Showing papers in "Fatigue & Fracture of Engineering Materials & Structures in 1999"

There is no infinite fatigue life in metallic materials

Abstract: Generally, the shape of the S-N curve beyond 10 7 cycles is unknown except in some statistical approaches, and this is also true for the fatigue limit. In the case of a statistical approach, the standard deviation applied to the average fatigue limit is certainly not the best way to reduce the risk of rupture in fatigue. Only the exploration of the life range between 10 6 and 10 10 cycles will create a safer basis for modelling. Today, some piezoelectric fatigue machines are very reliable, capable of producing 10 10 cycles in less than 1 week. We based our research on accelerated fatigue tests which were performed at 20 kHz in the gigacyclic fatigue regime in order to study several typical alloys from the aeronautical and space industries.

Factors influencing the mechanism of superlong fatigue failure in steels

Abstract: When the fatigue life N f of a specimen of 10 mm in thickness is longer than 10 8 cycles, the average fatigue crack growth rate is much less than the lattice spacing (∼0.1 A or 0.01 nm) that is 10 -11 to 10 -12 m/cycle. In the early stage of the fatigue process, the crack growth rate should be much less than the average growth rate, and accordingly we cannot assume that crack growth occurs cycle by cycle. In this paper, possible mechanisms for extremely high cycle fatigue are discussed. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of the fracture origin (non-metallic inclusions) of a heat-treated alloy steel, SCM435, which was tested to N ≥ 10 8 . The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of the fatigue limit by the √area parameter model are ∼ 10% unconservative for a fatigue life of N f = ∼10 8 , though it successfully predicts the conventional fatigue limit defined for N = 10 7 . Thus, the fatigue failure for N ≥ 10 8 is presumed to be caused by a mechanism which induces breaking or releasing of the fatigue crack closure phenomenon in small cracks. In the vicinity of a non-metallic inclusion at the fracture origin, a dark area was always observed inside the fish-eye mark for those specimens with a long fatigue life. Specimens with a short fatigue life of N f = ∼10 5 do not have such a dark area in the fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from the usual fatigue fracture surface in a martensite lath structure. Considering the high sensitivity of high-strength steels to a hydrogen environment and the high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high-strength steels from non-metallic inclusions is caused by environmental effects, e.g. hydrogen embrittlement coupled with fatigue.

Stepwise S-N curve and fish-eye failure in gigacycle fatigue

Abstract: Fatigue failure is normally initiated at the surface of a material. For some materials, failure can be initiated both at the surface and the interior. This twofold materials behaviour in fatigue is represented by a stepwise shape in the S-N curve. An internal failure mode is especially important for fatigue life in the gigacycle range, as this mode is predominant at low stress ranges. Materials with a hardened surface fail from the surface only at high stresses, and at low stresses from the inside, forming a fish-eye facet on the fracture surface. Exactly the same behaviour can be observed for materials without a hard surface, even at elevated temperatures. This paper displays some of the results obtained at NRIM and discusses possible interpretations.

Load interaction effects during fatigue crack growth under variable amplitude loading—a literature review. Part II: qualitative interpretation

Abstract: The current understanding of the underlying reasons behind the load interaction effects in fatigue crack growth under variable amplitude loading is presented. Mechanistic arguments proposed to control the load interaction phenomena are reviewed and evaluated based on their capability to qualitatively explain empirical trends in variable amplitude fatigue crack growth summarized in Part I [Fatigue Fract, Engng Mater. Struct. 1998. 21(8), 987-1006] of the present paper. Mechanisms linked to plastic straining at the crack tip enable an interpretation of the majority of the experimental results. Some observations, however, which cannot be understood in terms of plasticity-induced crack closure, or which are even in contradiction with the crack closure approach, indicate a possible role of other factors. A general conclusion is that conditions under which various phenomena can affect variable amplitude fatigue crack growth and interactions between them are insufficiently recognized.

Gigacycle fatigue of ferrous alloys

Abstract: The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 x 10 10 cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 10 5 and 10 7 cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 10 7 cycles, and the fatigue endurance stress S max continues to decrease with increasing number of cycles to failure between 10 6 and 10 9 cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 10 6 and 10 7 cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra-high-cycle fatigue (> 10 7 cycles). A surface-subsurface transition in crack initiation location is described for the four low-alloy high-strength steels and a SG cast iron.

Energy criteria of multiaxial fatigue failure

Abstract: This paper contains a review of energy-based criteria of multiaxial fatigue. The criteria have been divided into three groups, depending on the kind of strain energy density per cycle which is assumed as a damage parameter. They are: (i) criteria based on elastic strain energy for high-cycle fatigue; (ii) criteria based on plastic strain energy for low-cycle fatigue; and (iii) criteria based on the sum of plastic and elastic strain energies for both low- and high-cycle fatigue. The criteria which take into account strain energy density in the critical plane seem to be the most promising. In the energy approach to multiaxial fatigue there is an important unsolved problem, i.e. the evaluation of energy, especially plastic strain energy density, from closed stress-strain hysteresis loops under random loadings.

A critical plane-strain energy density criterion for multiaxial low-cycle fatigue life under non-proportional loading

Abstract: A series of multiaxial low-cycle fatigue experiments was performed on 45 steel under non-proportional loading. The present evaluations of multiaxial low-cycle fatigue life were systematically analysed. A combined energy density and critical plane concept is proposed that considers different failure mechanisms for a shear-type failure and a tensile-type failure, and from which different damage parameters for the critical plane-strain energy density are proposed. For tensile-type failures in material 45 steel and shear-type failures in material 42CrMo steel, the new damage parameters permit a good prediction for multiaxial low-cycle fatigue failure under non-proportional loading. The currently used critical plane models are a special and simple form of the new model.

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

Abstract: An attempt has been made to characterize high-cycle fatigue behaviour of high-strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction-tempered ultra-high-strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation. The fatigue strength of the steel wires between 106 and 109 cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 107 cycles. For Cr–V spring wire, the stress–life (S–N ) curve becomes horizontal at a maximum stress of 800 MPa after 106 cycles, but the S–N curve of the Cr–Si steel continues to drop at a high number of cycles (>106 cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high-cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high-cycle regime for the steels studied in this investigation.

The fatigue limit and its elimination

Abstract: The classical fatigue limit of ferrous metals is a consequence of testing materials at a constant range of cyclic stress and determining the cyclic stress range below which fatigue failures do not occur. This classical fatigue limit of a material is equated to the condition for which fatigue cracks can not propagate beyond microstructural barriers. This paper discusses the causes, leading to the elimination of this fatigue limit, including the introduction of transitory cyclic-dependent mechanisms and time-dependent processes that will permit a previously non-propagating crack to grow across the different threshold states expressed in terms of linear-elastic fracture mechanics (LEFM), elastic-plastic fracture mechanics (EPFM) and microstructural fracture mechanics (MFM). These transitory mechanisms and processes include different loading and environmental conditions, which in a long-life engineering plant (e.g. 30 years lifetime) can lead to apparently premature failures. Of greater concern is the creation of a new crack-initiation zone, i.e. a transfer from a surface-generated crack to an internal-generated crack that eventually dominates the fatigue failure event. The impact of these conditions on the elimination of the classical fatigue limit necessitates changes in Design Codes of Practice, and such changes are discussed in relation to the extremely long-lifetime regime (10 7

On the life‐controlling microstructural fatigue mechanisms in ductile metals and alloys in the gigacycle regime

Abstract: The high-cycle fatigue (HCF) behaviour of ductile metals and alloys, and the life-controlling microstructural fatigue mechanisms known from HCF are reviewed critically with respect to their possible role in the gigacycle or ultra-high-cycle fatigue (UHCF) regime. Arguments are presented to support the hypothesis that, at the very low amplitudes of the UHCF regime, fatigue crack initiation, resulting from cyclic strain localization, and slow early Stage I fatigue crack propagation are the life-controlling mechanisms and that these processes can essentially be described in terms of the microstructurally irreversible portion of the cumulative cyclic plastic strain. Emphasis is placed on the important role of the so-called slip irreversibility which decreases as the amplitude becomes lower and lower. Finally, the Manson-Coffin law is reformulated for very low amplitudes in terms of microstructurally relevant parameters, and a fatigue life diagram is developed, based on these preceding microstructural considerations. Important features of this diagram are: (i) the plastic strain fatigue limit in the HCF regime which is related to the threshold for cyclic strain localization in persistent slip bands; and (ii) the transition from this plastic strain fatigue limit to a threshold of negligible slip irreversibility at still lower amplitudes in the UHCF regime.

High-cycle fatigue of Ti-6Al-4V

Abstract: The definition of the critical levels of microstructural damage that can lead to the propagation of fatigue cracks under high-cycle fatigue loading conditions is a major concern with respect to the structural integrity of turbine-engine components in military aircraft. The extremely high cyclic frequencies characteristic of in-flight loading spectra, coupled with the presence of small cracks resulting from fretting or foreign object damage (FOD), necessitate that a defect-tolerant design approach be based on a crack-propagation threshold. The present study is focused on characterizing such near-threshold fatigue-crack propagation behaviour in a Ti-6Al-4V blade alloy (with ∼60% primary α in a matrix of lamellar α + β), at high frequencies (20-1500 Hz) and load ratios (0.1-0.95) in both ambient temperature air and vacuum environments. Results indicate that 'worst-case' thresholds, measured on large cracks, may be used as a practical lower bound to describe the onset of naturally initiated small-crack growth and the initiation and early growth of small cracks emanating from sites of simulated FOD.

From a local stress approach to fracture mechanics: a comprehensive evaluation of the fatigue strength of welded joints

Abstract: This paper investigates the possibility of unifying different criteria concerned with the fatigue strength of welded joints In particular, it compares estimates based on local stress fields due to geometry (evaluated without any crack-like defect) and residual life predictions in the presence of a crack, according to LEFM Fatigue strength results already reported in the literature for transverse non-load-carrying fillet welds are used as an experimental database Nominal stress ranges were largely scattered, due to large variations of joint geometrical parameters The scatter band greatly reduces as soon as a 03-mm virtual crack is introduced at the weld toe, and the behaviour of the joints is given in terms of ΔK I versus total life fatigue Such calculations, not different from residual life predictions, are easily performed by using the local stress distributions determined near the weld toes in the absence of crack-like defects More precisely, the analytical expressions for K I are based on a simple combination of the notch stress intensity factors K N 1 and K N 2 for opening and sliding modes Then, fatigue strength predictions, as accurate as those based on fracture mechanics, are performed by the local stress analysis in a simpler way

Fatigue life assessment of nodular cast iron containing casting defects

Abstract: The fatigue behaviour of a a nodular cast iron containing casting defects has been investigated in the high-cycle fatigue regime. In this paper, we propose a fatigue life assessment model for flawed materials based on a fracture mechanics approach which takes into account the position and size of the defect, short crack behaviour and the notch effect introduced by the defect. The fatigue behaviour of smooth samples, and long and short crack behaviour have been experimentally determined in order to identify the relevant mechanical parameters; these being introduced into the model. An experimental study has been made both in air and in vacuum in order to account for the position of the defect, noting that internal defects are supposed to be under vacuum conditions. Experimental results, which are based on a two-crack front-marking technique specially developed for this study, show that the propagation of natural cracks is controlled by the effective stress intensity factor in air as well as in vacuum. The K calculation for a short crack in the stress field of a notch is analysed using numerical clastic-plastic results. Comparison between experimental results and the computation of fatigue life for fatigue lives less than 10 6 cycles shows that the fatigue behaviour of nodular cast iron is controlled by a propagation process. The model proposed is thus relevant for fatigue lives less than 10 6 cycles so that the defect can be considered as a crack and the initiation stage neglected. Closer to the fatigue limit, this study shows that the initiation stage should be considered in the assessment of fatigue life of nodular cast iron, because a single macroscopic propagation assessment is not enough to describe the whole fatigue life. The defect cannot be considered as a pre-existent crack in the high-cycle fatigue range (> 10 6 cycles), and the initiation stage that contains microcrack propagation around the defect should be evaluated when assessing the high-cycle fatigue life of nodular cast iron.

A Weibull stress model to predict cleavage fracture in plates containing surface cracks

Abstract: This study applies recent advances in probabilistic modelling of cleavage fracture to predict the measured fracture behaviour of surface crack plates fabricated from an A515-70 pressure vessel steel. Modifications of the conventional, two-parameter Weibull stress model introduce a non-zero, threshold parameter (σ w-min ). The introduction of σ w-min brings numerical predictions of scatter in toughness data into better agreement with experimental measurements, and calibration of this new parameter requires no additional experimental data. The Weibull modulus (m) and scaling parameter (σ u ) are calibrated using a new strategy based on the toughness transferability model, which eliminates the non-uniqueness that arises in calibrations using only small-scale yielding toughness data. Here, the Weibull stress model is calibrated using toughness data from deep-notch C(T) and shallow-notch SE(B) specimens, and is then applied to predict the measured response of surface crack plates loaded in different combinations of tension and bending. The model predictions accurately capture the measured distributions of fracture toughness values.

An apparatus for quantitative fretting testing

Abstract: The design and construction of an apparatus for performing quantitative fretting fatigue experiments is described. The device allows accurate ineasurement and control of normal contact force, tangential contact force, relative displacement between contacting surfaces and bulk fretting loads, as well as measurement of average friction coefficients. Its design is simple, and includes interchangeable fretting contact pads, allowing the use of various pad geometries without major adjustment. The device incorporates many points of adjustment for alignment and compliance, making it a robust frame for a wide variety of fretting fatigue conditions involving different materials. The capabilities of this device are also verified by results of fretting fatigue experiments conducted on a 7075-T6 aluminium alloy.

Fracture mechanisms and fracture mechanics at ultrasonic frequencies

Abstract: Performing fatigue tests at ultrasonic frequencies, e.g. 20 000 Hz, allows one to perform experiments beyond 10 9 and 10 10 cycles within half a day or a week, respectively. The testing technique has led to the construction of fatigue machines of high technical standard. Use of the ultrasound technique to study the mechanisms of crack initiation in pure metal single crystals, in cast alloys with voids being crack initiation sites, and in complicated fibre-reinforced laminates is reported. Likewise, use of ultrasonic loading to study the mechanisms of crack propagation is discussed, as well as LEFM principles; especially when these principles cannot be applied. It is shown how crack growth retardation with increasing crack length is attained in fibre-reinforced laminates by the effect of fibre bridging. Additional experimental possibilities, e.g. random loading, variation of mean load, superposition of shear loads, variation of temperature and environment, and not only axial but also torsional loading at ultrasonic frequency, and recent research results are discussed.

The influence of cross‐sectional thickness on fatigue crack growth

Abstract: For thin structures, fatigue crack growth rates may vary with the structure's thickness for a given stress intensity factor range. This effect is mainly due to the change in the nature of the plastic deformation when the plastic zone size becomes comparable with, or greater than, the cross-sectional thickness. Variations in the constraint affect both the crack tip plastic blunting behaviour as well as the fatigue crack closure level. Approximate expressions are constructed for the constraint factor based on asymptotic values and numerical results, which are shown to correlate well with finite element results. It is demonstrated that the present results not only permit predictions of the specimen thickness effects on fatigue crack propagation under spectrum loading, but also eliminate the need to determine the constraint factor by curve-fitting crack growth data.

Interfacial fatigue crack growth in foam core sandwich structures

Abstract: This paper deals with the experimental measurement of face/core interfacial fatigue crack growth rates in foam core sandwich beams. The so-called ‘cracked sandwich beam’ specimen is used, slightly modified, which is a sandwich beam that has a simulated face/core interface crack. The specimen is precracked so that a more realistic crack front is created prior to fatigue growth measurements. The crack is then propagated along the interface, in the core material, during fatigue loading, as is assumed to occur in a real sandwich structure. The crack growth is stable even under constant amplitude testing. Stress intensity factors are obtained from the FEM which, combined with the experimental data, result in standard da/dN versus ΔK curves for which classical Paris’ law constants can be extracted. The experiments to determine stress intensity factor threshold values are performed using a manual load-shedding technique.

Application of a weakest‐link concept to the fatigue limit of the bearing steel SAE 52100 in a bainitic condition

Abstract: The influence of notches and load conditions on the fatigue limit of the high-strength steel SAE 52100 is investigated. The behaviour can be described quantitatively by a weakest-link concept which describes competitive crack initiation at the surface and at inclusions. This includes the possibility of calculating local and total endurance probabilities depending on the nominal stress amplitude. Crack initiation sites and the fatigue limit can also be predicted. The basic relation between the size distribution of crack-initiating inclusions and the fracture probability was proved for smooth specimens under tension-compression.

Application of ultrasound for fatigue testing of lightweight alloys

Abstract: The use of aluminium and magnesium alloys offers a great potential for weight reduction in automotive applications. Load-bearing car components are subjected to 10 8 cycles and more during service, and the high-cycle fatigue properties of construction materials are therefore of great interest. The time-saving ultrasound fatigue testing method has been used to study the fatigue properties of a high-pressure, die-cast magnesium alloy AZ91 hp and a post-forged, cast-aluminium alloy AlSi7Mg0.3 in ambient air and saltwater (5wt% sodium chloride) spray. In ambient air, fatigue cracks in AZ91 hp emanate from voids, and it is possible to correlate void areas with the numbers of cycles-to-failure. Post-forging of AlSi7Mg0.3 reduces the numbers and size of voids. The remaining small voids (void areas smaller than 9000 μm 2 ) do not significantly reduce lifetimes. Saltwater deteriorates the fatigue properties of both the lightweight alloys. With increasing numbers of cycles, the influence of the corrosive liquid on fatigue strength becomes more pronounced.

Measuring residual stress and the resulting stress intensity factor in compact tension specimens

Abstract: The measurement of residual stress through the remaining ligament of a compact tension specimen was studied. In the crack compliance method, a slot or notch is successively extended through the part, and the resulting strain is measured at an appropriate location. By using a finite element simulation of a specimen preloaded beyond yield, three techniques for determining the original residual stress from the measured strains were compared for accuracy and sensitivity to measurement errors. A common beam-bending approximation was substantially inaccurate. The series expansion method proved to be very versatile and accurate. The fracture mechanics approach could determine the stress intensity factor caused by the residual stresses with a very simple calculation. This approach offers the exciting possibility of determining the stress intensity factor prior to a fatigue or fracture test by measuring strains during the specimen preparation.

Influence of pitting on the fatigue life of a turbine blade steel

Abstract: The role of pits as stress raisers and their influence on fatigue life has been investigated for a 12Cr turbine blade material. A particular feature of this work was the establishment of an electrochemical procedure for generating pits with ‘controlled’ pit depth and low density. Pits grown under laboratory conditions were partially spherical in shape and simulated, in general appearance, those observed in service. In terms of the threshold stress intensity factor, the results supported the concept of pits acting as effective cracks of the same depth, provided that a short crack model based on an effective crack length is used.

High-cycle fatigue crack initiation and propagation behaviour of high-strength spring steel wires

Abstract: An attempt has been made to characterize high-cycle fatigue behaviour of high-strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction-tempered ultra-high-strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation. The fatigue strength of the steel wires between 10 6 and 10 9 cycles was determined at a load ratio R = -1. The experimental results show that fatigue rupture can occur beyond 10 7 cycles. For Cr-V spring wire, the stress-life (S-N) curve becomes horizontal at a maximum stress of 800 MPa after 10 6 cycles, but the S-N curve of the Cr-Si steel continues to drop at a high number of cycles (> 10 6 cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high-cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high-cycle regime for the steels studied in this investigation.

The influence of single and multiple overloads on fatigue crack propagation

Abstract: The effects of single and multiple overloads on fatigue crack propagation were studied experimentally using the aluminium alloy Al 7475-T7351. The crack propagation stress intensity factor, K PR , was determined after each single and multiple overload sequence. The variation of K PR with the overload parameters can be described by an empirical relationship that unifies the experimental data obtained under a variety of loading conditions. Observations made in the study suggest that load interaction effects are controlled by residual compressive stresses ahead of the crack tip, with fatigue crack closure playing only a secondary role.

Fatigue strength of crack‐healed Si3N4/SiC composite ceramics

Abstract: Si 3 N 4 /SiC composite ceramics were sintered and subjected to three-point bending on specimens made according to the appropriate JIS standard. A semi-circular surface crack of 110 μm in diameter was made on each specimen. By using three kinds of specimen (smooth, cracked and crack healed), crack-healing behaviour, cyclic and static fatigue strengths were determined systematically at room temperature and 1000°C. The main conclusions are as follows: (i) Si 3 N 4 /SiC composite ceramics have the ability to heal after cracking; (ii) crack-healed specimens showed similar cyclic and static fatigue strengths as smooth specimens, this being caused by crack healing; (iii) crack-healed zones had a sufficient fatigue strength and most fractures occurred outside the pre-cracked zone in those crack-healed specimens.

The significance of fractography for investigations of fatigue crack growth under variable-amplitude loading

Abstract: Fatigue crack growth tests were carried out on 2024-T3 and 7075-T6 centrally cracked specimens. Variable-amplitude (VA) load spectra were used with periodic overload (OL) cycles added to constant-amplitude (CA) cycles. The fatigue fracture surfaces were examined in the SEM to obtain more detailed information on crack growth contributions of different load cycles. The striation patterns could be related to the load histories. SEM observations were related with (i) delayed retardation, (ii) the effect of 10 or a single OL on retardation, (iii) crack growth during the ()L cycles, and (iv) crack growth arrest after a high peak load. Fractographs exhibited local scatter of crack growth rates and sometimes a rather tortuous 3D geometry of the crack front. Indications of structurally sensitive crack growth under VA loading were obtained. Fractography appears to be indispensable for the evaluation of fatigue crack growth prediction models in view of similarities and dissimilarities between crack growth under VA and CA loading.

Probabilities of occurrence and detection of damage in airframe materials

Abstract: The reliability of airworthiness assessment and the effective management of ageing fleets of aircraft depend critically on the quality of tools for predicting damage nucleation and accumulation and its detection, i.e. on the interrelationship between the probabilities of occurrence and detection. To illustrate these interrelationships, a mechanistically based probability approach involving localized pitting corrosion and subsequent fatigue cracking is presented. A probability of detection based on a typical state-of-the-art technique for non-destructive evaluation is used for comparison and probabilistic assessment. The results suggest that the probability of detection is inadequate, and information on damage size should be included as part of an effective airworthiness assurance methodology. An appropriate target for detecting and sizing damage of ≈ 0.10 mm with a probability of detection and a confidence level of at least 90% is suggested, versus the current capability of 1.27 mm at only 50%

Determination of S–N curves with the application of artificial neural networks

Abstract: The present paper describes the application of artificial neural networks for estimating the finite-life fatigue strength and fatigue limit. A comprehensive database with results of single-stage tests on specimens which simulate structural components is evaluated and prepared for processing with the use of neural networks. The available data are subdivided into different classes. A total of six different data classes are specified. The results of the prediction by means of neural networks are superior to those obtained with conventional methods for calculating the fatigue strength. The experimental results are estimated with high accuracy.

An investigation into the location of crack initiation sites in alumina, polycarbonate and mild steel

Abstract: The effects of notch root radius on fracture toughness and crack initiation sites have been investigated in this paper using three different classes of materials. Data on alumina which represent ceramics, mild steel from the metals family and polycarbonate representing plastics were obtained and analysed. The locations of crack initiation sites have been pinpointed by scanning electron microscopy. These identified sites more or less are located within the critical process zone or the theoretical plastic zone. The critical process zone size (D c ) or the theoretical plastic zone size (R YF ) are independent of the notch root radius unlike the plain-strain fracture toughness of notched specimens [K 1C (ρ)]. The authors emphasize why the parameters D c and R YF are useful for a quantitative evaluation of the reliability of structural materials.

Effects of frequency on fatigue crack growth at elevated temperature

Abstract: The effects of frequency on fatigue crack growth behaviour have been studied in a prealloyed powder material, Udimet 720Li, at 650 °C. Fracture mode and fatigue crack growth behaviour were studied at frequencies ranging from 0.001 to 5 Hz using a balanced triangular waveform. Tests were carried out under constant ΔK control, with load ratio and temperature being held constant. A mechanism map was constructed where predominantly time, mixed and cycle-dependent crack growth behaviour were identified. The results were verified by SEM analyses. Cycle-dependent crack growth data were obtained at room temperature, while fully time-dependent crack growth data were generated under sustained loads at 650 °C. It was found that mixed time/cycle-dependent behaviour is of most significance for this material at the temperature and frequencies studied. Data for other nickel-based superalloys from various sources in the literature were compiled and compared with those of U720Li alloy at a given stress intensity and temperature in the mixed regime. An analysis was developed to rationalize the observed effect of frequency on fatigue crack growth rate.