🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Building & Construction › Wind Turbine Technician › Blade and Nacelle Composites Repair › Lesson 7
BLD-WIND70-06 · Lesson 7 · 22 clock hours · lab

Wet Layup Structural Repair

Objective. Students will complete a wet-layup structural repair on a scarfed composite panel, placing plies in the correct orientation and sequence per a provided laminate schedule, and produce a cured laminate that passes tap-test and meets thickness tolerance.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Wet Layup Structural Repair

Why Wet Layup Matters in Wind Energy

Blade shells and nacelle fairings are built from fiber-reinforced polymer (FRP) composites. When a blade suffers an impact crack, delamination, or erosion damage that penetrates into the structural laminate, a wet-layup repair restores load-carrying capacity. Wet layup means you saturate dry fabric plies with liquid resin—usually a two-part epoxy—and build them up in a controlled stack directly on the prepared surface. Done correctly, the cured repair transfers stress across the damage zone just as the original laminate does. Done incorrectly, a hidden void or wrong ply orientation can cause the repair to fail under load, creating a blade or nacelle failure risk at height.

Understanding the Laminate Schedule

A laminate schedule is the engineering document that specifies, ply by ply, the fabric type, areal weight, fiber orientation, and stacking order. Orientations are given in degrees relative to a reference axis—typically the blade span direction. Common orientations in wind blade repairs include 0°, ±45°, and 90°. The schedule is not a suggestion; deviating from the specified sequence changes the stiffness and strength of the repair. Always read the schedule before cutting a single ply.

For a stepped scarf repair the plies are stepped outward in size from the deepest point of the scarf to the surface, so each successive ply overlaps the one below. The largest ply—usually the last one placed—covers the full repair footprint.

Scarfing and Surface Preparation

Before layup begins the damage area must be scarfed: ground into a gradual taper so the repair plies can be bonded without a stress concentration at the edge. Typical scarf ratios for structural composite repairs are specified by the OEM or repair manual—ratios such as 1:20 or 1:30 (rise over run) are common in the industry. The scarf must be dry, clean, and free of grease or release agent. Wipe with an approved solvent per the material safety data sheet (MSDS/SDS), allow to dry, and confirm surface cleanliness before mixing resin.

Mixing and Working with Epoxy

Epoxy systems consist of a resin and a hardener. The mix ratio—by weight or by volume—is specified by the manufacturer and must be followed exactly. An incorrect ratio produces an under-cured or brittle laminate. Mix thoroughly for the time stated on the technical data sheet (TDS), scraping the sides and bottom of the container, then pour into a fresh container and mix again to avoid unmixed material at the edges.

Epoxy has a working time (pot life) after which it begins to gel. Keep your mixed batch small enough to use within the pot life. Curing generates heat (exothermic reaction); a large mass in a cup gels faster than a thin spread layer. If the batch feels warm, work quickly or discard and mix fresh material.

Ply Cutting and Marking

Use the provided ply template and scissors to cut each glass-fiber ply to the correct shape and size. Mark the fiber orientation on each ply with a dry-erase marker before you move it to the layup area—once the ply is wet with resin, re-identifying orientation is nearly impossible. Number each ply to match the laminate schedule. Lay them out in order on a clean table so you can pick them up in sequence during layup.

Layup Procedure

1. Apply a thin coat of catalyzed epoxy to the scarfed surface with a foam roller.

2. Place Ply 1 (the smallest, deepest ply) onto the wet surface.

3. Wet out the ply thoroughly with more epoxy using the foam roller—work from the center outward.

4. Use the grooved aluminum consolidation roller to press out air bubbles and excess resin. Roll in multiple directions.

5. Repeat for each successive ply in schedule order. Keep resin content consistent—dry plies and resin-starved spots are as harmful as excessive resin.

6. After the final ply, apply peel ply, then perforated release film, then breather fabric.

7. Seal the vacuum bag film with sealant tape; make sure no creases bridge the bag at the seal.

8. Connect the vacuum pump, draw down to the target vacuum level specified in the repair procedure, and check for leaks by isolating the pump. A bag that holds vacuum without the pump running is properly sealed.

Vacuum Bag Cure

Vacuum pressure consolidates the stack, removes entrapped air, and holds the laminate in contact with the substrate during cure. Monitor the gauge periodically. Maintain cure temperature per the epoxy TDS—most structural blade repair epoxies cure at ambient shop temperature but some require a heat blanket for elevated-temperature post-cure. Record cure start time and temperature on the repair card.

Post-Cure Inspection

Tap test: After full cure, remove the bag, breather, release film, and peel ply. Using the tap-test hammer and a grid drawn on the surface, tap every square of the grid systematically. A clear, high-pitched ring indicates a well-bonded area. A dull, hollow sound indicates a delamination or void—a defect that requires further evaluation or re-repair. Mark any dull areas immediately.

Thickness measurement: Measure repair thickness at five specified points using a calibrated digital caliper. Record each measurement on the repair card. Compare against the laminate schedule target; all five readings must fall within the specified tolerance.

Safety

Key terms10

Laminate scheduleAn engineering document specifying the fabric type, fiber orientation, and stacking order for each ply in a composite laminate or repair.
Wet layupA composite fabrication method in which dry reinforcement fabric is saturated with liquid resin and placed on the substrate before the resin cures.
Scarf repairA repair geometry in which the damaged material is ground into a tapered taper so that repair plies can overlap progressively, transferring load without a sharp stress concentration.
Pot life (working time)The length of time after mixing a two-part resin system during which it remains fluid enough to apply and consolidate.
Tap testA non-destructive inspection technique in which a small hammer or coin is tapped across a composite surface; a dull or hollow sound indicates a delamination or void beneath the surface.
Consolidation rollerA grooved or ribbed roller used to press wet composite plies against the substrate, expelling trapped air and excess resin.
Peel plyA tightly woven release fabric placed on the last ply of a wet layup; when peeled away after cure it leaves a clean, bondable surface texture.
Fiber orientationThe angle at which the reinforcement fibers in a fabric ply are aligned relative to a defined reference axis, controlling the directional stiffness and strength of the cured laminate.
Exothermic reactionA chemical reaction that releases heat; relevant to epoxy cure—large mixed volumes in a confined container can overheat, gel prematurely, or in extreme cases cause fire.
Vacuum bagAn airtight film sealed over a composite layup and connected to a vacuum pump to consolidate plies and remove air during cure.

Worked examples4

1. Your laminate schedule calls for seven plies of 400 g/m² woven glass fabric. The smallest ply (Ply 1) is a circle 80 mm in diameter. Each successive ply increases in diameter by 20 mm. What is the diameter of Ply 7, the largest ply?
  1. Ply 1 diameter = 80 mm.
  2. Each additional ply adds 20 mm: Ply 2 = 100 mm, Ply 3 = 120 mm, Ply 4 = 140 mm, Ply 5 = 160 mm, Ply 6 = 180 mm, Ply 7 = 200 mm.
  3. Calculation: 80 + (7 − 1) × 20 = 80 + 6 × 20 = 80 + 120 = 200 mm.
Answer: Ply 7 has a diameter of 200 mm.
2. After cure you measure repair thickness at five points with a digital caliper. The laminate schedule target thickness is 4.20 mm and the tolerance is ±0.50 mm. Your readings are: 4.05 mm, 4.60 mm, 3.68 mm, 4.45 mm, and 4.10 mm. Which readings, if any, are out of tolerance?
  1. Acceptable range: 4.20 − 0.50 = 3.70 mm (minimum) to 4.20 + 0.50 = 4.70 mm (maximum).
  2. Check each reading against the range:
  3. Point 1: 4.05 mm — within range (3.70–4.70). PASS.
  4. Point 2: 4.60 mm — within range (3.70–4.70). PASS.
  5. Point 3: 3.68 mm — below 3.70 mm. FAIL.
  6. Point 4: 4.45 mm — within range. PASS.
  7. Point 5: 4.10 mm — within range. PASS.
Answer: Point 3 (3.68 mm) is out of tolerance by 0.02 mm. All other points pass.
3. A two-part epoxy has a mix ratio of 3 parts resin to 1 part hardener by weight. You need 200 g of mixed epoxy. How many grams of resin and how many grams of hardener do you weigh out?
  1. Total parts = 3 + 1 = 4 parts.
  2. Each part = 200 g ÷ 4 = 50 g.
  3. Resin = 3 × 50 g = 150 g.
  4. Hardener = 1 × 50 g = 50 g.
  5. Check: 150 g + 50 g = 200 g. ✓
Answer: Weigh 150 g of resin and 50 g of hardener.
4. Your repair area is a rectangle 300 mm × 200 mm and the repair card requires you to draw a tap-test grid with lines every 50 mm in both directions. How many grid squares will you need to tap?
  1. Number of columns (along 300 mm side): 300 ÷ 50 = 6 columns.
  2. Number of rows (along 200 mm side): 200 ÷ 50 = 4 rows.
  3. Total grid squares = 6 × 4 = 24 squares.
Answer: You will tap 24 grid squares.

The math in this lesson1

Learners use 80 + (n − 1) × 20 to size each ply, finding Ply 7 = 200 mm diameter.
F-BF.2
Brush up in the math halls at newSchoolWorks.com:
In class: Ply-cutting exercise: students cut seven glass-fiber plies to the correct shape and dimension for a stepped scarf repair using scissors and a ply template; each ply is numbered and its fiber orientation marked with a dry-erase marker before layup begins. · Layup: students wet out each ply with mixed epoxy using a foam roller, apply in correct sequence to the scarfed coupon, consolidate with a grooved roller to remove air bubbles, apply peel ply and breather, then close the bag for vacuum-bag cure; instructor observes and records any bagging defects. · Post-cure inspection: after cure, students remove bag, peel ply, and perform tap-test over the entire repair area using a grid; they measure repair thickness at five points with a calibrated digital caliper and record results on the repair card.
Materials: Woven and biaxial glass fabric, pre-mixed epoxy system, foam rollers, grooved aluminum consolidation roller, laminate schedule template, ply-cutting template, scissors, peel ply, perforated release film, breather fabric, vacuum bag film, sealant tape, vacuum pump and gauge, digital caliper, repair-card form, tap-test hammer
Check: Repair card checked by instructor: tap-test shows no dull areas in the repair footprint (100% clear); thickness at all five measurement points within ±0.5 mm of the laminate schedule target; ply sequence verified from photo log = pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Wind Turbine Tech – Composite Repair and Maintenance
1. When performing a stepped scarf repair, Ply 1 is placed at the deepest point of the scarf and each successive ply is larger. What is the main structural reason for this sequence?
NCCER Wind Turbine Tech – Composite Repair and Maintenance
2. A student marks fiber orientation on each ply with a dry-erase marker before the layup begins. Why is this step critical?
GWO Basic Safety Training – Workplace Safety and Hazard Identification
3. During tap testing, a dull or hollow sound under the hammer indicates:
NCCER Wind Turbine Tech – Composite Materials and Repair
4. A two-part epoxy specifies a 2:1 mix ratio by weight (resin to hardener). A technician needs 120 g of mixed epoxy. How many grams of hardener are required?
OSHA 10 – Personal Protective Equipment
5. Which PPE combination is required when mixing and applying epoxy resin during a wet-layup repair?
NCCER Wind Turbine Tech – Composite Repair Inspection
6. After cure, five thickness measurements are taken with a calibrated digital caliper. The laminate schedule target is 5.00 mm with a tolerance of ±0.50 mm. Which reading is out of tolerance?
NCCER Wind Turbine Tech – Composite Fabrication and Repair
7. What is the primary function of the vacuum bag during a wet-layup cure?
OSHA 10 – Hazard Communication and Chemical Safety
8. A technician notices that the mixed epoxy batch in the mixing cup is becoming warm shortly after mixing. According to safe work practice, the technician should:
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →