POST MECHANICAL FAILURE FIRE DAMAGE CHARACTERIZATION OF GRAPHITE/EPOXY COMPOSITES

Fire damage involving mechanically failed composite aircraft structures can dramatically alter their exposed surface characteristics in ways that inhibit fire forensic analyses. In this work, the effects of fire exposure on mechanically failed Cytec T40- 800/Cycom® 5215 graphite/epoxy composites were examined. Coupon level vertical fire tests were performed on mechanically failed unnotched compression and in-plane shear graphite/epoxy specimens. The fire damage was characterized by visual inspection and scanning electron microscopy. The fire damage development in the specimens involved a concurrent and sequential interaction between multiple physical, chemical, and thermal processes. This damage included melt dripping, matrix decomposition, char, soot, matrix cracking, delamination, and residual thickness increases due to explosive outgassing. The composite thermal degradation due to heat conduction, combustion, and/or thermal deformation was significantly affected by the specimen layup, ply orientation relative to the heat source, and the fracture surface morphology. Plies burned with fibers oriented parallel to the flame axis conducted heat into the interior of the composite. This resulted in melt dripping, internal pockets of matrix decomposition, and surface char deposition that, in some cases, completely obscured pertinent aspects of fiber fracture surface morphology. In contrast, plies burned with fibers oriented perpendicular to the flame axis acted like a thermal protection layer that impeded (slowed) heat transfer to the specimen’s interior. Furthermore, the thermal damage development was influenced by the specimen layup and the total available free surface area created during mechanical failure. Specimens with more free surface area promoted better airflow and oxygen availability for combustion and sustained far more thermal degradation for given fire exposure. Key fractographic features in exposed fiber bundles were destroyed due to severe thermal oxidation and thinning. A thorough understanding of these coupon-level fire tests represents a critical first step in developing a coherent strategy for the Federal Aviation Authority post-crash forensic analysis of composite aircraft structures.

Hydrogen Embrittlement of the Additively Manufactured High-Strength X3NiCoMoTi 18-9-5 Maraging Steel

The main aim of this study was to determine the susceptibility of the additively manufactured high strength X3NiCoMoTi 18-9-5 maraging steel to hydrogen embrittlement. For this purpose, samples produced by selective laser melting technology, before and after heat treatment, were used. The examined samples were electrochemically charged with hydrogen in NaCl + NH4SCN solution at a current density of 50 mA/cm2 for 24 h. The H content increased from about 1 to 15 ppm. Heat treatment did not affect the amount of H trapped in the maraging steel. Tensile testing revealed that the tensile strength of the H-charged samples was much lower than that of the uncharged samples. Moreover, the material became brittle after charging compared to the ductile as-printed and heat-treated samples with elongation values of 7% and 2%, respectively. The loss of plasticity was confirmed by fractography, which revealed transformation of the fracture surface morphology from dimple-like in the as-produced state to a brittle one with smooth facets in the H-charged state.

Application of r-GO-MMT Hybrid Nanofillers for Improving Strength and Flame Retardancy of Epoxy/Glass Fibre Composites

The application of nanomaterials as a strengthening agent in the fabrication of polymer nanocomposites has gained significant attention due to distinctive properties which can be utilised in structural applications. In this study, reduced graphene oxide (r-GO) and montmorillonite (MMT) nanoclay were used as filler materials to fabricate hybrid epoxy-based nanocomposites. The synergistic effect of nanomaterials on flammability and mechanical behaviour of nanocomposites were studied. Results revealed that the addition of nanofiller showcases 97% and 44.5% improvement in tensile and flexural strength. However, an increment in the percentage of filler material over 0.3% exhibits a decremental mechanical property trend. Likewise, the addition of nanofiller increases the nonignition timing of the glass-fibre-reinforced epoxy composites. Fracture surface morphology displays the occurrence of the ductile fracture mechanism owing to the presence of hybrid fillers.

Effect of peanut and walnut shells powders on tensile properties and morphological test by using heat cured PMMA as a matrix for prosthetic Denture

In the current Research , the heat cured matrix material powder of PMMA was reinforced with peanut and walnut shells (natural powders) which are chemically treated with 5% (w/v) (NaOH) to improve the matrix bonding (PMMA) before being used as a reinforcing powder and adding to exactly similar averages particle sizes ≤ (53µm), with different weight fractions of (4, 8, and 12 wt.%). The ASTM D638 is used for composite specimens of the tensile test. The results indicated that the Elastic modulus values reached its maximum value at (8 wt.%.) when reinforced with peanut shells particles (1.053Gpa) , while ,the values of tensile strength, elongation percentage at break, decrease as the weight fraction of peanut and walnut shells powder increase and the lowest values is obtained by reinforcing with peanut shells particles to reach their minimum values at (12 wt.%.) where the lowest values of them are (29 MPa, 2.758% ) respectively. The fracture surface morphology of pure PMMA seemed to be homogenous morphology in (SEM) test, whereas the fracture surface morphology of PMMA composite reinforced by (peanut and walnut shells) powders and shows a roughness fracture surface morphology this refer to semi ductile to ductile materials.

Experimental Reproducibility and Natural Variability of Hydraulic Transport Properties of Fractured Sandstone Samples

Flow and transport processes in fractured systems are not yet fully understood, and it is challenging to determine the respective parameters experimentally. Studies on 10 samples of 2 different sandstones were used to evaluate the reproducibility of tracer tests and the calculation of hydraulic transport properties under identical boundary conditions. The transport parameters were determined using the advection–dispersion equation (ADE) and the continuous time random walk (CTRW) method. In addition, the fracture surface morphology and the effective fracture aperture width was quantified. The hydraulic parameters and their variations were studied for samples within one rock type and between both rock types to quantify the natural variability of transport parameters as well as their experimental reproducibility. Transport processes dominated by the influence of fracture surface morphology experienced a larger spread in the determined transport parameters between repeated measurements. Grain size, effective hydraulic aperture and dispersivity were identified as the most important parameters to evaluate this effect, as with increasing fracture aperture the effect of surface roughness vanishes and the experimental reproducibility increases. Increasing roughness is often associated with the larger effective hydraulic aperture canceling out the expected increased influence of the fracture surface morphology.