Measured Crack Growth Rates Research Articles

The environmentally small/short crack growth effect: current understanding

The key factors that impact on the evolution of stress corrosion and corrosion fatigue cracks in the small crack regime are highlighted and a detailed description is given of the mechanistic basis of the environmentally small/short crack effect. Stress corrosion and corrosion fatigue crack growth rate measurements in the small and short crack regime, principally on steels, are then reviewed and the results discussed in the context of current understanding of the controlling processes.

New Experimental Techniques for In-situ Measurement of the Damage Characteristics of Piezoelectric Ceramics under High Cycle Fatigue Testing

To better understand the failure characteristics of lead titanate zirconate (PZT) piezoelectric ceramics, in-situ measurements of displacement and crack growth rates were conducted during high cycle fatigue testing, e.g., 5 kHz. A commercial PZT ceramic (used in a buzzer) was employed as the specimen. To examine the failure characteristics, two newly proposed systems were used: (i) a high speed camera system and (ii) a condenser microphone system. The former system consisted of two high speed cameras with an analytical system, which could measure the displacement of the PZT ceramic during the cyclic loading. The maximum displacement value of the ceramic was found to be approximately 20 μm at 0.5 kHz. The three-dimensional shape of the PZT ceramic during cyclic loading could be clearly observed. With the latter system, the displacement intensity arising from the ceramic vibration was detected continuously. It was found that the crack growth rate was not correlated with the fatigue frequency due to the resonance caused by the ceramic oscillation. There is a linear relationship between the crack growth rate and sonar intensity. On the basis of the crack growth behavior, the failure characteristics of the PZT ceramic could be clearly determined.

Effect of initiation feature on microstructure-scale fatigue crack propagation in Al–Zn–Mg–Cu

High fidelity measurements of constituent particle or corrosion topography nucleated fatigue crack growth rates (da/dN) are established for 7075-T651 in humid air. Values of microstructure-scale da/dN are determined by microscopy of programmed load-induced crack surface markers, rather than surface-only measurements. Both pristine and corroded specimen da/dN from various applied stress levels are successfully correlated using continuum-elastic stress intensity (ΔK or ΔK and Kmax) or dislocation-based (Bilby–Cottrell–Swindon) crack tip opening displacement (cyclic ϕ and ϕmax), with the former accounting for the gradient of elastic stress concentration due to the initiating feature. Values of da/dN vary by an order of magnitude at each fixed driving force due to microstructural influences that result in a locally irregular crack front. Grain-scale models using stress intensity closure or slip-based crystal plasticity do not capture experimental da/dN variability. Due to an inadequate mechanistic basis, mechanics-inspired models of da/dN do not predict multiple growth regimes that are typical of environment enhanced cracking. An elastic ΔK-based description of long crack da/dN data for a given alloy-environment can be transformed to a continuum elastic–plastic ϕc basis to provide a mean crack growth rate description. Coupling mean rates with a statistical description of microstructure sensitive variability, and dislocation or crystal plasticity-finite element modeling of component ϕc for non-continuum cracking, will enhance prognosis in the MSC regime.

Cyclic compression behavior of a Cu–Zr–Al–Ag bulk metallic glass

The cyclic compression behavior of a Cu 45Zr 45Al 5Ag 5BMG was investigated in order to elucidate the damage initiation and growth mechanisms. The present Cu 45Zr 45Al 5Ag 5 BMG was found to have a fatigue-endurance limit of 1418 MPa and fatigue ratio of 0.77. Fracture under cyclic compression occurred in a pure shear mode. The fracture surface forms an angle of 41° with respect to the loading axis. This angle was similar to the monotonic compressive fracture angle for the present BMG. The cyclic compression fracture surface displays a morphology nearly identical to the monotonic compression fracture surface. In addition to many shear bands and cracks, areas of “chipping” were commonly found on the outside surfaces of the fatigue specimens. An attempt was made to measure crack growth rates, and two types of crack growth behavior were found. With the first type, the growth rate decreased with cycles due to the decrease in the driving force for crack propagation. With the second type, the crack growth rate increased with cycles after chipped areas developed. The fatigue deformation process for BMGs under cyclic compression was carefully studied and rationalized.

An inverse method for evaluating weld residual stresses via fatigue crack growth test data

This paper presents an inverse method for calculating the thermal residual stresses in welded specimens via measured fatigue crack growth rates. Firstly, fracture-mechanics superposition law has been used to extract the stress intensity factor due to residual stress contribution from measured crack growth rate. Secondly, a so-called B matrix has been established by performing finite element analysis. Residual stress distribution is then determined by solving linear algebraic equations relating the B matrix and residual stress intensity factors obtained from crack growth test data. The inverse method has been validated by a well-established residual stress distribution and corresponding stress intensity factor, and then applied to an M(T) sample in 2024-T3 alloy with a longitudinal weld. Agreement with the measured residual stresses is reasonably good and reasons for certain differences between the calculated and measured are discussed.

Damage analysis of reinforced concrete buildings by the acoustic emission technique

This paper presents a research work in which the stability of a multi-storeyed building in reinforced concrete, with two visible macrocracks periodically subjected to visual inspection, is assessed by the acoustic emission (AE) technique. The observed proportionality between the rates of recorded AE activity from the cracks and the measured crack growth rates confirms significantly the effectiveness of the AE technique for damage evolution assessment in structural elements. The AE activity has been correlated with the size of the source crack advancements by some fitting relationships established using models from Damage Mechanics, Fracture Mechanics and Geophysics. Copyright © 2010 John Wiley & Sons, Ltd.

An analytical model of plasticity induced crack closure

Accurate and non-conservative measurements of fatigue crack growth rate are essential when designing efficient structures to be subjected to cyclic loading. Crack closure is a main mechanism of fatigue crack propagation and must be included. Numerical models have been successfully developed to predict plasticity induced crack closure (PICC), however a full understanding of the links between physical parameters, residual plastic wake and PICC has not been achieved yet. The objective of the present paper is the identification of the main micromechanisms involved in PICC and the establishment of qualitative and quantitative links between plastic deformation and the level of PICC. An M(T) specimen with 200×60×0.2 mm3 and an initial crack of 10 mm was studied. It was found that the linear superposition applies to the effect of individual plastic wedges on the PICC level. The vertical elongation of the plastic wedge, Δy, was considered and found adequate to quantify the weight of individual plastic wedges in the residual plastic wake. The effect of an individual plastic wedge was found to have an exponential decrease with the distance to the crack tip, d. An empirical model was developed relating the PICC level of individual plastic wedges with the distance d and the plastic deformation level, Δy, and was applied successfully to predict PICC evolution from residual plastic wakes.

Prediction by Genetic Algorithm and Measurement by Center Hole Drilling of Residual Stresses of MAG Weldment

In the present work, specimens were cut out from St-37 plates with 19 mm thickness. The thickness of plates was reduced to 12.5 mm by milling and grinding operations. Then a standard V-shaped fillet was made on one edge of the plates. Two plates were butt-welded by standard metal arc gas (MAG) welding process. Residual stresses induced by welding were measured on 20 specimens by centre hole drilling. Load controlled axial fatigue tests were carried out to determine the fatigue life of specimens. Crack growth rates were obtained from experiment. Fractography of specimens was performed. Genetic Algorithm (GA) was employed for prediction of residual stress value in weldments using the crack growth rates obtained from experiments. The results show that, by using the measured crack growth rates and GA model, residual stresses can be estimated with a good approximation.

Three-dimensional fatigue crack growth behavior in an aluminum alloy investigated with in situ high-resolution synchrotron X-ray microtomography

The fatigue crack propagation process in an Al–Mg–Si alloy was investigated using in situ high-resolution synchrotron radiation X-ray microtomography. Tomography datasets were obtained at periodic intervals throughout the 120,000 fatigue cycles. Three-dimensional rendering of the through-thickness crack shape indicates that in a number of regions the adjacent sides of two branched cracks tend to overlap with fatigue cycling and form a crack overlapping region. Measured crack growth rates in each tomographic slice show that crack growth retardation generally occurs in these crack overlapping regions. The through-thickness variation in crack tip opening displacement was also measured and was used to account for the observed crack propagation behavior. Crack morphologies were observed at different load levels in a fatigue cycle. The crack closure level varied for two selected regions comprising different overlapping cracks. The correlation of the crack growth rate with both crack opening and closure levels was discussed and interpreted.

Modification of the FRI Crack Growth Model Formulation from a Mathematical Viewpoint

The FRI model of crack growth, which incorporates mechanical properties into the slip oxidation mechanism of crack advance, is an extension of the well-known Ford-Andresen model. When the exponent of the oxidation current decay curve is set close to 1, however, the FRI model gives an infinite crack growth rate. Here, the oxidation decay curve integral is revised to eliminate this divergence, and modified crack growth rate equations are derived. Also presented here is a procedure for determining the oxidation current parameters from the curve-fitting to measurements of crack growth rate. Parameter value determination and crack growth calculations are illustrated for cold-worked Type 316L stainless steel.

Modification of the FRI Crack Growth Model Formulation from a Mathematical Viewpoint

The FRI model of crack growth, which incorporates mechanical properties into the slip oxidation mechanism of crack advance, is an extension of the well-known Ford-Andresen model. When the exponent of the oxidation current decay curve is set close to 1, however, the FRI model gives an infinite crack growth rate. Here, the oxidation decay curve integral is revised to eliminate this divergence, and modified crack growth rate equations are derived. Also presented here is a procedure for determining the oxidation current parameters from the curve-fitting to measurements of crack growth rate. Parameter value determination and crack growth calculations are illustrated for cold-worked Type 316L stainless steel.

Fatigue crack growth through a residual stress field introduced by plastic beam bending

ABSTRACTWe present predictions and measurements of fatigue crack growth rates in plastically bent aluminium 2024‐T351 beams. Beam bending and fatigue were carefully controlled to minimize factors other than residual stress that could affect the fatigue crack growth rate, such as large plastic strains or residual stress relaxation. The residual stress introduced by bending was characterized by a bending method and by the slitting method, with excellent agreement between the two methods. Crack growth rates were predicted by three linear elastic fracture mechanics (LEFM) superposition based methods and compared to experimental measurements. The prediction that included the effects of partial crack closure correlated with experimental data to within the variability normally observed in fatigue crack growth rate testing of nominally residual stress free material. Therefore, we conclude that crack growth through residual stress fields may be predicted using the concept of superposition as accurately as crack growth through residual stress free material, provided that the residual stress is accurately known, the residual stress remains stable during fatigue, the material properties are not changed by the introduction of residual stress, and that the effect, if any, of partial crack closure is taken into account.

Compression Precracking to Generate Near Threshold Fatigue Crack Growth Rates in an Aluminum and Titanium Alloy

Abstract The scope of this paper is to determine the fatigue-crack growth rates in the near-threshold regime for both an aluminum and titanium alloy using the compression precracking constant-amplitude (CPCA) test method on compact specimens. Tests were conducted over a wide range of stress ratios (R=Pmin/Pmax=0.1 to 0.9). Results were compared with threshold and near-threshold data generated on the same materials using the ASTM E647, “Standard Test Method for Measurement of Fatigue Crack Growth Rates,” load-shedding test procedure. On the 7075-T651 alloy, very little difference was observed in threshold values between the load-shedding and CPCA test methods. In contrast, the titanium alloy (Ti-6Al-4V) showed very large differences between the CPCA and load-shedding test results in the near-threshold and threshold regimes for the low stress ratios. Results under high R conditions (R≥0.7) agreed well between the two-threshold test methods for both materials. On the titanium alloy, the load-shedding test method also produced a specimen width effect on near-threshold behavior, whereas the CPCA test method produced results that were independent of specimen width and produced “steady-state” constant-amplitude data in the near-threshold regime, after the crack had grown several compressive plastic-zone sizes.

Obtaining the J–Δ a curves of an X-750 alloy from rising load test results and iso- a curves obtained by means of finite elements model

The rising load test (RLT) allows the susceptibility of the X-750 alloy to intergranular stress corrosion cracking (IGSCC) to be assessed. It consists of a three-point bending test of notched and precracked small specimens, in which the response in air and in an aggressive environment (water at 93 °C), under displacement control, is compared. The test classifies each material depending on the value of the stress intensity factor for the maximum load, P max, and, also, on the time for the load to drop from P max to 1/2 · P max during propagation. It is, in summary, a measurement of toughness at initiation together with a measurement of crack growth rate. In this paper, the response to IGSCC of an X-750 alloy under different heat treatments by means of the RLT is presented. In addition, a finite element model has been developed to obtain the iso- a curves (without propagation) for different crack lengths. By combining the experimental curves with the results of the numerical model it is possible to obtain the J–Δ a curves ( R curves) of the X-750 material, in air and RLT environment, respectively. In this paper, a detailed analysis methodology is presented.

Corrosion monitoring application in Taiwan's nuclear power plants

Corrosion monitoring methods have been applied extensively for system condition diagnosis, material degradation resistance evaluations, and water chemistry optimisation in Taiwan's boiling water and pressurised water reactor nuclear power plants. The online corrosion monitoring systems developed and currently applied in Taiwan are reviewed. Examples of applications discussed include electrochemical potential monitoring, online crack growth rate measurements, electrochemical noise monitoring, linear polarisation resistance monitoring, and potential drop pipe thinning monitoring. Online corrosion monitors have proven useful tools within the nuclear power plant monitoring systems currently installed and have achieved their specific objectives in the context of Taiwan's power plants.

Fatigue Crack Growth in Integrally Stiffened Panels Joined Using Friction Stir Welding and Swept Friction Stir Spot Welding

Abstract Fatigue crack growth rates in AA2024-T3 sheet joined to AA7075-T6 stiffeners by friction stir welding have been compared with rates measured in panels joined with rivets and in unstiffened parent material panels. Friction stir panels were prepared with both continuous friction stir welding (FSW) and swept friction stir spot welding (FSSW). Fatigue cracks in edge crack panels with continuous FSW joints tended to grow into the parent material away from the stiffeners. This behavior was attributed to the reduced stress levels corresponding to the increased thickness of continuous FSW joints and the support of the attached stiffener. Fatigue cracks in edge crack panels were found to follow the joint line in panels made with discrete fasteners (both rivets and swept FSSW joints). The measured crack growth rates in riveted panels increased at an increasing rate as the crack approached the riveted joints. In contrast, crack growth rates in panels joined with FSSW decreased at a decreasing rate as the crack approached the swept spot joints. Test results indicate that the swept FSSW process produced favorable residual stresses that inhibited fatigue crack growth along the joint line.

Hydrazine and Hydrogen Coinjection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors (VII)—Effects of Bulk Water Chemistry on ECP Distribution inside a Crack

Water chemistry in a simulated crack (crack) has been studied to understand the mechanisms of stress corrosion cracking in a boiling water reactor environment. Electrochemical corrosion potential (ECP) in a crack made in an austenite type 304 stainless steel specimen was measured. The ECP distribution along the simulated crack was strongly affected by bulk water chemistry and bulk flow. When oxygen concentration was high in the bulk water, the potential difference between the crack tip and the outside of the crack (ΔE), which must be one motive force for crack growth, was about 0.3V under a stagnant condition. When oxygen was removed from the bulk water, ECP inside and outside the crack became low and uniform and ΔE became small. The outside ECP was also lowered by depositing platinum on the steel specimen surface and adding stoichiometrically excess hydrogen to oxygen to lower ΔE. This was effective only when bulk water did not flow. Under the bulk water flow condition, water-borne oxygen caused an increase in ECP on the untreated surface inside the crack. This also caused a large ΔE. The ΔE effect was confirmed by crack growth rate measurements with a catalyst-treated specimen. Therefore, lowering the bulk oxidant concentration by such measures as hydrazine hydrogen coinjection, which is currently under development, is important for suppressing the crack growth.

Interlaminar fatigue crack growth of cross-ply composites under thermal cycles

Abstract The fatigue growth of a fiber reinforced composite laminate was characterized under thermal cycling using a combined experimental and computational investigation. Twenty-four ply composite laminates ( [ 0 ° 12 / 90 ° 12 ] ) are fabricated with a pre-existing delamination, and subjected to thermal cycling in an environmental chamber. The large mismatch in the coefficients of thermal expansion is used to grow an interlaminar crack at the interface of the 0° and 90° laminae. This thermal fatigue crack growth behavior is investigated for different amplitudes of temperature change (ΔT = 30–140 °C). The inspection of fracture surfaces, after completion of the fatigue tests, reveals an angled or kinked crack front growth with greater propagation distances near the free-surfaces/edges. Due to the non-uniform crack growth across the specimen thickness, three-dimensional finite element analyses are performed to investigate the fatigue growth mechanisms under thermal load. From the analysis, the energy release rate as well as the mixed-mode stress intensity factors is calculated and the variations of these fracture parameters are found to be consistent with the observed crack front configuration. Using the computed results, the experimentally measured crack growth rates are also correlated with the amplitude of energy release rate, and a power law form of the fatigue law is established. The relevant coefficients as well as the threshold energy release rate are also determined. The present analysis is useful for not only understanding the fatigue delamination mechanisms under thermal cycling but also for estimating the threshold temperature variation that is needed to drive crack growth.

Fatigue Crack Characteristics of Friction Stir Welded Aluminum Alloy Joints

Fatigue strength of Al5083-O FSW joints was evaluated in the relation of the FSW conditions. Static strength of the joints in a range of FSW conditions was the same as that of base metal. Because the fracture of the joints occurred at the base metal in the tensile tests. Therefore the joint efficiency in tensile strength of joints with the FSW condition of tool rotation: 800 rpmwelding speed: 100 mm/min (800-100), 800-200 and 500-100 are all hundred percent. However, the fatigue strength of those joints with fine static strength varied very much. The joint efficiency for fatigue strength varies from 75% for 500-100 to 31% for 800-200. Crack path in fatigue test was always initiated at the center of back surface of FSW weld zone, and propagated through stir zone. Crack growth rate measurement and fracture surface analysis resulted the difference of the joint efficiency has a connection with the incomplete welding at back surface of joints.

Simulation of crack growth under low cycle fatigue at high temperature in a single crystal superalloy

Crack growth tests have been performed at 950 °C with Single Edge Notch specimens of the Ni-based single crystal superalloy PWA1483. In particular, several orientations and frequencies have been investigated, thus allowing the assessment of the influence of these parameters on the crack growth rate. In addition, oxidation experiments have been carried out to characterize the kinetics of the outer oxide scale growth at the same temperature. On the other side, crack growth has been simulated with the Finite Element program ABAQUS in real test conditions by the node release technique. The nodes are released according to the measured crack growth rate. The simulation results are compared with the test results on the basis of the computed Crack Tip Opening Displacement (CTOD). For this purpose, the crack is propagated until a stabilized value of the CTOD is obtained. This is usually the case when the crack has crossed the initial plastic zone. The procedure provides an evaluation of the effects of cycle frequency, crystal orientation, plasticity and oxide induced crack closure.