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29 pages, 11178 KiB

Open AccessArticle

Plasticity and Formability of Annealed, Commercially-Pure Aluminum: Experiments and Modeling

by Jinjin Ha, Johnathon Fones, Brad L. Kinsey and Yannis P. Korkolis

Materials 2020, 13(19), 4285; https://doi.org/10.3390/ma13194285 - 25 Sep 2020

Cited by 13 | Viewed by 2512

The plasticity and formability of a commercially-pure aluminum sheet (AA1100-O) is assessed by experiments and analyses. Plastic anisotropy of this material is characterized by uniaxial and plane-strain tension along with disk compression experiments, and is found to be non-negligible (e.g., the r-values vary [...] Read more.

The plasticity and formability of a commercially-pure aluminum sheet (AA1100-O) is assessed by experiments and analyses. Plastic anisotropy of this material is characterized by uniaxial and plane-strain tension along with disk compression experiments, and is found to be non-negligible (e.g., the r-values vary between 0.445 and 1.18). On the other hand, the strain-rate sensitivity of the material is negligible at quasistatic rates. These results are used to calibrate constitutive models, i.e., the Yld2000-2d anisotropic yield criterion as the plastic potential and the Voce isotropic hardening law. Marciniak-type experiments on a fully-instrumented hydraulic press are performed to determine the Forming Limit Curve of this material. Stereo-type Digital Image Correlation is used, which confirms the proportional strain paths induced during stretching. From these experiments, limit strains, i.e., the onset of necking, are determined by the method proposed by ISO, as well as two methods based on the second derivative. To identify the exact instant of necking, a criterion based on a statistical analysis of the noise that the strain signals have during uniform deformation versus the systematic deviations that necking induces is proposed. Finite element simulation for the Marciniak-type experiment is conducted and the results show good agreement with the experiment. Full article

(This article belongs to the Special Issue Formability of Materials)

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29 pages, 18654 KiB

Open AccessArticle

Novel Approach and Interpretation for the Determination of Electromagnetic Forming Limits

by Koray Demir, Siddhant Goyal, Marlon Hahn and Erman Tekkaya

Materials 2020, 13(18), 4175; https://doi.org/10.3390/ma13184175 - 19 Sep 2020

Cited by 6 | Viewed by 2324

Abstract

A new method to determine electromagnetic forming limits curves (EM-FLCs) for sheet metals is proposed. The different strain paths (between uniaxial and biaxial tension) are achieved by specific tool coil and specimen designs. It is ensured that the apex of the specimen deforms [...] Read more.

A new method to determine electromagnetic forming limits curves (EM-FLCs) for sheet metals is proposed. The different strain paths (between uniaxial and biaxial tension) are achieved by specific tool coil and specimen designs. It is ensured that the apex of the specimen deforms on a constant strain path, and excess bending at the apex is avoided. This is done so that the determined EM-FLCs are comparable to their quasi-static counterparts. The method determines the EM-FLCs for the aluminum alloys AA-1050a-H24 and EN AW-5083-H111 and the magnesium alloy Mg AZ31-O. Overall, it is observed that the necking limits in electromagnetic forming (EMF) are higher compared to quasi-static forming. The fracture surfaces of electromagnetically deformed specimens are examined to reveal the existence of out-of-plane shear stresses. A numerical analysis corroborates this observation and their variation with strain rate. The presence of such stresses is proposed as a possible reason for the increased necking limits in EMF. As reasons for higher forming limits, previous research has identified inertial stabilization, strain rate hardening, die impact, and change in deformation mechanism. The current study reaffirms the positive effect of inertial stabilization and makes key observations in the increase of twinning in EMF of Mg AZ31-O. Full article

(This article belongs to the Special Issue Formability of Materials)

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19 pages, 8263 KiB

Open AccessArticle

On the Use of Strain Path Independent Metrics and Critical Distance Rule for Predicting Failure of AA7075-O Stretch-Bend Sheets

by Andrés Jesús Martínez-Donaire, Domingo Morales-Palma and Carpóforo Vallellano

Materials 2020, 13(17), 3660; https://doi.org/10.3390/ma13173660 - 19 Aug 2020

Cited by 2 | Viewed by 1999

Abstract

The strain-based forming limit curve is the traditional tool to assess the formability of metal sheets. However, its application should be restricted to proportional loading processes under uniform strain conditions. Several works have focused on overcoming this limitation to characterize the safe process [...] Read more.

The strain-based forming limit curve is the traditional tool to assess the formability of metal sheets. However, its application should be restricted to proportional loading processes under uniform strain conditions. Several works have focused on overcoming this limitation to characterize the safe process windows in industrial stretch-bend forming processes. In this paper, the use of critical distance rule and two path-independent stress-based metrics are explored to numerically predict failure of AA7075-O stretch-bend sheets with 1.6 mm thickness. Formability limits of the material were experimentally obtained by means of a series of Nakazima and stretch-bending tests at different thickness-over-radius ratios for inducing controlled non-uniform strain distributions across the sheet thickness. By using a 3D calibrated finite element model, the strain-based forming limit curve was numerically transformed into the path-independent stress and equivalent plastic strain polar spaces. The numerical predictions of necking strains in the stretch-bending simulations using the above approaches were successfully compared and critically discussed with the experimental results for different values of the critical distance. It was found that failure was triggered by a critical material volume of around the half thickness, measured from the inner surface, for the both path-independent metrics analyzed. Full article

(This article belongs to the Special Issue Formability of Materials)

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17 pages, 948 KiB

Open AccessArticle

Temporal and Spatial Detection of the Onset of Local Necking and Assessment of its Growth Behavior

by Christian Jaremenko, Emanuela Affronti, Marion Merklein and Andreas Maier

Materials 2020, 13(11), 2427; https://doi.org/10.3390/ma13112427 - 26 May 2020

Cited by 2 | Viewed by 1886

Abstract

This study proposes a method for the temporal and spatial determination of the onset of local necking determined by means of a Nakajima test set-up for a DC04 deep drawing and a DP800 dual-phase steel, as well as an AA6014 aluminum alloy. Furthermore, [...] Read more.

This study proposes a method for the temporal and spatial determination of the onset of local necking determined by means of a Nakajima test set-up for a DC04 deep drawing and a DP800 dual-phase steel, as well as an AA6014 aluminum alloy. Furthermore, the focus lies on the observation of the progress of the necking area and its transformation throughout the remainder of the forming process. The strain behavior is learned by a machine learning approach on the basis of the images when the process is close to material failure. These learned failure characteristics are transferred to new forming sequences, so that critical areas indicating material failure can be identified at an early stage, and consequently enable the determination of the beginning of necking and the analysis of the necking area. This improves understanding of the necking behavior and facilitates the determination of the evaluation area for strain paths. The growth behavior and traceability of the necking area is objectified by the proposed weakly supervised machine learning approach, thereby rendering a heuristic-based determination unnecessary. Furthermore, a simultaneous evaluation on image and pixel scale is provided that enables a distinct selection of the failure quantile of the probabilistic forming limit curve. Full article

(This article belongs to the Special Issue Formability of Materials)

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22 pages, 13499 KiB

Open AccessArticle

Design and Optimization of the Local Laser Treatment to Improve the Formability of Age Hardenable Aluminium Alloys

by Antonio Piccininni and Gianfranco Palumbo

Materials 2020, 13(7), 1576; https://doi.org/10.3390/ma13071576 - 29 Mar 2020

Cited by 19 | Viewed by 2567

Abstract

The research of innovative methodologies to improve the Aluminium alloys formability at room temperature still remains an open question: the local modification of the material properties via short-term heat treatments followed by the stamping at room temperature is reported to be an effective [...] Read more.

The research of innovative methodologies to improve the Aluminium alloys formability at room temperature still remains an open question: the local modification of the material properties via short-term heat treatments followed by the stamping at room temperature is reported to be an effective alternative to the forming in warm conditions. In the present work, such a methodology has been applied to the deep drawing of an age-hardenable Aluminium alloy (AA6082-T6) using an experimental/numerical approach. A preliminary extensive material characterization was aimed at investigating the material behaviour: (i) in the as-received condition (peak hardening), (ii) in the supersaturated condition (obtained by physical simulation) and (iii) after being locally solutioned via laser heating. A Finite Element based approach (Abaqus CAE, v. 6.17) was then used to design the laser treatment of the blanks to be subsequently deep drawn at room temperature: a 2D axisymmetric model of the deep drawing process was coupled with the optimization platform modeFRONTIER in order to define the radial extent of the laser heat treated area able to maximize the Limit Drawing Ratio. The experimental tests were finally conducted for validation purposes and revealed the effectiveness of the adopted approach which allowed to improve the drawability of more than 20% with respect to the as received condition (T6). Full article

(This article belongs to the Special Issue Formability of Materials)

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18 pages, 6993 KiB

Open AccessArticle

On the Determination of Forming Limits in Polycarbonate Sheets

by Ana Rosa-Sainz, Gabriel Centeno, Maria Beatriz Silva, Jose Andrés López-Fernández, Andrés Jesus Martínez-Donaire and Carpoforo Vallellano

Materials 2020, 13(4), 928; https://doi.org/10.3390/ma13040928 - 19 Feb 2020

Cited by 17 | Viewed by 2334

Abstract

By proposing an adaptation of the methodology usually used in metal forming, this paper aims to provide a general procedure for determining the forming limits, by necking and fracture, of polymeric sheet. The experimental work was performed by means of Nakajima specimens with [...] Read more.

By proposing an adaptation of the methodology usually used in metal forming, this paper aims to provide a general procedure for determining the forming limits, by necking and fracture, of polymeric sheet. The experimental work was performed by means of Nakajima specimens with different geometries to allow to obtain strains in the tensile, plane, biaxial and equibiaxial states for Polycarbonate sheet with 1 mm of thickness. The application of the time-dependent and flat-valley approaches used in metals has been revealed appropriate to characterize the onset of necking and obtain the forming limits of polycarbonate, despite the stable necking propagation typical of polymeric sheets. An analysis of the evolution of the strain paths along a section perpendicular to the crack allowed for a deeper understanding of the steady necking propagation behaviour and the adoption of the methodology of metals to polymers. The determination of the fracture strains was enhanced with the consideration of the principal strains of the DIC system in the last stage, just before fracture, due to the significant elastic recovery typical of polymeric sheets. As a result of this analysis, accurate formability limits by necking and fracture are obtained for polycarbonate sheet, together with the principal strain space, providing a general framework for analysing incremental sheet forming processes where the knowledge of the fracture limits is relevant. Full article

(This article belongs to the Special Issue Formability of Materials)

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15 pages, 6702 KiB

Open AccessArticle

Formability Limits, Fractography and Fracture Toughness in Sheet Metal Forming

by João P. Magrinho, Maria Beatriz Silva, Luís Reis and Paulo A. F. Martins

Materials 2019, 12(9), 1493; https://doi.org/10.3390/ma12091493 - 8 May 2019

Cited by 21 | Viewed by 4278

Abstract

This paper is focused on the utilisation of double edge notched tension, staggered and shear tests to determine fracture toughness and the formability limits by fracture in principal strain space. The experiments were performed in test specimens with different geometries and ligament angles, [...] Read more.

This paper is focused on the utilisation of double edge notched tension, staggered and shear tests to determine fracture toughness and the formability limits by fracture in principal strain space. The experiments were performed in test specimens with different geometries and ligament angles, and the influence of strain hardening was taken into consideration by selecting two materials (aluminium AA1050-H111 and pure copper), with very different strain hardening exponents. Results are plotted in principal strain space, and the discussion is focused on the link between formability limits, fracture toughness and macroscopic fractography characteristics of the specimens that fail by mode I, mode II or mixed-mode. Full article

(This article belongs to the Special Issue Formability of Materials)

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17 pages, 624 KiB

Open AccessArticle

Determination of Forming Limits in Sheet Metal Forming Using Deep Learning

by Christian Jaremenko, Nishant Ravikumar, Emanuela Affronti, Marion Merklein and Andreas Maier

Materials 2019, 12(7), 1051; https://doi.org/10.3390/ma12071051 - 30 Mar 2019

Cited by 16 | Viewed by 5516

Abstract

The forming limit curve (FLC) is used to model the onset of sheet metal instability during forming processes e.g., in the area of finite element analysis, and is usually determined by evaluation of strain distributions, derived with optical measurement systems during Nakajima tests. [...] Read more.

The forming limit curve (FLC) is used to model the onset of sheet metal instability during forming processes e.g., in the area of finite element analysis, and is usually determined by evaluation of strain distributions, derived with optical measurement systems during Nakajima tests. Current methods comprise of the standardized DIN EN ISO 12004-2 or time-dependent approaches that heuristically limit the evaluation area to a fraction of the available information and show weaknesses in the context of brittle materials without a pronounced necking phase. To address these limitations, supervised and unsupervised pattern recognition methods were introduced recently. However, these approaches are still dependent on prior knowledge, time, and localization information. This study overcomes these limitations by adopting a Siamese convolutional neural network (CNN), as a feature extractor. Suitable features are automatically learned using the extreme cases of the homogeneous and inhomogeneous forming phase in a supervised setup. Using robust Student’s t mixture models, the learned features are clustered into three distributions in an unsupervised manner that cover the complete forming process. Due to the location and time independency of the method, the knowledge learned from formed specimen up until fracture can be transferred on to other forming processes that were prematurely stopped and assessed using metallographic examinations, enabling probabilistic cluster membership assignments for each frame of the forming sequence. The generalization of the method to unseen materials is evaluated in multiple experiments, and additionally tested on an aluminum alloy AA5182, which is characterized by Portevin-LE Chatlier effects. Full article

(This article belongs to the Special Issue Formability of Materials)

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