BIZOMAT MAKEOS · Fasteners
Bolted Joint Fatigue Check
Will the bolt in this preloaded joint survive the fluctuating external load?
Alternating and mean bolt stress for a preloaded tension joint under a load cycling between zero and a maximum, and the Goodman fatigue safety factor using the bolt’s endurance strength.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Tensile stress area | mm² | Tensile stress area of the bolt (see the metric thread tool). |
| Preload | kN | Installed preload. |
| Maximum external load (cycling from zero) | kN | Peak external tensile load per bolt; the load is assumed to cycle between zero and this value. |
| Joint constant C = kb ÷ (kb + km) | Joint constant from the bolted joint tool (stiff joints have small C). | |
| Bolt endurance strength (fully corrected) | MPa | Fully corrected endurance strength of the bolt, including thread effects (published values for rolled threads are about 129 MPa for class 8.8, 162 MPa for 10.9). |
| Bolt tensile strength | MPa | Minimum tensile strength of the bolt. |
| Required fatigue safety factor | Required factor of safety against fatigue. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Tensile stress area
- What it is
- The tensile stress area of the bolt thread, the effective area used to work out bolt stress, from thread standards or the metric thread tool.
- Units and format
- Enter a number in mm²: square millimetres. The form accepts 0.000001 mm² to 100000 mm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 84.3 mm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- All bolt stresses are inversely proportional to this area.
- Get it right
- Use the tensile stress area, not the shank area or the minor-diameter area.
What this means: Preload
- What it is
- The tension in the bolt after tightening, before the external load is applied, from the tightening method and torque.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 35 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Tightening torque gives only an approximate preload; allow for scatter in the installation method.
What this means: Maximum external load (cycling from zero)
- What it is
- The largest external tensile load on one bolt. The tool assumes the load cycles from zero up to this value and back.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The alternating bolt stress rises in direct proportion to the load.
- Get it right
- This is the load per bolt, not the total load on the joint; divide by the number of bolts sharing it.
What this means: Joint constant C = kb ÷ (kb + km)
- What it is
- The share of the external load that the bolt takes, set by bolt stiffness divided by the sum of bolt and clamped-member stiffness. The bolted joint tool calculates it.
- Units and format
- Enter a plain number with no unit. The form accepts 0.01 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The alternating bolt stress rises in direct proportion to the joint constant.
- Get it right
- Stiff clamped parts give a small constant; gaskets or soft members raise it a lot.
What this means: Bolt endurance strength (fully corrected)
- What it is
- The endurance strength of the bolt itself, fully corrected for thread stress concentration and the way the thread was made (rolled or cut), from published fastener fatigue data.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 129 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the fully corrected bolt value, not the endurance limit of a smooth test piece of the same steel.
What this means: Bolt tensile strength
- What it is
- The minimum specified ultimate tensile strength of the bolt for its property class or grade, from the fastener standard.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 830 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use tensile strength, not proof or yield strength.
What this means: Required fatigue safety factor
- What it is
- The minimum Goodman safety factor against fatigue failure that your design standard or customer specification requires.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- σa = C P ÷ 2At; σm = σa + Fi ÷ At; σi = Fi ÷ At.
- Goodman (load line from preload): n = Se (Sut − σi) ÷ [Sut σa + Se (σm − σi)].
- Example: the default joint has σa = 11.862 MPa and a fatigue factor of 4.7039.
Limitations
- Concentric joint without separation; bending and prying raise bolt stress amplitudes.
- The endurance strength must already include thread stress concentration.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check cylinder head, flange and con-rod bolts under cyclic loads
- Show how preload and joint stiffness protect the bolt
- Compare rolled and cut threads
Who it is for
- Design engineers
- Maintenance engineers
What you get
- Alternating and mean bolt stress
- Preload stress
- Goodman fatigue safety factor
Interpreting the result
- A stiffer joint (smaller C) or a softer bolt (longer, reduced shank) cuts the alternating stress.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1535. Topic reference; no source tables or recommended settings are reproduced.
- VDI 2230 Part 1
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Bolts that cycle: fatigue in tension joints
In Bolted Joints by the Numbers
What it covers: How a cyclic external load becomes a small alternating stress in a well-preloaded bolt, why separation and soft members destroy that protection, and how to check bolt fatigue.
Method explained here · Library collection
Bolted joints that stay tight: preload, torque, fatigue and locking
In Machine Elements and Mechanical Design
What it covers: A bolt holds a joint by clamping it. How preload works, why torque is an unreliable proxy, how to resist fatigue and loosening, and how to specify bolted joints that last in service.
Method explained here · BIZOMAT original book
Bolted joints: preload is the defence
In Metal Fatigue by the Numbers
What it covers: Why a properly preloaded bolt sees only a fraction of a cycling load, how to check bolt fatigue by Goodman from the preload, and why a turn-of-nut rule failed on a short, stiff joint.
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