Bolts can be made from materials other than iron and steel.
In this regard, tensile strength and lower yield stress of the material were taken into account when classifying the bolts in terms of quality. In cases where the lower yield stress could not be determined, nonproportional elongation strain was taken into consideration. Bolt quality classes are stated with two numbers as follows: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9. What is meant by these two numbers is:
The first number multiplied by 100 gives the tensile strength of the material.
If the second number is divided by 10 and then multiplied by the tensile strength (which is the first digit), the lower yield stress is obtained up to 8.8, and the nonproportional elongation strain is obtained after 8.8.
Tensile Strength: It is the maximum strength per unit area that a material can endure.
Lower Yield Stress: For soft materials, there is a fluctuation in the graph when moving from the elastic deformation area to the plastic deformation area. The minimum point of this fluctuation is the lower yield stress.
Nonproportional Elongation Strain: In a graph belonging to a hard material, the above-mentioned fluctuation is not observed when moving from elastic deformation to plastic deformation; the tensile value corresponding to this switching point is called the nonproportional elongation strain or yield stress.
Example-1: What is the tensile strength of a bolt that has a quality degree of 8.8?
8x100=800 N/mm²
What is the tensile strength of a bolt that has a quality degree of 12.9?
12x100=1200 N/mm²
Example-2: What is the lower yield stress for an 8.8 bolt?
8/10x800= 640 N/mm²
What is the nonproportional elongation strain for a a12.9 bolt?
9/10x1200=1080 N/mm²
Example-3: What is the maximum Newton that an M12 bolt with an 8.8 quality class can withstand?
Tensile strength of an 8.8 bolt was stated to be 800 N/mm² in Example-1. The required strength is obtained by multiplying the tensile strength by the area of the circle.
800 N/mm² x π x R²/4= 800 N/mm² x π x (12 mm)²/4= 90,477.87 N= 90.48 kN