Showing papers on "Paris' law published in 1999"

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.

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.

Fatigue crack growth and threshold measurements at very high frequencies

Abstract: Fatigue testing at ultrasonic frequencies makes it possible to investigate the fatigue properties of materials in an effective and time saving manner. The mechanical and electrical components of ultrasound equipment are described in detail. Different testing and evaluation procedures are reviewed. Recommendations on how to perform high frequency fracture mechanics studies and to evaluate experimental results are described. Recent developments for multiaxial loading, for performing in-service loading fatigue experiments, and for realising high frequency torsional vibrations are described. A survey of fatigue investigations of metallic materials, composites, metallic glass, and ceramics, determined with the ultrasound method, is presented. Intrinsic frequency effects are reviewed, emphasising the influences on dislocation structures, crack initiation, and fatigue crack growth for pure metals and for technical materials. The advantages and disadvantages of the ultrasound method are compared with thos...

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.

The effect of microstructure on fracture toughness and fatigue crack growth behavior in γ-titanium aluminide based intermetallics

Abstract: Ambient-temperature fracture toughness and fatigue crack propagation behavior are investigated in a wide range of (γ+α 2) TiAl microstructures, including single-phase γ, duplex, coarse lamellar (1 to 2 mm colony size (D) and 2.0 µm lamellar spacing (λ)), fine lamellar (D ∼ 150 µm, λ=1.3 to 2.0 µm), and a powder metallurgy (P/M) lamellar microstructure (D=65 µm, λ=0.2 µm). The influences of colony size, lamellar spacing, and volume fraction of equiaxed γ grains are analyzed in terms of their effects on resistance to the growth of large (>5 mm) cracks. Specifically, coarse lamellar microstructures are found to exhibit the best cyclic and monotonic crack-growth properties, while duplex and single-phase γ microstructures exhibit the worst, trends which are rationalized in terms of the salient micromechanisms affecting growth. These mechanisms primarily involve cracktip shielding processes and include crack closure and uncracked ligament bridging. However, since the potency of these mechanisms is severely restricted for cracks with limited wake, in the presence of small (<300 µm) cracks, the distinction in the fatigue crack growth resistance of the lamellar and duplex microstructures becomes far less significant.

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.

Creep and fatigue properties of high temperature silicides and their composites

Abstract: A review of creep and fatigue behavior of high temperature silicides and their composites is presented along with new results pertaining to titanium silicides and functionally graded materials. The emphasis was placed on the molybdenum disilicides and their composites in comparison to other silicides and high temperature ceramics. It was shown that the grain size effects on creep are significant in these materials. The effects were present not only in the Newtonian creep but persists even in the power-law creep, with grain size exponents of the order of 4.2. Factors that influenced the grain size effects were examined. This anomalous behavior, which appeared to be exhibited by some stoichiometric intermetallics, could not be explained by the currently known theories of grain boundary creep. Effects of alloying and reinforcement on creep rates were also discussed. The creep rates in other high temperature silicides were compared. It was shown that molybdenum disilicide with silicon nitride has a potential for high temperature applications in terms of its creep and oxidation resistance, although its creep resistance appears to be much less than that of an advanced Si3N4, such as AS800. Published results on fatigue crack growth in MoSi2 and its composites were analyzed using the recently developed Unified Approach for Fatigue.

Micromechanisms of fatigue crack growth in a forged Inconel 718 nickel-based superalloy

Abstract: The micromechanisms of fatigue crack propagation in a forged, polycrystalline IN 718 nickel-based superalloy are evaluated. Fracture modes under cyclic loading were established by scanning electron microscopy analysis. The results of the fractographic analysis are presented on a fracture mechanism map that shows the dependence of fracture modes on the maximum stress intensity factor, K max , and the stress intensity factor range, Δ K . Plastic deformation associated with fatigue crack growth was studied using transmission electron microscopy. The effects of Δ K and K max on the mechanisms of fatigue crack growth in this alloy are discussed within the context of a two-parameter crack growth law. Possible extensions to the Paris law are also proposed for crack growth in the near-threshold and high Δ K regimes.

Engineering against fatigue

Abstract: The mechanics and materials approach to fatigue problems in engineering materials aspects of fatigue threshold stress range for short crack growth fatigue strength assessment of AlSi7Mg castings micromechanical modelling of fatigue in nodular cast iron direct observation of the formation of striations fatigue fracture toughness of steels a model for multiaxial small fatigue crack growth evaluation of fatigue life prediction under ideal circumstances compression fatigue damage in notched CFRP laminates surface roughness and crack initiation in fretting fatigue initiation, growth and branching of cracks in railway track prevention of fatigue by surface engineering atmmospheric influence on fatigue crack propagation.

Fatigue life analysis of fiber reinforced concrete with a fracture mechanics based model

Abstract: Fatigue life of fiber reinforced concrete (FRC) is theoretically analyzed with a fracture mechanics based model. The model accounts for the effect of cycle-dependent crack bridging properties in FRCs and predicts the number of cycles to failure defined by final fracture subsequent to stable fatigue crack growth in Mode I. The resulting theoretical S–N diagram was compared with experimental data reported in literature. The general agreement supports the validity of the current model, and reveals that cycle-dependent degradation of crack bridging controls fatigue life of FRCs. S–N diagrams are essential for material evaluation and structural design, and the model establishes the link between material structure and S–N diagram in an explicit manner, while the conventional stress-life approach realizes the link in an empirical manner.

Determining Fatigue Crack Opening Loads from Near-Crack-Tip Displacement Measurements

Abstract: The aim of this research was to develop a near-crack-tip measurement method that quantifies crack closure levels in the near-threshold fatigue crack growth regime--a regime where crack closure is not well characterized by remote compliance methods. Further understanding of crack closure mechanics was gained by performing novel crack growth experiments in conjunction with numerical simulations of three-dimensional crack-front propagation. Steady-state (i.e., constant growth rate) fatigue crack growth rates were characterized by performing constant cyclic stress intensity range ({Delta}K) experiments over a wide range of stress ratios (R). Near-crack-tip (less than 0.3 mm behind) load-versus-displacement measurements were conducted on the specimen surface using a novel noncontact experimental technique (Digital Imaging Displacement System--DIDS). The experiments and simulations revealed that the three-dimensional aspects of fatigue crack closure must be considered to determine correct opening load levels from near-crack-tip load-versus-displacement data. It was shown that near-crack-front, but increase near the free surface. The interior opening load was found to collapse closure-affected data to intrinsic rates, and thus shown to relate to the true crack-front driving force parameter. Surface opening load DIDS measurements made at an optimal distance behind the crack tip were used to correlate da/dN with {Delta}K{sub eff}. Opening load determinations made lessmore » than the optimal distance behind the crack tip were shown to be too high to correlate fatigue crack growth rates.« less