M.S. Theses

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Browsing M.S. Theses by Subject "Alloys -- Thermomechanical properties."

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    Effect of loading rate and thermomechanical coupling on fracture of shape memory alloys
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Özbaş, İsmail Yiğit.; Anlaş, Günay.
    This work is on the investigation of the effect of loading rate and thermomechan ical coupling on mechanical behavior and fracture of shape memory alloys (SMA). The constitutive model of Zaki and Moumni (ZM) is studied in detail and re-derived. With the changing loading rates, the ZM model does not capture correctly the martensite start stress. Based on experimental works of Tobushi et al., Mutlu and Shaw, a relation is proposed for the martensite start stress, σms to account for a change in loading rate. Some model parameters contain σms as variable, so, they are also proposed to be func tion of local strain rate. For numerical calculations ABAQUS is used and therefore the user subroutine UMAT is rewritten accordingly. Finite element simulations are conducted with modified model parameters using the specimen of Shaw. The stress strain behavior and hysteresis area are compared to the results obtained by using the unchanged ZM model. The fracture specimen, used in Mutlu’s experiments is modeled using original and modified model. Transformation regions, crack mouth opening dis placements (CMOD), temperature fields and stress intensity factors are obtained using original coupled/uncoupled and modified models to investigate effect of loading rate and thermomechanical coupling; the results are compared to experimental ones.
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    Shape-memory effect in a Cu-Al-Ni alloy
    (Thesis (M.S.)- Bogazici University. Institute for Graduate Studies in Science and Engineering, 1986., 1986.) Selimbeyoğlu, Erol.; Altıntaş, Sabri.
    In this study, the microstructure and mechanical property relationship of Cu-13.98Al-2.94Ni (wt.pct.) alloy that exhibits shape-memory effect is investigated. A brief review of thermoelasticity, pseudoelasticity, and memory effects are presented together with the complementary relationship of shape-memory effect and pseudoelastie effect. Then the mechanical behaviour, mechanisms, and some applications of shape-memory alloys are given. After these, the Cu-Al-Ni alloy system is explained in detail. In the experimental work, the Cu-Al-Ni alloy system is produced and the mechanical properties are measured with bending and hardness testing. The microstructures are examined using the techniques of optical microscopy. For the present alloy, room temperature is between Ms and As· Therefore, it is either in the parent phase or in the martensitic state at room temperature; The shape-memory effect occured on deformation in both states. Deformation caused the formation or reorientation of the martensite plates dependingon the initial position of the alloy. When the specimen was heated, the martensite plates formed in both states disappeared and the specimen reverted back to its original shape.