Fatigue failure happens when the bolts have not been tightened properly, or have loosened up during its service life. If enough force is acting on the loosened joint during use of the product, bending stresses can weaken the fastener, eventually causing it to fail. This can normally be diagnosed by a fastener expert by close examination of the broken fastener and the mating components. A third, less common type of failure is caused by hydrogen embrittlement.
This type of failure is considered a delayed failure and will always happen after assembly. The hydrogen embrittlement time to failure is typically within 48 hours. The break will almost always be directly under the head of the fastener and not in the threads. The head may break off completely, or it may simply crack enough to relieve clamp load, and remain attached. For cast materials and aluminum, this is far more challenging.
You will need 2X the diameter for engagement plus you will need to skip using the first 2 full threads. Common solutions are to change the design to use a nut, add a thicker washer to the head or make a counter-bore into the top material for a socket head capscrew. This problem is the least common and often leaves engineers scratching their head wondering what is going on.
This was also a case of poor thread engagement as it was only mild steel. We took the machine out for a drive and the screws loosened. They loosened up again. We were all stumped. If you look at the governing equation, the one I think that all engineers have memorized but rarely use, you will see the answer.
We cannot change P, A or E in most cases unless we add fasteners or change the size, so we need to change L. If we lengthen L, we increase the minute deflections allowable under the given preload.
Fasteners are not designed for bending loads. A fastener has a relatively small diameter in comparison to what is being joined. It is foolishness to design a fastener to take a bending load, but all so often bolts fail in this fashion. A telltale sign of this type of failure is seeing a large asymmetrical velvet section on the break. The strength of a fastener comes from its area and not the area moment of inertia.
As an engineer make your joints a combination of axial and shear loads. The formulas for calculating he stress are as follows:. Yes, both are a function on the area! This is how we get maximum strength from the fastener. Quite simply, the figure on the left shows an upward force on the right side and the top material barely larger than the washer.
This load will pry the parts apart and cause a slight slope to develop under the head of the screw. The slope with unevenly load the screw causing it to have higher tensile load on the side closet to the load, aka bending.
To avoid this, change load on the fastener from a moment to one part of a couple. In the figure on the right, there is much more space on the opposite side of the load. The more space allowed, the more like a couple it is. There are no hard and fast guidelines as to the proportions, but the further they are apart, and the closer the fastener is to the load, the better.
One thing to consider is the thickness of each part. If the top plate is too thin, it will deflect too much and you basically end up with the case on the left anyway. It almost goes without saying that a fastener will fail prematurely if it is installed incorrectly. There are usually several obvious causes to this:. Try to solve these problems first. If you have multiple fasteners in a joint, try to have both plates CNC cut laser, plasma so that they will lineup every time.
If you are mating several surfaces in a joint, can these surfaces be held better in a welding process? Perhaps the entire joint needs to be machined after welding. I will give you one glimmer of hope. This type of thing is already accounted for in testing standards. If you imagine doing a standard tension test on a fastener, you would expect the bolt and nut to be on surfaces parallel to each other.
This wedge test requires an angle to be put on the side of the part to be tested. These plates are not parallel anymore and the bolt will now have a bending component. The standard requires different angles based on the diameter of the screw.
This is where most failures occur. Since the analysis for both are the same, we will tackle them together. In design, I like to be a little more conservative and look for a design factor from hand calculations. This usually keeps me farther away from low preload issues without causing excessively larger or a high number of fasteners needed. Most bolts fall into this category. The remaining fasteners fall into the, engineered fasteners category.
An example of this is a rotation bearing with a crane or the gearbox that causes it to rotate. For fasteners like these, it would be best suited to aim no higher than 75 percent of the rated proof load. The reasoning behind this is that we do not want to go too low which forces us to use a larger bolts or higher grades. Also, we do not want to have a situation where the bolt is overloaded and be at a disadvantage due to stretching the bolt which in turn creates a propensity for it to loosen.
If and when this occurs, it can cause another bolt near it to pick up the extra load. This is generally undesirable. With fasteners, the tensile area is not equal to the area of the minor diameter. No matter where you take a section, you will find that the cross section will contain some thread and that area should be counted in your calculation.
However, when the thread is included, an area of 0. This is 9. The other thing to consider is that we are applying torque to a bolt and not directly applying a tensile load. This introduces data spread into our system that needs to be accounted for. Unfortunately, we need to introduce statistics into our calculations.
The value of K is the most difficult to estimate making a very simple equation complex. The general range is 0. It is important to note that the strength and security of bolted connections are largely dependent on the functioning of a bolted joint. Though far from a common occurrence, bolt failures do happen. Overstressing is perhaps the most common cause for bolt failures, and the easiest to determine as well.
The longevity of your bolts, in their functioning at full capacity, depends significantly on their bearing the right amount of stress without having to withstand an excess amount.
There are three main forms of stress on bolts:. Bolts seldom last you a lifetime, and they are prone to failure caused by fatigue. The efficiency of bolts diminishes over prolonged use. As is the case with excess stress, the application of internal and external forces are factors that determine the failure or strength of a bolt - they can be estimated roughly, according to the number of load-preload cycles the bolt is put through.
Bolts need to be replaced every couple of years, mainly due to fatigue. This is especially crucial when you are dealing with projects where stress levels are regularly high. As bolts are exposed to substances that are incompatible like oxygen, metal products, and even naturally-blending chemicals, they are prone to the risk of corrosion. Bolts used to secure automobile components are in danger of being exposed to fluid and engine leaks, causing them to deteriorate over time.
Corrosion also occurs in structural, construction, and mechanical applications. Pre-applications specially designed for specific types of fasteners should be used in order to safeguard your application against corrosion. This is a fairly simple and economical method that also happens to be highly efficient. With the tightening of a bolt, as the pairing of two threads occurs, a certain amount of shear stress gets added to the threaded part of the bolt.
Thread stripping is always a risk, where the section shears because of an excess amount of stress. To avoid joint failure arising from this phenomenon, you should assess the possibility and scope of damage well before using the bolt, and take measures to ensure that the threading is utilised accordingly.
In certain cases, the design of a bolt fails to take into consideration the possibility of excess force acting on the bo;t; i. For example, if a bolt is capable of handling one tonne of force, but its design is in such a way that 1. It is imperative, therefore, that the design of a fastener takes into consideration the amount of force acting on a bolt.
Bolt metal can become brittle from being exposed to atomic hydrogen - this is a difficult challenge to overcome.
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