PSI - Issue 13

Reza H. Talemi et al. / Procedia Structural Integrity 13 (2018) 775–780 Author name / Structural Integrity Procedia 00 (2018) 000–000

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grades are called sensitive grades. Currently, there is no specific criterion available in the literature for the brittle fracture assessment of steel slab materials. Sometimes for a same steel grade, steel producers have different practice regarding the cold/hot charging, which are based on empirical equations calculated by trial and error procedure. This is explainable due to logistic facts, different casting process parameters and in general different production conditions. Therefore, there is a need to define a fracture criterion which could be used to set different process parameters for cold, warm or direct charging of brittle steel slabs. To find such a criterion, the main objective of this study is to estimate the cleavage fracture behavior of steel slabs by means of both experimental and numerical approaches. In general, to prevent brittle fracture, materials are selected based on the ductile-to-brittle transition temperature (DBTT), which should be lower than the minimal design and service temperatures encountered during operation. To do so, Charpy V-Notch (CVN) test and Drop Weight Tear Test (DWTT) are widely used to find the ductile to brittle transition temperature for different steel grades. In order to qualify for variety of applications, such as slab materials, steel slabs could fulfil requirements from the DWTT, an impact test on a notched three-point bending sample, usually performed on thick specimens. The DWTT has been used in this study to monitor the ductile and the brittle fracture response of steel slab at different elevated temperatures ranging from 25°C to 500°C. There are many studies aimed to develop numerical models to describe the dynamic fracture propagation of impact phenomena Wu et al. (2013), Nonn et al. (2013) and Scheider et al. (2014). More recently, Hojjati-Talemi et al. (2016) and Talemi (2016) have implemented the XFEM-based cohesive segment technique, using a 2D Finite Element (FE) model, to simulate the dynamic brittle fracture of pipeline steel subjected to the CVN and the DWTT loading conditions, respectively. In the present study, a 3D FE model is developed to represent the actual practice of DWTT. To this end, the proposed XFEM-based model has been used to simulate the dynamic brittle fracture of steel slabs at different elevated temperatures. After validation of the developed model against experimental observations, significant results from the simulation are presented and discussed to find a set of critical cleavage stresses for cold/hot charged steel slabs. 2. Experiment In a DWTT, the test specimen is a rectangular bar with a length of 305mm, a width of 76mm and of a 16mm thickness. The specimen has a notch and is subjected to three-point bending impact load. The standards specify a 5mm deep notch made by a sharp indenter with a 45° included angle resulting in a tip radius that is normally between 0.0127mm to 0.0254mm. In this study, a series of steel slab specimens were broken under impact loading at various temperatures and the proportions of ductile fracture (shear) and brittle fracture (cleavage) on the fracture surfaces are measured. The specimens were tested at different temperatures by using the DWTT testing machine with maximum energy capacity of 30 kJ and hammer velocity of 6.5 m/s. The DWTT samples were manufactured with two different configurations. The main difference between the two configurations are the crack propagation planes. These planes are along slab thickness (ST) and slab width (SW) for Config1 and 2, respectively. The specimens were tested according to the API5L3 standard, which is normally used for pipeline steel applications, using the instrumented DWTT. The specimens were heated to the desired test temperature using a Paddeltherm resistor furnace. A maximum of five samples and one dummy sample (all at room temperature) were loaded inside the furnace. A thermocouple was mounted inside the dummy sample (center of specimen). All samples were heated up until the dummy sample indicates a stable temperature close to the requested temperature. The material used in this research was AHSS slab with modulus of elasticity, E= 210GPa. The mechanical properties of the steel slab were measured using tensile test at room temperature (25°C). To machine the tensile samples, a test plate was taken from steel slab materials. Fig. 1(a) illustrates the true stress versus strain curves for different temperatures i.e. 25°C, 100°C, 200°C, 300°C, 400°C and 500°C. It is worth mentioning that the tensile properties of the slab material at elevated temperatures were estimated using nonlinear regression equations based on the chemical composition and physical test results. Fig. 1(b) indicates that by increasing the test temperature the yield stress of the steel slab material decreases. This variation, however, is almost constant from 100°C to 400°C.