Fatigue Crack Growth Measurements Research Articles

Analysis of Fatigue Crack Growth Rate Data in an Aircraft 7175-T7351 Al-Alloy Generated at Different Accredited Laboratories with Probability Life Assessment Example

Estimation of safety and reliability of engineering structures and components containing cracks or crack-like defects is one of the most important application field of fracture mechanics particularly in components, where limited defects can be accepted. Such design philosophy is usually called “damage tolerance”, formerly “safe life”. In such cases, safety and reliability of further operation, residual life assessment and specification of interval of damage development inspections are important issues. Existing exact knowledge on fatigue crack growth (FCG) parameters is an essential condition. Results of FCG measurement in an Al 7175-T7351 alloy of a particularly high homogeneity, carried out as a part of the Proficiency Test Program organized by Exova in France are presented in this work. Results were evaluated in terms of parameters C and m of the Paris dependence in the stable growth region. To pass the test program successfully, a particular attention was paid to improve and verify direct current potential drop (DCPD) method being used for the crack length measurement. In the paper, the results of the FCG measurements generated by different participating laboratories were analyzed. A distinct correlation between C and m values, so called “coupling”, was found and demonstrated. Some paradoxes of the assessment of laboratories were addressed, namely the fact that a laboratory assessed as unacceptable concerning one of the parameters generated much more accurate and useful data than another laboratory with a better assessment. Eventually, the results including their scatter were used to provide an example of probabilistic assessment of a simple beam residual life to show practical actual impacts of the scatter on the life assessment.

A finite element study of fatigue crack propagation in single lap bonded joint with process-induced disbond

This paper presents a method for predicting fatigue crack propagation in adhesive bonded composite joints with an initial full-width disbond using finite element analysis and numerical integration of the material's fatigue crack growth rate law. Fatigue tests were conducted on single lap joints. Crack lengths were monitored from four runout corners. In-situ crack growth measurements were performed by ink injection to identify the crack front profile during fatigue loading. The crack growth was modelled using a fracture mechanics criterion considering two different crack propagation patterns. The material's fatigue crack growth rate law was determined experimentally using the standard double cantilever beam and end notch flexure specimens. Using the total strain energy release rate and the two crack scenarios, the numerical model predicted the lower and upper bounds of the measured fatigue crack growth rates of the lap joint.

Study on Fatigue Crack Propagation Behavior in Corrosion Environment of a Cold-Rolled Austenitic Stainless Steel

Fatigue crack growth testing and measurement of fracture toughness with single edge notch tension (SENT) specimens of American Iron and Steel Institute (AISI) 301L-MT stainless steel were performed in the air, distilled water, and 3.5% and 9% NaCl solutions. Type 301L-MT steel exhibited the same resistance to corrosion for various environments in near-threshold region. The crack growth in the Paris region could be accelerated in a [Cl−] containing environment by decreasing the threshold of stress corrosion KISCC, but independent of [Cl−] concentration. When specimens were fatigue precracked in different conditions, it was found that the fracture toughness of SENT specimens in the aqueous environment decreased 23% to 30% compared with that in the air. The normalized formula was established to describe the behavior of fatigue crack propagation of type 301L-MT steel. Three parameters, ΔKthcf, β, and JIQ, were used to clarify the effect of H-embrittlement mechanism on fatigue crack propagation.

Measured fatigue crack growth increments versus predictions for small cracks in 7XXX aluminium alloys

Whilst crack growth retardation following the application of an overload during variable amplitude loading is a well-known spectrum effect, the order of smaller amplitude cycles can also affect crack growth. One method of accounting for such spectrum effects in crack growth prediction algorithms is by correcting the estimated effectiveness of each cycle or group of cycles. Such predictions are usually verified by comparison to demonstrated total fatigue lives. These corrections have been shown to be suitable for cracks that are at least several millimetres long. However, for physically small cracks: here defined as less than a few mm in depth, such predictions are often far less accurate. In the case of aluminium alloy aircraft structures, where cracks typically grow approximately exponentially from small discontinuities, these predictive algorithms should also be evaluated for their ability to predict small crack growth rates, rather than total life alone.To examine the more subtle crack growth acceleration and retardation effects from individual load cycles in typical aircraft structural 7XXX aluminium alloys, this paper presents the results of quantitative fractography measurements of very small fatigue crack growth increments produced under the application of specially engineered spectra. One such spectrum that contained short segments of constant amplitude loads of various mean stresses, was designed to examine the extent of crack growth retardation, such as from closure, at a range of small crack lengths. Other spectrum effects were also explored through use of test spectra with constant amplitude cycles that were either separate to or embedded within small segments of variable amplitude loading. The local growth produced in each case was then measured and compared to predictions produced by two closure/retardation correction methods; based on wake and plastic zone related phenomena. The results showed that the prediction methods, although they made allowances for the presence of subtle retardations and accelerations, did not accurately predict these effects over the increments of growth examined.

A modified loading method for separating the effect of residual stress on fatigue crack growth rate of welded joints

AbstractA novel loading method is proposed to remove the effect of residual stress on measured fatigue crack growth rate in welded joint, using which the influence of empirical crack growth rate correlation equations on the results can be avoided. The total stress intensity factor range and the effective stress ratios corresponding to the combined effect of applied and residual stress are kept constant during the fatigue crack growth test, by changing the applied maximum and minimum loads with the increase of crack length and changes in residual stress based on the superposition rule. The feasibility of the proposed method was validated by tests of compact tension specimens containing residual stresses introduced by four‐point bending. The loading method was then applied to a test of welded compact tension specimen with coupled effect of residual stress and microstructure change on fatigue crack growth rate. The fluctuation in the separated crack growth rate reflects the effect of microstructure change on fatigue crack growth rate.

Cyclic behavior and modeling of small fatigue cracks of a polycarbonate polymer

The fatigue behavior of a polycarbonate (PC) thermoplastic material was experimentally investigated and modeled using a MultiStage Fatigue (MSF) model that evaluates fatigue crack incubation, Microstructurally Small Crack (MSC) growth, and Long Crack (LC) growth. A set of fully reversed strain controlled tests were conducted, and an analysis of the fracture surfaces was performed using Scanning Electron Microscopy (SEM) in order to quantify the structure-property relationships for the MSF model. Fractography of the microstructure revealed that incompletely melted PC pellets were present in the polymer material that nucleated the cracks along with crazes generated on the surface. Crack lengths and fatigue crack growth rates for the MSC regime were measured from striation observations on the fracture surfaces. Discontinuous crack growth (DCG) cycles between fatigue striations, for the current iteration of the model, are accounted for by the MSF crack incubation regime. Finally, the microstructure sensitive MSF model was implemented using the observed fatigue crack growth measurements. In addition, a Monte Carlo (MC) Simple Random Sampling (SRS) routine was implemented to quantify the model uncertainty for crack growth.

Measuring fatigue crack growth and closure in Polyether Ether Ketone (PEEK)

Throughout industry, polymers are being used to replace parts that were previously fabricated from metals. Thus, the fatigue resistance of polymers needs to be well understood. Polyether Ether Ketone (PEEK) is often chosen for its good mechanical properties and high glass transition temperature. In this paper, the experiments and methods to study fatigue crack growth and subsequent crack closure are presented. Compact specimens made of PEEK were used to experimentally monitor fatigue crack growth at R=0.1 and R=0.7. Some unique aspects of the experiments include the use of compression pre-cracking and a back-face strain gauge to measure crack length. While measuring crack closure was the primary focus of the experiments, crack growth rate versus stress intensity factor range results were also generated. Using remote methods of measuring crack closure show that PEEK experiences a small level of crack closure, though the effects are minimal. However, indirect methods of measuring crack closure show that the phenomenon is more prevalent in the material than what was measured. Further research for continuing the study of fatigue crack closure in PEEK are discussed.

Fatigue performance of additive manufactured TiAl6V4 using electron and laser beam melting

Increasingly, additive manufacturing is considered to reduce weight and lead time of aerospace components. Despite the high potential of this manufacturing technology, reliable mechanical properties are an essential prerequisite for future series production. Therefore, the aim of this work was to contribute to an improved understanding of the factors influencing the mechanical properties of laser powder bed and electron beam melted TiAl6V4. Therefore, tensile tests, fracture toughness tests, fatigue crack growth measurements for different r-ratios as well as high cycle fatigue properties were determined for different surface roughness and heat treatments.

A dedicated illumination system for fatigue crack-growth measurement

This letter describes the design and development of a dedicated lighting system for the automation of fatigue crack-growth measurements. Crack growth testing is important to the characterization of the expected behaviour during service. Characteristic da/dN vs. ΔK enables the Mechanical Engineer’s understanding of the material state in the presence of stress, and assists in the calculation of the remaining fatigue life expected behaviour. Similar to the darkfield illumination principle, the developed system uses a grazing incidence angle to enhance even the smallest crack detail, a necessary feature to enable the successful application of image processing methodologies for crack-growth monitoring. A poor or wrong illumination fails to reveal the true extent of hairline cracks, greatly influencing calculations that depend on large stress gradients. The device is being used in combination with a vision system for continuous monitoring of crack-growth in metallic specimens with great success. The results obtained to date are in full agreement with the expected data for the utilized specimens, which increases the confidence in the use of the device and the continuous work towards automation of fatigue experimental procedures.

Fatigue Crack Growth Behavior of Bonded Aluminum Joints

The current paper presents a research aiming at characterizing the fatigue behavior of adhesively bonded aluminum joints. In particular, Double Cantilever Beam (DCB) specimens and End Notch Flexure (ENF) specimens built using the 6061-T651 aluminum alloy substrate and the Araldite 2015 epoxy adhesive were used to evaluate the pure mode I and mode II fatigue crack propagation rates. Besides the fatigue crack growth rates the monotonic quasi-static fracture behaviors of the adhesive joints are also characterized for both loading conditions. Numerical finite element models of the joints are also proposed for the evaluation of the compliance calibration curves, avoiding the need of direct fatigue crack growth measurements, which is a very complex task mainly for the ENF tests. Critical fracture energies (GIc and GIIc) from experimental tests were in the same order of magnitude of the values published in the literature. Concerning the fatigue crack growth behavior, it was verified a higher fatigue crack growth resistance under pure mode II loading. A good agreement was found between the experimental fatigue crack growth data and the modified Paris law that accounts for fatigue crack propagation regimes I and II.

Revisiting fatigue crack growth in various grain size regimes of Ni

The fatigue crack growth characteristics of high purity nickel with grain sizes ranging from the micro- down to the nanocrystalline regime were examined. Fatigue crack growth measurements for different load-ratios combined with extensive fracture surface analyses revealed that dislocation motion is the dominating damage mechanism in the near-threshold region up to the Paris regime, irrespective of the grain size. Grain boundaries serve as preferential crack paths in the entire investigated grain size range, depending only on the loading conditions. The controlling factor is mainly the size of the cyclic plastic zone in comparison to the grain size. As a further peculiarity crack closure disappears for microstructures with a grain size below one micron. Along with the grain size variation the influence of the grain aspect ratio was in focus, which can cause a strong orientation dependence of the fatigue crack growth behavior. Elongated microstructures aligned perpendicular to the crack growth direction can considerably improve the overall resistance against fatigue crack growth.

Influence of Particulate Reinforcement and Equal-Channel Angular Pressing on Fatigue Crack Growth of an Aluminum Alloy

The fatigue crack growth behavior of unreinforced and particulate reinforced Al 2017 alloy, manufactured by powder metallurgy and additional equal-channel angular pressing (ECAP), is investigated. The reinforcement was done with 5 vol % Al2O3 particles with a size fraction of 0.2–2 µm. Our study presents the characterization of these materials by electron microscopy, tensile testing, and fatigue crack growth measurements. Whereas particulate reinforcement leads to a drastic decrease of the grain size, the influence of ECAP processing on the grain size is minor. Both reinforced conditions, with and without additional ECAP processing, exhibit reduced fatigue crack growth thresholds as compared to the matrix material. These results can be ascribed to the well-known effect of the grain size on the crack growth, since crack deflection and closure are directly affected. Despite their small grain size, the thresholds of both reinforced conditions depend strongly on the load ratio: tests at high load ratios reduce the fatigue threshold significantly. It is suggested that the strength of the particle-matrix-interface becomes the critical factor here and that the particle fracture at the interfaces dominates the failure behavior.

Influences on the thermomechanical fatigue crack growth of the nickel alloy 617*

Abstract The following contribution deals with the experimental investigation and theoretical evaluation of fatigue crack growth under isothermal and non-isothermal conditions at the nickel alloy 617. The microstructure and mechanical properties of alloy 617 are influenced significantly by the thermal heat treatment and the following thermal exposure in service. Hence, a solution annealed and a long-time service exposed material condition is studied. The crack growth measurement is carried out by using an alternate current potential drop system, which is integrated into a thermomechanical fatigue (TMF) test facility. The measured fatigue crack growth rates results in a function of material condition, temperature and load waveform. Furthermore, the results of the non-isothermal tests depend on the phase between thermal and mechanical load (in-phase, out-of-phase). A fracture mechanic based, time dependent model is upgraded by an approach to consider environmental effects, where almost all model parameters represent directly measureable values. A consistent description of all results and a good correlation with the experimental data can be achieved.

Application of Digital Image Correlation (DIC) in resonance machines for measuring fatigue crack growth

This paper presents a simple experimental procedure that greatly facilitates the use of digital image correlation (DIC) techniques in fatigue test conducted in resonant testing machines, without the need of test interruptions. This is possible due to the implementation of USB interface optical microscopes of very small dimensions, so that they can be mounted on the specimen as a contact extensometer. Thus, the microscopesample assembly oscillates at the resonance frequency of the test. This is how, although the resonant testing machine is in motion, and although the specimen is subjected to fatigue, changing its dimensions according to the applied load, the acquired image is completely static. This allows the evaluation of the plastic deformations generated by the crack growth, avoiding the elastic ones. Preliminary results on monitoring cracks with this technique on flat specimens with cylindrical notches of 1050 aluminium alloy are also presented.

Stochastic nonlinear fatigue crack growth predictions for simple specimens subject to representative ship structural loading sequences

Recent work by the authors investigated an extension of the finite element analysis of plasticity-induced crack closure to non-stationary, ship structural loading sequences by taking advantage of their inherent time-dependent nature in which the larger loading cycles tend to be clustered together. In doing so, first-order load interactions are presumed to arise from the random occurrence and severity of physical storms encountered by ships and offshore structures throughout their service lives. This material hysteresis is captured through a time-dependent crack “opening” level (Kop) which is based on the evolution of a rate-independent, incremental plasticity model simulating combined nonlinear kinematic and isotropic hardening. The result is a mechanistic rather than phenomenological numerical model requiring only experimentally measured fatigue crack growth rates under constant amplitude, cyclic loading (e.g., ASTM E647-13) and a full material constitutive model defined through experimental push–pull tests for the same material. This approach permits a consideration of material behaviors which are physically relevant to structural steels, yet necessarily omitted in the similar application of a strip-yield model.The present paper generalizes the model originally proposed by the authors to now consider arbitrary storm model loading sequences taken from high-fidelity, time-domain seakeeping codes. To predict the fatigue fracture induced by variable amplitude stress records with upwards of 5×106 time-dependent cycles, a consistent modeling reduction is applied based on the Ordered Overall Range (OOR) or racetrack counting method. The resultant crack growth behavior is demonstrated to converge remarkably well for sufficiently small refined mesh sizes. Using this model, and by considering different arrangements of the same stress record, the importance of nonlinearities (i.e., those associated with ship response as well as material hysteresis) are emphasized.

Influence of trisamide-based additives on the morphological and mechanical properties of isotactic polypropylene

The influence of two different trisamide-based additives on the morphological and mechanical behavior of isotactic polypropylene (PP) is investigated. Morphological investigations by wide-angle X-ray scattering (WAXS) show that only 0.04 wt.-% of one of the additives is enough to induce 80% beta modification in the crystalline part of the polymer. The materials are investigated with quasistatic (tensile testing) as well as short-time dynamic (Charpy impact strength) methods and for the first time fatigue crack growth measurements are performed on alpha- and beta-nucleated PP. In all cases a much higher toughness of the material containing the beta-nucleating agent (beta NA) can be observed. The crack growth propagation rate is one order of magnitude smaller with the use of the beta-nucleating agent compared to the alpha polymer. Scanning electron microscopy (SEM) pictures are used to support the correlation of the mechanical behavior with the morphological changes.

Standardized Test Method for Corrosion Pit-to-Fatigue Crack Transition for AA7075-T651 Aluminum Alloy

Corrosion damage (pit) is a stress raiser that can have deleterious effects on the fatigue life of airframe structural components. A better understanding and method for modeling the corrosion pit to fatigue crack transition would advance the fidelity of aircraft structural integrity estimates and fleet management decision making. Here, the focus is on developing a standardized fatigue test method for investigating the transition of a corrosion pit to fatigue crack in aluminum alloy AA 7075-T651 specimens. The standardized test method requires the development and validation of two sub-protocols (1) a pitting protocol to create a corrosion pit less than 200 μm diameter at the intersection of the central hole bore and planar surface of sheet and (2) a spot welding protocol for attaching the direct current potential drop (dcPD) probes on either side of the corrosion pit for fatigue crack growth measurement. A dcPD fatigue test method coupled with a unique 10-4-6 marker load sequence is used to measure the fatigue crack growth. The crack shape evolution and crack growth life are predicted using AFGROW.

A Modified Compliance Method for Automatically Measuring Fatigue Crack Growth <i>In Situ</i> during Spectrum Fatigue Testing

The fatigue critical structures of military aircraft are generally subjected to variable amplitude flight spectrum loading. Maintaining aircraft structural integrity to ensure safe operation of the fleet is critically dependent on accurate analysis and reliable prediction of fatigue crack growth in those structures under service operating conditions. To achieve this goal, laboratory experimental methods that can accurately measure and monitor fatigue crack growth under variable amplitude loading are required. This can be challenging as no test standard exists to guide the process of fatigue crack growth measurement under variable amplitude loading conditions to ensure the accuracy of the test results. This challenge was addressed by developing a modified compliance method as described in this paper. The results presented employed a modified compliance method complemented with a travelling microscope technique and marker band loads. The modified compliance method developed is able to measure in-situ, fatigue crack growth of standard compact-tension specimens under a fighter flight spectrum loading. The marker band loads and microscope readings were used to assist the post-test validation using quantitative fractography. The results from this study have demonstrated that the modified compliance method can produce consistent and accurate fatigue crack growth data under variable amplitude loading conditions.

Effects of Different Microstructure on Resistance of EA4T Railway Axle Steel of Equal Strength to Fatigue Crack Growth

Fatigue crack growth (FCG) rates in an EA4T railway axle steel heat treated by two different methods in near threshold and stable regions of growth were evaluated. Quite significant differences were observed, when the obtained results were compared with those published in the literature. Participation of the laboratory in an Exova (GE Aviation) FCG measurement qualification round robin programme with very good results practically excluded errors in the experimental methodology used. Strength of the two different evaluated series of the experimental material was equivalent. Nevertheless, there were substantial differences in fatigue crack growth rates, about 5-times in stable FCG region and even more than 10-times in the near threshold region, when oil quenching and air hardening treatments were compared. The differences were explained by different microstructures of the two groups of materials. Some minor differences between the character of the FCG curve in the threshold region evaluated using SEN(B) and M(T) specimens, published in the literature, are discussed considering crack closure phenomenon.

An Automated Method for the Measurement of Fatigue Crack Progression

This letter describes the work conducted at our laboratory for the implementation of an automated vision system for fatigue crack growth measurement. The system relies on a dedicated illumination system with grazing incidence and optimized feature extraction by morphological image processing and continuous calculation of the crack growth, for adjustment of the optimal time interval for image registration.