Análisis de la competencia

The uniaxial fracture specimen for a strain rate of was tested on both low and high speed machines. This corresponds to a loading velocity of or . Figure 66 shows effective strain and alpha versus time for both machines. The data for the two machines is synchronised on the x axis such that the high strain portions of the tests

Page | 88 are co-linear. There is a gap in the high speed machine’s data at , though this is an artefact of the pattern applied to this particular specimen and is not indicative of the machine’s performance. The two alpha signals are very similar, particularly as the strain increases and the error in the calculation of alpha decreases.

The major difference between the two machines is in the strain signal. This is particularly evident from to – the low speed machine has a lower strain. This is because the machine is still accelerating thus the grip displacement is lower for the same time. The difference reduces as the machine reaches the correct speed and the material necks.

Figure 66 – Comparison of low and high speed test machines with effective strain and alpha versus time for a DP800 uniaxial fracture specimen loaded at 2.2 mm.s-1

As the strain rate is measured from the latter portion of the test, the acceleration has little impact on the recorded results. However, for a suitably strain rate sensitive material, it may not be appropriate to use such a machine for tests at a higher rate than quasi-static.

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.2 0.4 0.6 0.8 1.0 1.2 al p h a Eff e ctiv e str ai n time, s

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4.3.5 – Thermal imaging

Thermal imaging of DP800 uniaxial fracture specimens was carried out, similar to the plane- strain specimens in Section 3.4.4. In this case, the two strain rates used were and

_{. Figure 67 shows the contour plots at peak temperature for a specimen at each of}

these strain rates. As with the plane-strain specimens, the peak temperature is reached at some point after there is a clear gap between the two halves of the specimen. This means that much of the heat is generated in the centre of the specimen and only conducts to the surface after fracture.

Figure 67 – Thermal imaging of DP800 uniaxial fracture loaded at 1 s-1 (upper) and 100 s-1 (lower) at peak temperature

Figure 68 shows temperature versus displacement. The measurement is taken from the one point where the peak temperature is observed for the whole test, assuming that this

Page | 90 corresponds to the peak strain and fracture initiation location. The x axes for the curves have been adjusted such that the peak temperatures are aligned. The curves for the lower strain rate are very consistent and the peak temperature recorded is approximately . The higher strain rate tests are much more variable in both the temperature reached and the shape of the curve. The peak temperatures for this strain rate range from to

. As with the plane-strain tests, it is assumed that these temperatures are not high enough to cause a significant change in the mechanical properties of a dual phase steel. The peak temperature measurement is misleading for the higher strain rate tests because it is highly dependent on the location chosen and the peak temperature is reached a considerable time and displacement after fracture. However, it can still be used to determine broad changes in temperature with strain rate.

Figure 68 – Graph of temperature versus displacement for DP800 uniaxial fracture loaded at 1 s-1 and 100 s-1

0 20 40 60 80 100 120 140 160 0 0.5 1 1.5 2 2.5 3 Tem p e ratu re , ° C Displacement, mm 1 s⁻¹ #1 1 s⁻¹ #2 1 s⁻¹ #3 100 s⁻¹ #1 100 s⁻¹ #2 100 s⁻¹ #3

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4.3.6 – Fractography and microscopy

Figure 69a) shows a fracture surface of a DP800 uniaxial fracture specimen loaded at at magnification. Figure 69b) is of a specimen loaded at at the same magnification. Both of these have the hole and thus the crack initiating at the left of the image.

It is evident that the fracture mechanism does not change between these strain rates. There is significant necking through the sheet’s thickness as well as across the width. Similar to the plane-strain specimens, the failure surface is typical of ductile failure with cups and cones from void coalescence. There is no morphological change in the fracture surface with strain rate.

Figure 70a) and b) show sections through the same specimens at magnification. There are two distinct regions on these specimens. The top region is highly polished though the bottom region is rough from ductile fracture. This is because this section is not in the same plane as the polished cross section. It is at due to shear bands forming through the thickness of the sheet. In both specimens, the damage is confined to a short distance of the fracture surface – approximately . This is because the strain is highly localised due to the necking and so the majority of the voids nucleate and coalesce in this small region.

There does appear to be a difference in the amount of damage done to the material around the fracture surface and the distance from the edge of the specimen to the shear band. The amount of damage could have increased due to the increased energy applied to the specimen due to strain hardening. However, it is more likely that the damage varies through the sheet thickness as these specimens are ground and polished to a smooth finish rather than to a target depth. As such, it is assumed that there are no strain rate dependent phenomena that would influence the fracture strain.

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Figure 69 – SEM images of fracture surfaces of DP800 uniaxial fracture specimens, loaded at a) 1 s-1 and b) 100 s-1

a)

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Figure 70 – SEM images of cross sections of DP800 uniaxial fracture specimens, loaded at a) 1 s-1 and b) 100 s-1

a)

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Chapter 5 – Shear fracture