Forming limit diagrams (FLDs) are widely used to assess metal sheet formability. Experimental FLDs are obtained by performing formability tests and determining failure strains. The standard method for detection of forming limits is based on the spatial distribution of the strains and requires formation of a single local neck. Some aluminium alloys, such as AA6016, have a tendency to form multiple strain localizations in formability tests, which can be interpreted as multiple local necks. Thus, use of the standard method is questionable for these aluminium alloys. The present paper presents an alternative, digital-image-correlation-based method for experimental detection of the onset of local necking in an aluminium sheet. The method is based on monitoring the sheet-thickness evolution, and is developed to be user independent and resistant to noise in the measurements. The method can be used in combination with different types of formability tests. The main requirement is that digital image correlation is used for strain measurements. Here, the method is initially tested on uniaxial tension tests of AA6016 aluminium alloy sheets and then extended to formability tests.
Forming limit strains are used to construct a forming limit diagram (FLD), which is a diagram in the principal strain space, traditionally used for designing forming operations of sheet metals. A line indicating the boundary between safe and unsafe strains is often called the forming limit curve (FLC). FLDs are also used to evaluate results from finite element simulations. Therefore consistency and reproducibility are important. This paper deals with the experimental determination of forming limit strains from Marciniak-Kuczynski (MK) tests. The material tested is AA6016 aluminum alloy in three different conditions: virgin material and material subjected to 5% and 8% deformation by rolling. Strains were measured by the use of digital image correlation (DIC) technique. Forming limit strains were determined by the use of two automated methods. The results from the two methods are compared and evaluated regarding their applicability to the Marciniak-Kuczynski test and ability to capture actual forming limit strains.
This paper is part of ‘through process modelling of welded aluminium’ project. It describes experimental and numerical investigation on butt-welded specimens of aluminium alloy AA6060. In the experiments, tensile test was used with Digital image correlation (DIC) technique to obtain full field strain measurement on the transversely loaded specimens. The tensile properties of these specimens are presented in terms of response curves. A user defined material was implemented in the explicit finite element code for the numerical calculations. The concept of non-local approach for plane stress analyses and the Cockroft Latham fracture criterion were used respectively to reduce mesh dependence of strain localization and to predict ductile fracture. The numerical results were compared to the experimental data and the measured and predicted response was evaluated.
Growing concerns for economy, environment and functionality have led to increased use of light-metals in the load carrying structure and safety components of cars. With High Pressure Die Casting (HPDC) of magnesium and aluminium alloys, components with very complex, thin-walled geometry, like instrument panels, A and B pillars and front end structures, can be cast with a high production rate. The challenge with HPDC is to optimise the process parameters with respect to the part design and the solidification characteristics of the alloy in order to obtain a sound casting without casting defects. Unbalanced filling and lack of thermal control can cause porosity and surface defects due to turbulence and solidification shrinkage. These defects can give low ductility compared to for instance extruded materials. To explore the possibilities for energy absorption in HPDC magnesium alloys, it was decided to investigate a different energy absorption principle, namely bolt shearing. In the automotive industry, this generic mechanism is already in use in extruded crash-boxes, but has not yet been utilised with respect to cast components.
Tensile tests are carried out for the aluminum alloys AA1200 and AA3103 at various strain-rates in the range from 10−4 s−1 to 1 s−1. Tests with constant nominal strain-rate and strain-rate jump tests are conducted, and the instantaneous rate sensitivity and the rate sensitivity of strain hardening are investigated. For both materials, the instantaneous rate sensitivity is found to be rather independent of strain, while the rate sensitivity of the strain hardening is important and the saturation stress increases with increasing strain-rate. A phenomenological constitutive model is described that comprises a kinetic equation governing the instantaneous rate sensitivity of the flow stress and a structural parameter that determines the mechanical state of the material. The evolution of the structure parameter is assumed to depend on strain-rate. The model parameters are determined for the two materials using the available experimental information. It is found that the constitutive model provides a good representation of the experimental results.
