91430 SPS cover edited - Electronic Fasteners Inc

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91430 SPS cover edited - Electronic Fasteners Inc

SCREW FASTENER THEORY & APPLICATIONSThis is the accepted practice for high fatigue performancebolts such as those used in aircraft and spaceapplications.FASTENER POINT END0 1 2 3 4RELATIVE INTERNAL STRESSAT FIRST ENGAGED THREADFASTENER HEAD ENDEVALUATING PERFORMANCEMechanical TestingIn the fastener industy, a system of tests and examinationshas evolved which yields reliable parts with provenperformance.Some tests are conducted on the raw material; someon the finished product.There always seems to be some confusion regardingmechanical versus metallurgical properties. Mechanicalproperties are those associated with elastic or inelasticreaction when force is applied, or that involve the relationshipbetween stress and strain. Tensile testing stressesthe fastener in the axial direction. The force at whichthe fastener breaks is called the breaking load or ultimatetensile strength. Load is designated in pounds, stress inpounds per square inch and strain in inches per inch.When a smooth tensile specimen is tested, the chartobtained is called a Stress-Strain Curve. From this curve,we can obtain other useful data such as yield strength.The method of determining yield is known as the offsetmethod and consists of drawing a straight line parallel tothe stress strain curve but offset from the zero point by aspecified amount. This value is usually 0.2% on the strainordinate. The yield point is the intersection of the stressstraincurve and the straight line. This method is notapplicable to fasteners because of the variables introducedby their geomety.When a fastener tensile test is plotted, a load/elongation curve can be obtained. From this curve, ayield determination known as Johnson‘s 2/3 approximatemethod for determination of yield strength is used toestablish fastener yield, which will be acceptable fordesign purposes. It is not recommended for qualitycontrol or specification requirements.Torque-tension testing is conducted to correlate therequired torque necessary to induce a given load in amechanically fastened joint. It can be performed by handor machine. The load may be measured by a tensilemachine, a load cell, a hydraulic tensile indicator or bya strain gage.Fatigue tests on threaded fasteners are usually alternatingtension-tension loading. Most testing is done atmore severe strain than its designed service load butususally below the material yield strength.Shear testing, as previously mentioned, consistsof loading a fastener perpendicular to its axis. All sheartesting should be accomplished on the unthreadedportion of the fastener.Checking hardness of parts is an indirect method fortesting tensile strength. Over the years, a correlation oftensile strength to hardness has been obtained for mostmaterials. See page 83 for more detailed information.Since hardness is a relatively easy and inexpensive test,it makes a good inspection check. In hardness checking,it is very important that the specimen be properly preparedand the proper test applied.Stress durability is used to test parts which havebeen subjected to any processing which may have anembrittling effect. It requires loading the parts to a valuehigher than the expected service load and maintainingthat load for a specified time after which the load isremoved and the fastener examined for the presence ofcracks.Impact testing has been useful in determining theductile brittle transformation point for many materials.However, because the impact loading direction istransverse to a fastener's normal longitude loading, itsusefulness for fastener testing is minimal. It has beenshown that many fastener tension impact strengths donot follow the same pattern or relationship of Charpyor Izod impact strength.Metallurgical TestingMetallurgical testing includes chemical composition,microstructure, grain size, carburization and decarburization,and heat treat response.The chemical composition is established when thematerial is melted. Nothing subsequent to that processwill influence the basic composition.The microstructure and grain size can be influencedby heat treatment. Carburization is the addition of carbonto the surface which increases hardness. It can occur ifheat treat furnace atmospheres are not adequately controlled.Decarburization is the loss of carbon from thesurface, making it softer. Partial decarburization is preferableto carburization, and most industrial standards allowit within limits.In summary, in order to prevent service failures, manythings must be considered:The Application RequirementsStrength Needed – Safety Factors Tension/Shear/Fatigue Temperature Corrosion Proper PreloadThe Fastener Requirements Material Fabrication Controls Performance Evaluations55

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