Investigation of Alternative Evaluation Spaces for Failure Prediction in Sheet Metals
Author
Barlo, Alexander Bendix Krukow
Term
4. term
Education
Publication year
2019
Pages
98
Abstract
This thesis investigates how to better predict the onset of localized necking (failure) in sheet metals subjected to combined tension and bending, focusing on AA6016 aluminium with supporting examinations of a dual-phase steel (CR440Y780T-DP). The work first evaluates the GISSMO approach as a digital post-processing criterion by transforming the conventional Forming Limit Curve (FLC) and simulated strain fields from principal strain space to a stress-triaxiality space using relations derived from the von Mises constitutive model; differences relative to the BBC05 framework were found negligible, so von Mises was adopted for its explicit formulation. Finite element simulations of bending-under-tension over a 6 mm tool radius showed that GISSMO predicts the onset of necking well for the aluminium alloy but poorly for the dual-phase steel, attributed to pronounced bending effects. Motivated by this, the thesis proposes a bending correction of the FLC based on failure strains from bending-under-tension experiments with punch radii of 3, 6, and 10 mm; by introducing tool curvature as a third parameter, a Bending Corrected Forming Limit Surface (BC-FLS) is constructed and implemented in AutoFormplus R8. Validation on a Volvo Cars test panel made from AA6016 accurately predicted the failure state in eight zones and clearly outperformed the standard FLC. While the BC-FLS improves predictions for stretch-bending cases, the approach still has important limitations and does not fully resolve the overall problem.
Denne afhandling undersøger, hvordan begyndelsen af lokal halsdannelse (svigt) i plademetal under kombineret træk og bøjning kan forudsiges bedre, med fokus på AA6016 aluminium og supplerende undersøgelser af en to-fase ståltype (CR440Y780T-DP). Udgangspunktet er at validere GISSMO-metoden som digital efterbehandling ved at transformere den konventionelle formbarhedskurve (FLC) og simulerede tøjningsfelter fra hovedtøjningsrum til et spændingstriaxialitetsrum via relationer afledt af von Mises’ konstitutive model; forskelle i forhold til BBC05-rammen blev vurderet negligerbare, og von Mises blev valgt pga. en enklere, eksplicit formulering. Finite element-simuleringer af bøjning-under-træk over en 6 mm værktøjsradius viste, at GISSMO forudsiger begyndende halsdannelse godt for aluminiumlegeringen, men utilfredsstillende for to-fase stålet, hvilket blev tilskrevet stærke bøjeeffekter. Derfor foreslås en bøjningkorrektion af FLC baseret på brudtøjninger målt i bøjning-under-træk-forsøg med stempelradier 3, 6 og 10 mm; ved at indføre værktøjskrumning som tredje parameter opbygges en Bending Corrected Forming Limit Surface (BC-FLS) og implementeres i AutoFormplus R8. En validering på et Volvo Cars testpanel i AA6016 forudsagde svigt i otte zoner korrekt og overgik den standard FLC markant. Metoden forbedrer dermed forudsigelser under stræk-bøjning, men har stadig væsentlige begrænsninger og løser ikke hele problemstillingen fuldt ud.
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