Fastener Testing

Here, Volker Hiller, head of business unit mobile testing at Foerster Group, explains the importance of hardness testing for fasteners and how the Group is staying ‘one step ahead’ with its UCI measuring technology for the mobile hardness testing of fasteners – providing maximum flexibility and reliable results.

The non-destructive UCI test method (Ultrasonic Contact Impedance) was first used in 1965 under the name ‘SonoDur’ and the process electronically evaluates and digitally displays the Vickers hardness test indentation within fractions of a second, by a frequency shift. Due to the small indentation size and depth, the process is considered to be non-destructive.

Why test for hardness?

Screws and fasteners must hold components together securely under a wide range of mechanical loads, with environmental conditions placing additional demands on the material properties, structure and surface condition. In addition, there are standards and regulations that fastener manufacturers must take into account, as well as material mix-ups when the raw material is received. This is where surface defect inspection for cracks plays an important role. 

The final product is tested for strength, which is usually determined destructively on a selection of parts in a tensile testing machine. The bolts are sorted into property or safety classes with corresponding markings according to the standard. These markings are very extensive, as the many different types of bolts suggest. 

Structural steel screws, for example, are divided into strength classes according to tensile strength and yield strength – with a uniform designation on the screw head or on the screw shaft. Since the tensile strength correlates positively with the hardness of the material, and empirical conversion rules exist – e.g according to EN ISO 18265, the hardness test is also repeatedly used for quality assessment for practical reasons. This is particularly important when a larger number of screws or bolts have to be inspected and/or the geometric dimensions are opposed to a tensile test. Although the classic Vickers, Brinell and Rockwell hardness test is more flexible, it usually requires complex surface processing or the bolt must be cut up and frequently embedded for the testing process. 

The situation becomes particularly volatile if, despite all precautionary measures, safety relevant bolts are already installed in the vehicle or on the construction site, for example, and subsequent removal is very difficult. This is where hardness measurement with UCI comes into play for confusion testing, as the measurement can simply be carried out directly on the installed part in any direction quickly and safely.

How is hardness testing done?

Screws and bolts are manufactured so they are optimised for the application. This includes surface coatings against corrosion or for optimum sliding properties and even local hardness or strength properties through thermal treatment. Thus, at predetermined points, such as bolt head, shaft, and end, the core hardness and surface hardness must be tested with defined test forces between HV 0.3 and HV 10. 

Accordingly, the hardness testing methods must be adjusted to these tasks. Before measuring the material, surface coatings must be removed. Measurements on the shaft are difficult for Vickers hardness testing because the surface usually must be ground flat to properly capture the diagonals of the indentation. The test location on the screw must also be easily accessible, which often requires disassembly of the screw due to the complex set-up of a testing machine. Finally, the screw must then also be correctly aligned (embedded and polished) in the case of a conical body, because otherwise, depending on the pitch, the optics cannot be focused well enough to evaluate the test indentation.

Depending on the requirements, various test methods are available. The most common methods include Martens (universal hardness), Vickers (HV), Rockwell (HR), Brinell, Knoop, Shore, Barcol and Buchholz.

One step ahead with UCI measuring technology

The essence of UCI hardness testing is the measurement of the acoustic impedance of a body via the frequency shift of a rod vibrating in resonance with Vickers diamonds. The hardness of the material is determined from the frequency shift (Δf) at a certain predefined test force. 

Here, the Vickers hardness scale forms the reference for each material. The UCI devices are adjusted to materials with a modulus of elasticity of approximately 210 GPa and can be precisely set to the new task by means of a comparative measurement with a Vickers hardness testing device. This is needed only once because the setting data can be retrieved from the memory again in a reproducible manner.

Thus, screw testing with UCI is very easy and fast through one time comparison with the Vickers scale. Since the UCI probes are pin shaped with thin test tips, they are suited for hardness testing in hard to reach positions.

Nevertheless, the boundary conditions from the application of the UCI method must be observed at this point. The surface must be clean or polished, but not necessarily bright. In addition, the measurement location must be accessible for the probe tip. The component must also be solid and decoupled from possible component resonances. Resonances can usually be recognised by strongly scattered and excessively high or much too low measured values, which are far outside the expected range.

Shafts can still be measured up to 3mm in diameter without special precautions, according to standard DIN 50159-1. With test forces between HV 0.1 UCI (motor measuring probes) and HV 10 UCI (handheld measuring probes), screws can be measured as a whole or also embedded, both manually and in a stand. 

Due to the rapidity of UCI measurement, combined with its high precision, a large number of measurements are possible compared to classical methods. As a result, entire measurement series can provide a detailed picture of the treatment result achieved by scanning the shaft.

A suitable tool for hardness testing 

Foerster Group’s product portfolio includes tools for mobile on-site hardness testing, as well as automated mass production. In addition, the Group offers hand and motor probes of the SONO series with different test forces from 1N to 98N, according to the UCI method, as well as hand sensors for different applications with the Leeb rebound method.

Among the UCI hardness testers, the SONODUR 3 for mobile testing is predestined for difficult testing tasks. It currently has the widest range of probes and accessories in the market. The mobile SONODUR 3 is robust, suitable for industrial use and specially designed and built for rough everyday use. It has a high functionality level similar to that of modern smartphones. The powerful energy concept also has up to eight hours of battery life in continuous operation.

Using the touchscreen, extensive series of measurements can be quickly searched for faulty measurements and evaluated –SONODUR 3
will locate the suspicious measured value. In addition, both the test management (conversion tables, CAL adjustment function and penetration time) and the documentation possibilities (data formats CSV, .txt, HTML, wireless, USB, photo) form a strong argument for the use of SONODUR 3.

The mobile high-tech all rounder is used for quality control in incoming goods and during production, as well as for rapid hardness testing of metallic materials after heat treatment or surface processing. The system is also excellently suited for hardness testing with positions that are difficult to access and for mobile weld seam testing in confined spaces. The layer hardness measurement with several measuring points and its easy adjustment functions on almost all fine grained homogeneous materials round off the SONODUR 3’s range of applications.