Enrique A. Medina

Optimization of microstructure during deformation processing using control theory principles

Abstract A two stage approach based on modem control theory has been proposed to control the microstructure development during hot working. This method was utilized for optimal design of hot extrusion process. In the first stage, equations for dynamic recrystallization of plain carbon steel were utilized to obtain an optimal deformation path such that the grain size of the product would be 26 μm. In the second stage, the geometric mapping was utilized to develop an extrusion die profile such that the strain rate profile during extrusion matches with the optimal trajectory computed in the first stage. An extrusion experiment was performed to validate the proposed methodology, by utilizing the extrusion die geometry obtained in the second stage. The as-extruded grain size was observed to be in close agreement with the optimal design performed in the first stage. The results of the present investigation revealed that the principles of control theory can be reliably applied for the optimization and control of microstructure during deformation processing.

Demonstration study for reliability assessment of SHM systems incorporating model-assisted probability of detection approach

This paper presents the results of a demonstration featuring the application of a validation protocol to a vibration-based structural damage sensing system. The results of the full validation study highlight the general protocol feasibility, emphasize the importance of evaluating key application characteristics prior to the POD study, and demonstrate an approach to quantify varying sensor durability on the POD performance. Challenges remain to properly address long time-scale effects with accelerated testing and large testing requirements due to the independence of the inspection of each flaw location.

MODEL‐ASSISTED PROBABILISTIC RELIABILITY ASSESSMENT FOR STRUCTURAL HEALTH MONITORING SYSTEMS

This paper describes a model‐assisted probabilistic methodology to ensure the reliability of SHM systems for damage detection, localization, and sizing. A hierarchical approach is presented that attempts to minimize the number of samples, the length of time, and degree of full‐scale testing required for statistically meaningful characterization results. The feasibility of applying this approach to typical sensing methods found in SHM systems is investigated, and additional challenges concerning model reliability and uncertainty propagation are addressed.

Optimization of Microstructure Development: Application to Hot Metal Extrusion

A new process design method for controlling microstructure development during hot metal deformation processes is presented. This approach is based on modern control theory and involves state- space models for describing the material behavior and the mechanics of the process. The challenge of effectively controlling the values and distribution of important microstructural features can now be systematically formulated and solved in terms of an optimal control problem. This method has been applied to the optimization of grain size and certain process parameters such as die geometry profile and ram velocity during extrusion of plain carbon steel. Various case studies have been investigated, and experimental results show good agreement with those predicted in the design stage.

Probabilistic Risk Assessment: Impact of Human Factors on Nondestructive Evaluation and Sensor Degradation on Structural Health Monitoring

Managing human factors in nondestructive evaluation is critical for maintaining inspection reliability. Reliability of structural health monitoring systems is particularly sensitive to sensor degradation over time. To investigate the impact of these issues, probabilistic models for risk assessment and cost‐benefit analysis tools have been developed. Quantitative studies are presented evaluating the effects of variations in probability of detection associated with human factors, plus in‐situ sensor degradation effects on life cycle measures such as cost and probability of failure.

ANALYTICAL MODEL OF THE SCATTERING OF GUIDED WAVES FROM FASTENER SITES WITH A STIFFNESS INTERFACE

In guided wave crack detection for structural health monitoring, there is a critical need to understand the sensitivity to joint interface conditions. Analytical models are presented for the interaction of ultrasonic waves from a cylindrical hole with an elastic insert coupled by a stiffness interface. Parametric studies are presented investigating the effect of variation in the contact conditions on the reflection and generation of secondary waves from the fastener.