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Author
Name Avendaño Rodríguez, Diego Fernando
Research field Material science and engineering
Career stage doctoral researcher
Home university/institution Universidad Nacional de Colombia
Department/Research unit at home university/institution Mechatronic & Mechanical Engineering
Chair/Working group at home institution INNOVATION IN MANUFACTURING PROCESSES AND MATERIALS ENGINEERING

International activity
Country Germany
Location Bochum
University Ruhr-Universität Bochum (RUB)
Fund Research School PhD-Exchange
Type of activity research stay
Period starts 01-03-2018
ends 31-12-2018
Keywords Dual-phase steels, damage evolution, fracture mechanisms, martensite volume fraction
Report The new modern challenges of the automotive industry related to the reduction of fuel consumption and increased passengers safety promote the development of new lightweight and more substantial materials. Advanced high strength steels were the first response of the steel industry to meet these new needs. The dual-phase (DP) steels are the first generation of AHSS and have been widely used in the automotive industry since 1995; however, in Colombia, these materials are only beginning to be implemented in the manufacturing industry and are a matter of research work. For that reason, there is a great interest in knowing the properties and limitations of DP steels.

Because improving the security and the resistance of the manufactured products is one of the main objectives of the automotive industry, the development of new lightweight materials and high-strength steels, such as DP, is being widely researched; however, the mechanical behavior of DP steels under crack propagation and damage evolution conditions has not been studied in sufficient depth. The research goal is to determine the effect of the microstructure and strain rate on the mechanical properties of damage evolution and crack propagation on advanced high-strength steels. It is of particular interest to understand how microstructural properties such as grain size, morphology, orientation, distribution, phase volume fraction, and voids density affect the material's mechanical properties. Several researchers have reported that DP steels exhibit strain rate sensitivity due to their microstructure characteristics; therefore, with higher strain rates, an increase in the steel's ability to resist damage and a rise in the energy absorption capability is expected. Furthermore, the strain hardening rate of DP steels (dependent on the martensite volume fraction) indicates that these steels have better properties regarding crack propagation and fracture behavior.

The Dual-Phase steels will be developed through intercritical heat treatments starting from a low carbon (0.05 to 0.15 wt. %) and manganese (1 to 1.5 wt. %) steel. Mechanical properties of the steel will be evaluated through tensile: testing, loading-unloading uniaxial tensile test, cyclic loads test, and fractography. Steel parameters like elastic modulus, yield stress, tensile strength, elongation, and strain hardening exponent at different strain rates can be determined through tensile testing. Furthermore, it is possible to obtain the curves of damage strain at different strain rates employing the cyclic uniaxial tensile test; therefore, a relationship to the DP steel damage evolution can be established. Furthermore, fatigue tests establish the crack's growth rate and, subsequently, the fracture toughness. Finally, the fractography will establish the type of fracture and the failure mechanism of the material. Through electron backscatter diffraction (EBDS) is expected to analyze the phases present in the material and its texture, including grain orientation; phase distribution, and volume fraction of the phases, also, the accumulated damage will be studied through the detection, and volume fraction of voids and then this information will be correlated with the results of unloading modulus degradation.

The most relevant expected results can be summarized: First, define the intercritical heat treatment parameters needed to obtain DP steels from locally low-carbon steels. Second, identify the effect of martensite volume fraction on increasing the mechanical properties of DP steels. Third, confirm if, during the damage evolution process due to cyclic uniaxial tensile test, the unloading modulus of the DP steels decreases with the plastic strain; this means that the accumulation of damage increases with increasing of voids and its coalescence until fracture. Finally, validate the effect of strain rate on increasing yield and tensile stresses on DP steels and increase the knowledge frontier of the experimental analysis of AHSS, clarifying the effect of strain rate and microstructure on the damage evolution and fracture behavior on DP steels.
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