Mechanism of crack branching in the fatigue crack growth path of 2324-T39 Aluminium alloy

The crack growth behavior in 2324-T39 aluminium alloy was experimentally investigated. Two types of crack branching were observed. The mechanism of an uncommon crack branching which results from the link up of secondary crack with the main crack was focused on. The crack paths were observed with optical microscope and in-situ SEM. Finite element crack tip simulation was performed to investigate the relationship between plastic zone size and the location of the secondary crack. Tests and analysis results indicate that the secondary crack initiates near the plastic zone boundary and from the subsurface. The mechanism of this kind crack branching relates to the interaction of grain size and plastic zone size

Experimental Investigations On Near-Threshold Events On Fatigue Crack Growth

In the past, the disagreement of near-threshold fatigue-crack growth (FCG) rate data generated from constant Kmax tests, high load ratio (minimum to maximum load) constant R tests, and ΔKeff based data was a mysterious issue. Because of the disagreement, a variety of test or analysis methods were created to correlate FCG rate data. It was suspected that the ASTM threshold test method using load reduction was inducing remote crack closure due to plastically deformed material, which caused elevated thresholds and slower rates than steady-state behavior. The first goal of this study was the development of a test method to eliminate remote closure during threshold testing. In order to avoid/minimize remote closure effect, compression-precracking methods were used to initiate a crack from a starter notch on compact specimens. Two materials with different fatigue crack surface profiles (flat or very rough) were tested and the results generated from the conventional ASTM precracking method and the compression-precracking test method were compared. In order to understand the disagreement of near-threshold data, crack-opening load measurements were performed from locally (near crack tip) installed strain gages instead of the remote gage (i.e., back face gage). Some careful specimen preparations were performed to avoid out-of-plane bending, to maintain straight crack fronts, and to ensure testing system linearity. It was known that remote gages, such as crack-mouth- opening-displacement-gages were insensitive to measuring load-strain records near threshold. By using local gages, the crack closure effects were clearly observed even in high load ratio (R) tests, like or higher than R = 0.7, and constant Kmax tests, which were believed to be crack closure free. By measuring load-reduced-strain records from local gages, crack-opening loads were able to correlate FCG rate data and showed that ΔKeff-rate data was unique for a wide variety of materials. By comparing (ΔKeff)th values, it may provide reasonable guidance for the material resistance against FCG. Because of “high R crack closure”, some theories considered in the past may need to be reconsidered. First, constant Kmax tests are not entirely crack-closure free. Second, there is no critical load ratio, Rc, to indicate the transition from crack-closure affected to crack-closure free data, and Kmax effects that appear in ΔKth-Kmax relations. Research has shown that the three dominate crack-closure mechanisms (plasticity-, roughness- and debris-induced crack closure) FCG rate behavior in the threshold regime from low to high load ratios

Experimental Investigations On Near-Threshold Events On Fatigue Crack Growth

In the past, the disagreement of near-threshold fatigue-crack growth (FCG) rate data generated from constant Kmax tests, high load ratio (minimum to maximum load) constant R tests, and ΔKeff based data was a mysterious issue. Because of the disagreement, a variety of test or analysis methods were created to correlate FCG rate data. It was suspected that the ASTM threshold test method using load reduction was inducing remote crack closure due to plastically deformed material, which caused elevated thresholds and slower rates than steady-state behavior. The first goal of this study was the development of a test method to eliminate remote closure during threshold testing. In order to avoid/minimize remote closure effect, compression-precracking methods were used to initiate a crack from a starter notch on compact specimens. Two materials with different fatigue crack surface profiles (flat or very rough) were tested and the results generated from the conventional ASTM precracking method and the compression-precracking test method were compared. In order to understand the disagreement of near-threshold data, crack-opening load measurements were performed from locally (near crack tip) installed strain gages instead of the remote gage (i.e., back face gage). Some careful specimen preparations were performed to avoid out-of-plane bending, to maintain straight crack fronts, and to ensure testing system linearity. It was known that remote gages, such as crack-mouth- opening-displacement-gages were insensitive to measuring load-strain records near threshold. By using local gages, the crack closure effects were clearly observed even in high load ratio (R) tests, like or higher than R = 0.7, and constant Kmax tests, which were believed to be crack closure free. By measuring load-reduced-strain records from local gages, crack-opening loads were able to correlate FCG rate data and showed that ΔKeff-rate data was unique for a wide variety of materials. By comparing (ΔKeff)th values, it may provide reasonable guidance for the material resistance against FCG. Because of “high R crack closure”, some theories considered in the past may need to be reconsidered. First, constant Kmax tests are not entirely crack-closure free. Second, there is no critical load ratio, Rc, to indicate the transition from crack-closure affected to crack-closure free data, and Kmax effects that appear in ΔKth-Kmax relations. Research has shown that the three dominate crack-closure mechanisms (plasticity-, roughness- and debris-induced crack closure) FCG rate behavior in the threshold regime from low to high load ratios

Effect of flexural crack on plain concrete beam failure mechanism A numerical simulation

The flexural failure of plain concrete beam occurs along with development of flexural crack on beam. In this paper by using ABAQUS, mechanism failure of plain concrete beam under three steps have been simulated. The cracking moment has been analytically calculated and applied on the both sides of the fixed beam, and flexural crack has been simulated on beam. Displacement, von Mises, load reaction, displacementcrack length, von Mises-crack length and von Mises-displacement of beams have been graphical depicted. Results indicated that, the flexural crack governs beam mechanism failure and its effects on beam resistance failure. It has been found that the flexural crack in initial stage it developed slowly and changes to be fast at the final stage of collapsing beam due to reduction of the flexural resistance of beam. Increasing mechanical properties of concrete, collapse displacement is reduced

Assessing Variability in Microstructural Influence on Fatigue Crack Growth Behavior

The effect of microstructural variability has long been recognized as a major contributing factor in the scatter of published fatigue data. It is also acknowledged that these effects are generally more prevalent for short cracks and in the threshold region. A number of models exist to explain individual microstructural effects such as grain boundary influence, grain cluster, average grain size, porosity etc. It is the aim of the Aeronautics and Astronautics Fatigue Lab to develop an encompassing model that accurately predicts these effects. In order to develop this model a range of material data will be required to inform and validate the model simulation. It is the aim of this thesis to develop the methods required to generate suitable fatigue crack data and also image the crack propagation and strain fields. The methodology from ASTM E647 was used for the determination of crack growth data with the notable exception of the use of compression pre-cracking and relevant crack growth models for the ESE(T) specimen. Compression pre-cracking methods have been utilised as data have shown that standard pre-cracking methods may affect crack growth rate data and the determination of threshold values. High and low load ratio tests were conducted with closure accounted for, allowing for accurate determination of the fatigue crack growth threshold. High resolution DIC imagery was captured for a range of loads over a range of crack lengths and enabled the visualization of material strain fields. The imagery also allowed correlation between fatigue crack growth variability, closure data and the tortuosity of the crack surface

Assessment of stable tearing on fatigue fracture surfaces in aluminium alloy

Fractographic analysis of fatigue fracture surfaces is used extensively in aircraft accident investigation to correlate various progression markings, associated with the crack front position, with the load cycle history which was experienced by the failed component in service. Matching the results of this analysis to the predicted fatigue crack growth, however, is often complicated by stable tearing crack growth. Bands of stable tearing are often observed on fracture surfaces in a range of structural metals but their growth is not incorporated in fatigue predictive models. Therefore, the key challenge in fracture surface analysis is to relate the multiple tears, of different lengths, to the loads present in the load history. The main objective of this research is to develop improved analytical and prognostic models for predicting the stable tearing jump length [Delta] a in aluminium alloys. This research involved a series of tests which produced stable tearing in 7075 aircraft aluminium alloy under constant amplitude (CA) and variable amplitude (VA) loading. Macroscopic and microscopic characteristics of CA and VA tearing were studied and the main conclusion relates to the notable differences between tearing under CA and VA loading. This study revealed that the stress intensity factor was one of the key controlling parameters in tearing onset and arrest. The loading conditions also have been observed to impose different effects on the size of tearing. This study suggests that for similar K , the CA tearing at initiation has smaller tearing crack jump length [Delta] a , than the VA tearing, but as the crack progresses, the size of [Delta] a under VA conditions is markedly larger than that sustainable under CA conditions. The CA condition seems to confer apparent resistance to tearing, which results in smaller tearing crack jumps, than in VA loading conditions. The static tearing curve is developed based on the standard K R curve test method. This study shows that the K -value at which the static tearing commences is approximately equivalent to the first onset stress intensity factor for VA tearing and the R -curve method can be used to estimate the [Delta] a of both CA and VA tearing during fractographic analysis, but this technique requires the R -curve to be developed for particular configurations. The complex crack front curvatures observed at tearing arrest distort simple estimates of stress intensity factor and hence a three-dimensional (3D) finite element (FE) analysis has been undertaken to estimate the through thickness stress intensity factor K 3D variation. Based on the parametric finite element analysis of the stress-intensity factor K 3D at the mid-thickness of three VA tearing, this study presents a new validated stable tearing model for predicting the crack jump length [Delta] a during stable tearing. The main features of this new model are that the tongue­ shaped region of stable tearing is idealised as a trapezoidal shape and the average of areal ratio of tearing is approximately constant. Comparisons between the model predictions and experimental results indicate that this new model produces satisfactory prediction of stable tearing crack jump length [Delta] a in aluminium alloys of different cross-sectional thickness

Fatigue Crack Propagation Under Variable Amplitude Loading in Steels Used in Francis Turbine Runners

RÉSUMÉ Les turbines hydrauliques sont soumises à de très grands nombres de cycles à faible amplitude de contrainte et à haute fréquence. Ces petits cycles sont générés par des phénomènes hydrauliques et sont superposés à une contrainte statique de tension. Aussi, dépendant des conditions de fonctionnement, il est possible d’avoir superposé aux petits cycles un plus faible nombre de grands cycles à forte amplitude de contrainte et basse fréquence. On a ainsi en pratique une superposition de petits cycles, de grands cycles et d’une contrainte statique de tension durant les 70 ans de durée de vie de la turbine. Les turbines hydrauliques qui sont fabriquées à partir des aciers AISI 415, ASTM A516, et AISI 304L (notés 415, A516, et 304L pour simplification) sont soumises à de telles contraintes cycliques et statique. Ces contraintes ont pour effet de favoriser la propagation des défauts existants dans les roues des turbines et peuvent mener à leur rupture. Pour éviter la propagation des fissures, les petits cycles doivent induire un ΔK qui est en dessous du seuil de fatigue. Néanmoins, les grands cycles peuvent contribuer à propager ces fissures. Ainsi, pour prédire la vitesse de propagation des fissures dans de telles conditions de cycles superposés, on a recours à la sommation linéaire de dommage (SLD). Il a été observé que les grands cycles superposés aux petits cycles peuvent induire une diminution du seuil de fatigue des petits cycles. Différentes procédures ont été proposées dans la littérature pour mesurer les seuils associés au petits cycles seuls et avec superposition des grands cycles. Cependant, la plupart des procédures ne minimise pas la fermeture induite lors de la mesure du seuil conduisant ainsi à une surestimation de leur valeur. La présente étude propose de nouvelles procédures d’essais pour réduire la fermeture lors de la mesure du seuil de fatigue pour les aciers mentionnés précédemment. De plus, différentes études ont démontré que les fissures peuvent se propager plus rapidement sous l’effet des grands cycles que ce que prédit la SLD. Nous vérifierons ainsi la précision de la prédiction LDS par rapport aux mesures de propagation. Dans une première étude, la propagation des fissures par l’interaction de petits et de grands cycles est caractérisée dans les trois aciers. Les cycles de base sont entrecoupés par les grands cycles. Les vitesses de propagation des fissures par les cycles de base et les grands cycles de sous-charges sont additionnées dans la SLD pour évaluer la vitesse de propagation de fissure.----------ABSTRACT Hydraulic turbine runners are subjected to a very large number of cycles with small stress amplitudes at high frequencies. These cycles are generated by hydraulic phenomena and are superimposed to a tensile static stress. Depending on the operating conditions, much lower number of large cycles are generated with large stress amplitudes at low frequencies. As a summary, the whole stress spectrum consists of small cycles superimposed to a tensile static stress that is intercut with large cycles during the 70 years design life of turbine runners. Turbine runners, which are fabricated from AISI 415, ASTM A516, and AISI 304L steels (i.e. called 415, A516 and 304L for simplicity), are subjected to the aforementioned stress cycles. The imposed stress spectrum propagates the existing defects or cracks in turbine runners and may lead to their failure. In order to avoid crack propagation, the small cycles should induce a ΔK that is lower than the fatigue threshold. Nonetheless, the crack can grow due to large cycles. As a result, linear damage summation (LDS) is employed to predict the crack growth. The large cycles superimposed to small cycles can also induce a decrease in fatigue threshold of the small cycles. Different test procedures have been proposed to measure the fatigue threshold of small cycles and the ones superimposed to large cycles; however, most of them do not minimize the crack closure while reaching the fatigue threshold leading to an overestimation of fatigue thresholds. In this study new test procedures are proposed in order to minimize crack closure while reaching the fatigue thresholds in turbine runner steels. Different studies have shown that crack can grow faster than the LDS prediction due to the interaction between large cycles. Therefore, we verify the precision of LDS prediction compared to the measured crack growth rates. In this first study, crack growth due to the interaction between two large cycles is investigated in the three aforementioned turbine runner steels. Baseline cycles are periodically intercut by an underload cycle. This variable amplitude loading is hereafter called periodic underloads. Crack growth rates of baseline cycles and underload cycles are summated in LDS to predict crack growth under periodic underloads. Crack growth measured under periodic underloads is then compared to LDS prediction. A ratio between the measured and predicted crack growth, that is greater than unity, is defined as an acceleration factor

Fracture, Fatigue, and Structural Integrity of Metallic Materials and Components Undergoing Random or Variable Amplitude Loadings

Most metallic components and structures are subjected, in service, to random or variable amplitude loadings. There are many examples: vehicles subjected to loadings and vibrations caused by road irregularity and engine, structures exposed to wind, off-shore platforms undergoing wave-loadings, and so on. Just like constant amplitude loadings, random and variable amplitude loadings can make fatigue cracks initiate and propagate, even up to catastrophic failures. Engineers faced with the problem of estimating the structural integrity and the fatigue strength of metallic structures, or their propensity to fracture, usually make use of theoretical, numerical, or experimental approaches. This reprint collects a series of recent scientific contributions aimed at providing an up-to-date overview of approaches and case studies—theoretical, numerical or experimental—on several topics in the field of fracture, fatigue strength, and the structural integrity of metallic components subjected to random or variable amplitude loadings

Aeronautical Engineering: A continuing bibliography with indexes (supplement 205)

This bibliography lists 517 reports, articles and other documents introduced into the NASA scientific and technical information system in September 1986

Aeronautical engineering: A cumulative index to a continuing bibliography

This bibliography is a cumulative index to the abstracts contained in NASA SP-7037 (197) through NASA SP-7037 (208) of Aeronautical Engineering: A Continuing Bibliography. NASA SP-7037 and its supplements have been compiled through the cooperative efforts of the American Institute of Aeronautics and Astronautics (AIAA) and the National Aeronautics and Space Administration (NASA). This cumulative index includes subject, personal author, corporate source, foreign technology, contract, report number, and accession number indexes