Composite parts are remarkable for their strength-to-weight ratio, but that same structure can hide damage in ways metals never do. A knock that leaves barely a mark on the surface might have caused significant delamination underneath. Knowing how to assess that damage properly is a core workshop skill, and it often makes the difference between a safe, cost-effective repair and an unnecessary replacement.

Why Composite Damage Is Different

Metals tend to bend, dent or crack in ways you can see and measure. Composites, by contrast, are layered. An impact can cause matrix cracking, fibre breakage, and delamination between plies without any obvious external sign. Sandwich panels add another layer of complexity: the core can crush or absorb moisture, while the skins look perfectly fine. That is why assessment needs to be systematic, not just a quick glance.

Start with a Proper Visual Inspection

Before you reach for any tool, clean the area and light it well. A bright LED torch at a low angle will reveal surface irregularities that a direct light misses. Use a magnifying glass or a loupe for small details. Look for:

• Cracks in the gelcoat, paint or resin — especially star-shaped or radiating cracks.
• Fibre breakout, where individual fibres are broken or protruding.
• Discoloured or cloudy patches that might indicate delamination or moisture ingress.
• Loose, missing or corroded fasteners around the damaged area.
• Previous repairs that may be hiding underlying problems.

Pay close attention to edges, holes, and areas that take the brunt of impacts. A visual check will not find everything, but it tells you where to focus your tapping and moisture testing.

The Tap Test: Your First Line of Defence

The tap test is simple, quick, and surprisingly effective. Use a small coin, a tap hammer, or a dedicated tap tool. Tap lightly in a grid pattern — say, every 25 mm — across the suspect area and a known good reference area. Listen carefully. A clear, ringing tap usually means a well-bonded, solid laminate. A dull thud or a change in pitch suggests delamination, disbonding, or a crushed core.

Mark the boundaries of any dull-sounding areas with masking tape or a chinagraph pencil. Then tap inside those boundaries at closer intervals to map the extent of the damage. For sandwich panels, the tap test can also reveal disbonds between skin and core. But be aware: the tap test is best at finding near-surface damage. Deep delamination may not change the sound enough for your ear to catch.

Soft Spots and Subsurface Damage

A soft spot is an area that feels spongy or less stiff when you press it with a rounded probe or your thumb. Do this gently — you do not want to cause further damage. Compare the suspect area with a mirror-image location on the opposite side of the part, if one exists. Soft spots often point to core crush, severe delamination, or moisture ingress that has weakened the structure.

If you find a soft spot, tap around it and use a moisture meter. Also look for swelling, bulging, or a wavy surface that was not there before. In thin laminates, a soft spot might indicate a disbond between plies. In thick sections, it can mean the damage goes deeper than the tap test suggests.

Using Moisture Meters Wisely

Moisture is the silent enemy of composite structures, especially those with foam or honeycomb cores. A moisture meter — capacitance or resistance type — can help you decide whether water has entered the core or the laminate. Take readings in the suspect area and in a dry reference area nearby. Follow the meter's instructions for the material and thickness you are testing.

Clean the surface first. If possible, avoid measuring through thick paint or coatings, as they can skew readings. Temperature and humidity also affect some meters, so let the tool acclimatise. A significantly higher reading in the suspect area suggests moisture ingress. That matters because moisture can cause freeze-thaw damage, reduce bond strength, and corrode metallic fasteners or inserts. If the core is wet, a simple patch repair may trap that moisture and lead to further damage. In many cases, high moisture content tips the decision towards replacement.

Repair or Replace? Making the Call

Once you have gathered your evidence — visual findings, tap test map, soft spot locations, and moisture readings — you can make a reasoned decision. A repair is usually safe when the damage is small, confined to a non-critical area, and there is no significant moisture in the core. Typical repairable damage might be a shallow impact less than 25 mm across, or a delamination that is well within the limits set by the original equipment manufacturer or a qualified engineer.

Replacement becomes the safer option when delamination extends beyond repairable limits, when the damage sits in a primary load path, when the core is wet, or when a previous repair has already compromised the area. Cost and downtime matter, of course, but a repair that fails in service is far more expensive — and dangerous — than a replacement done right. If you are unsure, consult a composites engineer. Document everything: photographs, tap test results, moisture readings, and your reasoning. That record will help with future inspections and repairs.