You needt o be careful there with that term “various degrees of cracking”? A crack may NOT be just cosmetic!
“Boat hulls are usually made from the outside-in, by applying the gel coat to a waxed mold and then adding the layers of glass reinforcement and polyester to complete the hull. The gel coat, the pigmented and filled polyester, is used to hide' the underlying glass composite structure, to color the hull, to produce a flexible surface which acts as a shock absorber and to help keep water from diffusing into the composite. Blisters and de-lamination are caused by water diffusing into the hull and reacting with water soluble material to form a droplet of solution which, because of osmotic pressure, grows in volume and creates a force which results in a blister or a de-lamination.
The integrity of the hull surface - ie: gelcoat - is essential in preventing this disastrous water diffusion into the laminate. The Gelcoat is basically aresin-rich surface (much like a person’s skin), which is designed to:
1. protect the lainate from the environment
2. reduce fibre pattern
3. provide a smooth aesthetic finish
4. eliminate the need for painting
Hairline cracks in a gelcoat surface (hereafter crazing) are often (mistakenly)considered merely a cosmetic problem. However, on occasion, gelcoat cracking may be an indication of underlying structural problems, or a result of manufacturing defects, environmental, or operating conditions.
Allowing cracks to remain “open” would be a serious misjudgment, likely
resultingin future serious structural & cosmetic problems, including de-laminationand osmotic blistering.
Whatever the underlying causes (or source); the mechanisms causing gelcoat crazing are always
STRESS and MOVEMENT.
Gelcoat, by nature of being on the outer surface of a structure, is subject to the highest strain of the entire laminate. The tensile or compressive strain in a loaded laminate increases with distance from the neutral axis of the load.Under a flexural load, the highest tensile strain is recorded at the top surface, while the highest compressive strain is at the bottom surface. There is no strain at the interior of the laminate, at the neutral axis. Because of the critical positioning of the gelcoat film in a laminate structure, both the laminate and the supporting structure must take into account the strain imposedby anticipated operating loads.
There are a number of sources of localized stress in a boat hull, all of which could first appear as crazing or hairline cracks, and may ultimately lead to structural de-lamination, and/or blister formation and growth.
1. Stresses are produced by polymer shrinkage during curing. As the laminating resin cures it bonds to the solid gel coat and then shrinks on curing producing a tensile stress in the laminate near the gel coat interface. After the gelcoats are cured on a mold, the resin is applied. It bonds to the gel coat before it cures. The resin near the gel coat interface goes into tension as the resin away from the interface cures and shrinks. Undercure, resulting from under-catalization, low shop temperature or too thin a film, will usually produce a flexible gel coat. While this flexible gelcoat is not prone to cracking, it may be inclined to premature color degradation, loss of gloss,chalking or chemical attack. On the other hand, over-catalization can easily lead to a brittle gel coat which cracks with little provocation.
2. Stresses are produced by swelling of the resin due to water diffusion. Theamount of water present causes swelling of the polymer. The resin can swell as much as 10 percent by volume, and this is greatly affected by the degree of cross linking. Stresses are generated by differential swelling. If the entirehull swells uniformly, no differential stress will result. However, if one layer swells and the adjacent layer does not, the adjacent layer will be pulled apart (put in tension) by the swelled layer. The level of differential stress generated will be determined by the water gradient and discontinuities in the gradient and not by the absolute amount of water present.
The stress is transient. The maximum tension will move inward and decrease in magnitude as water diffuses. If the resin has high strength, that is, it is well cured, highly cross-linked, and reinforced with glass, it can survive the passing stress field and not crack. If a disk crack forms, it constitutes a vacuum. Any local WSM units will be drawn toward the crack to increase the pressure. This is a mechanism for concentration of WSM units in the vicinity of the crack. Stress cracks can create blister centers.
3. Stresses are produced during boat use. Peak stress is produced by waveaction, rigging stresses, impact stresses and buoyancy stress.
4. Internal cracks produce stress concentration sites at the crack tips which can lead to further cracking or accelerated chemical attack. Strictly speaking,the crack does not produce a new stress but intensifies one of the above three stresses. Cracks can magnify a stress by hundreds of times.
5. Thermal shock or direct sunlight can heat darker colored composites to beyond the heat distortion temperature of the resin causing warpage, creeping of built in stresses, over expansion of trapped air or moisture - causing laminate separation (de-lamination), blistering, or even catastrophic collapse of entire structure.
Two or more of the above five types of stress can interact at a particular point in time and space. For example, if a modest shrinkage stress combines with a small water swelling stress and at the same time, severe wave impact flexes the hull, localized disk cracking can take place. Furthermore, the reaction of the polyester resin to the stresses applied is dependent on the flexibility and toughness, i.e. resistance to cracking, of the resin. If the resin is brittle cracking will occur. A flexible resin can deform under peak stress loads without cracking. Resin flexibility depends on the type and number of links in the polyester chain and, very importantly, on the number of cross-links between the chains.
To reiterate: the mechanisms causing gelcoat crazing are always STRESS and MOVEMENT. Movement in one form or another can have a number of causes. Many times the cause of the movement can be determined from the pattern of cracking.
There are a number of types of cracks that are evidenced in gelcoat, and each type may signify a particular problem or set of problems. Various crack configurations may indicate the underlying causes, and are vital in troubleshooting the problem. In some cases the root problem has nothing to do with the gelcoat, and is a manifestation of a structural problem or unanticipated movement of the substrate.
Radial Cracks:
Usually associated with impact, radial cracks are a good indicator of the direction of the impact. The classic "spider" crack is a result of a reverse impact or sharp, localized stress riser. a frontal impact is indicated by a concentric circle pattern, with the diameter of the inner circle having a relationship to the size of the impacting object.
Linear Cracks:
There are two groups of linear cracks: stress field patterns and parallel stress cracks. The primary cause of these cracks is flexural strain. However,in the case of stress field cracking, either structural elements or local stress risers modify the parallel pattern into a more complex structure.
Parallel stress cracks indicate flexural movement perpendicular to the direction of the cracks. Parallel curvillinear cracks often indicate a distribution of stress over a supported panel surface. If the surface is restrained in two 90-degree planes, the flexural strain will "fan out," creatinga "palm leaf" effect.
Parallel stress cracks radiate from a localized nucleation. The main effect is the deflection of the laminate inward toward the restraining member. The parallel stress crack is interrupted by a stress concentration around a point
Convergent stress field cracks may result when flexural strain is interruptedby a structural member.
Divergent stress fields occur when the laminate is deflected away from the supporting member and the crack propagation is consolidated through a localized lack of movement.
B]Thermal fatigue Cracks[/B]:
Thermal fatigue cracks are a result of repetitious expansion and contraction of the gelcoat film. Whether in a parallel pattern or an isotropic (nondirectional) configuration, thermally induced cracks are characterized by short discontinuous sections, and are usually grouped in forming in a dominate stress field.
Isotropic thermal cracks are a result of the surface expanding and exerting a tensile strain within the gelcoat film in a unidirectional fashion.
Parallel thermal fatigue cracks usually are propagated by expansion of the surface in conjunction with localized flexural stress.
Form stress risers:
This type of crack is a result of an intervening shape, usually a cutout, in the surface of a panel. The form or shape serves to concentrate strain into a localized area.
In the case of a hard point riser, a low-level strain may result in cracking due to high-level stress concentration in a very small area. A square shape with sharp corners is a prime candidate for creation of a hard point riser.
A radial riser may have a different origin. In this case, often a bolt or hardware fitting exerts a tensile force in the area around a hole. The edge of the hole distends causing a tensile failure of the gelcoat in the surrounding area.”
GELCOAT CRAZING and HAIRLINE STRESS CRACKS’
http://www.cruisersforum.com/forums/f55/gelcoat-crazing-part-1-a-944.html
