This document discusses carbon fiber repair techniques for vehicles. It covers various repair methods like bolted doublers, co-bonded doublers, stepped co-bonded repairs, and tapered-scarf co-bonded repairs. The pros and cons of each method are presented. Additional topics include non-destructive testing, heat curing equipment, and the challenges of repairing larger primary vehicle structures made of composites. The overall message is that standardized repair materials and training will be needed to support working on new automotive structures made of carbon fiber composites.

1. Louis C. (Lou) Dorworth
Abaris Training Resources, Inc.

2.  Lighter weight possible - weight savings can
help to compensate for battery weight in
electric vehicles.
 Molded to complex shapes compared to
stamping sheet metal.
 Crashworthiness ability demonstrated by
Formula 1 for 20+years.
 Fuel economy mandates

3. Prepreg hand layup is time consuming and quality is quite operator
dependent

4. Chassis made from “forged
composites”

5. Calloway RAZR Hawk
Head
Sesto Elemento monocoque chassis
Made with Short
Carbon Fiber
Tows
Rocker arms
Joint development by Lamborghini and Callaway Golf

6. Press-molded carbon-fiber
fenders, roof, roof bow,
hood, air splitter, and rocker
panels supplied by Plasan

7. i3 Chassis
i8

8. Note orientation of fibers tailored to distribute loads
on-axis throughout structure

9.  Structural Repair Vs Cosmetic Repair
◦ Structural repairs can be made flush…
 Class “A” surface is possible if painted afterwards
 More difficult, but not impossible w/o paint
◦ “Tricks” may be employed to restore the “look”
 For example: repair to corner of CFRP hood-might
require “mock repair” to opposite side for symmetry!

10.  Rebuild Load Path Through Structure
 Ideal is to Match Original Properties:
◦ Strength
◦ Stiffness
◦ Weight
 Trade-offs:
◦ To match strength, repair is stiffer & heavier
◦ To match stiffness, repair is weaker & heavier
◦ Cannot match all original properties

12.  Hidden or Undetected Damage
◦ Large area blunt impact:
 Internal damage to matrix from impact
 Large area matrix fracture?
◦ Internal damage to fibers from impact
 Backside fiber breakage?
 Repair or not?
◦ Repairs may do more damage than not…
 Weigh alternatives-remove good structure to repair damage?
 Will the structure still perform? If so, than maybe no repair is
the option, but…
 Once discovered, regular interval inspection is warranted to
note whether there is damage stabilization or propagation.

13.  Visual Inspection
 Tap Testing
 Ultrasonic
◦ “A”- Scan:
 Pulse Echo
◦ “C”- Scan:
 Through Transmission
 Thermography
Delamination
Eye level above
reflected light.
Flashlight
Shadow
Flashlamps
IR
camera
defect
PC

14.  Bolted and Bonded Doubler Repairs
◦ Pre-cured composite doublers
◦ Stainless steel or Titanium doublers on Carbon
 Cobonded or Secondary Bonded Doubler
Repairs
◦ Double Vacuum Debulk (DVD) patches
 Stepped Cobonded Repairs
◦ Wet layup and prepreg materials & processes
 Tapered-Scarf Cobonded Repairs
◦ Wet layup and prepreg materials & processes

15.  Pros:
 Can be faster and easier to accomplish compared
to tapered-scarf or stepped cobonded repairs.
 Requires less removal of original structure
(compared to step or tapered-scarf repairs) to
accommodate the repair.
 Most mechanics are already trained in drilling,
sealing, & fastener installation procedures.

16.  Cons:
 Doublers must be made to closely match the
surface contour.
◦ Can be problematic in compound contoured areas,
especially when using metals.
 Tooling may be required to facilitate fabrication
of repair patch.
 Autoclave cure of patch may be required to
achieve desired laminate properties.
◦ Optional DVD Process/cure Vs Autoclave Cure

17.  Cons:
 Risks associated with drilling into underlying
structure for fasteners.
 Expensive Ti fasteners required in carbon
reinforced structures.
 Sealing of the repair can be questionable and
not often durable.

18. Damage
Removed
Inside Doubler
Outside Doubler
Laminate Structure
Fasten as prescribed
Plug
Seal all around
the patch

19. To
Vacuum
Draw
Vacuum
to Box
By pulling vacuum inside the box, the pressure is equalized, thus the layers under the bag are no longer under
pressure yet are still under vacuum. This allows for the repair laminate to “de-gas” as if it were in a vacuum
chamber. Heat is applied during the process to lower the resin viscosity and promote the de-gassing event.
After sufficient time the box is vented and the layers are then compacted within the initial vacuum bag.
Tool
Vacuum box

20. To
Vacuum
Open Box
Vent to
Atmosphere
Venting the box allows the inner vacuum bag to now pull down tight against the repair laminate effectively
consolidating the freshly de-gassed layers against the tool surface. The resulting repair layup is now ready for co-
bonding or molding to a prepared structure.
Vacuum box
Tool

21. Filler
Backside patch
(non-structural)
Symmetric pre-compacted (DVD) repair patch
Adhesive

22.  Idea comes from metal lap-joint repair design
concepts.
 Each damaged layer is removed in “steps” so
as to provide a landing for each replacement
layer in the repair.
 Each repair ply then overlaps the
corresponding exposed layer in the structure.

23.  Pros:
 Easy to conceptualize & design.
◦ (It looks good on paper!)
 Feasible for flat panel repairs made with
discernible materials.
 Co-bonded layers makes for intimate repair
contact at faying surfaces.

24.  Cons:
 Requires meticulous machining of each layer
in the damaged laminate so as not to damage
the underlying plies. (This takes exceptional
skill!)
◦ Difficult to do on curved surfaces.
◦ Damage to underlying layers is common and often
overlooked in the industry.

25.  Cons:
 Requires careful fabrication and placement of
repair plies so that they fit into each step w/o
gaps or overlaps.
◦ Requires precise templates or tracings to ensure
repair-plies fit properly and are aligned on the
correct axis.
◦ Unwanted gaps and overlaps often result in defects
in the final repair.

26.  Cons:
 Takes a trained technician approximately 2 x
the time to perform a stepped repair
compared to a tapered-scarf repair.
◦ Tech’s are not likely to do step repairs as a result of
the extra labor involved.
◦ Many wind blade repair schemes involve a step
repair approach prior to tapering.
 This is very time consuming for larger repairs

27. Loads are distributed through
the repair via a lap joint into the
underlying layers
The resulting repair sits above the surface
Filler Ply

28. Filler Ply
Shear stress distribution in a stepped repair
Note peak stress concentrations at edges of each
step-lap within the repair

29.  A tapered-scarf angle is machined through
the composite structure so as to expose each
layer along a gently-angled slope.
◦ Can be tapered from both sides for best results in
solid laminate structures
 Each repair ply then lays over the
corresponding exposed layer along the
tapered angle.

30. Typical ½ inch per ply scarf removal shown. Note
orientation of each ply. Matching orientation with
repair plies is mandatory for regaining original
load path.

31.  Pros:
 Fast & easy to machine taper-angle and to
fabricate repair plies.
 Perfect size and fit of repair plies is less
important to performance.
 Flush surface is attainable.
◦ Important for critical “Class A” surfaces.

32.  Pros:
 Can machine a taper with a die grinder or
“jitter-bug” sander.
◦ No special tools or devices necessary.
 Less risk of damaging underlying layers in
tapered area.
 Co-bonded layers makes for intimate repair
contact at faying surfaces.

33.  Cons:
 Potential to remove too much “good”
structure at lower taper angle.
 Preferred two-sided taper scarf may be
difficult for technician to execute

34. Loads are transferred directly through the
edges of the repair plies, in plane, on axis,
in shear, to the underlying structure
The resulting repair is flush with the surface

35. Uniform shear stress distribution through a
tapered scarf joint

36. Loads are transferred directly through the
big ply laid down first, fully loading this ply
in the axis of its orientation prior to
transferring load to adjacent plies
Not recommended for highly loaded long fiber structural repairs!

37. Big Ply Down Repair
1607
1791
1931
1905
1876
1940
1831
1981
1858
110.64
Little Ply Down Repair
1944
1809
1921
1978
1975
1959
1986
1876
1931 Avg.
57.15 Std. Dev.
Tensile breaking strength of 1 inch wide coupons cut
from GFRP panels repaired with each method

38.  Used to accurately control a variety of
different heat sources & functions
 Documents actual time, temperature, &
vacuum data from each run
 Provides alarms for low vacuum, high or low
temperatures, open circuit T/C’s, heat source
faults, etc.
 There are a handful of equipment
manufacturers in the U.S. & Europe

39.  Heat blankets
◦ Silicone rubber with or without fiber/fabric
reinforcement
 Heat lamps
◦ Infrared lamps
 Hot air Machines
 Radiant Heaters

40. Kapton Blanket for
>450 F Cures
Important for any repair shop to have many different
sizes and shapes available to their technicians

41.  Size limits:
◦ 120 volt blankets are limited to about 5 square feet
on a 30-amp circuit.
◦ 240 volt blankets can cover up to 10 square feet of
area.
◦ 440 volt blankets can cover up to 20 square feet of
area.
 However:
◦ Multi-zone capabilities allow for larger coverage
area.

42.  Outer 2 inches of the
blanket runs much
cooler.
 Temperature is easily
30% cooler than center
of the blanket.
 All T/C’s & repair must
be placed within this
boundary.
All T/C’s and entire
repair must be
inside this area
2 inch cool zone
Buss tab
Cord

43.  Common in many repair shops
 Should Always be controlled with a controller
 Difficult to distribute heat uniformly to the
repair area
 Better to use an array of lamps rather than
one or two lamps for better uniformity
 Foil wrap and conductive breather assist heat
transfer/uniformity

44. Lamps are controlled with thermocouples via
the process control unit

45.  More effective for uniform heat distribution
than heat lamps.
 Can be purchased with infrared temperature
sensor for independent temperature control.
◦ Separate portable controller not required

46. Note the use of carbon fabric as a
breather to effectively transfer heat.

47.  Designed to work with a localized “tent” or
“oven” environment with portable repair unit
 Provides a hot-air convection source for more
uniform heating of structure
 Works exceptionally well for complex shapes
where a blanket might not conform well
 Excellent for secondary heating of heat sinks
in underlying structure

48. Hot air machine from Heatcon
Composite Systems, Inc.

54. Backside abraded for co-bonded “doubler” ply.

57. Trimmed repair, ready for final bodywork,
nutplates, and clear coat

58.  Larger & more complex composite primary
automotive structures are today’s big challenge!
 Innovative repair techniques and methodologies
will need to be explored & implemented
 Standardized repair materials & methodologies
are necessary to support these new structures
 Training the global workforce these new skills
will require a concerted effort
◦ Standardized curriculum to be used for certification of
mechanics and technicians. (I-CAR)

59. Thank You!
Louis C. Dorworth
Division Manager, Abaris Direct Services
Abaris Training Resources, Inc.
Reno, Nevada
lou@abaris.com
www.abaris.com