Concepts of strength¶
Strength refers to the the ability of a material or a structure to withstand a load without failure.
Failure occurs when
- The applied load exceeds the elastic limit, leading to permanent deformation and/or damage, or
- The applied load surpasses the ultimate load-bearing capacity, resulting in fracture, collapse, or any other complete inability to support the load.
Strength versus resistance¶
Strength and resistance are frequently used interchangeably in mechanical engineering. However, there is a formal distinction that is worth discussing:
The common understanding of resistance is the ability of a material or structure to oppose or withstand a load. The term has a broad yet context-dependent meaning. For instance, the stiffness of a structure or the modulus of elasticity of a material may represent resistance in contexts not directly related to failure. Resistance is often measured in quantities such as forces and moments acting on structures and components, whereas strength is most commonly measured in terms of stress or other normalized quantities such as strain and strain energy density.
In TMM4175, the terms will be used interchangeably, relying on informed readers with sufficient knowledge and the ability to contextualize their usage.
To be discussed in lectures¶
Failure modes¶
Abrasive wear Brittle fracture Buckling Compressive failure
Corrosion Creep Ductile fracture Dynamic fatigue failure
Elastic instability Erosion Excessive deformation Fatigue fracture
Flexural failure Impact failure Inelastic deformation Oxidation
Plastic deformation Resonance failure Shear failure Static fatigue failure
Tensile failure Thermal fatigue Thermal shock Torsional failureMeasures of strength/resistance¶
Buckling strength Compressive strength Ductility Hardness
Stiffness Failure strain Fatigue strength Flexural strength
Fracture energy Fracture toughness Impact strength Shear strength
Tensile strength Torsional strength Toughness Resilience
Yield strengthDesign strength and safety factors¶
See also Safety and uncertainty
To be discussed in lectures:
High degree of certainty Low degree of certainty
High degree of confidence Low degree of confidence
Failure has minor consequences Failure has severe consequences
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Small SF required Large SF required
