characterize & prevent
gel coat cracks
The cracks themselves are a roadmap leading to the
By Bob Lacovara
Marina Business Today, June 2002
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.
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.
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
|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
|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.
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.
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
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
email@example.com. The CFA's Web site can be accessed at