1.3 Conceptos fundamentales
1.3.5 Los juegos verbales
8.2.1 – DP800
DP800 was found to have a fracture strain that increased by across the strain rate range tested for the uniaxial and plane strain specimens. The shear specimen decreased by
across the same range. This effect is minor and similar in magnitude to the variation in fracture strain at each strain rate. This variation is larger than can be accounted for by either the random or systematic errors in the methodology, which indicates that the
Page | 171 fracture strain itself is variable. This is comparable to the variation in forming limit curves. Further testing is required to see if a statistical form to the variation can be found.
As the strain rate sensitivity for fracture in DP800 is limited, it is not necessary to model the effect of strain rate. If the strain rate is modelled, it potentially allows a lower factor of safety to be applied to the fracture and therefore extra weight savings. However, this effect will be limited in comparison to the spread in the fracture strain at each strain rate. It is possible to model fracture in DP800 using the previously existing quasi-static data with some degree of accuracy. This is not the most conservative approach, due to the reduction in fracture strain in shear at high strain rate. Therefore, to model fracture accurately and conservatively with a minimum number of tests, it is recommended to perform quasi-static tests for tension ( ) and high strain rate tests for shear ( ). The high strain rate to use for shear depends on the application, though is suitable for automotive crash safety.
An example of this strain rate insensitive fracture locus for DP800 is shown in Figure 117. As noted previously, there is a discontinuity at and an improved shear specimen is required to have greater confidence in the left half of the curve. If this locus were shown with a third axis for strain rate, it would be a horizontal extrude of the curve shown below. The temperature rises associated with high strain rate loading were not high enough to significantly affect a dual-phase steel. Likewise, no strain rate dependency for the fracture mechanism was observed. This is consistent with the lack of strain rate sensitivity in fracture strain.
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Figure 117 – Proposed strain rate insensitive fracture locus for DP800
8.2.2 – DX54 and AA5754
A forming grade of steel, DX54, and a structural grade of aluminium, AA5754, were briefly tested. Both materials exhibited a much greater response to strain rate. However, further tests are necessary to establish a strain rate dependency as a limited set of rates was tested and the high fracture strains required the data to be extrapolated. The DX54 results imply that a correlation may be found between the magnitude of the effects of strain rate on the work hardening and the fracture strain. However, no such link was found for the AA5754. If such a link exists, it may be limited to ferritic steels.
0.0 0.2 0.4 0.6 0.8 1.0 -1.0 -0.5 0.0 Eff e ctiv e fr ac tu re str ai n alpha
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8.3 – Further work
In this section, a range of potential extensions to this investigation are proposed. These include different materials, loading and manufacturing processes.
8.3.1 – Range of materials
The most obvious extension to this work is to investigate other materials. This could include other grades of steels or aluminium alloys, both of which are commonly used in similar automotive applications to dual-phase steels.
Other steels
Boron steels are of significant interest for reducing the mass of some safety-critical automotive components. (Lanzerath, et al. 2007) states that the conventional ductility of such grades from global strain measurement is approximately while local fracture strains are between and , depending on the stress state. These strains are significantly lower than those found in dual-phase steels, which means that it is even more important to understand the local fracture properties under crash conditions. As the fracture strain decreases, the probability of failure in the bulk material rather than at stress concentrators, such as joints and notches, increases.
Transformation induced plasticity - TRIP- grades of steel are also of interest. These are typically of similar strength to the dual-phase grades considered in this work. Unlike dual- phase steels, they have a microstructure that contains meta-stable austenite that transforms to martensite under loading, which increases the ductility. This transformation, and hence the ductility, is dependent on several factors, including stress state, strain rate and temperature (Choi et al. 2009). It is a diffusion-less process involving lattice distortion (Olson and Owen 1992), meaning that it occurs rapidly and is relevant to high strain rate loading.
Page | 174 It has previously been found that TRIP steels have an higher elongation to failure with increasing strain rate (Huh, Kim and Lim 2008) but that higher temperatures make austenite more stable (Tian, et al. 2006). Simulations have found that less of the austenite transforms to martensite at higher temperatures and that heat from adiabatic plastic work has the same effect, through a separate mechanism (Zaera, et al. 2009).
Another point to consider is the paint baking process in the automotive industry. The elevated temperatures cause a decrease in uniform elongation in TRIP steels (Durrenberger, Lemoine and Molinari 2011). Furthermore, while TRIP steels bake harden, unstrained DP steels do not (Zhang, et al. 2008).
In short, while dual-phase steels exhibit low strain rate sensitivity for fracture, TRIP steels show a much more complicated relationship between temperature, strain rate and fracture. Therefore, their crash performance cannot accurately be modelled without further materials characterisation.
Aluminium
A brief study of an aluminium grade, AA5754, was discussed in Section 7.3. A sensitivity to strain rate for fracture was observed, which is surprising given the existing work showing a lack of strain rate sensitivity for plastic flow. There is a general trend in the automotive sector to reduce the weight of vehicles by using composites and lightweight alloys, including aluminium and magnesium. It is therefore important to characterise the strain rate dependent fracture properties of these materials in addition to the work presented here on high strength steel.
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