A Modified Approach for Predicting Fracture of Steel Components Under Combined Large Inelastic Axial and Shear Strain Cycles

Separation of material, known as fracture, is one of the ultimate failure phenomena in steel elements. Preventing or delaying fracture is therefore essential for ensuring structural robustness under extreme demands. Despite the importance of fracture as the final stage during inelastic response of elements, the underlying mechanisms and the factors influencing the onset and progression of fracture have not been fully investigated. This is particularly the case for ductile fracture where significant pre-crack deformations are present. Existing approaches geared at predicting brittle fracture, marked by little to no plastic deformation, have been proven inadequate for capturing ductile fracture. Ductile fracture is dependent on two stress state parameters, the stress triaxiality and Lode parameter, which correspond, respectively, to two kinds of work hardening damage, which are hydrostatic and deviatoric stress components. The role of stress triaxiality on ductile fracture has been well defined and implemented in various models over the past several decades. Only until recently, however, has the role of Lode parameter been identified as an important factor for accurate prediction of ductile fracture. In general, no reliable fracture prediction methods are present that are consistent throughout the whole range of stress states, where the stresses are dominated by either tension loading, shear loading, or a combination of both. In this study, a new ductile fracture criterion based on monotonic loading conditions is first developed based on analysis and definitions of the two stress state parameters and subsequently extended to the reverse/cyclic loading conditions. The extension from monotonic to cyclic loading is based fundamentally on the fact that as long as large pre-crack plastic strain fields exist, the inherent mechanism in both loading cases can be viewed as the same. Although the inherent mechanism is the same for both loading cases, extending the model to the reverse loading conditions required the inclusion of the effects of nonlinearity of the damage evolution rule as well as the loading history. The two criteria, monotonic and cyclic, are then validated on the coupon specimen level through comparisons between predicted fracture strains and their experimental equivalents for various metal types and steel grades that are available in the literature. The newly developed models offer improvements to existing known ductile fracture criteria in terms of both accuracy and practicality. Following the validation of the fracture model on the coupon specimen level, the model is employed on the connection level, up to and including failure, to evaluate block shear failure for gusset plate and coped beam connections under monotonic loading and shear links under cyclic loading. The chosen connection types are dependent on stress triaxiality (tension) and Lode parameter (shear) and are therefore appropriate for the validation of the ductile fracture model. For the block shear failure, prediction accuracy is verified through comparisons with results from corresponding laboratory tests, in the perspective of load versus displacement curves, fracture profiles, and fracture sequences. Some underlying mechanism of block shear is also explored and explained for the first time. Following the same modeling procedure, parametric studies on geometric effects on block shear failure is conducted. Three different block-shear failure modes and one bolt hole tear out mode are captured in the simulations and suggestions on design code changes are provided. For the shear links, which are typically employed in eccentric braced frames, simulation of fracture under reverse/cyclic loading is also conducted and verifications are performed through comparisons with their previous experimental results. The fracture-associated variables are included in the cyclic loading analysis through deriving an implicit integration algorithm for the material constitutive equations with combined hardening, which was integrated in the simulation using a user-defined material subroutine VUMAT.

Language

  • English

Media Info

  • Media Type: Digital/other
  • Features: Figures; Photos; References; Tables;
  • Pagination: 70p

Subject/Index Terms

Filing Info

  • Accession Number: 01673787
  • Record Type: Publication
  • Report/Paper Numbers: MPC 18-345
  • Files: UTC, TRIS, ATRI, USDOT
  • Created Date: Jun 13 2018 9:50AM