FATIGUE LIFE BEHAVIOR OF COPPER SINGLE CRYSTALS. PART I: OBSERVATIONS OF CRACK NUCLEATION

In low strain fatigue, cracks often form in persistent slip bands (PSB) in a wide variety of materials. Observations of crack nucleation involving PSB have been made in copper single crystals by careful interferometric studies. The width, length, step height and thus strain localization in the PSBs remain very stable during saturation, even up to three-quarters of the life. For a given applied strain amplitude, the specimen will form PSBs having a certain distribution of slip offsets. The fatal crack is found to nucleate in the group of micro-PSBs having the largest offsets, and the largest strain localization. The PSB volume fraction for a given strain amplitude depends only very slightly on crystal orientation, and the slip offsets not at all. Thus the kinetics of crack nucleation are not affected by orientation. The mechanism of nucleation is concluded to be one of random walk in the slip offsets of PSBs.     

A finite element model of persistent slip band interaction with strengthened surface films during low cycle fatigue

Hardened surface layers such as those produced by ion implantation can inhibit the emergence of persistent slip bands (PSBs) at the free surface during low cycle fatigue, thereby extending crack initiation life. They have little effect, however, on bulk PSB formation. PSBs nucleated in the interior eventually impinge on the underside of the surface layer, producing localized cyclic shear strains in the film which lead to film rupture and rapid crack initiation. As a result, increases in fatigue life are modest. The present finite element model analysis shows that PSBs carrying plastic strains characteristic of fatigued f.c.c. metals produce stresses in an elastic surface layer which may exceed 1% of the surface film shear modulus. The detailed results are consistent with phenomenological data on surface damage accumulation during low cycle fatigue of ion-beam-modified nickel. Implications for the design of surface microalloys for inhibition of fatigue crack initiation are discussed.     

Half-cycle slip activity of persistent slip bands at different stages of fatigue life of polycrystalline nickel

The slip activity of persistent slip bands (PSBs) in polycrystalline nickel was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The half-cycle slip activity as well as the local shear strain amplitudes was investigated after half-cycle deformation at different numbers of cycles in the domain of stress saturation. Moreover, the fraction of grains containing cumulated PSBs and the accumulated volume fraction of PSBs was estimated depending on the number of cycles during fatigue life. The volume fraction of active PSBs during half-cycle deformation is significantly lower than the cumulated PSB volume and decreases with increasing number of cycles. Additionally, an increasing localization of cyclic plastic strain within the PSBs was observed. However, with increasing number of cycles the average local shear strain amplitude remains almost unchanged. Thus, PSBs in polycrystals are subjected to a life history which is characterized by active and inactive periods of their half-cycle slip activity during cyclic deformation at different stages of the saturation state.     

Life time and cyclic slip of copper in the VHCF regime

Studies were performed on the fatigue properties of polycrystalline copper at very low cyclic stress amplitudes and very high numbers of cycles. The experiments were carried out with the time-saving ultrasonic fatigue technique. Thus, experiments up to more than 10¹⁰ cycles could be performed within reasonable testing time. Main result is that below the PSB threshold, irreversible cyclic slip along crystallographic planes still occurs, as evidenced by changes in the surface slip line pattern down to strain and stress amplitudes that are as low as approximately only half of the PSB threshold values. These values are named SB (slip band) thresholds. No fatigue limit in the conventional sense could be detected below 1 × 10¹⁰ cycles. Thus, a threshold value at 1 × 10¹⁰ cycles is defined (named fatigue life threshold in this study) which is roughly 50% higher than the PSB threshold. The question whether the SBs are actually PSBs or whether continued cycling below the PSB threshold would ultimately lead to PSB formation and finally failure after extremely long testing has to be answered in future studies.     

FATIGUE DAMAGE IN AUSTENITIC-FERRITIC DUPLEX STAINLESS STEELS

Fatigue damage in two austenitic-ferritic duplex stainless steels, with the structure of a natural composite and different levels of nitrogen content, was studied in low-cycle fatigue. Both steels show initial cyclic hardening followed by softening and a long stabilisation period. The cyclic stress-strain curve increases with the nitrogen content while Manson-Coffin curves of both steels intersect at medium fatigue lives. The study of the surface relief reveals intensive slip markings both in ferrite and in austenite. Their density is influenced by the nitrogen content. Both the intensity and density of the persistent slip band (PSB) markings are higher in the ferrite. Crack initiation was found to appear predominantly in PSBs in the ferritic grains at the low strain amplitudes, and in the ferritic and austenitic grains at the highest strain amplitudes. The level of the cyclic stress-strain response and the fatigue lives are discussed in terms of the cyclic strain localisation and of the effect of texture and nitrogen content on the strength and fatigue damage. The increased strength of the austenitic phase, due to high nitrogen alloying, results in cyclic slip localisation in the ferrite, and the decrease of fatigue life, compared with the steel with the lower nitrogen content.     

Ultrasonic back reflection evaluation of crack growth from PSBs in low-cycle fatigue of stainless steel under constant load amplitude

The objective of this study is to develop a method for evaluating crack growth from persistent slip bands (PSBs) in low-cycle fatigue of stainless steel, using an ultrasonic back reflection wave during the early stages of its fatigue life. Changes in the back reflection intensity from surface of the material under cyclic loading are measured. Back reflection intensity decreased due to the evolution of PSBs before the start of fatigue crack growth from the crack initiated along PSBs with increase in the number of cyclic loads. The average dislocation density in a grain including PSBs corresponds to the attenuation change measured during the fatigue test, from the initial state to the nucleation and growth of the fatigue crack. The attenuation is caused by the movement of dislocation due to ultrasonic waves, whose mechanism was considered quantitatively. In this study, micromechanical modeling was conducted as a prediction method for remaining fatigue life to start crack growth from PSBs based on the changes in ultrasonic back reflection intensity.     

Effects of grain size on cyclic plasticity and fatigue crack initiation in nickel

Fatigue experiments were conducted on polycrystalline nickel of two grain sizes, 24 and 290 μm, to evaluate the effects of grain size on cyclic plasticity and fatigue crack initiation. Specimens were cycled at room temperature at plastic strain amplitudes ranging from 2.5×10⁻⁵ to 2.5×10⁻³. Analyses of the cyclic stress–strain response and evolution of hysteresis loop shape indicate that the back stress component of the cyclic stress is significantly affected by grain size and plastic strain amplitude, whereas these parameters have little effect on friction stress. A nonlinear kinematic hardening framework was used to study the evolution of back stress parameters with cumulative plastic strain. These are related to substructural evolution features. In particular, long range back stress components are related to persistent slip bands. The difference in cyclic plasticity behavior between the two grain sizes is related to the effect of grain size on persistent slip band (PSB) morphology, and the effect this has on long range back stress. Fine grain specimens had a much longer fatigue life, especially at low plastic strain amplitude, as a result of the influence of grain size on fatigue crack initiation characteristics. At low plastic strain amplitude (2.5×10⁻⁴), coarse grain specimens initiated cracks where PSBs impinged on grain boundaries. Fine grain specimens formed cracks along PSBs. At high plastic strain amplitude (2.5×10⁻³), both grain sizes initiated cracks at grain boundaries.     

Slip behaviour near a grain boundary in high-cycle fatigue of poly-crystal copper

In order to investigate the characteristic slip behaviour near a grain boundary in high‐cycle fatigue, a high‐cycle fatigue test is carried out using a copper poly‐crystal specimen, which consists of several tens of grains. Seventeen persistent slip bands (PSBs) are observed along the grain boundaries. Their location and the activated slip system are different from those expected by the Schmid factor. After the fatigue test, the crystalline orientation and the three‐dimensional shape of each grain are specified by the repetition of polishing and observation by means of an orientation‐imaging microscope (OIM). A finite‐element method (FEM) analysis is also conducted for the specimen with the same orientation and shape of grains taking into account the anisotropy. This analysis reveals that the shear stress concentrates near the grain boundaries where the PSBs are formed. The activated slip systems predicted by the maximum resolved shear stress agree well with those observed in the fatigue experiment. Thus, the characteristic slip near the grain boundary in the poly‐crystal is governed by the concentrated resolved shear stress on the specific slip system due to the deformation constraint by neighbouring crystals.     

FATIGUE CRACKING POSSIBILITY ALONG GRAIN BOUNDARIES AND PERSISTENT SLIP BANDS IN COPPER BICRYSTALS

In this paper, the fatigue cracking possibility in different kinds of copper bicrystals with large-angle grain boundaries (GBs) and copper multicrystals containing some low-angle GBs are compared. The results showed that the fatigue cracks, in the copper bicrystals, always nucleated firstly along GBs no matter whether the GBs are perpendicular or parallel to the stress axis. Whereas, for the copper multicrystals containing low-angle GBs, the persistent slip bands (PSBs) are always the preferential sites to initiate fatigue cracks no matter whether low-angle GBs are perpendicular or parallel to the stress axis.
Additionally, the fatigue lives of the GBs, and the [1¯23] and [3¯35] grains in the [1¯23] ⊥ [3¯35] and [5¯913] ⊥ [5¯79] bicrystals were measured at different cyclic stresses and strain amplitudes. The results show that intergranular fracture always occurred prior to transgranular fracture in those bicrystals. The fatigue lives increased in the order of the GB, the [1¯23] and the [3¯35] grains in the [1¯23] ⊥ [3¯35] bicrystal under cyclic tension–tension loading. On the other hand, the fatigue life of the GB in the [5¯913] ⊥ [5¯79] bicrystal is about two to three times higher than that in the [1¯23] ⊥ [3¯35] bicrystal. Based on these experimental results from the copper bicrystals and multicrystals, it is indicated that the possibility of fatigue cracking increased in the order of low-angle GBs, PSBs and large-angle GBs. It is suggested that both the PSB–GB mechanism and the step mechanism required for GB fatigue cracking were questionable, and the interaction modes of PSBs with GBs may be more important for intergranular fatigue cracking.     

FATIGUE LIFE BEHAVIOR OF COPPER SINGLE CRYSTALS. PART II: MODEL FOR CRACK NUCLEATION IN PERSISTENT SLIP BANDS

A crack nucleation model is constructed based on the random slip of the dislocations within the fatal persistent slip band (PSB). From such a model, we derive the notch-peak probability function. The crack is considered to nucleate when a critical notch depth is attained. This depth is constant for applied strains corresponding to the plateau of the cyclic stress- strain curve because the stress is constant. The PSBs behave in such a way that the more concentrated their localized slip the fewer the cycles required to attain the critical notch depth. Using the experimental relation between the applied strain and the magnitude of the slip offsets (reported in Part I) in conjunction with observations of the notch distributions along the gage surface, we have obtained the relation between the applied strain and the cycles for crack nucleation. A log-log plot of this relation shows a slightly different slope from that of the Coffin-Manson life data, previously reported. However, by allowing for cycles spent in Stage II propagation, good agreement is obtained.     

核电主管道不锈钢在高温高压水环境下的疲劳裂纹萌生行为

利用腐蚀疲劳测试系统研究了高温高压水环境下两种压水堆核电站一回路主管道用不锈钢的腐蚀疲劳裂纹萌生行为。结果表明,316LN奥氏体不锈钢的裂纹主要在材料表面的驻留滑移带处萌生,少量裂纹在两簇驻留滑移带交界的亚晶界面处。含有少量铁素体的Z3CN20.09M奥氏体不锈钢的疲劳裂纹依次在试样表面的驻留滑移带处、相界处和点蚀坑处萌生,但主要是在驻留滑移带处。通过研究高温高压水环境下氧化膜的组成和腐蚀疲劳试样横截面的形貌,分析了疲劳裂纹在滑移带处萌生的机理。最后对比分析两种不锈钢裂纹萌生机制的异同,并讨论了铁素体对材料腐蚀疲劳性能的影响。     

Grain boundary effects on cyclic deformation and fatigue damage

The cyclic deformation behaviors of single- and polycrystals were well documented in the past three decades. It has been recognized that there existed great difference in the fatigue damage mechanisms between single- and polycrystals, which can be mainly attributed to the effects of grain boundaries (GBs) and the crystallographic orientations. In the present work, a series of research work mainly on copper bicrystals with various GBs and different component crystals have been systematically investigated, including the macroscopic cyclic stress-strain responses and fatigue damage mechanisms in micro-scale. Firstly, direct evidence is offered to show the obvious strengthening effect caused by the large-angle GBs during cyclic deformation. The data of cyclic stress-strain responses will be presented to show the effects of the GBs and the crystallographic orientations in the macro-scale. Next, the influence of various GBs on fatigue cracking behavior will be considered for the crystalline materials. Clear evidences are shown that the interactions of persistent slip bands (PSBs) with various GBs play a decisive role in the intergranular fatigue cracking during cyclic deformation. It is suggested that the intergranular fatigue cracking strongly depends on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. The underlying fatigue damage mechanisms were further discussed in terms of the different interactions between PSBs and three types of GBs.     

FATIGUE LIFE BEHAVIOR OF MONOCRYSTALLINE COPPER IN 0.1 M PERCHLORIC ACID

We have studied the fatigue lives of single crystals of copper in 0.1 M HClO₄under different polarization potentials. Perchloric acid was chosen for the aqueous environment because it allows us to control the corrosion reactions rigorously. Persistent slip band (PSB) behaviour and crack nucleation were studied during life, and fracture surfaces after failure. Different behavior was observed depending on strain amplitude. At 2 × 10^-3 plastic shear strain amplitude, anodic potential was observed to decrease life, whereas cathodic potenlial was found to be less damaging than laboratory air. Crack nucleation and propagation occurred along the primary slip plane for both conditions. The reduction of fatigue life under anodic potential is explained by enhanced localized strain at the PSB's and preferential dissolution within them. However, for a strain amplitude of 4 × 10^-3, cracks nucleated and propagated along the secondary slip system. We observed crack nucleation to be associated with deformation-induced stress concentrations, and the aqueous solution showed no aggressive effect under either anodic or cathodic potential.     

Gigacycle fatigue initiation mechanism in Armco iron

Microstructure irreversibility plays a major role in the gigacycle fatigue crack initiation. Surface Persistent Slip Bands (PSB) formation on Copper and its alloy was well studied by Mughrabi et al. as typical fatigue crack nucleation in the very high cycle fatigue regime. In the present paper, Armco iron sheet specimens (1 mm thickness) were tested under ultrasonic frequency fatigue loading in tension–compression (R = −1). The test on the thin sheets has required a new design of specimen and new attachment of specimen. After gigacycle fatigue testing, the surface appearance was observed by optical and Scanning Electron Microscope (SEM). Below about 88 MPa stress, there is no PSBs even after fatigue cycle up to 5 × 10⁹. With a sufficient stress (above 88 MPa), PSBs in the ferrite grain was observed by optic microscope after 10⁸ cycles loading. Investigation with the SEM shows that the PSB can appear in the body-centered cubic crystal in the gigacycle fatigue regime. Because of the grain boundary, however, the local PSB did not continually progress to the grain beside even after 10⁹ cycles when the stress remained at the low level.     

Can public service broadcasting survive Silicon Valley? Synthesizing leadership perspectives at the BBC,PBS, NPR,CPB and local U.S. stations

Public Service Broadcasting (PSB) is generally characterized by a remit to provide high-quality news, educational content, cultural enrichment and entertainment as free public goods with as broad a public reach as possible while also filling critical gaps in the media ecosystem (a multifaceted principle called universality). However, in the current online environment, the ability to reach audiences is increasingly intermediated by online platforms managed by powerful technology companies who do not necessarily share the same objectives or values as PSBs. Interviews were conducted with PSB executives and managers working on digital products and strategy in the U.K. and U.S. to examine the challenges and tensions PSB entities face in the context of growing commercial platform power and the strategies emerging in response, such as investing in new platforms and digital infrastructures rooted in traditional public service values. The study also discusses differences in capacities to respond to platform power between the decentralized U.S. PSB system and centralized U.K. system. Finally, the study discusses some theoretical and practical implications, and suggests some ways PSB services might survive and thrive with robust fidelity to democratic needs.     

驻留滑移带与晶界和孪晶界的交互作用

对含有退火孪晶的多晶铜进行了不同塑性应变幅下的应变疲劳实验，利用扫描电镜及其电子通道衬度技术（SEM-ECC）观察了表面滑移形貌、疲劳裂纹和位错组态，研究了驻留滑移带与晶界和孪晶界的交互作用．结果表明，在晶界附近和远离晶界处观察到位错组态分布的不均匀现象．这种不均匀性导致多晶铜中疲劳裂纹首先沿着普通大角晶界开裂，在孪晶界处由于应变相容性较好而难以产生疲劳裂纹．     

Cyclic Saturation Dislocation Pattern Observation in the Vicinity of Grain Boundaries by Electron Channeling Contrast Technique

State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Chinese Academy of Sciences,Shenyang, 110015, China)Abstract:The cyclic saturation dislocation patterns within grains and in the vicinity of low-angle grain boundaries in fatigued copper crystal were successfully observed by electron channeling contrast technique in SEM. The results show that the dislocation patterns within grains consisted of typical two-phase structure, i.e. persistent slip bands (PSB) and veins. With increasing plastic strain amplitude (γp1 ≥1.7×10-3), large amount of PSBs and regufar dislocation walls were observed.The dislocation walls and PSBs could cross through the low-angle grain boundaries continuously except that the dislocation-free zone (DFZs) appeared at some local regions. Combining with the cyclic stress-strain response and dislocation patterns, the effect of low-angle grain boundaries on cyclic deformation behavior was discussed.     

Profiles of persistent slip markings and internal structure of underlying persistent slip bands

Focused ion beam sections and lamellae for transmission electron microscopy were prepared from fatigued specimens of polycrystalline copper and austenitic Sanicro 25 steel. The profiles of persistent slip markings developed on the surface were observed and documented simultaneously with the underlying dislocation structure. In copper fatigued at room temperature and close to liquid nitrogen temperature, persistent slip markings consisting of pronounced extrusions and parallel intrusions appeared at locations where persistent slip bands having the ladder‐like dislocation structure egress on the surface. Stage I cracks initiated from the tip of the intrusions. In Sanicro 25 steel, more planar character of the dislocation structure led to thin extrusions and intrusions and several stage I cracks running parallel to the primary slip plane. Exceptionally, ladder‐like structure and generally alternating dislocation‐rich and dislocation‐poor volumes were observed in the PSBs. Dislocations bands of secondary slip systems in the matrix disappeared on intersections with PSBs. Experimental findings were compared with predictions of recent physically based models of fatigue crack initiation.     

EFFECT OF RELATIVE SLIP AMPLITUDE ON FRETTING FATIGUE OF HIGH STRENGTH STEEL

Abstract— The effect of relative slip amplitude on fretting fatigue in high strength steel was studied at various contact pressures using fretting pads of various lengths. Under a given contact pressure, the fretting fatigue life showed a minimum at a certain relative slip amplitude. Under a fixed pad length, the life also showed a minimum at a certain contact pressure. A map of fretting fatigue life versus contact pressure and relative slip amplitude was obtained using the data of this study. The map indicated that both the phenomena which showed a minimum life in relation to slip dependence and contact pressure dependence were the same, as were the underlying mechanisms. The minimum life was interpreted in terms of local stress concentration at the fretted area.     

Influence of thermomechanical aging on fatigue behaviour of 2014 Al-alloy

The fatigue behaviour of 2014 Al-alloy has been studied in various thermomechanically aged conditions. It is observed that fatigue properties can be improved by a thermomechanical treatment, which would reduce the concentrations of dispersoids, provide a relatively uniform deformation structure and produce fine distribution of θ’ precipitates. Fine θ’ particles inhibit dynamic recovery and produce uniform deformation structure, which improves fatigue behaviour. Presence of dispersoids and coarse precipitate particles leads to the formation of persistent slip bands (PSBs) and a highly heterogeneous deformation structure, which cause damage to fatigue properties.