Fatigue Crack Closure Research Articles

Fatigue crack closure assessment by wavelet transform of infrared thermography signals

The occurrence of crack closure significantly impacts the fatigue life of materials and structural components. Whether it is induced by the nature of the loading, the fabrication process or the geometry of the structure, its magnitude and effect should be considered to further improve predictive models of fatigue crack propagation. However, the definition of reliable experimental methods for the observation and assessment of fatigue crack closure, and in particular suited to structure testing, remains a challenge. The present study aims to provide a novel approach for the assessment of fatigue crack closure via the continuous wavelet transform of infrared thermography data. The processing of the temperature signal close to the crack in a coherent time–frequency space allows for the identification of crack closing and opening instants associated with high-frequency components. The method is meant to be suited to any testing configuration (conventional compact tension specimen or full-scale structures) with minimum operator-dependent parameters.

Short Fatigue-Crack Growth from Crack-like Defects under Completely Reversed Loading Predicted Based on Cyclic R-Curve.

Understanding short fatigue-crack propagation behavior is inevitable in the defect-tolerant design of structures. Short cracks propagate differently from long cracks, and the amount of crack closure plays a key role in the propagation behavior of short cracks. In the present paper, the buildup of fatigue-crack closure due to plasticity with crack extension from crack-like defects is simulated with a modified strip yield model, which leaves plastic stretch in the wake of the advancing crack. Crack-like defects are assumed to be closure-free and do not close even under compression. The effect of the size of crack-like defects on the growth and arrest of short cracks was systematically investigated and the cyclic R-curve derived. The cyclic R-curve determined under constant amplitude loading of multiple specimens is confirmed to be independent of the initial defect length. Load-shedding and ΔK-constant loading tests are employed to extend the cyclic R-curve beyond the fatigue limit determined under constant amplitude loading. The initiation stage of cracks is taken into account in R-curves when applied to smooth specimens.

3D characterization of closure of surface and internal fatigue cracks in nodular cast iron using Digital Volume Correlation of laboratory X-ray tomography images

Digital volume correlation (DVC) of laboratory X-ray tomography images has been used to characterize the closure of artificial internal fatigue cracks in nodular graphite cast iron. DVC allows to produce crack opening displacement (COD) map from 3D tomography images at the sub-voxel scale thanks to the graphite nodules which can be used as internal natural markers. Fatigue samples containing controlled internal defects have been produced in a nodular cast iron by diffusion bonding plates containing laser-machined sharp notches. Specimens with internal cracks and surface cracks were respectively characterized by laboratory tomography at different loads during fatigue crack propagation tests. The local stress intensity factors (SIFs) have been extracted from the measured displacement fields by using Williams series, based on those results the closure levels of internal cracks and surface cracks were comparatively analyzed and discussed. Crack closure levels were found to be linearly related to the location of the surface crack front, and the level of internal crack closure was comparable to that of the surface crack in bulk.

Investigation of fatigue crack closure effect on the evaluation of edge cracks with the fundamental mode of edge waves

Fatigue cracks often initiate and propagate from edges of structural components. Detection and evaluation of edge cracks could be very challenging, specifically, due to the crack closure phenomenon, which makes fatigue cracks to be partially closed when the applied loading is removed; this usually corresponds to maintenance and NDE inspection conditions. Despite that fatigue crack closure is well investigated, past experimental and theoretical studies related to guided wave-based NDEs largely ignored this phenomenon. In this article, the fundamental symmetric mode of edge waves (ES0) is used to evaluate crack closure effects on the evaluation of fatigue cracks. The experimental studies have demonstrated that the reflected and transmitted signals at different frequencies correlate very well with the length of the open region of fatigue cracks. However, an accurate evaluation of the total crack length can only be conducted under an applied loading, which fully separates the crack faces. Finally, a new FE model has been proposed to simulate the fatigue crack closure and its effects on propagation of ultrasonic bulk and guided waves. The outcomes of FE modelling and experimental study were found in a good agreement.

50 Years of Controversy on Fatigue Crack Closure

In this article the 50 years of observations, implications and debates related to fatigue crack closure are discussed. New insights related to plasticity, oxide, and roughness induced crack closures and their role in shielding effects of the fatigue crack tip are re-examined. Supporting evidence for these insights comes from the lack of ΔKth dependence on R in a high vacuum (with partial pressure of 10−5 Pa or less). The presented new critical chemical-mechanical analyses are based on experimental results reported in the literature that demonstrate the marginal R-ratio effect on ΔKth of long cracks in vacuum for both planar/wavy slip alloys but show R-dependence in the lab air and in chemical environment. The latter is due to the formation of viscous nature of the oxide, which forms in humid air at the newly expose fresh fracture surfaces. It is demonstrated that the dominant factor related to the experimentally observed R-ratio effects on fatigue crack growth (FCG) behavior (on several alloys) in not related to crack closure but the access of the environment to the crack tip region that affects fatigue damage. In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when crack flanks contact occurs, most of the oxidized metallic species will exist as aquo-complexes, gel, or colloids, that have insufficient shear strength to wedge crack faces during unloading. Dislocation-based models have indicated that the crack tip shielding effect from a single asperity is small. The roughness induced crack closure has been suggested as a mechanical obstruction in the wake of the crack during cyclic unloading for planar slip alloys at the threshold region, the emphasis on the access of the environment to the crack tip for an environmental damage was not considered.

A Study on Fatigue Crack Closure Associated with the Growth of Long Crack in a New Titanium Alloy

In this article, the fatigue crack closure in a new titanium alloy is carried out under constant amplitude cyclic loading with the combination of the simulation method and experiments. A numerical simulation of the crack closure effect on a new titanium alloy under different load ratios was carried out to analyze the effect of grid size on the crack closure level and to obtain the fatigue crack opening load level under different load ratios. The experimental study of the fatigue crack growth rate of the new titanium alloy under different load ratios has demonstrated that the fatigue crack growth rate is affected by the stress ratio. With the increase in load ratio, the crack expansion rate of new titanium also increases. Combined with the simulation, the experiments show that the crack closure effect induced by the plastic region at the crack tip is the main reason for the load ratio effect on the fatigue crack expansion rate of the new titanium alloy.

On 50 years of fatigue crack closure dispute

AbstractIn this article, we examine the 50 years of observations and interpretations into the subject of fatigue crack closure in lab air and aqueous solutions: disputes and implications. Over this period, several closure concepts have been invoked to explain why the crack propagation threshold ΔKth should reduce with load ratio R. Among these, three types are the most popular and these are closure induced by crack (i) plasticity (PICC), (ii) oxides (OICC), and (iii) roughness (RICC). Therefore, these three and their role in shielding the fatigue crack tip are critically examined. This analysis finds that the reduction in ΔKth with increasing R‐ratio in lab air and aqueous solutions is weakly linked to decreasing crack closure behind the crack tip but strongly related to the access of the environment to the crack tip region and its influence on fatigue damage at and ahead of the crack tip. Supporting evidence for this interpretation comes from the lack of ΔKth dependence on R in a good vacuum (with partial pressure of 10−5 Pa or less). In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when closure occurs at the crack tip, most of the oxidized metallic species will exist as aquo‐complexes, gel, or colloids that have insufficient shear strength to wedge crack faces during unloading.

Effect of microstructure and stress ratio on fatigue crack propagation behavior of A7N01 aluminum alloy fiber laser-VPTIG hybrid butt welded

In this paper, the fatigue crack growth rates of A7N01 aluminum alloy welded joint in weld zone (WZ) and heat affected zone (HAZ) are compared under the stress ratio of 0.1. At the same time, the fatigue crack growth rate of WZ under different stress ratios is studied. The results show that the difference of microstructure leads to different fatigue crack growth rates in different areas of welded joints. The HAZ shows better fatigue crack growth resistance and slower fatigue crack growth rate. However, due to the burning loss of Zn element in the WZ, the precipitation of strengthening phase after solidification in the molten pool is less, and the fatigue crack propagation resistance is weak. After 60 days of natural aging, the number of precipitated phases in the WZ and HAZ increased, and the fatigue crack propagation resistance is improved compared with that in the welded condition. Compared with grain boundary strengthening, precipitation strengthening is more important. With the increase of stress ratio, the fatigue crack closure effect in WZ gradually weakens, and the fatigue crack propagation rate increases.

Numerical modelling of fatigue crack closure and its implication on crack front curvature using ΔCTODp

One of the widely used approaches to characterize fatigue crack propagation is the use of the effective stress intensity factor range ΔKeff, which relies on determination of crack closure value Kcl. The used models of crack closure are most frequently-two-dimensional, however, for real cracks, the 3D effects should be taken into account. The paper presents 3D finite element analyses of the influences of crack front shape, inserted cycles (loading cycles between node releases) and crack closure on the crack driving force in terms of ΔKeff and ΔCTODp (plastic part of crack tip opening displacement range). Numerically obtained crack closure depended on the simulation strategy. In the case of inserted cycles, crack closure disappeared in the internal part of the specimen and remained only near the free edges. The use of ΔCTODp had the advantage of a well-defined parameter in situations where ΔKeff was problematic, namely at the corner points, which did not allow finding of equalized crack driving force along the whole crack front using ΔKeff. Equalized crack driving force in terms of ΔCTODp was found for crack front curvature with the edge angle 15.4° in simulation with crack closure, which was in good agreement with the experimentally measured value of 16°. Loops produced by loading and unloading branches of the force vs CTOD diagrams helped to describe the crack closure process and magnitude. Actual values of CTOD did not agree with the classical idea of 2D solutions under plane strain and plane stress. CTOD was larger in the internal part of the specimen than at the free edges, even in simulations with no crack face contact.

Experimental investigation of crack opening loads in an aircraft load spectrum

The concept of plasticity-induced fatigue crack closure is indispensable in explaining many fatigue crack growth phenomena; and it is now widely applied in advanced fatigue life evaluation procedures. This concept has been extensively investigated experimentally over the past sixty years using different methods. However, most of these investigations are related to constant amplitude loading or simple load cases such as single or multiple overloads or block loading. In contrast, there are currently no experimental studies investigating crack closure under variable amplitude loading or load spectra, which are more realistic scenarios. This can be attributed to the many difficulties associated with the evaluation of crack opening loads for large numbers of fatigue cycles, including sensor sensitivity, signal noise, data limits, etc. Recently, the authors proposed an advanced piezoelectric sensor for cycle-by-cycle crack opening load measurements, which addresses the above limitations. In this work, the piezoelectric sensor is utilised to investigate crack closure under a typical transport aircraft load spectrum. In particular, it is demonstrated that crack opening load levels can change significantly from cycle-to-cycle, and these levels are typically larger than for constant amplitude loading at the same R-ratio. The relative density plot of crack opening load levels reveals three distinct regions, which can be associated with certain load sequences. The outcomes of this research are important for the development and validation of spectrum compression algorithms as well as advanced fatigue life evaluation procedures.

Fatigue Reliability Characterisation of Effective Strain Damage Model Using Extreme Value Distribution for Road Load Conditions

The aim of this paper is to characterise the fatigue reliability for various random strain loads under extreme value distribution while considering the cycle sequence effect condition in fatigue life prediction. The established strain-life models, i.e., Morrow and Smith-Watson-Topper, considered a mean stress effect and strain amplitude; nevertheless, it excluded the load sequence effect, which involves the fatigue crack closure that is subjected to overload or underload. A FESEM-EDX analysis is conducted to characterise the failure features that occurred on the leaf spring. A finite element is simulated to determine the critical region in order to obtain the strain load behaviour. In addition, the strain signal is captured experimentally at 500 Hz for 100 s under operating conditions for three different road loads based on the critical location obtained from the finite element analysis. The fatigue life correlation shows that the Pearson correlation coefficients are greater than 0.9, which indicates the effective strain damage model is linearly correlated with the strain-life models. The fatigue life data are modelled using extreme value distribution by considering the random strain loads as extreme data. The reliability rate for the fatigue life is reported to be more than 0.59 within the hazard rate range of 9.6 × 10-8 to 1.2 × 10-7 based on the mean cycle to the failure point. Hence, the effective strain damage model is proposed for a fatigue reliability assessment under extreme conditions with higher reliability and provides fatigue life prediction when subjected to cycle sequence effects.

Residual opening of fatigue cracks due to the wake of plasticity

The concept of a breathing crack and the clapping mechanism, when a crack periodically opens and closes subject to a cyclic or harmonic excitation, are widely utilised in studies concerning contact acoustic nonlinearity (CAN) and nonlinear phenomena. The investigation of these phenomena is particularly important for the development of reliable non-destructive fatigue crack detection techniques. The aim of the current contribution is to demonstrate the characteristic behaviour of a fatigue crack under excitation loading using a simple analytical model that accounts for the wake of plasticity behind the crack tip, which leads to the plasticity-induced fatigue crack closure phenomenon. It is believed that incorporation of these features and more realistic behaviour of fatigue cracks can help to improve the methods for the detection of nonlinear phenomena associated with CAN. The results of the present work indicate that ultrasonic inspections based on the CAN using a bilinear stiffness model may underestimate the length of the detected crack(s) due to the plasticity-induced crack closure phenomenon. Despite many modelling assumptions, the present work can serve as a starting point for the development of accurate models for different crack geometries and load histories. The latter may help to develop effective nondestructive techniques for detection and identification of fatigue cracks and prevention of fatigue failures.

Direct observation of three-dimensional short fatigue crack closure behavior in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography

Local 3D short fatigue crack closure behavior was investigated in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography (XMT). The results show that the inhomogeneous distribution of plasticity-induced and roughness-induced crack closure is caused by the variation of crack path morphologies. These crack path morphologies exhibited heterogeneous plastic deformation at the crack-tips due to the anisotropic nature of α grains and varying extents of crack tilting and twisting caused by the interaction of the crack front with α/α boundary or α/α + β interface. It was revealed via surrogate-based statistical analysis that the Schmid factor has the strongest effect on crack growth rate.

The influence of bending loading on surface fatigue crack growth life

Engineering structures often serve under complex fatigue loading and the most common of which are tension and bending loading, or their combination. Fatigue cracks always initiate as surface and other three dimensional (3D) cracks and most of the fatigue life of structures spends on 3D crack growth, but fatigue crack propagation curves of materials are widely obtained using through-the-thickness specimens. In this work, a 3D fracture mechanics based method is developed to evaluate surface crack propagation life under combined tension and bending loadings by considering the plasticity induced fatigue crack closure. The method is validated firstly against available experimental data of surface fatigue cracked plates under the combined loadings, and then applied to predict fatigue crack propagation life of surface cracked railway axles under rotating bending. It is shown that the method can make prediction of life within 0.80 to 1.08 of the test life, and the predicted shape evolution of surface crack agrees well with experimental observation. With a modification of short crack closure being made and an equivalent initial crack being assumed, the proposed method can be extended to fatigue life prediction. It is found that the influence of bending loading on fatigue life increasing for larger initial cracks, especially in the submillimeter range. This fracture mechanics based method can serve as a unified way to calculate fatigue crack growth life as well as fatigue life of practical structures under both tension and bending loadings by use of the material crack growth curves obtained with straight-through cracked standard specimens.

Research on the Evolution Law Physical Short Fatigue Crack and Tip Deformation Fields during Crack Closure Process of the Q&P Steel.

In this paper, a novel dual microscopic fatigue-crack and tip-deformation-fields measurement method based on a hybrid image-processing technique is proposed that was used to research the physical short fatigue crack (SFC) closure effect and the evolution law of the tip deformation fields of Quenching–Partitioning (Q&P) steel during the crack-closure process. The measurement problems are solved, such as the small SFC tip region, large deformation gradient, and strong material anisotropy. Microscopic crack and speckle images are acquired simultaneously on both sides of a compact tensile (CT) specimen of Q&P steel by dual microscopic cameras. A digital image processing (DIP) method is used to identify crack-growth morphology and measure crack length in Q&P steel, and the SFC growth rates are analyzed under different stress ratios. Microscopic digital image correlation (Micro-DIC) is used to analyze displacement fields at the crack tip of SFC and, combined with virtual extensometer technology, analyze the evolution law of crack closure and the evolution of crack-growing morphologies during the closure process under different lengths and stress ratios. Accordingly, the evolution of strain fields at the crack tip in one load cycle for different crack lengths and stress ratios during the SFC closure process is analyzed. The results show that the stress ratio affects the crack-closure behavior and crack growth rate of Q&P steel in the physical SFC crack-growing stage. The crack-closure effect has an obvious influence on the evolution process of displacement and strain fields at the crack tip. The evolution of short-fatigue-crack-tip morphology and strain field of Q&P steel conforms to the crack-closure law. The research results provide experimental and theoretical support for the further study of the SFC growth mechanism and fatigue life prediction of Q&P steel.

Towards three dimensional aspects of plasticity-induced crack closure: A finite element simulation

The mutual interactions between intrinsic (damage) and extrinsic (shielding) mechanisms are essential for understanding fatigue crack growth in ductile materials. In the latter case, plasticity-induced crack closure is the dominating retardation mechanism in the Paris regime. The transition between plane strain and plane stress states leads to a locally different fracture surface contact, which is difficult to access during experiments. In this work, plasticity-induced crack closure under constant amplitude loading of an AlCu4Mg (AA2024) aluminium sheet material is studied from a three-dimensional perspective. An elastic–plastic 3D finite element model of an M(T)160 specimen with a bilinear isotropic hardening model is used to study the evolution of plasticity during fatigue crack growth. Cyclic crack propagation is simulated with the releasing-constraint method.In the results, plasticity-induced crack closure is present up to a load ratio R = 0.3. Detailed investigations of the contact pressure distributions indicate a strong dependence on the stress intensity factors and the sheet thicknesses. The contact pressure distributions on the fracture surface can be divided into three characteristic regions. Near the plastic zone, plastic energy is still induced during the unloading process after the crack is already closed. However, the crack surface contact does not affect the shape of the plastic zone. Additionally, further plastic energy accumulates in the plastic wake region during crack closure within subsequent load cycles, described here as cyclic plastic wake. In summary, the finite element model can reproduce the major features of fatigue crack closure like Kop or Kcl and the three-dimensional and load-dependent evolution of plasticity-induced crack closure.

Crack closure of Ni-based superalloy 718 at high negative stress ratios

Fatigue crack growth damage mechanisms for nickel-based superalloys at high, negative stress ratios challenge classical crack tip shielding and closure concepts. Fatigue crack growth rate and crack closure for single-edge notch Ni-based superalloy 718 specimens at 100°C were evaluated for several stress ratios (0.5, 0.1, −1, and −2). Multiple methods were used to calculate the effective driving force for crack growth rates in region II (i.e., Paris or power law regime). The relationships between the effective driving forces, the experimentally-measured crack closure levels, and the fractographic evidence of damage mechanisms are discussed.

A finite element study on the influence of the hardening behavior on plasticity-induced fatigue crack closure

In this paper, the influence of the material hardening behavior on plasticity-induced fatigue crack closure is investigated for strain-controlled loading and fully plastic, large-scale yielding conditions by means of the finite element method. The strain amplitude and the strain ratio are varied for given Ramberg–Osgood material properties representing materials with different hardening behavior. The results show a pronounced influence of the hardening behavior on crack closure, while no significant effect is found from the considered strain amplitude and strain ratio. The effect of the hardening behavior on the crack opening stress cannot be described by existing crack opening stress equations.

Elasto-Plastic Fatigue Crack Growth Behavior of Extruded Mg Alloy with Deformation Anisotropy Due to Stress Ratio Fluctuation.

Fatigue crack growth (FCG) experiments were performed using a low-temperature extruded magnesium alloy AZ31 with texture. Under a constant maximum stress intensity factor (Kmax), the stress ratio R was changed from 0.1 to −1 during the fatigue crack growth process, and the FCG behavior before and after the R change was investigated. As a result, tensile twins were generated owing to the fatigue load on the compression side of R = −1, and the FCG velocity was accelerated. In addition, when the maximum compressive stress at R = −1 (|(σmin)R = −1|) exceeded the compressive yield strength of the material (σcy), the FCG velocity after R fluctuation greatly accelerated. On the other hand, under the condition |(σmin)R = −1| < σcy, the degree of acceleration of the FCG velocity due to R fluctuation was small. In either case, the degree of acceleration in the FCG increased as the Kmax value increased. The above FCG acceleration mechanism due to the R fluctuation was considered based on the observation of the deformation and twinning states of the fatigue crack tip, the fatigue crack closure behavior, and the cyclic stress–strain curve of the fatigue process. The FCG acceleration mechanism was as follows: First, the driving force of the FCG increased owing to the increase in crack opening displacement due to the generation of tensile twins. Second, the coalescence of the main crack and a plurality of microcracks were generated at the twin interface. The elasto-plastic FCG behavior after the stress ratio fluctuations is defined by the effective J-integral range ΔJeff.

Near-component testing of materials for cylinder heads to determine thermomechanical fatigue under superimposed high-frequency mechanical loads

Abstract Due to higher combustion chamber temperatures and pressures in efficient combustion engines, both the high-cycle and thermomechanical fatigue loads on service life-critical components, such as the cylinder head, are increasing. Material comparisons and analysis of damage behavior are very expensive and time-consuming using component tests. This study therefore develops a test method for cylinder head materials that takes into account the combined loading conditions from the above-mentioned loads and allows realistic temperature transients and gradients on near-component samples. The near-component cylinder head sample represents the failure-critical exhaust valve crosspiece and is tested in a test rig specially designed with the aid of conjugate heat transfer simulations. In the test rig, the sample is subjected to thermal stress by a hot gas burner and to mechanical stress by a high-frequency pulsator. Optical crack detection allows permanent observation of fatigue crack growth and crack closure during the test. Fractographic and metallo-graphic examinations of the fracture areas as well as analyses of the damage patterns show that loads close to engine operation can be set in this way and their influences on the damage can be monitored.