Objectives

Testing of the hull material was required to determine the quality of the lay-up of the various components of the vessel. 

Investigation

Four areas were marked for testing - an area near the fracture site on the port side, an area opposite this area on the starboard side, and two areas on the port and starboard side at the stern of the vessel (Figure 1). Tensile tests were conducted from samples from each area, as well as burn tests and a microstructural examination.

Figure 1: Testing Areas

Results

The results of the tensile tests show that the variability in tensile strength is within reasonable limits.  The lack of significant variability shows that the laminates were consistently laid up. 
Burn tests were conducted to determine the volume fraction of fibres to resin in the laminate.  In each case two tests were conducted and an average of these results was calculated.  The results indicate that the laminate is compliant with specification.   
A section through the laminate in the area of failure (port side) and the opposite side of the vessel (starboard) at the same areas as the fracture site were taken to view the microstructural properties of the laminate (Figure 2).  Further, the development of the crack which resulted in failure of the vessel could be evaluated (Figure 3). 

Figure2 – Image showing the position of the samples taken for microstructural analysis

Figure 3 – Section through the vessel in the region of fracture (port side)

Figures 4 shows the lay-up of the laminate, while Figures 5 and 6 show the differences in density of the fibres in the area with roving fibres and that with chopped strand.  The area where roving fibres have been used for the laminate is much more dense with respect to fibre volume fraction than the area where chopped strand has been used.  The burn tests evaluate the laminate as a whole, representing an average of fibre density. The microstructural investigation shows that it is only the two outer surfaces of the laminate that show a high volume fraction of fibres. The centre part shows a low volume fraction of fibres. Therefore there is a significant variance between the outer surfaces and centre part. 

Figure 4 – An example of a microstructural evaluation in the region on the roving fibres

Figure 5 – An example of a microstructural evaluation in the region on the roving fibres

Figure 6 – An example of the microstructure of the samples in the area of the chopped fibres

Evaluation of a number of samples taken from the fracture site, and a similar area on the starboard side, show that cracking developed in the centre part of the laminate which was made up of chopped strain and a high volume fraction of resin.  Figure 8 shows a section through the corner of the hull of the vessel and it indicates that a longitudinal crack developed in the centre of the laminate and facilitated a process of insidious delamination.  This probably accounts for the development of the failure observed on the port side.  Similar features were observed in the samples taken from the fracture site and in many cases a number of cracks could be observed within the centre of the laminate. 
The laminate is essentially made up of two parts.  The top and bottom layers are made up of roving fibres, and in some cases two layers of glass roving fibre mat appear to be used and in others one.  The centre part of the laminate is made up of chopped strand and resin.  The outer layers show a high density of fibres, while the centre part shows a low density.  The cracks observed on the port side of the vessel probably developed in the centre part.  This part of the laminate would be relatively weaker and more prone to fracture than the outer part of the laminate because of the lower fibre density.  Similar cracks were observed in similar areas of the laminate on the starboard side. 

Conclusions

The microstructural studies show further that the crack that lead to failure of the vessel developed in the centre part of the laminate.  This part of the laminate would be more prone to fracture than the outer layers as the volume fraction of fibres in this region is lower.  Similar features were observed on the starboard side of the vessel, where failure had not yet manifested itself.