Edoardo Mancini

Surface Defect Generation and Recovery in Cold Rolling of Stainless Steel Strips

Previously researchers investigated the mechanism of surface defect evolution in rolling. It was highlighted how the lubricant plays an essential role for the final strip surface quality. In some cases the lubricant can be entrapped in pits or in other defects where hydrostatic pressure tends to prevent its elimination; however, when some favorable conditions are satisfied, the lubricant can be drawn out by hydrodynamic actions and defects can be recovered. This mechanism has been described as microplastohydrodynamic lubrication (MPHL) and recent studies report a suitable parameter (the ratio of the oil drawn out from the pit to the initial pit volume) as MPHL characterization coefficient. The present paper deals with the recovery of isolated surface defects in the Sendzimir rolling process of AISI 304 stainless steel; the analyses have been conducted on two rolling conditions, which although quite similar, regularly showed opposite capability of defect recovery, moreover, with a trend that is in contrast with the predictions made by standard MPHL. Two effects, which are usually ignored in literature modeling, have been considered in this work: The former is the back-tension, which has relevant outcome on the contact pressure and the latter is the position of the neutral point, which cannot be assumed to lie at the end of the roll bite. The analytical treatment was supported by FEM simulations, which permitted to put realistic data into the MPHL equations, thus, to explain the experimental behavior. The analysis was then validated with two further rolling schedules that seem to confirm the proposed approach.

Comparison Between Two Experimental Procedures for Cyclic Plastic Characterization of High Strength Steel Sheets

In finite element analysis of sheet metal forming the use of combined isotropic-kinematic hardening models is advisable to improve stamping simulation and springback prediction. This choice becomes compulsory to model recent materials such as high strength steels. Cyclic tests are strictly required to evaluate the parameters of these constitutive models. However, for sheet metal specimens, in case of simple axial tension-compression tests, buckling occurrence during compression represents a serious drawback. This is the reason why alternative set-ups have been devised. In this paper, two experimental arrangements (a cyclic laterally constrained tension-compression test and a three-point fully reversed bending test) are compared so as to point out the advantages and the disadvantages of their application in tuning the well-known Chaboche’s hardening model. In particular, for tension-compression tests, a new clamping device was specifically designed to inhibit compressive instability. Four high strength steel grades were tested: two dual phases (DP), one transformation induced plasticity (TRIP) and one high strength low alloy material (HSLA). Then, the Chaboche’s model was calibrated through inverse identification methods or by means of analytical expressions when possible. The proposed testing procedure proved to be successful in all investigated materials. The achieved constitutive parameters, obtained independently from the two experimental techniques, were found to be consistent. Their accuracy was also been assessed by applying the parameter set obtained from one test to simulate the other one, and vice versa. Clues on what method provides the better transferability are given.

High Strain Rate Behaviour of AA7075 Aluminum Alloy at Different Initial Temper States

The aim of this work is to study the mechanical properties of alloy AA7075 in both T6 and O temper states, in terms of visco-plastic and fracture behavior. Tension and compression tests were carried out starting from the quasi-static loading condition 10-3 up to strain rates as high as 2 x 103 s-1. The high strain rate tests were performed using a Split Hopkinson Tension-Compression Bar (SHTCB) apparatus. The tensile specimens were also subjected to micro-fractography analysis by Scanning Electronic Microscope (SEM) to evaluate the characteristics of the fracture. The results show a different behavior for the two temper states: AA7075-O showed a significant sensitivity to strain rate, with a ductile behavior and a fracture morphology characterized by coalescence of microvoids, whilst AA7075-T6 is generally characterized by a less ductile behaviour, both as elongation at break and as fracture morphology. Brittle cleavage is accentuated with increasing strain rate. The Johnson-Cook viscoplastic model wad also used to fit the experimental data with an optimum matching.

Evaluation of friction at high strain rate using the Split Hopkinson Bar

The present work aims at studying the influence of strain rate on the frictional behaviour of AA7075 aluminium alloy in the O-annealed temper state. To this purpose, ring compression tests were performed both under quasi-static and dynamic loading conditions. The high strain rate tests were carried out by means of the Split Hopkinson Tension-Compression Bar in the direct version. In both cases, hollow cylindrical samples, characterised by an initial outer diameter to inner diameter to height ratio of 6:3:2, were tested under dry condition and by lubricating with molybdenum disulphide grease. The different frictional behaviour exhibited by AA7075-O under quasi-static and dynamic loading conditions can be attributed to the strain rate effect both on the plastic flow behaviour of the deformed material, and on the thickness of the lubricant film.

Experimental and Numerical Analysis of Pressure Waves Propagation in a Viscoelastic Hopkinson Bar

In this paper, the viscoelastic behaviour of PET is assessed in order to study the wave propagation in long SHPB made of polymeric materials.

Computer-aided engineering for sheet metal forming: Definition of a springback quality function

Computer-aided engineering methods are extensively applied to sheet metal forming integrated design. The adoption of a new class of materials, the advanced high strength steels, has increased the occurrence of springback, and consequently the request for tools oriented to springback reduction and optimization. This paper presents an approximated formulation to compute the springback field after stamping through the finite element analysis of the process. This can be found assuming that the residual field of nodal forces after stamping produces a springback shape referable to a linear combination of n modes of vibration of the nominal shape of the component. The aim of this formulation is not that of substituting the finite element analysis of the springback but rather to make use of the coefficients of the linear combination, so to define a global quality function for springback. In this way, Robust Design methods or other current optimization procedures to improve the stamping process as for structural defects (such wrinkling, necking and flatness) can be applied also for the reduction of springback. The meaning of these coefficients will be shown through three test cases and the consistency of the formulation will be discussed according to the number of modes of vibration included in the computation.

Dynamic testing and constitutive modelling of NBR rubbers

The present work describes the compression behaviour of NBR rubber. Experimental tests have been conducted both in dynamic conditions. The latter ones, performed by a polymeric Split Hopkinson Bar, range from 100 to 500 1/s of strain rate. The long lasting pressure wave generated by the adopted SHB permitted to obtain a relatively high strain level in all the tests, up to 0.7–1.0 logarithmic strain. The experimental stress-strain curves were used to fit hyperelastic-perfect viscoelastic constitutive models; in particular, the Ogden and Mooney-Rivlin models were used for the hyperelasticity, while the Prony series was used for the viscoelastic part.

Identification of Constitutive Model Parameters in Hopkinson Bar Tests

In this work, tension and compression tests have been carried out on aluminium samples at low and high strain rate, the latter performed by means of a direct tension Hopkinson bar equipment. The parameters of the Johnson-Cook constitutive model have been identified using different approaches; the first method consists in the classical Finite Element Model Updating, where numerical simulations are repeated with different material parameters until the mismatch between the experimental and numerical load–displacement curves falls below an acceptable threshold.

Identification of the plastic zone using digital image correlation

In this paper Digital Image Correlation (DIC) is used to study the evolution of the plastic zone close to a crack tip. A modified CT-specimen was used in order to fulfill the plane stress condition. The strain field around the crack tip was measured using two cameras and stereo DIC, so that out-of-plane movements are taken into account. Then, the Virtual Fields Method was used to identify the plastic zone, looking at the parts of the specimen which deviates from the linear elastic behavior. With such approach, it was possible to individuate the onset of plasticity close to the crack tip and follow its evolution. A comparison with FEM results is also provided.

A Computer-Aided Design Module to Analyze Manufacturing Configurations of Bent and Hydroformed Tubes

This paper presents a computer-aided design (CAD) module able to analyze different manufacturing configurations of tubes used in mechanical assemblies, such as exhaust system manifolds. It can be included in the knowledge-based expert system category and has been implemented into a CAD platform as a dedicated module able to take into account manufacturing requirements related to tube bending, hydroforming, and cutting. The experts knowledge, in terms of set of rules and criteria, has been implemented by means of the automation tools of CATIAV5R10 according to the socalled methodological formal approach. The resulting module is able to join different tubes starting from their geometrical models, obtaining a set of manufacturing alternatives. Each of them is verified with respect to collisions with a bending machine and also in terms of hydroforming process feasibility. Only those solutions that satisfy these checks are accepted as feasible and ranked according to three evaluation criteria related to manufacturing cost and easiness. The system is completely automatic and able to analyze more than 100 different configurations in <10 min. The feasible solutions are saved as CAD model to allow FEA of hydroforming and other possible CAE activities. Unfeasible solutions are deleted but reported and documented in a log file. The feasible solution rank is given in a table and has been developed according to a multicriteria approach to make optimal solution detection easier. The proposed test case aims to show and discuss these capabilities. By this module, two or more components of the exhaust system manifold can be manufactured in one stroke as a single component, starting from the same pipe and next trimmed to obtain the desired final parts. This capability can be used to reduce scraps and improve cycle time of the manufacturing process.