Damage in single lap joints of woven fabric reinforced polymeric composites subjected to transverse impact loading

Single lap joints of woven glass fabric reinforced phenolic composites, having four different overlap widths, were impacted transversely using a hemispherical impactor with different velocities in the low velocity impact range. The resulting damage was observed at various length scales (from micro to macro) using transmission photography, ultrasonic c-scan and x-ray micro tomography (XMT), in support of each other. These experimental observations were used for classification of damage in terms of damage scale, location (i.e. ply, interfaces between plies or bond failure between the two adherends) and mechanisms, with changing overlap width and impact velocity. In addition, finite element analysis was used to simulate delamination and disbond failure. These simulations were used to further explain the observed dependence of damage on overlap width and impact velocity. The results from these experiments and simulations lead to the proposal of a concept of lower and upper characteristic overlap width. These bounds relate the dominant damage pattern (i.e. scale, location and mechanism) with overlap width of the joint for a given impact velocity range.National University of Sciences and Technology, NUST, Pakistan: The University of Manchester, EPS fellowshi

Fabrication and test of lightweight honeycomb sandwich structures Final report

Fabrication and testing of lightweight honeycomb sandwich structure

Research in textile composites at KU, Leuven

An overview is presented of the research on textile composites at Katholieke Universiteit Leuven. Three dimensionally woven sandwich fabric preforms are investigated for delamination resistant sandwich structures, velvet woven 2.5 dimensional fabrics for delamination resistant laminates, and knitted fabrics with good drapability for laminates of complex shape

Ballistic impact on composite armour

Armoured vehicles in current military service are requiring ever more protection to enable them to carry out their mission in a safe, effective manner. This requirement is driving vehicle weight up to such an extent that the logistics of vehicle transport is becoming increasingly difficult. Composite materials are an important material group whose high specific properties can enable structures to be manufactured for a far lower weight than might otherwise be possible. Composite materials in an armoured vehicle will require structural performance as well as ballistic performance. The mechanical and ballistic performance of tl-kk armour and structural composites has been investigated against dcformable and armour-piercing ammunitions, over a range of impact velocities. Testing has indicated that heavy/coarse reinforcement weaves perform well against deformable ammunition and light/fine weaves well against armour piercing ammunition. The effect of individual mechanical properties on ballistic performance has been investigated as has the damage morphology of impacted materials. High tensile strength combined with low fracture toughness has been identified as an important requirement. Failure mechanisms have been identified from sections of ballistic impacts and through the use of mechanical test data the energy absorbed by each mechanism has been calculated. An energy audit has been carried out of all materials tested and a modelling procedure developed based on mechanical characteristics, damage morphology and failure mechanisms. This model has been tested against literature results and found to give very satisfactory performance

Hierarchically structured paper-based composite laminates

In this study, refined dissolving bagasse fibers was used to reinforce poly(lactic acid) (PLA) to produce hierarchically structured paper-based composite laminates. Initially, a recirculating colloidal mill was used to modify the surface of virgin cellulosic fibers from dissolving bagasse fibers. Accordingly, the gap between refiner disks was kept constant while the refining times were varied. Through this, “hairy” fibers with micron and sub(micron) dimensions was successfully liberated from the surface of the primary fibers at short time (e.g. after 6 min). This promoted areas for hydrogen thus produced dense and strong fiber networks (termed as re-engineered paper). As the use of the recirculating colloidal mill was found useful to produce “hairy” fibers, I extended the approach as a route to manufacture hierarchically structured paper-based composite laminates. My first strategy was to create the multi-layered composite laminates to manufacture ordered composites. Using laminating and papermaking like process, I studied composites with different magnitude of the hierarchy: 1, 2, and 4 denoted the number of re-engineered papers used in the composite laminates. Through morphological, mechanical, and thermal analysis of the multi-layered composites, the composites made perform better as the layers of the papers used increased. However, the limitation was the thinner re-engineered paper suffered from local stress concentration because of poor dispersibility henceforth creating gradient fibers distribution. This, without careful control, caused the properties to fall, hence, the synergism effect of the hierarchy structure was not achieved. Also, “hairy” fibers enhanced hydrogen bonding upon drying, hence, residual stress in re-engineered paper built-up creating micro-compression. Albeit, the effect not distinct on the mechanical performance of the papers and their composites. However, it is important to consider that when micro-compression overrides the bond strength of fibers loss of strength and modulus may occur. In another strategy, I combined cellulose nanofibers (e.g. cellulose nanofibrils (CNFs) and bacterial cellulose (BC)) with modified cellulosic fiber 7 to obtain fiber networks and composites with ordered higher. This allowed studies on the nano-confinement effect in the modified fiber networks. The composites structure utilising a simple laminate structure whereby the re-engineered (nano)paper sandwiched in between PLA films. The CNFs enhanced the mechanical and thermal properties of the composites, whereas with BC the performances were poor. This is due to BC agglomeration in the fiber networks owing to difficulty in disrupting three-dimensional network of BC fibers in pulp suspension. Consequently, preventing adjacent fibers for hydrogen bonding hence the advantages of nanofibers reinforcement effect not attained. To sum up, it is possible to produce hierarchically structured paper-based composite laminated using the aforementioned strategies. Introducing the recirculating colloidal proved useful as an alternative to existing mechanical equipment to fibrillate cellulosic fibers. Through “hairy” fibers hydrogen bonds were enhanced thereby creating dense and strong fiber networks. Nevertheless, the modified cellulosic fibers introduce itself as a hierarchical internal structure fiber networks model. This, in combination with the laminating technique, proved to level up the complexity of multiscale length in the hierarchy strategy. Nevertheless, nano-reinforcement effect abled to increase the order of hierarchy in composites, however, good dispersibility shall be attained to effect overall performances.Open Acces

Dynamic response of hybrid carbon fibre laminate beams under ballistic impact

This novel hybrid fibre composites combining stiff composites with soft composites are developed to improve the ballistic impact resistance of composite beams while maintaining good quasi-static loading bearing capacity. The ballistic impact performance of the hybrid beams have been investigated experimentally at a projectile velocity range of , including ballistic limits, failure modes, energy absorption capacity and the interaction between stiff and soft composite parts. For each type of monolithic beams, i.e. stiff, soft and hybrid monolithic beams, three categories of failure modes have been identified: minor damage with rebound of projectile at the low impact velocities, fracture of beam at the medium impact velocities and perforation of beam at the high impact velocities. The critical velocity of hybrid monolithic beam was similar to that of the soft monolithic beam under the same failure mode, and higher than that of the stiff monolithic beam. For the sandwich beams with stiff, soft and hybrid face sheets, the failure modes were similar to those of the monolithic beams. Among the monolithic beams, the hybrid and soft monolithic beams exhibited better energy absorption capacity than the stiff monolithic beams. As for the sandwich beams, the hybrid-face sandwich beams absorbed more kinetic energy of projectile than the soft-face sandwich beams at higher projectile velocity. The advantages of the stiff/soft hybrid construction include: (i) at lower impact velocity, the soft composite part survived with negligible damage under impact; (ii) due to the buffer effect of the soft part at the front face, stress distribution within the stiff part of the hybrid monolithic beams is more uniform than that of the stiff monolithic beams

Development of aircraft lavatory compartments with improved fire resistance characteristics. Phase 4: Sandwich panel decorative ink development

Five chemically different resin systems with improved fire resistance properties were studied for a possible screenprinting ink application. Fire resistance is hereby defined as the cured ink possessing improvements in flammability, smoke emission, and thermal stability. The developed ink is for application to polyvinyl fluoride film. Only clear inks without pigments were considered. Five formulations were evaluated compared with KC4900 clear acrylic ink, which was used as a baseline. The tests used in the screening evaluation included viscosity, smoke and toxic gas emission, limiting oxygen index (LOI), and polyvinyl fluoride film (PVF) printability. A chlorofluorocarbon resin (FPC461) was selected for optimization studies. The parameters for optimization included screenprinting process performance, quality of coating, and flammability of screenprinted 0.051-mm (0.002-in.) white Tedlar. The quality of the screenprinted coating on Tedlar is dependent on viscosity, curing time, adhesion to polyvinyl fluoride film, drying time (both inscreen and as an applied film), and silk screen mesh material and porosity

Effects of molding variables, glass-fiber reinforcement and aging on the fatigue behavior of polycarbonate in aqueous and saline environments

Includes vita.The effects of molding temperatures, glass-fiber reinforcement, stress concentration, aging and saline environment on the fatigue behavior of injection molded polycarbonate were investigated. Fatigue tests were made with specially designed machines in which statistical samples of specimens were subjected to alternating fluid loading. The fluid loading permitted temperature control, high testing speeds, and aqueous or other environments. The fatigue lives fell on Weibull distributions with nonzero minimum life parameters. S-N equations werg fitted at 5%, 50% and 95% probabilities of failure. Lower cylinder temperature during molding resulted in substantially higher fatigue strengths for glass-reinforced polycarbonate. This was attributed primarily to orientation. The increased viscosity at lower temperature caused steeper velocity gradients for flow in the mold. The resulting shearing strains oriented both reinforcing fibers and molecular chains. Higher mold temperature produced slightly higher fatigue strengths for glass-reinforced polycarbonate. This was explained by premature cooling of the surface layer in the colder mold, so that orientation was incomplete. The fatigue strength of glass-reinforced polycarbonate increased with increase in percentage of reinforcing fibers. However, 40% glass-reinforced polycarbonate required higher cylinder temperatures than 20% glass-reinforced. Also, the higher fiber content tended to inhibit orientation. As a result, highly oriented 20% glass-reinforced polycarbonate had fatigue strengths almost as high as 40% reinforced. The notch sensitivity of glass-reinforced polycarbonate was quite low. This was primarily due to increased orientation in the notched specimens, resulting from steeper velocity gradients during flow through the abrupt change in cross-section. It was possible to separate the effects of fiber orientation and molecular orientation by means of annealing. In the specimens without stress concentration, fiber orientation caused an increase in fatigue strength of about 40%. Molecular orientation caused an additional increase of about 25%. Thus the total increase in strength due to orientation was about 65%. In the notched specimens, total increase in strength due to orientation was about 90%. There was no evidence that residual stresses had any significant effect on fatigue strength of reinforced polycarbonate. Aging for periods of up to 5 years did not cause crazing or other degradation of polycarbonate resin specimens. The fatigue strengths of polycarbonate resin specimens were substantially reduced by cyclic stressing in sodium chloride solution. The reduction in fatigue strength was attributed to solvent action of the saline solution on certain polycarbonate molecular bonds.Includes bibliographical references