Fatigue Crack Initiation Process Research Articles

Corrosion fatigue crack initiation in 12% chromium stainless steel

Characteristics of corrosion fatigue failures of steam turbine blades derived from failure analysis are summarized. Then corrosion fatigue variables on corrosion fatigue strength of 12% chromium stainless steel are briefly reviewed. The paper emphasizes initiation and growth of corrosion pits in the corrosion fatigue crack initiation process. A recent investigation of the early stages of corrosion pit initiation by use of electrochemical noise measurement is demonstrated. Finally, some recommendations are given how to clarify the corrosion fatigue crack initiation process.

Fatigue Crack Initiation and Small-Crack Propagation in Zr-Based Bulk Metallic Glass

Fatigue tests on Zr-based bulk metallic glass were conducted under fully reversed cyclic bending (R ¼� 1), and the surfaces of the fatigued specimen were observed to elucidate the fatigue crack initiation mechanisms. The fracture surface was also observed to examine the crack propagation mechanism. In contrast to most brittle materials, the metallic glass showed fatigue behavior. Its fatigue strength was much lower than its tensile strength, and it had a fatigue limit. In the S-N curve, the number of cycles at the knee point was much lower than that for crystalline metals. The fatigue crack initiation process observed by atomic force microscopy (AFM) showed that cracks were initiated at the bottom of shear steps just after their formation. [doi:10.2320/matertrans.MJ200758]

The effect of cold work and heat treatment on the fatigue behaviour of 3CR12 corrosion resistant steel wire

The fatigue behaviour of 3CR12 corrosion resistant steel wires has been investigated as a function of combined effects of heat treatment, such as annealing, quenching as well as mechanical treatments, such as a cold drawing. The samples were heat-treated, cold drawn and subjected to fatigue testing under load control mode. The results obtained for cold drawn 3CR12 steel wires having initial annealed microstructure, showed that the drawing strain increases both static mechanical properties and fatigue limit. Further drawing above 0.47 strain does not increase the fatigue strength significantly. However, an application of quenching treatment followed by cold drawing, improves the fatigue limit more significantly than in case where the 3CR12 steel was annealed and cold drawn. Dual-phase microstructure obtained by quenching treatment acts beneficially in terms of delaying fatigue crack initiation and propagation processes and most importantly, a notch effect on the fatigue behaviour of 3CR12 steel wires treated in this way become insignificant.

Corrosion fatigue phenomena learned from failure analysis

The purpose of this paper is to present corrosion fatigue phenomena learned from failure analysis. The emphasis is focused upon the importance of the role of corrosion pit in the corrosion fatigue crack initiation process and intergranular fracture surface in the corrosion fatigue crack propagation process of steam turbine blade. The hydrogen effect is also stressed in the crack propagation process in the corrosion fatigue crack propagation of the cargo oil tank ship structural members for very large crude oil carrier. Finally the future problems should be solved in corrosion fatigue failure are touched in brief.

Effects of Glass-Forming Metallic Film on the Fatigue Behavior of C-2000 Ni-Based Alloy

AbstractGlass-forming Zr47Cu31Al13Ni9 (in atomic percent) films of various thicknesses were deposited on the C-2000 Ni-based alloy substrate by magnetron sputtering. Four-point-bending fatigue tests were conducted on the above system with the coated surface on the tensile side. It has been found that both fatigue life and fatigue-endurance limit can be considerably improved, while the degree of fatigue resistance enhancement depends on the maximum applied stress and the film thickness. Mechanisms of fatigue-resistance enhancements of the coated Ni-based alloy are discussed from the following aspects: reduction of surface roughness by the thin-film coating, good adhesion between thin film and substrate, development of residual compressive stress, and excellent ductility of glass-forming thin film (which would be otherwise brittle in bulk form). Of particular interest, we examine the interaction of substrate slip bands and the thin film ductile property, which would delay fatigue crack initiation process and thus extend the fatigue life.

Characterization of the cyclic deformation behaviour and fatigue crack initiation on titanium in physiological media by electrochemical techniques

Abstract In the present work, the cyclic deformation behaviour as well as fatigue crack initiation processes of commercially pure (cp) titanium grade 2 were characterized in axial single step and stepwise load increase tests by mechanical hystereses, corrosion potential and current measurements. Scanning electron microscopic investigations were used to correlate fatigue induced surface damages with the results of electrochemical investigations. Potential measurements proved to be very sensitive to indicate surface damages in situ at the very beginning. The corrosion potential is strongly related to the development of the plastic strain amplitude. Thus, cyclic softening and hardening processes are represented by a characteristic decrease and increase of the potential curve. Current measurements are less sensitive, but allow a detailed characterization of crack propagation processes.

Fatigue Crack Initiation Behavior in Ultrafine-Grained Steel Observed by AFM and EBSP

Ultrafine-grained specimens with the average grain size of less than 2µm and medium-grained specimens were prepared from steel plates produced by an advanced thermo-mechanical control process. The smooth specimens were fatigued under axial tension compression at room temperature in air. The fatigue crack initiation process was investigated by orientation imaging microscopy based on EBSP in combination with atomic force microscopy and scanning electron microscopy. The results show that fatigue cracks initiated in the simple slip lines in the medium-grained specimens. On the other hand, in the ultrafine-grained specimens, complex slip deformation was formed in the vicinity of the grain boundaries prior to the fatigue crack initiation. Fatigue cracks were nucleated at the boundary between the grains with the concentrated complex slip deformation. The crystallographic orientation analysis revealed that the complex slip deformation was formed by the cross slip of active slip systems with the largest Schmid factors. It was found that the relation between the fatigue limit and grain size of the ultrafine-grained specimens does not conform to the Hall-Petch relation of conventional ferritic-pearlitic steel due to the texture and cross slip.

Characterization of fretting fatigue crack initiation processes in CR Ti–6Al–4V

A study was conducted to quantify fretting fatigue damage and to evaluate the residual fatigue strength of specimens subjected to a range of fretting fatigue test conditions. Flat Ti–6Al–4V specimens were tested against flat Ti–6Al–4V fretting pads with blending radii at the edges of contact. Fretting fatigue damage for two combinations of static average clamping stress and applied axial stress was investigated for two percentages of total life. Accumulated damage was characterized using full field surface roughness evaluation and scanning electron microscopy (SEM). The effect of fretting fatigue on uniaxial fatigue strength was quantified by interrupting fretting fatigue tests, and conducting uniaxial residual fatigue strength tests at R=0.5 at 300 Hz. Results from the residual fatigue strength tests were correlated with characterization results. While surface roughness measurements, evaluated in terms of asperity height and asperity spacing, reflected changes in the specimen surfaces as a result of fretting fatigue cycling, those changes did not correspond to decreases in residual fatigue strength. Neither means of evaluating surface roughness was able to identify cracks observed during SEM characterization. Residual fatigue strength decreased only in the presence of fretting fatigue cracks with surface lengths of 150 μm or greater, regardless of contact condition or number of applied fretting fatigue cycles. No cracks were observed on specimens tested at the lower stress condition. Threshold stress intensity factors were calculated for cracks identified during SEM characterization. The resulting values were consistent with the threshold identified for naturally initiated cracks that were stress relieved to remove load history effects.

107 α黄銅の組合せ応力下における疲労き裂発生機構の AFM による観察

Since the surface morphology of materials can be observed with atomic-scale resolution, scanning atomic force microscopy (AFM) is a powerful technique to study mechanisms of fatigue and fracture of solid materials. In the present study, slip-band formation and fatigue crack-initiation processes in α-brass were observed by means of AFM. The depth of the intrusion drastically increased with its outgrowth to a crack. For crack-initiation from slip-band, the intrusion depth took a critical value. That is, slip distance exceeds a critical value, slip-band changes for a crack. The critical depth was almost identical between cyclic shear stress and cyclic normal stress.

Evaluation of Fatigue Damage and Fatigue Crack Initiation Process by Means of Atomic-Force Microscopy

Researches on fatigue crack-initiation processes in metals by using atomic-force microscopy (AFM) were reviewed. For fatigue of a low-carbon steel and α-brass in air, morphologies and sizes of slip-bands and crack initiation processes were discussed, and it was found that the depth of an intrusion drastically increased with its outgrowth to a crack. With coalescence of cracks, the width of cracks increased rapidly. Transgranular cracks were initiated when their accumulated sliding distances, those were evaluated from measured intrusion depth as a function of intrusion angle relative to the stress-axis, reached a critical value. The critical value was independent of stress amplitude, mean stress, and grain-size. Change of surface roughness during fatigue process was also presented.In corrosion fatigue crack initiation study of a stainless steel and a high-strength aluminum alloy, even for a crack whose optical micrographs showed that it was initiated from corrosion pit, a micro-crack was sometimes found from AFM images before pit formation. On the other hand, for a crack whose optical micrographs showed that it was initiated from grain-boundary without corrosion pit, AFM image sometimes indicated that there was a small pit at the crack initiation site.These results showed that AFM is a very useful and powerful tool for elucidating micro-mechanisms of fatigue crack initiation process.

Study on Fatigue Crack Initiation Process by Using Crystalline FEM analysis (4th Report)

Study on Fatigue Crack Initiation Process by Using Crystalline FEM analysis (4th Report)

Recent Progress of Experimental and Measuring Technology. Quantitative Evaluation of Slip-Band Growth and Crack Initiation in Fatigue of 70-30 Brass by Means of Atomic-Force Microscopy.

Since the surface morphology of materials can be observed with atomic-scale resolution, scanning atomic force microscopy (AFM) in a powerful technique to study mechanisms of fatigue and fracture of solid materials. In the present study, slip-band formation and fatigue crack-initiation processes in 70-30 brass were observed by means of AFM. The slip-band angle relative to the stress-axis at the specimen surface varied from 15 to 90°, and it appeared most frequently around 60°. The depth of the intrusion drastically increased with its outgrowth to a crack, and with coalescence of cracks, the width of cracks increased rapidly. The critical value of the intrusion depth for crack initiation was given as a function of the slip-band angle through a geometrical model for the slip-direction and the slip-step, and the critical value was independent of stress amplitude. From presice observations of slip-band tips, slip-bands had steep slope when they were blocked by grain boundaries, and the slip-bands descended by gradual slopes to plain surfaces when they terminated within grains.

506 高強度Al合金における疲労き裂発生機構のAFMによる観察(GS-3 疲労(2))

Fatigue tests of a high-strength aluminum alloy 7075-T657 were conducted under the constant amplitude in air. The fatigue crack initiation and propagation process were observed by using a optical microscopy and an atomic forced microscopy (AFM). Before crack initiation, no slip-bands were observed either by optical microscopy or AFM. After crack initiation, micro cracks were observed in front of the main crack.

349 低サイクル疲労微小表面き裂の超音波非破壊評価 : 検出限界と周波数の関係(OS2-1 欠陥検出)(OS2 超音波と材料評価)

This present study was done with the aim of improving the reliability of quantitative ultrasonic nondestructive testing. The water immersion ultrasonic method, using an oblique longitudinal wave with large incidence angle (over the critical angle), was applied to measure small surface fatigue crack due to low-cycle loading. Surface fatigue crack initiation process, beginning from persistent slip band initiation, was measured using ultrasonic microscope. To estimate the precision of measurement, four transducers with different frequency, they are 5,10,20 and 50MHz, respectively, were used to make clear the relation between the ultrasonic frequency and its minimum detectable size of surface fatigue crack.

A controlled-notch specimen to study fatigue crack initiation in bone cement.

Despite the extensive literature on the mechanical characteristics and failure properties of poly(methyl methacrylate) bone cement, little is known of its fatigue crack initiation process. The most likely in vivo bone cement fatigue crack initiation sites are internal flaws and irregularities on the bone cement surface. The stress concentration created by a flaw, and subsequently the stress state at that flaw, depends on the flaw geometry. To model the fatigue crack initiation process of a flaw, it is necessary to reproduce the stress state at that flaw. In this study, a special mold was designed to introduce notches with specific tip radii into fatigue specimens. The notch was molded into the specimen to simulate the in vivo flaw formation process. The molding method allows control of the stress concentration by specifying the notch tip radius. We created notched specimens where the tip radii of the notches ranged from "sharp" (< 3 microm) to 400 microm. The results demonstrated that notched specimens created by the special mold satisfied two necessary requirements for fatigue crack initiation studies: (1) the material microstructure at the notch tip must not be disrupted by the notching process, and (2) the notch tip stress field, determined by the notch tip geometry, must be reproducible.

On fatigue lifetimes and fatigue crack growth behavior of bone cement.

Bone cement is used to develop a mechanical bond between an artificial joint and the adjacent bone tissue, and any degradation of this bond is of serious concern since it can lead to loosening and eventually malfunction of the artificial joint. In the present study, the fatigue lives and fatigue crack propagation behavior of two bone cements, CMW Type 3 and Zimmer, were investigated, and it was found that the size and distribution of pores played a major role in influencing both the fatigue crack initiation and propagation processes. The fatigue lifetimes of CMW exceeded those of Zimmer because of a lesser density of large pores. When the fatigue lifetimes were plotted as a function of Klimax, the maximum initial stress intensity factor based upon the initiating pore size, the difference in fatigue lifetimes between CMW and Zimmer bone cements was greatly reduced. The fatigue crack growth behavior of both bone cements were similar. This is a further indication that the noted differences in fatigue lifetimes were related to the size of the pore at the crack initiating site.

113 高強度アルミニウム合金における腐食疲労き裂発生機構のAFMによる観察(GS-3 疲労き裂の発生と伝ぱ)

Corrosion fatigue crack initiation process of a high-strength aluminum alloy, 7075-T651 was observed by means of a scanning atomic force microscopy (AFM). Plane bending fatigue tests were carried out either ill air or in 3% NaCl solution. The fatigue strength in aqueous environment was much lower than that in air. Grain boundaries around specific grains were preferential site of anodic dissolution, and cracks were initiated from these grain boundaries.

Study on Fatigue Crack Initiation Process by Using Crystalline FEM analysis (2 nd Report)

A crystalline FEM theory which incorporates the Bausinger's effect is applied to investigate the relationship between the crystal direction and the progress of plastic deformation localization and irreversible slip generation, which are part of the fatigue crack initiation process, by calculating the cyclic deformation behavior of a f.c.c. crystal with high Schmid's factor which is buried in a large crystal with low Schmid's factor.As results, the followings are found : 1) In the same manner as the results for f.c.c. single crystal analyzed in the previous reports, plastic deformation localization and irreversible slip generation in the course of loading history can be simulated by using the crystalline FEM code developed in the previous reports when the buried crystal is set in the specific direction.2) Plastic deformation localization and irreversible slip generation appears only when all of the following conditions are satisfied : a) direction of crystal is set so that the coplaner or non-coplaner double slip occurs ; b) the slip directions of two activated systems meet the free surface at the same angle ; c) the slip planes of two systems meet the free surface at the same angle.3) The properties of cyclic deformation behavior in the coplaner double slip cases are affected more seriously by the deviation of loading direction than the ones in the non-coplaner double slip cases.

Microscopic and Mesoscopic Evaluations of Materials. Observation of Fatigue Crack Initiation Process in .ALPHA.-Brass by AFM.

Since the surface morphology of materials can be observed with atomic-scale resolution, scanning atomic force microscopy (AFM) is a powerful technique to study mechanisms of fatigue and fracture of metallic materials. In the present study, slip band formation and fatigue crack initiation processes in α-brass were observed by means of AFM. Surface of a fatigued specimen was observed under maximum stress. unloading state, and minimum stress at fatigue cycles. In the initial stage of the fatigue process, slip bands which formed only under tension stress or compression stress were observed. These kinds of slip bands, however, disappeared shortly. Under tension stress, cracks could be detected easily just after initiation. For specimens at the unloading state, cracks just after initiation were hardly detected from the AFM images, By measuring the size of slip bands at the unloading state, however, crack initiation from slip bands could be detected. Before crack initiation heights and width of extrusions and the depth and width of intrusions gradually increased with the number of cycles. After crack initiation, one of these values changed drastically, depending on the slip band angle relative to the stress axis and the shape of slip bands.

Study on Fatigue Crack Initiation Process by Using Crystalline FEM analysis (3rd Report)

Study on Fatigue Crack Initiation Process by Using Crystalline FEM analysis (3rd Report)