[D2] METAL DEGRADATION AND FAILURE MERGED

A comprehensive set of 100 practice flashcards in Question and Answer style covering mechanical failure, fracture mechanics, fatigue, creep, corrosion, root cause analysis, and non-destructive testing based on lecture materials.

How is mechanical failure by fracture broadly classified?

Fracture is classified based on the ability of a material to experience plastic deformation, into ductile and brittle modes.

What are the two primary fracture modes in metals?

Ductile fracture and brittle fracture.

Which ASTM standards cover tension testing of metallic materials at standard and elevated temperatures?

ASTM E8/E8M-22 covers Standard Test Methods for Tension Testing of Metallic Materials, and ASTM E21-20 covers Elevated Temperature Tension Tests.

What are the two fundamental steps involved in the process of fracture?

Crack formation and crack propagation.

How do ductile and brittle fractures differ in terms of plastic deformation ahead of the crack?

Ductile fracture involves extensive plastic deformation ahead of the crack, while brittle fracture exhibits very little plastic deformation.

How do ductile and brittle fractures compare in terms of stability and energy absorption?

Ductile fracture is stable and features high energy absorption; brittle fracture is unstable and features low energy absorption.

What visual behavior differentiates brittle fracture from ductile fracture at the atomic level under tensile force?

Brittle behavior involves sequential breaking of atomic bonds along a sharp crack tip without plastic distortion, whereas ductile behavior involves significant shear deformation and dislocation movement ahead of the crack tip.

What macroscopic fracture surface markings characteristic of brittle fracture point back to the crack origin?

Chevron markings (V-shaped patterns) or fan-like patterns.

What is the difference between transgranular and intergranular fracture?

Transgranular fracture propagates through the grains (cleavage), whereas intergranular fracture propagates along the grain boundaries.

What type of fracture propagation path is depicted in this microstructural diagram?

Transgranular fracture, where the path of crack propagation goes directly through the individual grains.

What type of fracture propagation path is depicted in this grain boundary schematic?

Intergranular fracture, where the path of crack propagation follows the grain boundaries between adjacent grains.

How is fracture mechanics defined?

The quantification of relationships between material properties, stress level, the presence of crack-producing flaws, and crack propagation mechanisms.

What formula calculates the maximum stress $\sigma_m$ at the tip of a crack?

$\sigma_m = 2\sigma_0 \left(\frac{a}{\rho_t}\right)^{\frac{1}{2}}$ where $\sigma_0$ is the nominal applied stress, $a$ is the length of a surface crack (or half internal crack length), and $\rho_t$ is the crack tip radius of curvature.

What is the stress concentration factor $K_t$ and how is it expressed?

A measure of the degree to which an external stress is amplified at the tip of a crack: $K_t = \frac{\sigma_m}{\sigma_0} = 2\left(\frac{a}{\rho_t}\right)^{\frac{1}{2}}$.

What equation defines critical stress $\sigma_c$ required for crack propagation according to Griffith's theory?

$\sigma_c = \left(\frac{2E\gamma_s}{\pi a}\right)^{\frac{1}{2}}$ where $E$ is the modulus of elasticity, $\gamma_s$ is the specific surface energy, and $a$ is crack dimension.

In Griffith's critical stress formula, how does parameter $a$ differ for a surface crack versus an internal crack?

For a surface crack, $a$ is the total length of the crack; for an internal crack, $a$ is half of the total crack length.

If aluminum oxide has a specific surface energy of $0.90\,J/m^2$ and elastic modulus of $393\,GPa$, what is the critical stress $\sigma_c$ to propagate an internal crack of length $0.40\,mm$?

$33.6\,MPa$ (using $a = 0.2\,mm = 0.2 \times 10^{-3}\,m$).

A glass plate ($E = 69\,GPa$, $\gamma_s = 0.3\,J/m^2$) is subjected to a tensile stress of $40\,MPa$. What is the maximum allowable surface flaw length $a$ without fracture?

$0.0082\,mm$ (or $8.2\,a$ where $a = \frac{2E\gamma_s}{\pi \sigma_c^2}$).

What is fracture toughness $K_C$?

A property measuring a material's resistance to brittle fracture when a crack is present, expressed as $K_C = Y\sigma_c \sqrt{\pi a}$.

What are the approximate values of dimensionless parameter $Y$ for a plate of infinite width versus semi-infinite width with an edge crack?

$Y = 1$ for a plate of infinite width with a through-thickness crack, and $Y \cong 1.1$ for a plate of semi-infinite width containing an edge crack.

What is plane strain fracture toughness $K_{Ic}$?

The fracture toughness value measured when specimen thickness is much greater than crack dimensions, where $I$ denotes Mode $I$ crack displacement.

What three major factors influence plane strain fracture toughness $K_{Ic}$?

Temperature, strain rate, and microstructure.

What three modes of crack surface displacement exist in fracture mechanics?

Mode $I$ (Opening or Tensile mode), Mode $II$ (Sliding mode), and Mode $III$ (Tearing mode).

What three variables must be evaluated together relative to the structural failure possibility of a component?

Fracture toughness ($K_c$ or $K_{Ic}$), imposed stress ($\sigma$), and flaw size ($a$).

Will a 4340 steel alloy specimen ($K_{Ic} = 54.8\,MPa\sqrt{m}$, largest surface crack $0.5\,mm$, $Y = 1.0$) fracture under an applied stress of $1030\,MPa$?

No, because its critical failure stress is $\sigma_c = \frac{54.8}{(1.0)\sqrt{\pi (0.0005)}} = 1382\,MPa$, which exceeds the applied stress of $1030\,MPa$.

What test conditions are imposed during impact fracture testing?

Deformation at a relatively low temperature, a high strain rate (rate of deformation), and a triaxial stress state (introduced by a notch).

Which ASTM standard specifies notched bar impact testing for metallic materials?

ASTM E23-18.

What are the two standardized impact test methods used to measure notch toughness?

The Charpy test and the Izod test.

What is the difference in specimen mounting between Izod and Charpy impact tests?

In the Izod test, the specimen is mounted vertically as a cantilevered beam; in the Charpy test, the specimen is supported horizontally at both ends as a simple beam.

What key variables influence impact fracture test results?

Specimen size and shape, notch configuration and depth, and temperature.

What is the ductile-to-brittle transition?

A phenomenon where a metal undergoes a transition from ductile to brittle fracture behavior with a decrease in temperature.

How does increasing carbon content affect the Charpy V-notch energy versus temperature curve for steel?

Increasing carbon content lowers the impact energy across temperatures and raises the ductile-to-brittle transition temperature.

What is mechanical fatigue?

Failure occurring under conditions of repetitive or fluctuating stress at stress levels significantly lower than the static tensile strength.

What percentage of metallic failures is attributed to fatigue?

Approximately $90\%$ of all metallic failures.

Which ASTM standard governs force-controlled constant amplitude axial fatigue tests of metallic materials?

ASTM E466-15.

What is the formula for mean stress $\sigma_m$ in cyclic loading?

$$\sigma_m = \frac{\sigma_{\text{max}} + \sigma_{\text{min}}}{2}$$

What is the formula for stress range $\sigma_R$ in cyclic loading?

$$\sigma_R = \sigma_{\text{max}} - \sigma_{\text{min}}$$

What is the formula for stress amplitude $\sigma_a$?

$$\sigma_a = \frac{\sigma_r}{2} = \frac{\sigma_{\text{max}} - \sigma_{\text{min}}}{2}$$

What is stress ratio $R$ in fatigue loading?

$$R = \frac{\sigma_{\text{max}}}{\sigma_{\text{min}}}$$

What is an S-N curve (Wöhler curve)?

A plot of stress amplitude ($S$) versus the logarithm of the number of cycles to failure ($N$).

What is fatigue limit (or endurance limit)?

The limiting stress level below which fatigue failure will not occur regardless of the number of cycles.

For many steels, what fraction of static tensile strength corresponds to the fatigue limit?

Approximately $35\%$ to $60\%$ of the static tensile strength.

What is the difference between fatigue strength and fatigue life?

Fatigue strength is the stress level at which failure occurs for a specified number of cycles, whereas fatigue life ($N_f$) is the number of cycles to cause failure at a specified stress level.

What are the three stages of fatigue failure?

1. Crack initiation, 2. Crack propagation, 3. Final failure (rupture).

Where do fatigue cracks almost exclusively initiate?

From stress concentrators located on the outer surface of a component.

What microscopic surface slip features serve as crack nucleation sites during fatigue crack initiation?

Persistent slip bands (PSBs) forming surface extrusions and intrusions.

What two types of fatigue propagation markings can be identified on a fracture surface?

Beachmarks (clamshell marks) and striations.

How do beachmarks and striations differ in observation scale?

Beachmarks are macroscopic features visible to the naked eye; striations are microscopic features requiring high magnification (e.g., TEM or SEM).

Does the absence of beachmarks or striations on a fracture surface rule out fatigue failure?

No, their absence does not conclusively rule out fatigue as the cause of failure.

What design modification reduces stress concentration at section changes to prolong fatigue life?

Designing generous fillets with larger radii of curvature at section transitions.

What three surface treatment methods are commonly employed to increase fatigue resistance?

Polishing, shot peening, and case hardening.

How does shot peening improve fatigue life?

High-velocity shot particles deform the surface layer to induce residual compressive stresses, which counteract applied external tensile stresses.

How does case hardening enhance fatigue performance?

Carbon or nitrogen is diffused into the outer surface layer at high temperature to form a hard, wear-resistant case with residual compressive stresses.

How does component size affect fatigue strength?

Smaller components generally exhibit higher fatigue strength because they have a lower probability of containing critical flaws or defects.

What is thermal fatigue?

Fatigue induced at elevated temperatures by fluctuating thermal stresses, occurring even without external mechanical loading.

What equation gives thermal stress $\sigma$ caused by temperature change $\Delta T$ when expansion is constrained?

$\sigma = \alpha_l E \Delta T$ where $\alpha_l$ is the linear coefficient of thermal expansion and $E$ is the modulus of elasticity.

What is corrosion fatigue?

Fatigue failure caused by the simultaneous action of a cyclic stress and chemical attack.

How is creep defined?

Time-dependent, permanent deformation of materials subjected to a constant load or stress at elevated temperature.

At what temperature threshold relative to absolute melting temperature ($T_m$) does creep become significant for metals?

At temperatures $T > 0.4 T_m$.

Which ASTM standard specifies methods for conducting creep and stress-rupture tests?

ASTM E139-11.

What is the Equi-Cohesive Temperature (ECT)?

The temperature at which grains and grain boundaries possess equal strength.

Which grain structure exhibits higher strength above versus below the Equi-Cohesive Temperature (ECT)?

Coarse-grained materials are stronger above the ECT, while fine-grained materials are stronger below the ECT.

What fracture path predominates in metallic materials during low-temperature applications?

Transgranular fracture, because grains and slip planes are weaker than grain boundaries.

What fracture path decision occurs in high-temperature creep applications?

Intergranular fracture, because grain boundaries become weaker than grains and slip planes.

What are the three stages of a standard creep curve?

Primary (transient) creep, secondary (steady-state) creep, and tertiary creep.

Why does creep rate continuously decrease during primary creep?

Because strain hardening (work hardening) increases the material's resistance to deformation.

Why is the creep rate constant during secondary (steady-state) creep?

Because a balance is achieved between competing processes of strain hardening and recovery.

How is steady-state creep rate $\dot{\varepsilon}_s$ defined and calculated?

It is the slope of the secondary creep stage: $\dot{\varepsilon}_s = \frac{\Delta \varepsilon}{\Delta t}$.

How do increases in stress or temperature affect the creep curve characteristics?

Increases in stress or temperature increase instantaneous strain, increase steady-state creep rate, and reduce rupture lifetime.

What is the difference between Coble creep and Nabarro-Herring creep diffusion paths?

Coble creep ($T < 0.7 T_m$) involves atomic diffusion along grain boundaries, while Nabarro-Herring creep ($T \approx T_m$) involves atomic diffusion through the bulk grain lattice.

What material choices and microstructural methods enhance creep resistance?

Using high-melting-point metals, superalloys, and coarse-grained, columnar, or single-crystal structures.

What progression of turbine blade grain structures improves creep performance?

Conventional casting (polycrystalline) to columnar grain structure to single crystal structure.

How is corrosion defined in metals?

The destructive and unintentional attack of a metal due to an electrochemical reaction starting at its surface.

What two distinct half-cell reactions constitute an electrochemical corrosion process?

Oxidation (losing electrons) and reduction (gaining electrons).

At which electrode does reduction occur during corrosion?

At the cathode.

At which electrode does oxidation occur during corrosion?

At the anode.

What formula calculates the standard cell potential $E^0_{\text{cell}}$ of a galvanic cell?

$E^0_{\text{cell}} = E^0_{\text{reduction, cathode}} - E^0_{\text{reduction, anode}}$ or $E^0_{\text{cell}} = E^0_{\text{reduction, cathode}} + E^0_{\text{oxidation, anode}}$.

What equation links standard cell potential $E^0_{\text{cell}}$ to standard Gibbs free energy $\Delta G^0$?

$\Delta G^0 = -nFE^0_{\text{cell}}$ where $n$ is moles of electrons and $F$ is Faraday's constant.

What equation expresses electrode potential under non-standard concentration conditions?

The Nernst equation: $E = E^0 - \frac{RT}{nF} \ln \left(\frac{[C]^c [D]^d}{[A]^a [B]^b}\right)$.

What four essential components are required to form an electrochemical corrosion cell?

1. Anode, 2. Cathode, 3. Electrolyte, 4. Electrical contact.

What is galvanic corrosion (composition cell)?

Corrosion resulting from electrical contact between two dissimilar metals in an electrolyte, in which the less noble (anodic) metal corrodes.

What is a stress corrosion cell?

A corrosion cell where localized accelerated attack occurs at regions of highest stress or plastic deformation.

What is crevice corrosion?

Localized corrosion occurring in narrow crevices or stagnant solution zones where oxygen becomes depleted.

What is pitting corrosion?

An intensive localized corrosion attack producing small holes or pits that propagate downward into the metal.

What is selective leaching (dealloying)?

A process occurring in solid solution alloys where one constituent element is preferentially dissolved by corrosion (e.g., dezincification).

What is erosion-corrosion?

Accelerated material degradation caused by combined chemical attack and mechanical abrasion/wear from fluid motion.

What is stress corrosion cracking (SCC)?

Failure resulting from the combined simultaneous action of an applied tensile stress and a specific corrosive environment.

What is passivation?

The spontaneous formation of a thin, transparent, adherent oxide film on a metal surface that protects it against further corrosion.

How does cathodic protection via a sacrificial anode protect a metal structure?

The protected metal is electrically connected to a more reactive (anodic) metal, which corrodes preferentially and supplies electrons to the structure.

How does impressed current cathodic protection operate?

Direct current (DC) is supplied from an external power source through an inert anode to make the protected structure cathodic.

What is the formula for Corrosion Penetration Rate ($CPR$)?

$CPR = \frac{KW}{DAT}$ where $W$ is weight loss, $D$ is density, $A$ is exposed area, $T$ is exposure time, and $K$ is a constant.

What value of $K$ gives Corrosion Penetration Rate ($CPR$) in millimeters per year (mm/yr)?

$K = 87.6$ (with $W$ in mg, $D$ in $g/cm^3$, $A$ in $cm^2$, and $T$ in hours).

If a submerged metal plate ($A = 800\,cm^2$, density $D = 4.5\,g/cm^3$) loses $7.6\,kg$ under a CPR of $4\,mm/yr$, what is the estimated submersion time?

$5.27\,years$ (or $46233.33\,hours$).

What is the primary objective of failure analysis?

To determine the fundamental root cause of a failure to assign responsibility, prevent recurrence, evaluate design/reliability, and establish maintenance protocols.

What does the acronym FRACAS stand for?

Failure Reporting, Analysis, and Corrective Action System.

What are the three categories of root causes identified in a Logic Tree Analysis?

Physical root (failure mechanism), Human root (human factors/actions), and Latent root (organizational/cultural rules).

What is the Toyoda (5 Whys) root cause analysis technique?

An iterative questioning technique that asks 'Why?' repeatedly (typically 5 times) to drill down to the fundamental root cause of a component or system failure.

What is an Ishikawa (Fishbone) diagram?

A graphical tool mapping potential causes along categorized branches off a central spine to identify inputs creating output variability.

What are the six most common Non-Destructive Testing (NDT) methods?

Visual, Liquid Penetrant, Magnetic Particle, Radiographic, Eddy Current, and Ultrasonic Testing.

How does Liquid Penetrant Testing highlight surface flaws?

A fluid with high surface wetting penetrates surface cracks, excess is cleaned, and a powder developer draws out trapped fluid to form a visible indication.