Fatigue life is the number of loading (stress or strain) cycles a component can withstand before failure occurs. For some metals, i.e. steels, there is a theoretical number of cycles for a given stress range. If kept below this number of cycles, there is a confidence that the component will not suffer a fatigue failure. This is known as fatigue strength or fatigue/endurance limit. SN (stress/number of cycles) curves are used to predict the fatigue strength of materials and joint types.
It is reported that over 90% of mechanical failures fail by the fatigue mode. Often with very little or no warning before fatigue failure occurs, the consequences are often considered catastrophic. Of the five main failure modes (ductile fracture, brittle fracture, fatigue, corrosion, and creep), fatigue is the second most dangerous mechanism (corrosion tops the list).
One of the most infamous incidents attributed to a fatigue failure was during the 1994 San Marino Grand Prix crash involving Ayrton Senna, who was tragically killed and was ultimately caused by fatigue failure of the steering column in his Williams-Renault FW16 race car.
When experts examined the accident, it was clear that fatigue was the main culprit. Senna had reported feeling awkward while grabbing the steering wheel, so the engineering team welded an inner tube to the steering wheel to expand it. However, microcracks formed on the steering component during the attachment of the inner tube. These cracks propagated over time, leading to the failure of the steering column during the race. Senna could not control the car, and the crash ultimately resulted in his death. This disaster sparked changes in Formula One safety regulations and led to a greater focus on car design and the materials used to prevent such accidents in the future.
The extension tube was welded before the start of the race. Fatigue cracks have occurred on the welded pipe, and the final fracture caused the loss of the steering control. Bastianini, M. (2020). Process – Ayrton The Magic. Retrieved November 2020, from http://www.ayrtonthemagic.com/pages_eng/ayrtonilpilota/incidente/processo.php
So, what is it?
Fatigue cracks initiate and propagate under repeated fluctuating loads. Fatigue failure occurs as a crack’s steady progression until the component’s remaining cross-section cannot withstand the load, and the final failure mode is either a ductile or brittle fracture.
Fatigue can occur at much lower nominal maximum stress values than the typical quoted ultimate tensile strength and yield strength for given materials. In other words, lower cyclic stresses than the material’s tensile strength may cause crack initiation and propagation in the material.
It is not just the peak stress that needs to be considered, but the stress range—the more extensive the stress range, the higher risk and lower the fatigue life that can be expected. In addition to the stress range, the type of stress, tensile or compressive, is also a factor. If the stress range includes compressive and tensile stresses, the component’s fatigue life will be further reduced.
Whilst unwelded parts can fail due to fatigue, there are several reasons why welded components have a lower fatigue life and make up the vast majority of fatigue failures. It should be noted that fatigue life does not increase with steel strength; whichever grade of steel is used in welded construction, the fatigue performance will be approximately the same, with some suggesting lower grade steels perform better as they cannot retain as much residual stress.
Fatigue failure is influenced by the following
Orientation of stress concentrations to the load.
It is widely recognised that the orientation of the stress concentration to the load is a significant influence. In the image below, if we have an undercut at the toe of the weld and the load is longitudinal, it has much better fatigue performance. When the load is transverse to the stress concentration, the risk of fatigue failure is much greater.
This is why it is suggested that grinding and sanding marks should be parallel to the loading direction, not transverse. Surface finish is a factor in fatigue performance as grinding and sanding marks are stress concentrations, so it is always preferable to sand using a fine grit disk and, where possible, ensure the sanding marks are longitudinal/parallel to the loading, not transverse. In later articles, we will discuss fatigue life improvement techniques.
Ref: Microscopic view of grind marks
In welding, residual stress is caused by the uneven heating and cooling of the material during the welding process. The residual stress can significantly affect the fatigue performance of welded structures, leading to premature failure of the structure due to fatigue cracking. The residual stress is considered a part of the total stress range when calculating the fatigue performance of a welded structure. The whole stress range includes both the applied stress and the residual stress.
Tensile residual welding stress results from the welding process, where the welding heat causes the material to expand and cool, causing it to contract. The contraction of the material generates internal stresses that remain in the material after the welding process is completed.
The tensile residual stress is directly related to the yield strength of the material. The yield strength is the stress at which a material deforms permanently. In other words, the yield strength is the point at which the material can no longer return to its original shape after being deformed. Often residual stresses are in the order of magnitude of the yield strength of the material.
The magnitude of residual stress can be affected by the welding process and the material properties. Factors such as the welding speed, the size and shape of the weld, and the type of material can all affect the magnitude of the residual stress.
High tensile residual stress has a significant effect on fatigue. The residual stress retained in the component gets added to the mean stress, and the stress range can become much more significant. The stress range (the stress range it oscillates within) is another key determining factor of fatigue life.
This is why many dynamically loaded components are post-weld treated (stress relieved) to relieve retained residual stress so it does not increase the mean stress and stress range. Whilst it is suggested by many technical authorities that PWHT does not remove all residual stress but reduces it by approximately 30%. The likes of DNV allow no extension of fatigue life if the component is stress relieved.
Stress concentrations significantly affect the fatigue life of components as soon as a part is welded, its fatigue category drops. Stress concentrations can take many forms. They can be undercut, lack of fusion, weld profile, weld toe shape, or other weld defects; a lesser-known fact is that arc welding unavoidably produces micro intrusions during solidification at weld toes.
Stress concentrations refer to areas in a structural component where the stress is significantly higher than the surrounding areas. In a welded component, stress concentrations can occur at the edges of the weld, where the component’s geometry changes abruptly. These areas are typically subject to higher stress levels and are more likely to fail under load. Stress concentrations can also occur due to defects in the weld, such as porosity or lack of fusion. To minimise stress concentrations in a welded component, it is essential to use proper welding techniques and inspect the welds for defects.
The shape and geometry of the weld toe (which is the point where the weld meets the base material) can significantly affect stress concentrations in a welded component. A smooth, rounded weld toe will have a lower stress concentration than a sharp, angular one. This is because a sharp corner or edge will create a concentration of stress at that point, making the component more susceptible to failure.
Additionally, a fillet weld with a larger size will also have a lower stress concentration than a smaller fillet weld as the stress is distributed over a larger area. The same goes for the depth of the throat of a fillet weld; a deeper throat will have a lower stress concentration.
To reduce stress concentrations at the weld toe, it is essential to use proper welding techniques to ensure a smooth, rounded toe. The welding process should be designed to create a smooth transition between the weld and the base material. The welding parameters should be carefully controlled to prevent defects such as porosity or lack of fusion.
Location of the stress concentration
Near-surface defects refer to imperfections or flaws present at or near the surface of a material, such as cracks, lack of fusion, inclusions, or porosity. They can significantly impact the strength and fatigue life of a welded component. The K factor is a parameter used in fracture mechanics to describe the stress intensity at the tip of a crack. It is defined as the ratio of the applied stress to the square root of the crack size. Surface-breaking defects, such as cracks or notches, have a higher K factor than subsurface defects because the stress intensity at the tip of a surface-breaking defect is more elevated.
Surface breaking defects are more likely to be loaded in a way that causes the crack tip to be under tensile stress, increasing the K factor. Subsurface defects are usually packed in a way that forces the crack tip to be under compressive stress, which will decrease the K factor.
Lack of fusion at the toe of a weld, undercut, micro intrusions from arc welding and other imperfections are all present at the material’s surface, therefore, having the most significant influence on fatigue performance.
To summarise, fatigue failures occur at stresses much lower than the material tensile strength due to fluctuating loads. Fatigue life is influenced primarily by stress concentrations, particularly surface-breaking weld defects such as undercut and lack of fusion. Weld toe geometry, weld profile, residual stress, and micro intrusions from arc welding are stress concentrations that cause fatigue failure to occur.
Fatigue life improvement is all about extending the fatigue life of a component.
Technoweld are experienced in increasing and improving fatigue life
There are several well-known ways, and some not so well-known, to reduce and improve the fatigue performance of welded components. Technoweld has developed the knowledge and skills to extend the fatigue life of welded components to reduce the risk of your assets failing prematurely.
In the following articles, we will discuss these methods, their effectiveness and factors that should be considered when applying them.
In the meantime, to learn more about how our team can assist you with fatigue life improvement, contact us at 1300 00 WELD or visit technoweld.com.au.