6.1 Failure Modes and Criteria
2 min read•august 6, 2024
Failure modes in mechanical engineering are crucial to understand when designing components. From yielding and fracture to fatigue and , each mode affects materials differently. Knowing these helps engineers prevent catastrophic failures and ensure safety in their designs.
Material properties like stress-strain behavior and strength characteristics are key in predicting how materials will respond under load. Understanding these properties allows engineers to select the right materials and design parts that can withstand expected stresses without failing.
Failure Modes
Types of Failure
Top images from around the web for Types of Failure
Bridge failure - All this View original
Is this image relevant?
Stress–strain curve - Wikipedia View original
Is this image relevant?
Bridge failure - All this View original
Is this image relevant?
Stress–strain curve - Wikipedia View original
Is this image relevant?
1 of 2
Top images from around the web for Types of Failure
Bridge failure - All this View original
Is this image relevant?
Stress–strain curve - Wikipedia View original
Is this image relevant?
Bridge failure - All this View original
Is this image relevant?
Stress–strain curve - Wikipedia View original
Is this image relevant?
1 of 2
- Yielding occurs when a material permanently deforms under stress exceeding its
- Fracture happens when a material breaks or separates into two or more pieces due to applied stress (brittle materials like glass)
- Fatigue is the progressive, localized structural damage that occurs when a material is subjected to cyclic loading (repeated bending of a paperclip)
- Creep refers to the tendency of a solid material to slowly deform permanently under the influence of persistent mechanical stresses, especially at high temperatures (sagging of a plastic ruler over time)
- is a sudden sideways or lateral deflection of a structural member under compressive stress, causing it to lose its load-carrying capacity (collapsing of a thin metal column under weight)
Surface Degradation
- is the erosion of material from a solid surface by the action of another surface through mechanical interaction or chemical reactions (abrasion of a metal gear against another gear)
- is a process where a material, usually a metal, electrochemically reacts with its environment causing deterioration and loss of material properties (rusting of steel exposed to moisture and oxygen)
Material Properties
Stress-Strain Behavior
- graphically represents the relationship between stress (force per unit area) and strain (relative deformation) of a material under load
- : material returns to its original shape when the load is removed
- : material permanently deforms and does not return to its original shape
- is the maximum stress a material can withstand before breaking or failing ( on stress-strain curve)
- is the stress at which a material begins to plastically deform and no longer behaves elastically ( on stress-strain curve)
- Determined by the 0.2% offset method: a line is drawn parallel to the elastic portion of the curve at 0.2% strain; the stress where this line intersects the curve is the yield strength
Strength Properties
- measures a material's resistance to pulling forces that try to stretch it apart (steel cable suspending a bridge)
- measures a material's ability to withstand pushing forces that try to crush it (concrete pillar supporting a building)
- is a material's resistance to forces trying to slide one part of the material over another (bolted joint holding two metal plates together)
- quantifies how much twisting force a material can handle before failure (drive shaft transmitting power in a vehicle)
Key Terms to Review (17)
Buckling: Buckling is a structural failure mode that occurs when a member subjected to compressive stress deforms and ultimately collapses, often suddenly and without significant plastic deformation. This phenomenon is critical in the design of slender structural elements, where stability is compromised under axial loads, making it essential to understand its implications in various stress contexts.
Compressive Strength: Compressive strength is the capacity of a material to withstand axial loads without failure, characterized by its ability to resist compression until yielding or fracturing occurs. This property is crucial in determining how materials behave under loads, especially in structures where compression is a primary mode of stress. Understanding compressive strength helps engineers select appropriate materials and design components that can safely support anticipated loads.
Corrosion: Corrosion is the gradual destruction of materials, usually metals, through chemical reactions with their environment. This process often leads to material degradation, affecting the integrity and performance of structures and components, making it a significant factor in assessing failure modes and criteria.
Creep: Creep is the gradual and time-dependent deformation of a material under constant load or stress, particularly at elevated temperatures. This phenomenon is critical in understanding how materials behave over time when subjected to prolonged exposure to stress, especially in high-temperature environments. Creep can lead to significant changes in material properties and can ultimately contribute to failure modes in structural applications.
Elastic Region: The elastic region refers to the range of stress and strain in which a material can deform elastically, meaning it will return to its original shape and size once the applied load is removed. Within this region, the deformation is reversible, and the relationship between stress and strain is typically linear, following Hooke's Law, where stress is directly proportional to strain until the yield point is reached.
Fracture failure: Fracture failure refers to the separation of a material into two or more pieces due to stress exceeding its strength, leading to a loss of structural integrity. This phenomenon is critical in understanding how materials behave under load and can significantly impact the safety and performance of mechanical systems.
Plastic region: The plastic region refers to the part of a material's stress-strain curve where the material undergoes permanent deformation under applied load. In this phase, the material does not return to its original shape once the load is removed, indicating that it has exceeded its elastic limit. Understanding the behavior of materials in this region is crucial for evaluating failure modes and criteria in engineering design.
Shear Strength: Shear strength is the maximum stress a material can withstand when subjected to shear forces before failure occurs. It plays a crucial role in understanding how materials behave under various loading conditions, impacting design decisions to ensure structural integrity and reliability.
Stress-strain curve: The stress-strain curve is a graphical representation of the relationship between stress (force per unit area) and strain (deformation) in materials when subjected to loading. This curve is crucial in understanding material behavior under different load conditions, revealing important characteristics such as elastic and plastic deformation, yield strength, ultimate tensile strength, and failure points.
Tensile strength: Tensile strength is the maximum amount of tensile (pulling) stress that a material can withstand before failure or breaking. Understanding this property is crucial for engineers as it helps in selecting the right materials for structures and components that will face pulling forces, ensuring safety and performance in various applications.
Tensile Strength: Tensile strength is the maximum amount of tensile (stretching) stress that a material can withstand before failure. This property is crucial as it helps in determining how materials will perform under load, impacting design decisions and material selection across various applications. Understanding tensile strength is essential for predicting how components will behave when subjected to forces, and it informs decisions on safety, durability, and efficiency in engineering applications.
Torsional strength: Torsional strength refers to the ability of a material or structural element to withstand twisting forces without failing. This property is crucial in applications where components are subjected to torque, as it determines how much load a material can handle before it experiences torsional failure, which is often characterized by shear stress leading to deformation or fracture.
Ultimate strength: Ultimate strength is the maximum stress a material can withstand before failure occurs. This key property is crucial when analyzing the behavior of materials under load, as it indicates the limit of strength that a material can handle while being deformed. Understanding ultimate strength helps in designing safe structures and components by ensuring they can endure expected loads without failing.
Ultimate Strength: Ultimate strength is the maximum amount of stress a material can withstand while being stretched or pulled before failing or breaking. It is a crucial concept in understanding how materials behave under load, particularly in assessing their failure modes and determining safe design limits.
Wear: Wear refers to the gradual removal of material from a solid surface due to mechanical action, such as friction or contact with another surface. This phenomenon is critical in understanding how components perform over time, especially in terms of their longevity and reliability. Different types of wear mechanisms can lead to various failure modes, significantly affecting the performance of elements like bearings and other moving parts.
Yield Strength: Yield strength is the stress at which a material begins to deform plastically, meaning it will not return to its original shape once the load is removed. This property is crucial because it indicates the maximum stress that a material can withstand without undergoing permanent deformation, impacting how materials are selected and used in engineering applications.
Yield strength: Yield strength is the amount of stress that a material can withstand without undergoing permanent deformation. It marks the transition from elastic behavior, where the material returns to its original shape after the load is removed, to plastic behavior, where permanent changes occur in the material's shape. Understanding yield strength is crucial for evaluating how materials perform under loads, predicting failure modes, ensuring safety, and making informed design choices.