Effect Of Cold Expansion Research Articles

A fractographic study of fatigue crack growth from a cold expanded fastener hole at an engineering “crack initiation” scale

AbstractThe effects of cold expansion on fatigue crack growth from fastener holes in aircraft structure are a well‐studied problem, with many contributions focusing on both residual stresses and “long crack” experimental data. In this study, the authors have used quantitative fractography techniques to measure fatigue crack growth from naturally occurring discontinuities on an engineering “crack initiation” scale. Fatigue crack growth rates were measured through the full range of crack depths, allowing the effects of beneficial residual stresses to be directly observed. The almost identical early crack growth rates in cold expanded and non‐cold expanded specimens were demonstrated up to crack depths of approximately 0.1–0.2 mm. Beyond this depth, fatigue crack growth in cold expanded specimens was markedly slowed to the point at which failure ultimately arose from cracks away from, or growing into, the fastener hole. The available life improvement factor for cold expansion is therefore maximized through a crack growth‐based lifing approach.

Effect of cold expansion on high-temperature low-cycle fatigue performance of the nickel-based superalloy hole structure

Low-cycle fatigue properties before/after cold expansion under multiple conditions were comparatively researched. The results showed the fatigue cycles of the cold-expanded specimens are more than doubled compared to the original, and the fatigue gain decreased as the stress increased. Surface smoothing, surface compressive residual stress field, and cold work-hardened structure with the sub-grain refinement were recognized as the main reasons for increasing the initiation life. The relatively stable compressive residual stress under 400 °C aging and 820 MPa/RT limited the crack propagation, making the width of the fatigue band and the crack propagation speed of cold-expanded specimen smaller than hole-reamed at the same depth.

Effect of cold expansion on fatigue life of double lap structure

The finite element simulation analysis was used to determine the weak parts of the fatigue life of a double lap structure, for which the cold expansion finite element model was established. The effect of the cold expansion of slotted bushings on the residual stress distribution around the hole of the double lap structure under different interferences was deeply studied, and the optimal interference amount was determined to be 2.5%. The effect of reaming on the residual stress distribution around the hole was studied. The residual stress distribution around the edge of the hole under actual loading conditions before and after cold expansion was compared to provide a theoretical analysis basis for extending the fatigue life of the specimen after cold expansion. The China-made and free- forged 7A85 aluminum alloy specimen was used to obtain the fatigue life and DFR value of the double lap structure after ordinary hole making and the cold expansion of slotted bushing. The results show that the fatigue life under 95% confidence and 95% reliability after cold expansion strengthening increases by about 49% and that the tested DFR value increases by about 9.8%.

Effect of cold expansion on room-temperature low cycle fatigue life of GH4169 superalloy with hole structure

Effect of cold expansion on room-temperature low cycle fatigue life of GH4169 superalloy with hole structure

Effect of cold expansion on fatigue life of anodized Al alloy

Anodizing treatment can improve corrosion resistance of aluminum alloy but decrease the fatigue life significantly. Introducing cold expansion before anodizing can increase the fatigue life of the specimen with open-hole structure by 2582% than the anodized one. Scanning electron microscopy (SEM) was employed for microstructure and fracture surface observation. Residual stress distribution near the sample hole edge was measured to elucidate the strengthening mechanism. The results revealed that although anodizing treatment decreases the surface integrity, the cold expansion is an effective method to improve the open-hole structure specimen remarkably.

Fatigue crack nucleation and propagation in aluminum alloy plates with cold expanded holes

Analysis of the literature shows that in some cases existing technological methods of processing, including cold expansion, are the effective means to improve fatigue durability of structural elements with holes. The effect of cold expansion (1−3 %) on the stress state and fatigue durability of aluminum plates with functional holes under uniaxial cyclic loading was studied. The fatigue crack growth in the plates with a thickness t = 6 mm and width of 60 mm with a central hole diameter of 8 mm and 10 mm of aluminum alloy D16chТ (2024-T3) was investigated. Fatigue test was carried out under constant amplitude loading at stress ratio R = σmin/σmax = 0 and room temperature. Here σmin, σmax are the minimum and maximum stresses, respectively. The stress range was 147 MPa, loading frequency was 25 Hz. The mechanical properties of the alloy at room temperature were: yield strength σY = 300 MPa, tensile strength σU = 430 MPa. For all tested specimens with cold expanded holes the cracks initiation from the edges of holes on the entrance faces mandrel and from middle part for plain hole. Using the finite element method, the distribution of local residual stresses in the vicinity of the expanded hole was calculated. For specimens with cold expanded holes the least compressive stresses were near the entrance face and the largest were in the middle of the hole and near the exit face. With the increase of the cold expansion hole, the difference in values of compressive residual stress along the plate thickness is reduced. The width zone of residual compressive stresses increase with the increase of cold expansion. The dependencies of the number of cycles to crack initiation in aluminum specimens on the local maximum stress and local stresses range in the vicinity of surface hole were obtained. Cold expansion degree of 1% increases the lifetime of the plates to the initiation of fatigue crack length of 0.25 mm in 1.5-3 times as compared with plane plates. A similar dependence is observed for cold expansion of 2%. With further increase of cold expansion degree up to 3% the lifetime to fatigue crack initiation is increased in 7−10 times as compared with plane plate. By using the finite element method, the distributions of local stress range, maximum and minimum local stress near the hole depending on the number of loading cycles and cold expansion were built. In the middle section of the specimen the local stress ratio in the second half-cycle of load decrease with increasing of distance from the edge of the hole and with the increase of cold expansion. The lowest value of local stress ratio was obtained at cold expansion degree of 3% and it is constant across thickness of plate.

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.

Effect of Cold Expansion on Crack Opening Displacement and Stress Intensity Factor

Two-dimensional (2D) finite element analyses have been carried out to study the crack opening displacement and stress intensity factor of a cracked centre-hole plate. With the different interference levels (2%, 4%, 6% and 8%) and acted by residual stress caused by cold expansion, the crack opening displacement, the stress intensity factor and J-integral around the crack tip is presented. With the different interference levels and acted by different applied loads , the crack opening displacement, the stress intensity and J-integral around the crack tip is also presented. The results of the 2D FEM analysis show, 1) with the different interference levels and acted by residual stress caused by cold expansion, the crack mouth is throughout closed and the middle part is open, but the opening displacement is constant; 2) under some applied loads, with the interference levels up, the stress intensity factor on the crack tip and J-integral, become weak gradually, then they keep constant values; 3) with the interference increases, the stress intensity factor and J-integral decreases. When the interference increases a certain value, the stress intensity factor and J-integral do not decrease and keep constant values.

The Effect of Cold Expansion on the Fatigue Life of the Chamfered Holes

The cold expansion method is one of the most popular techniques in the fatigue enhancement processes, and it has been widely used as a means of improving the fatigue resistance for aircraft structures with holes. Cold expanded holes have lower compressive residual stresses on the entry surface rather than the middle and exit surfaces. Due to the nonuniform residual stress distribution, fatigue crack initiation often occurs on the entry surface. This study proposes a new approach to increase the compressive residual stress magnitude at the entry of the hole. The new method is to apply chamfers into holes before the cold expansion process. Split mandrel process was used to cold work the hole with and without chamfers. Both numerical and experimental studies were done to verify the effects of hole chamfers on the residual stress distribution of the cold expanded holes. Finite element analysis (FEA) was conducted in order to see the effects of the chamfer geometries on the residual stress distributions. The FEA results showed an improvement of compressive residual stress magnitudes at the entry position of the cold expanded hole. The numerical results were compared with X-ray diffraction measurements. Fatigue tests were done to compare the fatigue life of the holes with various chamfer sizes and angles. The cold expansion chamfered holes showed a clear improvement in fatigue life over cold expanded holes without chamfers.

Effect of cold expansion on fatigue performance of open holes

In this paper, residual stress measurements were made on aluminium alloy 2A12T4 open holes after cold expansion. By contrast, a finite element method (FEM) was also used to analyze the residual stress distribution around cold expansion hole. Then a series of uniaxial fatigue tests were carried out using specimens containing non-cold expanded holes and cold expanded holes to assess the effect of direct cold expansion process on fatigue performance of open hole. At last the fracture morphology of the specimens after fatigue test was examined by scanning electron microscopy. The results reveal that direct cold expansion technique could elevate about six times fatigue life. After cold expansion, fatigue crack of open holes always initiates on entrance face.

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.

The effect of cold expansion on fatigue crack growth from open holes at room and high temperature

In this paper a series of residual stress measurements and fatigue crack growth tests have been carried out using aluminium alloy 2650 specimens containing cold expanded and non cold expanded holes. Residual stress measurements have been done after cold expansion and after various loading and temperature conditions. In order to measure an angular variation of residual stresses, X-ray and a new technique called the Garcia–Sachs method have been employed. Results revealed that residual stress relaxation occurred as a result of exposure at 150°C. The magnitude of relaxation was shown to be dependent on the level and the sign of externally applied load. Fatigue crack growth tests have been carried out at 20°C and 150°C for both cold expanded and non-cold expanded conditions. Fatigue crack growth rates in specimens containing cold expanded fastener holes were affected significantly by elevated temperature exposure. Depending on the exposure time and loading conditions the fatigue life improvement was found to be between one and greater than 10 for tests at 20°C.