Materials Science on the FE Exam
Materials science appears on the FE Mechanical exam (5-8%), the FE Other Disciplines exam (5-8%), and as part of the engineering sciences section on other disciplines. Understanding material behavior, testing methods, and selection criteria is fundamental to engineering design and analysis.
Mechanical Properties
The stress-strain diagram is the most important tool for understanding mechanical behavior. Key points on a typical stress-strain curve for a ductile metal include:
- Proportional Limit: The highest stress at which stress is proportional to strain (Hooke's Law applies)
- Elastic Limit: The maximum stress that can be applied without permanent deformation
- Yield Strength: The stress at which permanent deformation begins (often defined using the 0.2% offset method)
- Ultimate Tensile Strength: The maximum stress the material can withstand
- Fracture Strength: The stress at which the material breaks
Key material properties include elastic modulus (slope of the linear portion of the stress-strain curve), ductility (percent elongation or percent reduction in area at fracture), toughness (total area under the stress-strain curve), resilience (area under the elastic portion of the stress-strain curve), and hardness (resistance to indentation).
Material Testing Methods
The FE exam may test your knowledge of common material testing methods:
- Tensile Test: Measures yield strength, ultimate strength, elastic modulus, elongation, and reduction in area
- Hardness Tests: Brinell, Rockwell, and Vickers methods measure resistance to indentation
- Impact Test: Charpy and Izod tests measure toughness and ductile-to-brittle transition temperature
- Fatigue Test: S-N curves show the relationship between cyclic stress amplitude and number of cycles to failure
- Creep Test: Measures time-dependent deformation under constant load at elevated temperature
Phase Diagrams
Binary phase diagrams show the phases present at equilibrium for different compositions and temperatures. The iron-carbon phase diagram is the most important for engineering applications. Key features include:
- Ferrite (alpha iron): BCC crystal structure, soft and ductile, limited carbon solubility
- Austenite (gamma iron): FCC crystal structure, exists at higher temperatures, greater carbon solubility
- Cementite (Fe3C): Iron carbide, hard and brittle intermetallic compound
- Pearlite: Lamellar mixture of ferrite and cementite, moderate strength and ductility
- Eutectoid point: 0.76% carbon at 727 degrees Celsius, where austenite transforms to pearlite
Know how to use the lever rule to calculate the fraction of each phase present at a given composition and temperature. The lever rule applies to any two-phase region on a phase diagram.
Heat Treatment
Heat treatment processes alter the microstructure and properties of metals. Key processes include annealing (heating above the critical temperature, then slow cooling to soften the material), quenching (rapid cooling to produce martensite, a hard and brittle phase), and tempering (reheating quenched steel to reduce brittleness while maintaining some hardness).
Failure Modes
Fatigue
Fatigue failure occurs under cyclic loading at stress levels below the ultimate tensile strength. The S-N curve shows the endurance limit (for ferrous metals) or fatigue strength at a specified number of cycles (for non-ferrous metals). Factors that reduce fatigue life include surface roughness, stress concentrations, corrosive environments, and elevated temperatures.
Creep
Creep is time-dependent deformation under constant load, significant at temperatures above roughly 40% of the melting temperature (in Kelvin). The creep curve shows three stages: primary (decreasing strain rate), secondary (constant strain rate), and tertiary (accelerating strain rate leading to fracture).
Corrosion
Corrosion is the degradation of materials through chemical reactions with the environment. Common types include uniform corrosion, galvanic corrosion (when dissimilar metals are in electrical contact), pitting, crevice corrosion, and stress corrosion cracking. Understanding galvanic series helps predict which metal in a couple will corrode.
Material Selection
FE exam questions may ask you to select an appropriate material based on requirements like strength, weight, corrosion resistance, temperature capability, and cost. Understand the general categories of engineering materials (metals, polymers, ceramics, composites) and their typical property ranges.