The effects of solidification conditions and heat treatment on the microstructure and mechanical properties of EN-AC 44400 alloy

Abstract. Improved mechanical and physical properties of an Al-Si alloy as a well-known casting alloy is strongly dependent upon the morphology of silicon particles, Al grains and also type of intermetallics which are in turn a function of alloy composition, solidification rate and heat treatment. This study aims at investigating the influence of the different solidification conditions (high pressure die, gradient and sand cast) and heat treatment on the microstructure (dendrite parameters, silicon particle morphology, intermetallic compounds), mechanical properties and fracture surface appearance of Al-9Si-4Mn alloy. To identify the features of microstructure and fracture surface analysis, a combination of optical metallography, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) have been employed. The results show that the cooling rate has a strong effect on the evolution of intermetallics, morphology of the silicon and dendrite parameters. Introduction

The effects of solidification conditions and heat treatment on the microstructure and mechanical properties of EN-AC 44400 alloy

Abstract. Improved mechanical and physical properties of an Al-Si alloy as a well-known casting alloy is strongly dependent upon the morphology of silicon particles, Al grains and also type of intermetallics which are in turn a function of alloy composition, solidification rate and heat treatment. This study aims at investigating the influence of the different solidification conditions (high pressure die, gradient and sand cast) and heat treatment on the microstructure (dendrite parameters, silicon particle morphology, intermetallic compounds), mechanical properties and fracture surface appearance of Al-9Si-4Mn alloy. To identify the features of microstructure and fracture surface analysis, a combination of optical metallography, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) have been employed. The results show that the cooling rate has a strong effect on the evolution of intermetallics, morphology of the silicon and dendrite parameters. Introduction

Effect of Strain rate on Stress-Strain Properties and Yield Propagation in Steel Reinforcing Bars

Report No. CCEER-07-02Dynamic loading due to earthquakes alters the stress-strain properties and strain distribution in steel reinforcement. Past research on dynamic load effects has been under constant strain rates, and the applicability of results to variable strain rates caused by earthquakes is uncertain. In the study presented herein, the effect of variable strain rate and distribution of strain along the length of steel bars were studied experimentally. The specimens were ASTM Grade 60 with #3, #8, and #11 sizes, and the strain rates ranged from 0.0001 to 0.1 strain per second. The analysis of data showed that the rate of change in strain rate (second derivative of strain) is an important factor. The data also revealed concentration of strain at higher strain rates. Based on the results new simple equations to estimate the yield and ultimate strength of the bars were proposed and compared with other available data

Analytical Study of NEESR-SG 4-Span Bridge Model Using OpenSees

Report No. CCEER-07-03This study focuses on the development of a 4-span bridge analytical model using OpenSees. Three ¼ -scale 4-span bridge models are included for testing on the University of Nevada, Reno (UNR) shake tables as a part of a collaborative Network for Earthquake Engineering Simulation Research-Small Group (NEESR-SG) study entitled Seismic Performance of Bridge Systems with Conventional and Innovative Design. The columns of the first bridge model include conventional steel and concrete, while in the other two models, innovative materials will be incorporated in the bridge columns. This pre-test analytical study is intended to predict the performance of the first bridge model during shake table testing and develop motions to be used at the abutments during shake table testing. The bridge piers are of drop-cap type with a continuous post-tensioned deck connected to three, two-column bent caps and roller type connections at the abutments. The bridge model will be tested under bilateral horizontal motions applied by the UNR shake tables at the bridge pier footings and longitudinal motions applied at the abutments through actuators to simulate the abutment interaction with the bridge. To estimate the bridge-abutment interaction and its effect on the bridge response, three different models are developed. The first model (Model 1) represents a bridge with no abutment interaction. In this model the bridge deck ends are supported on rollers so the bridge deck is free to move in both horizontal directions. The abutment interaction is included in the second bridge model (Model 2). In this model, the bridge deck ends are assumed to be supported on the roller. The abutment consists of the backwall and the backfill soil behind it. The soil stiffness is represented by a nonlinear spring. An initial gap of 0.5 in (12.7 mm) is assumed between the deck and the backwall. The third bridge model (Model 3) is developed to represent the actual bridge test setup with the abutment springs replaced by the actuators at the ends. The displacement histories recorded at the bridge deck end nodes in Model 2 are used as the actuator input at the bridge deck end nodes in Model 3. This report presents the modeling assumptions and the predicted results. To evaluate the adequacy of the analytical modeling method, an analytical model of a two-span bridge for which test data were available was used. Results were found to be reasonably close. In addition to the response of the two-span bridge, this report summarizes important calculated response parameters for the 4-span bridge

Experimental Evaluation of Performance of Conventional Bridge Systems

Report No. CCEER-07-04As part of a multi-university research project utilizing the Network for Earthquake Engineering Simulation (NEES), a quarter-scale 110 ft (33.5 m) long asymmetric conventional reinforced concrete bridge model was tested using the shake table system at the University of Nevada, Reno. The model consisted of four spans supported on three, two-column bents of a drop-cap design and two abutment seats driven by hydraulic actuators capable of dynamic excitation. The objective of shake table testing was to study the response and performance of a contemporary reinforced concrete bridge model subjected to biaxial horizontal earthquake excitation. This included the effects of in-plane rotation, force distribution among bents, and abutment interaction. The model was designed in accordance with the provisions of the National Cooperative Highway Research Program (NCHRP) document 12-49. It represented an assumed prototype located in the Los Angeles area. The ground motion that was simulated was the Century City record of the 1994 Northridge earthquake. It was applied in six test runs with increasing amplitudes. At the end of Test 6, with a PGA of 1.00 g in the transverse direction and 1.20 g in the direction, some of the columns showed sign of imminent failure. A seventh test was conducted repeating the same amplitudes as those of Test 6. The model met all performance requirements for both the expected and rare earthquakes when subjected to equivalent excitations. Failure occurred during Test 7 in bent 1. The bent failed in flexure with rupture of both longitudinal and transverse steel reinforcement and crushing of core concrete in the bottom plastic hinge of the east columns. Bents 2 and 3 only exhibited spalling and cracking of the cover concrete in the plastic hinge regions, revealing some of the reinforcing bars in bent 3. Displacement ductilities of 11.7, 3.4, and 8.3 were reached in bents 1, 2, and 3 respectively. Closing of the abutment backwall-superstructure gap occurred first during Test 2, which somewhat limited the longitudinal displacement of the superstructure

A Study of Concrete Bridge Columns Using Innovative Materials Subjected to Cyclic Loading

Report No. CCEER-07-01In an attempt to improve performance of concrete bridge columns subjected to strong earthquakes, three 0.2 scale circular columns using innovative materials in the plastic hinge zone were tested under cyclic loading. The first column (RSC) utilized conventional concrete and steel reinforcement, the other two (RNC and RNE) incorporated shape memory alloy (SMA) longitudinal reinforcement, and the third (RNE) utilized engineered cementitious composites (ECC) in the plastic hinge. The average ratio of residual to maximum displacement in RSC, RNC, and RNE was 0.82, 0.27 and 0.14, respectively, indicating substantial benefits of using innovative materials. RNE experienced the least damage and highest drift capacity among the three columns. The test results showed that SMA and ECC are very effective in improving serviceability of bridges after earthquakes. Analytical studies using the program OpenSees led to reasonable estimates of residual drifts, overall cyclic response, and forces when compared to the experimental results

Seismic Load Path in Steel Girder Bridge Superstructures

Report No. CCEER-05-3Past earthquakes have shown that the inherent strength assumed in a bridge superstructure does not always prevent damage to components of steel plate girder superstructures during moderate to large earthquake excitation. Cyclic and shake table experiments on a 2/5th scale model of a simply supported steel girder bridge superstructure, with two steel plate I-girders and a reinforced concrete deck slab, were performed to determine the effects of transverse seismic loading on this type of bridge superstructure in high seismic regions. From experiments it is shown the superstructure deforms in a flexural-torsional mode with transverse seismic forces distributed along the span and resisted at the supports. At the supports, the loads are transferred from the deck slab into the girders through the shear studs located near the girder supports or through a top chord connecting the deck slab to the end cross frames. The transverse loads are then distributed from the top into the base of the girders through the end cross frames and into the substructure through the bearings and transverse bearing restraints. Each of these critical components should be designed for transverse seismic loading

Seismic Vulnerability Evaluation and Retrofit Design of Las Vegas Downtown Viaduct

Report No. CCEER-05-6This study focused on the seismic vulnerability assessment and development of retrofit methods for Las Vegas Downtown Viaduct. The viaduct was built in 1960�s and similar to other bridges designed prior to 1970, lacks the necessary details to provide sufficient ductility capacity for dynamic loading caused by earthquakes. Several aspects of the behavior of the bridge were studied both at the system level and at component level. The studies at the system level included pushover analysis of the entire structure and an evaluation of the ductility demand. Another aspect of the system behavior was a general study of the effects of incoherent ground motions on the forces and displacements of the bridge. Other segments of the study included the seismic evaluation of the ramp structures, the development of retrofit strategies for single-column bents, and the development of retrofit details for multi-column bents. Several research reports providing the details of the above aspects of the study have been prepared. This report presents a brief summary of the highlights of those studies. The study showed that the majority of the columns are in need of seismic retrofit but the as-built cap beams are adequate. For a thorough evaluation of the seismic demand the effect of incoherent ground motions has to be included. The column and pedestal retrofit methods developed and verified in this study proved to be effective and more economical than standard retrofit. Retrofit design methods are presented for both single-column and multi-column bents