Fatigue Life Of Fastener Holes Research Articles

Finite Element Analysis of Split Sleeve Cold Expansion Process on Multiple Hole Aluminum Alloy.

Multiple cold expansion holes are widely used in connection areas of aircraft structures, in order to achieve uniform load transfer of the skin or connection parts. Split sleeve cold expansion (SSCE) is widely used to enhance the fatigue life of fastener holes by applying compressive residual stresses around the holes. In this study, the finite element method (FEM) was used to research the distribution and variation of residual stresses along the hole edges of 7075AA single-hole and multi-hole cold expansion (CE) specimens. Full-field strain measurements of single-hole and multi-hole specimens were performed using two-dimensional digital image correlation (DIC), and the residual stress and strain at the hole edge of the specimens measured by FEM and DIC were compared. FEM results shows that the maximum circumferential and radial residual stresses of three-hole specimens with three-hole spacing are increased by 5.37% and 31.53% compared with single-hole specimens. The maximum circumferential residual stress of three-hole specimens with four-hole spacing increases by 7.25% compared with a single hole, but the radial residual stress decreases by 12.98%. In addition, for three-hole specimens with hole spacing three times the hole diameter, the strengthening effect of SSCE in the order of middle hole, then left hole, and, finally, right hole is better than that of SSCE in the order of left to right hole. FEM and DIC full-field strain results are basically consistent.

Mitigating Cutting-Induced Plasticity Errors in the Determination of Residual Stress at Cold Expanded Holes Using the Contour Method

BackgroundThe split sleeve cold expansion process is widely used to improve the fatigue life of fastener holes in the civil and military aircraft industry. The process introduces beneficial compressive residual stresses around the processed hole, but uncertainties remain about the character of the stress field immediately adjacent to the bore of the hole.ObjectiveThe primary objective of this study was to implement the contour method with minimising error associated with cutting-induced plasticity to provide detailed and reliable characterisation of the residual stress introduced by the split sleeve cold expansion process.MethodsA systematic FE study of plasticity effects by simulating different contour cutting strategies (a single cut, two sequential cuts and a 6-cut sequence) for a cold expanded hole in an aluminium alloy coupon was conducted. The identified ‘optimum’ cutting strategy was then applied experimentally on coupons containing a hole that had been processed to 3.16% applied expansion.ResultsThe FE study of different cutting simulations show that the locations of the stress error is consistent with the location where cutting-induced plasticity accumulated and that the magnitude and locations of stress re-distribution plasticity can be controlled by an optimised cutting strategy. In order to validate this hypothesis a high quality contour measurement was performed, showing that accurate near bore stress results can be achieved by the proposed 6-cut approach that controls cutting induced plasticity.ConclusionsThe present work has demonstrated that detailed FE simulation analysis can be a very effective tool in supporting the development of an optimum cutting sequence and in making correct choices of boundary conditions. Through optimizing these key aspects of the cutting sequence one is much more likely to have a successful, low error contour residual stress result.

Effect of Ultrasonic Impact Treatment on the Fatigue Performance of Cold Expanded AA6061-T6 Hole

Cold expansion is widely used to improve the fatigue life of fastener holes. However, fatigue crack often initiates at the entrance edge of the hole due to lower compressive or tensile residual stress as a result of cold expansion. In the current study, the entrance edge of the cold expanded hole has been ultrasonically impacted with a specially designed pin, and comparative fatigue tests are performed to evaluate the influence of the ultrasonic impact treatment on the fatigue performance of the cold expanded holes. The findings show that the ultrasonic impact treatment imparted a larger compressive residual stress and created a fillet arc at the entrance edge, which resulted in an obvious reduction in the fatigue crack propagation rate in the earlier stage of the fatigue crack growth and extended the fatigue life of the cold expanded holes further.

Study on the effect of laser peening with different power densities on fatigue life of fastener hole

Laser shock processing can improve the fatigue life of fastener holes. The purpose of this study is to investigate the effect of laser power density on fatigue life and fracture characteristics of 7050-T7451 aluminum alloy fastener hole specimens with 6 mm thickness. ABAQUS software is used to analyze the three-dimensional stress distribution. The fatigue test was carried out and the treated specimens were evaluated by scanning electron microscope. The results show that with the increase of laser power density, the fatigue life gain of fastener hole specimens increases firstly, then decreases, and then increases again. The fatigue life gain of the fastener hole specimen is related to the initiation position of the fatigue crack and the direction of the crack propagation. It is speculated that this is closely related to the three-dimensional stress distribution of the fastener holes after laser shock processing in different laser power density.

A new conception for enhancement of fatigue life of large number of fastener holes in aircraft structures

AbstractA new conception for increasing fatigue life of large number of fastener holes in aircraft structures is developed. It is accomplished by a new method, called friction stir hole expansion (FSHE). This method not only reduces labour and time consumption, but it also decreases the overall cost for processing a large number of holes in structures made of aerospace grade 2024‐T3 aluminium alloy. FSHE combines the advantages of friction stir processing with these of mandrel cold working methods in two ways: a micro effect, expressed in hole surface modification, and a macro effect, expressed by the introduction of beneficial compressive residual macro stresses. The effectiveness of the method has been assessed by fatigue tests. Finite element simulations have been carried out. It has been proven that the greater fatigue life of fastener holes, processed by FSHE, is a consequence of the obtained micro effect.

The Influence of Corrosion Pits and Cold Expanded Fastener Holes on the Fatigue Life Aluminium 7075-T651

The Royal New Zealand Air Force (RNZAF) utilised the split sleeve cold expansion process to increase the fatigue life of fastener holes in the wings of the C130 transport fleet. As part of the validation of the fatigue improvements offered by the process the Defence Technology Agency conducted a series of fatigue tests on cold expanded fastener holes in aluminium 7075-T651, including specimens with corrosion induced after the cold expansion process had been performed. This research conducted an analysis of fatigue crack origins and modelled the stress concentration factors generated as a result of the corrosion pits. These results were used to explain the differing fatigue life and s-n curves produced by corroded and non-corroded fatigue specimens and the location of crack initiation sites around corroded cold expanded fastener holes.

Fatigue Crack Growth Life Prediction around Cold Expanded Hole Using Finite Element Method

Cold expansion process is widely used to enhance fatigue life of fastener holes in aircraft industry. The benefit derived from cold expansion treatment is attributed to residual stresses induced around hole. This work is aimed at establishing a simple procedure for predicting Fatigue Crack Growth (FCG) life around cold expanded holes. Cold expansion process is simulated for uncracked and cracked holes using Finite Element method. The effect of residual stresses is accounted to predict FCG life using Willenborg's model. As a result of cold expansion, significant retardation in FCG rate and life improvement is achieved.

Residual Stresses Induced by Cold Expansion of Adjacent and Cut-Out Holes

Fatigue life of fastener holes can be enhanced via a cold-expansion process to introduce a compressive residual stress field around the hole edge and to reduce crack growth propagation. Considering that aerospace components contain multiple rows of holes, the present investigation focuses on the evaluation of the three-dimensional residual stress distribution in adjacent cold-expanded (CE) holes. The redistribution of residual stresses caused by a cut introduced between two adjacent holes was also investigated. Finite element (FE) analysis and experimental technique were used to assess the residual stress distribution in a 6082-T6 aluminum plate with two adjacent holes expanded sequentially at 4 % nominal interference. The influence of center-to-center distance between holes was explored to assess the optimal level of separation between adjacent holes. Results suggested that residual stresses near second CE hole are markedly lower than those of first CE hole and that a cutting process does not affect the beneficial compressive residual stress around CE holes. These effects may delay fatigue crack propagation from CE holes or cut-out holes.

Numerical Simulation of the Cold Expansion and Residual Stress Relaxation for Aluminum Alloy 7050

Cold expansion is a well-known technique for improving the fatigue life of fastener holes in aeronautical structures by introducing a compressive residual stress field around them. In this paper, a 3-D finite element model is used to analyze the residual stress distribution and relaxation around an expanded hole for aluminum alloy 7050. The results reveal that the cutting process of split sleeve cold expansion and creep are main reason for residual stress relaxation in room temperature, which may limit the beneficial effects of cold expansion.

Prediction of fatigue performance of fastener holes with bolt clamping force based on critical plane approach

In the present study, a series of fatigue tests are conducted on aluminum alloy 2024 to investigate fatigue behavior of fastener holes in reverse double bone specimens. With reference to the finite element results, the fatigue lives of fastener holes in reverse double bone are predicted based on critical plane approaches, including Smith–Watson–Topper (SWT) model and Wang–Brown (WB) model. After that, the predicted values are compared with that from tests. At last, under different far field stress levels, the effect of bolt clamping force on fatigue life of fastener holes is predicted based on the aforementioned two models. The results show that the SWT approach achieves a better accuracy than the WB approach for high cycle fatigue (HCF) while the opposite holds for low cycle fatigue (LCF). The fatigue life of fastener holes decreases with the increase of bolt clamping force.

Effect of detail design on fatigue performance of fastener hole

In the present study, a series of tests were conducted on aluminum alloy 2024 to investigate the effect of detail design on the fatigue behavior of fastener holes in specimens. Two types of detail designs were concerned. One was the mode of fastener holes (countersunk rivet or countersunk bolt), the other was drilling process (traditional air-drilling process or one step compound cutting process). The fracture surfaces were observed by means of an optical microscope. Finite element method (FEM) was employed to analyze the distribution of stress around the fastener holes. The results showed that crack always initiated at the hole edge where the stress concentration occurred. Crack initiation was induced by stress concentration. Crack initiation life accounted for 80% of total fatigue life of fastener holes. The fatigue life of fastener hole using countersunk rivet was longer than that using countersunk bolt. Contrasted to traditional air-drilling process, the fatigue life of fastener hole could be improved by 44–55% using one step compound cutting process. However, the dispersibility of fatigue life became increasingly severe when fatigue life was prolonged.

Modelling of hardening behaviour of cold expanded holes in medium-carbon steel

Cold expansion is a well-known approach for the enhancement of the fatigue life of fastener holes in structural components by introducing residual compressive circumferential stresses around them. In this work, the hardening of a medium-carbon structural steel is studied experimentally and numerically from the point of view of the material behaviour during the cold expansion process of holes in structural steel components. Six material hardening models obtained on the basis of symmetric strain-controlled experiments and half-cycle test data from unidirectional tension experiments have been used in a two-dimensional axisymmetric finite element model of the process. Parallel with this an x-ray diffraction analysis of the residual stresses at the entrance and exit faces of the structural component has been carried out to prove the finite element result’s authenticity. It is established that the nonlinear kinematic hardening model obtained by a strain-controlled cyclic test to achieve a stabilized cycle for this steel secures finite element results close to the experimental ones. The constitutive hardening model obtained can be applied to a corresponding finite element model for stress analysis of steel structural component with cold expanded holes, subjected to an external load.

Fatigue life of an aerospace aluminium alloy subjected to cold expansion and a cyclic temperature regime

This paper describes experimental work that was carried out to study the influence of cyclic temperature variation on the fatigue life of fastener holes in an aerospace aluminium alloy after they had been subjected to cold expansion. Three different batches of specimen were produced for fatigue testing. The first samples had the fastener hole left in the ‘as drilled’ state, the second and third batches had their drilled holes cold expanded by directly force fitting an oversized pin through the hole. In addition, the third batch with cold expanded holes was then subjected to a representative range flight cycle temperature variations. The results reveal that the cyclic temperature variation, rather than relaxing the residual stress and thus the cold expansion effect, as was feared, has actually extended the fatigue life of the cold expanded holed specimens tested. It is believed that this is because the temperature changes have tended to neutralise the local peak residual stresses which can occur at the surface faces.

The effect of cold expansion on improving the fatigue life of fastener holes

An analytical and experimental study has been completed to assess the effectiveness of a direct cold expansion technique on the fatigue strength of fastener holes. Such methods are used in the aircraft construction industry to make holes more resistant to failure but recent research findings have reported that sometimes the anticipated benefits of cold expansion are not always achieved. This research work has sought to clarify the situation.

Numerical analysis of bonded repairs for fastener holes with three-dimensional cracks

Recent finite element analysis has shown that stress intensity values for two-dimensional cracked fastener holes can be significantly reduced through the use of bonded sleeves. Experimental work has also shown that adhesively bonded inserts can significantly increase the fatigue life of both cracked and uncracked fastener holes. In this paper we present the results of elastic finite element stress analyses carried out on fastener holes with three-dimensional crack geometries and consider the use of both bonded sleeves and bonded fibre composite patches. It is shown that the stress intensity factors for these geometries are significantly reduced by using the bonded sleeves and bonded patches separately or together. A limiting factor is considered to be the level of the stresses in the sleeve adhesive, leading to the possibility of fracture or yielding in the adhesive. It has been found that first order estimates of reductions in the stress intensity factor can be obtained for the three-dimensional crack geometries by comparison with simpler finite element or handbook solutions. The reductions in stress intensity factors obtained indicate that fatigue crack growth rates will be significantly reduced and hence the repair approaches considered show great promise for increasing the fatigue life of fastener holes with realistic crack geometries.

Analysis of bonded inserts for the repair of fastener holes

Recent experimental work has shown that adhesively bonded inserts can significantly increase the fatigue life of fastener holes. This paper concentrates on developing an understanding of the mechanisms which give rise to the observed increase in life. Cracked as well as uncracked fastener holes are considered. It is shown that both the stress concentration factors and the stress intensity factors are significantly reduced by the use of either bonded rivets or bonded sleeves. Furthermore the stress intensity factor, for a cracked hole repaired by a bonded insert, can be obtained by analogy with readily available solutions. The ability to repair cracked structures using this method is then confirmed by a series of experimental tests.