M. Bruhis

A Suitable Criterion for Precise Determination of Incipient Necking in Sheet Materials

Forming limit diagram (FLD) is a measure of the formability of a sheet material. The major-minor strain pairs that are closest to the neck on multiple specimens of various strain paths are utilized to construct a boundary between safe and unsafe zones. The challenge to obtain the FLD is the determination of incipient necking. Three approaches to determine the limit strains have been investigated and compared in this research in order to establish the optimal one for implementation: (1) commonly used Bragard criterion ( 1)e Br with periodic grids; (2) tracking the region of large local strains from strain history to locate the instance when critical major strain ( 1)e cr happens; (3) post-processing of strain history to locate the inflection in the major strain rate curve 1 max (e&&) at the onset of localization. The last criterion of inflection in strain rate 1 max (e&&) carries both a numerical and a physical meaning towards developing an understanding of flow localization, formability and fracture.

Studies in Cup Drawing Behavior of Polymer Laminated Sheet Metal

Axisymmetric deep drawing behavior of a polymer laminated sheet metal (PLSM) is investigated using an axisymmetric cup drawing test. PLSMs are of interest as a replacement for painted finishes for automotive applications as they have the potential to achieve good quality long lasting and aesthetically appealing surfaces on stamped parts. However, there is limited understanding of PLSMs in automotive deep drawing situations to produce complex 3-D parts. The tests are carried out using well-controlled, laboratory-based, dual-action, servo-hydraulic forming presses under blank-holder force and punch displacement control conditions. An optical strain mapping system is used to measure the surface strains (and to construct 3D strain maps) from the film side of the deformed samples for a range of forming conditions. Deep drawing characteristics such as punch load versus punch displacement traces, strain distribution along the cup profile, flange wrinkling and fracture characteristics are experimentally assessed for stainless steel-plastic film laminated sheet materials. Also the effect of lamination pressure on wrinkling and delamination is investigated for a decorative pressure sensitive adhesive film affixed to the stainless steel sheet.

Experimental and Numerical Study of Intense Shear Banding for Al-Alloy under Uniaxial Tension

The occurrence of intense shear band is a prelude to failure in many Al-sheet materials. In the present study, a full field optical system measurement technique (digital image correlation) and the finite element method are used to characterize the sequence of deformation in uniaxial tension before and after the intense shear band formation in AA6111-T4. The results indicate good agreement between the measurement and the predictions in terms of shear band width, strain distribution along the gauge length and the failure mode.

Obtaining Formability Characteristics Of Automotive Materials Using On‐line Strain Imaging System

The formability of automobile sheet material AA6111‐T4 was investigated in the hemispherical punch test. Specimens with various geometry and lubrication conditions were utilized to obtain a continuous strain map from a biaxial stretching. The data was processed to obtain strain path and limit strain values as measures of formability. The Strain imaging system, ARAMIS, in which a speckle pattern is utilized instead of the conventional grid system, was employed to capture the strains during the forming process. Features of the evolving dome surface, such as development of shear bands and strain localization were accurately captured and studied. Specimen profile with non‐symmetric notches for shearing test in biaxial loading was designed and investigated in both the experimental and the FE approach. The on‐line strain imaging method offers a useful approach towards developing an understanding of flow localization, formability and fracture under biaxial loading conditions.

Characterization of Damage in Magnesium Using Digital Image Correlation and Electron Backscattered Diffraction Patterning

The microscopic strain distribution of commercially pure magnesium has been investigated during room temperature deformation. Digital image correlation (DIC) coupled with electron backscattered diffraction (EBSD) patterning has been used to relate strain measurements with the microstructure. Sheet magnesium tensile samples were etched and a random pattern of points were machined onto the sample surface using a focused ion beam (FIB). The samples were pulled in tension under an optical microscope. Images were obtained in increments and the DIC method was used to calculate the strain distribution. The strain distribution was compared to the EBSD pattern and the results show that strain localization is associated with twin and grain boundaries.

Effect of Carbon Gradient on the Microstructure and Mechanical Properties of Fe-22Mn-C TWIP/TRIP Steels

High-manganese austenitic steels are promising emerging automotive steels demonstrating high strength and ductility. The main deformation products observed in these steels are mechanical twins and ε-martensite, where the dominant deformation products vary quite strongly with stacking fault energy (SFE), which in turn is a very strong function of the alloy carbon content. In this research, a Fe-22Mn-0.6C sheet steel was decarburized to achieve a variety of through-thickness C gradients, thereby varying the dominant deformation products through the sheet thickness, with the overall objective of producing unique microstructures and mechanical properties. Microstructural analyses after interrupted tensile testing indicated that the amount of both mechanical twins and ε-martensite increased with increasing true strain, where the deformation products changed from mechanical twins at the higher-C core to ε-martensite at the lower-C surface. The spring-back properties of the C graded steels were also compared with reference to the effect of differential carbon concentration gradient.