Post on 11-Apr-2017
transcript
Sky High Solutions, LLC Cameron Wong Chinmay Parikh Eric Hernandez July Aye (Project Manager) Luis Salazar Nimish Dave Okechi Kwem Sam Heller Sam Richesson Victor Ma
Spring 2015
HYDRAULIC CONTROLLER
VALVE GROUP 3
HYDRAULIC CONTROLLER VALVE page 2
Table of Contents Background Summary
Service and Application Condition of Part
Interaction with Skydrol Oil
In Service Stress Conditions
Material Identification
SEM Analysis and Determination of Cause Redesign and Conclusion References
HYDRAULIC CONTROLLER VALVE page 3
Background Summary
Catastrophic failure reports were experienced on commercial aircrafts using a hydraulic
check valve Part No. H61C0552M1 manufactured by the Parker Hannifin Corporation. The check
valves fractured under pressure and the fractures occurred in the first external thread of the valve
nearest the hex head, according to a service letter released by Boeing in 2010 (Boeing).
The following parts of the hydraulic check valve were given to Sky High Solutions, LLC to
investigate the fracture and provide evaluation of the probable cause of the part failure as well as
the type of loading conditions, determine who is at fault, and resolve the issue by implementing
critical design changes for future applications of the part.
Figure 1-Fragments of Failed Hydraulic Check Valve
Service and Application Condition of Part
The part under examination is a hydraulic check valve for use with Skydrol hydraulic oil
designed by the Parker Hannifin Corporation. A check valve is an automatic fluid flow regulator that
opens to forward flow and closes to backward flow, making it useful in isolating sections of a
hydraulic circuit. This particular design is referred to as a “poppet type” cartridge valve by Parker
which refers to the cylindrical barrier inside the valve that restricts flow; the valve itself is essentially
a large hex-bolt with moving innards, hence the “cartridge”. The valve interior is pre-loaded with a
spring of known tension that seats the poppet against the entry port, sealing it to backflow. When a
HYDRAULIC CONTROLLER VALVE page 4
pressure greater than this tension is exerted by the fluid at the entry port the poppet is unseated
allowing the high-pressure fluid to enter through the valve and exit through the side ports relieving
the pressure in the line.
Figure 2- A similar hydraulic check valve setup from Parker Hannifin Corporation (Series
CVH121P)
Interaction with Skydrol Oil
Its also important to look into the typical fluid with which this part is in constant contact,
especially given the elevated service pressures. Skydrol is a fire-resistant hydraulic fluid made by
adding certain chemicals to a phosphate ester chemical suspension which helps prevent corrosion
and erosive damage to hydraulic components; Skydrol also contains either green or purple dye for
identification purposes. The Skydrol line of oils was specially developed in the late 1940’s for use
in aircraft hydraulic systems by both the Monsanto Company and the Douglas Aircraft Company in
order to reduce the hazard of ignition from traditional mineral oil based fluids.
Skydrol oil turns a dark amber color when it has been thermally stressed. The remnants of
the hydraulic fluid on the part, specifically inside the chamber could be described as a dark amber
color, which could hint that this part has been subject to high thermal stresses. The proper
HYDRAULIC CONTROLLER VALVE page 5
maintenance of a material is particularly important when lubricants are involved, due to the fact that
improper lubricants can be extremely damaging to certain mechanisms, as well as to the seals and
gaskets that are intended to keep them from leaking.
In Service Stress Conditions
In an aircraft, the hydraulic controller valve moves in a cyclic motion as it moves depending
on whether the flow needs to be sealed or not in order to control the internal pressure of the
system. This cyclic motion promoted fatigue failure in this particular failed component as the
external threads experienced a high concentration of stress. As the pressure drops again, the
stress would be relieved on these threads and continue to experience episodes of highly stressed
stages.
Material Identification
The hydraulic check valve was split into three pieces after fracture. There is the hex head
casing, the O-ring casing, and a threaded O-ring. After further investigation, it was determined that
the grooved part of the O-ring was intentionally cut off from the O-ring casing after the fracture
occurred so it could be examined for the fracture initiation. As a result of this information, testing
was done only on the hex head and the O-ring casing to determine what sort of material was being
used in the part. A density test of the pieces yielded 2.92 g/m. This value points in the direction of
Aluminum rather than Steel being used in the pieces, as the density of Aluminum is 2.7 g/mL
(Reference 1). The material was also not magnetic, which confirmed that it was Aluminum. Due to
the density of the hydraulic check valve having some discrepancy with the actual density of
Aluminum, additional hardness testing was performed on the pieces.
The hardness scale for Aluminum is Rockwell B, determined through the charts provided in
the lab testing area. Three tests were done in each area, and the average hardness will be
reported. The interior of the hex head casing yielded a hardness of 81.0. Similarly, the outside of
HYDRAULIC CONTROLLER VALVE page 6
the casing gave a hardness of 80.6. The O-ring casing resulted in a hardness of 76.3. The slight
difference in all of these values may be attributed to small human errors when placing the sample
under the testing machinery. Overall, these hardness values are quite similar and can be used to
determine a more specific Aluminum alloy that may have been used in these parts. Conducting
further research, Aluminum 7050 appears to fit the data collected on this part so far. Aluminum
7050 is approximately 88.8 weight percent Aluminum and 12.2 weight percent of other chemicals
like Magnesium, Chromium, Copper, and Iron (Reference 2). Additionally, its density is 2.83 g/mL,
which is very close to the value recorded above. The hardness of Aluminum 7050 is 84. The
hardness measurement of the hydraulic valve pieces may be slightly off due to machining
calibration error. The ultimate tensile strength of Aluminum 7050 is 524 MPa or 76000 psi
(Reference 2).
The hydraulic check valve is composed of aluminum, as determined by the density and
hardness test values. ASM International states that aluminum does not have a true endurance limit
with regards to fatigue tests. Furthermore, the material’s fatigue strength is usually reported as the
total number of cycles it can survive. In this case, the material’s total number of cycles could have
been significantly reduced because of the small surface flaws on the external thread. The crack
then propagated through the material, which is shown on some of the SEM images as the slip lines
occur perpendicular to the main tensile axis. The material also did not experience fully reverse
bending and instead was subject to random or spectrum loading.
HYDRAULIC CONTROLLER VALVE page 7
SEM Analysis and Determination of Root Cause
Figure 3- Fracture Surface that has seven distinct failure zones
In Figure 3, the seven areas where the SEM images correspond to are shown. It is
observed in these failure zones that the part had undergone some type of consecutive failure mode
that happened around the connection between the first thread and the valve chamber. Already
from an initial glance, the shiny lines visible to the naked eye, near area 6, show a twisting motion.
The three stages of fatigue are observed in the SEM images, which conclude to the theory that the
root cause of failure must be due to fatigue fracture.
Figure 4- A zone on the fracture surface showing between the thread and inside of the part
HYDRAULIC CONTROLLER VALVE page 8
The fracture features observed microscopically support the theory of fluctuating stresses
causing fatigue failure of this hydraulic check valve. The high number of micro cracks observed in
the threads in the Figure 4 suggests that fatigue failure would have been a preferred mode of
failure. In this figure, the top shows the thread, where some sliding wear and spalling can be seen,
but still is not serious enough to be the root cause of failure. In addition, this image was probably
taken in the zone that was between the initial and final areas of failure.
While none of these micro cracks seem severe enough to facilitate Stage I Fatigue, their
high numbers suggest that this is highly possible in another location along the threads. The
inclusion-extrusion pairs seen near the center of Figure 5, similar to Stage I Fatigue, reinforce this
idea.
Figure 5- Image showing both ductile and brittle features, close to the final failure zone
The failure surface exhibits some ductile features but simultaneously appears to be brittle at
other points. This could be attributed to a quasi-cleavage due to the appearance of the rosette
pattern in the top right corner as well as the bottom left corner. Perhaps the quasi-cleavage is due
to the outside surface of the threads being harder which would result from case hardening. The
ductile dimples here are sheared in a direction from top-right to bottom-left which eludes to a shear
HYDRAULIC CONTROLLER VALVE page 9
stress applied to the part. Thus the part most likely had a catastrophic failure while being twisted or
unscrewed and not while in service.
Another piece of evidence that points to fatigue fracture is the following SEM image
composed of shallow dimples, which are ductile rupture features, indicative of a Stage III fatigue
failure. On the left of the image, it is noted that there are also three circular areas consisting of fold-
like features.
Figure 6- Dimples showing ductile rupture
The intergranular fracture surface shown on the right side of this SEM image also points to
a case hardened zone where a fatigue crack would propagate through the grain boundaries, but in
a transgranular way in a less hardened zone. This suggests that there is a non-uniform hardness in
the part and the shallow dimple features possibly confirm that the threads were cold worked too
much. However, the fact that this failure happened in a fairly ductile manner concludes that this
was a case of high cycle fatigue with lower applied stresses.
The following area portrays how the thread was fractured in a twisting motion through
multiple steps, with the left side of the image at a higher elevation than the right side. In the center
of the image, ductile failure is shown through the observed shear dimples occurring in layers,
HYDRAULIC CONTROLLER VALVE page 10
which is where the actual rupture may have occurred. On the left, slip lines, a feature of Stage II
fatigue, are visible as the material experienced some torque. The motion of the part can be
visualized as this area sheared with a force that traveled from right to left.
Figure 7- Slip lines as well as dimples shown
A secondary crack can be seen in the bottom left of Figure 8. Thus when comparing to a
theoretical diagram provided by the Atlas of Metal Damage by Lothar Engel that describes fatigue
cracking, one can see yet another correlation of fatigue fracture features and our check valve.
Figure 8- Ductile rupture observed by shallow dimples shearing towards the right
HYDRAULIC CONTROLLER VALVE page 11
Figure 9- Textbook image showing secondary cracks and crack propagation
Figure 9 shows some slip bands occurring on the lower left side, which identify as Stage II
fatigue features. There are some more brittle fracture features on the right side of the image.
Figure 10- Stage II Fatigue Features
Slip bands can be observed further in the next SEM image, occurring at what could possibly
be the source of the failure. Stage II fatigue failure is apparent in this image with the multiple
intrusion and extrusion pairs occurring in a mostly parallel pattern. Different layers of elevation can
also be seen as the right side of the image is on a higher elevation than the left. The lower right
area of the image also appears brighter, confirming that it is the last place that fractured. In
HYDRAULIC CONTROLLER VALVE page 12
comparison to the diagram taken from the Lothar Engel Atlas of Metal Damage (which directly
follows the SEM image) one can obviously see a similar trend thus, it can be inferred that the
direction of the failure occurs from left to right in an almost clockwise direction.
Figure 11- Multiple slip lines showing Stage II Fatigue
Figure 12- Image from textbook showing crack propagation and slip lines
HYDRAULIC CONTROLLER VALVE page 13
Redesign and Conclusion
In conclusion, this failure can be attributed to a poor design, where the cyclic stresses felt
by the aluminum part in this particular application was greater than the part could withstand. It is
undetermined by our investigation whether or not this part was published to have a greater
resistance to fatigue cracking or if this particular application exceeded its designed value. Further
investigation and information would be needed to find who is liable.
In order to resolve this issue in future aircrafts that will use the same hydraulic check valve,
the material used and the geometric properties of the part must be altered. First, the material
should be made out of steel in order to provide better fatigue strength. Not only does steel have the
higher fatigue strength, but it also will fail less due to cyclic loading alone, provided it is operating
below a certain stress value.
Another option is to change the geometry of the material by increasing the radius and
thickness of the threads. By providing a larger radius, the internal pressure will apply a smaller
value of stress at the external threads due to the larger area.
An additional change that needs to be made is proper maintenance of the hydraulic check
valve. The hydraulic fluid needs to be changed properly as required due to its operation life. The
fact that a dark amber color was found inside the chamber of the valve shows that the part had not
been properly maintained and that the hydraulic fluid was not changed regularly.
HYDRAULIC CONTROLLER VALVE page 14
References
Aerospace Specification Metals, Inc. “Aluminum 7050 -T7451.”
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7050T745
ASM International. “Fatigue.” 2008.
http://www.asminternational.org/documents/10192/1849770/05224G_Chapter14.pdf
Boeing. “Service Letter.” 27 April 2010.
http://www.crissair.com/BoeingSB/SB-737-SL-29-108-C.pdf
Electron Microscope. Englewood Cliffs, NJ: Prentice-Hall, 1981. P
Engel, Lothar, and H. Klingele. An Atlas of Metal Damage: Surface Examination by Scanning.
Ophardt, Charles E. “Aluminum.” Virtual Chembook. 2003.
http://elmhcx9.elmhurst.edu/~chm/vchembook/102aluminum.html
Parker Hannafin. “Hydraulic Cartridge Systems.”
http://www.parker.com/literature/Literature%20Files/IHD/CVsection.pdf
Skydrol. “Type IV Fire Resistant Hydraulic Fluids.” 2003.
http://skydrol-ld4.com/technical_bulletin_skydrol_4.pdf