Initiation and growth behaviour of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation microcomputed tomography

Small internal fatigue cracks initiated in Ti‐6Al‐4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR‐μCT) at SPring‐8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN‐ΔK relationship was measured and compared with that obtained in a high vacuum environment. SR‐μCT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10^?10 m/cycle, and extremely slow rates ranging from 10^?13 to 10^?11 m/cycle were measured in a lower ΔK regime below 5 MPa√m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum‐like environment inside the internal cracks.     

Direct observation of tensile and fatigue cracks

Crack growth in aluminum foil specimens 1 mil thick has been studied under monotonie tension and low cycle fatigue. Crack tip regions were observed during loading by optical microscopy. Following final fracture, the crack path and fracture surface were examined by scanning electron microscopy.

Both tensile and fatigue cracks are typically preceded by a narrow necked region. Voids develop in the neck, crack growth proceeding by their growth and coalescence. As the voids form and grow, the foil necks down to zero thickness along the path of the crack.

Crack growth in both tension and fatigue results from concentrated plastic strain. The plastic flow is heterogeneous to an extent that voids precede the crack. In monotonic tension new voids are continually nucleated in the region of high strain ahead of the growing crack, while in fatigue, the voids result from plastic instabilities under cyclic loading.     

SURFACE AND IN-DEPTH CRACK PROPAGATION IN HIGH TEMPERATURE LOW-CYCLE FATIGUE OF SUS 316 STAINLESS STEEL

Abstract— The low-cycle fatigue crack propagation behaviour of surface cracks in SUS316 stainless steel at 700°C, in both the surface direction and the in-depth direction, has been studied with special emphasis on the role of oxidation. The coalescence behaviour of surface cracks is essential for the process of crack propagation in high temperature low cycle fatigue, irrespective of the existence of oxidation effects. For sub-surface cracks the process of crack propagation is divided into two stages characterized by differences in fracture mode. In both stages, the in-depth crack propagation rate in air is higher than that in vacuum. This difference in crack propagation rate is the main reason for the decrease of fatigue life in air compared with that in vacuum. The crack propagation behaviour in the in-depth direction can be estimated from the conversion of the surface crack length into the subsurface depth by the use of an aspect ratio.     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

Initiation and propagation behaviour of fatigue cracks in hard-shot peened Type 316L steel in high cycle fatigue

Observations of fatigue crack growth behaviour were made during rotating‐bend testing of hard‐shot peened Type 316L steel. From the results of these observations, the crack that developed in the axial direction was observed and the mechanism of the fatigue crack properties was clarified as follows: (1) Small circumferential surface fatigue cracks were detected at 60% of the fatigue lifetime. These cracks propagated very slowly in both the circumferential and radial directions. (2) When a radial crack reached a depth of between 150 and 350 μm, axial fatigue cracks were formed. (3) In the next stage, either the radial or the axial fatigue cracks continued propagating, or an inwards growing radial crack formed from the axial crack. (4) In the final stage, the circumferential surface crack began to grow rapidly and resulted in fracture. (5) The fracture type of hard‐shot peened Type 316L is a particular type of surface fracture.     

Crack interaction modelling

The growth characteristics of short fatigue cracks under axial loading were investigated using specimens of the ferritic–martensitic steel F82H-mod. Interest focused on crack propagation due to coalescence, which proved to be the dominant mechanism of crack growth. Crack propagation due to coalescence under a certain loading state is strongly influenced by the microstructure of the material on the one hand and by the interaction of cracks on the other. This study deals with an elasto-plastic fracture mechanics analysis of two interacting cracks neglecting the microstructural influence. Finite element calculations based on a Ramberg–Osgood model for the material properties were performed to quantify the interaction of two cracks in terms of an interaction function Y depending on the material and crack configuration. Finally, a neural network was trained to determine the interaction function for two cracks within the range of interest.     

Some Peculiarities of Fatigue Crack Growth at Various Stages of Its Development

The author considers some peculiarities of fatigue crack growth in metals at the stages of its initiation and initial development, and stable and unstable growth that precedes final fracture. It is shown that at the stage of initial growth of fatigue cracks, the stress state, nonlocalized fatigue damage that precedes initiation of the main fatigue crack, residual surface stresses, surface manufacturing and in-service defects, and contact interactions are the factors that determine the crack paths. Stable growth of a fatigue crack is primarily determined by the stress-strain state of a structure as a whole and by the stress-strain state at the crack tip with allowance for its variation due to crack propagation, which is evaluated by the criteria of fracture mechanics. The author also studied peculiarities of fatigue crack development in compressor blades of marine gas turbines. It is shown that for embrittled steels, when fatigue cracks develop under plane strain conditions, final fracture occurs at very small crack sizes. In this case, the characteristics of fatigue fracture toughness are appreciably lower than the static values. The paper also considers peculiarities of unstable fatigue crack propagation.     

Micromechanisms of fatigue crack growth in aluminium alloys

The fatigue crack growth characteristics of high-strength aluminium alloys are discussed in terms of behaviour during mechanical testing and fracture surface appearance. For a wide range of crack growth rates, the crack extends both by the formation of ductile striations and by the coalescence of micro-voids. Dimples are observed at stress intensities very much less than the plane strain fracture toughness, and this is explained in terms of the probability of inclusions lying close to the crack tip. The striation formation process is described as a combination of environmentally-enhanced cleavage processes and plastic blunting of the crack tip.     

Fatigue life prediction of additively manufactured material: Effects of surface roughness,defect size,and shape

In this paper, the effects of process‐induced voids and surface roughness on the fatigue life of an additively manufactured material are investigated using a crack closure‐based fatigue crack growth model. Among different sources of damage under cyclic loadings, fatigue because of cracks originated from voids and surface discontinuities is the most life‐limiting failure mechanism in the parts fabricated via powder‐based metal additive manufacturing (AM). Hence, having the ability to predict the fatigue behaviour of AM materials based on the void features and surface texture would be the first step towards improving the reliability of AM parts. Test results from the literature on Inconel 718 fabricated via a laser powder bed fusion (L‐PBF) method are analysed herein to model the fatigue behaviour based on the crack growth from semicircular/elliptical surface flaws. The fatigue life variations in the specimens with machined and as‐built surface finishes are captured using the characteristics of voids and surface profile, respectively. The results indicate that knowing the statistical range of defect size and shape along with a proper fatigue analysis approach provides the opportunity of predicting the scatter in the fatigue life of AM materials. In addition, maximum valley depth of the surface profile can be used as an appropriate parameter for the fatigue life prediction of AM materials in their as‐built surface condition.     

Fatigue and fracture of a Ni–P amorphous alloy thin film on the micrometer scale

Fracture and fatigue tests have been performed on micro‐sized specimens for microelectromechanical systems (MEMS) or micro system technology (MST) applications. Cantilever beam type specimens with dimensions of 10 × 12 × 50 μm³, approximately 1/1000th the size of ordinary‐sized specimens, were prepared from a Ni–P amorphous thin film by focused ion beam machining. Fatigue crack growth and fracture toughness tests were carried out in air at room temperature, using a mechanical testing machine developed for micro‐sized specimens. In fracture toughness tests, fatigue pre‐cracks were introduced ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface, with closure effects on the fatigue crack growth also being observed for micro‐sized specimens. Once fatigue crack growth occurs, the specimens fail within one thousand cycles. This indicates that the fatigue life of micro‐sized specimens is mainly dominated by a crack initiation process, also suggesting that even a micro‐sized surface flaw may be an initiation site for fatigue cracks which will shorten the fatigue life of micro‐sized specimens. As a result of fracture toughness tests, the values of plane strain fracture toughness, K_IC, were not obtained because the criteria of plane strain were not satisfied by this specimen size. As the plane strain requirements are determined by the stress intensity, K, and by the yield stress of the material, it is difficult for micro‐sized specimens to satisfy these requirements. Plane‐stress‐ and plane‐strain‐dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro‐sized specimens. The results obtained in this investigation should be considered when designing actual MEMS/MST devices.     

Numerical modelling of fatigue crack initiation and growth of martensitic steels

This paper presents a numerical simulation of micro‐crack initiation that is based on Tanaka‐Mura micro‐crack nucleation model. Three improvements were added to this model. First, multiple slip bands where micro‐cracks may occur are used in each grain. Second improvement deals with micro‐crack coalescence by extending existing micro‐cracks along grain boundaries and connecting them into a macro‐crack. The third improvement handles segmented micro‐crack generation, where a micro‐crack is not nucleated in one step like in Tanaka‐Mura model, but is instead generated in multiple steps. High cycle fatigue testing was also performed and showed reasonably good correlation of proposed model to experimental results. Because numerical model was directed at simulating fatigue properties of thermally cut steel, edge properties of test specimens were additionally inspected in terms of surface roughness and micro‐structural properties.     

Investigating environmental effects on small fatigue crack growth in Ti–6242S using combined ultrasonic fatigue and scanning electron microscopy

In situ ultrasonic fatigue with a cyclic frequency of 20 kHz was employed in an environmental scanning electron microscope (ESEM) to characterize fatigue crack formation and growth in the near alpha titanium alloy Ti–6242S. The role of environment on small fatigue crack initiation and growth was investigated in vacuum and in variable pressures of saturated water vapor, as well as in laboratory air. Small crack growth behavior from cracks initiated at FIB-produced micro-notches indicated a significant environmental dependence, with fatigue crack growth rates increasing with increasing partial pressures of water vapor. Environment also influenced crack initiation lifetime in that cracks initiated earlier in laboratory air than in vacuum or saturated water vapor environments. Transgranular, crystallographic crack growth was observed in each environment, with the crack path in primary α grains producing facets parallel to basal planes when crack size was small. Small crack growth resistance had a marked sensitivity to microstructural features, such as α/α grain boundaries with high misorientation and α/α + β boundaries. These initial investigations demonstrate the usefulness of in situ ultrasonic fatigue instrumentation (UF-SEM) as a new tool for the characterization of environmental and microstructural influences on very high cycle fatigue (VHCF) behavior.     

Effects of microstructural through‐thickness non‐uniformity and crack size on fatigue crack propagation and fracture of rolled Al‐7075 alloy

Some of the fatigue tests performed using the standard compact tension (CT) and a non‐standard specimen made of rolled 7075 aluminium alloy exhibit fatigue crack growth (FCG) lagging in a small region along the crack front. Through‐thickness microstructural evaluation shows that material grains in this region did not flatten as much as other regions. In the non‐standard specimen, surface cracks are either grown under fatigue loading or broken under monotonically increasing quasi‐static loads at different crack sizes. The aforementioned lagging also exists in a narrow region of 3‐D FCG for specimens with microstructural through‐thickness non‐uniformity. A more important feature for this type of specimen with surface crack is the deflection of fast fracture direction into the grain interfaces, namely from L‐T orientation to S‐L and S‐T directions. It is proved that this is due to significant levels of second principal stresses near the free surface for small cracks and lower fracture toughness of the material in S‐L and S‐T directions.     

Non-destructive observation of internal fatigue crack growth in Ti–6Al–4V by using synchrotron radiation μCT imaging

The propagation of an internal fatigue crack in Ti–6Al–4V was non-destructively observed by synchrotron radiation μCT imaging to clarify the crack growth rate in very high cycle fatigue. The results show that the internal crack propagated quite slowly at a rate of less than 10⁻¹⁰ m/cycle. The propagation rate of an internal crack was compared with that of a surface crack in air and in high vacuum to examine the internal fracture process in terms of the environment around the crack. The rate of the internal crack was similar to that of the surface crack in high vacuum, but was significantly lower than that in air. This led us to conclude that the low propagation rate of the internal crack is due to the vacuum-like environment inside the crack.     

Development of fatigue cracks from mechanically machined scratches on 2024‐T351 aluminium alloy—part I: experimentation and fractographic analysis

Clad and unclad 2024‐T351 aluminium alloy sheets, weakened by mechanically machined scratches, were fatigued to investigate the effect of small surface damage, like scribe marks, on aircraft fuselage joints. The role of scratch cross section geometry on fatigue life of scribed components was analysed. Scratches between 25 and 185 µm deep, with 5, 25 and 50 µm root radii, were cut on sample surface by using diamond‐tipped tools. After testing, fracture surfaces were examined using a scanning electron microscope, and crack growth rates were measured by striation counting. Scratches reduced aluminium fatigue life under tensile and bending load up to 97.8% due to multiple crack nucleation at their roots. Short cracks nucleated from sharp scratches coalesced to form unique elongated cracks growing through sample thickness. Cracks initiated from scratches were typical short cracks, growing faster than conventional long cracks. Despite the different scribing process, fatigue data of regular diamond tool cut scribes can be used to conservatively predict life reduction owing to ploughed in‐service scribe marks on fuselage joints. Finite element analyses on scribed samples and the fatigue life prediction models are described in Part II of this paper.     

Near‐threshold fatigue crack growth properties of wrought magnesium alloy AZ61 in ambient air,dry air,and vacuum

Environmental influences on near‐threshold fatigue crack growth in wrought magnesium alloy AZ61 were investigated. Fatigue tests were performed in ambient (humid) air, dry air, vacuum, and dry nitrogen gas at 19 kHz cycling frequency and load ratio R = ?1. Threshold stress intensity factor amplitudes, K_th, determined for limiting growth rates below 5 × 10^?13 m/cycle were 1.1 MPam^1/2 in ambient air and 1.2 MPam^1/2 in dry air. A much higher K_th of 1.9 MPam^1/2 was measured in vacuum and dry nitrogen gas. This suggests oxygen to be the most detrimental constituent of ambient air that increases near‐threshold crack propagation rates and decreases K_th. The deleterious effect of humidity is comparatively small. Corrosive influences are effective at ultrasonic cycling frequency for growth rates below approximately 3 × 10^?9 m/cycle. The crack propagation curves in ambient and dry air show a plateau‐like regime where the fracture mode changes from purely ductile to a mixed ductile and brittle mode. In vacuum and dry nitrogen gas, a ductile crack path is found for all investigated crack growth rates.     

Fatigue as a process of cyclic brittle microfracture

While fatigue crack growth in vacuum may occur by slip alone, environmental fatigue including crack growth in air is strongly influenced by crack‐tip surface chemistry that adversely affects ductility. Cumulative diffusion, combined with adsorption and chemisorption in the loading half‐cycle may promote instantaneous crack extension by brittle microfracture (BMF). Unlike slip, the BMF component will be sensitive to parameters that affect near‐tip stresses, such as load history and constraint. BMF dominates near‐threshold environmental fatigue. Being a surface phenomenon, it loses its significance with increasing growth rate, as slip‐driven crack extension gains momentum and growth becomes less sensitive to environment. The BMF model provides for the first time, a scientific rationale for the residual stress effect as well as the related connection between stress–strain hysteresis and load‐sequence sensitivity of metal fatigue including notch response. Experimental evidence obtained on a variety of materials under different loading conditions in air and vacuum appears to support the proposed model and its implications.     

Investigation of small fatigue crack initiation and growth behaviour of nickel base superalloy GH4169

Surface replication method was utilized to monitor the small fatigue crack initiation and growth process of single‐edge‐notch tension specimens fabricated by nickel base superalloy GH4169. Three different stress levels were selected. Results showed that small fatigue cracks of nickel base superalloy GH4169 initiated from grain boundaries or surface inclusions. The small fatigue crack initiation and growth stages took up about 80–90% of the total fatigue life. Multiple major cracks were observed in the notch root, and specimen with more major cracks seemed to have smaller fatigue life under the same test conditions. At the early growth stage, small crack behaviour might be strongly influenced by microstructures; thus, the crack growth rates had high fluctuations. However, the stress level effect on the small fatigue crack growth rates was not distinguishable for the three different stress levels. And no clear differences were found among the crack initiation lives by using replication technique.     

淬火微观组织对重轨钢疲劳裂纹扩展速率的影响

以U75V重轨钢CT(Compaction test)试样为研究对象,分别研究了冷速为3℃/s、5℃/s及空冷轧态的组织对稳定区(疲劳扩展Ⅱ区)疲劳裂纹扩展速率的影响,疲劳试验在高频共振疲劳试验机上进行。研究结果表明:疲劳裂纹扩展速率随着珠光体片层间距的减小而降低,冷速由大到小(5℃/s、3℃/s、空冷)所对应的n值分别为3.603 05、3.631 18和3.885 28;珠光体组织的疲劳裂纹断裂形式主要以穿晶断裂为主,同时伴随部分沿晶断裂;片层间距影响裂纹扩展路径的偏折程度,偏折程度随片层间距的减小而增大,增大的裂纹偏折路径对裂纹扩展的阻碍作用增强,有利于疲劳裂纹扩展速率的降低。     

An engineering approach to fatigue analysis based on elastic‐plastic fracture mechanics

Nowadays cast iron components are widely used in highly stressed structures. Component lifetime is strongly influenced by inhomogeneities caused by the material's microstructure and the manufacturing process (graphite particles, (micro‐)shrinkage pores, inclusions). Inhomogeneities often act as a fatigue crack starter. Lifetime until failure may be divided into stages for crack initiation, short and long crack growth. Initiation of a crack of technical size (a ≈ 1mm) is often dominated by the growth of short cracks. The paper presents an approach to analyse the mechanically short fatigue crack growth based on elastic‐plastic fracture mechanics considering the closure behaviour of short cracks. The effective J‐integral range is used as a crack driving force. Finite element analysis results as well as analytical solutions to approximate the crack driving force are presented. The application of the approach is successfully demonstrated for cast iron material EN‐GJS‐400‐18‐LT using data from fatigue tests, microstructure and fracture surface analyses to assess the fatigue life.