Engine Fastener Safety
Michael Adams
Embry Riddle Aeronautical University

As everyone in the Aviation industry knows safety is paramount to all aspects of flight operations.one aspect that gets overlooked by most people is how exactly the little bits and pieces create an aircraft that can operate safely. The importance of aircraft hardware is often overlooked because of the small size of most items. However, the safe and efficient operation of any aircraft depends upon the correct selection and use of aircraft hardware. One of the most overlooked aspects of aviation safety is the fastener and more specifically the engine fasteners. Anyone who has been around aircraft engines know that they are a unique engineering marvel and are capable of some pretty spectacular feats, but to accomplish this the engines have to produce tremendous amounts of stress and heat. To put it in perspective it is not uncommon for an aircraft engine to produce 30,000 lbs.’ of thrust with temperatures at the exhaust ports exceeding 2000° F. To hold these engines and power plants together we depend upon various types of fasteners which leads to the question how do these small bits of hardware survive under the extreme amount of heat and stress that an engine produces? In our course we learned that there are 4 methods of controlling safety with the hierarchy being engineering, administrative, PPE and interim fixes. Engine fasteners have their safety controls engineered into them and validated with extensive testing. This is because if an engine fastener fails it can easily produce a catastrophic failure that can result in loss of life, limb and valuable property. Because engineering controls are the best way to make engine fasteners safely they are controlled by engineering specifications. Most high strength, high heat fasteners fall under manufacture specifications or SAE International AMS (Aerospace Material Standards) Specifications. SAE has over 100 specifications relating to Bolts, screws, washers and nuts alone so you can imagine how many diverse engineering designs exist out there. Certain manufacturers who are industry giants, such as Pratt and Whitney, GE Engine and Lockheed Martin own their own specifications due to the tight engineering controls they implement and their need to retain proprietary information. When you look at these tight engineering requirements you can begin to understand why you cannot just go to the local hardware store and buy any old fastener and stick it in an Airbus or C-130 engine they would have an immediate failure and cause an accident. This is also why these fasteners, especially military application ones, cost anywhere from $1.50 to $75.00 each. The first step to properly engineering an engine fastener is ensuring the proper raw materials are procured for fabrication. There are close to 100 different blends of raw materials for fasteners each having its own unique metallurgical properties so utilizing the proper blend per specification is paramount to the safety of the fastener. The various factors used in considering raw material are: tensile strength, proof load, shear strength, hardness, ductility, and toughness. Tensile strength is the most widely associated mechanical property associated with standard threaded fasteners. Tensile strength is the maximum tension-applied load the fastener can support prior to or coinciding with its fracture or in layman’s terms it is the biggest load a fastener can support pulling from each end without fracture, this is also called the ultimate tensile strength (UTS) and can easily have requirements as high as 180,000 kilo pounds per square inch (ksi) for engine applications. The proof load represents the usable strength range for certain standard fasteners and represents how much load can be applied without a permanent deformation of the material. Shear strength is defined as the maximum load that can be supported prior to fracture, when applied at a right angle to the fastener’s axis. Shear strength is the same concept as UTS except the load is applied horizontally across the material instead of vertically. Hardness is a measure of a material’s ability to resist abrasion and indentation. A Brinell or Rockwell hardness test can be used to estimate tensile strength properties of the material. Ductility is a measure of the degree of plastic deformation that has been sustained at fracture. In normal terms, it is the ability of a material to deform before it fractures. A material is considered brittle if it experiences very little or no plastic deformation upon fracture. One interesting aspects of raw materials is all specifications require the business that has melted and provided the raw material to issue a legally binding certification that it has been melted using the proper procedures and that the metallurgical blend is correct by percentage weight. This basically means that at any given time any fastener with the proper documentation can be traced to the original raw material blend and any failures of that nature can have legally binding ramifications for the raw material manufacturer.
Once the proper raw material is procured then the actual manufacturing of the fastener can begin. Every specification has an order of operations that it has to follow for the proper manufacture of that particular fastener. Even though the order can differ the following operations are always present: head upsetting, heat treatment, grind, fillet and thread roll and lastly, plating if required. Raw material is normally delivered on a wire roll or a round stock bar. The raw material is then cut to size to make what is referred to as a blank. The blanks then have the heads created using a head upsetting operation, this upset can either be cold formed or hot formed but, are machine formed head is never allowed due to structural weaknesses inherent in the operation. The blanks are input into a heading machine that has a die on one end and a rod on the other. The blanks are pushed using the rod into the die which upsets the material into the shape of the head without causing any distortion of the material. Although both hot and cold head upsets are allowed the only time most manufacturers use a hot head is when the material will not allow cold hardening at ambient temperatures, such as the metal Indium. The cold heading creates a higher yield strength material due to dislocation movements of the raw materials crystal structure. The next steps can vary by specification so I will list them in what is the most common order of operations. Heat treatment is the act of hardening and tempering the fastener to increase its hardness and strength. The fasteners are heated to a specified temperature, normally over 1500° F and held at that temperature for a specific amount of time. Once the proper time and temperature is achieved the fasteners are cooled using air, inert gases such as argon or an oil mixture. If the fastener is properly heated and cooled the metal has undergone a transformation of its crystal structure generally resulting in small eutectoid crystal formations. These small crystals help enhance mechanical properties such as toughness, shear strength and tensile strength. Heat treatment has to be carefully controlled because over heating a fastener or cooling one to quickly will change the crystal structure to a undesirable form that most likely is not as strong as a eutectoid style structure. Depending on specification a fastener may go through multiple heat cycles for aging purposes. Grinding is not normally allowed because it has a risk of ruining the shank, but if allowed by specification its purpose is to remove small traces of surface contamination and ensure the surface is low stress prior to thread rolling. Grinding is simply moving a small about of abrasive material over the shank to remove these contaminants and create a smoother surface. There are multiple ways to make threads on a fastener but in aerospace applications threads are only allowed to be made by a rolling process. In this processes threads are formed into a blank by pressing a shaped die against the blank. As the blank rolls up and down the thread rolling machine the die pushes into the blank causing the ductile metal to push itself outward and form a thread using cold working methods. There are four main types of thread rolling, named after the configuration of the dies: flat dies, two-die cylindrical, three-die cylindrical, and planetary dies. The last manufacturing method is normal plating and again is only required if the specification or drawing calls for it. Plating is simply the act of putting a thin metal plate over the fastener to change some of its metallurgical properties. Plating can be done to reduce corrosion, reduce friction, prevent oxidation, improve lubricity, and improve malleability. The most important thing to remember about plating is all specification and drawing requirements for dimensions require that those dimensions are met with the plating included so you have to plan to manufacture your fasteners with those tolerances in mind otherwise the products will be non-conforming standards.
As you can see there is quite a bit of engineering analysis done to make conforming engine fasteners but verification of most of these aspects requires specialized testing, which covers the second half of their safety features. All aerospace fasteners are required to be subjected to rigorous testing both non-destructive and destructive to validate their characteristics prior to installation in an aircraft. This is also another reason why these fasteners generally cost more than normal hardware store ones. Most of these tests are conducted by independent test labs that are certified national standards such as National Aerospace and Defense Contractors Accreditation Program (NADCAP), SAE AMS specs and Aerospace Industries Association, National Aerospace Standards (AIA-NAS). The most common testing required for aerospace applications are nondestructive inspection, tensile load, stress rupture, fatigue strength, hardness, microscopic and macroscopic analysis. The most important thing to remember in this testing is that all this testing is either detailed in the specification or detailed in a related specification referenced in the manufacturing specification. It is not uncommon to have a manufacturing specification reference 20 other specifications for various operations or requirements so the data line can get quite deep and staying compliant to all these requirements can be quite troublesome.
Nondestructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. The purpose of nondestructive testing is to detect surface and sub-surface defects that are not detectable with a human eye visual inspection. In the United States Aerospace industry all nondestructive testing houses and inspectors have to be certified and trained in accordance with AIA-NAS-410. There are multiple methods of nondestructive testing but in the aerospace fasteners the two most common are magnetic particle and liquid penetrant. Magnetic particle inspection is a process for detecting surface and slightly subsurface discontinuities in ferroelectric materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part that magnetizes them by direct or indirect magnetization. The magnetic lines of force are perpendicular to the direction of the electric current which causes magnetic flux to leak out showing the presence of a surface or subsurface discontinuities in the material. Liquid penetrant is a form of nondestructive inspection which uses a liquid mixture to detect surface discontinuities on ferrous and nonferrous metals. Liquid Penetrant is based upon capillary action where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied which will allow the penetrant to draw out of the flaw and improve the visible indications. Inspection is performed under ultraviolet or white light, depending upon whether a fluorescent or non-fluorescent dye was used in the penetrant. Tensile testing is a destructive test in which a small sample of fasteners is subjected to a tensile force to help determine the ultimate tensile strength of the fastener. Tensile testing of fasteners helps us understand the amount of force required to pull the fastener out of the base material. While straightforward, the many shapes and sizes of fasteners complicate the testing. Most manufacturers must not only test for ultimate tensile strength but also to validate that no permanent deformation has occurred once the proof load is removed. Tensile testing is achieved by placing the fastener in a testing machine that applies force to both the head and end of the fastener. The applied force is recorded versus the elongation of the fastener, and once it fractures those values is recorded. From there a standardized formula is applied to validate the UTS and make sure it is within the specification requirements for UTS. Stress-rupture is also a destructive test in which a sample is subjected to constant load for a given amount of time at a given temperature to induce the sudden and complete failure of the fastener. This test is used by fastener manufacturers to determine how their products will perform when subjected to constant loads at both ambient and elevated temperatures. Fatigue is a measure of the stress that a material can withstand repeatedly without failure. A fatigue failure is particularly catastrophic because it occurs without warning. Fatigue tests are performed on fasteners by alternating loading and unloading the part. Most testing is done at more severe strain than its designed service load but usually below the material yield strength. Fatigue testing equipment is usually designed to induce cyclic loading and unloading to a known (peak) stress and measure the number of such cycles to failure of the specimen. Hardness testing is destructive test used to determine the hardness of a material to deformation. Hardness testing is an indentation test where a tester uses a diamond tipped applicator to indent into the shank of the fastener. The indentation that is left after the indenter and load are removed is known to "recover", or spring back slightly which is known as shallowing. There are actually two forms of indention testing, microindention and macroindention, which depend on the force applied. The biggest difference between them is microindention testing typically has an applied force less than 2 Newton’s. One thing that is always called out in the specification is what type of hardness test is required and what scale it is measured to. Like most other things in the testing world there are multiple types of test and each has its own operations to determine and measure the hardness rating. In aerospace applications the three most common are a Vickers hardness test (HV), Brinell harness test (HB) and a Rockwell hardness test (HR). The last form of testing, and most interesting to me, is the Micro and macro analysis of the fastener. The basis of these test are that a sample is taken and cut in half to be analyzed under a microscope. Most specifications call for an etching agent which is listed in the specification to help microstructures appear under the microscope. Once the etching agent is applied the fasteners are placed under a microscope with a magnification of 20x to 200x to have various characteristics visually inspected. The flow lines of the material are normally viewed at a 20x resolution and need to be analyzed to verify they follow the contour of the shank, head-to-shank fillet, and the load bearing surfaces of the fastener. The fastener is then normally inspected at a 200x magnification looking for intergranular attack (IGA), thread and head defects. IGA is a form of corrosion where the boundaries of crystals are depleted of their corrosion inhibiting elements and are more susceptible to corrosion. This will show up on the microanalysis as spots in the boundaries that appear as black areas. At the same magnification the threads and head of the fastener are analyzed for any small imperfections in them such as laps or cracks that are not visible to the naked eye. The last microanalysis is normally conducted at 100x magnification and it is a grain size analysis. Once the fastener is magnified and the actual grain structure of the crystals is visible it is then compared to standard examples of grain sizes for conformance to specification requirements. As we can see aircraft engine fasteners have extensive engineering controls designed into their drawing and specifications which require extensive, and expensive, testing to validate conformance to. This may seem like overkill to the average person but there have been many incidents of engine failure due to a bad fastener and the famous F117 crash of 1997 was traced to a fastener defect on the wings. The majority of these crashed have been in private aircraft and one could deduce that is primarily because of the quality controls the military and commercial aerospace has implemented. Safety is something that begins with the smallest pieces and can affect every aspect of aircraft so the requirements for manufacturing may seem excessive but they do save lives.

References
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H.Herring, D. (2010, 02). Testing of Heat-Treated Fasteners. Retrieved 12 01, 2011, from The Heat Treat Doctor: http://www.heat-treat-doctor.com/documents/Testing%20of%20Heat%20Treated%20Fasteners.pdf
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