Fastener Testing

Here we speak to Cesare Certini, owner of S.M.T Srl, about the independent Italian-based laboratory’s vast knowledge and experience and how the company is able to provide the tests, machinery, and services, required by the fastener and fixing industry.

How was S.M.T Srl established and what have been the key points in its development?

“Sider Metro Tests (S.M.T) was founded in 1979 and initially carried out the calibration, repair and sale of used test machines. In 1983 I took over and a few years later I moved the company to its current headquarters in Pozzo d’Adda in the province of Milan. From here the business expanded into the mechanical processing of specimens for tests on mechanical material.

Together with my brother Piergiocondo, we made S.M.T Srl as it currently stands today. One of the key points of the development was by happy intuition, when we replaced the dials of the traction machines and the pendulums for resilience. We updated them and switched from kg force to Newton, without the need to change the machine. Doing this created a fruitful collaboration with the academic world, which enabled the business to develop know-how and to acquire new customers.

Subsequently, by paying constant attention to the needs of the market, S.M.T has developed customised software for its testing machines – making old machines more current. Another key element in the company’s growth is represented by our mechanical workshop, which is equipped with numerous machining centres that can produce specimens without altering the state of the material and therefore provide reliable results. The mechanical workshop also adds a qualitative aspect to our service and helps reduce the response time to the customer. This was especially the case after the implementation of the machine park and the expansion of the workshop area in 2014.

Added to all this, since 2005, is our sister company – CERMAC Srl – which is specialised in the maintenance and overhaul service of laboratory test equipment, such as traction machines, pendulums and durometers. The need to satisfy a constantly expanding market saw CERMAC, in 2008, transform itself and specialise in the production and marketing of its own equipment. Over the years, my three children, Laura, Elena and Paolo have joined the business and assist me in managing the company.”

What are the key tests/services you are able to offer fastener companies?

“The services we are able to offer concern the execution of a large number of tests, from tensile and impact tests, to dynamic tests, as well as chemical and metallographic tests. In addition to this, our laboratory is configured to not only carry out tests but to also be an ‘active support’ for the customer. We can do this thanks to the knowledge of our job managers, who act as a direct contact for customers and enable them to interface with the laboratory. Through this structure we are able to further understand the needs and requirements of customers and ensure we have constructive dialogue with customers aimed at optimal solutions.

Our machine park also allows us to cover a wide range of tests for fasteners in compliance with international standards. In particular, we are able to perform static tests up to M39, directly on the component, to characterise both the tensile strength and yield strength, as well as the resistance of the head by wedge test.

In the field of dynamic tests, we distinguish ourselves once again for our flexibility as we have the ability to perform fatigue tests on small-sized fasteners with a 250kN vibrophore and on large dimensions or with high mechanical characteristics, with a vibrophore 700kN.

Other fundamental tests we are able to offer regarding the characterisation of fasteners include the verification of the decarburisation of the threads – both through micrographic analysis and by means of micro-hardness; control of the microstructure of the reclaimed fasteners; as well as the checking of thread profiles.

These tests, performed in accordance with international standards, guarantee both the manufacturer and the user a high-level of product quality.”

You mention tensile and impact testing, why are these important when it comes to fasteners?

“The tensile test is a fundamental test for both those who produce and those who use fasteners. The main information that can be obtained from this test are the yield strength, the tensile strength and the elongation, which are all key parameters as they define the class to which the fasteners belong. Furthermore, they are of vital importance for designers, as they allow, along with other parameters, the choice of the optimal solution for a correct and safe realisation of a joint.

The test can be performed both on the finished part; to evaluate the ultimate tensile strength and the integrity of the transition area between the head and body of the fasteners; as well as on machined specimens to evaluate the characteristics of the material with which the fastener is made.

It is performed with uniaxial testing machines, which must comply with the quality standards required for the execution of the test. A tensile test consists of applying an axial load to a sample, causing a break. The sample must be correctly bound to the machine through the use of the most appropriate equipment, depending on the geometry and size of the sample itself. Once the sample has been correctly mounted in the machine, the operator proceeds by setting the test parameters for the execution of the test. Particular attention must be paid to identifying the correct speed of application of the load. The value of this parameter is bound by legislation and must therefore be respected so as not to invalidate the test. At the end of the test the management software returns the significant values, such as tensile strength, yield strength, modulus of elasticity and elongation.

Today, much attention is paid to misalignment because non-axial loading is one of the most common causes of incorrect tensile measurements. Even very small off-centre loading when using a load sensor or force gauge can result in measurement errors of up to 0.5%. It is therefore important to ensure that the alignment of the testing string, i.e the load cell, top test fixture, sample, and bottom test fixture, are perpendicular to one another.

Alongside tensile testing, impact testing is also very important, as a key property for a fastening element is ductility, as working conditions of fasteners are varied. It is therefore very important that during the design and use of the fasteners the following factors are taken into account – the operating environment, in particular the temperature; the state of the multiaxial stress due to the thread; as well as possible dynamic overloads during operation.

The most advanced method for studying the ductile behaviour of fasteners is the Charpy pendulum impact test with a V-notch specimen. The test is performed using an oscillating pendulum. It is very important that the machine used is calibrated in accordance with international standards.

The test is performed on specimens that have a geometry and dimensions also imposed by international standards. For instance, even the dimensions of the notch are indicted by international standards – the impact test sample can have a notch that is either a ‘V’ or ‘U’ geometry.

For the Charpy test procedure the specimen is thermally conditioned since the impact values of many metallic materials vary with temperature. The tests must therefore be carried out at a specified temperature. For temperatures other than ambient ones, climatic cells are used to obtain the desired test temperature.

Once the test conditions are reached the sample is placed in the machine, the notched surface will go against the anvil and the opposite surface will be hit by the hammer. The positioning must be done quickly to ensure the test conditions in terms of temperature of the specimen.

Moreover, the sample must be correctly positioned on the anvil.  To do this, special self-centring pliers can be used or centring devices are mounted on the pendulum. As soon as the specimen is positioned, the hammer is released, which hits the specimen. It is very important that the movement of the hammer is free from vibrations in order not to invalidate the test. The test machine will provide the absorbed energy value and an analysis of the failure surfaces of the specimen will allow the user to determine the morphology of the failure.”

How do you ensure you stay up to date with the latest requirements and standards regarding fasteners?

“We are very curious, dynamic and in step with the times. We focus a lot on the professional growth of our people as a lever for business success. This includes organising training courses dedicated to staff; participating in trade fairs; and researching relevant studies and material. We are in close contact with the academic world, moreover, we are associated with Centro Inox and AIM, the Italian Metallurgy Association – entities that allow a continuous exchange of information and experiences. This helps us understand the needs and dynamics of the market and enables us to transform according to these needs by investing in machinery, strategies and people.”

Why should a fastener company work with S.M.T?

“A fastener company should work with us because we have acquired a great deal of experience in the material testing sector and we are equipped with modern cutting-edge machines, which enables us to offer a high-quality service quickly. The extremely flexible structure of S.M.T, and the presence of a structured mechanical workshop, also allows us to customise the test to meet the individual needs of the customer. Finally, we are able to perform non standard tests using the synergy with CERMAC, our sister company, who can also manufacture customised machines, and related software, according to customers’ needs.”