Jeom Yong Choi

High Strain-Rate, Small Strain Response of a NiTi Shape-Memory Alloy

The compressive response of a NiTi shape-memory alloy is investigated at various strain rates using UCSDs modified 1/2-in. Hopkinson pressure bar and a conventional Instron machine. To obtain a constant strain rate during the formation of a stress-induced martensite in a Hopkinson test, a copper tube of suitable dimensions is employed as a pulse shaper, since without a pulse shaper the strain rate of the sample varies significantly as its microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. The NiTi shape-memory alloy shows a superelastic response for small strains at all considered strain rates and at room temperature, 296 K. At this temperature and below a certain strain rate, the stress-strain curves of the NiTi shape-memory alloy display two regimes: an elastic austenite regime and a transition (stress-induced martensite) regime. The transition stress of this material and the work-hardening rate in the stress-induced martensite regime increase with increasing strain rate, the latter reaching a steady state level and then rapidly increasing.

Quasi-Static and Dynamic Buckling of Thin Cylindrical Shape-Memory Shells

To investigate the buckling behavior of thin and relatively thick cylindrical shape-memory shells, uniaxial compression tests are performed at a 295 K initial temperature, using the CEAM/UCSDs modified split Hopkinson bar systems and an Instron hydraulic testing machine. The quasi-static buckling response of the shells is directly observed and recorded using a digital camera with a close-up lens and two back mirrors. To document the dynamic buckling modes, a high-speed Imacon 200 framing camera is used. The shape-memory shells with an austenite-finish temperature of A f =281 K, buckle gradually and gracefully in quasi-static loading, and fully recover upon unloading, showing a superelastic property, whereas when suitably annealed, the shells do not recover spontaneously upon unloading, but they do so once heated, showing a shape-memory effect. The thin shells had a common thickness of 0.125 mm a common outer radius of 2.25 mm (i.e., a common radius, R, to thickness, t, ratio, R/t, of 18). A shell with the ratio of length, L, to diameter, D (LID) of 1.5 buckled under a quasi-static load by forming a nonsymmetric chessboard pattern, while with a LID of 1.95 the buckling started with the formation of symmetrical rings which then changed into a nonsymmetric chessboard pattern. A similar buckling mode is also observed under a dynamic loading condition for a shell with LID of 2. However, thicker shells, with 0.5 mm thickness and radius 4 mm (R/t=8), buckled under a dynamic loading condition by the formation of a symmetrical ring pattern. For comparison, we have also tested shells of similar geometry but made of steel and aluminum. In the case of the steel shells with constrained end conditions, the buckling, which consists of nonsymmetric (no rings) folds (chessboard patterns), is sudden and catastrophic, and involves no recovery upon unloading. The gradual buckling of the shape-memory shells is associated with the stress-induced martensite formation and seems to have a profound effect on the unstable deformations of thin structures made from shape-memory alloys.

Extended strain hardening by a sequential operation of twinning induced plasticity and transformation induced plasticity in a low Ni duplex stainless steel

Extended strain hardening was realized by a sequential operation of twinning induced plasticity (TWIP) followed by transformation induced plasticity (TRIP) in a Fe-20Cr-3Mn-2Cu-1Ni-1Si-0.2N duplex stainless steel (DSS). As a result, the present DSS exhibited an excellent combination of strength — ductility of 900 MPa which was 75% superior to that of conventional DSSs. The deformed microstructures of the present DSS revealed that strain induced martensite (SIM) causing TRIP primarily nucleated at intersections of mechanical twins without formation of ε martensite which is an intermediate phase during SIM transformation. In addition, the sequential operation of TWIP-TRIP enables strain hardening to be extended to higher strains compared to the operation of TWIP alone.

Experimental observation of high-rate buckling of thin cylindrical shape-memory shells

We investigate the buckling behavior of thin cylindrical shape-memory shells at room temperature, using a modified split Hopkinson bar and an Instron hydraulic testing machine. The quasi-static buckling response is directly observed using a digital camera with a close-up lens and two back mirrors. A high-speed Imacon 200 framing camera is used to record the dynamic buckling modes. The shape-memory shells with an austenite-finish temperature less than the room temperature, buckle gradually and gracefully in quasi-static loading, and fully recover upon unloading, showing a superelastic property, whereas when suitably annealed, the shells do not recover spontaneously upon unloading, but they do so once heated, showing a shape-memory effect. The gradual and graceful buckling of the shape-memory shells is associated with the stress-induced martensite formation and seems to have a profound effect on the unstable deformations of thin structures made from shape-memory alloys.