Papers by Keyword: Cold Expansion

Abstract: Split-sleeve cold expansion processing was employed on the 7050-T7451 aluminum alloy plate. Fatigue lives were compared according different expansion, then the relationship of fatigue life and expansion was analyzed. Residual stresses were measured with different expansion, and the fatigue fractograph was analyzed by SEM. The results show that the split-sleeve cold expansion can obtain longer life compared with the non-strengthen hole. When over the optimum expansion, fatigue life began to decrease. The maximum fatigue life increased to 2.92 times with 4.1% expansion. The maximum values of radial residual stresses grew with expansion. The depths of residual compressive stresses were more than 6mm with 2.6% and 4.1% expansion. The fatigue fractograph shows mixed transgranular fracture.

Abstract: Cold expansion was employed on 7050-T7451 aluminum. Residual stresses were measured, fatigue tests were carried out for joints with different expansion, and the fracture surfaces of specimens were observed by scanning electron microscope (SEM). The results show that residual compressive stresses generate by cold expansion process. The maximum values of radial residual compressive stresses locate the middle of the inner wall of the hole and residual compressive stresses extend radially up to one diameter from the edge of the hole. Axial residual compressive stresses distribute near the exit. The fatigue lives N₅₀ and N_95,95 for specimen with strengthened hole with 5% expansion are respectively 3.54 and 3.68 times compared with non-strengthened hole. Fatigue fracture morphology shows the characteristics of mixed transgranular fracture and the propagation speed of the crack along horizontal direction is larger than vertical direction.

Abstract: The primary focus of this investigation is to determine the distribution of thermal residual stresses that result during composite bonding processes, and the effect on stresses generated during the subsequent cold expansion of holes. Residual stress measurements were carried out using neutron diffraction techniques. Results show that the cold expansion process resulted in radial compressive stresses 3-4 mm from the edge of the hole and there was no significant effect of thermal residual stresses from the bonding process on the cold expansion and bolted stresses.

Abstract: Over the past 50 years, mitigating cracks from holes has been a major focus of aerospace research, design, and stress engineers. Technology and design philosophies were developed to focus on predicting and increasing fatigue life of metal aircraft structure. One of the innovations to have the biggest impact to slow or arrest crack growth was to induce beneficial residual stresses around a hole. The zone of residual compressive stresses shields the hole from the effects of cyclic loads. The split sleeve cold expansion method was pioneered in the early 1970s by Boeing and Fatigue Technology. The split sleeve cold expansion process has become the bench mark method in mitigating fatigue cracks and enhancing durability and damage tolerance of aerospace metal structures. This paper will review the history of cold expansion methods and the evolution of this technology to the controlled and widely accepted methods used by industry today.

Abstract: Cracking in steel road or rail bridges is a major concern for continued safe operation of the bridge. Traditional crack arrest hole (CAH) or drill stops placed at the end of the detectable crack have been shown to be ineffective in stopping cracks. A method adapted from the aerospace industry, which utilizes a high interference fit bushing expanded into the CAH, has been tested in large A36 steel coupons under typical cyclic bridge loading. Expansion of the bushing into the hole yields the surrounding steel and induces a residual compressive stress around the hole to shield it from the cyclic load and prevent further crack growth. Not only did this method completely arrest the crack with a life improvement factor exceeding 60:1 over the conventional CAH but it also increased the load factor capability of the coupon by over 20%. This paper will describe the methodology, present the results of the test program and show how this innovative method dramatically increases the effectiveness of a typical CAH that could forestall major structural failure.

Abstract: A numerical investigation was conducted to apply cold expansion methods on composites structures under the guidance of cold expansion techniques of metal materials and interference-fit techniques of composites structures. Finite element expansion models with sleeves or without sleeves are established to study the stress distributions and failure conditions around cold expanded holes with different expansion values (0.5%, 1% and 1.5%). Results show that no damage occurs with expansion value 0.5% and using a sleeve in the expansion process can cause uniform stress distributions.

Abstract: 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.

Abstract: The enhancement method of fatigue life and the crack initiate and growth behavior of a holed specimen was investigated by using the 2024 Aluminum alloy and 0.45% Carbon steel. The purpose of present study is to propose a simple technical method for enhancement of fatigue life in a notched specimen. Also, the effect of local plastic deformation by cold work on fatigue crack initiation behavior was examined. This paper presents a basic experimental kinematic cold expansion method by inserting and removing a pin through the specimen hole. The shape of cross-section of pin was a circle or an ellipse. It was shown that the fatigue life of the specimen with the cold-worked hole was longer than that of the specimen with non-cold-worked hole for the case of same stress level in aluminum alloy and carbon steel. Also, the fatigue strength was higher in the case of the cold expanded hole. In this study, a methodology of lengthening of fatigue life of holed specimen is shown. Also, the improvement conditions of fatigue life were significantly affected by shape of pin, local hardening and residual stress conditions. The fatigue life improvement of the damaged component of structures was studied.

Abstract: In this paper, four axisymmetrical finite element models have been established to study effect of thinckness on residual stress fields of cold expansion hole, experiments were carried out to measure the residual stress of cold expansion hole and verify simulation results. The experiment results also show that cold expansion processing can decrease surface roughness of holes and improve surface quality. The FEM results show, at the top(bottom) surface, the volue of the radial residual stress varies with the plate’s thickness, and the regularities of distribution of the radial residual stress are identical. The volue and the regularities of distribution of the circumferential residual stress vary with the plate’s thickness. At the mid-thickness surface, the volue and the regularities of distribution of radial residual stress vary with the plate’s thickness. The volue of the circumferential residual stress varies with the plate’s thickness, and the regularities of distribution of the radial residual stress are identical.

Abstract: 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.