of ±10.1%. When we consider the fatigue stress range this average error was reduced to only some ±4%. This was good news to both failure analysts and other workers involved in the field of component remnant life and life extension since such predicted fatigue stress ranges use real fracture characteristics observed at some point on the actual component fracture surface.These findings were applied to a real cracking problem recently reported in a steam raising plant, viz., a cracked attemperator reducer weld. In this case an NDT assessment indicated that the maximum crack depth was 7 mm while the lower bound critical crack depth was estimated at 10 mm. As such, remnant life assessments can be estimated for a series of fatigue stress ranges through the use of a reported 450 °C fatigue crack growth law for C–Mn steels.Remnant life estimates of a 7 mm deep crack for a range of stress ranges varied from 3000 to 4000 starts where the chances of the real remnant life values being greater than the calculated values was only 1 in 2. However when a realistic failure probability which reflected the serious implications of a failure event of E?4 was taken the remnant life values were reduced to around 100 starts or some 6 months of normal service.     

Fracture mechanics of surface fatigue crack growth by ductile striation space measurement in notched Waspaloy

Ductile striation space (DSS), a parameter to predict actual cracks in both direction of length and depth, is proposed for the surface fatigue crack behaviors on notched Waspaloy. Three different lengths (1, 2 and 4 mm) of artificial notches are formed as the initial surface crack for an applied maximum stress of 1,103 MPa at the stress ratio R of 0.05. These notches are similar with the appearance of the surface cracks found from the survey of compressor disk. The results show that, all initial crack sites in the depth direction started from the multiple origination sites. The DSS parameter was clearly confirmed, and it also proves the high effectiveness of the measurement in the range of the stress intensity factors for acquiring the crack growth rate on the fractured surface. The surface cracks on Waspaloy at room temperature in an atmosphere perfectly follow the relation of ΔK versus da/dN and db/dN, even though there are, respectively, earlier and later timing differences on the initiation of cracks for the notch sizes of 1 and 4 mm. The results of ΔK versus da/dN and db/dN relations show a similar slope for three different kinds of notches.     

The effect of frequency on fatigue crack propagation rate and striation spacing in 2024-T3 aluminium alloy and SM-50 steel

The effect of loading frequency on fatigue crack propagation rate and striation spacing in 2024-T3 aluminium alloy and SM-50 steel was studied at room temperature. The fatigue crack propagation rate is expressed experimentally by the equation: $\text{dc/dN = AΔK}{}^3\text{?}{}^{\mathrm{?\lambda }}$ and striation spacing, s by the following formula: $\text{s = BΔK}{}^5\text{s?}{}^{\mathrm{?\lambda }}$, where ?= loading frequency, λ = 0.08~0.14 and γ = 0'06~0?12.Fractographical studies were made.The ln dc/dN?lnΔK straight line and the ln s?lnΔK straight line intersect each other. That is, dc/dN < s below the stress intensity factor at the intersecting point, and dc/dN >s above the critical stress intensity factor. It is to be noted that dc/dN and s will not coincide except the very narrow region near the intersecting point.The formula of dc/dN experimentally obtained in this article has quite the same type as indicated by the dislocation dynamic theory of fatigue crack propagation.     

Relationship between fatigue striation height and stress ratio

In order to obtain stress ratio as well as stress intensity factor range which was applied on the material before failure from the fracture surface observation, the effect of stress ratio on fatigue striation height has been investigated on two different alloys (an Fe-3% Si single crystalline alloy and a 7075 polycrystalline aluminium alloy). Striation height has been measured both microscopically using a scanning tunnelling microscope and macroscopically using a scanning electron microscope. Striation height increased with decreasing stress ratio on both the materials at a given crack growth rate. This tendency was found by all the measuring methods used. This suggests that not only stress amplitude but also stress ratio which were applied on the material before the failure could be obtained by post-failure analysis.     

A new type of fatigue striation on PMMA immersed in organic agents

Fatigue crack growth tests of PMMA were undertaken at 295 K in organic agents with relatively high viscosity. Regular wavy striations, the wave length and amplitude of which were dependent on the characteristics of the agent, appeared on the fracture surfaces at low values of the stress intensity factor ranges. This type of striation has not been reported elsewhere. The formation mechanism of these was analysed based on the two dimensional viscous-fluid flow through a narrow gap between the crack planes. The theoretical suggestion that the wave length of the striation tends to increase with increase in the surface tension of the liquid and a decrease in its viscosity was in good agreement with the experimental trend.     

Effect of surface stress concentrators and microstructure on the fatigue limit of the material

A calculation model is developed that makes it possible to predict the fatigue limit σ₋₁ of specimens with technological surface defects based on the data on the material microstructure and stress concentrator geometry. The fatigue test results are presented for specimens with technological defects under symmetric lateral bending that are made of Ti–6Al–4V titanium alloy condensate produced by the electron-beam physical vapor deposition method. The results obtained are used to calculate σ₋₁ based on the developed model and are in very good agreement with the experimental data.     

Effects of laser carburised striation units on rail steel fatigue wear

Carburised striation units were prepared by laser processing on the surface of train rail steel specimens. The microstructures and microhardness of the units were then characterised utilising scanning electron microscopy and a hardness tester. The specimens were tested for rolling contact fatigue resistance. The test results were compared and pitting development trends were evaluated with a plotting curve. The resistance enhancement related to the addition of carbon and laser processing was examined, and specimens featuring treated units presented superior wear resistance over the untreated sample. The transformation of unit metallographic structure, grain refinement and alternating hardness distribution of the hard unit and soft substrate were determined to positively affect the resistance status.     

Standing contact fatigue testing of a ductile material: surface and subsurface cracks

During standing contact fatigue testing of case hardened steel plates, four different fatigue crack types are found: ring/cone; lateral; radial; and median cracks. Fatigue results are presented as load versus cycle number, with endurance limits and initiation laws for the ring/cone and lateral cracks. The behaviour of the radial surface strain outside the contact is altered by the presence of cracks. In particular this makes in situ crack detection possible for the lateral crack. The ductility of the tested material is found to be important for fatigue crack initiation. Numerical elastoplastic computations are used to derive the stress cycles responsible for each crack type. Stress cycles at different locations and in different directions are compared in order to explain why a particular crack type initiates. It is noted that cracks are produced normal to principal stresses of sufficient range, which are tensile sometime during the load cycle. Implications for spalling are discussed.     

Analysis of crack propagation during tooth interior fatigue fracture

Tooth interior fatigue fracture is a failure mode that is initiated as a fatigue crack in the interior of the tooth of a gear. TIFF cracks have been observed in case hardened idler gears. A fracture mechanical analysis of a TIFF crack is performed utilising FEA. A 3D TIFF crack is modelled at a position in the tooth that corresponds with an observed crack surface. The different material properties in the case and the core, determined by mechanical testing, are considered, as well as the residual state of stress due to case hardening. Various crack lengths are analysed to estimate crack propagation both into the core and into the case. The following results have been found:

• A TIFF crack initiated slightly under the case layer will propagate into the case layer where it stops.

• The main crack propagation will take place in the core.

• The crack propagation is only a small portion of the total life (order of 10⁵ cycles).

• After reaching the case layer the TIFF crack eventually deflects toward the tooth root and the upper part of the tooth falls off. The crack deflection is due to redistribution of contact loading. Several gear teeth pairs are simultaneously in contact and the cracked tooth is loaded less than the uncracked during this stage of life.

Analysis of powder steel material,laser sintering technology and machining on surface parameters and fatigue

The technology of selective laser melting (SLM) is booming in all engineering applications today and tends to expand production of statically and dynamically loaded parts, not only for the prototypes but also for direct metal end–use parts. However, apart from design and type of loading, there still is a number of real material and technological parameters that contribute to the final quality of the produced parts and affect or improve the surface integrity and life of the parts. This paper expands the results and conclusions made in some previous works and presents all data from the tests expressed in their technological relations. The main conclusion is that a suitable machining process did not affect the surface integrity and fatigue of the tested samples, and the key role in assessments were the material structure, its homogeneity and defects.     

Optimization of interior roller burnishing process for improving surface quality

Improving the surface characteristics of roller burnishing processes is one of effective approaches to decrease the machining costs and time. This paper systematically investigates the nonlinear relationships between machining parameters and surface characteristics, including surface roughness (R_a), surface hardness (H), and hardness depth (HD) of the interior roller burnishing using response surface method (RSM) model. Three process parameters considered include spindle speed S, feed rate F, and burnishing depth D. A set of physical experiments was carried out with AISI 1045 steel on a computer numerical control (CNC) milling machine using the roller burnishing tool. The target of the current complex optimization is to enhance the surface hardness and hardness depth, while the surface roughness is considered as the constraint. Finally, an evolutionary algorithm entitled archive-based micro genetic algorithm (AMGA) was used to generate a set of feasible optimal solutions and determine the best machining conditions. The results show that an appropriate trade-off solution can be drawn with regard to the low surface roughness and high the surface hardness as well as hardness depth. Furthermore, the integration of RSM model and AMGA can be considered as a powerful approach for modeling and optimizing interior roller burnishing processes.     

Initiation and propagation of fatigue crack in pre-strained material

Experimental investigations of the effect of pre-straining on the crack initiation and propagation in fatigue have been carried out on mild steel and stainless steel. X-ray analysis of the material has clearly revealed that pre-straining of the material introduces surface compressive residual stress. With increasing degree of pre-strain the crack nucleation period is delayed and the rate of crack propagation is decreased. The maximum stress intensity factor K _max is related to the rate of crack propagation by a relation da/dN=CK _max ^q , where the value of the exponent q is approximately 2.5. The constant C decreases with increasing pre-strain. Thus it has been observed that the increase in fatigue life is contributed by the delay in crack nucleation and a decrease in the rate of crack propagation. The increase in the endurance limit appears to bear a linear relation with the degree of pre-strain.

---

Résumé Des études expérimentales sur l'effet d'un écrouissage sur l'amorçage et la propagation d'une fissure de fatigue ont été effectuées sur de l'acier doux et sur de l'acier inoxydable. Par analyse aux rayons X, on a pu clairement établir que l'écrouissage introduisait en surface du matériau des tensions résiduelles de compression. En augmentant le taux d'écrouissage, la période de nucléation de la fissure est retardée et la vitesse de propagation de la fissure décroît.Le facteur d'intensité des contraintes maximum, K _max, est lié à la vitesse de propagation par la relation da/dN=CK _max ^q où l'exposant q vaut environ 2.5.La constante C décroît lorsque l'on augmente l'écrouissage. On observe donc que l'augmentation de l'endurance résulte à la fois du retard dans la formation de la fissure et de la diminution de sa vitesse de propagation. L'accroissement de la limite d'endurance semble être une fonction linéaire du taux d'écrouissage.

     

A material and environment-dependent criterion for the prediction of fatigue crack paths in metallic structures

Crack growth under mode II cyclic loading was investigated in maraging steel, ferritic–pearlitic steel and TA6V. When ΔK_II exceeds a threshold value, cracks do not bifurcate but grow in mode II over a distance which increases with ΔK_II. Shear mode crack growth was much more extensive in maraging steel than in TA6V and ferritic–pearlitic steel. This result is discussed in relation with the cyclic behaviour of the materials and the importance of friction along the crack faces. The maximum growth rate criterion is shown to be suitable for the prediction of crack paths when shear mode crack growth is likely to occur.