Slow Crack Growth Rate Research Articles

Atomic force microscopy monitoring of subcritical slow crack growth in soda lime silicate glass

Prediction of brittle fracture in silicate glasses requires a fundamental understanding of slow crack growth rates, far below the critical stress intensity (KIC) of the material. Here, atomic force microscopy (AFM) was used to characterize slow crack propagation in soda lime silicate glasses. Using large-sample AFM systems with specialized tips and scanners, fractures less than 20 nm wide were measured as a function of relative humidity (RH). The results were found to be sensitive to the type of AFM tip used and to the dryness of the surface. Continuous scanning and image capture over several days allowed the determination of the crack tip velocity. The dependance of crack tip velocity on RH was measured by enclosing the sample and AFM head in a humidity-controlled chamber. The crack tip velocity was measured as low 4.0×10-12 m/s at 15 % RH and as high as 3.7×10-10 m/s at 40 % RH. Overall crack growth was smooth during the AFM scan. Additionally, recession of the crack tip was noted as the sample dried, indicating crack tip closure may be occurring. Further studies on alternative compositions will provide details on subcritical fracture processes in a range of different environmental conditions.

Assessing fatigue crack growth thresholds for a Ti-6Al-4V (STOA) alloy using two experimental methods

Fatigue crack growth rate behavior in the threshold and near-threshold regimes for a Ti-6Al-4V solution-treated and over-aged (STOA) alloy was investigated using two test procedures: i) accelerated load reduction (ALR) utilizing faster load-shed rates than recommended in the ASTM standard E647 load-reduction (LR) test procedure, and ii) compression pre-cracking constant-amplitude (CPCA) or load-increasing (CPLI) and load-reduction (CPLR). Fatigue crack growth tests were conducted on compact, C(T), specimens (B = 6.35 mm) with two different widths (W = 51 and 76 mm) at two stress ratios (R = 0.1 and 0.7). The crack-growth-rate test data were compared to previous test data produced from the same batch of material using the ASTM LR and CPCA test procedures that were tested on C(T) specimens (W = 25, 51 and 76 mm) over a wide range in stress ratios (R = 0.1, 0.4, and 0.7).Although none of the load-reduction test procedures provided a good representation of the threshold value (ΔKth) at low R, the CPCA test procedure was the most promising. The LR, ALR, and CPLR test procedures were influenced by prior loading history, leading to higher ΔKth values and slower crack growth rates in the threshold regime due to a rise in the crack-opening loads. The LR and ALR tests at R = 0.1 and shed rates of −0.08, −0.32, and −0.95 mm−1 yielded ΔKth values that were 15 to 38 % higher than the estimated threshold stress-intensity factor range (ΔK*th)R=0.1 based on the crack-closure model (FASTRAN). The compression pre–cracking test procedures are more accurate alternatives for developing threshold and near-threshold fatigue crack growth rates. Specifically, the CPLR test procedure produced a ΔKth value that was 8–18 % higher than (ΔK*th)R=0.1. Additionally, LR and ALR tests produced different ΔKth values as a function of the specimen width for a given load ratio, which violates classical fracture mechanics principles. In addition, the spread in the ΔK-rate data in the low-rate regime was larger than the spread in the mid-rate regime as a function of R, a term referred to as “fanning”. In contrast, the CPCA test procedure produced consistent crack growth rates over the same range of ΔK values examined, independent of the specimen width and these data did not exhibit “fanning” with R. Further research is necessary to develop standard test procedures capable of providing a more definitive representation of the ΔKth value in the threshold regime. However, FASTRAN can serve as a crucial tool in aligning the actual ΔKth values with those obtained through CP testing, thereby bridging the gap and enhancing the reliability of the results.

Determining SCC resistance of stainless steel claddings in high- temperature water by constant load crack growth tests and slow strain rate tests

Stress corrosion cracking (SCC) behaviors of 309 L and 308 L weld overlays are characterized in oxygen-free and oxygen-bearing high-temperature water using slow strain rate tests (SSRTs) and crack growth rate (CGR) tests with compact tension specimens. There is no apparent SCC indication on 308 L, while local SCC is observed on 309 L after SSRT in oxygen-free water. The SCC sensitivities of 308 L and 309 L increase with the increase of dissolved oxygen (DO) content, while 308 L shows relatively higher SCC resistance than 309 L in water with 200 ppb and 8000 ppb DO. SSRT and CGR test results show a similar effect of DO concentration on SCC of 309 L and 308 L. The fracture surface of 309 L exhibits extensive intergranular SCC, while 308 L shows only local interdendritic SCC after CGR tests. The crack tip blunting decreased the CGR in the low alloy steel. The highly island-shaped δ-ferrites and moderate strain hardening in 308 L contribute to its SCC resistance over 309 L.

Effect of recycled material on failure by slow crack growth in multi-layer polyethylene pipes

Among polymer materials, high-density polyethylene (HDPE) is one of the most demanded polymers for packaging as well as in sectors with higher material requirements, such as the plastic pipe industry. The extensive use of HDPE means it is often present in municipal waste. Therefore, there is a high potential for reprocessing and exploring new applications for recycled HDPE, aiming for a circular economy. In the plastic pipe industry, recyclates have found practical use in non-pressure applications. However, the high structural and loading requirements do not allow using it for pressurized pipes yet. The presented study is focused on the possible utilization of recycled HDPE in pressurized pipes in the form of a layer in a multi-layer pipe with two protective layers made of virgin HDPE. The performance of the pipes is assessed based on numerical simulations, coupled with lifetime estimations. The lifetime assessment is based on linear elastic fracture mechanics using experimentally measured slow crack growth (SCG) rates in the virgin and recycled HDPE grades. Thanks to the numerical models, it was possible to describe damage by SCG of the multi-layer pipe as well as to find the dependency of the pipe performance on the thickness ratio and material properties of the layers.

Influence of Retrogression Time on the Fatigue Crack Growth Behavior of a Modified AA7475 Aluminum Alloy

This paper investigates the effect of retrogression time on the fatigue crack growth of a modified AA7475 aluminum alloy. Tests including tensile strength, fracture toughness, and fatigue limits were performed to understand the changes in properties with different retrogression procedures at 180 °C. The microstructure was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The findings indicated that as the retrogression time increased, the yield strength decreased from 508 MPa to 461 MPa, whereas the fracture toughness increased from 48 MPa√m to 63.5 MPa√m. The highest fracture toughness of 63.5 MPa√m was seen after 5 h of retrogression. The measured diameter of η' precipitates increased from 6.13 nm at the retrogression 1 h condition to 6.50 nm at the retrogression 5 h condition. Prolonged retrogression also increased the chance of crack initiation, with slower crack growth rate in the long transverse direction compared to the longitudinal direction. An empirical relationship was established between fracture toughness and the volume fraction of age-hardening precipitates, with increasing number density of precipitates seen with increasing retrogression time.

Deriving exponential and power law slow crack growth parameters from dynamic fatigue of indented or as-polished window material coupons

Knowledge of slow crack growth rate in a window material is required to determine the design life of a window in which cracks grow under the influence of applied service stress. Parameters that characterize crack growth rate can be measured by dynamic fatigue in which coupons are taken to failure at several constant stress rates. The slower the stress rate, the more time is available for cracks to grow during the strength measurement, and the lower the stress at which the coupon fails. That is, coupons tested at a slower stress rate are weaker than coupons tested at a faster stress rate. The rate of slow crack growth is commonly fit to either a power law or an exponential law. The exponential law provides the more conservative estimate of design life and is prescribed for use by the U.S. National Aeronautics and Space Administration to design windows for manned vehicles. Well-established analytical equations are used to derive the power law crack growth parameters from dynamic fatigue measurements of as-polished (unindented) coupons. There are no exact analytical equations to derive power law parameters from indented coupons or to derive exponential law parameters from unindented or indented coupons. Approximate procedures have been used to derive crack growth parameters when there are no exact equations. We now describe a numerical method that gives the power and exponential laws for both unindented and indented coupons by least-squares fitting of dynamic fatigue measurements. A MATLAB code is provided to carry out the calculations.

Slow crack growth in multispectral zinc sulfide for use in aircraft window design

The slow crack growth rate and inert strength of multispectral zinc sulfide were measured for use in aircraft window lifetime analysis. Dynamic fatigue data for uncoated multispectral zinc sulfide were fit to a power law by linear regression and to an exponential law by computer-aided numerical integration to obtain best-fit slow crack growth rate parameters. For the power law slow crack growth equation, crack velocity ( v ) = A * (KI/KIc)n, we find A * = 0.00769 m / s and n = 14.65, where KI is the stress intensity factor, KIc = 0.72 MPam is the critical stress intensity factor, and we assign the value of the geometric factor to be Y = 2 / π. Parameters for the exponential law slow crack growth equation, crack velocity ( v ) = vo exp ( βKI ) are vo = 3.02 × 10 − 14 m / s and β = 43.64 (MPam) − 1. There were only small differences in the crack growth rate between uncoated and antireflection-coated material. The crack growth rate that we observe is 10 to 200 times faster than previously reported for a stress intensity factor KI = 0.25 MPam, which is a representative value of KI for an aircraft window in service. The faster crack growth rate predicts a correspondingly shorter window lifetime. The inert strength of uncoated biaxial flexure disks (38.1 mm diameter with 15.88 mm load diameter and 31.75 mm support diameter) measured in dry nitrogen exhibited a 50% Weibull failure probability at 115 MPa with an unbiased ASTM C1239 Weibull modulus of 4.68. Antireflection-coated material had a 50% Weibull failure probability at 100 MPa with a Weibull modulus of 5.25.

Nanovoids in fracture surface of unsaturated polyester/vinyl ester blends resulting from disruption of the cross-linking of the polymer chain networks

A weakness of unsaturated polyester is brittle due to the cross-linking of the UP chain network. Improving the toughness may be carried out by blending with appropriate additives. This work aims to characterize the fracture surface of unsaturated polyester (UP)/vinyl ester (VE) blends with a ratio of 70% and 30% respectively. Addition of VE in UP increases the toughness of the blends resulting from disruption of the cross-linking of the UP chain network. The disrupted network was characterized by the FESEM morphology from fracture surface of the blend which exhibits three different regions i.e., the smooth zone corresponding to the slow crack growth rate (1); a transition zone, where the surface roughness steadily increases (2); a final failure zone with much rougher marks (3). The presence of more nanovoids in region 2 and 3 confirmed the bigger disruption of the crosslinking in UP networks increasing UP chain mobility and toughness of the blends. The nanovoids result in a dimpled appearance of the fracture surface and become points of high-stress concentration from which cracks can initiate. When subjected to a sudden load, many cracks can grow simultaneously forming beach marks where the cleavage from several cracks meets.

Effect of styrene content on mechanical and rheological behavior of styrene butadiene rubber

Styrene butadiene rubber(SBR) consists of two random polymerization units, styrene and butadiene. As a rigid segment, styrene chain provides strength and rigidity to the rubber. The content of styrene in SBR directly affects its strength and stiffness. In this paper, the effect of styrene content on the fatigue characteristics and rheological properties of SBR have been investigated. The stress relaxation of SBR was reduced and the cross-linking density of butadiene was increased with the increasing styrene content. Especially the dynamic fatigue life of precracked SBR samples was obviously prolonged via slow down crack growth rate when the styrene content increased, which was accompanied by an augment in the value of J-integral and tearing energy in the case of a same gap extension length. At the initial stage of crack growth, the growth of secondary crack played an important role in the growth of main crack, and when the main crack accelerates, the growth of main crack itself dominated. On the other hand, the glass transition temperature obviously increased with the increase of styrene content.

Microstructure, mechanical properties and IASCC susceptibility of stainless steel baffle bolts after 30 years of operation in a PWR

Neutron irradiation in the core of a nuclear power plant can change the material properties of stainless steel significantly. In this paper cold worked AISI 316 baffle bolts retrieved from Doel 1 PWR were investigated with the aim to assess the structural degradation due to neutron irradiation by analyzing microstructure, mechanical behavior and stress corrosion susceptibility. The baffle bolts had received neutron doses ranging from 6 to 59 dpa covering a major part of a nuclear power plant's lifetime. First, microstructural investigations were carried out to reveal the defect and dislocation density and the amount of void swelling. Then mechanical properties were determined using tensile tests and fracture toughness tests. The irradiation assisted stress corrosion cracking (IASCC) susceptibility was investigated by slow strain rate testing and crack growth rate testing. Results were compared with literature values and found to be in good agreement. Main conclusions were that void swelling was small even for the highest dpa values and that the fracture toughness values saturated above 15 dpa. The material was susceptible to IASCC as shown by SSRT and crack growth rate measurements exhibiting intergranular fracture morphology.

Modeling and application of discontinuous slow crack growth behaviors of high-density polyethylene pipe with various geometries and loading conditions

A slow crack growth model based on the crack layer theory for various geometries of a high-density polyethylene (HDPE) pipe is developed to investigate the discontinuous slow crack growth (SCG) behavior of HDPE pipes. Discontinuous SCG patterns, jump lengths, and periods can be accurately simulated using the proposed model. In addition, effects of soil embedding height, internal pressure, and ground pressure on the discontinuous SCG behavior and consequent lifetime are investigated. It is confirmed that the additional ground pressure increased the discontinuous SCG rates of the buried HDPE pipes, thereby reducing the final lifetime considerably.

Hydrogen Embrittlement of Low Alloy Steels Under Cathodic Polarization

Hydrogen embrittlement of low alloys steels at three different strength levels (745 Mega Pascals [MPa], 904 MPa, and 1,166 MPa) were evaluated under cathodic polarization. Crack growth rate measurements were performed under constant stress intensity (K) conditions, as a function of applied K values as well as applied potential to characterize the behavior of the three different steels. At −1,050 mVSCE saturated calomel electrode (SCE), the threshold stress intensity (Kth) value increased from 44 MPa√m to 60 MPa√m as the yield strength decreased from 1,166 MPa to 745 MPa. The crack growth rate at 66 MPa√m and −1,050 mVSCE decreased from 3 × 10−5 mm/s to 4 × 10−8 mm/s as the yield strength decreased from 1,166 MPa to 745 MPa. For the 1,166 MPa steel at low values of K, the crack growth rate decreased by two orders of magnitude as the potential decreased from −1,000 mVSCE to −950 mVSCE. At higher values of K, the effect of potential on the crack growth rate was not as significant. The 745 MPa steel in general exhibited slow crack growth rate values (2 to 4 × 10−8 mm/s) over the range of K values and applied potentials in which it was evaluated. Water adsorption on fresh metal surfaces in the estimated crack tip chemistry was modeled using density functional theory. The variation in crack growth rate with applied potential at low and intermediate values of K correlated with the fractional coverage of water adsorption on the fresh metal surface. It is proposed that the water reduction reaction and the subsequent generation of hydrogen are the rate limiting steps in the slow subcritical crack growth rate processes for low alloy steels under the conditions evaluated. For the higher values of K, where the crack growth rate showed a weak dependence on applied potential, water reduction, and generation of hydrogen are likely not the rate limiting steps.

A decohesion pathway for hydrogen embrittlement in nickel: Mechanism and quantitative prediction

Hydrogen embrittlement (HE) is a ubiquitous and catastrophic mode of fracture in metals. Here, embrittlement is considered as an intrinsic ductile-brittle transition at the crack tip, where H at the crack tip can reduce the stress intensity KIc for cleavage below the value KIe required for ductile dislocation emission and blunting. Specifically, cleavage fracture along (111) planes in Ni occurs due to the formation of just 3 planar layers of H interstitial occupation at a sharp crack tip. During the cleavage process, the sub-surface H in the upper and lower layers can rapidly diffuse to the fracture surface, lowering the net fracture free energy to KIc < KIe and enabling brittle fracture. Details of the process are demonstrated using both first-principles density functional theory and a new interatomic potential for Ni-H. Thermodynamic and kinetic models show that the 3 layers of H can form at the crack tip in equilibrium at room temperature with bulk H concentrations and loading rates where H embrittlement in Ni is observed. The kinetic model also predicts the slow crack growth rate in agreement with experiments. The energetics of the mechanism is then shown to apply to cleavage along grain boundaries. All together, these results show that a version of “Hydrogen enhanced decohesion” is the operative embrittlement mechanism in Ni.

A fatigue assessment approach involving small crack growth modelling for structural alloy steels with interior fracture behavior

A fatigue assessment approach for interior inclusion-FGA (fine granular area)-fisheye induced fracture was proposed through experimental investigation of very high cycle fatigue properties of two high-strength low-alloy steels. The rougher morphology of FGA reveals the rather distorted crack growth path and slow crack growth rate. A strength evaluation model related to defect size and fatigue life was developed, using which the predicted results are in agreement with the experimental results. Based on the modelling of micro-crack growth rate, the evaluated value can be less than the magnitude of one Burger’s vector, which reflects the discontinuous crack growth behavior within FGA.

VISCOELASTIC RESPONSE DURING CRACK PROPAGATION OF UNFILLED AND FILLED SBR

ABSTRACT The crack propagation behavior of unfilled and filled styrene–butadiene rubber (SBR) in steady and dynamic tearing was investigated using tensile tests on trousers samples and tear fatigue measurements on single edge notched tension (SENT) samples, respectively. For the unfilled sample, both types of measurements indicated that the tearing energy is dominated by the viscoelastic response of the polymer. This was demonstrated by the creation of crack growth master curves using the horizontal shift factors obtained from mastering the complex modulus. The tearing energy increased with increasing crack velocity and decreasing temperature as required by the time-temperature superposition principle. The crack growth master curves followed a power law with the same exponent for steady and dynamic tearing. This exponent agrees fairly well with the exponent predicted by linear viscoelastic crack propagation theory. For steady tearing at high crack propagation rates, systematic deviations from the master curves appeared, which were attributed to flash temperature effects. For the filled SBR, which contained 50 phr carbon black, only dynamic tearing was found to be dominated by the viscoelastic response of the polymer. For steady tearing, filler networking seemed to alter crack propagation rates significantly, so that the viscoelastic fingerprint was no longer visible. For the filled sample, additional measurements with a high preload were conducted. It was demonstrated that a single master curve can be constructed, only if the (static) preload contribution is neglected and the dynamic contribution of the energy density is used for the evaluation of the tearing energy. This master curve showed two distinct slopes at high and low crack velocities. It is argued that the higher slope at low crack speeds is relevant for lifetime predictions based on the integration of the Paris–Erdogan power law. Under conditions in which the viscoelastic crack propagation holds, very slow crack growth rates can be explored at reasonable testing times by measurements at elevated temperatures.

Numerical simulations of cracked round bar test: Effect of residual stresses and crack asymmetry

The Cracked Round Bar (abbreviated as CRB in the following) test is used as standardized accelerated testing method for simple ranking of polyethylene pipes based on measuring number of cycles to failure of the cyclically loaded CRB specimen. It can also be used for the slow crack growth rate evaluation of the tested material. This paper deals with the issue of asymmetrically growing cracks in CRB specimens and possible influence of the asymmetry on the results of CRB tests, both the number of cycles to failure and the crack growth rates. It is assumed that the asymmetry is caused by presence of residual stress in the CRB specimens. In the first part of the paper, residual stress in two polyethylene pipes (produced using different extrusion speed) is investigated. Then CRB specimens made from these pipes are also examined regarding the residual stress and the results are compared. The found residual stress distributions are used as input for the following finite element method (FEM) simulation of crack growth in the CRB specimens. The FEM model results are then evaluated and fatigue crack growth rates for asymmetrically growing cracks are calculated. They are compared to the results for symmetrically growing cracks and influence on the results and its significance is discussed. In the end, CRB specimen lifetime prediction is carried out and the influence of the asymmetry on the obtained number of cycles to the failure is also addressed.

Polypropylene/organically modified‐grafted mica/organoclay hybrid nanocomposites: Preparation, characterization, and mechanical properties

Polypropylene (PP) was reinforced using both an organically modified‐grafted mica (OMGM) and an organically modified montmorillonite (OMMT) as debonding rigid nanoparticles to improve its impact resistance with the advantage of increasing its Young's modulus and crack growth resistance. For this purpose, mica was organically modified using diacetone acrylamide, a double bond‐ended modifier. To improve further the polymer/nanoparticle interfacial interactions and delamination of layered particles, PP graft maleic anhydride was grafted on the modified mica. Various amounts of OMMT, 0–3 wt% along with 1 wt% OMGM were used to reinforce the PP matrix. The effect of OMMT content on the crystallinity degree, fracture morphology and mechanical properties of resultant hybrid nanocomposites was investigated. The results showed that the incorporation of 1 wt% OMGM and 0.5 wt% OMMT resulted in the highest impact resistance (68%) and fracture toughness (5.7%) when compared with neat PP. In addition, the elastic modulus and yield stress were increased 12 and 14.5%, respectively. Transmission electron microscopy exhibited a fairly good dispersion of smaller OMMT and longer OMGM tactoids in the resultant hybrid nanocomposite. A slower crack growth rate was observed for the hybrid nanocomposite because of the barrier property of the silicate layers against the crack propagation when compared with the neat PP. However, increasing of the OMMT content caused the nanoparticle agglomeration and lowering the impact resistance in the resultant hybrid nanocomposites. POLYM. COMPOS., 40:1718–1730, 2019. © 2018 Society of Plastics Engineers

Improving the fracture toughness of 3D printed thermoplastic polymers by fused deposition modeling

This work describes a new filament deposition in fused deposition modeling process through criterion based on mechanical stress. This criterion requires that the filaments’ directions to follow the principal directions of the stress in the sample. The article also presents several Crack-test specimens that have been printed with and without respect to this criterion. The fracture behavior of these specimens has been investigated. The results show that criterion leads to an improvement of 30% in the fracture toughness. Digital image correlation has been extensively used to study the local strain field in the specimens. The strain cartographies reveal a drastic change in fracture behavior. The modification of filament direction leads to “ductile-like behavior” in crack extension which is characterized by a large deformation zone associated with a slow crack growth rate during the crack propagation.

Separating plasticity-induced closure and residual stress contributions to fatigue crack retardation following an overload

The introduction of an overload or underload within a constant amplitude loading fatigue test leads to a retardation or acceleration of the Fatigue Crack Growth Rate (FCGR). The understanding of the causes of these effects is essential in the context of variable amplitude fatigue loading, since in principle any loading history can be represented as a sequence of overloads and underloads. In the case of overload, along with some other minor causes, the residual stress changes at the crack tip and crack closure behind the tip can be thought of as the main factors that affect the fatigue crack growth rate. Whilst this has been recognised and accepted for many decades, controversy persists regarding the relative significance and presence of these two effects, and consensus is yet to emerge. The effect of crack closure, when the baseline loading ratio is high enough, can be inhibited so that the main cause of retardation becomes doubtless the residual stress present ahead the crack tip.In the present paper we report our attempt to deconvolve the contributions of crack closure and residual stress on crack retardation following an overload. To accomplish this task we analyse the results of fatigue tests run at two baseline load ratios, namely R=0.1 and R=0.7. At the load ratio of R=0.7 the crack closure effect is not operative, as confirmed by Digital Image Correlation analysis of the crack flanks close to the tip, and post mortem fractographic analysis of crack surfaces. Therefore, for R=0.7 the compressive residual stress region created by the overload ahead of the crack tip is the sole mechanism causing crack retardation. Therefore, for R=0.7 the focus must be placed entirely on the strain field around the crack tip. To this end, line profiles along the crack bisector of elastic strain in the crack opening direction were collected at several stages of crack propagation past the overload using in situ Synchrotron X-ray Powder Diffraction (SXRPD) technique.By performing comparison between the two loading conditions (R=0.7 and R=0.1), information was extracted regarding the role of residual stress alone, and then, by subtracting this effect for the R=0.1 sample, for crack closure alone. To enable this analysis, we propose a introducing the concept of equivalent effective stress intensity factor range, ∆Keq,eff proposed by Walker. Afterwards, the SIF range reduction ratio, β, which represents the “knock down” factor with respect to the steady state growth was assessed. It is in terms of these newly introduced parameters that the magnitude and extent of the overload-induced crack growth rate retardation can be plotted, fitted and decomposed into closure and residual stress effects, respectively. It is concluded that although the residual stress effect is present at all values of the load ratio R, its effect is relatively short-lived, whilst the closure effect that is dominant at low values of R causes longer range retardation.

High Cycle Fatigue in the Transmission Electron Microscope.

One of the most common causes of structural failure in metals is fatigue induced by cyclic loading. Historically, microstructure-level analysis of fatigue cracks has primarily been performed post mortem. However, such investigations do not directly reveal the internal structural processes at work near micro- and nanoscale fatigue cracks and thus do not provide direct evidence of active microstructural mechanisms. In this study, the tension-tension fatigue behavior of nanocrystalline Cu was monitored in real time at the nanoscale by utilizing a new capability for quantitative cyclic mechanical loading performed in situ in a transmission electron microscope (TEM). Controllable loads were applied at frequencies from one to several hundred hertz, enabling accumulations of 10(6) cycles within 1 h. The nanometer-scale spatial resolution of the TEM allows quantitative fatigue crack growth studies at very slow crack growth rates, measured here at ∼10(-12) m·cycle(-1). This represents an incipient threshold regime that is well below the tensile yield stress and near the minimum conditions for fatigue crack growth. Evidence of localized deformation and grain growth within 150 nm of the crack tip was observed by both standard imaging and precession electron diffraction orientation mapping. These observations begin to reveal with unprecedented detail the local microstructural processes that govern damage accumulation, crack nucleation, and crack propagation during fatigue loading in nanocrystalline Cu.