6 Common Modes of Mechanical Failure in Engineering Components

Understanding Engineering Failures

Overview of Engineering Material Failures

• The tutorial introduces the topic of engineering materials and components, focusing on their failure in service and methods to prevent such failures.

• Six common mechanisms of mechanical failure are outlined: overload, buckling, impact, creep, fatigue, and wear. This serves as a recap for those familiar with the unit.

Mechanism 1: Overload Failure

• Overload failure occurs when stress exceeds the material's failure stress. This can happen due to inappropriate material selection or design calculation errors.

• Improper use of components can lead to overload failures; for example, exceeding specified pressure limits in air receivers can cause catastrophic failures.

• Manufacturers specify maximum operational pressures (e.g., 220 psi), while higher test pressures (e.g., 330 psi) incorporate safety factors against malfunctions.

• Flaws or defects in materials may also contribute to overload failures. Non-destructive testing methods can help identify these issues before they lead to failure.

• Environmental changes like increased ambient temperatures can affect material properties and potentially lead to overload failures.

Types of Overload Failure

• Ductile fractures occur under tensile loads where significant necking is observed before failure; brittle fractures show little plastic deformation prior to sudden breakage.

• The differences between ductile and brittle fractures are illustrated through UTS tests showing distinct behaviors under tension.

• Compression-related overload failures also exist; ductile materials exhibit significant plastic deformation while brittle materials fail suddenly without much deformation.

Mechanism 2: Buckling Failure

• Buckling occurs under compressive stresses in long slender structural members that do not yield solely from compression but fail due to instability.

Understanding Buckling and Impact Failure in Structural Members

Buckling of Slender Members

• Long structural members with a narrow cross-section are prone to failure due to buckling, while those with a larger cross-section relative to their length are more likely to fail from overload.

• A pin-ended structural member subjected to compressive force becomes unstable, leading to sideways deflection at the center.

• An analogy is made using a ruler held between hands; applying axial load causes bending rather than just compression, illustrating buckling behavior.

• Bending stresses can exceed the material's failure stress even when compressive stresses do not, leading to failure on the tension side of the component.

• The phenomenon of buckling results in bending stresses that surpass the material's capacity, causing structural failure.

Impact Failure Overview

• Impact failures are unpredictable due to varying forces, velocities, and impact parameters like time and direction affecting outcomes.

Types of Impact Scenarios

1. Low Velocity Collision

• A spherical object colliding with a soft component may embed into it if its velocity is low, absorbing impact energy without immediate rupture.

2. Moderate Velocity Penetration

• As velocity increases, penetration occurs where the projectile embeds but does not cause full rupture; however, the component may still be unserviceable.

3. High Velocity Rupture

• At high velocities, projectiles can exit through materials entirely causing complete rupture; shear strength plays a critical role in this process.

4. Brittle Material Response

• In brittle materials like ceramics, cracks propagate upon impact; supporting plates can prevent full rupture by absorbing some energy and spreading it over a larger area.

5. Energy Absorption Mechanism

• The amount of energy per unit area from an impacting object must exceed the toughness of the material for failure to occur.

Vehicle Crash Testing Considerations

Vehicle Safety and Material Failure

Understanding Impact Energy Absorption

• The discussion emphasizes the importance of assessing vehicle safety, particularly how materials fail under impact. It highlights that failure is often a design feature, as seen in crumple zones of cars.

• While the amount of impact energy absorbed cannot be changed, increasing the time over which this energy is absorbed can reduce the force experienced by passengers.

Design for Protection

• Components like cycle helmets and crash helmets are designed to fail upon impact, thereby protecting the wearer. Once involved in an impact, these helmets are no longer safe for use and must be replaced.

Creep Failure Explained

• Creep failure occurs when materials experience prolonged loading at stresses below their failure stress. This leads to gradual deformation over time until rupture occurs.

• The relationship between stress and strain is discussed; even with constant stress, strain increases over time due to creep until material failure happens.

Temperature Dependency of Creep

• The temperature significantly affects creep behavior; higher temperatures decrease the time to rupture under a given stress level. For example, at 649°C with 80 MPa stress, failure occurs in about 100 hours compared to much longer times at lower temperatures.

Implications of Creep on Component Interaction

• When components like bolts experience creep or plastic deformation, it can increase stress on neighboring components. This interconnectedness means that one failing component can lead to cascading failures in others.

Fatigue as a Mode of Failure

Cyclic Loading Effects

• Fatigue failure arises from cyclic loading where stresses may be lower than material failure limits but repeated cycles lead to eventual breakdown.

• The number of cycles before fatigue failure varies based on applied stress levels; smaller stresses allow more cycles before failure while larger stresses reduce cycle capacity significantly.

Stages of Fatigue Failure

• Fatigue involves three stages: initiation (where cracks first form), propagation (where cracks grow due to continued loading), and fracture (final breakage).

Understanding Mechanical Modes of Failure

Rapid Fracture in Materials

• The final stage of material failure is rapid fracture, which occurs when the area resisting stress becomes too small, equating the applied stress to the material's failure stress.

• In fatigue loading, while the cyclic force remains constant, a reduction in cross-sectional area increases stress, leading to potential failure after just one cycle.

• The process of rapid fracture includes crack initiation and propagation through beach marks, culminating in a brittle fracture at the final fractured area.

Erosion and Wear Types

Necessary Wear

• Erosion results from surface-to-surface contact; necessary wear occurs between components like brake discs and pads where friction converts kinetic energy into heat for stopping vehicles.

• This type of wear is unavoidable due to design requirements; however, it necessitates monitoring as brake pads thin over time.

Unnecessary Wear

• Unnecessary wear can lead to inefficiencies and performance degradation; long-term effects may result in seizing mechanisms within components.

• An example includes pitting wear on gear teeth caused by surface movement that breaks down material integrity, increasing friction and reducing gearbox efficiency.

Gear Wear Mechanisms

• Scuffing is another form of gear tooth wear characterized by deep scores that can significantly impact performance over time.

• Accumulated debris from worn gears can exacerbate failures within the gearbox itself, potentially requiring reconditioning or replacement once sufficient wear occurs.

Summary of Mechanical Failure Modes