Ganadería

THE MECHANICAL EVALUATION of components requires an engineer to use many sources of information. It requires an under-standing of service conditions, design, and manufacturing variables. While there are many types of component tests for a multitude of prod-ucts, this chapter focuses on three examples of engineering components that undergo signifi-cant loading in tension: threaded fasteners and bolted joints; adhesive joints; and welded joints.

For some components, tensile loading is not the primary concern. For example, rolling contact fatigue is the most important consideration for rolling-element bearings. Gears, in addition to rolling contact fatigue tests, are tested for resis-tance to wear, bending fatigue, and impact. Pres-sure vessels, piping, and tubing are tested for their creep and fracture resistance.

An overview of mechanical properties for component design can be found in Ref 1. Prop-erties and design for static (tensile and com-pressive) loads, dynamic (impact and fracture toughness) loads, and cyclic (fatigue) loads are addressed.

Testing of Threaded

Fasteners and Bolted Joints

Fastener engineering and the mechanical test-ing of threaded fasteners and bolted joints is an important specialty within the field of mechan-ical engineering. With the wide variety of fas-teners and bolted joints available for use, no one set of tests can be specified to cover all appli-cations. Fasteners are routinely tested for hard-ness, tensile strength, and torsional strength, as well as corrosion and hydrogen embrittlement.

Before describing the standardized tensile test for externally threaded fasteners, some brief background information is provided to help the

reader understand the relationships between torque, angle-of-turn, tension, and friction.

Torque, Angle, Tension, and Friction A proper amount of tension, or clamping force, must be developed to ensure that a bolted assembly will function in a safe and reliable manner. The most common attempt to indirectly estimate fastener tension is to take torque mea-surements either dynamically as the fastener is tightened or with a breakaway audit after the fact. The torque that is required to produce the desired tension in a fastener is dependent on sev-eral factors, with frictional characteristics being the most important. Angle-of-turn measure-ments combined with torque measuremeasure-ments can help overcome the unknown friction-induced variability in the torque-tension relationship.

Tension. The tension that is created in a threaded fastener when it is tightened represents the clamping force that holds the assembly to-gether. Once the assembly is brought together, the fastener responds like a tension spring, and the assembly acts like a compression spring. The interaction between the fastener and the assem-bly is illustrated in Fig. 1. As the fastener is turned and load is applied, the fastener is stretched, and the parts are compressed. This compression results in an elastic joint in which the fastener is normally the more flexible mem-ber, and the assembly is the more rigid member.

The amount of clamping force that the fastener must provide to hold the assembly together must be sufficient to both maintain preloading and pre-vent slipping of the parts or opening of the joint when the service loads are applied. The factors that primarily establish the preload requirement are the stiffness of the materials in the joint and the loads that are placed on the assembly.

Fastener tension can be measured using dif-ferent devices, such as strain-gaged bolts or

fas-Fig. 2 Typical distribution of energy from torque applied to a bolted assembly

Fig. 1 Spring effect of fastener and assembly under load

tener force washers, or by using special tech-niques, such as ultrasonic bolt measurement.

Although these devices and methods are useful in research and engineering efforts, they are of-ten impractical or costly for evaluating fasof-tener tension in production quality-control efforts.

Torque. The most common way to estimate clamping force is to observe the amount of torque applied to the fastener, either as the fas-tener is tightened or with a breakaway audit of the tightened fastener. This procedure assumes that the relationship between torque and tension is known, such that, for example, the nut factor, or K, from the simple equation T⳱ KDF (where T is torque, D is diameter, and F is clamping force) is established and known to have accept-able variability. The truth of the matter is that if torque alone is measured, it can never be known with certainty whether the desired tension has been achieved. Thus, unfortunately, it must be concluded that torque is a highly unreliable, to-tally inaccurate measurement for evaluation of the preload on a threaded fastener. However, for many noncritical fasteners, where safety or the functional performance of an assembly is not compromised, it may be acceptable to specify and monitor torque alone. The most common measurement tools are hand torque wrenches that are used for installation and torque audit measurements and rotary torque sensors that are used to measure installation torque dynamically.

In order for tension to be developed, the torque applied to a fastener must overcome

fric-tion under the head of the fastener and in the threads, and the fastener or nut must turn. Be-cause the friction may absorb as much as 90 to 95% of the energy applied to the fastener, as little as 5 to 10% of the energy is left for gen-erating fastener tension as shown in Fig. 2. If the amount of friction varies greatly, wide variations in clamping force are produced, which can mean loose or broken bolts leading to assembly fail-ures. To ensure proper assembly of critical fas-teners, more than torque must be measured.

Angle. The amount of fastener tension can be correlated to fastener rotation once the parts of an assembly are drawn firmly together. The clamping force that is developed in this zone of the assembly process, called the elastic tighten-ing region, has been proven to be proportional to the angle-of-turn. This proportional relation-ship is based on the helix of the threads and is not influenced by the frictional characteristics of the joint once sufficient clamping force has been produced to firmly align the components such that a linear torque-angle signature slope is at-tained. More detailed information on the rela-tionship between torque and angle-of-turn can be obtained by torque-angle signature analysis described in Ref 2.

Friction Measurements. Whereas fastener engineering analysis of threaded fasteners must consider material strength, surface finishes, plat-ing, and coatings to ensure reliable performance, for predictable and repeatable assemblies it is also necessary to understand, measure, and con-trol the frictional characteristics in both the thread and underhead regions. This is

particu-Fig. 3 Torque-tension research head, 800 kN capacity

larly true when developing fastener-locking de-vices such as locknuts, serrated underheads, spe-cial thread forms, and thread-locking adhesives and friction patches. Achieving a specific clamp force during installation is always the desired result, and the roles of thread friction and un-derhead friction must be analyzed and under-stood to ensure joint integrity.

To determine both thread friction and under-head friction, measurements are taken using a torque-tension research head, as shown in Fig.

3. This device is a special load cell designed to simultaneously measure both thread torque and

clamp load. When used with torque sensors that measure the input torque, it is possible to deter-mine the underhead friction torque and the thread friction torque. With this measurement equipment, the fastener can then be tested to es-tablish and maintain standards for friction per-formance.

For example, in the test plot illustrated in Fig.

4, a locknut is initially driven onto a bolt. The thread friction torque is equal to the input torque until contact with the underhead-bearing surface is made. Once contact is made with the under-head area, the underunder-head friction torque is mea-sured as the difference between the total input torque and the thread torque. As clamp force is developed, the pitch torque is calculated and subtracted from the thread torque to compute the thread-friction torque. Note that for prevailing torque locknuts, the elastic origin is located at the prevailing torque level as shown in Fig. 4, not at the zero torque level used for fasteners without prevailing torque characteristics.

Considerations in Testing. There are a num-ber of factors that can affect the tension created in a bolt when torque is applied. Depending on the fastener and joint configuration, direct mea-surement of tension is not always practical or even possible by any means. Fortunately, torque and angle measurements can be taken for most bolted joints and then analyzed to assist in de-termination of important characteristics and properties related to strength and reliability.

Fig. 4 Determining friction forces for prevailing torque locknut

When tightening a threaded fastener, it is al-most always important to know both how much torque is applied and how far the fastener is turned. Similarly, it is always important to fully understand how friction affects the relationship of torque, angle, and tension.

To ensure that critical joints are tightened properly, it must be kept in mind that it is the control of tension that is most important, not the control of torque. This fact must always be con-sidered when choosing and setting up tools, when monitoring production, and when per-forming quality control audits. The fastener-tightening process is dependent upon the energy transfer from the tightening tool into the fastener and bolted joint. The integrated area under the torque-angle signature curve is a measure of the energy absorbed by the assembly.

Standard Test Methods for Determining Materials Properties of Fasteners

The materials properties of the fastener must be known before a more detailed analysis of the bolted joint is possible. Many standards exist for the testing of fasteners. ASTM F 606M (Ref 3), a specification developed through the proce-dures of ASTM for metric fasteners, is consid-ered to be one of the most complete. The cor-responding standard for English threaded fasteners is ASTM F 606. More complete de-scriptions of the methods can be found in the standard. The text following in this section is a summary of the basic test methods according to ASTM F 606M.

The test methods described in ASTM F 606M establish procedures for conducting mechanical tests to determine the materials properties of ex-ternally and inex-ternally threaded fasteners. For externally threaded fasteners, the following test methods are described:

● Product hardness

● Proof load by length measurement, yield strength, or uniform hardness

● Axial tension testing of full-sized products

● Wedge tension testing of full-sized products

● Tension testing of machined test specimens

● Total extension at fracture testing Product Hardness

The hardness of fasteners and studs can be determined on the ends, wrench flats, or un-threaded shanks after removal of any oxide, de-carburization, plating, or other coating material.

Rockwell or Vickers hardness standards may be used at the option of the manufacturer. Hardness is determined at midradius of a transverse sec-tion of the product taken at a distance of one diameter from the point end of the product. The reported hardness is the average of four hardness readings located at 90⬚ to one another. Accept-able alternative methods of determining hard-ness for bolts are either at midradius, one di-ameter from the end, or on the side of the head of a hex-head or square-head product of all prop-erty classes after adequate preparation to remove any decarburization. As explained subsequently, uniform hardness measurement is one method for determining the proof load.

Tensile Tests

Fasteners and studs should be tested at full-size and to a minimum ultimate load in kilonew-tons (kN) or stress in megapascals (MPa). Such testing includes proof-load tests (by length mea-surement, yield strength, or uniform hardness), axial tensile tests, wedge tensile tests, and total extension-at-fracture tests.

Proof-Load Tests. The basic proof-load test consists of stressing the product with a specified load that the product must withstand without any measurable permanent set and evaluating the fastener in terms of any change in length. Alter-native tests to determine the ability of a fastener to pass the proof-load test are the yield-strength test and the uniform hardness test. Although any of the alternative test methods described may be used, the proof-load test is the arbitration method used in case of any dispute.

Method 1, Length Measurement. The overall length of the specimen is measured at its true centerline with an instrument capable of mea-suring changes in length of 0.0025 mm with an accuracy of 0.0025 mm in any 0.025 mm range.

Measuring the length between conical centers on the centerline of the fastener or stud with mating centers on the measuring anvils is preferred. The head or body of the fastener or stud should be marked so that it can be placed in the same po-sition for all measurements.

The product is assembled in the fixture of the tension-testing machine so that six complete threads are exposed between the grips. Tests for heavy hex structural bolts are based on four threads. This is obtained by freely running the nut or fixture to the thread runout of the speci-men and then unscrewing the specispeci-men six full turns. For continuous threaded fasteners, at least