CBEMS 175 report

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


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


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


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


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.


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


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


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,


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


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


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


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.


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

Documents

CBEMS 175 report