Tensile properties of carbon fibres and carbon fibre-polymer composites in fire

The effect of fire on the tensile properties of carbon fibres is experimentally determined to provide new insights into the tensile performance of carbon fibre-polymer composite materials during fire. Structural tests on carbon-epoxy laminate reveal that thermally-activated weakening of the fibre reinforcement is the dominant softening process which leads to failure in the event of a fire. This process is experimentally investigated by determining the reduction to the tensile properties and identifying the softening mechanism of T700 carbon fibre following exposure to simulated fires of different temperatures (up to 700 degrees C) and atmospheres (air and inert). The fibre modulus decreases with increasing temperature (above similar to 500 degrees C) in air, which is attributed to oxidation of the higher stiffness layer in the near-surface fibre region. The fibre modulus is not affected when heated in an inert (nitrogen) atmosphere due to the absence of surface oxidation, revealing that the stiffness loss of carbon fibre composites in fire is sensitive to the oxygen content. The tensile strength of carbon fibre is reduced by nearly 50% following exposure to temperatures over the range 400-700 degrees C in an air or inert atmosphere. Unlike the fibre modulus, the reduction in fibre strength is insensitive to the oxygen content of the atmosphere during fire. The reduction in strength is possibly attributable to very small (under similar to 100 nm) flaws and removal of the sizing caused by high temperature exposure

Tensile strength modelling of glass fiber-polymer composites in fire

A thermal-mechanical model is presented to calculate the tensile strength and time-to-failure of glass fiber reinforced polymer composites in fire. The model considers the main thermal processes and softening (mechanical) processes of fiberglass composites in fire that ensure an accurate calculation of tensile strength and failure time. The thermal component of the model considers the effects of heat conduction, matrix decomposition and volatile out-gassing on the temperature-time response of composites. The mechanical component of the model considers the tensile softening of the polymer matrix and glass fibers in fire, with softening of the fibers analyzed as a function of temperature and heating time. The model can calculate the tensile strength of a hot, decomposing composite exposed to fire up to the onset of flaming combustion. The thermal-mechanical model is confined to hot, smoldering fiberglass composites prior to ignition. Experimental fire tests are performed on dry fiberglass fabric and fiberglass/vinyl ester composite specimens to validate the model. It is shown that the model gives an approximate estimate of the tensile strength and time-to-failure of the materials when exposed to one-sided heating at a constant heat flux. It is envisaged the model can be used to calculate the tensile softening and time-to-failure of glass-polymer composite structures exposed to fire

Hierarchical surface features for improved bonding and fracture toughness of metal-metal and metal-composite bonded joints

Structural adhesive joints involving Selective Laser Melting (SLM) titanium bonded to titanium or to a composite material have significant potential for weight and cost saving in aerospace and other industries. However, the bonding potential of as-manufactured SLM titanium is largely unknown, and the use of additional hierarchical surface features has not been explored or characterised. Here we demonstrate with the use of SLM that a hierarchy of two surface features at different length scales can improve the fracture toughness of metal-metal and metal-composite bonded joints. At one length scale (10-15 μm), we established through fracture toughness testing that the intrinsic irregular roughness of the SLM surface maximises the bonding potential for both metal-metal adhesive joints (KIc=1.38 kJ/m2) and hybrid metal-composite co-cured joints (KIc =1.20 kJ/m2). We then combined this with surface features at a larger length scale (200 μm). For metal-composite joints, the use of groove surface features was found to deflect the crack path, which increased the fracture toughness of the joint by up to 50% for outward protruding grooves to a value of KIc=1.65 kJ/m2. We identified the rise in fracture toughness as a combination of an increase in the crack path length and a shift from pure mode I to a mixed-mode crack growth. We found that the relative contributions of these two factors were approximately equal. This work demonstrates that SLM-manufactured titanium can have significant advantages over conventional titanium for bonded joints. In comparison with conventional techniques, SLM surfaces can be used in adhesive bonds without the need for expensive and time-consuming surface preparation, and the design freedom allows for surface features that can significantly improve performance

Interaction of laminate damage and adhesive disbonding in composite scarf joints subjected to combined in-plane loading and impact

Impact tests were carried out on composite laminates and composite scarf repairs, while both were subjected to in-plane loading with tensile pre-strain levels up to 5000 microstrain. The results show that pre-straining of the composite laminates has no noticeable influence on the size of the delamination area for the given impact energy of 8 J, which represents a typical barely-visible impact on thin-skin composite structures. For composite scarf joints, however, resulting damage has been found to be a combination of adhesive disbonding and matrix cracking (delamination and intraply cracking) in the composite laminate. The size of this mixed type of damage increases significantly with increasing pre-strain levels. A finite element model was developed to investigate the interaction between adhesive disbonding and composite delamination. The computational results reveal that both delamination and adhesive disbonding are dominated by the mode II fracture. Since the critical mode II fracture energy release rate for composite laminates (GIIC = 1.08 kJ/m2) is much less than that pertinent to the adhesive (GIIC = 3.73 kJ/m2), delamination tends to occur first in the composite laminates, which then shield the growth of disbonding in the adhesive

Application of selective laser melting (SLM) for hybrid aerospace structures

The study investigates the adhesion properties of SLM manufactured Titanium (Ti-6Al-4V) alloy surfaces for metal-metal and metal-composite hybrid joints using Mode I Double Cantilever Beam (DCB) specimens. The results indicate that the inherent micro-topology of the SLM surface is able to maximise the bonding potential of the adhesive. With the introduction of additional macro-surface features, the crack path is deflected from a straight mode I path and follows the design of the surface features. Selected macro-features were found to increase the fracture toughness by up to 50%. Finite element analysis indicates that the rise in fracture toughness is due to two factors: 1) the increase in the effective crack path length and 2) a shift from mode I to mode II crack growth

Loaded carbon composite scarf joints subject to impact

Bonded composite scarf repairs are often used when a flush surface is required for aerodynamic or stealth reasons. Such repairs on the external surface of an aircraft are subject to the same impact risk as that of the parent structure. Consequently, it is essential to assess their durability in the case of impact. A previous preliminary experimental study found an instance of catastrophic failure of a composite scarf joint subject to impact whilst prestrained to 3000 μ. It was postulated that this phenomenon is a result of failure in the joint due to the combination of the prestrain and global structural oscillations resulting from the impact event. In this investigation, a previously applied finite element model is extended to more accurately replicate such catastrophic failure. The effect of lay-up sequence on adhesive failure is studied

Compression load failure of aluminum plates due to fire

An experimental study was performed to quantify the response and failure of 5083-H116 and 6082-T6 aluminum plates under compression load while being subjected to a constant heat flux representing a fire exposure. Using an intermediate scale loading frame with integrated heating, the study evaluated the effects of geometry, aluminum type, fire exposure, load, and fire protection. Intermediate scale aluminum panels which were more than 0.7 m high and 0.2 m wide were used to gain insights into the structural behavior of large structural sections exposed to fire. Failure temperatures were measured to range from 100 to 480 C and were dependent on applied stress and aluminum type. This indicates that the use of a single temperature criterion in fire resistance without load as typically done is not sufficient for evaluating structural response during fire. An empirical failure model was developed to account for fire exposure conditions, aluminum type, and geometr