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Ductile fracture locus of AC4CH-T6 cast aluminium alloy

Mae H., Teng X., Bai Y., Wierzbicki T.

Archives of Computational Materials Science and Surface Engineering

2009

Vol. 1, nr 2

100-105

Purpose: Cast aluminium alloys have found wide application to manufacture lighted-weight components of complex shape in automotive and aerospace industries. To improve the strength and ductility of cast aluminium alloys, it is necessary to study their fracture properties by conducting a series of tests. This study addresses calibration of ductile fracture property of the cast aluminium alloy (AC4CH-T6) made by the gravity die casting with sand mold. Design/methodology/approach: 6 round bar specimens and 6 butterfly specimens are machined from the actual cast component. The tensile tests on the smooth and notched round bar specimens are performed to calibrate the fracture strain in the range of high positive stress triaxialities. The combined loading tests on the butterfly specimens are carried out using a uniquely designed Universal Biaxial Testing Device (UBTD). These tests cover the fracture properties in the rage of low and negative stress triaxialities. Detailed finite element models of all the tests are developed. The fracture locus in the space of the effective plastic strain to fracture and the stress triaxiality are constructed in a wide rage from -1/3 to 1.0. Findings: It is found that material ductility sharply decreases with the stress triaxiality. The material ductility at the negative stress triaxiality is much higher than that in the positive stress triaxiality. Research limitations/implications: Large spread of data is observed for those tests repeated on the same loading configuration, necessitating the statistical analysis of the fracture processes. Practical implications: It is expected that such a fracture criterion would be able to correctly predict the fracture response of actual cast aluminum components under complex loading in the practical applications. Originality/value: The conventional researches focused on the material ductility at the stress triaxiality larger than +1/3. The present study showed the material ductility at the wide range of stress triaxiality from -1/3 to 1.0.

Tensile mechanical properties in PP/SEBS/Microballon composites under impact loadings

Mae H.

Journal of Achievements in Materials and Manufacturing Engineering

2008

Vol. 31, nr 2

341-347

Purpose: The mechanical properties of the syntactic polymer foams at the intermediate and high strain rates were not understood comprehensively. Then, this study characterizes the tensile mechanical properties of the polymer syntactic composites at high strain rates. Design/methodology/approach: Eight kinds of syntactic foams and one neat PP/SEBS specimens are prepared at the same manufacturing process: 0, 2, 4, 8, 10, 20, 30, 40 and 50 volume percents of microballoons in the PP/SEBS blend matrix. Tensile tests are conducted at strain rates ranged from 0.3 to 100 s-1. Apparent elastic modulus, yield stress and rupture strain are measured and the effects of microballoons on the mechanical properties are studied. In addition, the experimental results are compared with analytical model for closed-cell foam and the effects of the density of the PP/SEBS/microballoon composite on both apparent elastic modulus and yield stress are discussed. Findings: The apparent elastic moduli of PP/SEBS/microballoon syntactic composites follow the Gibson-Ashby law at the nominal strain rate of 100 s-1. The yield stress of PP/SEBS/microballoon syntactic composites follow the simple rule of mixture at the relative densities larger than 0.9. The material ductility decreases drastically once the microballoons are blended in the matrix material. Research limitations/implications: The influence of the local strain rate caused by the heterogeneous microstructure on the mechanical properties is to be further explored. Practical implications: In the automobile applications, the thermoplastic polymer syntactic foams are believed to have many advantages because the usual commercial extruders or injection moulding machines are applicable for producing them, leading to the more light-weight polymeric components. Originality/value: The present study investigates the effects of the strain rate and density on the tensile mechanical properties comprehensively in the polymer syntactic foams.