ABSTRACT
In this paper, the time- and temperature-dependent cyclic ratchetting plasticity of the nickel-based alloy IN100 is experimentally investigated in strain-controlled experiments in the temperature range from 300 °C to 1050 °C. To this end, uniaxial material tests are performed with complex loading histories designed to activate phenomena as strain rate dependency, stress relaxation as well as the Bauschinger effect, cyclic hardening and softening, ratchetting and recovery from hardening. Plasticity models with different levels of complexity are presented that consider these phenomena, and a strategy is derived to determine the multitude of temperature-dependent material properties of the models in a step-by-step procedure based on sub-sets of experimental data of isothermal experiments. The models and the material properties are validated based on the results of non-isothermal experiments. A good description of the time- and temperature-dependent cyclic ratchetting plasticity of IN100 is obtained for isothermal as well as non-isothermal loading with models including ratchetting terms in the kinematic hardening law and the material properties obtained with the proposed strategy.
ABSTRACT
Maintaining appropriate sedation and analgesia in pediatric burn patients can be quite challenging and often requires high doses of analgesics and anxiolytics because tolerance quickly develops. Escalating doses of opioids and benzodiazepines provide little additional benefit while increasing the incidence of side effects. Dexmedetomidine (DEX) is a novel alpha2-adrenergic agonist that provides sedation, anxiolysis, and analgesia with much less respiratory depression than other sedatives. In addition, DEX stimulation of alpha2 receptors on pancreatic beta cells may inhibit insulin secretion. Hyperglycemia has not been studied specifically in patients receiving DEX. Therefore, we hypothesized that DEX would improve sedation compared with our standard sedation regimen. In addition, we studied the effects of DEX on blood glucose levels. We performed a retrospective chart review of 65 pediatric burn patients (42 boys, 23 girls) in the intensive care unit admitted between 2001 and 2004 who received DEX infusion because of failure to achieve adequate sedation with our standard regimen of opioids and benzodiazepines. We recorded age, TBSA burn size, weight, dose and duration of infusion, adequacy of sedation before and after initiation of DEX, blood glucose levels before and after DEX, and the presence or absence of mechanical ventilation. The mean age was 5 years (range, 0.6-17), burn size was 36% TBSA (range, 3-94), and weight was 26 kg (range, 8-100). All patients were rated "inadequately sedate" before DEX infusion was initiated at 0.2 mug/kg/hr and titrated to effect. Twenty-six patients received a loading dose of 1 mug/kg. The average duration of DEX infusion was 11 days (range, 2-50), and no tachyphylaxis was noted. The mean dose was 0.5 mug/kg/hr (range, 0.1-2). Infusions were weaned over the course of 12 to 24 hours without evidence of rebound hypertension or withdrawal. With DEX titration, all patients were rated "adequately sedate," even though all were sedation failures with opioids and benzodiazepines. Eleven of 42 patients receiving ventilatory support were extubated while on DEX infusion, and no patient showed evidence of DEX induced respiratory depression. Patient's blood glucose levels averaged 121.2 +/- 8.9 mg/dl while on DEX infusion and 117.1 +/- 12.1 mg/dl while off, a nonsignificant difference.
