Browsing by Subject "hysteresis"
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Item Engineering the Martensitic Transformation Hysteresis of Ni-Rich NiTi Alloys(2014-12-18) Franco, Brian EelanThe shape memory behavior in NiTi alloys can be exploited for a wide variety of applications that require active materials. The application dictates the transformation temperatures and hysteresis of the alloy. NiTi alloys with high Ni contents can be precipitation heat treated to produce large changes in the transformation temperatures, as well as increases in strength and dimensional stability. The effect of aging on the Ms temperature has been previously studied in the literature; however, few studies have investigated long duration aging, and little attention has been paid to the effect of precipitate formation on the thermal hysteresis. In the current work, a systematic study of heat treatments was performed to study the effect of aging time, aging temperature, and initial Ni composition on the transformation temperatures and the thermal hysteresis under zero stress conditions using differential scanning calorimetry. Heat treatments and NiTi compositions were chosen in order to ensure that only Ni4Ti3 type precipitates formed during the aging process. The results showed aging led to three different transformation paths; single step B2-B19`, B2-R-B19`, and multiple step transformation. At low aging temperatures, the transformation temperatures were initially suppressed but increased after sufficient aging durations. At higher aging temperatures the transformation temperatures only increased with aging time. In high Ni content materials the transformation temperatures were suppressed more than in the low Ni content materials. The thermal hysteresis was highest in materials aged at short times at low temperatures, when the average spacing between adjacent precipitates was small. As the materials continued to aged, the thermal hysteresis decreased with time. As the aging temperature increased, the thermal hysteresis decreased.Item Experimental Investigation of Wind-Forced Drop Stability(2012-10-19) Schmucker, JasonThe stability of drops forced by both wind and gravity is a fluid mechanics problem relevant to heat exchangers, fuel cells, and aircraft icing. To investigate this phenomenon, drops from 15 micro-liters to 400 micro-liters were placed on the rough aluminum (RA = 3.26 micrometers) floor of a tiltable wind tunnel and brought to critical conditions, when the drop begins to run downstream. Various combinations of drop size, inclination angle, and flow speed were employed. A measurement technique capable of measuring full 3D drop profiles was implemented to investigate the drops' evolution toward runback. The measurement requires the comparison of the speckle pattern captured by an overhead drop image with a corresponding image of the dry surface. Stability limits for 235 drops are measured as functions of drop volume and surface inclination. Drops experiencing airflow alone are found to shed at a Weber number of 8.0 +/- 0.5. From measurement sequences of reconstructed drop profiles, the evolution of contact lines, drop profiles, and contact angle distributions are detailed. Contact line integral adhesion forces are calculated from contact angle distributions and related to the forcing air velocity. Drops whose stability limits are dominated by gravity are found to exhibit significantly different evolution toward runback than those dominated by airflow.Item Nonlinear dynamics of hysteretic oscillators(2009-05-15) Shekhawat, AshivniThe dynamic response and bifurcations of a harmonic oscillator with a hysteretic restoring force and sinusoidal excitation are investigated. A multilinear model of hysteresis is presented. A hybrid system approach is used to formulate and study the problem. A novel method for obtaining exact transient and steady state response of the system is discussed. Simple periodic orbits of the system are analyzed using the KBM method and an analytic criterion for existence of bound and unbound resonance is derived. Results of KBM analysis are compared with those from numerical simulations. Stability and bifurcations of higher period orbits are studied using Poincar?e maps. The Poincar?e map for the system is constructed by composing the corresponding maps for the individual subsystems of the hybrid system. The novelty of this work lies in a.) the study of a multilinear model of hysteresis, and, b.) developing a methodology for obtaining the exact transient and steady state response of the system.Item Structural Thermomechanical Models for Shape Memory Alloy Components(2014-04-18) Rao, AshwinThermally responsive shape memory alloys (SMA) demonstrate interesting properties like shape memory effect (SME) and superelasticity (SE). SMA components in the form of wires, springs and beams typically exhibit complex, nonlinear hysteretic responses and are subjected to tension, torsion or bending loading conditions. Traditionally, simple strength of materials based models/tools have driven engineering designs for centuries, even as more sophisticated models existed for design with conventional materials. In light of this, an effort to develop strength of materials type modeling approach that can capture complex hysteretic SMA responses under different loading conditions is undertaken. The key idea here is of separating the thermoelastic and the dissipative part of the hysteretic response by using a Gibbs potential and thermodynamic principles. The dissipative part of the response is later accounted for by a discrete Preisach model. The models are constructed using experimentally measurable quantities (like torque?twist, bending moment?curvature etc.), since the SMA components subjected to torsion and bending experience an in-homogeneous non-linear stress distribution across the specimen cross-section. Such an approach enables simulation of complex temperature dependent superelastic responses including those with multiple internal loops. The second aspect of this work deals with the durability of the material which is of critical importance with increasing use of SMA components in different engineering applications. Conventional S-N curves, Goodman diagrams etc. that capture only the mechanical loading aspects are not adequate to capture complex thermomechanical coupling seen in SMAs. Hence, a novel concept of driving force amplitude v/s number of cycles equivalent to thermodynamical driving force for onset of phase transformations is proposed which simultaneously captures both mechanical and thermal loading in a single framework. Recognizing the paucity of experimental data on functional degradation of SMAs (especially SMA springs), a custom designed thermomechanical fatigue test rig is used to perform user defined repeated thermomechanical tests on SMA springs. The data from these tests serve both to calibrate the model and establish thermodynamic driving force and extent of phase transformation relationships for SMA springs. A drop in driving force amplitude would suggest material losing its ability to undergo phase transformations which directly corresponds to a loss in the functionality/smartness of SMA component. This would allow designers to set appropriate driving force thresholds as a guideline for analyzing functional life of SMA components.