Crack Growth Measurements Research Articles

On the application of laser shock peening for retardation of surface fatigue cracks in laser beam‐welded AA6056

AbstractThe present study aims to investigate the extent to which the fatigue behaviour of laser beam‐welded AA6056‐T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as‐welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as‐welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing.

Fatigue life prediction of pipeline with equivalent initial flaw size using Bayesian inference method

Majority of pipeline infrastructure are old and susceptible to possible catastrophic failures due to fatigue. Timely maintenance is the key to keep pipeline in serviceable and safe condition. This paper proposed a Bayesian inference methodology based on the observed crack growth measurements and cycle data that predicts the probability density of failure after initially estimating the equivalent initial flaw size (EIFS). The model was first developed based on one-dimensional crack growth problem in plate with edge crack. Then the model was expanded to two-dimensional crack growth problem in pipe wall. Stress intensity factors (SIF) at the crack tip in pipe model were calculated using finite element (FE) analysis for different crack lengths and depths. Polynomial function and Gaussian process were used to develop surrogate models of SIF. The analysis demonstrated that the proposed Bayesian inference method with hyperparameters generated accurate inferred results for probability density function (PDF) of both EIFS and the number of cycles to failure.

Methods for automated crack length detection in fracture mechanical fatigue tests of unreinforced polymers

The continuous measurement of crack growth during fracture mechanical fatigue tests is of great importance for an accurate lifetime estimation of components. Different methods for an automatic crack length detection exist for linear elastic materials and electrically conductive materials like metals (ASTM International E 647-11, 2011; International Standard ISO 15850, 2002a) or fiber reinforced composites (ASTM International D5528-13, 2013; International Standard ISO 15024, 2001). However, there is no satisfying automatic system for the crack length detection in polymers. For polymers, it is still state of the art to measure the crack length manually by the use of a travelling microscope. Besides other drawbacks, this way of measuring provides only a limited number of data points. Therefore, the aim of this work was to develop methods for the automated crack length detection in these materials. Two measuring techniques were examined regarding their general applicability: (i) infrared thermography (IRT) and (ii) digital image correlation (DIC). For the validation, three types of thermoplastics (Polyvinylchloride (PVC-U), Polymethylmethacrylate (PMMA) and Polyoxymethylene (POM)), which show rather different mechanical behavior, were tested. Satisfactory correlations were obtained for both DIC and IRT method. In general, all tested systems showed good results and the determined average difference in the measured crack lengths was smaller than 5.5 %. Hence, the investigated methods based on DIC and IRT are highly interesting for an automation of fatigue crack growth measurements of polymers.

Hydrogen embrittlement analysis in a CrMoV steel by means of sent specimens

In this study, single-edge notched tensile (SENT) specimens machined from a quenched and tempered CrMoV steel plate were hydrogen pre-charged using two different methods: in a high pressure and high temperature hydrogen reactor (19.5 MPa and 450 °C); and using an electrochemical method (2 M H2SO4 + 0,25 g/l As2O3, under a current density of 2 mA/cm2). The thickness of the specimens was only 1 mm in order to facilitate hydrogen entrance in a short time and the fracture J resistance curves were obtained using digital image correlation (DIC) to measure crack growth. The effect of both hydrogen pre-charged methods on the fracture toughness results was analysed, as was the influence of the applied displacement rate. Embrittlement indexes increase as the displacement rate decreases, with maximum embrittlement indexes of 20% and 84% being respectively measured for gaseous and cathodic pre-charging in tests performed at a displacement rate of 0.01 mm/min. Significant differences in the operative failure micromechanisms in the samples pre-charged via both methods were also detected and hydrogen embrittlement theories (hydrogen-enhanced localized plasticity, HELP, and hydrogen-enhanced decohesion, HEDE) were evaluated by mapping the strain distribution in the crack tip process region by means of DIC.

Fatigue crack growth lessons from thirty-five years of the Royal Australian Air Force F/A-18 A/B Hornet Aircraft Structural Integrity Program

As the Royal Australian Air Force (RAAF) retires its fleet of F/A-18 A and B model Hornets, it is opportune to reflect upon some of the in-service fatigue crack growth lessons accrued from 35 years of the aircrafts Aircraft Structural Integrity Program (ASIP). In this paper, the authors have selected a small number of the key lessons and observations arising from real-world fatigue problems in the RAAF Hornet fleet that have contributed to advances in the state of the art of fatigue crack growth measurement and prediction. The importance of the lead crack concept, quantitative fractography, fatigue testing, nucleating discontinuities, the physically short crack regime, as well as individual aircraft tracking are given prominence. It is hoped that this review of the metal fatigue challenges for a highly stressed lightweight structure that have driven research into fatigue crack growth will be valuable to those aspiring to make impactful contributions to the field and the sustainment of aircraft fleets worldwide.

Fatigue Crack Propagation under Mixed Mode I and III in Polyoxymethelene Homopolymer

While mixed-mode loading conditions became hot topic recently, it is still quite unexplored area, even in case of metals. In case of polymers, this is even more complex problem, thanks to their higher sensitivity on higher temperatures connected with friction. Since polymer materials are very popular, this topic is of high interest. One of the best experimental specimens for achieving mixed mode loading conditions is the CRB specimen, as it provides clear mixed-mode I + III conditions under tension + torsion. Furthermore, for lifetime predictions it is necessary to calculate some material constants based on measured crack growth rates. However, these crack growth rates are not as easily and accurately measured on CRB specimens, as on CT specimens. Therefore, the main focus of this study is to observe difference between lifetime predictions based on CT and CRB data and possible application of CT data for CRB specimens.

The edge-notched clamped beam bend specimen as a fracture toughness test geometry

Edge-notched clamped beam bend fracture specimens provide the required geometric stability for controlled crack growth in brittle materials, enabling the measurement of R-curves and cyclic crack growth. In this paper, finite element simulations were carried out on this test geometry to extract the normalized stress intensity factor KI solutions for different useful specimen dimensions, loading tip radius and loads. Based on these results, geometric factors have been proposed that can fit various beam aspect and crack depth ratios. Crack stability analysis is carried out in terms of the R locus for various beam aspect ratios for the clamped beam geometry and compared to other fracture toughness tests. Experimental verification of the same has been carried out at the macro-scale using poly-methyl methacrylate (PMMA) and WC-Co as model materials.

A phase-field model for fatigue crack growth

A phase-field model is presented for the study of fatigue crack growth. A crack growth viscosity parameter is introduced into the standard phase-field model for brittle fracture to account for rate- or cycle-dependent crack growth phenomena. A modified J-integral is developed to demonstrate how the phase-field approach can be used to generate Paris-law type crack growth rates. In order to model more general crack growth versus applied loading behaviors that are not a single simple Paris-law, steady-state finite element calculations are performed to calibrate fits of the phase-field model to measured crack growth rates found in da/dN versus ΔK curves. Transient time- or cycle-dependent calculations are performed and compared to experimental measurements on samples where crack turning is induced by the presence of a hole in the vicinity of the crack. A three-dimensional example with turning of the crack front is also computed illustrating the capabilities of the phase-field approach for complex crack paths.

Fatigue Behavior and Fracture Mechanism of a Hot Rolled AA7020 Aluminum Alloy

Fatigue tests of smooth specimens and CT specimens of a hot rolled AA7020 Aluminum alloy have been performed in laboratory at ambient temperature. Fatigue strength and fatigue crack propagation (FCP) characteristic were evaluated and fracture mechanism was discussed on the basis of crack initiation, small crack growth, and fracture surface analysis. The growth behavior of small and large cracks has been investigated on 7020 alloy.The FCP resistance was found significantly lower than that of other aluminum alloys. The crack growth measurements were performed in CT specimens at constant load ratios ranging from R = 0.1–0.5. The fatigue strength at 104 cycles was 110 MPa that led to a considerably fatigue ratio of 0.1, fatigue failure did not occur. There existed two different modes of crack initiation depending on applied stress level. Above 200 MPa, cracks initiated at the specimen surface in transgranular or intergranular manner due to cyclic slip deformation, while below that stress subsurface crack initiation took place. The growth of small cracks initiated at the surface coincided with the FCP characteristic after allowing for crack closure for large cracks, but the operative fracture mechanisms were different between small and large cracks.

A numerical model to simulate ductile tearing-creep crack growth interaction

Ductile tearing and creep crack growth are generally treated independently but there are situations in which they can occur simultaneously. First, creep crack growth calculations in components are often continued to large defect sizes in order to determine when limiting conditions for short-term fracture are reached. Secondly, in order to obtain data in reasonable timescales, experimental creep crack growth tests are often performed at high loads such that plasticity occurs in the tests, particularly as the crack grows to larger sizes. This paper presents a numerical model to simulate the interaction of ductile tearing and creep crack growth to address such cases. A strain-based damage model is introduced with total damage assumed to be the linear summation of creep and plastic damage. The model is applied to Type 316H stainless steel at 550 °C with the parameters in the damage model determined from tensile, creep and fracture toughness test data. Predictions using the proposed model are then compared with notched creep tensile and creep crack growth test results and shown to be in good agreement with experimental measurements of creep deformation and crack growth.

Observations of compression and fracture in polymer networks subjected to impact loading

Compression and fracture during high rate impacts were quantified in a series of model transparent polymer networks using high speed videography with sub-microsecond temporal resolution. Sample transparency enabled direct quantification of the compression front velocities, as well as time-resolved measurements of crack initiation and growth. Compression front propagation reveals multiple fronts and reflections, indicating a complex loading profile under these conditions. The front velocity is found to correlate linearly with the longitudinal sound velocity. Radial crack growth was measured both in the radial direction as well inward growth towards the sample impact surface. Measurements of the crack length reveal a deceleration period at short times, transitioning to a steady-state velocity at longer times, and crack arrest at still longer times. Crack lengths were found to correlate with quasi-static fracture toughness, and crack velocities were compared to longitudinal sound velocities.

Fatigue crack growth of a hot mix asphalt using digital image correlation

The objective of this study is to demonstrate that digital image correlation can be employed to track crack propagation and produce more accurate measurements of the fatigue crack growth of hot mix asphalts. Indirect tensile, fracture, and fatigue crack growth (FCG) experiments are performed. Digital image correlation is shown to capture the crack propagation, crack tip remodeling, fatigue process zone, and strain distribution on the surface of the specimens. Microstructurally-small cracking is detected in the FCG data; where cracking is extremely sensitive to microstructural features. Failure analysis finds on the macroscale, the crack bypasses aggregates and propagates primarily through the mastic (bitumen and mineral filler). On the microscale, the temperature-dependent cohesive strength of the mastic governs FCG resistance.

Fatigue crack initiation and subsequent crack growth in fillet welded steel joints

The fatigue damage evolution in fillet welded steel joints where cracks are emanating from the weld toe is investigated. Based on existing experimental data for as-welded joints including crack depth measurements of the early crack growth it is proposed to make a distinction between the crack initiation phase and the subsequent crack growth phase. The welded detail in question is an F class detail with plate thickness 25 mm made of medium strength carbon steel. It is found that the crack initiation phase defined at a crack depth of 0.1 mm is close to 25% of the fatigue life even at a relatively high constant stress range of 150 MPa. At lower stress ranges it is concluded that the initiation phase is the dominating part of the fatigue life. The present work is focusing on the crack propagation phase that is defined from a crack depth of 0.1 mm to final failure of the detail. It is demonstrated that the recommendation given in rules and recommendations (DNVGL and BS 7910) for applying Linear Elastic Fracture Mechanics (LEFM) for the crack propagation phase is valid for the propagation of such small surface breaking cracks. A model based on the rule-based formulas for the Stress Intensity Factor Range (SIFR) and the growth parameters C and m in Paris law agree well the with the measured crack growth curves. For these small semi-elliptical cracks at the weld toe notch important topics like the existence of a threshold limit for the SIFR and the influence of the stress ratio R are discussed. Due to the inherent scatter in the variables characterizing the fatigue damage evolution stochastic approaches are applied for the analyses. Observations and measurements are presented by descriptive statistics and simulations are carried out using Monte Carlo techniques.

An Optical Crack Growth Sensor Using the Digital Sampling Moiré Method.

High-accuracy crack growth measurement is crucial for the health assessment of concrete structures. In this work, an optical crack growth sensor using the digital sampling moiré (DSM) method is developed for two-dimensional (2D) crack growth monitoring. The DSM method generates moiré fringes from a single image through digital image processing, and it measures 2D displacements using the phase difference of moiré fringes between motion. Compared with the previous sensors using traditional photogrammetric algorithms such as the normalized cross-correlation (NCC) method, this new DSM-based sensor has several advantages: First, it is of a higher sensitivity and lower computational cost; second, it requires no prior calibration to get accurate 2D displacements which can greatly simplify the practical application for multiple crack monitoring. In addition, it is more robust to the change of imaging distance, which is determined by the height difference between two sides of a concrete crack. These advantages break the limitation of the NCC method and broaden the applicability of the crack growth sensor. These advantages have been verified with one numerical simulation and two laboratory tests.

Short crack growth behavior and its transitional interaction with 3D microstructure in Ti-6Al-4V

Currently, short crack growth behavior and rate variations are not well understood in the literature. This is due to lack of studies regarding the interaction between 3D short crack and microstructure and its effect on crack growth. In order to study this interaction, in situ computed tomography was performed to measure crack growth at sub-grain level (every 5 µm) during a fatigue test in a bimodal Ti-6Al-4V alloy for two crack front regions. This was followed by serial sectioning coupled with electron backscattering diffraction (EBSD) to identify the short crack growth in the microstructure, i.e. α, α + β phases and interface. Results show that crack growth has the highest rate in α phase as compared to the α + β phase and the interface in both regions. The crack grows preferably into α phase when compared to the average microstructural fraction in the first region, but it decreases below this fraction in the second region. The crack grows mainly close to crystallographic planes in α grains with the maximum shear stress (favorable planes) in the first region. As the short crack grows into the second region, there is an increase in number of grains enclosed in the plastic zone size. As a result, there is a decrease in the mismatch angle between neighboring cracked grains, which leads to higher deviation from favorable planes causing a local variation in crack growth rate.

Comparative evaluation of two different methods for thermal shock resistance of laminated ZrB2-SiCw/BN ceramics

The thermal shock resistance (TSR) of laminated ZrB2–SiCw/BN ceramic was evaluated through indentation-quench and quenching-strengthening methods. It was correspondingly compared to monolithic ZrB2–SiCw ceramic. In the indentation-quench method with consideration to crack propagation on the surface layer, the critical thermal shock temperature of laminated ZrB2–SiCw/BN ceramic with surface residual tensile stress was 550 °C, which was lower than monolithic ZrB2–SiCw ceramic (600 °C). Unlike the microscopic method of crack growth measurement through indentation-quench testing, the quenching-strengthening method, which was based on the macroscopic properties of the material, mainly characterizing the residual strength subsequently to thermal shock, the critical thermal shock temperatures of the laminates and monolithic were 609 °C and 452 °C, respectively. Compared to the brittle fracture of ZrB2–SiCw ceramics, the deflection, bifurcation and delamination of the cracks as the main TSR mechanisms of the laminated ceramics, were revealed through quenching-strengthening method, which was more suitable for the TSR characterization of laminated ceramics.

Modelling of the fatigue crack growth of a coated single crystalline nickel-based superalloy under thermal mechanical loading

The focus of this paper is the simulation of fatigue crack growth of the coated single crystalline nickel-based superalloy PWA 1484 under thermal mechanical loading. Thus, two physical models are superimposed in terms to firstly calculate the deformation behavior under instationary thermal and mechanical loading (TMF) and secondly to model crack propagation after initial brittle cracking of the coating layer on the basis of cyclic crack-tip opening displacement (CTOD). All material parameters implemented in the models were evaluated from monotonic isothermal tensile and creep tests as well as from isothermal low cycle fatigue (LCF) experiments. The calculated fatigue crack growth was validated by in situ crack growth measurements using the beachmark technique. Hence, crack propagation initiated by the brittle coating system closely to the experimental results using rectangular flat specimen geometry instead of corner-crack (CC) specimens. The comparison of the simulated lifetimes to the experimental results provides remarkable accuracy of the physically-based lifetime model.

Mixed mode II and III fatigue crack growth in a rail steel

Rolling contact fatigue cracks in rails undergo non-proportional mixed-mode I + II + III, in variable proportions along their front. In order to determine the crack growth thresholds and kinetics in mixed-mode II/III, asymmetric four point bending tests are run on a rail steel with different angles between the crack front and the shearing load, so as to vary the mode mixity ratio. For sufficiently high loading ranges, these tests give rise to coplanar shear-mode crack growth. The effective stress intensity factors (SIFs) are derived by an inverse method from the measured crack face relative displacements. It appears to be 10–70% lower than the nominal SIFs and to allow a reasonable correlation of the measured crack growth rates. The local application, ahead of the crack front, of shear-driven or tension-driven fatigue damage models – after 3D elastic-plastic computations of local stress and strain ranges – allows a successful prediction of crack fronts paths and growth rates.

Assessment of fatigue crack growth in 316LN stainless steel based on acoustic emission entropy

This paper presents an investigation of assessment of fatigue crack growth (FCG) of 316LN stainless steel using the acoustic emission (AE) monitoring technique. A new AE parameter, i.e. AE entropy, was used for accurate assessment of fatigue damage and prediction of fatigue crack length. The effects of peak load on both FCG and AE behaviors were also discussed by fractographic observations and AE parameter analyses. The results showed that AE entropy is effective for accurately identifying the point of crack initiation and assessing the fatigue damage during FCG of 316LN stainless steel under high noise loading environment. Moreover, two distinct damage stages were distinguished by the characteristics of AE entropy. The source mechanisms of AE entropy for stage 1 were fatigue crack initiation and small crack growth, while the mechanisms responsible for stage 2 were the plastic activities in plastic zone at the crack tip, the formation of micro-voids and the ligaments shearing between these micro-voids. The results also indicated that AE entropy is available for identifying the transition in stress condition from plane strain to plane stress state of specimens under high peak load. In addition, a probabilistic model based on the quantitative relationship between the crack growth size and cumulated entropy was proposed for prognosis of crack growth. Bayesian inference was used to estimate the model parameters and the posterior predictive distribution of crack length. The predictive results showed that the model could accurately predict the measured crack growth lengths in all cases. The proposed model is applicable to stable long crack growth and can be applied under high noise loading environment.

The signatures of acoustic emission waveforms from fatigue crack advancing in thin metallic plates

The acoustic emission (AE) waveforms from a fatigue crack advancing in a thin metallic plate possess diverse and complex spectral signatures. In this article, we analyze these waveform signatures in coordination with the load level during cyclic fatigue. The advancing fatigue crack may generate numerous AE hits while it grows under fatigue loading. We found that these AE hits can be sorted into various groups based on their AE waveform signatures. Each waveform group has a particular time-domain signal pattern and a specific frequency spectrum. This indicates that each group represents a certain AE event related to the fatigue crack growth behavior. In situ AE-fatigue experiments were conducted to monitor the fatigue crack growth with simultaneous measurement of AE signals, fatigue loading, and optical crack growth measurement. An in situ microscope was installed in the load-frame of the mechanical testing system (MTS) to optically monitor the fatigue crack growth and relate the AE signals with the crack growth measurement. We found the AE signal groups at higher load levels (75%–85% of maximum load) were different from the AE signal groups that happened at lower load levels (below 60% of load level). These AE waveform groups are highly related to the fatigue crack-related AE events. These AE signals mostly contain the higher frequency peaks (100 kHz, 230 kHz, 450 kHz, 550 kHz). Some AE signal groups happened as a clustered form that relates a sequence of small AE events within the fatigue crack. They happened at relatively lower load level (50%–60% of the maximum load). These AE signal groups may be related to crack friction and micro-fracture during the friction process. These AE signals mostly contain the lower frequency peaks (60 kHz, 100 kHz, 200 kHz). The AE waveform based analysis may give us comprehensive information of the metal fatigue.