Xuming Su

Numerical Simulation of Water Quenching Process of an Engine Cylinder Head

Presented is a simulation of an engine cylinder head undergoing water quenching process using a recently developed approach for modeling quenching cooling of metal parts (Wang et al., 2002). The approach is based on the AVL SWIFT Eulerian two-fluid method with special emphasis on handling high mass exchange rate associated with quenching. A tetrahedral grid of 830,000 cells is generated for the computational domain, which includes the solid part of the cylinder head immersed in the fluid. Detailed vapor and temperature distributions are obtained which offer valuable information for the thermal stress analysis. It is observed that the temperature field within the cylinder head is highly non-uniform. The computed cylinder head monitoring point temperature versus time is compared with that registered by the thermal couple measurement. Reasonable agreement is observed. The simulation exercise may potentially be used to identify the cause of cracks often encountered in quenching heat treatment thereby lead to a better design of the process.Copyright

Tensile and fatigue behaviour of self-piercing rivets of CFRP to aluminium for automotive application

In this study, the tensile and fatigue behaviour of self-piercing rivets (SPRs) in carbon fibre reinforced plastic (CFRP) to aluminium 6111 T82 alloys were evaluated. An average maximum lap-shear tensile load capacity of 3858 N was achieved, which is comparable to metal-to-metal SPR lap-shear joints. The CFRP-Al SPRs failed in lap-shear tension due to pull-out of the rivet head from the CFRP upper sheet. The CFRP-Al SPR lap- shear specimens exhibited superior fatigue life compared to previously studied aluminium-to- aluminium SPR lap-shear joints. The SPR lap-shear joints under fatigue loads failed predominantly due to kinked crack growth along the width of the bottom aluminium sheet. The fatigue cracks initiated in the plastically deformed region of the aluminium sheet close to the rivet shank in the rivet-sheet interlock region. Scatter in fatigue life and failure modes was observed in SPR lap-shear specimens tested close to maximum tensile load.

Mechanical Properties of Thermoplastic Olefin Composites: Effect of Fillers Content, Strain Rate and Temperature

The mechanical properties of thermoplastic olefin (TPO) composites with rubber content ranged from 17 to 23 wt% and talc content ranged from 19.5 to 25.5 wt% have been investigated at different strain rates and temperatures. Tensile tests were conducted at temperature of −30, 23, and 80°C, respectively, and strain rate from 10−3 to 10−1/s, respectively. The results showed that that the mechanical properties were modified significantly with the addition of rubber and talc, strain rate and temperature. The necking process of the TPO was rate dependent. At low strain rate, the deformation was close to the isothermal drawing.

Fatigue Predictions of Various Joints of Magnesium Alloys

In this project, a front shock tower of a passenger vehicle is developed with various magnesium alloys and joining methods. To predict the fatigue life of the joints in the structure, fatigue tests of various joint specimens including friction stir linear welding, self-piecing rivet joint with and without adhesive, and friction stir spot welding were conducted. The magnesium alloys used for the specimens are AM60 (cast), AM30 (extrusion), and AZ31 (sheet). Various finite element modeling techniques were attempted for simulating the various joints. Fatigue life prediction method for the joints was performed using the stress-life curve approach. The finite element modeling technique and the fatigue prediction method will be verified with fatigue tests of the actual front shock tower structure subjected to variable amplitude loadings in near future.

An integrated computational materials engineering method for woven carbon fiber composites preforming process

An integrated computational materials engineering method is proposed in this paper for analyzing the design and preforming process of woven carbon fiber composites. The goal is to reduce the cost and time needed for the mass production of structural composites. It integrates the simulation methods from the micro-scale to the macro-scale to capture the behavior of the composite material in the preforming process. In this way, the time consuming and high cost physical experiments and prototypes in the development of the manufacturing process can be circumvented. This method contains three parts: the micro-scale representative volume element (RVE) simulation to characterize the material; the metamodeling algorithm to generate the constitutive equations; and the macro-scale preforming simulation to predict the behavior of the composite material during forming. The results show the potential of this approach as a guidance to the design of composite materials and its manufacturing process.

Fatigue Behavior of AM60B Subjected to Variable Amplitude Loading

Magnesium alloys are considered as an alternative material to reduce vehicle weight due to their weight which are 33% lighter than aluminum alloys. There has been a significant expansion in the applications of magnesium alloys in automotives components in an effort to improve fuel efficiency through vehicle mass reduction. In this project, a simple front shock tower of passenger vehicle is constructed with various magnesium alloys. To predict the fatigue behavior of the structure, fatigue properties of the magnesium alloy (AM60B) were determined from strain controlled fatigue tests. Notched specimens were also tested with three different variable amplitude loading profiles obtained from the shock tower of the similar size of vehicle. The test results were compared with various fatigue prediction results. The effect of mean stress and fatigue prediction method were discussed.

Numerical Modeling of Quench Cooling Using Eulerian Two-Fluid Method

A numerical approach is presented for modeling quenching cooling of solid metal parts. The approach is based on the Eulerian two-fluid model with modifications toward specific needs of boiling heat transfer. Numerical aspects are discussed on handling high phase change rate of boiling, flux formulation in the presence of strong body force, and conjugate heat transfer of solid immersed in a liquid bath. The modeling approach is assessed for a quenching cooling problem with a simple geometry. Comparisons are made between the computed temperature history within the solid and that of measured data using thermocouples. Reasonably well agreement is observed. The study demonstrated that the present modeling approach is able to meet the numerical challenges entailed in quenching cooling and able to predict the quenching cooling phenomena with satisfactory accuracy. Areas for further improvement are also noted.© 2002 ASME

Influence of Low Temperature Forging on Microstructure and Low Cycle Fatigue Behavior of Cast AZ31B Mg Alloy

The effect of low temperature forging on the microstructure, quasi-static response and stress-controlled fatigue behavior of cast AZ31B Mg alloy was investigated. The forging process was conducted at a temperature of 275 °C and a forging rate of 20 mm/s. Fully reversed stress controlled cyclic tests were performed on cast and forged material under total stress amplitudes of 120–160 MPa. Neckless type bimodal grain structure, an indication of incomplete dynamic recrystallization was observed in the forged microstructure in addition to the development of a sharp basal texture. The obtained mechanical test results show that the forged material achieved significantly improved yield and tensile strengths along with longer fatigue life. The improvement in the quasi-static properties was attributed to the strengthening effect of partial grain refinement and activation of non-basal slip modes due to texture modification.

The effect of voids on the quasi-static tensile properties of carbon fiber/polymer-laminated composites

Static longitudinal/transverse tensile tests for unidirectional carbon fiber/polymer (T300/924) laminates and laminates with lay-ups [ 0 / ± 45 / 0 / 90 ] s at various void levels were conducted, and degradations in stiffness/strength were observed with the presence of voids. The void levels were controlled by compression pressure during the compression molding process. The characterization of voids was achieved by digital microscopy image analysis; the density distributions of equivalent diameters and aspect ratios were analyzed with respect to compression pressure. For the purpose of quantifying the effect of voids on the static mechanical properties of composites, a stiffness prediction method based on the Mori-Tanaka method and void geometric statistical data have been used with the implementation of a finite element model of the representative volume element for unidirectional composites; the prediction results show good correlation with experimental data. Finally, a modified continuum damage model for laminated composites with the presence of voids was proposed, the model is capable of capturing the effect of voids; and gradual damage analysis for carbon fiber/polymer composite laminates at different void levels was conducted to evaluate the effect of voids on their tensile properties.

Fatigue Behavior of Friction Stir Linear Welded Dissimilar Aluminum-to-Magnesium Alloys

In this paper, we present the results of fatigue testing and analysis of friction stir linear welded dissimilar aluminum-to-magnesium alloys in lap-shear configuration. The overlap linear welds were created by joining AA6022 aluminum alloy to AM60 magnesium alloy. In general, the test data exhibited significant scatter in the fatigue life results and the corresponding failure modes. In fact, observations from fractography analysis revealed two distinct modes of failure. In the first mode of failure observed, fracture occurred when the dominant fatigue crack propagated into either the magnesium or aluminum sheet in a kinked crack formation. Interestingly, frettinglike debris was observed at the initiation sites for this failure mode. In the second mode of failure observed, fracture occurred by interfacial weld separation. In this mode, fractography analysis suggests that the fatigue cracks initiated at weld defects and then propagated through the intermetallic phase.