Tensile Testing and Screw Strength: How Does It Work?
The wide range of screws available on the market is directly proportional to their possible and different uses. As we have seen in some other articles, the screw is a fastening element that finds a wide range of applications and, for this reason, requires in-depth knowledge.
In fact, to choose the most suitable screw for your needs, you should not only consider the diameter, length, threading, or type of head but also the mechanical characteristics such as the resistance class.
To measure this capacity, tensile tests come into play, which we have dedicated the following paragraphs to understand what they involve.
What is Determined by the Tensile Test?
The tensile test is a conventional test that measures tensile strength, or the breaking resistance, of materials subjected to a process of elongation.
It is therefore a mechanical test conducted on specimens which are subjected to an increasingly high monoaxial load. In other words, the element is subjected to forced deformation with the goal of evaluating its yield load or breaking load.
The test develops through several phases:
Elastic behavior, the 1st phase of the process where the element's configuration is still restorable. Returning the load value to zero, the element will return to its original length;
Yielding, in this 2nd phase the material's behavior is no longer linear and, consequently, an increase in load value corresponds to a slight deformation and a drop in resistance;
Plastic behavior, during the 3rd phase, the deformation becomes increasingly permanent. Indeed, even if the load value is zeroed, the material does not return to its original length;
Necking, in the 4th and penultimate phase, part of the tested element deforms quickly;
Specimen breakage, is the 5th and final phase of the process that occurs when the breaking load is reached.
The test results are reported in a stress-strain diagram showing all the steps taken and the material reactions, starting from a null value to a maximum value known as UTS (Ultimate Tensile Strength).
The UTS is the maximum stress applicable to the tested element.
Figure 1. Stress-strain diagram example
Thus, the two crucial points within the diagram and therefore the process are yielding and the breaking load.
In the following paragraphs, we will define in detail what is meant by these terms but, for completeness of information, we wanted to dedicate an in-depth analysis to further screw resistance tests.
Some Screw Resistance Tests
The main reference standards establish the minimum values to be respected for a screw to be considered standard. These values apply to both the physical characteristics of the element – measurable, for example, by using the caliper – and the mechanical characteristics.
Specifically, the latter are used to measure the screw's resistance and can be distinguished in:
Head resistance tests. The test is conducted on an inclined plane where the element's head is subjected to a breaking load relative to its diameter. The test is passed if no breakage of the head occurs;
Hardness tests. The hardness of the screws must be as homogeneous as possible, guaranteeing the use of adequate steels and correct heat treatment. The test is carried out on the surface of the screw or on a section. Using a durometer – a device capable of applying variable and increasing pressure over time – the material is indented and the size of the indentation will be measured: the smaller it is, the harder the material will be.
Microhardness tests. This test, provided for screws that have undergone a surface hardening process, is performed on the surface of the threading and on the section. Applying the same procedures as the hardness test, the test involves the screw being longitudinally sectioned. This examination, for functional reasons, is only carried out on elements with thread pitches greater than or equal to 1.25mm. If the pitch has a different value than indicated, it is not possible to proceed with the hardness test but a microscope analysis is performed.
What is Steel Yielding?
The standard definition of yield load indicates it as the stress that, in reference to the stress/strain diagram, causes a deviation from the initial length.
When a material is subjected to a force of a value lower than its yield load, it deforms but returns to its original shape as soon as the load is removed. Conversely, when the force is greater than the yield load, the material does not return to its original shape.
In general, the yield load varies depending on the material. For example, the yield load of steel is significantly higher compared to that of aluminum.
What Does Breaking Load Mean?
The term unit breaking load refers to the maximum load a material can withstand without breaking. It is thus an external stress greater than the yield load, beyond which the material fractures.
The breaking load, like the yield load, is determined through tensile tests from which different results emerge for different types of materials.
The breaking load is not defined in a uniform way but can vary depending on the mode of application.
For this reason, different tests are carried out to record the material's different behavior based on its final destination. There are therefore different types of tests, including:
Tensile breaking load;
Compressive breaking load;
Bending breaking load;
Torsional breaking load;
Shear stress breaking load.
For further useful information on tensile tests and the values related to them, subscribe to our newsletter.
Over time, classifications and nomenclatures for bolts have been defined to regulate their mechanical and physical characteristics, both at national and international levels. Specifically, the main standards for fasteners are ISO, DIN, and UNI....
In bolted connections subjected to continuous stresses, there is often a serious risk that the nut may loosen from the screw. For this reason, self-locking (or self-braking) nuts have been developed and designed....
Washers are a mechanical component that is generally placed between the nut of a bolt or under the head of a screw and the component to be locked. For many, this particular part is relegated to the task of increasing the locking of the bolted union t...
Anchor Bolts are elements used in various sectors, from construction to telecommunications, for the construction and securing of load-bearing structures. They can be made with different head and shank shapes, with various types of materials and surfa...
Washers are a mechanical item generally placed between the nut of a bolt and the object to be locked. Their main task is to distribute the clamping load over a larger surface than that covered by a bolt or screw (a very advantageous functionality wit...
Galvanizing is a process generally carried out on metallic artifacts, where a layer of zinc is applied as protection against corrosion. To achieve excellent results from the galvanizing process, it is fundamentally important to take into consideratio...
Belleville washers, a mechanical component within the washer family, are easily recognized by their distinctive conical shape, which gives the part elasticity and the capacity to absorb high loads even in a small housing space....
Tamper-proof screws, also known as security or anti-theft screws, are recognized by the special configuration of their head or type of drive. One of the advantages of this product is the vast array of combinations and options between the type of head...
Cotter pins are mechanical fastening elements that prevent a nut or pin from sliding out of its seat. There are several types of cotter pins, among the most commercially available we can find: standard, elastic, snap, and ring types....
Assembling objects, tools, or furniture is an activity that requires secure tightening, thus tightening the used elements to the right point. This activity cannot be left to chance but it is appropriate to know well the torque values of bolts, nuts, ...
