Showing papers on "Paris' law published in 2000"

Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-T7451

Abstract: Aluminum alloy 7050 was friction-stir welded (FSW) in a T7451 temper to investigate the effects on the microstructure and mechanical properties. Results are discussed for the as-welded condition (as-FSW) and for a postweld heat-treated condition consisting of 121 °C for 24 hours (as-FSW + T6). Optical microscopy and transmission electron microscopy (TEM) examination of the weld-nugget region show that the FS welding process transforms the initial millimeter-sized pancake-shaped grains in the parent material to fine 1 to 5 µm dynamically recrystallized grains; also, the FS welding process redissolves the strengthening precipitates in the weld-nugget region. In the heat-affected zone (HAZ), the initial grain size is retained, while the size of the strengthening precipitates and of the precipitatefree zone (PFZ) is coarsened by a factor of 5. Tensile specimens tested transverse to the weld show that there is a 25 to 30 pct reduction in the strength level, a 60 pct reduction in the elongation in the as-FSW condition, and that the fracture path is in the HAZ. The postweld heat treatment of 121 °C for 24 hours did not result in an improvement either in the strength or the ductility of the welded material. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW + T6 condition, at a stress ratio of R = 0.33, shows that the FCG resistance of the weldnugget region is decreased, while the FCG resistance of the HAZ is increased. Differences in FCGRs, however, are substantially reduced at a stress ratio of R = 0.70. Analysis of residual stresses, fatigue-crack closure, and fatigue fracture surfaces suggests that decrease in fatigue crack growth resistance in the weld-nugget region is due to an intergranular failure mechanism; in the HAZ region, residual stresses are more dominant than the microstructure improving the fatigue crack growth resistance.

Effect of Crystal Orientation on Fatigue Failure of Single Crystal Nickel Base Turbine Blade Superalloys

Abstract: High cycle fatigue (HCF) induced failures in aircraft gas turbine and rocket engine turbopump blades is a pervasive problem. Single crystal nickel turbine blades are being utilized in rocket engine turbopumps and jet engines throughout industry because of their superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities over polycrystalline alloys. Currently the most widely used single crystal turbine blade superalloys are PWA 1480/1493, PWA 1484, RENE' N-5 and CMSX-4. These alloys play an important role in commercial, military and space propulsion systems. Single crystal materials have highly orthotropic properties making the position of the crystal lattice relative to the part geometry a significant factor in the overall analysis. The failure modes of single crystal turbine blades are complicated to predict due to the material orthotropy and variations in crystal orientations. Fatigue life estimation of single crystal turbine blades represents an important aspect of durability assessment. It is therefore of practical interest to develop effective fatigue failure criteria for single crystal nickel alloys and to investigate the effects of variation of primary and secondary crystal orientation on fatigue life. A fatigue failure criterion based on the maximum shear stress amplitude /Delta(sub tau)(sub max))] on the 24 octahedral and 6 cube slip systems, is presented for single crystal nickel superalloys (FCC crystal). This criterion reduces the scatter in uniaxial LCF test data considerably for PWA 1493 at 1200 F in air. Additionally, single crystal turbine blades used in the alternate advanced high-pressure fuel turbopump (AHPFTP/AT) are modeled using a large-scale three-dimensional finite element model. This finite element model is capable of accounting for material orthotrophy and variation in primary and secondary crystal orientation. Effects of variation in crystal orientation on blade stress response are studied based on 297 finite element model runs. Fatigue lives at critical points in the blade are computed using finite element stress results and the failure criterion developed. Stress analysis results in the blade attachment region are also presented. Results presented demonstrates that control of secondary and primary crystallographic orientation has the potential to significantly increase a component S resistance to fatigue crack growth with- out adding additional weight or cost. [DOI: 10.1115/1.1413767]

Predicting the service-life of adhesively-bonded joints

Abstract: A fracture-mechanics approach has been used to predict the cyclic-fatigue performance of the adhesively-bonded single-lap joint and a typical bonded component, represented by an adhesively-bonded `top-hat' box-beam joint The joints were tested under cyclic-fatigue loading in either a `wet' or `dry' environment, respectively Several steps were needed to predict the cyclic-fatigue lifetime of these joints Firstly, fracture-mechanics tests were used to obtain the relationship between the rate of fatigue crack growth per cycle, da/dN, and the maximum strain-energy release-rate, Gmax, applied during the fatigue cycle for the adhesive/substrate system under investigation, in both a `dry' and a `wet' test environment Secondly, analytical and finite-element theoretical models were developed to describe the variation of the strain-energy release-rate with crack length, as a function of the applied fatigue loads, for the single-lap joint and the `top-hat' box-beam joint Thirdly, the experimental results from the short-term fracture-mechanics tests, obtained under similar test conditions and in the same environment as were used for the single-lap or bonded box-beam joints, were combined with the modelling results from the theoretical studies This enabled the cyclic-fatigue performance of the single-lap or bonded box-beam joints to be predicted over relatively long time-periods Finally, the agreement between the theoretical predictions and the experimentally-measured cyclic-fatigue behaviour for the joints was found to be very good

Linear Elastic Fracture Mechanics for Engineers: Theory and Applications

Abstract: Introduction: Summary References Fracture toughness: Introduction Energy approach to brittle fracture Stress intensity factors Some Mode I stress intensity factor solutions Limitations of the stress intensity factor approach Effects of small scale yielding Slant crack growth in thin sheets Effect of thickness on fracture toughness Effect of notch acuity on fracture toughness R-curves General yielding fracture mechanics Summary References. Plane strain fracture toughness testing: Introduction Specimen types Offset procedure Fatigue precracking Specimen size requirements Klc- Charpy correlations Chevron notch tests Summary References. Fatigue crack growth: Introduction Use of stress intensity factors Fatigue crack growth rate data Mechanisms of fatigue crack growth Threshold for fatigue crack growth Overall fatigue crack growth behaviour Crack closure Short cracks Fatigue crack growth under variable amplitude loading Weighted average stress range Cycle counting Summary References. Fatigue crack growth testing: Introduction Specimen types Specimen size requirement Fatigue precracking Crack path requirement Data reduction The threshold and near threshold crack growth rates Summary References. Fatigue crack paths: Introduction Crack path stability in two dimensions Crack growth from an initial mixed Modes I and II crack Constraints on crack paths in three dimensions Constraints on crack fronts in three dimensions Mode I crack growth in three dimensions Slant crack growth in thin sheets Summary References. Applications: Introduction Modelling of irregular cracks Static failure Constant amplitude fatigue crack growth Variable amplitude fatigue crack growth Proof test logic Leak-before-break Summary References.

Fatigue crack growth threshold conditions at notches. Part I: theory

Abstract: A micromechanical description of the fatigue crack growth process at notches is presented. Crack interaction with the plastic slip barriers of the material (e.g. grain boundaries) and the influence of the notch stress gradient are intrinsically taken into account in the model. Both the notch fatigue crack initiation limit and the limit for propagation up to failure (i.e. the conventional notch fatigue limit) are clearly identified and calculated. The formation of non-propagating cracks is also explained.

Failure Analysis of Industrial Composite Materials

Abstract: Part 1: fracture and failure mechanisms composite strength evaluation debonding and delamination failure mechanisms of fibre reinforced composites fracture toughness and fatigue crack growth failure and notch size effect constant and variable amplitude fatigue damage fatigue damage tolerant design of metal matrix composites. Part 2 Case studies: aerospace engineering construction engineering medical engineering.

Fatigue crack-growth in shape-memory NiTi and NiTi-TiC composites

Abstract: An experimental study was conducted to examine the room-temperature fatigue crack-growth characteristics of shape-memory NiTi matrix composites reinforced with 10 and 20 vol.% of TiC particles. Microstructural characterization of these hot-isostatically-pressed materials shows that the TiC particles do not react with the NiTi matrix and that they lack any texture. Overall fatigue crack-growth characteristics were found to be similar for the unreinforced and reinforced materials. However, a slight increase in the threshold for fatigue crack initiation was noted for the composites. The fracture toughness, as indicated by the failure stress intensity factor range, was found to be similar for all materials. Neutron diffraction studies near the crack-tip of the loaded fracture NiTi specimen detected no significant development of texture at the crack-tip. These results are explained by recourse to fractographic observations. Finally, a comparison is made between the micromechanisms of fracture of metal matrix composites, which deform by dislocation plasticity, and those of the present NiTi–TiC composites, which deform additionally by twinning.

Effect of strain rate on stepwise fatigue and creep slow crack growth in high density polyethylene

Abstract: The effects of frequency and R-ratio (the ratio of minimum to maximum stress in the fatigue loading cycle) on the kinetics of step-wise crack propagation in fatigue and creep of high density polyethylene (HDPE) was characterized. Stepwise crack growth was observed over the entire range of frequency and R-ratio examined. A model relating crack growth rate to stress intensity factor parameters and applied strain rate was proposed by considering the total crack growth rate to consist of contributions from creep and fatigue loading components. The creep contribution in a fatigue test was calculated from the sinusoidal loading curve and the known dependence of creep crack growth on stress intensity factor in polyethylene. At a very low frequency of 0.01 Hz, fatigue crack growth rate was found to be completely controlled by creep processes. Comparison of the frequency and R-ratio tests revealed that the fatigue loading component depended on strain rate. Therefore, crack growth rate could be modeled with a creep contribution that depended only on the stress intensity factor parameters and a fatigue contribution that depended on strain rate.

A study on fatigue crack growth in dual phase martensitic steel in air environment

Abstract: Dual phase (DP) steel was intercritically annealed at different temperatures from fully martensitic state to achieve martensite plus ferrite, microstructures with martensite contents in the range of 32 to 76%.Fatigue crack growth (FCG) and fracture toughness tests were carried out as per ASTM standards E 647 and E 399, respectively to evaluate the potential of DP steels. The crack growth rates (da/dN) at different stress intensity ranges (DK) were determined to obtain the threshold value of stress intensity range (DKth). Crack path morphology was studied to determine the influence of microstructure on crack growth characteristics. After the examination of crack tortuosity, the compact tension (CT) specimens were pulled in static mode to determine fracture toughness values. FCG rates decreased and threshold values increased with increase in vol.% martensite in the DP steel. This is attributed to the lower carbon content in the martensite formed at higher intercritical annealing (ICA) temperatures, causing retardation of crack growth rate by crack tip blunting and/or deflection. Roughness induced crack closure was also found to contribute to the improved crack growth resistance at higher levels of martensite content. Scanning electron fractography of DP steel in the near threshold region revealed transgranular cleavage fracture with secondary cracking. Results indicate the possibility that the DP steels may be treated to obtain an excellent combination of strength and fatigue properties.

Fatigue properties of aluminium foams at high numbers of cycles

Abstract: Endurance fatigue experiments have been performed with two kinds of Al–Mg–Si foams and one Al–Si foam under fully reversed loading conditions using the ultrasound fatigue testing method. Young’s modulus of the foams is 3.9 GPa. Constant amplitude fatigue data show endurance limits on the basis of 10 9 cycles between 1.1 and 1.4 MPa which is 16–23% of the plateau stress. Lifetimes exhibit a pronounced scatter, which is caused by the inhomogeneous structure of the foams. Fatigue damage is governed by the formation of cracks, which preferentially initiate in the interior sections of cell walls at initial defects, like precracks or holes. No strain localization and formation of deformation bands was found. Fatigue crack growth preferentially follows areas of cell walls with a minimum wall thickness, and eventually may stop near cell-nodes. The cyclic properties of foams can be improved, if initial defects are small, if the mean cell sizes are reduced, and if a more homogeneous foam is obtained.

An investigation of short and long fatigue crack growth behavior of Ti–6Al–4V

Abstract: This paper presents the results of a study of the effects of positive stress ratios on the propagation of long and short fatigue cracks in mill annealed Ti–6Al–4V. Differences between the long fatigue crack growth rates at positive stress ratios (R=Kmin/Kmax=0.02–0.8) are attributed largely to the effects of crack closure. Microstructurally short fatigue cracks are shown to grow at stress intensity factor ranges below the long crack fatigue threshold. Anomalously high fatigue crack growth rates and crack retardation are also shown to occur in the short crack regime. Differences between the long and the short crack behavior at low stress ratios are attributed to lower levels of crack closure in the short crack regime.

Modelling fatigue crack growth in shot‐peened components of Al 2024‐T351

Abstract: Microstructural fracture mechanics concepts are used to develop a model to incorporate shot-peening effects into crack propagation laws and fatigue life predictions. Shot peening produces a residual stress which resists opening of the crack and also produces a work-hardened layer within which the flow stress is raised. The model takes account of these effects to give an accurate prediction of the increase in fatigue life. The model was also used to derive the conditions for crack arrest, and the results are presented in the form of a fatigue damage map (FDM). The FDM can be used for the determination of safe loads in durability and maintainability analyses.

Effect of stress ratio on fatigue crack growth in TiAl intermetallics at room and elevated temperatures

Abstract: The fatigue crack growth behavior of γ-based titanium aluminides (TiAl) with a fine duplex structure and lamellar structure has been investigated by scanning electron microscope (SEM) in situ observation in vacuum at 750°C and room temperature. For the duplex structured material the fatigue crack growth rates are dominated by the maximum stress intensity, particularly at 750°C. The threshold stress intensity range for fatigue crack growth at 750°C is lower than that at room temperature for any corresponding stress ratio. The fatigue crack growth rate at 750°C is affected by creep deformation in front of the crack tip. The severe crack blunting occurs when the stress ratio is 0.5. For the lamellar structured material the scatter of fatigue crack growth data is very large. Small cracks propagate at the stress intensity range below the threshold for long fatigue crack growth. The effects of microstructure on fatigue crack growth are discussed.

Fatigue crack growth rates under sequential mixed‐mode I and II loading cycles

Abstract: One common mode of failure that occurs in rolling bodies such as gears, bearings and rails is due to the fatigue process. Several research workers suggest that rolling contact fatigue cracks are subjected to mixed mode I and II loading cycles. It is believed that the correct modelling of loading cycles can help us to study the mechanics of crack growth because fatigue comprises a major safety consideration in the design process. Experiments have been performed under nonproportional mixed-mode I and II loading cycles with fixed degrees of overlap, so that coplanar cracks were produced. Three empirical crack propagation laws have been established which are related to both mode I and mode II effective stress intensity factor ranges.