Testing times for structural integrity - GE Measurement & Control

ENGINEERING & MAINTENANCE 

Testing times for structural 

integrity: NDT to the rescue 

Non-destructive testing is a technique employed in a number of different industries to verify structural 

integrity, but what requirements are specific to aviation, and what equipment and expertise are used 

to satisfy them? Joanne Perry went in search of the answers. 

84 ❙ Aircraft Technology - Issue 113 ❙ 

In April this year, Southwest Airlines flight 812 

from Phoenix to Sacramento in the US came 

close to disaster when the skin of the 737- 

300 tore open at 34,000ft. The aircraft landed 

safely at Yuma International Airport, Arizona, 

with only two minor injuries sustained, but the 

incident sparked a major investigation which 

caused the cancellation of 300 Southwest 

flights. Seventy-nine Southwest 737 aircraft 

were subsequently checked for the lap-joint 

fatigue cracking which caused the depressurisation 

incident. 

The method stipulated by Boeing’s service 

bulletin (SB), enforced by a Federal Aviation 

Administration (FAA) airworthiness directive 

(AD), was high-frequency eddy current testing. 

This classic non-destructive technique proved 

its usefulness by detecting cracks in five other 

aircraft, which were then removed from service 

for repairs. 

Non-destructive testing (NDT) can be 

defined as the assessment of material integrity 

without compromising future use, for example 

by taking samples for analysis. It is a collection 

of processes used across a number of different 

industries, such as power generation and 

construction as well as transportation. The 

simplest form is a visual inspection, aided 

by remote visual inspection (RVI) equipment 

such as borescopes for areas that would be


Top and above: Technicians perform 

thermographic NDT. 


inaccessible without disassembly. However, 

this method is only useful for superficial problems 

and is heavily dependent upon the skill 

and dedication of the technician. 

The American Society for Nondestructive 

Testing (ASNT) lists six basic methodological 

categories: mechanical and optical; penetrating 

radiation; electromagnetic and electronic; sonic 

and ultrasonic; thermal and infrared; and chemical 

and analytical testing. The ASNT supplements 

these with image generation and signal 

image analysis. Varieties of NDT too numerous 

to mention branch out from each main type. 

In aviation, NDT is used not only during 

post-incident investigations as in the case of 

Southwest, but during component manufacture, 

to preclude flaws, and in the maintenance 

and repair of both airframes and engines to 

detect not only cracks but disbonding, corrosion, 

scratches and other problems or damage. 

Steven Shepard, president of Thermal Wave 

Imaging (TWI), based in Michigan, US, explains 

some of the requirements of aerospace NDT: 

“Speed and economy are essential for NDT 

equipment in almost any industry in today’s 

economy. However, aviation requires a higher 

degree of accuracy and reliability than most”. 

The Southwest incident provides a timely example 

of the important role NDT has to play in verifying 

the airworthiness of aircraft and the 

safety of passengers. Mike Fortman, president 

of NDT service provider Aerotechnics in 

Minnesota, US, says that “nearly all aircraft 

components require some sort of special attention 

or detailed inspection”. With a constant 

drive to reduce aircraft weight because of its 

impact on fuel burn, parts are called upon to 

perform their respective functions at high load 

relative to material strength. They must furthermore 

withstand the stress created by repeated 

loading and unloading of the aircraft, temperature 

and pressure changes, and other atmospheric 

conditions such as lightning strikes. 

The need for NDT does, however, vary 

across the aircraft. As Mark Ginn, chief inspector 

at Delta TechOps, explains, many components 

“require special attention when receiving 

NDT as a result of their criticality to safe flight”, 

whether they are located in the engines, landing 

gear or airframe. Albrecht Maurer, senior

product manager testing systems, at GE 

Sensing & Inspection Technologies agrees that 

the difference in safety requirements is the 

main factor in determining the intensity of the 

NDT which is conducted. “Primary structures 

receive more NDT attention than secondary or 

tertiary structures,” he states. 

Shepard adds that, in an additional complication 

for the aerospace industry, many aviation 

inspections “involve large areas in which the 

component construction may vary considerably”. 

A central aspect of this variation stems from the 

material used in manufacture, but further considerations 

are the nature of the suspected flaw 

and the conditions of inspection, says Philippe 

Boiteux, COO managing director of NDT Expert, 

an NDT solutions provider based in France. The 

latter include the expertise of the company and 

technicians concerned, and whether it is a manufacturing 

or maintenance operation performing 

the NDT. Maurer points out that the choice of 

NDT technique is also governed by manufacturer 

and certifier approvals. 

MRO provider Delta TechOps maintains a 

broad portfolio of NDT capabilities in order to 

balance out the strengths and weaknesses of 

the different methods: eddy current; magnetic 

FACT: Max rotational 

speed 2,550 rpm 

particle; fluorescent penetrant; ultrasonic; radiographic; 

and infrared testing. As an example, 

Ginn explains that the magnetic particle 

method, which involves the dusting of a magnetised 

surface with iron particles to highlight 

anomalies, cannot be applied to non-ferrous 

materials. In addition, although magnetic particle 

NDT can be used to identify subsurface 

defects, its effectiveness decreases with depth. 

It also requires the removal of paint from the 

test surface. Meanwhile, eddy current testing, 

which involves the generation of electrical currents 

by a changing magnetic field and the noting 

of any flow disruption, cannot be performed 

on non-metallic materials such as composites. 

NDT for composites 

“Infrared (thermal imaging), ultrasonic testing 

(resonance, low frequency pitch-catch, pulse 

echo, through transmission), and radiography (digital 

or film x-ray) need to be used to find flaws in 

composite materials,” according to Fortman. GE 

Sensing & Inspection’s Maurer believes that for 

such applications ultrasonic testing (UT) “is the 

most reliable method with least technical restriction, 

i.e. minimum/maximum thickness and complexity 

of structure”. 


Many aviation inspections 

involve large areas in which 

the component construction 

may vary considerably. 

— Steven Shepard, president, 

TWI 

AIRCRAFT TECHNOLOGY’S 

annual publication for the 

aero-engine professional 

JUST FACT, NO SPIN 

Engine Yearbook 2012 Deadlines: 

Ad Booking: October 10th 2011 Ad Copy: October 17th 2011 Publishing: November 7th 2011 

If you want to promote in the 2012 edition of The Engine Yearbook, or if you would like any other information, please 

contact any of the Sales Team: Simon Barker, Alan Samuel or Anthony Smith on Tel: +44 (0) 207 579 4845/ -46/ -75 or via 

Email: simon.barker@ubmaviation.com or alan.samuel@ubmaviation.com or anthony.smith@ubmaviation.com 

❙ Aircraft Technology - Issue 113 ❙ 

87


Top: Eddy current NDT can only be applied to 

metallic materials. 

Inset: “Bondtracer” from GE Sensing & 

Inspection Technologies. 


In UT, both geometric surfaces and internal 

integrity can be analysed by the transmission 

of high-frequency sound waves into the test 

material. Resonance testing in its most rudimentary 

form consists of a “tap” test, which is 

what the name suggests, but is computerised 

at the highest levels. In through transmission, 

a transmitter is positioned on one side of the 

test material and a receiver on the other, while 

pulse echo is a single-sided technique for less 

accessible areas, and in the pitch-catch 

method the transmission occurs at an angle 

and is useful for the testing of non-linear 

objects. Linear array UT involves a single 

source of transmission, while a more complex 

and more commonly used version with multiple 

pulsing elements — phased array — creates a 

kind of steerable “searchlight” for inspection. 

Newly launched NDT products in the UT category 

include the “Bondtracer” unveiled by GE in 

May this year in collaboration with Boeing. This 

is a portable composites inspection tool which 

is designed to enable mechanics to assess 

minor impact damage at an airport gate. The 

quick feedback produced by “Bondtracer” determines 

whether an aircraft is fit to fly or requires 

further investigation and repair. 

In radiographic testing (RT), gamma or X-rays 

— which one depends upon the thickness of 

the material in question — are directed through 

the test object onto a film, which produces a 

shadowgraph depicting internal features. As in 

the well-known medical application, variations in 

density are represented by lighter and darker 

areas. Radiography has the advantage of removing 

disassembly requirements, but brings with it 

the disadvantage of safety concerns. Owing to 

the hazardous nature of X-rays and radioactive 

isotopes such as iridium 192, which produce 

gamma rays, extra precautions must be taken 

when using this technique, such as protective 

equipment and warning systems. 

Shepard asserts that thermography is also 

a good option for composites, because “the 

cost of a large-scale thermography system is 

significantly less than the alternatives”. As a 

result, “many companies have replaced C-scan


Top: NDT is performed on an engine. 

Above: An NDT employee analyses results by 

computer. 


[UT] systems with thermography”. In thermography, 

the test material is heated so that the 

temperature decay can be observed over time, 

with structural anomalies disrupting normal 

cooling. However, Shepard notes that the 

unprocessed images from the standard 

infrared camera used to detect the thermal patterns 

“are not sufficient for many aviation NDT 

requirements”. 

TWI therefore uses a thermographic signal 

reconstruction (TSR) processing method which 

analyses the time evolution of each pixel, 

enabling not only the identification of anomalies 

but quantitative measurement of thickness, thermal 

diffusivity or porosity. He says this effectively 

allows a user to “drill down” into the test item. 

“This combination of fast area coverage and the 

ability to ‘self-validate’ image results using time 

response is a unique advantage of thermography,” 

he states. Other advantages include a fair 

tolerance of non-planar geometries, surface 

characteristics and ambient conditions. “The 

most fundamental limitation is the one imposed 

by the physics of diffusion, which requires that 

the diameter of a subsurface feature is larger 

than its depth,” states Shepard. 

Maurer says that, in summary, the NDT 

methods which can be used for composites are 

“ideal to detect lack of bonding (delamination) 

as well as porosity over the whole area of each 

component without impacting its properties”. 

This is because composites “rely strongly on 

the perfect bonding between individual layers 

and on absence of pores which may develop 

during the hot curing process”. 

Flaw characteristics and 

inspection conditions 

The properties of material flaws which 

impact on NDT include size, depth and accessibility, 

says Fortman. He gives the example of 

rotating engine parts such as turbine blades 

and discs, which may contain very small 

defects that require the use of special penetrants 

and/or automated scanning. In penetrant 

testing (PT), a simple NDT method based 

on the capillary action of liquids, a solution of

visible or fluorescent dye is applied to the test 

object, before the excess solution is removed 

to highlight any breaks in the surface. A developer 

is used to draw the penetrant out of the 

defects. Visible dyes rely on colour contrast 

between the penetrant and the developer, 

while fluorescent dyes are activated by ultraviolet 

light. However, as Ginn points out, a key 

drawback is that penetrants can only detect 

superficial discontinuities. The chemicals may 

also damage composite materials. Care must 

be taken in any case to properly clean off the 

penetrant, or risk misleading results. This 

method, like magnetic particle testing, also 

necessitates the removal of paint from the 

test material. 

As regards the differences between NDT for 

manufacturing and maintenance applications, 

Maurer explains: “Parts with MRO-testing 

requirements demand instant image output 

rather than time consuming scanning 

processes, e.g. impact analysis through 

shearography [optical NDT], remote visual 

inspection and in special cases UT testing with 

portable scanners.” TWI, for example, provides 

large-scale thermographic systems for manufacturing 

contexts and portable/ handheld 

products for in-service use. There is some 

crossover, however. GE’s Phasor XS and DM 

phased array UT products can be used for volumetric 

inspection during both manufacture 

and maintenance, as can the company’s range 

of RVI equipment. 

In light of the pros and cons of different 

NDT methods, it can be necessary to use multiple 

techniques during inspection. For example, 

the emerging NDT technique of process 

compensated resonance testing (PCRT) offers 

a high degree of objectivity through the compilation 

of statistical data — but relies on a 

known sample set to establish basic parameters. 

In this method, samples of defective and 

defect-free parts identified by destructive 

analysis or another NDT technique are used to 

build a customised software algorithm, based 

on the contrast between a series of natural frequencies 

or resonant responses from the two 

groups. As Greg Weaver, director of operations 

at Vibrant Corporation in New Mexico, US, 

explains: “The software is defining not only the 

absolute response differences, but more 

importantly the relationship difference across 

multiple responses.” The system can be 

“taught” to recognise acceptable and unacceptable 

differences between components of 

the same type, compensating for the unintentional 

variation generated by even the most 

modern manufacturing processes. 

Like other NDT types, PCRT possesses a 

mixture of good and bad points. On the negative 

side, although PCRT can detect a decline in 

structural integrity, it does not specify the defect 

type or location as would magnetic particle testing 

and phased array UT. However, Weaver says 

the technique does challenge FP and X-ray NDT, 

and at a comparable cost. One of the key advantages, 

he explains, is that PCRT can detect 

more than one defect type, internally and externally, 

in a single inspection. Additionally, it is 

“the only NDT method that can detect metallurgical 

issues such as alloy overtemp and intergranular 

attack”. The inspection time is also 

impressive — between four and six seconds for 

the resonance test itself. Importantly, PCRT can 

be applied to both metallic and non-metallic 

parts including composites, during either manufacture 

or maintenance processes. “A PCRT 

test at the front of an MRO receiving process 

could save companies an enormous amount of 

time and money,” states Weaver. “The same 

goes for blade manufacturing, where PCRT can 

not only be used as an inspection tool, but also 

as a process control measurement.” 

In aviation, PCRT is mainly used for turbine 

components, including solid and hollow turbine 

blades, silicon nitride bearing elements and forgings/castings. 

Weaver believes that PCRT 

“should become a dominant inspection in the 


GE’s “Phasor XS” phased array NDT solution. 

❙ Aircraft Technology - Issue 113 ❙ 

91


PCRT systems can recognise acceptable and unacceptable differences between components of the same type. 

Parts such as bolts may be tested by NDT. 


turbine blade world” and that it might be 

included in most OEM standard practice manuals 

within five years, bearing in mind the difficulty 

of making predictions. Delta TechOps received 

FAA approval for PCRT in September 2010, and 

Ginn views the technique as “an important capability 

in years to come and an important part of 

the Delta TechOps NDT portfolio”. 

The latest on NDT 

“Even conventional techniques are permanently 

moving on,” Boiteux observes. One of 

the biggest sources of change in the NDT business 

is the trend in airframe manufacturing 

toward greater proportions of composite materials. 

Jeff Stetson, senior product manager, 

ultrasonics, at GE, says that on the UT side, 

“composite airframes are driving some 

changes in equipment”. According to Boiteux, 

the increase over the past decade has led 

to an expansion of UT, thermographic and 

shearographic capabilities. As the composite 

level rises, “NDT methods such as ultrasonics, 

radiography, and infrared inspections become 

more valid,” notes Fortman. “Established methods 

such as penetrant testing, magnetic particle 

testing, and eddy current are nearly 

obsolete or unusable on composite materials”. 

At Delta TechOps, Ginn has witnessed the 

growing importance of UT, “with ultrasound 

being the method of choice for many composites”, 

but also a similar trend in eddy 

current testing. He adds that eddy current 

NDT, UT, RT and infrared testing have all “benefited 

from technological advancements over 

the last few years”. 

Referring to NDT as a whole rather than 

composites-focused NDT, Ginn says the use of 

mature methods such as magnetic particle 

testing and FP “has remained fairly constant”. 

Stetson adds that for engine inspections one 

of the most noteworthy technological advancements 

has been the “huge transition 

from film to digital RT”, for which GE has 

developed products such as the DXR 250P, a 

digital and portable RT solution for on-wing 

inspections. 

There is clearly a wealth of NDT equipment 

and expertise on the market. If the near-disaster 

of flight 812 is anything to go by, it is more 

a question of ensuring that these services are 

called upon as appropriate. Although eddy current 

testing detected cracks in a number of 

Southwest aircraft during the fleet-wide investigation, 

prior to the incident only visual inspections 

were required for the 737 Classic 

serving the flight. Nor was this the first such 

incident involving a Southwest aircraft; something 

of a track record had even led to a $7.5m 

settlement in 2009 for missed fuselage 

inspections. It seems safe to say that a little 

more non-destructive testing on the ground 

could prevent a whole lot more destruction in 

the air. ■

Testing times for structural integrity - GE Measurement & Control

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?