Total fatigue lives, crack growth paths and cycles in cold expanded adjacent holes

Cold expansion of rail-end-bolt holes: Finite element predictions and experimental validation by DIC and strain gauges

Lifetime of aircraft alloy plates with cold expanded holes

In order to reveal the formation mechanism of microstructural delamination in flange steel during punching, the microstructural characteristics of flange steels with 0.064, 0.085, and 0.101 wt% carbon contents were analyzed by an optical microscope, optical emission spectrometer, metal in situ analyzer, and electron probe microanalyzer. It was found that microstructural delamination occurred due to C and Mn segregation, especially the former. The intensive segregation degree of C was dependent on its relatively smaller partition coefficient. Moreover, the segregation degree gradually decreased with the decreasing carbon content. And the increased segregations of elements except carbon were related to the coupling effect between different alloying elements. Furthermore, C and Mn were prone to segregate at the 1/4th position along the transverse direction of the casting slab; hence, almost no segregation was noticed at the edge of the casting slab. The homogenization treatment had a substantial effect on the elimination of C segregation; however, Mn segregation was relatively hard to be removed.

Investigation on Microstructural Delamination and Compositional Segregation in Flange Steel with a High Stretch Ratio

Fatigue strength assessment of misaligned laser beam welded T-joints by effective stress method

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 = σ min /σ max = 0 and room temperature. Here σ min , σ max 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 σ Y = 300 MPa, tensile strength σ U = 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.

Sergii Volodymyrovych Glado

Papers

Lifetime of aircraft alloy plates with cold expanded holes

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