What is Non-Destructive Testing (NDT)?

Non-destructive testing (NDT) is any test method that does not remove or destroy the material tested. The FAA defines it for pavement as a test that “does not involve removal or destruction of pavement material”.

NDT stands for non-destructive testing. FAA aircraft maintenance guidance calls the same group of methods nondestructive inspection (NDI). The methods differ by material. This page covers pavement and concrete first, then metal parts.

Why the FAA uses it on pavements

The FAA names three advantages. NDT tests the pavement in place, under its real moisture and density. It disrupts traffic less. It reduces the need for destructive tests.

NDT methods for pavement and concrete

• FWD and HWD. These are the most common NDT tools for pavement. They record how the pavement responds to a controlled dynamic load. Engineers use the data for these jobs:
  • evaluate load-carrying capacity and remaining life
  • calculate load transfer at cracks and joints
  • detect voids at the corners and joints of rigid pavement
  • find the properties of the in-place layers and the subgrade, for overlay design
• Ground penetrating radar (GPR). It sends radar pulses into the pavement and records the reflections from layer boundaries. It relies on differences in dielectric constant between materials. Common uses are layer thickness, excess water, utilities or rebar in concrete, delamination between layers, and voids.
• Impact-echo. It finds concrete delamination, voids and honeycombing from the stress waves of a short mechanical impact. FHWA describes it as a method to detect delamination and debonding under overlays.
• Ultrasonic surface waves. FHWA describes them as a way to assess the quality and modulus of elasticity of concrete.
• Electrical resistivity. FHWA states that the resistivity of concrete is inversely related to its potential for corrosion.
• Infrared thermography. It observes differences in infrared emission to find certain physical properties of the pavement.
• Rebar location devices. On new concrete pavement, AC 150/5370-10H accepts these in place of cores, if the engineer approves them, to check dowel and tie bar placement.

The FAA classifies the dynamic cone penetrometer (DCP) as a minimally destructive test.

FHWA ranked GPR, impact echo and ultrasonic surface waves as the top technologies for concrete bridge decks. That ranking is for decks, not for airfield pavement.

Which methods need access to one side only

A concrete slab on the ground has one accessible face. These methods work from that face:

• GPR. One antenna sends the pulse into the pavement and a second antenna records the reflections. A vehicle or a cart carries the antennas. Air-coupled and ground-coupled antennas are both in use.
• Impact-echo. The impact and the receiving transducer sit on the same surface. The transducer records the surface motion after the impact.

GPR has limits. High frequency waves give resolution at shallow depth. Low frequency waves reach deeper with less resolution. GPR works well in coarse-grained soils, ice and frozen ground. It is less effective in fine-grained soils such as silt or clay.

How impact-echo measures slab thickness

ASTM C 1383 uses impact-echo to measure the thickness of plate-like concrete members. A plate has lateral dimensions of at least six times its thickness. The method has two procedures. The first measures the P-wave speed of the concrete with two transducers a known distance apart. The second finds the thickness frequency, and the thickness follows from the measured wave speed.

NIST reports that limited comparisons with drilled cores agreed within 3 % of the core length. An FHWA study on interstate pavement found a different result. The impact-echo thickness was about 6 to 16 mm below the rod-and-level reference and did not meet the 5 mm target. The study used an assumed wave speed that it calibrated on cores. The FHWA report notes that a 10 % error in wave speed gives about a 10 % error in thickness.

NDT methods for metal

AC 43.13-1B lists visual, eddy current, magnetic particle, penetrant, radiography and ultrasonic methods for aircraft parts. Each one has limits.

Radiography and ultrasonic testing are the two methods that find flaws completely internal and well below the surface. Radiography is more effective for flaws that are not planar. Ultrasonic testing is more effective for planar flaws.

Ultrasonic testing has two techniques. Pulse-echo uses one transducer to send and receive the pulse. Through-transmission uses two transducers, and a defect in the sound path between them interrupts the signal. For thickness, pulse-echo is the most common method. The FAA lists “checking part thickness when access to the back side is not available” as one use.

Who does the testing

AC 43.13-1B states that the success of any NDI method depends on the knowledge, skill and experience of the people involved. People who read indications, such as eddy current, X-ray or ultrasonic, must hold a qualification and a certificate. The FAA or another acceptable standard sets the requirements. For a given part, the aircraft manufacturer or the FAA usually specifies the method.

NDT and a visual survey

The FAA texts treat visual inspection and NDT as separate sources of data. A drone photo survey records the visible surface of a pavement. It does not measure what is below the surface, and it does not replace FWD or GPR. The visual survey and the NDT data answer different questions.

Records

AC 150/5380-6C advises airports to consider NDT data in addition to visual inspections and construction records. AC 150/5380-7B allows the NDT data to be stored in the pavement management database. This lets an airport compare NDT results with the condition history of the same pavement section.