scholarly journals The Evaluation of Front Shapes of Through-the-Thickness Fatigue Cracks

Metals ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 403
Author(s):  
Behnam Zakavi ◽  
Andrei Kotousov ◽  
Ricardo Branco
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Closure ◽  
Crack Front ◽  
Fatigue Cracks ◽  
Numerical Approach ◽  
Metal Fatigue ◽  
Underlying Mechanisms ◽  
Actual Shape

Fatigue failure of structural components due to cyclic loading is a major concern for engineers. Although metal fatigue is a relatively old subject, current methods for the evaluation of fatigue crack growth and fatigue lifetime have several limitations. In general, these methods largely disregard the actual shape of the crack front by introducing various simplifications, namely shape constraints. Therefore, more research is required to develop new approaches to correctly understand the underlying mechanisms associated with the fatigue crack growth. This paper presents new tools to evaluate the crack front shape of through-the-thickness cracks propagating in plates under quasi-steady-state conditions. A numerical approach incorporating simplified phenomenological models of plasticity-induced crack closure was developed and validated against experimental results. The predicted crack front shapes and crack closure values were, in general, in agreement with those found in the experimental observations.

scholarly journals Revisiting Classical Issues of Fatigue Crack Growth Using a Non-Linear Approach

Materials ◽  
2020 ◽  
Vol 13 (23) ◽  
pp. 5544
Author(s):  
Micael F. Borges ◽  
Diogo M. Neto ◽  
Fernando V. Antunes
Keyword(s):  
Plastic Deformation ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Closure ◽  
Crack Tip ◽  
Maximum Load ◽  
Numerical Approach ◽  
Load Range ◽  
Definition Of

Fatigue crack growth (FCG) has been studied for decades; however, several aspects are still objects of controversy. The objective here is to discuss different issues, using a numerical approach based on crack tip plastic strain, assuming that FCG is driven by crack tip deformation. ΔK was found to control cyclic plastic deformation at the crack tip, while Kmax has no effect. Therefore, alternative mechanisms are required to justify models based on ΔK and Kmax. The analysis of crack tip plastic deformation also showed that there is crack tip damage below crack closure. Therefore, the definition of an effective load range ΔKeff = Kmax − Kopen is not correct, because the portion of load range below opening also contributes to FCG. Below crack closure, damage occurs during unloading while during loading the crack tip deformation is elastic. However, if the maximum load is decreased below the elastic limit, which corresponds to the transition between elastic and elasto–plastic regimes, there is no crack tip damage. Additionally, a significant effect of the crack ligament on crack closure was found in tests with different crack lengths and the same ΔK. Finally, the analysis of FCG after an overload with and without contact of crack flanks showed that the typical variation of da/dN observed is linked to crack closure variations, while the residual stresses ahead of crack tip are not affected by the contact of crack flanks.

Steady fatigue crack growth: micromechanism and mathematical modeling

2018 ◽  
Vol 84 (11) ◽  
pp. 52-69 ◽  
Author(s):  
N. V. Tumanov
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Front ◽  
Fatigue Cracks ◽  
Crack Tip ◽  
Tensile Load ◽  
Metallic Materials ◽  
Fatigue Striations ◽  
Shaped Crack

A universal energy-intensive micromechanism of periodic splitting-rupture (PSR) is revealed which proceeds at the front of the fatigue cracks in metallic materials, providing their steady growth, forming T-shaped crack tip and striated microrelief of the fracture surface. The PSR micromechanism is caused by a critical (prior to fracture) fragmentated structure formed in the area of the crack front where the material is subjected to multiple and increasing plastic deformation. This universal prefracture structure is a final stage of the evolution of the deformational structures emerged in front of the fatigue crack at the stage of stable crack growth in metallic materials with different initial structural states. This is responsible for universality of PSR micromechanism and fatigue striations. Fatigue striations are the traces of extending crack front with T-shaped tip formed during brittle transverse microsplitting along the overstressed boundaries of critical fragmentated structure. Based on 3D finite element modeling of the stress-strain state in front of the cracks with T-shaped tip, it is established that the value and the location of maximum of normalized in-plain stresses (acting in front of crack tip in the plane of crack along the normal to its front) are close or coincide for the cracks of different configuration and different types of tensile load under condition that splitting in the T-shaped crack tip is considerably less than the crack length. Taking into account the PSR micromechanism and asymptotic stress distribution in front of T-shaped crack tip the physically based mathematical model for steady fatigue crack growth is developed along with the techniques for prediction of steady fatigue crack growth in full-scale components under simple and complex loading cycles.

scholarly journals Short/small fatigue crack growth, thresholds and environmental effects: a tale of two engineering paradigms

2021 ◽  
Vol 39 (2) ◽  
pp. 165-175
Author(s):  
Russell J. H. Wanhill ◽  
Stefanie E. Stanzl-Tschegg
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Steam Turbine ◽  
Crack Growth ◽  
Environmental Effects ◽  
High Cycle Fatigue ◽  
Materials Science ◽  
Fatigue Cracks ◽  
Cycle Fatigue ◽  
Metal Fatigue

Abstract This paper results from mutual discussions on the review ‘When do small fatigue cracks propagate and when are they arrested?’ in Corrosion Reviews, 2019; 37(5): 397–418. These discussions have arisen from the two engineering paradigms characterizing our fatigue research: (i) an aerospace research and technology remit for metallic airframes, and (ii) a materials science research programme supporting a methodology for steam turbine low pressure (LP) blade operations. In our opinion, this paper is of interest for other investigators of metal fatigue with respect to design requirements, life predictions and assessments. In more detail, the paper considers the fatigue design methodologies for airframes and steam turbine LP blades. This includes short/small fatigue cracks, fatigue crack growth thresholds, high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF), and the relevance of environmental effects (corrosion and corrosion fatigue).

Fatigue Crack Growth Prediction of Elliptical Corner Crack Using 3D FEM

2012 ◽  
Vol 248 ◽  
pp. 469-474
Author(s):  
M.H. Gozin ◽  
M. Aghaie-Khafri
Keyword(s):  
Finite Element ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Closure ◽  
Crack Front ◽  
Crack Opening ◽  
3D Fem ◽  
Corner Crack ◽  
Closure Method

Plasticity induced crack closure (PICC) simulation using finite element analyses has been concerned by many researchers. In the present investigation elliptical corner fatigue crack growth from a hole was predicted using PICC method. An elastic-plastic finite element model is built with a suitably refined mesh and time-dependent remote tractions are applied to simulate cyclic loading. In a 3D geometry the crack opening value will vary along the crack front. For simplicity this shape evolution is neglected and the crack front is extended uniformly. Predicted fatigue life using crack closure method for elliptical corner crack is in good agreement with the experimental data. The results obtained highlighted the sensitivity of crack closure method to the opening stress intensity values.

scholarly journals Fatigue Crack Growth from Notches: A Numerical Analysis

2020 ◽  
Vol 10 (12) ◽  
pp. 4174 ◽  
Author(s):  
Micael Borges ◽  
Manuel Caldas ◽  
Fernando Antunes ◽  
Ricardo Branco ◽  
Pedro Prates
Keyword(s):  
Fatigue Crack ◽  
Stress State ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Closure ◽  
Numerical Approach ◽  
Crack Tip Opening Displacement ◽  
Notch Radius ◽  
Opening Displacement ◽  
Global Trends

A numerical approach based on plastic crack tip opening displacement (CTOD) was followed to study fatigue crack growth (FCG) from notches. The identification of fundamental mechanisms was made considering notched and unnotched models, with and without contact of crack flanks. Different parameters were studied, namely, notch radius, crack length, stress state, and material. The notch increases the plastic CTOD, and therefore fatigue crack growth rate, da/dN, as expected. The reduction of notch radius increases da/dN but reduces the notch affected zone. Ahead of the notch affected zone, da/dN increases linearly with crack growth, with a rate that increases linearly with the plastic CTOD. The crack closure phenomenon has a dramatic effect under plane stress conditions but a limited effect on plane strain conditions. In the former case, the contact of crack flanks reduces substantially the effect of notch radius and the size of the notch affected zone. These trends are associated with the increase of crack closure level with notch radius. The material does not affect the global trends, but the reduction of yield stress increases the level of plastic deformation and therefore da/dN. The effect of material, and also of stress state, is mainly associated with crack closure.

Guided Wave Acoustic Emission from Fatigue Crack Growth in Aluminium Plate

2006 ◽  
Vol 13-14 ◽  
pp. 23-28 ◽  
Author(s):  
C.K. Lee ◽  
Jonathan J. Scholey ◽  
Paul D. Wilcox ◽  
M.R. Wisnom ◽  
Michael I. Friswell ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Elastic Waves ◽  
Fatigue Cracks ◽  
Guided Wave ◽  
Experimental Results ◽  
Multiple Reflections ◽  
Alloy Plate

Acoustic emission (AE) testing is an increasingly popular technique used for nondestructive evaluation (NDE). It has been used to detect and locate defects such as fatigue cracks in real structures. The monitoring of fatigue cracks in plate-like structures is critical for aerospace industries. Much research has been conducted to characterize and provide quantitative understanding of the source of emission on small specimens. It is difficult to extend these results to real structures as most of the experiments are restricted by the geometric effects from the specimens. The aim of this work is to provide a characterization of elastic waves emanating from fatigue cracks in plate-like structures. Fatigue crack growth is initiated in large 6082 T6 aluminium alloy plate specimens subjected to fatigue loading in the laboratory. A large specimen is utilized to eliminate multiple reflections from edges. The signals were recorded using both resonant and nonresonant transducers attached to the surface of the alloy specimens. The distances between the damage feature and sensors are located far enough apart in order to obtain good separation of guided-wave modes. Large numbers of AE signals are detected with active fatigue crack propagation during the experiment. Analysis of experimental results from multiple crack growth events are used to characterize the elastic waves. Experimental results are compared with finite element predictions to examine the mechanism of AE generation at the crack tip.

Comparison of DBEM and FEM Crack Path Predictions with Experimental Findings for a SEN-Specimen under Anti-Plane Shear Loading

2007 ◽  
Vol 348-349 ◽  
pp. 129-132 ◽  
Author(s):  
Roberto G. Citarella ◽  
Friedrich G. Buchholz
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Front ◽  
Crack Path ◽  
Shear Loading ◽  
Crack Growth Behaviour ◽  
3D Crack Growth ◽  
Corner Points ◽  
Experimental Findings

In this paper detailed results of computational 3D fatigue crack growth simulations will be presented. The simulations for the crack path assessment are based on the DBEM code BEASY, and the FEM code ADAPCRACK 3D. The specimen under investigation is a SEN-specimen subject to pure anti-plane or out-of-plane four-point shear loading. The computational 3D fracture analyses deliver variable mixed mode II and III conditions along the crack front. Special interest is taken in this mode coupling effect to be found in stress intensity factor (SIF) results along the crack front. Further interest is taken in a 3D effect which is effective in particular at and adjacent to the two crack front corner points, that is where the crack front intersects the two free side surfaces of the specimen. Exactly at these crack front corner points fatigue crack growth initiates in the experimental laboratory test specimens, and develops into two separate anti-symmetric cracks with complex shapes, somehow similar to bird wings. The computational DBEM results are found to be in good agreement with these experimental findings and with FEM results previously obtained. Consequently, also for this new case, with complex 3D crack growth behaviour of two cracks, the functionality of the proposed DBEM and FEM approaches can be stated.

Three-Dimensional Study of Fatigue Crack Growth in a Rotating Disc

2013 ◽  
Vol 3 (2) ◽  
pp. 45-62
Author(s):  
Eskandari Hadi ◽  
Nami Mohammad Rahim
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Front ◽  
Numerical Technique ◽  
Three Dimensional ◽  
Crack Orientation ◽  
Rotating Disc ◽  
Mesh Density ◽  
Three Dimensional Finite Element

The problem of fatigue-crack-growth in a rotating disc at different crack orientation angles is studied by using an automated numerical technique, which calculates the stress intensity factors on the crack front through the three-dimensional finite element method. Paris law is used to develop the fatigue shape of initially semi-elliptical surface crack. Because of needs for the higher mesh density and accuracy near the crack, the sub-modeling technique is used in the analysis. The distribution of SIF’s along the crack front at each step of growth is studied and the effect of crack orientation on the rate of crack-growth is investigated. The calculated SIF’s are reasonable and could be used to predict the probable crack growth rates in fracture mechanics analysis and can help engineers to consider in their designing and to prevent any unwanted failure of such components.

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