Considering Tap Testing - Aviation Non-Destructive Testing (NDT) process

Sofema Online (SOL) considers NDT testing of “Composite Material” using Tap Testing Method.

Introduction

Non-destructive testing (NDT) of composite materials typically involves the use of more than one inspection method, including both non-instrumental and instrumental methods.

Visual inspection and tap testing being “entry level” other commonly used methods include ultrasonic inspection, radiography, as well as advanced methods such as thermography and Laser Shearography  (Laser Shearography is an optical measurement technique for Non-Destructive Testing (NDT) and Quality Control applications, commonly used on composites and metallic materials).

The primary NDT technique used with a Part 145 environment and applicable to Composite Parts is ultrasound, so all parts which are considered critical should receive an ultrasound scan. We use Tap Testing In aviation as a backup Non Destructive Inspection (secondary) method also to be used in non critical applications.

Tap Testing is typically ok for finding dis-bonds in the composite skin to the honeycomb core greater than about 10mm in diameter.

Caution: Tap testing is not particularly accurate and you can be easily misled as to the size of a defect as the lines distinguishing bonded and unbonded areas are blurred by variable pitched sounds.

What is “Tap Testing”

The method involves gently tapping the part with a special tap hammer or even a coin, hence the use of the term coin tap inspection. Tap Testing is highly dependant on the skill and experience of the inspector.

It is simple for an experienced technician to give a part a quick "tap" at any obvious visual damage indications, any suspected damage found is then scanned with the primary ultrasound method to validate.

Note – Key word – Gentle as tapping too hard may actually create damage.

The impact energy travels through the part causing it to resonate or ring in the same way a tuning fork rings at a certain frequency or note. If there is an inconsistent condition such as an dis-bond or major delamination, the audible tone will be different. A typical well bonded area will produce an even pitch sound compared to a disbonded area which usually produces a dead or dull sound.

By listening to the tonal changes, we can achieve an indication that there may be potential damage.

Caution:  Background noise etc can also interfere with any results.

Tap testing is one of the most misunderstood methods in testing of composites and although tap testing is a viable method for the inspection of composite materials, it needs to be followed by another NDT method, such as ultrasonic and/or radiographic inspection to accurately report defect size and depth information.

Tap testing is a good tool for inspectors, but those using this method need to understand it's limitations.

Where Can We Use Tap Testing Effectively?

Tap Testing can be used as an NDT technique for finding flaws in bonded forms and flat panels, for example between the composite or metal skin and the honeycomb core.

Substantive flat areas with thin skins are very suitable for Tap Testing – be aware that as the skin thickness increases or the curvature increases then the results become less reliable.

Note - This technique is highly dependant on the form of the part and the ability of the  “Inspector” to discern the process by which the sound waves (or vibration) travels through the part.

When is Tap Testing Unreliable?

Tap testing is generally not recommended for laminate testing as it is unreliable due to the laminate construction.

For bonded parts when the damage is either below a critical size or the material thickness is above a limit also areas which are “Heavy” in resin as well as voids and porous parts will not be suitable.

Additionally where the Geometry of the structure does not support Tap Testing clearly it will not be suitable finally very light impact damage known as BVID, (barely visible impact damage) will usually not be detectable.

One of the main problems with composite materials, in that internal damage is not always evident on the surface of the material; there may be hidden damage.

Some of the limitations involve the depth and type of materials that you can use it on. Generally it is most satisfactory for testing bonded materials, if you have over 7-8 mm or more of material down to the bond line it is not recommended to use a Tap Test.

Visual Inspection

A trained composite visual inspector typically is looking for resin starvation, edge delamination, fibre break-out, and other types of anomalies present on the exterior of the item inspected.

Visual inspection can be supported by 10X magnification as well as the use of borescope’s.

A good visual inspection may identify variables that will hinder the NDT inspection and can identify obvious defects accessible to the surface.

Visual inspection can detect resin starvation which appears as porosity on the surface of the laminate.

Note - Porosity would have to be further evaluated using ultrasonic inspection to give accurate sub-surface information and to evaluate per-sound attenuation criteria.

Further Guidance

Sofema Aviation Services (SAS) (www.sassofia.com) and Sofema Online (SOL) www.sofemaonline.com provide training for aircraft inspectors delivered as classroom, webinar & online. For Details please email office@sassofia.com or online@sassofia.com