Energy absorbtion capability of damage affected composite structures
The aim of this project is to consider the effect of damage on the energy absorption potential of continuous filament random mat (CoFRM) E-glass / polyester composite tubes. Composite materials have been shown to absorb significantly higher specific energy levels than metals under axial crushing conditions. This property can be exploited in automotive crashworthiness applications. Replacing steel crash structures with composites can lead to significant weight reductions. However, damage in composite structures can be difficult to assess and may not be visible by casual inspection. There is a concern that damage may accumulate in the crash structures, as a result of in-service wear and tear or due to operator negligence. It is important to understand how much accidental damage the crash structures can sustain before they are no longer able to fulfil their requirements. Two wall thicknesses of circular and square tube geometries were tested, with over 650 samples crushed either quasi-statically at 5mm/min or dynamically at 5m/s. Damage was induced in three ways: drilled holes, delamination in the form of Melinex® inserts moulded into the samples, and out-of-plane impact damage of various energy levels. Cylindrical samples made from this low cost composite are able to absorb up to 87 kJ/kg when tested quasi-statically. Dynamic testing was carried out as it provides a better representation of the loading conditions the parts will see in operation. Dynamically tested samples absorbed less energy than the quasi-statically tested samples (up to 18%). This was due to the viscoelastic nature of the matrix causing a greater degree of fragmentation at the higher test speed, leaving the load bearing fibres less well constrained and therefore reducing their load bearing capability. Square section tubes absorb less energy (up to 31 %) than a circular section of the same cross sectional area and fibre volume fraction. This is due to geometric stress raisers at the comers causing intralaminar failure. This splitting at the comers leaves the fronds less constrained and allows them to splay at a lower load. A threshold level was found for each type of damage. Below the threshold level the damage zone had no effect on the progressive failure mode or the specific energy absorption (SEA). Above the threshold level unstable compressive failure occurs in the form of a crack initiating at the damage zone and then propagating around the tube. In this situation a portion of the tube breaks off uncrushed and therefore reduces the energy absorption capability of the structure. For this material tested, relatively small hole sizes (5mm) and relatively low impact energy levels (l.5J - 3J) can cause unstable failure to occur at quasi-static test speeds. However, it has been shown that the damage tolerance of the material increases (to 10mm and 3J - 9J) at higher test rates (5m/s). Having observed the failure modes and damage tolerance of the tubes under various testing parameters it was important to look at ways of improving the damage tolerance of the samples. Moulding a thermoplastic interleaf into the sample to increase the interlaminar fracture toughness increases the damage tolerance of the tubes. Increasing the wall thickness and adding an interleaf increases the damage tolerance by up to a factor of 9. However, the increased damage tolerance of samples with interleaf was offset by a reduction in SEA by up to 48% due to a reduction in coefficient of friction in the crush zone from 0.36 to 0.22. The ultimate compressive stress (UCS) increases at dynamic test speeds and the mean crush load observed decreases. Therefore the crushing stress of the dynamically tested samples is a far lower percentage of the UCS of the material than under quasi-static loading. A greater stress concentration is therefore required to cause unstable failure at higher rates. Improved damage tolerance is also seen by increasing the wall thickness of the sample, testing square rather than circular section samples, and moulding interleaves into the samples. In all of these cases the changes that lead to improved damage tolerance lead to a reduction in the crush load of samples as a percentage of the ultimate crush load. Understanding the work in this thesis will enable the design of damage tolerant composite crash structures for the automotive industry. Such a part will, even with the inclusion of accidental damage, be able to absorb the energy required in the event of a collision.