PSI - Issue 14

Sameerkant Behera et al. / Procedia Structural Integrity 14 (2019) 112–118 Sameerkant Behera, S. K. Sahoo, Lokesh Srivastava, A. S. Srinivasa Gopal / Structural Procedia 00 (2018) 000 -000

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2.3. Critical Defects and Modes of Failure

Composite pressure vessels are multilayer, multiphase and multi-interface structure. These are generally large sized contour shaped structure having variable thickness in the order of 10 to 15 mm at cylindrical region and 20 to 45 mm at aft end/fore end dome region. Here likely chance of various type of defects having overlapping signatures also can be encountered. Some critical defects which are likely to be observed due to process variations are shown in figure 3. Along with such defects, other physical properties variation may be seen such as resin content, fiber volume fraction, density and glass transition temperature. Over and above, most critical failure modes are described below.

Fig. 3. Critical defect mode

 Composite Casing  Composite skirts

: Hoop & Helical fibre failure, Compressive strength failure and Buckling.

: Compressive strength failure and Buckling. : End fitting Blow-out & Excessive stress in metal/rubber.

 Metallic end fitting Joint

 Composite casing and skirt joint : Strength failure of composite and Bond failure.  Metallic flange joint failure : Composite bearing failure & fastener failure. 3. Test procedure and experimental test set-up 3.1. Test Procedure

In through transmission technique, two transducers are located on opposite sides of the composite laminate. One transducer acts as a transmitter and the other as a receiver. Discontinuities in the sound path will result in a partial or total loss of sound being transmitted and be indicated by a decrease in the received signal amplitude [6]. Schematic for the above technique is mentioned in figure 4. The above method is chosen so as to cater the requirements of high attenuation of sound wave in composite, variation in thickness, geometry and multiple phases/interface in composite pressure vessel.

Fig. 4. Schematic of through transmission technique .

3.2. Experimental test set-up Here low frequency, dry coupling and point to point contact (A-scan) has been followed. Ultrasonic flaw detector Sonatest MS410D and probe of 0.5 MHz (fig. 5a) has been selected. Different reference blocks are made simulating varying thickness and defect such as delamination. Figure 5(b) shows reference thickness blocks having dimension of 150*120*100mm. Total thickness of 100 mm is made in to five steps and each step is having a height of 20mm. Figure 5(c) shows reference block having dimensions of 90*90*10mm showing 50% healthy zone and 50% defective zone (delaminated zone). Above reference blocks are made up of carbon epoxy material system as equivalent to full scale component.