Steels - Endurance Limits and Fatigue Stress
- Fatigue - when a material is subjected to repeated cycles of stress or strain and its structure breaks down and ultimately leads to fracture
- Creep - when a material is subjected to a load for a very long time it may continue to deform until a sudden fracture occurs
Fatigue
Fractures due to fatigue is common in cyclic loaded parts like connected rods, crankshafts, turbine blades, railroad wheels and so on. Fractures occur at stress less than the material Yield Stress.
- Yield strength is defined in engineering as the amount of stress (Yield point) that a material can undergo before moving from elastic deformation into plastic deformation.
Most steels have an endurance or fatigue limit about half the Tensile Strength.
- Tensile Strength - (Ultimate Tensile Strength) - of a material is the limit stress at which the material actually breaks, with sudden release of the stored elastic energy.
| Steel | BS | Treatment | Tensile Strength - σu - (MPa) | Endurance Limit - σe - (MPa) | σe / σu |
|---|---|---|---|---|---|
| 0.4% Carbon | BS970 080M40 | Normalized | 540 | 270 | 0.5 |
| 0.4% Carbon | BS970 080M40 | Hardened and tempered | 700 | 340 | 0.49 |
| Carbon, manganese | BS970 150M19 | Normalized | 540 | 250 | 0.46 |
| Carbon, manganese | BS970 150M19 | Hardened and tempered | 700 | 325 | 0.53 |
| 3% Chrome molybdenum | BS970 709M40 | Hardened and tempered | 1000 | 480 | 0.48 |
| Spring steel | BS970 735A50 | Hardened and tempered | 1500 | 650 | 0.43 |
| 18.8 Stainless | Cold rolled | 1200 | 490 | 0.41 |
- 1 MPa = 106 Pa = 1 N/mm2= 145.0 psi (lbf/in2)
- Fatigue limit, endurance limit, and fatigue strength are used to describe the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure.
Creep
The time dependent deformation due to heavy load over time is known as creep.
In general both stress and temperature influence on the rate of creep.Normally creep strength decreases with temperature.
Allowable creep strain can be specified - typical 0.1 %/year for steel bolts and piping
Related Topics
• Mechanics
The relationships between forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more.
• Statics
Forces acting on bodies at rest under equilibrium conditions - loads, forces and torque, beams and columns.
Related Documents
Barlow's Formula - Calculate Internal, Allowable and Bursting Pressure
Calculate pipes internal, allowable and bursting pressure.
Composition of Steels
Typical composition of AISI steels.
Compression and Tension Strength of some common Materials
Common materials and average ultimate compression and tension strength.
Engineering Materials - Properties
Some typical properties of engineering materials like steel, plastics, ceramics and composites.
Metals - Strength vs. Temperature
The influence of temperature on the strength of metals.
Metals and Alloys - Young's Modulus of Elasticity
Elastic properties and Young's modulus for metals and alloys like cast iron, carbon steel and more.
Modulus of Rigidity
Shear Modulus (Modulus of Rigidity) is the elasticity coefficient for shearing or torsion force.
Reliability of Machine Components
Mean Time Between Failure - MTB and reliability of machine components and systems.
Steel Bolts - SAE Grades
SAE steel bolts - grades ranging grade 1 to 8.2 - proof and tensile strength.
Steel Tempering Colors
The tempering process of hardening steel.
Stress
Stress is force applied on cross-sectional area.
Stress, Strain and Young's Modulus
Stress is force per unit area - strain is the deformation of a solid due to stress.
Threaded Bolts - Stress Area
Threaded bolts tensile stress area.
Threaded Rods - Loads in Imperial Units
Weight rating of threaded hanger rods.
US Bolts - Tensile Strength and Proof Loads
Tensile strength and proof loads SAE bolts.
Young's Modulus, Tensile Strength and Yield Strength Values for some Materials
Young's Modulus (or Tensile Modulus alt. Modulus of Elasticity) and Ultimate Tensile Strength and Yield Strength for materials like steel, glass, wood and many more.