JEC COMPOSITES MAGAZINE - Issue #75 - August/September 2012 - 39

Tab. 1: Comparison of glass/epoxy mechanical properties using different test methods

Material Et (MPa) Er (MPa) Ea (MPa) nrt nar nat Gar (MPa) Gat (MPa) Gtr (MPa)

Ultrasonic wave speed
33,250 20,500 22,140 0.121 0.320 0.277 0.121 0.320 0.277

Vibration analysis
29,100

18,520

7,500 12,200 7,600

and shear) directly on thick material coupons taken from the structure, then using an inverse method to access the 9 anisotropic elastic parameters of the material. - Vibration analysis: measuring the modal frequencies of a ring taken from the structure (in plane, out of plane, extension mode-shapes) then using an optimization method to determine five moduli of the material. All these values were compared with results from micro-mechanics models such as the homogenization method. Table 1 shows that it is difficult to establish reliable material properties for the 3D models needed for thick composite cylinders.

with carbon fibre, needs to be strictly controlled to verify Mechanical that no internal Model test defects caused by thermal reac27,700 25,330 tions and inter18,960 15,470 nal stresses are present. 17,410 14,040 An ultrasonic non0.139 0.128 destructive testing (NDT) method 0.275 0.256 using a 2-MHz 0.460 0.627 focused sensor was developed. 5,360 Filament-wound 12,700 carbon epoxy cylinders up to 5,340 60 mm thick were inspected and defects such as delaminations were clearly identified (Figure 2). The method consists in immersing the cylinder in water on a turn-table and following the amplitude level of the signal reflected by a perfect reflector (metallic plate) placed inside the tube. This method was calibrated in several European projects with structures having artificial internal defects; attenuation levels up to 30 dB were associated to delaminations.

Fig. 3: FE models of cylinder end closure contact (end contact, inner wall contact) Tab. 2: Collapse pressure (bar) with different types of end closure

Contact on end Linear FE Non-linear FE Test
477 340 345

Contact on inner periphery
598 440 436

End closures
End closures are a key element when designing containers (generally composed of cylindrical tubes) that should withstand deep sea hydrostatic pressure. Different solutions can be used such as: - flat plates with direct contact on the tube ends, - flat closures with direct contact on the inner periphery of the tube, - metal domes with watertight rings bonded on the tube ends, - composite domes. The first two solutions were investigated (Figure 3) and modelled with different finite element (FE) analyses, with significant gains in collapse pressure (up to 25%), see Table 2.

NDT control
The manufacture of thick composite materials, in particular those reinforced

Hemispheric domes were designed and tested for larger structures. Titanium was used to limit weight, yielding good results. A composite dome (epoxy resin, carbon fibre) manufactured by RTM (resin transfer moulding) was also tested recently, significantly improving the design and manufacture of closures for such structures (Figure 4).

Feature Marine

Fig. 2: Ultrasonic inspection of a thick (50 mm) carbon fibre/epoxy filament-wound cylinder (left, natural defect, right, artificial defects)

Fig. 4: 60-cm diameter structure with composite dome before pressure test

No75 August - September 2012 /

jec composites magazine 39
JEC COMPOSITES MAGAZINE - Issue #75 - August/September 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #75 - August/September 2012
