Can delamination be detected early?

Question

When a composite structure is hit, the impact causes shockwaves which travel through the structure, and cause delamination. Delamination reduces the strength of the composite structure and may cause the structure to break at a fraction of the load it would have carried before the impact.

Delamination and its ill-effects can be reduced, for instance by adding metal at strategic points of the aircraft. But once delamination occurs, how can it be detected? What measure (if any) have been implemented to detect delamination on aircraft?

Answer 1

There are multiple non-destructive methods to detect delamination in composite materials.

• In case the delamination occurs on the surface, it can be easily found by visual inspection. This is the easiest method to detect delaminations; however, this is useful only if the delaminations are visible from the outside (i.e. they are on the surface), which is rarely the case, at-least in aircraft.

• Ultrasonic testing- In this method, ultrasonic waves are passed through the surface of the material and the response is monitored. Depending on the condition of the material (whether it has defect or not), the ultrasonic waves behave differently.

Source: cnde.iastate.edu

The ultrasonic method used can be through scan (with the transmitter and reciever on the opposite sides of the part) or pulse echo method, where the transmitter/reciever 'listens' to the ultrasonic sound reflected back from the defect or the other end of the part. There are three main methods of ultrasonic scanning in use:

• The A-scan presentation displays the amount of received ultrasonic energy as a function of time.

• The B-scan presentations is a profile (cross-sectional) view of the part. Here, the travel time of the sound energy is displayed along the vertical axis and the linear position of the transducer is displayed along the horizontal axis.

• The C-scan presentation provides a plan-type view of the location and size of defects.

• Tap test, using a coin or an instrumented hammer- This is the most widely used method in the field as it requires minimal instrumentation. However, it requires a trained operator, who can distinguish between the normal and delaminated composite from the sound. Instrumented hammers are also available, where the frequency response of the part can be analyzed for identifying defects.

Source: amumagazine.com

• Thermography- Here, an energy source is used to produce thermographic contrast between tthe defect on the part. This is useful in detecting shallow subsurface defects, but is rarely used in field operations.

Infrared thermography Source: seattleinfrared.com

• X-Ray radiography (or CT scan in some cases)- This is a very accurate method for detecting internal defects; however, the equipment requirements mean that it is usually used in the place of the OEM, though some field equipment is available for military uses.

Fatigue crack under X-Ray, Source: vidisco.com

There are some other methods like the acoustic emission and lasers, which also can be used for detecting delamination. The table below gives some of the methods used for detecting defects in a manufacturing environment

Source: amumagazine.com

Answer 2

The most obvious way is to de-assemble the structure and look inside. This leaves the least amount of doubt.

But there are many ways to do this without destruction.

• Ultrasonic: The echo of the ultrasonic ping comes back early (reflection at an inner layer) and in multiple echoes (due to the many boundaries inside the delaminated area). This is easy to do and needs access only from one side, but the interpretation of the signal needs some training.
• X-ray, but now you need a receiver at the other side.
• Knocking on the surface will help with massive delamination
• Eigenfrequency Testing of resonant structures. If the structure is damaged, its stiffness is reduced and so is its eigenfrequency.

And there are even ways to monitor the structure in flight:

• You can install microphones internally and let them monitor the crackling due to damage. This allows to spot damage quite precisely when the signal is run through suitable computer software.
• Or you put optical fibers into the structure and test whether they still transmit light. This needs quite a lot of fibers and optical terminals, though. When a crack interrupts the transmission, you can time the return of the reflected light pulses and determine the location of the damage quite precisely.

Sorry, @aeroalias, but optical inspection from the outside will only work when damage is extensive. Most damage can not be spotted from the outside with the naked eye.