material science chapter 7

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Questions and Answers

What happens to the crack growth rate as the crack length increases?

  • It remains the same
  • It increases (correct)
  • It fluctuates unpredictably
  • It decreases

The limiting value of ΔK below which there is measurable crack growth is called ΔKth.

False (B)

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.

<p>crack</p> Signup and view all the answers

Match the following terms with their definitions:

<p>ΔK = Stress intensity factor range A = Constant depending on material m = Slope of the line in log(da/dN) vs log(ΔK) ΔKth = Threshold below which no crack growth occurs</p> Signup and view all the answers

What does each full rotation of the specimen in the fatigue test represent?

<p>A cycle (B)</p> Signup and view all the answers

Crack initiation occurs at a rapid pace during the fatigue process.

<p>False (B)</p> Signup and view all the answers

What is the effect of surface roughness on fatigue strength?

<p>Increased surface roughness reduces fatigue strength.</p> Signup and view all the answers

The specimen used for measuring fatigue crack propagation is __________.

<p>notched</p> Signup and view all the answers

Match the following factors with their effects on fatigue strength:

<p>Stress concentration = Reduces fatigue strength Surface roughness = Increases fatigue strength when smoother Surface treatments = Increase fatigue life Reactive environment = Decreases fatigue life</p> Signup and view all the answers

Which of the following phases occurs after crack initiation in the fatigue process?

<p>Crack growth (C)</p> Signup and view all the answers

Ductile failure occurs when the remaining cross-sectional area is sufficient to withstand the stress.

<p>False (B)</p> Signup and view all the answers

What is the primary reason for a reduction in fatigue strength?

<p>Increasing stress concentration.</p> Signup and view all the answers

Which of the following is NOT a type of failure in materials?

<p>Expansion (C)</p> Signup and view all the answers

Ductile fracture occurs without any plastic deformation.

<p>False (B)</p> Signup and view all the answers

What is defined as the formation of new surfaces under stress?

<p>Fracture</p> Signup and view all the answers

Ductile fracture is marked by extensive __________ deformation.

<p>plastic</p> Signup and view all the answers

Match the types of fracture with their characteristics:

<p>Brittle Fracture = Rapid crack propagation without plastic deformation Ductile Fracture = Involves extensive plastic deformation before failure Cup &amp; Cone Fracture = Characteristic of ductile fracture with a specific shape Fracture in metals = Separation into multiple parts under stress</p> Signup and view all the answers

What is the direction of crack propagation in ductile fracture?

<p>At a 45-degree angle (D)</p> Signup and view all the answers

Failure is defined as the ability of a component to perform its intended function safely.

<p>False (B)</p> Signup and view all the answers

What are the three stages of ductile fracture in a tensile specimen?

<p>Crack formed by coalescence of cavities, cavities form crack, crack propagates towards the surface.</p> Signup and view all the answers

What characterizes brittle fracture?

<p>Little or no plastic deformation and very fast crack propagation (B)</p> Signup and view all the answers

Brittle fracture primarily propagates between grains at the boundary.

<p>False (B)</p> Signup and view all the answers

Name one factor that can lead to brittle fracture.

<p>Hydrogen diffusion</p> Signup and view all the answers

Brittle fracture occurs under a stress normal to the __________ plane.

<p>cleavage</p> Signup and view all the answers

Match the following stages of brittle fracture to their descriptions:

<p>Formation of dislocations = Dislocations form and concentrate along slip planes Shear stress buildup = Nucleation of microcracks due to dislocation pileup Microcrack propagation = Release of stored energy as microcracks propagate Existing defects = Factors such as geometric stress risers contributing to fractures</p> Signup and view all the answers

Which of the following metals is likely to exhibit brittle fracture at room temperature?

<p>BCC metals (B)</p> Signup and view all the answers

Brittle fractures occur with advanced warning signs.

<p>False (B)</p> Signup and view all the answers

What is one of the dangerous characteristics of brittle fracture?

<p>It occurs with little advanced warning.</p> Signup and view all the answers

Which phase of creep shows a decreasing creep rate over time?

<p>Primary creep (A)</p> Signup and view all the answers

Tertiary creep is associated with a constant creep rate.

<p>False (B)</p> Signup and view all the answers

What does a creep test measure?

<p>The effect of temperature and stress on creep rate.</p> Signup and view all the answers

The minimum creep rate necessary for a given temperature is _____%/h.

<p>10-5</p> Signup and view all the answers

What happens during the tertiary stage of creep?

<p>Necking and fracture occur. (B)</p> Signup and view all the answers

Match the following parameters related to the Larsen-Miller parameter:

<p>P(Larsen-Miller) = T[log tr + C] T = Temperature in Kelvin tr = Stress-rupture time in hours C = Constant of order 20</p> Signup and view all the answers

The time for stress rupture increases with higher stress and temperature.

<p>False (B)</p> Signup and view all the answers

What is the purpose of a creep rupture test?

<p>To determine the stress levels at which a specimen fails.</p> Signup and view all the answers

In the fatigue life calculation, what does ΔK represent?

<p>Stress intensity factor range (A)</p> Signup and view all the answers

The equation Nf = (Ayσπ(-a2 + 1)) assumes that Y is dependent on crack length.

<p>False (B)</p> Signup and view all the answers

What does 'A' stand for in the fatigue life calculation equation?

<p>A represents a material constant.</p> Signup and view all the answers

To calculate the number of fatigue cycles, Nf, you need to integrate from the initial crack size a0 to the final crack size _____ .

<p>af</p> Signup and view all the answers

Match the variables in the fatigue life calculation with their meanings:

<p>da = Incremental crack growth dN = Incremental number of cycles Y = Geometric factor σ = Applied stress</p> Signup and view all the answers

Which equation represents the relationship between ΔK and other factors in the fatigue life calculation?

<p>ΔK = Yσπa (A)</p> Signup and view all the answers

The fatigue life calculation includes integrating the crack growth equation from initial to final crack sizes.

<p>True (A)</p> Signup and view all the answers

What is the role of 'σ' in the fatigue life calculation?

<p>σ represents the applied stress on the material.</p> Signup and view all the answers

Flashcards

Material Failure

Inability of a component to perform its intended function safely and reliably.

Ductile Fracture

Fracture accompanied by widespread plastic deformation and slow crack growth.

Brittle Fracture

Fracture with little or no visible plastic deformation and rapid crack propagation.

Fracture

Formation of new surfaces in a material under stress, leading to separation into parts.

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Yielding

Permanent deformation of a material under stress.

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Crack Propagation

The spreading of a crack through a material.

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Plastic Deformation

Permanent change in shape of a material without breaking.

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Cup and Cone Fracture

A specific type of ductile fracture characterized by a cone-shaped and cup-shaped fracture surfaces.

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Cleavage Planes

Planes within a metal along which brittle fracture occurs.

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Transgranular Fracture

A brittle fracture that propagates across the crystal structure of the metal.

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Intragranular Fracture

A type of brittle fracture that propagates within individual grains.

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Intergranular Fracture

A type of ductile fracture that propagates along the grain boundaries of a metal.

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Brittle Fracture Stages

Brittle fractures are often characterized by three stages of development: dislocation formation, shear stress buildup, and microcrack propagation.

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Factors Affecting Brittle Fracture

Factors such as existing defects, hydrogen diffusion, corrosion, stress risers, low temperatures, and fast loading can promote brittle fracture.

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S-N Curve

A graph that shows the relationship between the applied stress (S) and the number of cycles (N) to failure in a material under cyclic loading.

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Fatigue

A type of failure that occurs under cyclic loading, even if the applied stress is below the material's yield strength.

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Slipband Extrusion and Intrusion

Surface features that form during the early stage of fatigue crack initiation, caused by the repeated slip of material layers.

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Fatigue Crack Propagation

The process of how a fatigue crack grows and spreads through a material as it experiences cyclic loading.

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Stress Concentration

Areas in a material where the stress is higher than the average stress, often due to geometric features like notches or holes.

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Surface Roughness

The unevenness of a material's surface, which can affect its fatigue strength.

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Surface Treatments

Processes applied to the surface of a material to improve its properties, such as fatigue life.

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Environment's Effect on Fatigue

The surroundings, such as the presence of corrosive chemicals, can affect how long a material lasts under fatigue.

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Fatigue Crack Growth Rate

The rate at which a crack grows due to repeated cyclic loading.

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Stress Intensity Factor Range (ΔK)

The difference between the maximum and minimum stress intensity factors experienced during a cyclic loading.

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What does da/dN represent?

The rate of crack growth per cycle of loading.

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What factors affect fatigue crack growth rate?

The fatigue crack growth rate (da/dN) is affected by factors such as stress intensity factor range (ΔK), material properties, environmental conditions, and temperature.

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Stress Intensity Threshold (ΔKth)

The minimum stress intensity factor range below which no measurable crack growth occurs.

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Fatigue Life Calculation

The process of determining how many cycles of stress a material can withstand before failure due to fatigue.

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Stress Intensity Factor (ΔK)

A measure of the stress concentration at the tip of a crack, indicating the crack's tendency to grow under stress.

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Crack Growth Rate (da/dN)

The speed at which a crack grows per fatigue cycle.

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Fatigue Crack Growth Equation

An equation used to model the relationship between crack growth rate, stress intensity factor, and material properties. It is often represented as da/dN = A(ΔKm) where A is a material constant and m is a material exponent.

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Initial Crack Size (a0)

The original size of a crack before the fatigue loading begins.

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Final Crack Size (af)

The critical crack size at which the material fails due to fatigue loading.

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Number of Fatigue Cycles (Nf)

The total number of stress cycles the material can withstand before failure occurs.

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Fatigue Life (Nf)

The endurance limit of a material. It is proportional to the stress intensity factor and crack size.

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Creep

The gradual deformation of a material under constant stress, especially at high temperatures.

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Primary Creep

The initial stage of creep where the deformation rate decreases with time due to strain hardening.

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Secondary Creep

The steady-state stage of creep where the deformation rate is constant due to a balance between strain hardening and recovery.

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Tertiary Creep

The final stage of creep where the deformation rate increases with time, leading to necking and eventual fracture.

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Creep Strength

The stress required to produce a minimum creep rate of 10^-5%/hour at a given temperature.

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Creep Test

An experiment that determines the effect of temperature and stress on the creep rate of a material.

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Creep Rupture Test

A creep test where the goal is to determine the time it takes for a material to fracture under a specific stress and temperature.

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Larsen-Miller Parameter

A parameter used to represent creep-stress rupture data, relating temperature, stress, and rupture time.

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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.
  • 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.
  • 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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