This paper shows how two different penetrant
methods used in conjunction can provide a more complete inspection
coverage and minimized risk of failure. A process is described
where krypton gas penetrant imaging complements traditional
liquid penetrant to achieve higher detection reliability of
anomalies in aircraft engine components than either method can
Associate Contributing Editor
This year marks the 27th anniversary of the first successful use
of krypton gas penetrant imaging for the detection of cracks, casting
porosity, hot tears, cold shuts, dross, etc., in aircraft engine components.
In the late 1960s the US Air Force first applied this method to the
detection of porosity in sleeve bearings used in the nose cones of T-34
turboshaft gas turbine engines (Figure
These bearings, made of Babbit metal, were failing
unexpectedly. Millions of dollars were being spent on unscheduled maintenance
and repairs to subsystems. The Air Force felt that cavitation erosion
was the cause of these unexpected failures. However, the krypton method
identified the porosity as the cause of the fatigue failures and not
cavitation erosion. Subsequent corrections to the casting procedures
eliminated the porosity and the failures.
This original application of the krypton evaluation
technique, or simply KET as it's called today, involved the use of hand
wrapped photographic film. Since then numerous improvements have been
made to the process, most notably the replacement of the photographic
film with a sprayable emulsion. KET is now a robust, low cost production
process much like liquid penetrant inspection and is depicted in Figure
A Closer Look at the KET Process
KET is a batch process similar in throughput, procedure, and cost to
the familiar liquid penetrant method. The krypton gas used in the early
experiments is still the working fluid in today's process. It consists
of 95 percent inert krypton and 5 percent krypton 85.
Millions of dollars were being spent on unscheduled maintenance
and repairs to subsystems.
Initially the part is cleaned and placed in a chamber
where it is outgassed to remove the layer of air molecules adsorbed
along the surfaces of both the cracks and the outer surface. The evacuated
air in the vessel is replaced with the mixture of krypton gases in which
only the krypton 85 atoms are active.
The part is left in the krypton until all of the
exposed surfaces contain adsorbed krypton atoms. Following exposure,
the krypton gas is pumped out of the vessel and air is readmitted. During
this step, the krypton atoms along the free surfaces desorb more readily
than those trapped in the cracks. This results in relatively little
surface activity but high concentrations of activity at crack sites.
The average amount of gas retained in a crack is substantially greater
than that retained along outer surfaces. Krypton 85 atoms, because of
their extremely small diameter, diffuse into very tight, oxide filled
cracks or casting hot tears.
Imaging the concentrations of krypton gas trapped
in these microcracks is made possible with a liquid emulsion (sometimes
referred to as a dispersion). The dispersions used for the KET process
are sensitive film emulsions developed by Eastman Kodak Co., Rochester,
New York. They are liquid gelatin, a water soluble material, containing
silver halide particles. Dispersion coatings 250-500 µm (1-2 milli-in.)
thick are applied under darkroom conditions.
After the dispersion is dry, the part is allowed
to sit for an appropriate time while the krypton 85 gas in the cracks
exposes the dispersion. Finally, the exposed part is developed by normal
photographic techniques as if it were a sheet of exposed film.
The dispersion allows the inspector to readily identify
the microcrack. Indications appear black against a white background
and may be read under white light conditions. KET images are in exact
registry on the part where the cracks reside and will remain distinct
for some time.
The KET process can be adjusted to meet individual
customer requirements. Film exposure time for casting porosity, hot
tears, cold shuts, dross, etc., and low cycle fatigue cracks will vary.
Every effort is made to produce an optimum signal for each application
whether film, dispersion, or electronic sensors are used.
A unique feature of the KET process is its built-in
magnification factor. Beta particles emitted by the krypton 85 atoms
radiate laterally and can penetrate 500-1,250 µm (2-5 milli-in.)
of material. This allows one to see microcracks not normally visible
with the naked eye. This enlargement factor varies from 1,250 to 2,500
µm (5 to 10 milli-in.) so that a 25 µm (0.1 milli-in.) crack
opening would show up as a 1,250 to 2,500 µm (5 or 10 milli-in.)
KET indication. Frequently, tight microcracks or hot tears with a 25
µm (0.1 milli-in.) surface opening will behave in this manner.
The enhanced visibility of microcracks through KET increases the likelihood
that a technician will spot potentially life limiting microcracks in
a part before unnecessary investments are made in machining a part of
questionable quality or risking the shipment of such a part to a customer.
While KET is able to detect cracks not visible to
the unaided eye, it often misses visible cracks which have large crack
openings. Krypton atoms desorb rapidly from wide open cracks when the
krypton treated part is returned to the atmosphere. Frequently, the
activity emitted by a large crack is equivalent to that coming from
a good surface, and the crack signal blends in with the noise. Subsequent
KET inspection often will not show these large cracks, which are easily
detected by fluorescent penetrant inspection. For this reason, it is
advisable to use KET after parts have been liquid penetrant inspected.
Parts failing liquid penetrant need not be KET inspected; those that
areacceptable after liquid penetrant inspection should be KET inspected.
Comparison of Penetrant Systems
There are basically two kinds of penetrant systems: liquid penetrants
and gas penetrants. The liquid fluorescent penetrant inspection method
has become the industry standard since it was first discovered in 1942
by Joe and Bob Switzer (Flaherty, 1986). In many ways that original
process is very similar to the process we use today. Early applications
were in the maintenance and overhaul of aircraft engines. Over the intervening
years aircraft engines have changed considerably and the need to find
smaller cracks has increased immensely. KET answers this need. The krypton
gas penetrant system was never meant to compete with or replace the
current industry standard. KET effectively identifies the smaller cracks
that fluorescent penetrant inspection has difficulty imaging, hence
it is complementary. This paper describes how both methods used together
are much more effective than either method used alone.
3 depicts the penetrant problem. Liquid
molecules are much larger than krypton gas atoms. Frequently liquid
molecules are only absorbed by small portions of a casting porosity
as in this case, at the larger opening in the center of this field of
clustered microshrinkage (arrow). When only some of the liquid penetrant
is absorbed, as shown here, the inspector has no idea as to the size
of the microshrinkage cluster. When he dabs this area with alcohol and
the indication grows fainter, he's led to believe that it's an innocuous
Liquid penetrants are generally useful above 3 mm
(0.125 in.) crack length but their probability of detection falls off
rapidly below that size. If the crack is compressively stressed, as
is usually the case for gas turbine compressor and turbine disks, then
chances for detection are reduced further. And, of course, foreign debris
and oxide products are usually contained in cracks, thus making it still
more difficult. Liquid penetrants offer a four-sigma level of quality
over all size ranges.
To achieve a six-sigma level of quality or detection
reliability, one must obviously find smaller cracks with the same level
of proficiency that the larger cracks are found. Original equipment
manufacturers (OEM) use multiple inspections, acid etching, thermal
cycling, and even engine running of turbine blades to increase the probability
of detecting life limiting casting porosity, hot tears, cold shuts,
dross, etc., in new parts, but the results have been mixed. New environmental
regulations on cleaners pose new challenges for liquid penetrants (Rummel,
A far simpler and less costly approach to this problem
is krypton gas penetrant imaging of parts that pass the liquid penetrant
inspection. If in fact liquid was not absorbed by an existing crack
or porosity, hot tear, cold shut, dross, etc., then chances are excellent
that a gas would be adsorbed.
Our research has shown that compressively stressed
cracks or oxide filled hot tears actually getter the krypton gas. Signal
to noise ratios are enhanced several times by this effect.
This oxide gettering effect of krypton could also
be applied to airfoil repairs where the tail of a crack is densely packed
with oxide product and not detected by liquid penetrant inspection.
Weld or braze repairing an airfoil with residual oxide results in internal
voids and cracks. Expected life and repair cost savings are consequently
If one includes KET in the quality assurance strategy,
there are new options and possibilities. For example, acid etching and
thermal cycling prior to liquid penetrant are not necessary when KET
is part of the quality assurance strategy. Using KET for small crack
detection reduces the need for multiple penetrant inspections and could
very possibly allow the use of water soluble penetrants and environmentally
Process Demonstration and Validation
As one approaches the lower limit of sensitivity for any given inspection
method, the probability of detection decreases rapidly. The lower limit
of sensitivity for gas penetrant is much lower than that for liquid
penetrants. For this reason KET has a very high probability of detection
over the range of crack sizes liquid penetrant must detect at its lower
limit of sensitivity. This situation is shown graphically in Figure
Some case histories will now be discussed briefly
to show why KET should be used as a complement to fluorescent penetrant
Case 1. A large
quantity of retired B1900 turbine blades from an overhaul facility were
obtained and characterized by KET according to percent microshrinkage
prior to inspections by two major original equipment manufacturers (OEMs)
(Table 1). Ten blades representing
the best to worst cases were then selected for study.
It should be mentioned here that OEM and field reports
indicated that a high percentage of these turbine blades were cracking
and failing unexpectedly. In all cases microshrinkage was present at
the site of stress rupture cracking. The engine manufacturer had placed
low life limits on these blades after one failure occurred at 80 h.
Two large OEMs were contracted to inspect these
ten blades with their production fluorescent particle inspection lines.
Using their own accept-reject criteria, they were asked to pass judgment
on these blades. OEMs allow between 1-2 percent microshrinkage as does
most of the aerospace industry. KET rejected eight of the ten blades
based on this rejection criteria. Following the OEM's fluorescent penetrant
inspection, all ten blades were accepted. Subsequent metallurgy by Battelle
confirmed that the eight blades which KET rejected, but the OEMs accepted,
contained microshrinkage exceeding the OEM's own fluorescent penetrant
inspection standards. Blade #247 is shown in Figure
Three important conclusions were drawn from this
1. Repetitive fluorescent penetrant inspection does not ensure the detection
of microshrinkage. Prior to service these blades were inspected once
at the foundry and two times during manufacture by the OEM. During this
study they were inspected by two different OEMs. In all, they were inspected
five times by four different fluorescent particle inspection lines.
Failure to detect was not operator dependent but method dependent.
2. The KET inspection of fluorescent particle inspection
approved blades is a more accurate quality assurance strategy than the
standard practice of repetitive fluorescent particle inspection.
3. To ensure that investment castings are hippable
(i.e., can be hot isostatically pressed) they should be KET inspected
prior to hipping. Such inspections would offer assurance that the microshrinkage
is not surface connected and that hot tears, cold shuts, and other non-hippable
casting discontinuities are not present. The industry standard today
is fluorescent particle inspection prior to hipping.
Case 2. The Radian
Corporation in The Conduit (1995) reports how casting porosity
produced invisible leaks of a highly reactive gas from pressurized canisters
(Ellis, 1995). The leaks were in the stub portion of special gage assemblies
made of cast 316L stainless steel (Figure
6). Gas leaks became visible a few days after installation as the
escaping gas reacted with the atmosphere and caused discolorations.
In this case 20 percent of the cast 316L gage assemblies
leaked in the cast stub portion but had passed liquid penetrant inspection.
Under the scanning electron microscope (SEM) no visible surface perforations
were observed. Only after metallurgical analysis was the band of microshrinkage
responsible for the leaks visible. This example demonstrates the effectiveness
of gas as a penetrant. More importantly it points out the need to KET
inspect parts at the foundry after fluorescent particle inspection to
ensure that the investment casting process is optimized before parts
reach the customer.
Case 3. In 1993
the US Navy's Aviation Supply Office (ASO), now called the Naval Inventory
Control Point (NICP), completed a 6,000 blade study comparing fluorescent
particle inspection with KET (Mahorter, 1993). However, in this case
the Mar M-246 turbine blades used in this study received a two hour
cyclic engine test prior to fluorescent particle inspection (i.e. type
ZL22). This OEM green run requirement for hot tear detection was expensive
and resulted in the prime being the only source who could comply, thus
restricting acquisition to the prime. The ASO's goal was to qualify
KET as an alternative to the green run plus fluorescent particle inspection
The results of this study demonstrated the superiority
of gas penetrant over liquid penetrant in the identification of hot
- KET identified 12 blades with hot tears, none
of which were found by the foundry fluorescent particle inspection
nor the green run plus fluorescent particle inspection. A typical
blade is shown in Figure
- The KET hot tear blades were then subjected to
a high sensitivity fluorescent particle inspection (i.e. Magnaflux
ZL30) in a Navy Research Laboratory. Fluorescent particle inspection
could only find hot tears in four of these blades.
Navy researchers concluded that engine running turbine
blades first and then doing fluorescent particle inspection was ineffective
in detecting hot tears up to 2.5 mm (0.1 in.) long. KET is now used
by the Navy in place of the engine green run plus fluorescent particle
inspection. Savings to date are in the millions of dollars and no blade
failures have been reported.
Applying KET to Practical Problems
During the past several years the demand for industrial gas turbines
has grown rapidly worldwide. This growth has been fueled to a large
extent by the OEM's use of aero technology in the design, development,
and manufacture of industrial engines.
In one case, an industrial engine from an aero derivative
design having a 35,000 h design life began experiencing unexpected second
stage turbine blade failures. These failures occurred as low as 800
h in this unshrouded IN792 Hf blade design. Investigators identified
hot tears at the trailing edge above the blade platform as the cause.
The manufacturer investigated various inspection methods to find the
anomalous parts but had little success. Most methods were unreliable,
with some costly and impractical in a production environment.
After learning about KET through Battelle, the gas
turbine manufacturer conducted a lengthy evaluation program. Figure
8 shows one of the blades from this study. The tightness of the
hot tear makes viewing even at 25x impossible and precludes entry of
liquid penetrant. Krypton gas penetrant however, had no problem visualizing
these invisible cracks. For this application KET was implemented through
a process specification.
Risk Reduction With KET
Whenever a new engine design is taken through the manufacturing and
development cycle, there are many unknowns or risks. This process is
far from an exact science. New materials, cooling configurations, and
manufacturing processes compound these problems. When mistakes become
evident long after this cycle has been completed and production is in
full swing, corrections can and do become costly for both the OEM and
the customers. The costs associated with these risks can be avoided
when more is known rather than unknown. KET can serve such a purpose
when it becomes part of the manufacturing, development, and production
We have shown in this paper that there is a small
risk of parts cracking or failing prematurely when KET is not used.
Oldfield and Oldfield (1993) have shown a direct correlation between
casting anomalies and the incidence of cracking and failure of turbine
blades. The Federal Aviation Administration (Operational Systems Branch,
Oklahoma City, OK) has records of many such events. They are reported
under Service Difficulty Data and Accident/Incident Data. The Navy,
Air Force, and Army overhaul depots report similar events in their engineering
investigations of engine failures.
9 summarizes what we have discovered with
KET over a fourteen year period about turbine blades and life limiting
casting anomalies. This database included 15 different engines (i.e.,
military, commercial, and industrial). Approximately 100,000 blades
were examined after acceptance by fluorescent particle inspection. Findings
conclusively point to a small percentage of turbine blades (e.g. about
0.25 percent on average) that cause most of the reported problems. These
are the blades that contain potentially life limiting anomalies and
escape detection at the foundry. These blades represent a risk to the
customer (i.e., the anomaly may or may not lead to a failure). The risk
may be very low (it is never zero) or very high.
Because a turbine may contain up to 400 turbine
blades, a 0.25 percent risk factor is very significant. That's one anomalous
blade in 400. Figure 9 explains why turbine blade failures are not rare
events but expected events. It only takes one blade failure to cause
an engine failure.
Traditional approaches use service lives to measure
risk - a costly and time consuming process. Using KET on foundry product,
risk assessment is immediate for almost the cost of fluorescent particle
inspection. Potentially life limiting anomalies are accurately characterized
and casting parameters can be fine tuned to minimize risk. Design lives
can be verified earlier in the development process, thereby allowing
for less costly design changes before the production phase begins.
Acceptance of KET as the
The aero engine manufacturers agree that KET is superior to fluorescent
particle inspection at the lower end of the discontinuity size spectrum,
but they are reluctant to make KET a requirement for turbine blades.
Foundries are also reluctant to use KET because it isn't an engine manufacturer's
requirement. Imposing higher standards admittedly raises the cost of
doing business. However, experience shows that KET provides its users
with paybacks far in excess of the cost of using it.
The current standard will inevitably be raised by
market forces. Global competition, demands for six-sigma quality, and
new complexities in investment castings will drive the standard up.
For now, it's necessary for customers to specify KET on their purchase
orders for investment castings, hipping, repairs, and overhaul if they
want to minimize their risk.
Ellis, P.F., II, "The Case of the Invisible
Leaks," The Conduit, Radian Corporation, Austin, TX, 1995.
Flaherty, J.J., "Yesteryears: History of Penetrants:
The First 20 Years, 1941-61," Materials Evaluation, Vol.
44, No. 12, Nov. 1986, pp 1371-1374, 1376, 1378, 1380, 1382.
Mahorter, R.G., "Evaluation of Krypton Evaluation
Technique (KET™) for Hot Tear Detection in T56 Turbine Blades,"
Naval Air Warfare Center, Patuxent River, MD, Code: Air 4.4.3, Nov.
Oldfield, W., and F.M. Oldfield, "Service Failure
of Hot Stage Turbine Blades: The Role and Mechanism of Oxidation Ratcheting,"
Metallurgical Transactions, Oct. 1993.
Rummel, W.D., "Cautions on the Use of Commercial
Aqueous Cleaners in Fluorescent Penetrant Inspection Processes,"
Review of Progress in Quantitative NDE Conference, Bowdoin College,
Brunswick, ME, Jul. 28-Aug. 2, 1996.
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