An experimental and numerical study of the effects of length scale and strain state on the necking and fracture behaviours in sheet metals

Abstract

Within sheet metal forming, crashworthiness analysis in the automotive industry and ship research on collision and grounding, modelling of the material failure/fracture, including the behaviour at large plastic deformations, is critical for accurate failure predictions. In order to validate existing failure models used in finite element (FE) simulations in terms of dependence on length scale and strain state, tests recorded with the optical strain measuring system ARAMIS have been conducted. With this system, the stress-strain behaviour of uniaxial tensile tests was examined locally, and from this information true stress-strain relations were calculated on different length scales across the necking region. Forming limit tests were conducted to study the multiaxial failure behaviour of the material in terms of necking and fracture. The failure criteria that were verified against the tests were chosen among those available in the FE software Abaqus and the Bressan-Williams-Hill (BWH) criterion proposed by Alsos et al. 2008. The experimental and numerical results from the tensile tests confirmed that Barba's relation is valid for handling stress-strain dependence on the length scale used for strain evaluation after necking. Also, the evolution of damage in the FE simulations was related to the processes ultimately leading to initiation and propagation of a macroscopic crack in the final phase of the tensile tests. Furthermore, numerical simulations using the BWH criterion for prediction of instability at the necking point showed good agreement with the forming limit test results. The effect of pre-straining in the forming limit tests and the FE simulations of them is discussed.

Characteristics of material, ship side structure response and ship survivability in ship collisions

“…In Hogström et al (2009), novel uniaxial tensile tests are presented using the ARAMIS (2009) optical strain measuring system. This system enables a very accurate monitoring of displacements on the specimen surfaces and, therefore, also material characteristics.…”

“…The resulting facet displacement shows, for instance, regions of localised strains. The cameras used in Hogström et al (2009) allow for a constant sampling frequency of 1 Hz and the computer used had the capacity to store up to 500 images during one test.…”

“…Figure 3 presents results from the ARAMIS recordings made on the NVA mild steel: a long VE corresponds to a strain value measured over the entire length of the test rod (see the bold line in Figure 3a), while a small value of the VE corresponds to more local strain behaviour (see the dotted line in Figure 3a). The 'Aramis' stress in Figure 3 was calculated using the force recorded by the load cell in the test machine divided by the actual area of the cross-section where fracture (eventually) occurred (see Hogström et al 2009). This area was calculated using the displacement information recorded on the specimen.…”

“…This article gives an overview and summary of the achievements made by the author and co-authors in joint work in research on ship collision safety (see, for example, Hogström et al 2009;Karlsson 2009;Karlsson et al 2009;Schreuder et al 2009). Examples are presented for the calculations of structural damage by means of non-linear finite element (FE) analysis, experimental and numerical analysis of prediction of rupture, and an example of a crashworthy design.…”

Stresses and strains of geometrically complex structures of pipeline fittings

“… Za verifikaciju metode konačnih elemenata (MKE) [50,51,61,63,64,69,70,74] ili teorijskih analiza [52] i da premoste prazninu između eksperimenta, simulacije i teorije.…”