Mixed-mode fatigue crack growth of thin aluminium panels with single-side repair using experimental and numerical methods

In this paper, experimental and numerical fatigue crack growth of thin aluminium panels containing a central inclined crack of 45° with single-side glass/epoxy composite patch are performed. Effects of patch lay-up configuration on the restarting crack growth (crack re-initiation) life and crack growth rate of the repaired panels are investigated. The obtained experimental results are compared with those predicted using finite element analysis based on both mid-plane and unpatched surface fracture parameters. In the finite elements analyses, it is assumed that the crack-front remains perpendicular to the panel's surfaces during its propagation. It is shown that the finite element crack re-initiation and propagation lives predictions using the unpatched surface results are too conservative. However, the finite element mid-plane results lead to a non-conservative life prediction. It is experimentally shown that, the most effective patch lay-up configurations to retard the crack growth of the repaired panels is [−45/+45]₂; however, the most life extension including the crack propagation cycles belongs to the patch lay-up of [90₄]. It is also shown that using the asymmetric patch lay-up configuration similar to [90₂/0₂] with a proper bonding process may lead to a very slow crack growth rate, even slower than the patch lay-up of [90₄].     

Numerical and experimental fatigue crack growth analysis in mode-I for repaired aluminum panels using composite material

Crack-front shape is an important parameter influencing the stress intensity factor and crack propagation rate in asymmetric repaired panels. In this study, the numerical and experimental fatigue crack growth behaviour of centrally cracked aluminum panels in mode-I condition repaired with single-side composite patches are investigated. It is shown that the crack growths non-uniformly from its initial location through the thickness of a single-side repaired panel. There is a good agreement between the propagated crack-front shapes obtained from finite element analysis with those obtained from the experiments for various repaired panels with different patch thicknesses. Furthermore, effects of plate and patch thickness on the crack growth life of the repaired panels are investigated. The experimental results show that the crack growth life of thin panels may increase up to 236% using a 16 layers patch. However, for thick panels, the life may extend about 21–35% using a 4 layers patch. Implementing of 8 and 16 layers patches has not a significant effect on the life extension of thick panels with respect to the 4 layers patch life.     

Experimental investigations on fatigue crack growth of repaired thick aluminium panels in mixed-mode conditions

In this paper, experimental fatigue crack growth of thick aluminium panels containing a central inclined crack of 45° repaired with single-side glass/epoxy composite patch are performed. It is shown that, the technique of single-side repair using glass/epoxy composite patch is effective in the crack growth life extension of the thick panels in mixed-mode conditions. It is also shown that the crack-front of the propagated cracks of the repaired panels has a curvilinear shape which is the effect of the existed out-of-plane bending due to the asymmetry conditions in the single-side repaired panels. It is indicated that the crack propagation path at patched surface is different from the un-patched surface of the panels. In the primary stages of the crack growth, the crack surfaces through the thickness, in the vicinity of the mid-plane propagate without surface twisting. There are considerable differences between the obtained crack growth path at patched and un-patched surfaces of the panels which mean that the crack propagation surfaces have three-dimensional patterns. Using the various thin patch lay-ups has minor effects on the crack re-initiation life of the repaired thick panels. It is shown that using various four layers patch lay-up configurations, the crack propagation life of the cracked panels may increase by the order of 30–85%. The most fatigue crack growth life extension belongs to the repaired panel with the patch lay-up of [90]₄.     

Experimental fatigue crack growth and crack-front shape analysis of asymmetric repaired aluminium panels with glass/epoxy composite patches

In this study, we investigate the experimental fatigue crack-growth behaviour of centrally cracked aluminium panels in mode-I condition which have been repaired with single-side composite patches. It shows that the crack growths non-uniformly from its initial location through the thickness of the single-side repaired panels. The propagated crack-front shapes are preformed for various repaired panels with different patch thicknesses. It is shown that there are considerable differences between the crack-front shapes obtained for thin repaired panels with various patch thicknesses. However, the crack-front shapes of thick repaired panels are not significantly changed with various patch thicknesses. Furthermore, effects of patch thickness on the crack growth life of the repaired panels are investigated for two typical thin and thick panel thicknesses. It shows that the crack growth life of thin panels may increase up to 236% using a 16 layers patch. However, for thick panels, the life may extended about 21–35% using a 4 layers patch, and implementing 8 and 16 layers patches has not a significant effect on the life extension with respect to the 4 layers patch life.     

Modeling and validation of composite patch repair to cracked thick and thin metallic panels

A two-dimensional finite element analysis is presented to predict crack growth behavior of cracked panels repaired with bonded composite patch. Fatigue experiments were conducted with precracked aluminum specimens of two thicknesses (1 and 6.35 mm), with and without debond, and repaired asymmetrically. Fatigue lives of thick and thin repaired panels extended four and ten times relative to unrepaired cases, respectively. The predicted fatigue crack growth rates were in agreement with experimental values at the unpatched face but not at the patched face. Thus, the present analysis provides a conservative assessment of durability and damage tolerance of repaired thin and thick panels.     

Finite element fatigue propagation of induced cracks by stiffeners in repaired panels with composite patches

Fatigue crack growth analyses of aluminum panels with stiffeners repaired by composite patches have been rarely investigated. Generally, cracks may occur around the rivets which are capable to propagate under cyclic loadings. A composite patch can be used to stop or retard the crack growth rate. In this investigation, finite element method is used for the crack propagation analyses of stiffened aluminum panels repaired with composite patches. In these analyses, the crack-front can propagate in 3-D general mixed-mode conditions. The incremental 3-D crack growth of the repaired panels is automatically handled by a developed ANSYS Parametric Design Language (APDL) code. Effects of rivets distances and their diameters on the crack growth life of repaired panels are investigated. Moreover, the obtained crack-front shapes at various crack growth steps, crack trajectories, and life of the unrepaired and repaired panels with various glass/epoxy patch lay-ups and various patch thicknesses are discussed.     

Crack propagation behavior under creep conditions

The creep crack propagation behavior of a Cr–Mo–V rotor steel has been investigated at 538°C using time-dependent fracture mechanics concepts. The creep crack propagation lives and the creep deflection rates of double-edge-notched (DEN) specimens were estimated using previous data from compact-type specimens. The predicted crack growth lives and deflection rates compared favorably with the experimental data. When plotted as a function of the C _t parameter, the experimentally determined creep crack propagation rates of DEN specimens were found to be in agreement with those of compact and center-crack-tension specimens. These results provide further experimental verification for the validity of the C _t parameter for characterizing creep crack growth behavior. Some discrepancies between the predicted and the observed behavior are attributed to primary creep deformation behavior which was not considered in estimating the value of C _t .     

Multiscale fatigue crack growth modelling for welded stiffened panels

The influence of welding residual stresses in stiffened panels on effective stress intensity factor (SIF) values and fatigue crack growth rate is studied in this paper. Interpretation of relevant effects on different length scales such as dislocation appearance and microstructural crack nucleation and propagation is taken into account using molecular dynamics simulations as well as a Tanaka–Mura approach for the analysis of the problem. Mode I SIFs, K_I, were calculated by the finite element method using shell elements and the crack tip displacement extrapolation technique. The total SIF value, K_tot, is derived by a part due to the applied load, K_appl, and by a part due to welding residual stresses, K_res. Fatigue crack propagation simulations based on power law models showed that high tensile residual stresses in the vicinity of a stiffener significantly increase the crack growth rate, which is in good agreement with experimental results.     

A procedure for estimating the total life in fretting fatigue

ABSTRACT This paper proposes a procedure for estimating the total fatigue life in fretting fatigue. It separately analyses the fatigue crack initiation and propagation lives. The correlation between crack initiation and propagation is made considering a non‐arbitrary crack initiation length provided by the model. The number of cycles to initiate a crack is obtained from the stress distribution beneath the contact zone and a multiaxial fatigue crack initiation criterion. The propagation of the crack is considered using different fatigue crack propagation laws, including some modifications in order to take the short crack growth into account. The results obtained by this method are compared with the fatigue lives obtained in various fretting fatigue tests under spherical contact with 7075‐T6 aluminium alloy.     

Prediction of crack opening stress levels for 1045 quenched and tempered steel under service loading spectra

The opening stresses of a crack emanating from an edge notch in a 1045 quenched and tempered steel specimen were measured under two different Society of Automotive Engineers (SAE) standard service load histories having different average mean stress levels. The two spectra are the Grapple Skidder history (GSH), which has a positive average mean stress, and the Log Skidder history (LSH), which has a zero average mean stress. To capture the behaviour of the crack opening stress in the material, the crack opening stress levels were measured at 900X using an optical video microscope, at frequent intervals for each set of histories scaled to two different maximum stress ranges.A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behaviour of the steel specimens for the given load spectra and stress ranges. Crack growth analysis was based on an effective strain‐based intensity factor, a crack growth rate curve obtained during closure‐free loading cycles and a local notch strain calculation based on Neuber's rule.The crack opening stress (S_op) was modelled and the model was implemented in a fatigue notch model, and the fatigue lives of the specimens under the two different spectra scaled to several maximum stress levels were estimated. The average measured crack opening stresses were between 6 and 12% of the average calculated crack opening stresses. In the interest of simplifying the use of S_op in design, the average S_op was correlated with the frequency of occurrence of the cycle reducing the S_op to the average crack opening stress level. The use of an S_op level corresponding to the cycle causing a reduction in S_op to a level reached once per 10 cycles gave a conservative estimate of average crack opening stress for all the histories.     

Generation and analysis of FCG data using a single specimen and Kmax–ΔK testing matrix

A custom method to generate fatigue crack growth (FCG) data requires testing of multiple specimens at different load ratios, R, and the application of a load shedding procedure from pre-cracking level to threshold. In this paper, a novel method of testing has been investigated which utilizing a single specimen and a testing matrix in terms of K_max and ΔK values corresponding to predetermined R-ratios for which FCG data are recorded. Automatic K-controlled tests on 2324-T39 Al alloy were conducted using both increasing and decreasing ΔK procedures while K_max was kept constant. Results show that the increasing ΔK procedure gives less scatter than decreasing ΔK procedure. Also, fatigue crack growth curves near the threshold region obtained from increasing ΔK are above the curves obtained from decreasing ΔK procedure. These differences are explained by means of interaction between cyclic plastic zones and their effect on fatigue damage. The procedure with increasing ΔK demonstrated minimal interaction effects and hence it is recommended for efficient FCG data generation. The proposed procedure reduces testing time, the overall scatter associated with multiple samples and eliminates possible uncertainty linked to the load shedding procedure and its effects on threshold.     

A numerical study of fatigue crack closure induced by plasticity

Crack closure delays the intrinsic mechanisms responsible for crack growth, therefore, it must be considered in fatigue crack growth modelling. The objective of this work is to develop a numerical procedure to predict crack closure induced by plasticity. First the crack closure was experimentally measured on M(T) 6082‐T6 aluminium alloy specimens of 3 mm thickness. A pin microgauge was used with the compliance technique. Then different parameters of the numerical procedure were analysed, namely the finite element mesh and the crack propagation scheme. The size of crack‐tip elements has an important influence and it is recommended to be of the same order of cyclic plastic zone. Crack‐opening levels only 10% lower than experimental results were obtained considering kinematic hardening and two load cycles in each increment.     

Modelling strength of cracked friction stir welded panels by means of the crack tip opening angle (CTOA)

The objective of this work is to assess numerically the strength of cracked flat friction stir welded panels for aerospace applications, on the basis of simple experiments made on small coupons of material (Kahn tear test). The transferability from a geometry to another, in particular the results obtained from Kahn tear tests to the prediction of the R-curve of cracked FSW M(T) panels is performed using the crack tip opening angle (CTOA). The Kahn tear test is reproduced first by means of finite element analysis using a debond procedure based on the attainment of a critical CTOA as a function of crack length. The CTOA extracted from Kahn tests is then used to simulate the R-curve of M(T) panels of different widths. The material considered here is a 6013-T 6 aluminium alloy. Two series of values of CTOA are determined: (i) considering the material as homogeneous with strength equal to that of the parent material; (ii) introducing different strengths locally for the weld TMAZ/HAZ and nugget.     

Very high cycle fatigue properties of high‐strength spring steel 60SiCrV7

The very high cycle fatigue properties of spring steel 60SiCrV7 for automotive suspension system with different hydrogen contents were studied by using ultrasonic fatigue testing and fatigue crack growth testing. The results show that the S–N curves exhibit continuous drop of fatigue lives and no obvious horizontal line exists. Similar fracture surface features were observed for all the specimens that failed mainly from internal inclusions with surrounding granular bright facet (GBF). Fatigue strength decreases remarkably with increasing hydrogen content. The applied stress intensity factor range at the periphery of GBF ΔK_GBF is approximately proportional to 1/3 power of the square of GBF area. The average values of ΔK_GBF for uncharged specimens are close to crack growth threshold ΔK_th, which indicates that ΔK_GBF could be regarded as the threshold value governing the beginning of stable fatigue crack propagation. The increase of hydrogen content tends to reduce ΔK_GBF.     

Further developments in <Emphasis Type="Italic">J</Emphasis> evaluation procedure for growing cracks based on LLD and CMOD data

Laboratory testing of fracture specimens to measure resistance curves (J − Δa) have focused primarily on the unloading compliance method using a single specimen. Current estimation procedures (which form the basis of ASTM E1820 standard) employ load line displacement (LLD) records to measure fracture toughness resistance data incorporating a crack growth correction for J. An alternative method which potentially simplifies the test procedure involves the use of crack mouth opening displacement (CMOD) to determine both crack growth and J. However, while the J-correction for crack growth effects adopted by ASTM standard holds true for resistance curves measured using load line displacement (LLD) data, it becomes unsuitable for J-resistance measurements based upon the specimen response defined in terms of load-crack mouth opening displacement (CMOD). Consequently, direct application of the evaluation procedure for J derived from LLD records in laboratory measurements of resistance curves using CMOD data becomes questionable. This study provides further developments of the evaluation procedure for J in cracked bodies that experience ductile crack growth based upon the eta-method and CMOD data. The introduction of a constant relationship between the plastic components of LLD (Δ _p ) and CMOD (V _p ) drives the development of a convenient crack growth correction for J with increased loading when using laboratory measurements of P-CMOD data. The methodology broadens the applicability of current standards adopting the unloading compliance technique in laboratory measurements of fracture toughness resistance data (J resistance curves). The developed J evaluation formulation for growing cracks based on CMOD data provides a viable and simpler test technique to measure crack growth resistance data for ductile materials.     

Fatigue life and crack path predictions in generic 2D structural components

This paper proposes a reliable and cost-effective two-phase methodology to predict crack propagation life in generic two-dimensional (2D) structural components. First, the usually curved fatigue crack path and its stress-intensity factors are calculated at small crack increments in a specialized finite-element software, using automatic remeshing algorithms, special crack tip elements and appropriate crack increment criteria. Then, the computed stress-intensity factors are transferred to a powerful general-purpose fatigue-design program, which has been designed to predict both initiation and propagation fatigue lives by means of classical design methods. Particularly, its crack propagation module accepts any K_I expression and any crack growth rate model, considering sequence effects such as overload-induced crack retardation to deal with 1D and 2D crack propagation under variable amplitude loading. Non-trivial application examples compare the numerical simulation results with those measured in physical experiments.     

Progressive edge cracked aluminium plate repaired with adhesively bonded composite patch under full width disbond

In this study, the crack growth behaviour of an aluminium plate cracked at the tip and repaired with a bonded boron/epoxy composite patch in the case of full-width disbond was investigated. This effect is the imperfection which could result during the bonded patch of the repaired structure. Disbonds of various sizes and situated at different positions with respect to the crack tip as well as the effect of adhesive and patch thickness on repair performance were examined. An analysis procedure involving the efficient finite element modelling applied to cracked plate, adhesive and composite patch was used to compute the stress intensity factors. The crack growth rate is dominated by the stress intensity factor near the location and size of the pre-existing disbonds. The cracked plate and disbond propagation result in an increase in the patch deformation. The patch does not have an influence on the crack growth when the ratio 2a/d_R exceeds 0.8.     

The effect of T-stress on plane strain dynamic crack growth in elastic–plastic materials

In this paper, the effects of T‐stress on steady, dynamic crack growth in an elastic–plastic material are examined using a modified boundary layer formulation. The analyses are carried out under mode I, plane strain conditions by employing a special finite element procedure based on moving crack tip coordinates. The material is assumed to obey the J₂ flow theory of plasticity with isotropic power law hardening. The results show that the crack opening profile as well as the opening stress at a finite distance from the tip are strongly affected by the magnitude and sign of the T‐stress at any given crack speed. Further, it is found that the fracture toughness predicted by the analyses enhances significantly with negative T‐stress for both ductile and cleavage mode of crack growth.     

A fracture mechanics analysis on the fatigue behaviour of cruciform joints of duplex stainless steel

The paper presents the results of an experimental and numerical study on the fatigue behaviour of cruciform load carrying joints made from the duplex stainless steel and failing from the weld root through the weld metal. Fatigue crack growth (FCG) data, obtained in specimens of the weld metal, are presented, as well as threshold data, both obtained for R= 0 and 0.5. The influence of stress ratio is discussed, and the FCGR results are compared with data for low carbon structural steels. S–N data were obtained in the joints, both for R= 0.05 and 0.5, and the fatigue cracking mechanisms were analysed in detail with the SEM. It was found that the cracks propagated very early in the lifetime of the joints, under mixed mode conditions (I + II), but the mode I component was found to be predominant over mode II. The geometries of the cracks were defined in detail from measurements taken in the fracture surfaces. A 2D FE analysis was carried out for the mixed mode inclined cracks obtained at the weld root, and the J‐integral formulations were obtained as a function of crack length and crack propagation angle. The values of the crack propagation angle, θ_i, were obtained for the J_max conditions, and it was found that, in the fatigue tests, the cracks propagated in directions very close to the predicted directions of maximum J. K_I and K_II formulations were obtained, and the K_I data were compared with the formulations given in the PD6493 (BS7910) document, and some differences were found. A more general formulation for K under mixed mode conditions was derived. The derived K solutions were applied to predict the fatigue lives of the joints under crack propagation, and an extremely good agreement was found with the experimental results obtained in the fatigue tests.