Behaviour of short fatigue cracks in a naval steel (original) (raw)
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Critical review of the fatigue growth of short cracks
Engineering Fracture Mechanics, 1986
Retirement for cause (RFC) has become a popular design/analysis philosophy because it facilitates continued use of components which would otherwise be retired based on a safe life philosophy. RFC permits this continued service on the basis that service-induced damage tracked by periodic inspections will not develop to a critical size within a future operating interval. Successful implementation of RFC requires fracture mechanics technology to predict in-service growth behavior. Recent observations suggest that nonconservative estimates of crack growth rate (and therefore inspection interval) and critical flaw size arise when conventional linear elastic fracture mechanics (LEFM) is applied to predict the growth of physically small cracks. This paper summarizes the results of an extensive limited-circulation critical review of the phenomenology and mechanics of short crack growth with a view to identifying when this anomalous growth makes RFC analyses untenable in terms of LEFM. Factors which control the growth of short cracks are enumerated. It is shown for unnotched samples that the apparent effect may be traced to microcrack closure and the metallurgical. mechanical, and environmental transients which develop in the transition from initiation to steady-state growth. For notch samples the anomalous growth of cracks is traced to the inelastic action that develops a displacement-controlled notch field, which, contrary to LEFM analysis, dominates crack extension. Mechanics analyses relevant to characterizing the growth of short cracks are discussed. A crack tip opening displacement criterion is indicated to be appropriate.
Experimental investigations on the growth of small fatigue cracks in naval steel
Fatigue & Fracture of Engineering Materials and Structures, 2007
The concept of damage tolerance is now largely employed to evaluate the fatigue life of structures. However, part of this fatigue relies on the initiation and growth of small cracks. The fatigue behaviour of a naval structural steel (S355NL) was investigated. In order to characterize the behaviour of short and long cracks, tests were performed under constant amplitude loading for several load ratios between-1.0 and 0.5. A major part of fatigue life is constituted by short crack initiation and propagation.
A new approach to the evaluation of danger of short fatigue cracks
Strength of Materials - STRENGTH MATER-ENGL TR, 2000
We demonstrate the possibility of using new physically substantiated characteristics of toughness of metals and the embrittling action of stress concentrators proposed by the authors somewhat earlier in the analysis of brittle fracture initiated by short cracks. We describe a procedure for the experimental determination of a parameter characterizing the embrittling action of these defects and obtain an approximate expression for its evaluation in the presence of a short crack. For typical structural steels, we establish the dependence of the toughness margin of steel with a short crack on the level of strength of this material. We also formulate the requirements on the level of toughness of steels guaranteeing that defects of this type are not dangerous.
Prediction of the critical stress to crack initiation associated to the invest.PDF
Fatigue design is of vital importance to avoid fatigue small crack growth in engineering structures. This study shows that the critical fatigue design stress can be defined below the usual endurance limit, considered in rules and codes. The material constitutive behaviour is using linear isotropic elasticity. Lassere and Pallin-Luc [1-2] use the elastic energy and over-energy under uniaxial load (tension and rotating bending). The authors deduce the influencing critical stress value corresponding to σ*. It's a linear approach. We propose an over-energy under dissymmetrical rotating bending and expressed in the ellipse axes. An asymptotic approach is transformed the over-energy in polynomial function of critical stress. Unknown depend on experimental service conditions, endurance limit of tension and rotating bending of specimen. The fatigue database of 30NCD16 steel studied by Froustey and Dubar [3-13] is used. Critical stresses are evaluated . The research done by Manning and all [4] has shown the small crack effect to be as large as 0.3 mm. Small crack and critical stress are illustrated here in as resulting from pure bending approach expressed by Bazant law . It's reproduces well the Kitagawa diagram [6] . When the short cracks are hidden in the material, we shows that the number Citation: Marhabi, D., Benseddiq, N., Mesmacque, G., Azari, Z., Nianga, J.M., Prediction of the critical stress to crack initiation associated to the investigation of fatigue small crack, Frattura ed Integrità Strutturale, 38 (2016) 36-46.
A new fatigue domain diagram, recently introduced by one of the authors, makes it possible to demonstrate the fatigue behavior of specimens under varying stress amplitude loading both qualitatively and quantitatively. The diagram is briefly reviewed and crack propagation and damage summation of steel specimens under two level and multi level tension-compression loading are simulated and discussed. Typical patterns of low-high and high-low sequence levels are explained, predicted and, with careful classification, shown to follow certain cumulative damage trends. Correlation with experimental results is shown and discussed. The main conclusion is that one can show repeatable trends in H-L and L-H two-step and multi-step loading sequences, only for cases where the local material properties are not drastically changed, and the failure pattern is similar (critical crack propagation or gross yielding) in all stages of the tests. Damage accumulation, expressed as additional crack length, is clearly shown on the general fatigue diagram. NOMENCLATURE a = crack length da/dN = crack propagation rate (da/dN),, (da/dN), = crack propagation rate in the short cracks and LEFM regimes C, rn = material constants E = modulus of elasticity F,, F, = parametric functions K = stress intensity factor (SIF) n = number of stress reversals Ni = number of stress reversals till separation R = stress ratio ( = umin/umax) S, = endurance limit S, = yield strength S, = ultimate tensile strength K,, = plane strain fracture toughness K, , , K,, = maximum and minimum SIF AK, = threshold SIF range AKen, AK,,, = effective and effective threshold SIF range ua, a , = stress amplitude and mean stress a = constant
Experimental Evaluation of the Energy Responsible for the Crack Advancing
2013
Abstract. Many high performance ductile structures in civil engineering, automotive, aerospace and electrical industries are manufactured from high ductile thin wall materials. The mechanical behaviour of such materials is different from that of bulk (thick wall) materials. It is well known that measured value of fracture toughness strongly depends on many factors, namely geometry and type of loading. The existence of the plastic region in crack front is generally taken as a reason for dependence of fracture toughness on the body geometry. As appropriate determination of the fracture toughness evaluation of the energy responsible for fracture advancing can be considered. For such evaluation, the knowledge of several physical quantities is necessary: crack shape and processing zone behaviour, strain-stress field and plastic deformation. Combination of several experimental methods computed tomography, Digital Image Correlation and strain gauge measurements were employed for this purpose.
Propagation of short fatigue cracks
International Materials Reviews, 1984
E == elastic (Young's) modulus E' == effective value of Young's modulus under different loading conditions i == function of stress intensity factor range M and load ratio R (equation (15)) f ij == dimensionless function of polar angle e measured from crack plane (equation (3)) ib ,Iij == universal functions of both polar angle e measured from crack plane and work hardening exponent n (equation (10)) G == strain -energy release rate J == scalar amplitude .of crack tip stre ss and strain field under nonlinear elastic conditions J == cyclic component of J kf == fatigue strength reduction factor kt == theoretical elastic stress concentration factor 446 Suresh and Ritchie: Propagation of short fatigue cracks Meff R E , R a == strain and stress concentration factors K cl == closure stress intensity factor K~== critical value of microscopic stress intensity at tip of slip band K I == stress intensity factor in mode I loading I\Ic == critical stress intensity at failure (plane strain fracture toughness) Ki¥' == mode II value of microscopic stress intensity at tip of slip band (Fig. 17) Kl == limiting stress intensity for long crack emanating from notch == maximum and minimum stre sss intensities during cycle == limiting stress intensity for short crack emanating from notch == threshold stress intensity factor == long crack threshold stress in tensity factor == local mode I and mode II stress intensity factors for non-linear crack == nominal stress intensity factor range (K max -Kmin) == effective stress intensity factor range (Kmax -K cl) == equivalent stress intensity range for short crack Mi == initial stress intensity range Mth == threshold stress intensity factor range DJ( E == pseudo-elastic-plastic strain intensity range
Fatigue crack initiation at a notch
International Journal of Fatigue, 2011
A short crack approach has been developed to determine the fatigue crack initiation life at a notch tip, in which the initiation life is assumed to correspond to the period during which an initial crack, equivalent to a microscopic defect, grows to attain a detectable size. This approach is compared to conventional local strain approach and it is shown that the short crack approach gives acceptable predictions as compared to experiments, in the 2024 T351 alloy. In the 7449 alloy, a different approach, using marker-band technique has been applied to determine the short crack growth kinetics at a notch. The advantages and disadvantages of the conventional strain based approach and the short crack model are discussed.