PSI - Issue 64

Ali Jafarabadi et al. / Procedia Structural Integrity 64 (2024) 2059–2066 A. Jafarabadi et al./ Structural Integrity Procedia 00 (2019) 000 – 000

2061

3

couplers with a diameter of 30 mm, meeting the requirements for pipe jointing in general industrial applications. To assess the performance of the joints under dynamic loads, Cao et al. (Cao et al., 2023, Cao et al., 2022) conducted a comprehensive series of studies characterizing the axial and bending strength of shape memory pipe couplers under low and high velocity impact loads. Piotrowski et al. (Piotrowski, 2010) have experimentally investigated the tightening performance a NiTi SMA through quantification of the interface contact pressure. This study aims to shed light on the clamping capacity of iron-based shape memory couplers considering the effect of heat-treatment post-processing providing new insights on designing such joints in an optimal manner. 2. Design and experimental investigation In the case of a thick-walled open-ended Fe-SMA tube loaded under internal pressure, the strain and stress develop at radial (compressive) and circumferential (tensile) directions with non-uniform distribution. In the linear elastic problem, the principal stresses peak at the inner diameter for both the radial and circumferential components, denoted as and , respectively. However, when the cross-section is fully inelastic, the distribution of changes such that the max value peaks at the outer diameter (Chakrabarty, 2012). Through the pre-straining process of Fe-SMA tubes the cross section experiences a fully inelastic state with a non-uniform distribution. Thus, a non-uniform biaxial pre straining forms throughout the cross-section. That is, stress-induced martensite forms in both the radial and circumferential directions. Therefore, the SME is affected by the direction and interaction of biaxial martensite. As a result, a complex pre-straining state occurs in the material making it challenging to assess the SME for the case of Fe SMA tubes. However, it is feasible to interpret the overall SME performance through tracking of the resultant pressure that the Fe-SMA tube can apply on a substance, restricting its free recovery. For this purpose, an experimental setup was employed consisting of Fe-SMA and steel tubes, as shown in Fig.1. The contact pressure at the interface, denoted as P, was determined using the theory of elasticity. This calculation relies on the measured circumferential and axial strain components (ε θ and ε a ) at the inner diameter of the steel tube, which is formulated as: =− 2(1 + ) 2 − 2 2 ( − ) (1) where E and are the elastic properties of the steel tube —Young’s modulus and the Poisson’s ratio , and d i and d o are the inner diameter and outer diameter for the steel tube. Table 1. Fe-SMA tubes geometrical, expansion and post-processing parameters

Sample ID

Initial Inner Diameter ( mm )

Finished Inner Diameter ( mm)

Thickness ( mm )

Diameter expansion ( mm )

Treatment

IID26T2HT * IID26T2AR

26 26

28 28

2 2

2 2

HT a AR b

* Initial inner diameter (IID), Thickness (T), Heat-treated (HT) a Heat-treated b As-received

The outer diameter of the steel tube was selected to match the finished (expanded) inner diameter (FID) of Fe SMA tubes which is 28mm. This ensured that the tolerances for machining the steel tube and the FID of the Fe-SMA tube were adjusted to allow for a minimal gap of approximately 0.1mm for accommodating the steel tube inside the Fe-SMA tube. Additionally, the thickness of the steel tubes was selected to balance the sensitivity to low interface contact pressures, while preventing inelastic deformations under higher contact pressures during activation. With these considerations, a wall thickness of 3mm was deemed appropriate. To validate the design of the steel tubes, a basic finite element model was employed to ensure that the principal strains at the inner diameter of the steel tube, resulting from an outer pressure ranging from 20 to 150 MPa, remained within reasonable limits. After machining the as-received Fe-SMA tubes according to initial values mentioned in Table 1, samples were subjected to a two-step heat treatment (aged at 600 °C for 20 h followed by aging at 680 °C for 8 h) followed by air cooling. Then, for the purpose of pre-straining of as-received and heat-treated Fe-SMA tubes, a commercial tube end forming tool was utilized. The process involves uniform expansion of the Fe-SMA up to a certain target diameter —