material science chapter 7

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.