PredictionofDynamicMaterialFailure：GISSMOinLS-DYNA

2016-09-26 by:CAE仿真在线来源:互联网

材料失效动态预测Part II: GISSMO模型在LS-DYNA中的应用

摘要

GISSMO (Generalized Incremental Stress-State dependent damage MOdel) 模型是在戴姆勒的实际工程应用中而开发出来的. 在Forming材料成型方面,使用常规FLD方法是偏保守的,而使用GISSMO方法则其精度更高,更加符合试验值. 此外,在Forming和Crash计算时,通常使用的是不同的材料本构模型,而GISSMO模型可以和任意的材料本构进行组合,这为在Crash模型中考虑Forming时不同应力路径下材料的损伤和失效提供了可能.

A Generalized Scalar Damage Method (GISSMO)1Introduction▪ For forming simulations, a more sophisticated and anisotropic description of yield loci – often based on the Hill or Barlat (1989) criteria – is considered important, which makes it necessary to use different constitutive models for both parts of the process chain. ▪ A damage model suitable to be used for both disciplines therefore has to be able to correctly predict damage regardless of the details of the constitutive model formulation.▪ To fill this gap the damage model GISSMO (Generalized Incremental Stress-State dependent damage MOdel) has been developed at Daimler and DYNAmore (Neukamm et al. (2009), Haufe et al. (2010)). ▪ It combines proven features of damage and failure description available in crashworthiness calculations with the possibility of mapping various history data from sheet metal forming to final crash loading.
2GISSMO – a short description▪ GISSMO = Generalized Incremental Stress State dependent damage MOdel ▪ Separate treatment of plasticity formulation and damage/failure prediction▪ To be combined with arbitrary constitutive models, e.g. *MAT_024 (von Mises)▪ Coupling of damage to stresses pFailure strain in dependency of triaxiality ▪ triaxiality η:quotient of mean stress and von Mises-stress

3Different ways to realize a consistent modeling

One Material Model for Forming and Crash Simulation • Requirements for Forming Simulations: Anisotropy, Exact Description of Yield Locus, Kinematic Hardening, etc. • Requirements for Crash Simulation: Dynamic Material Behavior, Failure Prediction, Energy Absorption, Robust Formulation • Leads to very complex model Modular Concept for the Description of Plasticity and Failure • Plasticity and Failure Model are treated separately• Existing Material Models are kept unaltered• Consistent modeling through the use of one damage model for forming and crash simulation•*MAT_ADD….(damage)

Calibration of a complete material card1Isotropic (scalar) damageEffective stress concept (similiar to MAT_81/224 etc.)

2GISSMO – a short description Ductile damage and failure The input of this failure strain is realized as a tabulated curve definition of failure strain values vs. triaxiality.

Damage Evolution

Failure Curve

From the figure, Δεp of the UHSS B-Type isgreater than that of UHSS A-Type. This shows that the local ductility of the former is higher than thatof the latter.The difference of equivalent plastic strains between the fracture and instability curves, denoted by Δεp, is thought to correspond to the progress of local ductility.
3Failure criterion for Shell vs. Solid

Currently, modeling with shell elements is the main stream practice in crash safety simulation.

However, to predict crack initiation and propagation in detail, we need a modeling methodology with fine solid elements.

• For shells (2D with the assumption of plane stress ) triaxility and Lode angle depend on each other.

fracture strain is a function of the triaxiality

• For Solids (3D) both the Lode angle and triaxiality are independent fracture strain is a function of triaxiality and Lode angle
By increasing the thru-thickness integration points, one can increase the resolution to monitor the progress of damage.
• When stress in an element reaches its element erosion state
solid model will remove element. This step will form a cut (surface) or void (internal) and create a local stress concentration or even singularity within the model (color red), which may propagate either in thru-thickness or in-plane direction.
Unlike its shell approximation counterpart, it can only suppress one of its (Gaussian) integration point, weakening the structure locally but not enough to create geometrically singular feature.

Solid vs. Shell – Reality vs. Approximation

Example of a LS-DYNA input for GISSMOEInput of GISSMO The GISSMO damage model, chosen to describe damage and failure behavior, is implemented in card 3 and card 4 of the LS-DYNA keyword *MAT_ADD_EROSION and activated by the first flag IDAM=1. With DMGTYP=1 the damage is accumulated and element failure occurs for D=1.

Summary

Features of GISSMO: • The use of existing material models and respective parameters• The constitutive model and damage formulation are treated separately • Allows for the calculation of pre-damage for forming and crashworthiness simulations Characterization of materials requires a variety of tests Automatic method for identification of parameters is to be developed Offers features for a comprehensive treatment of damage in forming simulations Available in LS-DYNA V9.71 R5 Verification and validation of concept are under way