Left: phased array ultrasound inspection. An area of single skin pre-preg carbon is scanned by sliding an ultrasonic phased array probe over the inner surface. The scans are monitored during the inspection and are also recorded to be examined later. Water has been spread across the laminate surface to ensure a good connection with the probe. To interpret the signals generated requires a detailed knowledge of the structure that is being examined. Put another way, in the hands of specialist surveyors like Ben Pierrepont (Pierrepont Analysis) or Stefano Beltrando (QI Composites) you can learn more about a laminate than a raw ultrasound probe should be capable of detecting. Typically during a composite ultrasound inspection we are looking for: l Voids: air inclusion between layers
and/or between core panels l Delamination: total debonding between
adjacent layers l Disbond: a lack of bond between two substrates, most typically a composite
skin to core disbond in a sandwich panel l Blisters: delamination with deflection
of one layer l Porosity: many small voids present
between or within the layers l Inclusions: foreign material that is
not specified in the laminate schedule l Cracks: fibre interruption or other categories and types of discontinuity
Left: recorded phased array scan. With the phased array probe placed on the inner surface of the part, a cross-sectional side scan of the finished laminate has been recorded. Within the scan a high amplitude Back Wall Echo (BWE) from the outer surface is indicated by the broader yellow/brown down-sloping band; this indicates a well-consolidated laminate. A weak BWE could indicate excess porosity content. There are no signs of intermediate echoes which could indicate inclusions such as plastic pre-preg backer, voiding or other issues associated with the construction process
made with the same material batches and has been cured with the same cure schedules. We’ll send resin samples back to the lab to check for state of cure; we’ll check the bond between skin and core with a pull-out test apparatus; we’ll measure fillet size on honeycomb and air inclusions in adhesive film.
Destructive and Non-destructive testing tells us more about a laminate than ever before, and this has enabled builders to develop techniques to reduce potentially fatal flaws At this point we could delve into the detailed QA procedures all boatyards now employ, but there’s no space for that in this article so we’ll focus on build procedures. There are dozens of ways in which a laminate can be degraded and a builder must have a strategy to counter each one. Fibres must run straight; resin ratios must be within a given band; resin must be sufficiently cured; honeycomb
54 SEAHORSE
fillets must be consistent and sufficiently large, etc. Here are three examples of the steps we take to build better laminates…
1. Environmental controls It can take a number of weeks to laminate the skin of a large yacht. For a boat made from unidirectional carbon in particular, it is vital to keep humidity at a constant level, and the longer the length of boat we are laminating, the more important it becomes. Uncured epoxy resin is prone to absorb water; if we let the humidity rise overnight (by letting the temperature fall, for example), the resin will absorb water and the laminate will try to expand. We use large quantities of unidirectional
pre-preg and a good proportion of the fibre runs the full length of the boat. It only has to lengthen by a fraction of a millimetre for the fibre to try to move out of column. The site at which this happens will be the point at which the fibre bridges a ply overlap. We mentioned earlier that we won’t use heavy cloth as a first ply in a
hull mould, and this is the reason. If a ply overlap or drop-off is too big and/or the humidity is allowed to deviate too much, the fibre will form a wrinkle overnight. Few things upset structural engineers more than a wrinkle in unidirectional fibre.
2. Resin flow during cure It is simple enough to control the humidity in a small oven or autoclave, but the situation is different in a large hull oven that is 40m or 50m long and 5m high. Also there will usually be a difference between the temperature of the air in an oven and the mould tool surface. It’s up to the builder to control the temperatures in every part of the oven in order to achieve the best overall laminate. We are not just talking about curing the
resin – that happens anyway in the final 10-hour ‘cook’. The quality of the laminate is determined during the period that the oven temperature is rising – ramp rates and dwells determine air release, resin bleed and honeycomb fillet size.
PIERREPONT ANALYSIS
PIERREPONT ANALYSIS
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