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Questions and Answers
What happens to the crack growth rate as the crack length increases?
What happens to the crack growth rate as the crack length increases?
The limiting value of ΔK below which there is measurable crack growth is called ΔKth.
The limiting value of ΔK below which there is measurable crack growth is called ΔKth.
False
What does the variable 'ΔK' represent in the context of fatigue crack growth?
What does the variable 'ΔK' represent in the context of fatigue crack growth?
Stress intensity factor range
When 'a' is small, d a/dN is also small, but d a/dN increases with increasing ________ length.
When 'a' is small, d a/dN is also small, but d a/dN increases with increasing ________ length.
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Match the following terms with their definitions:
Match the following terms with their definitions:
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What does each full rotation of the specimen in the fatigue test represent?
What does each full rotation of the specimen in the fatigue test represent?
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Crack initiation occurs at a rapid pace during the fatigue process.
Crack initiation occurs at a rapid pace during the fatigue process.
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What is the effect of surface roughness on fatigue strength?
What is the effect of surface roughness on fatigue strength?
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The specimen used for measuring fatigue crack propagation is __________.
The specimen used for measuring fatigue crack propagation is __________.
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Match the following factors with their effects on fatigue strength:
Match the following factors with their effects on fatigue strength:
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Which of the following phases occurs after crack initiation in the fatigue process?
Which of the following phases occurs after crack initiation in the fatigue process?
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Ductile failure occurs when the remaining cross-sectional area is sufficient to withstand the stress.
Ductile failure occurs when the remaining cross-sectional area is sufficient to withstand the stress.
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What is the primary reason for a reduction in fatigue strength?
What is the primary reason for a reduction in fatigue strength?
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Which of the following is NOT a type of failure in materials?
Which of the following is NOT a type of failure in materials?
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Ductile fracture occurs without any plastic deformation.
Ductile fracture occurs without any plastic deformation.
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What is defined as the formation of new surfaces under stress?
What is defined as the formation of new surfaces under stress?
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Ductile fracture is marked by extensive __________ deformation.
Ductile fracture is marked by extensive __________ deformation.
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Match the types of fracture with their characteristics:
Match the types of fracture with their characteristics:
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What is the direction of crack propagation in ductile fracture?
What is the direction of crack propagation in ductile fracture?
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Failure is defined as the ability of a component to perform its intended function safely.
Failure is defined as the ability of a component to perform its intended function safely.
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What are the three stages of ductile fracture in a tensile specimen?
What are the three stages of ductile fracture in a tensile specimen?
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What characterizes brittle fracture?
What characterizes brittle fracture?
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Brittle fracture primarily propagates between grains at the boundary.
Brittle fracture primarily propagates between grains at the boundary.
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Name one factor that can lead to brittle fracture.
Name one factor that can lead to brittle fracture.
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Brittle fracture occurs under a stress normal to the __________ plane.
Brittle fracture occurs under a stress normal to the __________ plane.
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Match the following stages of brittle fracture to their descriptions:
Match the following stages of brittle fracture to their descriptions:
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Which of the following metals is likely to exhibit brittle fracture at room temperature?
Which of the following metals is likely to exhibit brittle fracture at room temperature?
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Brittle fractures occur with advanced warning signs.
Brittle fractures occur with advanced warning signs.
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What is one of the dangerous characteristics of brittle fracture?
What is one of the dangerous characteristics of brittle fracture?
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Which phase of creep shows a decreasing creep rate over time?
Which phase of creep shows a decreasing creep rate over time?
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Tertiary creep is associated with a constant creep rate.
Tertiary creep is associated with a constant creep rate.
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What does a creep test measure?
What does a creep test measure?
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The minimum creep rate necessary for a given temperature is _____%/h.
The minimum creep rate necessary for a given temperature is _____%/h.
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What happens during the tertiary stage of creep?
What happens during the tertiary stage of creep?
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Match the following parameters related to the Larsen-Miller parameter:
Match the following parameters related to the Larsen-Miller parameter:
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The time for stress rupture increases with higher stress and temperature.
The time for stress rupture increases with higher stress and temperature.
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What is the purpose of a creep rupture test?
What is the purpose of a creep rupture test?
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In the fatigue life calculation, what does ΔK represent?
In the fatigue life calculation, what does ΔK represent?
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The equation Nf = (Ayσπ(-a2 + 1)) assumes that Y is dependent on crack length.
The equation Nf = (Ayσπ(-a2 + 1)) assumes that Y is dependent on crack length.
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What does 'A' stand for in the fatigue life calculation equation?
What does 'A' stand for in the fatigue life calculation equation?
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To calculate the number of fatigue cycles, Nf, you need to integrate from the initial crack size a0 to the final crack size _____ .
To calculate the number of fatigue cycles, Nf, you need to integrate from the initial crack size a0 to the final crack size _____ .
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Match the variables in the fatigue life calculation with their meanings:
Match the variables in the fatigue life calculation with their meanings:
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Which equation represents the relationship between ΔK and other factors in the fatigue life calculation?
Which equation represents the relationship between ΔK and other factors in the fatigue life calculation?
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The fatigue life calculation includes integrating the crack growth equation from initial to final crack sizes.
The fatigue life calculation includes integrating the crack growth equation from initial to final crack sizes.
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What is the role of 'σ' in the fatigue life calculation?
What is the role of 'σ' in the fatigue life calculation?
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Study Notes
Chapter 7: Mechanical Properties of Metals - II
- Failure of Materials and Components: Failure is the inability of a component to perform its intended function safely and reliably. Failure can manifest as yielding, fracture, buckling, wear, and corrosion.
- Fracture: Fracture is a catastrophic failure mode characterized by the formation of new surfaces and separation into multiple parts under stress. It is the most destructive type of failure.
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Ductile Fracture: Ductile fracture involves extensive plastic deformation and slow crack propagation. It often exhibits a cup-and-cone fracture shape.
- The three stages of ductile fracture in a tensile specimen are neck formation, cavity formation within the neck, and crack propagation.
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Brittle Fracture: Brittle fracture occurs with little or no plastic deformation and rapid crack propagation. It usually follows cleavage planes perpendicular to the stress.
- Many HCP and BCC metals fracture in a brittle manner at room temperature.
- Brittle fracture can be transgranular (across the grains) or intragranular (between the grains).
- Brittle fracture can occur in three stages: dislocation formation and concentration, microcrack nucleation and propagation, and stored energy release.
- Dynamic or Impact Toughness: A measure of a material's ability to absorb energy before fracture. This is important in situations involving impact, such as collisions. Toughness is measured using impact testing machines like the Charpy V-notch specimen.
- Ductile vs Brittle Fracture Comparison: Ductile fracture is characterized by a cup-and-cone shape and extensive plastic deformation, while brittle fracture is characterized by a flat fracture surface with little deformation.
- Ductile-to-Brittle Transition (DBT): Materials can transition from being ductile at higher temperatures to brittle at lower temperatures. This transition is temperature-sensitive. The Titanic disaster is an example.
- Fracture Toughness: A measure of a material's resistance to crack propagation. Pre-existing cracks and flaws cause stress concentration (amplification). A crucial parameter for designers to consider and the critical value (Kic_{ic}ic) can dictate material usage.
- Measuring Fracture Toughness: Notch is machined in the specimen; Specimen is tensile-tested to failure; Higher K${ic}meansmoreductilematerial.TheK means more ductile material. The Kmeansmoreductilematerial.TheK{ic}$ value is important for design to find the allowable or critical crack size.
- Cyclic Stresses and Fatigue: In many engineering applications, stresses are continually fluctuating. Cyclic stress, even at low amplitudes, can cause fatigue damage or failure over time. The damage is often internal and hidden and grows slowly with cycles.
- Types of Cyclic Stresses: Axial, torsional, and flexural are possibilities. Parameters calculated from the applied stresses include mean stress, stress amplitude, stress ratio and stress range. Fully reversed or repeated loading can be distinguished according to mean stress (σm).
- Fatigue Failures: Failure at stress levels below yield strength. Cracks nucleate at regions with stress concentrations (like keyways, changing diameters, or internal flaws).
- Fatigue Testing: Laboratory tests (like the R. R. Moore test) apply alternating compression and tension loads, to count cycles to failure at constant stress. The S-N curve displays stress versus cycles-to-failure.
- Structural Changes in Fatigue Process: Crack initiation, slipband extrusion and intrusion (stage 1), and crack growth occurs. The cross-sectional area decreases as the crack propagates.
- Factors Affecting Fatigue Strength: Stress concentration, surface roughness, surface condition (like carburizing), and environmental factors (like chemically reactive environments that may cause corrosion). Smoother surfaces generally increase fatigue life.
- Notched Fatigue Crack Propagation Rate: Crack propagation rate measurements in notched specimens, relating to the change in potential produced by crack opening. Potentials are used to track crack length evolution. .
- Stress & Crack Length – Fatigue Crack Propagation: Relationship between stress, crack length, and cycles-to-failure. The fatigue crack growth rate increases as stress or crack length increases.
- Fatigue Crack Growth Rate: Plotting of log(da/dN) versus log(ΔK). The slope is m and the y-intercept is log(A). A threshold stress-intensity factor range (ΔKth_{th}th) exists below which the crack does not propagate.
- Fatigue Life Calculation: Calculating the number of cycles (Nf) for a material to fail, given conditions and initial and final crack sizes, using stress intensity factor range (ΔK).
- Creep in Metals: Progressive deformation under constant stress at high temperatures. Critical in high-temperature applications. Primary creep involves decreasing creep rate with time, secondary creep is a constant creep rate, and tertiary creep involves increasing creep rate that leads to necking and fracture.
- Creep Test: Creep test determines the effect of temperature and stress on creep rate; tested at constant stress or temperature with different stress.
- Larsen-Miller Parameter: Larson-Miller parameter is a method for representing creep data; log time to rupture is a function of stress and temperature. Used to determine the time to rupture under varying temperature-stress conditions.
- Case Study – Analysis of Failed Fan Shaft: Failure analysis in a fan system; considerations of material properties (like yield strength and material type) compared to actual properties, observed failure initiation locations, and the eventual cause for failure. The study noted the failed shaft had lower strength than spec and non-cold rolled material.
- Recent Advances in Strength and Ductility: Recent developments in metal processing to increase strength and ductility include nanocrystalline metals, coarse grained metals and cold rolling at low temperatures
- Fatigue Behavior of Nanomaterials: Nanomaterials and ultrafine metals, such as Ni, exhibit higher endurance limit and lower fatigue crack growth thresholds compared to larger-grained metals. Grain size seems to play a role.
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Description
This quiz covers Chapter 7, focusing on the mechanical properties of metals, particularly the failure modes including ductile and brittle fracture. You'll explore concepts such as yielding, fracture formations, and the stages of ductile fracture in materials. Test your understanding of how different metals respond under stress and the mechanisms behind material failure.
