Application of In Situ Neutron Diffraction to Characterize Transient Material Behavior in Welding

A fundamental understanding of the transient and nonequilibrium material behavior during welding is essential in the pursuit of process control and optimization to produce defect-free, structurally sound, and reliable welds. The deep penetration capability of neutrons into most metallic materials makes neutron diffraction a unique and powerful tool in understanding the material structures and properties. However, the inadequate neutron flux limits its application in time-resolved study of transient material behavior. This article highlights recent developments toward in situ time-resolved neutron diffraction measurement of material behavior during welding with two examples: (I) measurement of the transient temperature and thermal stresses during friction-stir welding of an aluminum alloy and (II) measurement of the solid-state phase transformation behavior of an advanced high-strength steel under thermal conditions comparable to the welding processes. These newly developed experimental approaches can be broadly applied to other welding or thermomechanical processes for time-resolved measurement of the fast-changing material state in structural metals.     

MICROCHANNEL COMBUSTOR EVAPORATOR THERMAL PROCESSES

This article presents the results of an experimental investigation of the performance of an integrated microchannel combustor evaporator. Separate experimental investigations were conducted on combustion in small channels and evaporation in microchannels. These results, along with the results of a simulation of heat transfer from the flue gas in microchannels, were used to design, and subsequently fabricate and test, microchannel combustor evaporators. Performance results showed that the combustor evaporator was capable of achieving high heat fluxes (up to 30 W/cm2) while maintaining a high combustor efficiency.     

Statistical tolerance synthesis using distribution function zones

Tolerance is one of the most important parameters in design and manufacturing. Tolerance synthesis has a significant impact on manufacturing cost and product quality. In the international standards community two approaches for statistical tolerancing of mechanical parts are being discussed: process capability indices and distribution function zone. The distribution function zone (DFZone) approach defines the acceptability of a population of parts by requiring that the distribution function of relevant values of the parts be bounded by a pair of specified distribution functions. In order to apply this approach to statistical tolerancing, one needs a method to decompose the assembly level tolerance specification to obtain tolerance parameters for each component in conjunction with a corresponding tolerance-cost model. This paper introduces an optimization-based statistical tolerance synthesis model based on the DFZone tolerance specifications. A new tolerance-cost model is proposed and the model is illustrated with an assembly example.     

An improved water wave optimization algorithm with the single wave mechanism for the no-wait flow-shop scheduling problem

In this article, a water wave optimization algorithm with a single wave mechanism, called single water wave optimization (SWWO), is proposed to solve the no-wait flow-shop scheduling problem (NWFSP) with the objective of minimizing the makespan. In the proposed SWWO, an improved Nawaz–Enscore–Ham (NEH) heuristic is applied to construct a high-quality initial candidate. In the propagation operation, a self-adaptive block-shift operation is employed. In the breaking operation, a variable neighbourhood search operation is utilized to explore the local optimal solution. According to the schema theory as presented in genetic algorithms, a crossover operation is adopted as the refraction operation. Finally, the computational results based on several benchmarks and statistical performance comparisons are presented. The experimental results demonstrate the effectiveness and efficiency of the proposed SWWO for solving the NWFSP.     

High temperature vacuum assisted resin transfer molding of phenylethynyl terminated imide composites

Vacuum Assisted Resin Transfer Molding (VARTM) has proven to be a cost effective process for manufacturing composite structures compared with prepreg/autoclave and traditional resin transfer molding (RTM) processes. However, VARTM has not been accomplished with high temperature resins (such as polyimides) until recently, primarily because no resins had low melt viscosity and long melt stability that are required by VARTM. With the recent invention of phenylethynyl terminated imides (PETIs), high temperature VARTM has been achieved. Two processing methods, in‐plane and through‐thickness resin flow, were proposed and tested. Both methods are capable of fabricating polyimide matrix composites; and the carbon fiber laminates yield good fiber‐resin interfacial bonding and comparable mechanical properties to those laminates fabricated using RTM. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Perception for collision avoidance and autonomous driving

The Navlab group at Carnegie Mellon University has a long history of development of automated vehicles and intelligent systems for driver assistance. The earlier work of the group concentrated on road following, cross-country driving, and obstacle detection. The new focus is on short-range sensing, to look all around the vehicle for safe driving. The current system uses video sensing, laser rangefinders, a novel light-stripe rangefinder, software to process each sensor individually, a map-based fusion system, and a probability based predictive model. The complete system has been demonstrated on the Navlab 11 vehicle for monitoring the environment of a vehicle driving through a cluttered urban environment, detecting and tracking fixed objects, moving objects, pedestrians, curbs, and roads.     

Emitter spacing effects on field emission properties of laser-treated single-walled carbon nanotube buckypapers

Carbon nanotube (CNT) emitters on buckypaper were activated by laser treatment and their field emission properties were investigated. The pristine buckypapers and CNT emitters' height, diameter, and spacing were characterized through optical analysis. The emitter spacing directly impacted the emission results when the laser power and treatment times were fixed. The increasing emitter density increased the enhanced field emission current and luminance. However, a continuous and excessive increase of emitter density with spacing reduction generated the screening effect. As a result, the extended screening effect from the smaller spacing eventually crippled the field emission effectiveness. Luminance intensity and uniformity of field emission suggest that the highly effective buckypaper will have a density of 2500 emission spots cm(-2), which presents an effective field enhancement factor of 3721 and a moderated screening effect of 0.005. Proper laser treatment is an effective post-treatment process for optimizing field emission, luminance, and durability performance for buckypaper cold cathodes.     

Influence of alcohol pre‐infusion on the quality of VARTM composites

In the vacuum assisted resin transfer molding (VARTM) process, part‐to‐part variations such as the uncertainty in the permeability and race tracking phenomenon make it difficult to achieve consistent mold filling and ensure part quality of composites. Alcohol pre‐infusion was presented in this study as a novel real‐time monitoring and control approach for the flow process in the VARTM process, alcohol test fluid is infused before the actual resin infusion to locate the potential dry spots without using the large quantity of sensors. Then corresponding process control strategy is designed, such as opening the auxiliary gate at specific moment on those predicted dry spot locations to compensate flow defects. Moreover, alcohol can be easily removed by heat without changing the local permeability. The influence of alcohol pre‐infusion on the quality of VARTM composites were investigated in this study. The mechanical tests were conducted to verify that the alcohol pre‐infusion approach has no significant effect on composite properties because alcohol can be removed from fiber by heat and air flow. Specifically, DMA, TGA, and FTIR spectrum proved that negligible difference existed on the resin–fiber interface between the composites with or without alcohol pre‐infusion. Finally, the microscopy results revealed a similar failure path in a resin matrix. TMA results also demonstrated similar dimension stability. This alcohol pre‐infusion approach was effective when compared with computer simulation and could eliminate the occurrence of dry spots and voids without using sensors or data‐acquisition system. The control schemes were shown in a case study to be capable of compensating the flow defects and achieving desired fill patterns in the face of permeability uncertainty. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Processing,characterization, and modeling of carbon nanotube-reinforced multiscale composites

Carbon fiber-reinforced epoxy composites modified with carbon nanotubes (CNTs) were fabricated and characterized. High-energy sonication was used to disperse CNTs in the resin, followed by infiltration of fiber preform with the resin/CNT mixture. The effects of sonication time on the mechanical properties of “multiscale” composites, which contain reinforcements at varying scales, were studied. A low CNT loading of 0.3 wt% in resin had little influence on tensile properties, while it improved the flexural modulus, strength, and percent strain to break by 11.6%, 18.0%, and 11.4%, respectively, as compared to the control carbon fiber/epoxy composite. While sonication is an effective method to disperse CNTs in a resin, duration, intensity, and temperature need to be controlled to prevent damages imposed on CNTs and premature resin curing. A combination of Halpin–Tsai equations and woven fiber micromechanics was used in hierarchy to predict the mechanical properties of multiscale composites, and the discrepancies between the predicted and experimental values are explained.