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newSchoolWorks.com › VoTechWorks › Building & Construction › Wind Turbine Technician › Blade and Nacelle Composites Repair › Lesson 4
BLD-WIND70-06 · Lesson 4 · 18 clock hours · lab

Damage Inspection Methods and Damage Classification

Objective. Students will inspect a set of pre-damaged composite panels and a decommissioned blade section using visual inspection, coin-tap test, and borescope, then classify each defect by type and severity using a four-level damage-category matrix.

⚠ 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.

Why Damage Inspection Matters

A wind turbine blade is a precision structural component. It carries aerodynamic loads, bending loads, and torsional loads every second it spins. When the composite laminate that forms that blade is damaged — even invisibly — those loads concentrate at the defect and can cause rapid, catastrophic failure. Your job as a technician is to find damage early, describe it accurately, and classify it so the right repair decision gets made. Misclassifying a structural defect as cosmetic has led to blade failures and fatalities in this industry. Accuracy is not optional.

How Composite Blades Are Built

Most utility-scale blades use fiberglass-reinforced polymer (FRP) laminates bonded over a foam or balsa core, with carbon fiber reinforcement in the spar cap. The outer skin is a thin gelcoat or erosion-resistant coating. Because the interior is not solid, damage can hide between laminate plies or inside bonded joints without breaking the surface. That is why visual inspection alone is never enough.

Safety Before You Touch Anything

Personal Protective Equipment (PPE): Wear nitrile gloves when handling damaged panels — composite edges are razor-sharp and fiberglass dust is an irritant. Safety glasses are required throughout the lab. When working around the decommissioned blade section, a hard hat is required if there is any overhead exposure risk. If you are working at elevation on an actual turbine, all GWO Working at Heights requirements apply; in this lab setting, keep blade sections secured on sawhorses with chocks.

Borescope access port: The drilled port in the blade model was made by the instructor. Do NOT drill additional access points without authorization. Drilling through a structural laminate at the wrong location can compromise a real blade's integrity.

Housekeeping: Loose carbon fiber or fiberglass debris from damaged panels must be collected and bagged as composite waste — do not blow it into the air or dispose of it in a regular trash bin without checking your facility's waste rules.

The Three Inspection Methods

Visual Inspection

Always start with a thorough visual survey in good lighting. Use a flashlight and hold it at a low angle (rake lighting) to reveal surface waviness, cracks, or erosion pitting that flat lighting hides. Look for paint lifting, moisture staining, gelcoat crazing, surface cracks, and leading-edge erosion. Record every anomaly on your damage-assessment form with its blade station (distance from root in meters or feet) and chord position (leading edge, mid-chord, trailing edge, or spar cap area).

Coin-Tap Test

The coin-tap test — also called acoustic tap testing — detects subsurface delaminations and disbonds. When you tap over a solid, well-bonded laminate the sound is a crisp, high-pitched "tick." When you tap over a delamination or void, the sound drops to a dull "thud" because the separated plies cannot transmit vibration normally.

Procedure in this lab: lay out a grid of marks at 50 mm intervals across the panel using a ruler and fine-tip marker. Work row by row, tapping each grid intersection and immediately noting "S" (solid) or "D" (dull) on your map. A dedicated tap-test hammer gives a more consistent strike force than a coin but both work; coins are used in the field when a proper hammer is unavailable. The boundary of a delamination is where the sound transitions from solid to dull. Sketch that boundary on your grid map.

Borescope Inspection

A video borescope is a flexible probe with a camera tip and a light source. It lets you see inside enclosed spaces — hollow blade trailing-edge cavities, access hatches, or drilled inspection ports — without cutting the structure open. In this lab you will feed the borescope through an existing port, navigate to the disbond location, capture a still image on the tablet display, and record the position using the blade station and chord reference marks already labeled inside the model. Write the position on your damage-assessment form; attach or sketch the image.

The Four-Level Damage Classification System

Manufacturers and major operators use damage categories to drive repair decisions. Although exact category names vary by OEM, the following four-level structure represents standard industry practice:

| Category | Common Name | Description | Typical Decision |

|---|---|---|---|

| 1 | Cosmetic | Surface coating damage only; structural laminate intact | Coat or leave; monitor |

| 2 | Structural Surface | Laminate damage that does not penetrate full thickness; limited delamination | Wet layup or infusion repair |

| 3 | Structural Through | Full-thickness laminate damage, large delamination, or bonded-joint disbond | Engineering-reviewed repair or blade removal |

| 4 | Beyond Field Repair | Extensive structural damage, crushed core, or buckling | Blade replacement |

You will use a decision tree that walks through questions: Is the coating breached? Is the laminate cracked or delaminated? Does damage reach the core? Is the spar cap involved? Each yes/no branch lands on a category number. Practice reading the tree carefully before classifying each photo case.

Recording and Reporting

Every finding must go on your damage-assessment form: date, technician name, blade serial or unit ID, blade station, chord position, inspection method used, sketch or photo reference, and damage category assigned. Incomplete forms are unacceptable in industry — insurance, warranty, and regulatory compliance all depend on a complete written record. Think of the form as a legal document.

Connecting to Your Credentials

GWO Basic Safety Training emphasizes hazard identification and safe work practices — the PPE and fall-arrest awareness in this lab connect directly. NCCER Wind Turbine Tech module content covers blade inspection and composite repair procedures. OSHA 10 gives you the framework for recognizing and reporting safety hazards, including the ergonomic and dust-exposure hazards of composite work.

Key terms10

DelaminationSeparation between adjacent plies (layers) of a composite laminate, often caused by impact or fatigue; not always visible on the surface.
DisbondLoss of adhesion at a bonded joint — for example, where the blade skin bonds to the shear web or spar cap — leaving a void between two previously joined surfaces.
Coin-tap test (acoustic tap test)A non-destructive inspection technique in which a coin or tap-test hammer is struck against a composite surface; a dull sound indicates a subsurface void or delamination.
BorescopeA flexible or rigid optical probe with a built-in light and camera used to inspect enclosed or inaccessible areas without cutting the structure open.
Blade stationA reference measurement along the blade's span, typically measured in feet or meters from the blade root, used to record the spanwise position of a defect.
GelcoatA thin pigmented resin applied as the outermost layer of a fiberglass composite surface; it protects the structural laminate from UV and moisture but carries no structural load.
LaminateMultiple plies of reinforcement fabric (fiberglass, carbon fiber, or both) bonded together with a polymer resin to form a structural composite material.
Damage categoryA numbered or named severity rating assigned to a defect — typically cosmetic, structural surface, structural through, or beyond field repair — that determines the repair action required.
Rake lightingHolding a flashlight at a low, oblique angle to a surface so that shadows reveal surface waviness, cracks, or depressions that straight-on lighting would hide.
Chord positionThe location of a defect across the width of the blade, from leading edge to trailing edge, used together with blade station to give a complete two-dimensional position reference.

Worked examples4

1. You complete a tap-test grid at 50 mm spacing on a 300 mm × 300 mm panel. Your map shows dull responses at the following grid intersections (using column, row notation in mm): (100,100), (100,150), (150,100), (150,150), (200,100), (200,150). The ground-truth delamination outline provided by the instructor is a rectangle with corners at (90,90), (210,90), (210,160), (90,160). Is your mapped delamination boundary within the required 25 mm accuracy?
  1. Step 1 — Identify your mapped boundary. Your dull hits span columns 100–200 mm and rows 100–150 mm, so your mapped boundary rectangle is approximately (100,100) to (200,150).
  2. Step 2 — Compare each edge to the ground-truth boundary. Left edge: your map = 100 mm, ground truth = 90 mm, difference = 10 mm. Right edge: your map = 200 mm, ground truth = 210 mm, difference = 10 mm. Top edge: your map = 100 mm, ground truth = 90 mm, difference = 10 mm. Bottom edge: your map = 150 mm, ground truth = 160 mm, difference = 10 mm.
  3. Step 3 — Check against the 25 mm threshold. All four edge differences (10 mm each) are less than 25 mm.
  4. Step 4 — State the result.
Answer: All mapped boundary edges are within 10 mm of the actual delamination boundary, which is well inside the 25 mm required tolerance. The tap-test map passes.
2. A defect is found at blade station 18.3 m from the root on a 45 m blade. Express the station as a percentage of total blade length and state which half of the blade the defect is in.
  1. Step 1 — Divide the blade station by total blade length: 18.3 ÷ 45 = 0.4067.
  2. Step 2 — Multiply by 100 to convert to a percentage: 0.4067 × 100 = 40.7%.
  3. Step 3 — Compare to 50%. Because 40.7% < 50%, the defect is in the inboard (root) half of the blade.
Answer: The defect is at 40.7% span, located in the inboard half of the blade.
3. Your tap-test map covers a 400 mm × 200 mm panel gridded at 50 mm intervals. How many individual grid intersection points will you need to tap and record?
  1. Step 1 — Count grid lines along the 400 mm dimension. Lines occur at 0, 50, 100, 150, 200, 250, 300, 350, 400 mm = 9 lines.
  2. Step 2 — Count grid lines along the 200 mm dimension. Lines occur at 0, 50, 100, 150, 200 mm = 5 lines.
  3. Step 3 — Multiply to find total intersections: 9 × 5 = 45 intersection points.
Answer: You will tap and record 45 grid intersection points on this panel.
4. During the photo-classification exercise, a student correctly classifies 9 of 10 photo cases. What is the student's percentage score, and does it meet the 80% pass threshold?
  1. Step 1 — Divide correct answers by total cases: 9 ÷ 10 = 0.90.
  2. Step 2 — Multiply by 100: 0.90 × 100 = 90%.
  3. Step 3 — Compare to threshold: 90% > 80%.
Answer: The student scored 90%, which exceeds the 80% threshold. The photo-classification portion is passed.

The math in this lesson2

Learners record tap-test hits as grid coordinates in millimeters to map a damage rectangle.
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Brush up in the math halls at newSchoolWorks.com:
In class: Tap-test grid: students use a coin and a dedicated tap-test hammer on flat panels containing artificially seeded delaminations of known size; they grid-mark the panel at 50 mm intervals, map sound vs. dull areas, and compare their map to the ground-truth delamination outline provided by the instructor. · Borescope inspection: students thread a video borescope through a drilled access port into a hollow blade-section model to locate an internal disbond, photograph it, and record GPS coordinates (blade station, chord position) on a damage-assessment form. · Damage classification exercise: students review ten color photograph sets of blade defects and assign each to a damage category (cosmetic, structural surface, structural through, or beyond field repair) using a manufacturer-style decision tree; instructor reviews answers as a group.
Materials: Pre-damaged fiberglass panel set, decommissioned blade section or training mock, coin and tap-test hammer, video borescope with tablet display, damage-assessment forms, ruler and depth gauge, photograph packet, damage-classification decision-tree reference
Check: Completed damage-assessment forms for three panels: student must correctly classify defect type and severity category on at least 8 of 10 photo cases (80%) AND produce a tap-test map with delamination boundary within 25 mm of actual boundary on the seeded panel.

Practice check8

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

NCCER Wind Turbine Tech — Blade Inspection
1. When performing a coin-tap test over a healthy, well-bonded composite laminate, the expected sound is:
NCCER Wind Turbine Tech — Blade Damage Classification
2. A technician finds a blade defect that has cracked the gelcoat and penetrated partway into the outer laminate plies but has NOT reached the core material. According to a standard four-level damage classification system, this defect is MOST likely classified as:
NCCER Wind Turbine Tech — Blade Inspection
3. Which of the following BEST describes why visual inspection alone is insufficient for composite blade inspection?
NCCER Wind Turbine Tech — Blade Damage Classification
4. A blade defect is found with full-thickness laminate cracking and evidence that the bonded shear-web joint has separated over a significant area. According to a standard four-level classification system, this is MOST likely:
GWO Basic Safety Training — Hazard Identification; OSHA 10 — PPE
5. Before inserting a video borescope into a blade inspection port, which of the following is the MOST important PPE and procedural check a technician should make?
NCCER Wind Turbine Tech — Documentation and Reporting
6. On a damage-assessment form, a technician records a defect at 'blade station 12 m, mid-chord, trailing-edge skin.' What do these two reference dimensions (blade station and chord position) together define?
NCCER Wind Turbine Tech — Blade Inspection Technique
7. During a tap-test exercise, a student's mapped delamination boundary on one edge is 30 mm away from the instructor's ground-truth boundary. According to the lab's pass criteria, this result:
OSHA 10 — Hazard Communication and PPE; GWO Basic Safety Training — Workplace Hazards
8. Why must fiberglass and carbon fiber composite debris from damaged panels be collected and bagged rather than swept into a regular trash bin or blown away with compressed air?
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