Fastener reliability and performance can only be ensured by testing. Since heat treatment can have a profound influence on final properties, it is especially important to conduct testing on the raw material, after heat treatment and on the finished product.
The diversity of fastener types, sizes and end-use applications presents a challenge in testing that often requires choosing appropriate test methods (Table 1), from simple dimensional checks to rigorous mechanical testing methods. Common types of tests include: fatigue, tensile strength, wedge tensile (10 degrees), double shear, hardness, stress rupture, stress durability, pull-out/push-out, vibration, engagement, wear, microstructure, chemical analysis and corrosion.
Mechanical Test Methods
In mechanical testing, fasteners are analyzed to determine their mechanical properties. Mechanical properties are those associated with elastic or inelastic behavior of a component when force is applied. It involves the relationship between stress and strain. A mechanical test shows whether the material or fastener is suitable for its intended application by measuring such aspects of performance as elasticity, tensile strength, elongation, hardness and fatigue limit.
Tensile testing of fasteners (Fig. 2) 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. High production demands necessitate a large number of tests, and the potential of violent fractures makes testing more complicated. Most manufacturers must not only test for ultimate tensile strength but also perform proof tests and ensure that no permanent deformation has occurred once the proof load is removed. In addition, they must find ways to test for thread quality and head strength. It is not uncommon to see tensile-testing equipment in the QC department or even out on the floor of a heat-treat shop processing fasteners.
Fasteners are typically installed by applying a torsional force to the head or nut. This force (or seating torque) causes the fastener to stretch and effectively applies a preload to ensure a snug fit between components. One purpose of a torque-tension test (Fig. 3) is to determine the appropriate tightening torque required. This type of test will allow the nut factor (sometimes referred to as the torque coefficient or k factor) to be determined as well as the overall coefficient of friction. Similarly, one often needs to determine the torque force at which the fastener will fail. By completing several similar tests, the variation in the torque-tension relationship, due to frictional variation, can be established for a given application.
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. Three basic factors are necessary to cause a fatigue failure: a maximum tensile stress of sufficiently high value, a large enough variation or fluctuation in the applied stress, and a sufficiently large number of cycles of the applied stress. Fatigue-life tests are performed on threaded fasteners by alternating loading and unloading of 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. Variants of the test include tensile, bending and rotating. The average stress at which a steel can withstand 10 million loading cycles without failure is reported as the fatigue strength (also called the endurance limit). As stress increases, the number of cycles to failure decreases.
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. Simply stated, it is the force required to pull the base material in one direction and the top material in the other direction until failure.
Modes of failure include deformation of the base material (i.e. the fastener pulls out of the base material) and fastener fracture. Bolted or riveted connections are those that are commonly subjected to shear stresses.
Unlike tensile testing, determining the ultimate shear load and detecting specimen failure can be difficult. The test system must be flexible enough to define different end-of-test criteria for each style of fastener. Most, if not all, shear testing is done on the unthreaded portion of the fastener.
Single-shear testing applies a load in one plane and results in the fastener being cut into two pieces, while double-shear (Fig. 4) produces three fastener pieces. Single-shear values for fasteners are typically calculated based on the nominal body diameter, or body shear area. There is a relationship between the tensile strength of a material and its shear strength. For example, in alloy steel the shear strength is approximately 60% of its tensile strength. In corrosion-resistant steels (e.g., 300-series stainless steels) the tensile/shear relationship is usually only 50-55%.
Creep and Stress-Rupture
Creep is time-dependent deformation of a material while under an applied load (below its yield point). Stress-rupture is the sudden and complete failure of a material held under a constant load for a given period of time at a specific temperature. These tests are used by fastener manufacturers to determine how their products will perform when subjected to constant loads at both ambient and elevated temperatures.
Stress durability is used to test parts that have been subjected to any processing operation (e.g., electroplating) that may have an embrittling effect. It requires loading the parts to a value higher than the expected service load and maintaining that load for a specified time after which the load is removed and the fastener is examined for the presence of cracks.
Microhardness testing (Fig. 5) is often done to measure the core hardness of a fastener or measure the depth of case hardening. This type of hardness test helps characterize the fastener’s durability or wear. For example, low core-hardness measurements may indicate a premature yielding of the fastener, leading to a ductile failure. By contrast, high core hardness may indicate the inability to properly yield and lead to a brittle-type fracture. In either case, the integrity of the fastener may be jeopardized.
Vibration tests are used to determine a fastener’s life span and compare its self-loosening characteristics under vibratory conditions. A transverse vibration test machine (commonly called a Junker machine) is used to produce a preload-decay graph, an indication of resistance to self-loosening.
Metallurgical (Structure) Analysis
Metallurgical testing can be performed to evaluate fastener microstructure. It yields invaluable information on grain size, surface condition (e.g., carburization or decarburization) and heat-treatment response. The microstructure and grain size are most often influenced by heat treatment.
The chemical composition of a steel is established at the mill and reported on a material certification sheet. It is highly desirable to know not only the principal elements but also the trace elements present before heat treatment. Grain size, prior processing (e.g., mill annealing) and hardenability are also commonly reported.
Steel fasteners exposed to sources of hydrogen (e.g., electroplating operations) can fail prematurely at stress levels well below the material’s yield strength. The effect is often a delayed one, meaning that it may occur in service. Higher-strength steels are more susceptible to hydrogen embrittlement than lower-strength steels. As a rule of thumb, steels below 30 HRC are considered to be far less susceptible. The problem can be controlled by careful selection of plating formulation, proper plating procedure and sufficient post-plate baking to drive off any residual hydrogen.
Fastener failure due to corrosion can be relatively slow or surprisingly rapid. It is usually defined as the amount of time before white or red rust appears on the surface of a fastener and is measured in terms of hours of resistance to a salt-spray (fog) test.
There are at least three reasons why a working knowledge of statistics is needed in mechanical testing.
• Mechanical properties are structure-sensitive, so they frequently exhibit considerable variability or scatter. This makes statistical techniques useful, and often necessary, for determining the precision of the measurements and enabling valid conclusions to be drawn from test data.
• Statistical methods can assist in designing experiments to provide the maximum amount of information at minimum cost.
• Statistical methods (that are based on probability theory) can be used to help explain certain problems or phenomena such as the size effect in brittle fracture and fatigue.
Fastener Testing Specifications
Some of the more common fastener testing specifications are as follows:
1. NASM 1312-5 (replaces MIL-STD-1312-5): Standard test method to determine Hydrogen embrittlement (stress durability) of internally threaded fasteners
2. NASM 1312-7: Standard for accelerated vibration testing of fasteners
3. NASM 1312-8 (replaces MIL-STD-1312-8): Standard test method to determine tensile strength of externally and internally threaded fasteners
4. NASM 1312-10 (replaces MIL-STD-1312-10): Standard test method for stress-rupture
5. NASM 1312-11 (replaces MIL-STD-1312-11): Standard test method for determining tension fatigue of bolts, screws and nuts at room temperature
6. NASM 1312-12 (replaces MIL-STD-1312-12): Standard test method for determining thickness of metallic coatings
7. NASM 1312-13 (replaces MIL-STD-1312-3A): Fastener test methods for double-shear tests
8. NASM 1312-15 (replaces MIL-STD-1312-15): Standard test method for determining the room temperature torque-tension relationship for threaded fasteners
9. NASM 1312-20 (replaces MIL-STD-1312-20): Standard test method to define the procedures and apparatus for testing fasteners in single shear
10. NASM 1312-28 (replaces MIL-STD-1312-28): Standard test procedure to determine double-shear strength, at elevated temperatures, for all types of structural fasteners
11. NASM1312-31 (replaces MIL-STD-1312-31): Standard test method for torque testing threaded fasteners
12. Tension Testing per ASTM A574
13. Fatigue Testing per: NASM8831 Rev.3; MIL-S-5000 REV. E; NASM14181; and NASM85604
14. ASTM F606: Standard test method to determine the mechanical properties of externally and internally threaded washers and rivets
15. ASTM F606M: Standard test method to determine the mechanical properties of externally and internally threaded washers and rivets (metric)
Determining the proper test methods for a particular fastener application and executing them in such a way as to make sure that the actual testing does not introduce variability into the results are important parts of any good quality-control system. Practical shop tests have frequently been devised that produce highly valid results for fastener systems and should not be discounted provided there is adequate historical and field data to support the validity of the tests. IH
For more information:Contact Daniel H. Herring, “The Heat Treat Doctor,” The HERRING GROUP, Inc., P.O. Box 884, Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: email@example.com. Richard D. Sisson, Jr., is George F. Fuller Professor, Director of Manufacturing & Manufacturing Engineering, Mechanical Engineering Department, Worcester Polytechnic Institute, Worcester, Mass.
1. Herring, D. H., and Richard D. Sisson, Jr., “Testing of Heat Treated Fasteners,” Fastener Technology International, February 2010.
2. Herring, D. H., “Basics of Mechanical Testing,” Heat Treating Progress, March/April 2005.
3. Instron (www.instron.com)
4. NIST (ts.nist.gov)
5. Collaborative Testing Services, Inc. (www.collaborativetesting.com)
6. Bolt Science (www.boltscience.com)
7. Westmoreland Mechanical Testing & Research, Inc. (www.wmtr.com)
8. Test Application Handbook Relating to Mechanical Fasteners, Division VI Aerospace Fasteners, 1995
9. Stork Materials Technology (www.storksmt.com).