Fastener Testing

Here John Cross, general manager at Rotech Laboratories Ltd – the UK-based leading UKAS accredited materials testing laboratory, which is also part of the wider Rubery Owen Group and RO Materials Testing Division – outlines the importance of mechanical testing and how it is used within many sectors of industry.

Mechanical testing ensures that products and manufactured goods meet particular benchmarks, such as customer requirements, as well as British/International standards.  This testing should verify the manufactured parts conform to, or do not conform to, those standards to avoid catastrophic failure. Mechanical testing is essential in design and manufacturing in establishing if a product is, or is not, fit for purpose. The consequence of not conducting these tests and fixtures failing, could lead to large-scale environmental, human and legal repercussions.

Mechanical properties describe characteristics of the material or product and determine how it will perform in the field. They form part of a series of of tests that assess the integrity and safety of components. There are several main mechanical testing areas, which will show fundamentally how a material is going to behave when subjected to various physical forces.

Tensile testing

Tensile testing is predominantly a destructive test in which a sample is subjected to a controlled tensional force until it ultimately fails. If you know when it fails, you know what forces are required to ensure that it is fit for purpose in whatever application needed. Basically, a sample is subjected to a physical force until it breaks. Tensile testing gives the Rotech team information about a sample’s strength, ductility and the forces needed to produce permanent deformation or failure.

A controlled load is applied to determine the strength of bolting or a coupling, so that when it is in the field as a fixing of two sections together, they will be safe to complete their function. The tensile strength is usually specified in MPa (megapascal) or N/mm², although this does depend on the standard and specification required. Tensile testing on materials is done on standardised test pieces turned from the supplied material.

A universally accepted property used to assess fastener components is its ultimate tensile strength, which is the maximum stress that a material can withstand in tension before breaking. However, another important property that goes hand in hand with the ultimate tensile strength is the yield or proof stress.

Before any fastener fails, it will start to plastically deform; this tells you the maximum load that a mechanical component can resist until it gets to a point of permanent plastic deformation.  At this stage the integrity of the fixture or joint would be compromised.

This yield stage is important for engineers and designers to understand, so they know what is the maximum force to which a sample can be subjected before it is irreversibly damaged.  Designers of anything in engineering applications, which require components or assemblies that need to be fixed together with fastener parts, must understand and consider this yield stage.

Some materials do not have a pronounced yield point, cold formed product being typical. For such materials, an offset yield of 0.2% (Rp0.2) is generally used to determine acceptance. The 0.2% is taken as being relative to the initial length of the specimen and generally defined by a stated gauge length.

The tensile properties of a fastener material are critical but sometimes it is also required to understand the properties of a material in its finished form. As such a significant amount of fastener tensile testing is conducted on parts that have completed all the stages of manufacture or are fully machined.

Tensile tests are also conducted on full size finished fasteners to determine their behaviour under stresses after manufacturing, as the manufacturing process itself can induce defects, stress raisers and compromise the strength of the component. Full size testing gives the customer confidence of a quality product in its finished form.

In this state the UTS and Proof/Yield stress tests can be performed as they would be on the raw material, but there are also other supplementary tests that are relevant to fasteners and fixings and how they perform in their finished fully machined condition.

Two such tests are proof load testing and wedge testing. In proof load testing a force is applied to the finished fastener to reveal if, at that required force, the component will show no or minimal plastic deformation. It can be judged by the change in length and/or continued free thread movement.

In wedge testing the head soundness of a bolt is assessed. A hardened steel wedge is positioned underneath the head. When the required tensile load is applied the screw must break in the thread or in the shank and so gives a measure of the head integrity.

The principal fastener product and test specifications worked to are ASTM A193, ASTM A962, ASTM A194, BS EN ISO 3506-1, BS EN ISO 3506-2, BS EN SO 898-1, BS EN SO 898-2, ASTM F606/M, ASTM F738, ASTM A370 and BS EN ISO 898-6.

Another test that can be applied is torque testing. This involves measuring the amount of torque being applied to a fastener. Torque is used to create tension in threaded parts. When a nut and bolt or any other fastener is tightened, generally two opposing faces are clamped together. The thread converts the torque into tension in the bolt/fastener. This then converts into a clamping force. The amount of tension created in the bolt is critical.

Bolts and fixings need a torque applied, so faces couple together satisfactorily but not to the point where the fastener’s integrity is compromised. Manufacturers need to understand that a bolt can withstand the required amount of torque without failing, yielding or cracking under the applied stresses.

Rotech works daily with international fastener companies to verify both existing parts waiting shipment and to assist on new product development and design modifications. The end users are extensive ranging from general engineering application products to petrochemical, nuclear and defence suppliers.  

Hardness testing

Hardness testing goes hand-in-hand with tensile testing in that it is a good indicator of mechanical properties of the material, although hardness itself is not a property but a characteristic of the material. It indicates a material’s ability to resist indentation and, as such, gives an indication of strength, wear resistance and toughness.

The process involves the application of a constant load on a rounded or pointed indenter to create an indentation in the material surface. The depth of penetration is then measured to provide an indication of hardness. Although it can be done as a stand-alone test, it is usually done in conjunction with other mechanical tests as an overall assessment of material properties.

There are a number of ways that hardness can be determined, usually governed by the size and geometry of the sample; the area to be tested; and ease of application. Rockwell, Vickers and Brinell are types of hardness test regularly used to assess fasteners in accordance with ASTM E18/ISO 6508, ASTM E92/ISO 6507 and ASTM E10/ISO 6506.

Many fastener specifications not only require a general surface hardness determination but also hardness assessments at the microstructural level to assess for incorrect heat treatments; incorrect grade of material; microstructural variation in the original bar stock; or excessive cross-sectional variability.  When assessing hardness at the micro level, testing is generally determined through Vickers or Knoop testing in accordance with ASTM E384/ ISO 6507.

Recently a major fastener supplier was requested to supply critical bolting for use in aggressive petrochemical applications. It would be required to have high tensile properties but combined with as lower hardness as possible – due to potential stress corrosion cracking. Working with the customer, Rotech advised on thermal treatments to achieve the desired properties but also conducted the necessary mechanical tests, to provide an independent UKAS test report to gain acceptance by the end user and secure further orders.

Impact testing

Impact testing is an important test that indicates the toughness of a material by determining the energy absorbed when attempting to fracture a standard test specimen by pendular fracture technique. Although tensile strength indicates how much force the material can withstand, it is the toughness test that will show how much energy a material will absorb before fracturing. In the process, a universally standardised notched test piece is placed on an anvil and using a large pendulum of a known length and design, the swinging force hits the sample. The energy needed to break the sample gives us the measurement of how tough the material is. It is usually expressed in Joules.

The two main recognised measures are either Charpy testing to ASTM A370/ISO 148 or Izod testing to ASTM D256/BS 131, which determine the energy absorbed in a pendular test fracture technique. The predominant method is usually the Charpy V-Notch test. According to ASTM A370/ISO 148 the standard specimen size for Charpy impact testing is 10mm × 10mm × 55mm, although subsize specimen sizes can be used in accordance with the standard if material size is a limiting factor.

The appearance of a fracture surface also gives information about the type of fracture that has occurred. If the material fractures on a flat plane, it would be judged as brittle appearing bright/crystalline. If the material fractures showing a cleaved face, dull/fibrous, with jagged edges or shear lips, it would be judged as ductile. Although material tends to not break in just one way or the other, so in comparing the jagged to flat surface areas of the fracture you can determine a percentage of crystallinity or brittle fracture.  Percentage crystallinity is sometimes a reporting requirement.

Lateral expansion is also another indicator of the ductility of a material. As a specimen piece is broken during testing the material deforms and is pushed out on the fractured face. The amount the specimen deforms in this way can be measured and is expressed as millimetres of lateral expansion. When impact testing, the absorbed energy (in J) is always reported, but the percentage crystallinity and lateral expansion is not always required, it is dependent on the specification.

Fatigue and thermo-mechanical fatigue testing

As assessing the general mechanical properties of fastener components helps provide reliability and quality assurance regarding a product, so does fatigue testing. Fatigue testing provides an understanding of how a component behaves with the application of cyclic loading. In extrapolating the test data, we can identify the critical failure area within any material. 

Our sister laboratory at Phoenix Materials Testing has the latest fatigue testing technology and any required accessories and instrumentation, with machines that range from 1kN to 1,600kN.

The role of TMF testing (thermo-mechanical fatigue) is to understand how a material behaves once temperature is added to create a more aggressive environment.  The temperature addition comes in the form of induction heating, furnaces and perhaps infrared emitters. We can conduct such testing via Phoenix Materials Testing and can determine the maximum temperature that any material can endure before it fails.  Our Phoenix engineers can measure temperatures up to 1,000°C for UKAS testing but up to 1,400°C in non-UKAS applications.

“Many TMF projects we work on require additional in-house designed features to ensure that a sample is ready for testing in temperature-controlled environments. This may include specially made grips or fixtures to make sure that any sample can be mounted into the test machine.  We also have an on-site TMF software suite with in-house experts who work in close partnership with our customers.

Our TMF testing up to 1,000°C is conducted on a dedicated 160kN hydraulic test machine, which features thermal imaging high temperature extensometry, environmental control, forced air cooling, induction heating and an emissivity compensating pyrometer. We also have the benefit of being able to phase the heating and cooling profiles in and out of sync with additional control modes (i.e load, strain) if necessary. 

For more information about Rotech Laboratories, please go to https://www.rotechlabs.co.uk/ or speak to the Rotech experts on +44 (0) 121 505 4050, quoting Fastener + Fixing