Tommaso Coppola

Prediction of ductile failure in materials for onshore and offshore pipeline applications

This paper shows the procedures needed to calibrate a numerical model intended for ductile damage estimation of bulk materials. For this purpose, an extensive experimental campaign has been carried out on three steels used for offshore/onshore pipe applications. Tests have been performed providing very different stress states: tensile and compressive uniaxial tests, multiaxial tensile tests on round notched bars, 3-point bend tests, again on notched geometries, and plane strain tensile tests on large grooved specimens. Based on the gathered results, a standard plasticity model has been tuned and then the damage model parameters have been identified for each investigated material. The chosen theoretical formulation can take into account all of the experimental evidence: hence, the numerical model represents a useful tool for finite element simulation of engineering problems where information concerning the materials ultimate resistance capability is needed. Moreover, the proposed calibration technique has general validity and can be used to tune other similar damage models.

A J2–J3 approach in plastic and damage description of ductile materials

This paper illustrates a methodology to improve the description of the plastic behavior and the fracture prediction for ductile materials under complex loading conditions. To this purpose, a plasticity model and a damage estimation model are proposed. The former, differently from the classic J2 plasticity theory, takes into account the effect of the third deviatoric invariant on the plastic flow. The latter assumes that damage accumulation is governed by both stress triaxiality and deviatoric parameters, and takes advantage of the new plasticity formulation. The two models rely on the same theoretical foundation, where a specific function is invoked to describe the subsequent yield surfaces and the damage accumulation up to fracture. Both have been implemented into a commercial finite element code via user subroutines. Three steel alloys have been tested under very different stress states: tensile tests on smooth and round notched bars, plane strain tests, torsion tests, and combined tension–torsion tests on hollow and solid cylindrical bars have been executed. For the last ones, several tension—torsion-loading ratios have been applied. These kinds of tests allow to explore a wide domain of the governing parameters for both models. The experimental results from tensile and torsion tests are used to calibrate the proposed plasticity model and the damage model; combined tests are used for validation purposes. The experimental–numerical comparison of global quantities made by using a standard plasticity approach confirms the need for a more accurate plastic description in the large strain range. The proposed plasticity model is able to provide a very good match until fracture for all tests available. Moreover, the damage model has the potential to take into account the experimental evidence, predicting the fracture initiation accurately. In particular, its validation by using tension–torsion tests shows to be really significant.

An Enhanced Plasticity Model for Material Characterization at Large Strain

An experimental campaign on some isotropic steels for pipeline applications has been put forth. It was based on tests with different stress states: tension on smooth and notched geometries, torsion, three point bending, plane strain, and combined tension-torsion. The aim was the characterization of the material elasto-plastic behavior up to large strain and the calibration of a ductile damage model for failure estimation.

Investigation of Possible Mechanisms for Developing Long Steel Products with Ultrafine Grained Microstructure

The obtainment of ultrafine grain microstructures, by the application of process parameters which are potentially feasible under industrial conditions, is attractive to develop a new generation of low alloy steel (Ultrafine Grain Steel, UFG) characterized by high strength and toughness, good cold/warm formability, environmentally-friendly process. The ferrite grain size refinement beyond existing levels by means of hot rolling mills, without requiring drastic plant changes, can be achieved by lowering the rolling temperature down to the range Ae3 - Ar3 in the finishing stands. In this temperature range different metallurgical mechanisms may take place. Austenite recrystallization is slower and there is a greater chance of obtaining non-recrystallized deformed austenite (pancake), which after phase transformation will give finer ferrite (Heavy Gamma Deformation). Or, in alternative, Deformation Induced Ferrite Transformation can occur especially in C-Mn steels, promoting the formation of ultrafine ferrite grains (DIFT). Most of the existing studies on UFG steel focus on flat products. In this paper the mechanisms to be exploited for producing UFG long products are identified and examined on different low and medium carbon non-alloyed steels, as the common grades used for fastener applications. In particular, Heavy Gamma Deformation and DIFT are investigated through laboratory tests aimed at determining the process parameters affecting the two mechanisms in different ranges of chemical composition. On the basis of the results found, some basic concepts for industrialization on modern hot rolling mills will be given.

Effect of Grain Refinement on Cold Formability Behavior on Medium C-Boron Steel

The effect of grain refinement on cold formability behavior on medium C-Boron steel has been studied. In particular a 30MnB4 steel long product has been considered to be used for screw production as final application. One of the issues during the cold forming of screw heads is given by the possible material failure in the most strained area, like the flange tip in the head. The probability of having a high scrap rate is strictly linked to the material cold formability limits. The study focused on the comparison between steel with conventional and ultrafine microstructure. Relationships between steel microstructure and plastic deformation behavior of the material were investigated by FEM modeling. Calibration of the model was carried out by analyzing the ultrafine grained steel, produced in pilot mill whose grain size was about 2-3 μm and the conventional steel industrially produced (grain size about 12 μm). Specific laboratory tests to characterize the material formability have been carried out. Finite element simulations have been performed to characterize the strain and stress state both in the forming process and in experimental tests to determine the material ductile fracture locus. Mechanical tests allowed to build the ductility curves related to the different microstructures to be used to simulate cold forming taking into account the material damage. Results put in evidence a significant improvement in the forming behavior of UFG steel, meaning that the refinement of grain size should produce a more evident effect in scrap reduction during the cold forming process.

Application of Ductile Damage Concepts in the Evaluation of Material Formability during Screw Head Cold Forming

An up to date approach to material cold formability, based on a conventional J2 theory for plasticity together an uncoupled linear plastic damage evolution theory, is applied to the study of steel bolt head cold forming. Specific laboratory tests to characterize the material formability have been used. Finite element simulations have been performed to characterize the strain and stress state both in the forming process and in experimental tests to determine the material ductile fracture locus. The influence of chemical composition and initial microstructure is discussed.