Prediction of short fatigue crack growth of Ti‐6Al‐4V

It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.     

The Prediction of Fatigue Crack Initiation Life in Spot Welds

Abstract: In this paper, strain‐based fatigue life prediction method has been used to estimate the fatigue crack initiation life of spot‐welded joints of Mild Steel JSC270D and Ultra‐High Strength Steel JSC980Y. To do so, the joints were simulated using three‐dimensional finite‐element (FE) models, and then nonlinear FE analysis was performed to obtain the local stress and strain ranges and finally, the Morrow equation was applied to estimate the crack initiation lives. The results have been compared with those obtained from experimental crack growth morphology. In addition, the difference between fatigue limits for smooth specimens and spot‐welded joints for mentioned materials has been briefly discussed. It has been shown that mean stress values in the Ultra‐High Strength Steel can significantly decrease the fatigue limit of spot‐welded joint because even at very low load level the stresses exceed the yield point at the root of nugget of spot‐welded joint, while the amount of mean stress in the Mild Steel for the same load level is much less than that of Ultra‐High Strength Steel. The comparison between numerical results of fatigue crack initiation lives and experimental data provided good agreement between numerical predictions and crack growth morphology observations. The results also shows that in some cases, depending on the joint type, the life spent in the nucleation phase can be an important part of the final failure lifetime.     

Analysis of fatigue crack growth in an attachment lug based on the weight function technique and the UniGrow fatigue crack growth model

A generalised step-by-step procedure for fatigue crack growth analysis of structural components subjected to variable amplitude loading spectra has been presented. The method has been illustrated by analysing fatigue growth of planar corner crack in an attachment lug made of Al7050-T7451 alloy.Stress intensity factors required for the fatigue crack growth analysis were calculated using the weight function method. In addition, so-called “load-shedding” effect was accounted for in order to determine appropriate magnitudes of the applied stress intensity factors. The rate of the load shedding was determined with the help of the finite element (FE) method by finding the amount of the load transferred through the cracked ligament. The UniGrow fatigue crack growth model, based on the material stress–strain behaviour near the crack tip, has been used to simulate the fatigue crack growth under two variable amplitude loading spectra. The comparison between theoretical predictions and experimental data proved the ability of the UniGrow model to correctly predict fatigue crack growth behaviour of two-dimensional planar cracks under complex stress field and subjected to arbitrary variable amplitude loading.     

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.     

Modellierung des Ermüdungsrisswachstums in Schweißverbindungen unter Berücksichtigung von Schweißeigenspannungen

The present paper contains research results determined within the framework of a project called IBESS (?Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“) by the Materials Mechanics Group of the Technische Universität Darmstadt [1]. Aim is to calculate the fatigue life of welded joints by taking into account the effect of residual stresses and the influence of the weld toe geometry. Here, the fatigue life is regarded as period of short fatigue crack growth. Two and three dimensional finite element models, with cracks as initial defects, are constructed for this purpose. Fatigue crack growth analyses are performed by using the node release technique together with the finite element program ABAQUS. The welding residual stresses as well as the plasticity induced crack closure effects are considered. Structural calculations are performed in order to introduce residual stress fields in finite element models. The calculated compressive residual stress field matches the measured one especially in the weld notch area. The effective cyclic J‐integral (ΔJ_eff) is used as crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a Python code was written for the determination of ΔJ_eff at every crack length phase. The calculated fatigue lives were compared with experimental data and a good accordance between both results was achieved. The impact of welding residual stresses on ΔJ_eff as well as on the fatigue life during short crack growth was investigated. As expected, results revealed that at lower stress amplitude, a compressive residual stress field is favorable to the fatigue life, whilst a tensile residual stress field is unfavorable. The influence of residual stresses can be neglected only for large load amplitudes.     

Fatigue crack initiation and growth in AlMg4.5Mn butt weldments

The fatigue life of full-penetration and partial-penetration 5 and 25  mm thickness AlMg4.5Mn (AA5083) aluminium alloy butt weldments was investigated under ( R = 0 and R = − 1) constant amplitude loading. The fatigue lives of the tested specimens were predicted using an analytical model which estimated both the crack initiation and crack growth portions of the total fatigue life. The fatigue life of partial-penetration weldments was found to be substantially less than that of full-penetration weldments because of the greater stress concentrations of the incomplete joint penetration and the consequent absence of a substantial crack-initiation life period. Tensile mean stresses ( R = 0 versus R = − 1-test conditions) markedly reduced the fatigue life of the weldments studied and greatly diminished the duration of the fatigue crack growth period. The extra material provided by the weld reinforcement noticeably increased the fatigue life of the partial-penetration weldments. Weld angular distortion-induced bending stresses greatly affected the smaller thickness (5  mm) full-penetration weldments offsetting the fatigue strength bonus anticipated for small-size weldments. Except for the predictions for R = − 1 full-penetration weldments at long life, which the analytical model underestimated, the agreement between experiment and analytical prediction was within a factor of 2, that is, as good as can be generally expected.     

Fatigue life prediction of offshore tubular joints using fracture mechanics

ABSTRACT Fatigue crack growth calculations were performed on offshore tubular joints using the Paris crack growth law. The stress intensity factors required for such calculations were obtained from T‐butt solutions previously proposed by the authors. The applicability of the solutions to tubular joints was first demonstrated by comparing the fatigue life of a base case with that obtained from a mean S–N curve, and the influence on fatigue life of various factors including load shedding, the size of initial defects, weld geometry, etc. was investigated. The solutions were then used to predict the lives of tubular T‐joints from an experimental database. The results show that the solutions underestimate the fatigue life; this underestimation was shown to be primarily due to ignoring the combined effects of load shedding and the intersection stress distribution. In general, however, the trends in the predicted fatigue lives with joint geometry and other details were seen to be superior to predictions from the S–N approach, with the solutions significantly reducing the dependency on loading mode exhibited by the test data.     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.     

Short‐crack thresholds and propagation in an AISI 4340 steel under the effect of SP residual stresses

The effect of residual stresses induced by shot‐peening in a high‐strength AISI 4340 steel has been studied with the purpose of deriving a consistent fatigue model incorporating the results of fatigue crack growth experiments in the threshold region for a broad range of load ratio (R‐ratio ranging from ?2.5 to 0.7), and the effect of short cracks by means of a modified El‐Haddad model. The proposed model, taking into account the effect of crack closure and being capable to assess the conditions for fatigue propagation of short cracks partially embedded in the shot‐peened surface layer, was validated against constant amplitude fatigue experiments conducted in the endurance strength region, ie, for fatigue lives up to 10⁷ cycles, with micronotched specimens in the presence of shot‐peening residual stresses. The proposed model was also validated by comparing the results of fatigue crack propagation simulations with fatigue crack growth experiments under variable amplitude loading, experimentally reproducing the combined effect of service fatigue loads and shot‐peening residual stresses.     

Crack growth predictions in a complex helicopter component under spectrum loading

Fatigue crack growth predictions have been made on a helicopter round‐robin crack configuration. The crack configuration was a small corner defect at the edge of a large central hole in a flanged plate made of 7010 aluminium alloy and the component was subjected to a simulated helicopter spectrum loading. The crack growth rate data and the stress‐intensity factor (K) solution for the crack configuration were provided in the round‐robin. The FASTRAN life‐prediction code was used to predict fatigue crack growth under various load histories on the aluminium alloy, such as Rotorix and Asterix, on both compact tension C(T) specimens and the complex crack configuration. A BEASY three‐dimensional stress‐intensity factor solution for the round‐robin problem was also provided for this paper and is compared with the original K solution. Comparisons are made between measured and predicted fatigue crack growth lives for both crack configurations. The predicted lives for the C(T) specimens were 15–30% longer than the measured lives; and crack growth in the round‐robin configuration agreed very well in the early stages of crack growth, but the life was 30% short of the test results at the final crack length.     

SMALL CRACK GROWTH AND FATIGUE LIFE PREDICTIONS FOR HIGH-STRENGTH ALUMINIUM ALLOYS: PART I—EXPERIMENTAL AND FRACTURE MECHANICS ANALYSIS

The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests were conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plasticity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions. Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminium alloys was developed and demonstrated using the crack-closure model.     

Investigation of small fatigue crack initiation and growth behaviour of nickel base superalloy GH4169

Surface replication method was utilized to monitor the small fatigue crack initiation and growth process of single‐edge‐notch tension specimens fabricated by nickel base superalloy GH4169. Three different stress levels were selected. Results showed that small fatigue cracks of nickel base superalloy GH4169 initiated from grain boundaries or surface inclusions. The small fatigue crack initiation and growth stages took up about 80–90% of the total fatigue life. Multiple major cracks were observed in the notch root, and specimen with more major cracks seemed to have smaller fatigue life under the same test conditions. At the early growth stage, small crack behaviour might be strongly influenced by microstructures; thus, the crack growth rates had high fluctuations. However, the stress level effect on the small fatigue crack growth rates was not distinguishable for the three different stress levels. And no clear differences were found among the crack initiation lives by using replication technique.     

Variable amplitude fatigue of spot welds

A new method for fatigue life prediction of spot welds subjected to variable amplitude loads is proposed. The method is based on the concept of crack closure and is experimentally verified with three different specimens and four different load signals with variable amplitude. Experimental fatigue lives were found to be within a factor of three from the predicted lives. To start with, the stress intensity factor history at the spot weld is calculated with a finite element analysis. Then, crack closure is taken into account: the crack opening stress intensity factor, which is assumed to be constant, is determined from the maximum and minimum in the history. All stress intensities lower than the crack opening level are filtered from the calculated history. The filtered history is then analysed with rain flow count. Finally, fatigue life is predicted with the Palmgren–Miner cumulative damage rule together with an effective (closure‐free) curve for spot welds. In addition, single overload tests were carried out to investigate the assumption of a constant crack opening stress.     

Fatigue life of cold‐expanded fastener holes with interference‐fit fasteners at short edge margins

The fatigue life of 7075‐T6 aluminium specimens with countersunk fastener holes with cold expansion and interference‐fit fasteners with short edge margins was studied. The study was performed experimentally and through finite element analysis. The experiments measured the total fatigue life and crack growth. The results from the finite element analysis consisted of tangential residual stress profiles, which were combined with applied cyclic stresses for fatigue analysis. The experiments showed that the fatigue life improved with interference‐fit fasteners and cold expansion at all edge margins. The fatigue life also increased with increasing edge margin. The finite element results were used to make fatigue life predictions that corresponded reasonably well with the experimental results.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

Effect of notches on creep–fatigue behavior of a P/M nickel-based superalloy

A study was performed to determine and model the effect of high temperature dwells on notch low cycle fatigue (NLCF) and notch stress rupture behavior of a fine grain LSHR powder metallurgy (P/M) nickel-based superalloy. It was shown that a 90 second (s) dwell applied at the minimum stress (“min dwell”) was considerably more detrimental to the NLCF lives than similar dwell applied at the maximum stress (“max dwell”). The short min dwell NLCF lives were shown to be caused by growth of small oxide blisters which caused preferential cracking when coupled with high concentrated notch root stresses. The cyclic max dwell notch tests failed mostly by creep accumulation, not by fatigue, with the crack origin shifting internally to a substantial distance away from the notch root. The classical von Mises plastic flow model was unable to match the experimental results while the hydrostatic stress profile generated using the Drucker–Prager plasticity flow model was consistent with the experimental findings. The max dwell NLCF and notch stress rupture tests exhibited substantial creep notch strengthening. The triaxial Bridgman effective stress parameter was able to account, with some limitations, for the notch strengthening by collapsing the notch and uniform gage geometry test data into a singular grouping.     

Fatigue crack initiation assessment of welded joints accounting for residual stress

The impact of residual stresses on the fatigue crack initiation life of welded joints is evaluated by the finite element method. The residual stresses of nonload‐carrying cruciform joints, induced by welding and ultrasonic impact treatment, are modelled by initial stresses, using the linear superposition principle. An alternative approach of using modified stress‐strain curves in the highly stressed zone is also proposed to account for the residual stress effect on the local stress‐strain history. An evaluation of the fatigue crack initiation life of welded joints based on the local strain approach is carried out. The predicted results show the effect of residual stresses and agree well with published experimental results of as‐welded and ultrasonic impact treated specimens, demonstrating the applicability of both approaches. The proposed approaches may provide effective tools to evaluate the residual stress effect on the fatigue crack initiation life of welded joints.     

Driving forces for localized corrosion‐to‐fatigue crack transition in Al–Zn–Mg–Cu

Research on fatigue crack formation from a corroded 7075‐T651 surface provides insight into the governing mechanical driving forces at microstructure‐scale lengths that are intermediate between safe life and damage tolerant feature sizes. Crack surface marker‐bands accurately quantify cycles (N_i) to form a 10–20 μm fatigue crack emanating from both an isolated pit perimeter and EXCO corroded surface. The N_i decreases with increasing‐applied stress. Fatigue crack formation involves a complex interaction of elastic stress concentration due to three‐dimensional pit macro‐topography coupled with local micro‐topographic plastic strain concentration, further enhanced by microstructure (particularly sub‐surface constituents). These driving force interactions lead to high variability in cycles to form a fatigue crack, but from an engineering perspective, a broadly corroded surface should contain an extreme group of features that are likely to drive the portion of life to form a crack to near 0. At low‐applied stresses, crack formation can constitute a significant portion of life, which is predicted by coupling macro‐pit and micro‐feature elastic–plastic stress/strain concentrations from finite element analysis with empirical low‐cycle fatigue life models. The presented experimental results provide a foundation to validate next‐generation crack formation models and prognosis methods.     

Numerical predictions and experimental measurements of residual stresses in fatigue crack growth specimens

Controlling macro residual stress fields in a material while preserving a desired microstructure is often a challenging proposition. Processing techniques which induce or reduce residual stresses often also alter microstructural characteristics of the material through thermo-mechanical processes. A novel mechanical technique able to generate controlled residual stresses was developed. The method is based on a pin compression approach, and was used to produce well-controlled magnitudes and distributions of residual stresses in rectangular coupons and compact tension specimens typically used in fatigue crack growth testing. Residual stresses created through this method were first computationally modeled with finite element analysis, and then experimentally reproduced with various levels of pin compression. The magnitudes and distributions of residual stresses in experimental specimens were independently assessed with fracture mechanics methods and good correspondence was found between residual stresses produced using the pin compression and processing techniques. Fatigue crack growth data generated from specimens with low residual stresses, high residual stresses resulting from processing, and high residual stresses introduced through the new pin compression technique were compared and validated. The developed method is proposed to facilitate the acquisition and analysis of fatigue crack growth data generated in residual stresses, validate residual stress corrective models, and verify fatigue crack growth simulations and life predictions in the presence of residual stresses.     

Effects of Shot‐Peening on Fretting Fatigue Crack Growth Behavior in Ti‐6Al‐4V

Abstract: The effects of shot‐peening on fretting fatigue crack growth behaviour in titanium alloy, Ti‐6A1‐4V were investigated. Three shot‐peening intensities: 4A, 7A and 10A were considered. The analysis involved the fracture mechanics and finite element sub‐modelling technique to estimate crack propagation lives. These computations were supplemented with the experimentally measured total fretting fatigue lives of laboratory specimens to assess the crack initiation lives. Shot‐peening has significant effect on the initiation/propagation phases of fretting fatigue cracks; however this effect depends upon the shot‐peening intensity. The ratio of crack initiation and total life increased while the ratio of the crack propagation and total life decreased with an increase of shot‐peening intensity. Effects of residual compressive stress from shot‐peening on the crack growth behaviour were also investigated. The fretting fatigue crack propagation component of the total life with relaxation increased in comparison to its counterpart without relaxation in each shot‐peened intensity case while the initiation component decreased. Improvement in the fretting fatigue life from the shot‐peening and also with an increase in the shot‐peening intensity appears to be not always due to increase in the crack initiation resistance from shot‐peened induced residual compressive stress.