Shape memory alloys are widely used in medicine, bioengineering, aerospace, mechanical, civil engineering and other areas. Though they haven't been used for a long period of time, nevertheless, there are already known cases of SMA failure in structural elements. Therefore, there arises the question: how long can such structural elements be under operation. To answer this question, it is necessary to predict the residual lifetime of such alloys. The development of science and technology demands new increased requirements for the safety of such important structural elements, and, in particular, to the used devices. Fatigue crack growth (FCG) diagrams are generally significantly scattered, that can be taken into account, for instance, by building the distribution of parameters, which are involved in the equation describing the FCG diagram. Therefore, it is necessary to be able to predict FCG taking into account the scatter of mechanical properties, preliminary determined statistical distribution of crack growth resistance parameters, particularly, parameters of C and n of Paris equation. It is known, that the parameters of Paris equation are mutually dependent. Therefore, considering C as random variable, that changes from the specimen to specimen, it is possible to take into account the existing data scatter. The defects, which are found in the structural elements, have frequently the form of semi-elliptical cracks. FCG rate of pseudo-ellastic NiTi alloy was studied experimentally under uniaxial tension of cyllindrical specimens with a dimater of 8 mm at room temperature on air at servo-hydraulic testing machine STM-100. A methodology for predicting the residual lifetime of SMA cylindrical specimens with a semi-elliptical surface crack is proposed. The methodology is based on solving the system of differential equations describing crack propagation, load parameters and cyclic crack grow resistance, taking into account their statistical scatter and change of crack front shape. There were plotted the cumulative distribution functions of lifetime of specimens with a diameter of 2r for different initial crack depth (b/r = 0.25; 0.36; 0.5; 0.75) and the crack shape factor.
The process of cold plastic deformation of a hole in a plate made of shape-memory alloy is simulated by the finite-element method by setting the mechanical characteristics and the temperatures of the onset and termination of direct and reverse phase transformations. By using the ANSYS Workbench software, we determine the distribution of residual stresses in the plate in the vicinity of a functional hole. The highest normal compressive and tensile stresses are formed in the middle part of the plate and at a distance of 2-4 mm from the edge of the hole, respectively.
The fatigue life of aircraft structure elements with operational damage in the vicinity of the hole was investigated. The plates 60 mm wide and 6 mm thick made of D16chT aluminum alloy with a central hole were taken for the study. Fatigue damage was examined with an corner quarter-elliptical fatigue crack with a length of 1,25 mm, which was initiated from an edge notch of 0,5 x 0,5 mm. The fatigue crack growth rate on the surface of the plate after mandrel hole with cold expansion degree i = 2,7% increases up to15 times and residual lifetime in three times compared to the virgin plate.
Superelastic behaviour of Ni-Ti alloy wire under variable amplitude loading was simulated and experimentally studied. Mechanical properties and phase transformation stresses ( , , , ) of the material were obtained in uniaxial tensile test. The wire of 55,8% Ni -44,2% Ti alloy with a diameter of 1,5 mm and a working length of 30 mm was tested at room temperature (+ 16°C) on the air. On the base of finite elements method, using ANSYS the stress-strain dependencies on each loading cycle were calculated. The stressphase transformation are changes under a variable amplitude loading. The simulated stress-strain dependencies were compared with the experimental ones. The maximum error, being compared with experimental data, does not exceed 10,9%. The calculated dependences of elastic strain and dissipated energies on the amplitude stress are well agreed with experimental data. The obtained results are of theoretical and applied interest for modelling the superelastic behaviour of SMA under variable amplitude loading.Statement of the problem. Development of science and technology promotes the appearance of new and advanced requirements for application of materials, their strength and durability characteristics. Shape memory alloys (SMA) are used in many branches of industry thanks to the shape memory effect, good damping properties, durability and strength characteristics [1 -6]. The nickel-titanium alloy can «remember» its original shape and return it in the original non-deformed state after unloading (the effect of pseudoelasticity) or heating (the effect of shape memory) during many cycles of loading and unloading.Analysis of available results of investigations. The shape memory alloy Nitinol -(Ni-Ti) was created in the 1960-ies at Naval Ordnance Laboratory (USA) and soon was widely used in medicine and different branches of engineering [1 -4]. Many papers are devoted to the pseudoelesticity modelling and the shape memory effect of such alloys, the study of which is presented in the paper [7] in particular. The 3D model of the shape of the shape memory alloys behaviour for ANSYS makes possible to take into account the decrease of rigidity of the structural element during transition from austenite to martensite [8]. The 3D SMA model [9], which presents the pseudoelestic behaviour and the shape memory effect and makes possible to model, using the finite elements method, the behaviour of free-extended stents and spring working elements, is presented.The Objective of the paper. To model the pseudoelastic behaviour of nickel-titanium alloy, using the finite elements method (FEM), under the variable amplitude of loading and compare it with the experimental results.Statement of the task. Mechanical behaviour of SMA is specified by the microstructure characteristics, containing two different phases, austenite and martensite.Modelling of mechanical behaviour of shape memory alloys using finite elements method 8 ….
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