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How to characterize & prevent
gel coat cracks
The cracks themselves are a roadmap leading to the solution.
By Bob Lacovara
Marina Business Today, June 2002

 

This article was reproduced by Island Coast Boat Works
with the permission of Marina Business Today

   
   There are a number of types of cracks that are evidenced in gel coat, and each type signifies a particular problem or set of problems. Even a rudimentary visual analysis of these crack modes can provide insight into the forces acting on the gel coat surface. Various crack configurations indicate the underlying causes and are vital in troubleshooting the problem. In some cases the root problem has nothing to do with the gel coat and is a manifestation of a structural problem or unanticipated movement of the substrate.
   In a simplistic and general view, it might be said there is only one cause of gel coat cracking -- movement. If the gel coat film or the laminate does not move, cracking can not occur. 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.
   
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," creating a "palm leaf" effect. Convergent stress field cracks may result when flexural strain is interrupted by a structural member.

   Parallel stress cracks radiate from a localized nucleation. The main effect is the laminate is deflected inward toward the restraining member. The parallel stress crack is interrupted yb a stress concentration around a point. In the case of a divergent stress field, the laminate is deflected away from the supporting member and the crack propagation is consolidated through a localized lack of movement.

Thermal fatigue Cracks

   Thermal fatigue cracks are a result of repetitious expansion and contraction of the gel coat 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 gel coat 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 gel coat in the surrounding area.

 

Crack severity level

   To develop a uniform characterization for cracking, the following crack severity scale is offered as a method of standardizing the description of a cracking problem. The level of penetration through the gel coat film or into the laminate affects the method of repair, which can range from cosmetic to structural. This crack severity scale differentiates between two levels of cosmetic, involving only the gel coat film and two levels of structural severity, from minor laminate incursion to serious structural penetration.

 
Scratches

   Cosmetic scratches can be defined by the depth of the scratch. The depth should not exceed the first laminate and have no apparent fracturing of the laminate. Deeper scratches or gouging will require some form of structural repair.

   Visually inspect the scratch; if the underlying laminate is white or milky in color, the damaged laminate will need to be removed before repairing. Scratches also should be examined from an angle of approximately 35 degrees from the surface -- this allows a reflection to be seen and any distortion is an indication of internal delamination or a core failure. If distortion is apparent, a tap test can be performed using a coin or small object, picking up minute audible differences to determine the damaged area.

 
How to prevent gel coat cracking

   Minimizing the possibility of gel coat cracking involves attention to a number of areas. First is specifying the proper gel coat for the application. a high Barcol gel coat (hard/brittle) formulation may be suitable for a mold component that is not dynamically loaded. Whereas a highly stressed part should use tough gel coat, formulated for appropriate elongation.

   The second item is to consider the effect of a structural design on the gel coat surface. Excessive flexing or a flexible panel with rigid corners may contribute to gel coat cracking. Keep in mind: the only cause of gel coat cracking is movement, although there are many contributing factors. The skill of the laminate designer determines the amount of acceptable surface movement.

   Third, gel coat application is critical. Thick gel coat is a major culprit in cracking. Gel coat film thickness should be a primary focus of quality control. accurately controlling thickness requires mil gauging every part, all of the time. Gel coat curing efficiency is another critical factor. Gel time, catalyst level, shop temperature, spray gun setup and spraying technique are critical factors in producing high quality crack resistant gel coat.

   Laminate sequence timing is another important element. There is an optimal window of timing from gel coat application to laminate application. Pushing the edges or going outside this window increases the chances of interface bonding problems, possibly resulting in cracking. Usually interface bonding problems associated with cracking are sporadic, occurring in groups. Optimal timing from gel coat application to laminate is .5 to 4 hours, and 8 hours is acceptable. Overnight is marginal, and letting the gel coat cure over a weekend before laminating is not acceptable for crack preventions.

   One often overlooked, and very basic principle, is proper mixing of gel coat in the drum. Unmixed gel coat may have a higher styrene content at the top of the drum as compared with the bottom. Styrene is inherently brittle, and an unmixed or inadequately mixed drum of material may affect the properties of the gel coat, causing cracking. This problem also will appear sporadically and be difficult to diagnose. The lesson is: The basic principles of proper material handling and application prevent a multitude of problems.

   Gel coat cracking may be one of the easier problems to diagnose in composites fabrication; the cracks themselves often lead to the proper solution.

 
   Bob Lacovara is director of technical services for the Arlington, VA-based Composites Fabricators Association (CFA). He is a recipient of the CFA President's Award for outstanding industry service, as well as an author and frequent lecturer. Having been in the composites industry for more than 30 years, he maintains an industrywide international consultancy. He can be reached at the group's technical office in Harleysville, PA, at (215) 513-7546 or via e-mail at blacovara@aol.com. The CFA's Web site can be accessed at www.cfa-hq.org.

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