The response of aluminium/GLARE hybrid materials to impact and to in-plane fatigue

Fibre metal laminates (FMLs), such as glass reinforced aluminium (GLARE), are a family of materials with excellent damage tolerance and impact resistance properties. This paper presents an evaluation of the low velocity impact behaviour and the post-impact fatigue behaviour of GLARE laminate adhesively bonded to a high strength aluminium alloy substrate as a fatigue crack retarder. The damage initiation, damage progression and failure modes under impact and fatigue loading were examined and characterised using an ultrasonic phased array C-scan together with metallography and scanning electron microscopy (SEM). After impact on the substrate, internal damage to the GLARE bonded on the opposite side of the substrate occurred in the form of fibre and matrix cracking. No delamination was detected at the GLARE/substrate bond. Before impact the bonded GLARE strap caused reductions in substrate fatigue crack growth rate of up to a factor of 5. After impact the retardation was a factor of 2. The results are discussed in terms of changes to the GLARE stiffness promoted by the impact damage.     

A comparison of fatigue crack growth performance of two aerospace grade aluminium alloys reinforced with bonded crack retarders

To improve the fail‐safety performance of integral metallic structures, the bonded crack retarder concept has been developed in recent years. This paper presents an experimental investigation on the effectiveness of bonded crack retarder on fatigue crack growth life in two aerospace aluminium alloys: 2624‐T351 and 7085‐T7651. M(T) specimens bonded with a pair of straps made of GLARE fibre‐metal laminate were tested under the constant amplitude load. Although the bonded crack retarders increased the crack growth life in both alloys, the magnitude of life improvement is very different between them. Compared to unreinforced specimens, application of crack retarders has resulted in 90% increase in fatigue life in AA7085, but only 27% increase in AA2624. The significant difference in fatigue life improvement is owing to the material's intrinsic fatigue crack growth rate property, ie, the Paris law constants C and n. Value of n for AA7085 is 1.8 times higher than that for AA2624. Therefore, AA7085 is much more sensitive to reductions in the effective stress intensity factor brought by the crack retarders, hence better life improvement.     

Residual stresses in structures reinforced with adhesively bonded straps designed to retard fatigue crack growth

The residual stresses induced when adhesively bonding patches to a 7085 alloy SENT (side edge notched tension) specimen in order to produce fatigue crack growth retardation have been investigated. Knowledge of the induced residual stresses is important as they affect the beneficial bridging effect of the strap. The strap materials studied were: Titanium, GLARE (fibre metal laminate), GFRP (glass fibre reinforced polymer) and CFRP (carbon fibre reinforced polymer). The residual stresses were measured using neutron diffraction and are compared with those predicated by FE (finite element) simulation. The measured and modelled residual stresses were in reasonable correlation. Tensile residual stresses were found close to the strap, whereas small compressive residual stresses were found on the un-bonded side. The residual stresses were induced due to the mismatch in the coefficient of thermal expansion (ΔCTE) between the SENT and the strap. The magnitude of the stresses induced by the bonded crack retarders depend both on the ΔCTE and the stiffness ratio between the reinforced structure and the strap. For the straps studied, the magnitude of the peak residual stresses found were in the following descending order: CFRP, titanium, GFRP and GLARE.     

Distortion and residual stresses in structures reinforced with titanium straps for improved damage tolerance

Distortion and residual stresses induced during the manufacturing process of bonded crack retarders have been investigated. Titanium alloy straps were adhesively bonded to an aluminium alloy SENT specimen to promote fatigue crack growth retardation. The effect of three different strap dimensions was investigated. The spring-back of a component when released from the autoclave and the residual stresses are important factors to take into account when designing a selective reinforcement, as this may alter the local aerodynamic characteristics and reduce the crack bridging effect of the strap. The principal problem with residual stresses is that the tensile nature of the residual stresses in the primary aluminium structure has a negative impact on the crack initiation and crack propagation behaviour in the aluminium. The residual stresses were measured with neutron diffraction and the distortion of the specimens was measured with a contour measurement machine. The bonding process was simulated with a three-dimensional FE model. The residual stresses were found to be tensile close to the strap and slightly compressive on the un-bonded side. Both the distortion and the residual stresses increased with the thickness and the width of the strap. Very good agreement between the measured stresses and the measured distortion and the FE simulation was found.     

Effect of Temperature on the Residual Stresses in an Integral Structure with a Crack‐Retarding Patch

Abstract: The application of bonded ‘straps’ has been proposed for extending the fatigue life of aircraft structures, particularly for integral structures with low inherent crack‐retarding capability. A potential disadvantage in the use of bonded crack retarders is the difference between the coefficients of thermal expansion between the strap and substrate materials, which causes residual stresses when the temperature deviates from the curing or assembly temperature. The evolution of these stresses in flight is important to take into account to accurately assess the fatigue crack growth behaviour of the structure. In this work, the residual strains in an aluminium compact‐tension sample adhesively reinforced with a titanium strap have been measured with neutron diffraction and modelled with a finite element approach at room temperature and at ?50 °C. It was found that a linear elastic approach could accurately be used to predict the residual stresses. The residual stresses were found to be about twice as large at ?50 °C as at room temperature.     

Crack closure in fibre metal laminates

GLARE is a fibre metal laminate (FML) built up of alternating layers of S2-glass/FM94 prepreg and aluminium 2024-T3. The excellent fatigue behaviour of GLARE can be described with a recently published analytical prediction model. This model is based on linear elastic fracture mechanics and the assumption that a similar stress state in the aluminium layers of GLARE and monolithic aluminium result in the same crack growth behaviour. It therefore describes the crack growth with an effective stress intensity factor (SIF) range at the crack tip in the aluminium layers, including the effect of internal residual stress as result of curing and the stiffness differences between the individual layers. In that model, an empirical relation is used to calculate the effective SIF range, which had been determined without sufficiently investigating the effect of crack closure. This paper presents the research performed on crack closure in GLARE. It is assumed that crack closure in FMLs is determined by the actual stress cycles in the metal layers and that it can be described with the available relations for monolithic aluminium published in the literature. Fatigue crack growth experiments have been performed on GLARE specimens in which crack growth rates and crack opening stresses have been recorded. The prediction model incorporating the crack closure relation for aluminium 2024-T3 obtained from the literature has been validated with the test results. It is concluded that crack growth in GLARE can be correlated with the effective SIF range at the crack tip in the aluminium layers, if it is determined with the crack closure relation for aluminium 2024-T3 based on actual stresses in the aluminium layers.     

FATIGUE BEHAVIOUR OF FIBRE ALUMINIUM LAMINATES WITH DIFFERENT RESIDUAL STRESSES

In this study two kinds of fibre aluminium laminates (aramid aluminium laminates, ARALL and glass aluminium laminates, GLARE) with different residual stresses in the aluminium layers were prepared. Fatigue crack propagation tests were performed. It is found that the residual stress condition plays an important role in the fatigue behaviour of fibre aluminium laminates. With a decrease of the tensile residual stress in the aluminium layers, the fatigue crack growth rate of the laminates is greatly reduced, and the shape of the curves of fatigue crack propagation rate as a function of the stress intensity factor changed. Compared to GLARE, the ARALL is more sensitive to the residual stress condition. The fatigue properties of non-prestressed GLARE are better than those of ARALL. The influence of the residual stress is discussed in detail.     

Effect of residual stresses on fatigue strength of high strength steel sheets with punched holes

The effect of residual stresses on the reverse bending fatigue strength of steel sheets with punched holes was studied for steels with tensile strength grades of 540 MPa and 780 MPa. Tensile and compressive residual stresses were induced around the punched holes. Heat treatment of the specimens with punched holes at 873 K for 1 h decreased the residual stresses around the holes and improved the fatigue strength of the sheets. This result means that the tensile residual stresses induced in the sidewalls of the holes and near the hole edges by punching reduced fatigue strength. The effect of the residual stresses on the fatigue limits of the edges was estimated by the modified Goodman relation using the residual stresses after cyclic loading and the ultimate tensile strength at the fatigue crack initiation sites.     

Finite element approach toward an advanced understanding of deep rolling induced residual stresses,and an application to railway axles

A full-scale railway axle, made of medium strength steel EA4T and adopted for high-speed applications, is deep rolled. The induced residual stresses were experimentally characterized by X-ray diffraction and hole drilling. A realistic finite element model is proposed to overcome some of the existing shortcomings in simulation of deep rolling. Deep rolling coverage is defined, formulated and incorporated into the simulation. The model is validated by the experimental measurements. A parametric study is performed to investigate the effect of rolling force (4–19 kN), rolling feed (0.1–0.7 mm/rev) and roll geometry (1.5–10 mm roll tip radius) on the distribution of residual stresses and the induced hardening. A fatigue crack propagation algorithm is used to analyze the influence of the technological parameters on the lifetime of railway axles. Lower feeds, higher loads and thicker rolls, all resulting in higher coverage, can result in higher improvement against fatigue crack propagation. However, extremely high coverage can deteriorate the performance of deep rolled components. Coverage can effectively serve as a master parameter in deep rolling. As a general rule of thumb, adopting deep rolling feed to get a coverage level of 500–900%, while avoiding too high rolling loads and too thin rolls, can induce a suitable compressive residual stress distribution; and effectively prevent/retard fatigue crack propagation.     

Analysis of fracture and delamination in laminates using 3D numerical modelling

The static failure behaviour of the fibre-metal laminate GLARE is examined using 3D finite element simulations. The configuration analysed is a centre-cracked tensile specimen composed of two aluminium layers sandwiching a cross-plied, fibre-epoxy layer. The crack and delamination growths are simulated by means of interface elements equipped with a mixed-mode damage model. The mode-mixity is derived from an energy criterion typically used in linear elastic fracture mechanics studies. The damage kinetic law is rate-dependent, in order to simulate rate effects during interfacial delamination and to avoid numerical convergence problems due to crack bifurcations. The numerical implementation of the interface damage model is based on a backward Euler approach. In the boundary value problem studied, the failure responses of GLARE specimens containing elastic aluminium layers and elasto-plastic aluminium layers are compared. The development of plastic deformations in the aluminium layers stabilizes the effective failure response, and increases the residual strength of the laminate. For a ‘quasi-brittle’ GLARE specimen with elastic aluminium layers, the residual strength is governed by the toughness for interfacial delamination, and is in close correspondence with the residual strength obtained from a closed-form expression derived from energy considerations. Conversely, for a ‘ductile’ GLARE specimen with elasto-plastic aluminium layers, the residual strength is also determined by the relation between the fracture strength and the yield strength of the aluminium. The amount of constraint by the horizontal displacements at the vertical specimen edges has a moderate to small influence on the residual strength. Furthermore, the ultimate laminate strength is lower for a larger initial crack length, and shows to be in good correspondence with experimental values.     

Thermal shock behavior of nano-sized SiC particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications. Thermal shock resistance is a major concern and an important performance index of high-temperature ceramics. While silicon carbide (SiC) particles have been proven to improve mechanical properties of AlON ceramic, the high-temperature thermal shock behavior was unknown. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of AlON ceramic and 8 wt% SiC–AlON composites over a temperature range between 175 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing quenching temperature and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. A linear relationship between the residual strength and thermal shock times was observed in both pure AlON and SiC–AlON composites. The addition of nano-sized SiC particles increased both residual strength and critical temperature from 200 °C in the monolithic AlON to 225 °C in the SiC–AlON composites due to the toughening effect, the lower coefficient of thermal expansion and higher thermal conductivity of SiC. The enhancement of the thermal shock resistance in the SiC–AlON composites was directly related to the change of fracture mode from intergranular cracking along with cleavage-type fracture in the AlON to a rougher fracture surface with ridge-like characteristics, crack deflection, and crack branching in the SiC–AlON composites.     

Fatigue failure investigation on anti-vibration springs

Fatigue failure investigation on anti-vibration springs, involving both metal and rubber materials, is presented. Rubber-to-metal bonded springs are widely used in industry as anti-vibration components giving many years of service. Recently a need to improve time and cost efficiencies has caused an unexpected early fatigue failure of the component with no immediate explanation. The required total fatigue life was 1.25 million cycles but only 0.7 million cycles achieved. There was an urgent need to investigate the causes of the fatigue failure and to modify the component design accordingly to meet the customer requirement and the supply schedule.The investigation, based on the actual fatigue loads, is carried out on these failed and modified products using a method of continuum mechanics. To simplify the simulation, a non-linear quasi-static analysis is carried out and then the residual stresses are superimposed to obtain the effective stress range to predict the metal crack initiation. For the rubber parts a three-dimensional effective stress criterion is employed to predict the fatigue crack initiation. The fatigue failure is taken as visual crack observation (normally 1–2 mm).The fatigue crack initiation for the metal parts of the failed component is predicted at 225 K cycles under specified fatigue load against total metal broken at 700 K cycles from the test. For the modified part the minimum total fatigue life for the metal parts of the component, estimated conservatively, is 2.1 million cycles against 1.75 million cycles from the test without any crack observed. The rubber fatigue crack initiation is predicted at 90 K cycles against crack onset around 79 K cycles and crack length 40 mm at 145 K cycles from the test. From design point of view it is important to optimize the rubber profile under this very tight allowable space to provide the maximum support of the metal interleaves and at the same time to meet the minimum requirements of the manufacture process. It is shown that this approach can be employed at a design stage for both metal and rubber fatigue evaluations on anti-vibration springs.     

Prediction of notched specimen behaviour at ambient and high temperatures in Ti6246

Life prediction methods are essential in the selection of materials for high performance applications. These design criteria allow safe lives to be predicted for areas of geometrical discontinuity where localized increases in stress lead to the early initiation and propagation of fatigue cracks under cyclic loading. This paper explains two methods for predicting the initiation life of a double edged notch specimen (K_t = 1.9), and applies them to the α + β titanium alloy Ti6246 over a range of temperatures. The Coffin–Manson equation is effective for fully reversed cyclic behaviour. However a Walker strain-based parameter was found to be more appropriate when mean stresses are introduced. The analysis encompasses traditional analytical approaches which are limited in relation to the determination of the stresses and strains at the notch root and a finite element analysis based on ABAQUS^®. The FEA is used to characterize loop generation at the notches and to optimize the stress and strain conditions in the critical root positions.The main focus of the paper, however, is high temperature behaviour where creep and environmental damage impact on fatigue crack development. The limitations of the modelling approach under these conditions are discussed.     

The relative effects of residual stresses and weld toe geometry on fatigue life of weldments

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.     

Fatigue and fatigue crack growth of aluminium alloys at very high numbers of cycles

The application of ultrasonic frequency (20 kHz) loading to test fatigue and fracture mechanical properties of materials is briefly reviewed and recent investigations on high strength aluminium alloys are reported. Very high cycle endurance tests and near threshold crack growth experiments were performed with the 2024-T351 aluminium alloy. Lifetimes are approximately 10–100 times lower, if cycled in distilled water instead of ambient air. Fatigue experiments under randomly varying loads showed that linear damage summation calculations overestimated lifetimes by approximately a factor 2. Fracture mechanics studies in ambient air, dry air and in vacuum served to investigate the role of air humidity on near threshold fatigue crack growth at ultrasonic frequency. The threshold value was 2.1 MPa√m in ambient air and 3.3 MPa√m in vacuum. The aluminium alloy AlZnMgCu1.5-T66 and the aluminiumoxyde particle reinforced alloy 6061-T6 were tested at 100 Hz and 20 kHz to investigate frequency influences on high cycle fatigue properties, and similar lifetimes were found at both frequencies.     

The role of oxidation damage in fatigue crack initiation of an advanced Ni-based superalloy

The effects of prior oxidation on the room temperature fatigue life of coarse-grained Ni-based superalloy, RR1000, have been investigated. High cycle fatigue tests were conducted, on both machined and pre-oxidised testpieces, at room temperature at an R ratio of 0.1. The oxidation damage was produced by pre-exposures at 700 °C for either 100 or 2000 h. Pre-oxidised testpieces tended to fail with shorter fatigue lives than those obtained from the as-machined testpieces although they were also observed to outperform the as-machined test pieces at peak stress levels around 900 MPa. The chromia scale and intergranular alumina intrusions formed during pre-oxidation are prone to crack under fatigue loading leading to early crack nucleation and an associated reduction in fatigue life. This has been confirmed to be the case both below and above a peak stress level of ∼900 MPa. The better fatigue performance of the pre-oxidised specimens around this stress level is attributed to plastic yielding of the weaker γ′ denuded zone, which effectively eases the stress concentration introduced by the cracking of the chromia scale and intergranular internal oxides. This γ′ denuded zone is also a product of pre-oxidation and develops as a result of the selective oxidation of Al and Ti. Over a limited stress range, its presence confers a beneficial effect of oxidation on fatigue life.     

Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

Fatigue life prediction based on crack closure for 6156 Al-alloy laser welded joints under variable amplitude loading

A fatigue prediction approach is proposed using fracture mechanics for laser beam welded Al-alloy joints under stationary variable amplitude loading. The proposed approach was based on the constant crack open stress intensity factor in each loading block for stationary variable amplitude loading. The influence of welding residual stress on fatigue life under stationary variable amplitude was taken into account by the change of crack open stress intensity factor in each loading block. The residual stress relaxation coefficient β = 0.5 was proposed to consider the residual stress relaxation for the laser beam welded Al-alloy joints during the fatigue crack growth process. Fatigue life prediction results showed that a very good agreement between experimental and estimated results was obtained.     

Fail-safe design of integral metallic aircraft structures reinforced by bonded crack retarders

This paper presents an investigation on the effectiveness of crack growth retarders bonded to integral metallic structures. The study was performed by both numerical modelling and experimental tests. It focuses on aluminium alloy panels reinforced by bonded straps made of carbon-epoxy, glass-epoxy composite materials or a titanium alloy. The goal was to develop a fail-safe design for integrally stiffened skin-stringer panels applicable to aircraft wing structures. The modelling strategy and finite element models are presented and discussed. The requirements that the models should meet are also discussed. The study has focused on establishing the extent of crack retarder benefits, in terms of fatigue crack growth life improvement, by numerical simulation and experimental tests of various crack retarders. The results of predicted fatigue crack growth retardation have been validated by tests of laboratory samples. This study concludes that by bonding discrete straps to an integral structure, the fatigue crack growth life can be significantly improved.     

Effect of nitriding and shot-peening on the fatigue behavior of 42CrMo4 steel: Experimental analysis and predictive approach

In this work, we are interested in the bending fatigue resistance of the nitrided 42CrMo4 steel improvement by shot-peening. The micro-structure, the micro-hardness, the residual stresses distribution and the crack resistance of the hardened steel are determined. The gains, expressed in term of endurance limit, brought by these treatments are established by three-points bending fatigue tests and discussed in relation to the residual stresses evolution under the cyclic loading conditions. The fatigue fracture resistance after a combined process of surface hardening including shot peening followed by nitriding is analyzed by methods of fracture mechanisms. This reveals that the gain provided by the nitriding is about 8% against 35% for the nitriding with shot-peening. This is primarily allotted to a high level of compressive residual stresses for nitrided + shot-peened state compared to the nitrided state. The fast relaxation of these stresses in the low cycle fatigue domain is at the origin of the fatigue fracture at surface, which leads to a lower fatigue fracture resistance compared to the untreated state. Based on multiaxial HCF criterion of Sines and taking into account the different surface properties, a local predictive approach was developed.