Bolts are widely used for assembling and joining different mechanical parts. What makes a strong connection without the use of welding or heat? If bolts are misused, they can break or be damaged. That’s where knowing bolt yield strength comes in handy. However, many people don’t know how to calculate bolt yield strength.
Remember, bolt yield strength is the maximum stress a bolt can withstand. Beyond this limit, the bolt will deform or even break, leading to failure of the joined parts. Once you know the bolt yield strength, you can use it to ensure it is used correctly. In this article, I will explain how to calculate bolt yield strength. So, let’s get started.
What is Bolt Yield Strength and Why Does it Matter?
Before going into calculations, let’s first understand the basics of bolt yield strength.
Bolt yield strength indicates the maximum stress a bolt can withstand. After this level, the bolt becomes overstressed and may deform and ultimately break. Bolts must be used within their yield strength. This ensures it does not face high pressure or stretch, which could cause the connection to break. Every size of bolt has a different level of yield strength.
As I said earlier, there are different types of bolts available with different grades. The examples include Grade 8.8 or Grade 10.9, and so on. Each of these bolts has a different grade and yield strength. Generally, Grade 8.8 has a lower yield strength than Grade 10.9. The selection of these bolt grades depends on your use and applications.
The right bolt grades help get joints between two parts that last for years. Suppose you need a very tight join with no looseness. In such a case, a higher-grade bolt will be used, as it will withstand higher stress. You’ll be able to tighten the bolt with greater force. Despite high stress, the bolt will not stretch or deform and will remain secured in its place.
Bolt generally tightens to around 70% of its yield strength. This ensures a strong connection between the two parts and a secure fastening. Additionally, when needed, you can easily remove the bolt without overloading or overclamping. 70% of yield strength also ensures the bolt does not exceed stress to permanently deform or break.
Importance of Bolt Yield Strength
Understanding bolt yield strength is very important for connections between two mechanical parts. If you don’t know the yield strength of a bolt, you’re more likely to use it incorrectly. Here are some key reasons why bolt yield strength matters:
- When connecting two parts, you’ll need bolts of different sizes. If you need a very firm connection, you’ll use a bolt with higher yield strength. This way, the bolt will withstand higher loads and keep the parts joined.
- As I said above, different bolt grades have different yield strengths. So selecting these bolt grades can be confusing if you don’t understand bolt yield strength. Once you understand the bolt’s yield strength, you can choose the right bolt grade for your needs.
- You’ll notice failures in structural components due to incorrect bolts. When such bolts face high stress, they stretch. As a result, the connection between parts becomes loose, resulting in their failure. When you know the right bolt yields strength, you choose the right bolt. This bolt will remain stable, preventing parts from failing.
How to Calculate Bolt Yield Strength?
Calculating bolt yield strength is straightforward if you know the formula. For calculating, you actually have three key parameters, which are as follows:
- Stress at the yield point.
- Cross-sectional area
Here is the exact formula you’ll use to find the yield strength of the bolt.
Bolt Yield Strength = Stress at yield point × Effective cross-sectional area
F(y) = A (t) × S (y)
F (y) is the yield force – a stress material can withstand. Beyond this limit, the bolt will either deform permanently or break. A (t) is the effective area where it faces tensile stress. As you know, every material has its unique yield strength, which is represented by S (y).
Let’s assume a case in which you have a bolt of grade 10.9. The bolt size in this case is M12. These two parameters help us understand the material’s yield strength and the cross-sectional area. For grade 10.9 bolt with size M12:
Yield strength = 940 MPa
Tensile stress area = 84.3 mm²
You can find the exact yield force by putting the values in the above formula:
F(y) = 84.3 × 940 = =79,242 N
This 79,242 N is the maximum yield force or force that this bolt can withstand. If the force exceeds this limit, the bolt can deform or break. Remember, you should use 70-80% of this force when tightening the bolt. This is the safest limit where the bolt will be tightened enough to hold the mechanical parts together. Under this stress, you’ll be able to remove the bolt as well.
Metric vs Imperial: How do they impact the Calculation of Bolt Yield Strength?
The metric and imperial systems are two different unit systems. They are used in different parts of the world. For example, the imperial system is typically followed in the United States and Canada. On the other hand, the metric system is preferred in other parts of the world, including Europe. Countries that use the imperial system follow the SAE or ASTM standards.
In contrast, metric systems rely on ISO standards. However, the formula to calculate the bolt yield strength remains the same for both of these unit systems, which is as follows:
F(y) = A (t) × S (y)
However, the value or parameters are written in different units. In metric systems;
Tensile stress area = mm²
Yield strength S (y) = MPa
Force = Newton (N)
On the other hand, units in the imperial systems are different, which are as follows:
Tensile stress area = in²
Yield strength S (y) = Psi
Force = Pounds-force (lbf)
Despite using the same formulas, you’ll get different results because you’re using different units. However, these units or values are interchangeable.
How to Know Cross-Section Area from Bolt Size?
As I said above, knowing the cross-sectional area is important for calculating bolt yield strength. However, each bolt size has a different cross-sectional area, which can be confusing. Below is the exact formula you can use to find the cross-sectional area:
A = (π ÷ 4) × d²
However, using this formula to find the cross-sectional area (Tensile stress area) can be tedious. Some ISO/SAE tables give exact cross-sectional areas for different bolt sizes. These include M6, M10, M12, and so on. This saves you from having to calculate the cross-section every time. Instead, you can refer to the table and get the exact area for different bolt sizes.
Here is the table for finding the cross-sectional areas:
| Bolt Size | Tensile Stress Area (Aₜ) |
| M6 | 20.1 mm² |
| M7 | 28.9 mm² |
| M8 | 36.6 mm² |
| M10 | 58.0 mm² |
| M12 | 84.3 mm² |
| M14 | 115 mm² |
| M16 | 157 mm² |
Yield Strength of Common Bolt Grades
Different bolt grades differ in their material composition. The bolt with a higher grade will have a higher yield strength, and vice versa. Remember, bolt grade and bolt size are two different parameters. Bolt grades only refer to the strength of the bolt and its composition.
On the other hand, bolt size refers to its size and dimensions. Two bolts with the same grade but different bolt sizes will have different yield strengths. Here is a table indicating different bolt grades and their yield strength. These grades are in ISO standards and don’t consider it fixed, as it can change with different bolt sizes:
| Bolt Grade | Yield Strength (MPa) |
| 4.6 | 240 MPa |
| 5.8 | 400 MPa |
| 8.8 | 640 MPa |
| 10.9 | 940 MPa |
| 12.9 | 1100 MPa |
As previously mentioned, the imperial unit system follows SAE standards rather than ISO standards. According to SAE standards, bolt grades are represented as Grade 2, 5, and 8. Below is the table that shows the yield strength of these grades:
| Bolt Grade | Yield Strength (Psi) |
| Grade 2 | 57,000 psi |
| Grade 5 | 92,000 psi |
| Grade 8 | 130,000 psi |
Frequently Asked Questions
What is the strength of a Grade 8.8 bolt?
Grade 8.8 bolts are considered stronger, with excellent tensile and yield strengths. The exact values of tensile and yield strength are 830 MPa and 640 MPa, respectively.
What is the difference between yield strength and tensile strength?
Yield and tensile strength are two different parameters. Yield strength indicates the level of stress beyond which materials start to deform. Tensile strength is the point (level of stress) at which a material or bolt breaks and becomes damaged.
Can a bolt be used beyond its yield strength?
No. Bolt cannot be used beyond its yield strength. If used, the bolt will deform and stretch, resulting in a loose connection. This will eventually fail the structural or mechanical components joined through the bolt.
What is the safest load for a bolt?
The safest load for the bolt is around 70-75% of its total yield strength. At this force or stress, the bolt securely holds the parts. Additionally, it remains durable longer, and removing the bolt is also easier at this optimal force or load.
Conclusion
Let’s conclude: bolts are important fasteners used to join two components. However, each bolt has its specific yield strength. If the bolt is used beyond its limit, it will start to deform. Once the bolt deforms, it will stretch and become loose. As a result, its performance will decline, and it won’t hold structural parts together.
It is worth noting that you must know the bolts’ yield strength before using them. This is even more important if the parts joined together are fragile. The reason is that if the bolt exceeds its yield strength, it will deform and break, resulting in joint failure. As a result, parts joined together will fall apart and eventually break. In this article, I have discussed everything about the calculation of yield strength.