Properties of engineering materials Flashcards
(25 cards)
What is the difference between density and unit weight?
Density is mass per unit volume (kg/m³), while unit weight is weight per unit volume (KN/m³). or N/m³
How is specific gravity defined?
Specific gravity = Density of material / Density of water (1000 kg/m³). It is dimensionless.
Define stress and its unit.
Stress (σ) = Force (N) / Cross-sectional area (mm²). Unit: MPa (N/mm²).
What is strain, and how is it calculated?
Strain (ε) = Change in length / Original length. Unit: Micro-strains (10⁻⁶).
What does the elastic modulus (E) represent?
The slope of the stress-strain curve in the elastic region (E = σ/ε). Unit: GPa.
Compare E for concrete, steel, and timber.
oncrete: ~30 GPa; Steel: ~200 GPa; Timber: ~20 GPa.
Define Poisson’s ratio (ν).
ν = -Lateral strain / Longitudinal strain.
What does a Poisson’s ratio of 0.3 indicate?
For every 1 mm of elongation, the material contracts 0.3 mm laterally.
What distinguishes ductile from brittle materials?
Ductile materials (e.g., steel) undergo large plastic deformation before fracture; brittle materials (e.g., concrete) fail suddenly with minimal deformation.
Why is ductility advantageous in structural design?
It provides warning (visible deformation) before failure.
Define creep in materials.
Time-dependent deformation under constant stress (e.g., concrete under sustained load).
What causes shrinkage in concrete?
Loss of moisture (drying) or internal chemical reactions (autogenous shrinkage).
What is the coefficient of thermal expansion?
Measures strain per degree temperature change (units: ×10⁻⁶ K⁻¹).
Compare thermal expansion of steel vs. concrete.
Steel: ~12 ×10⁻⁶ K⁻¹; Concrete: ~10–14 ×10⁻⁶ K⁻¹.
List three sustainability criteria for material selection.
Minimize resource/energy consumption.
Maximize recyclability/reusability.
Reduce environmental impact (e.g., low toxicity).
How does local material sourcing improve sustainability?
Reduces transportation energy/costs.
Why is characteristic strength (5% fractile) used in design?
Ensures 95% of test results exceed this value, accounting for natural variability.
What is the role of safety factors?
To cover uncertainties in material properties, loads, and workmanship. A factor of safety is the load-carrying capacity of a system beyond what the system actually supports.
Why can timber support a taller column than concrete under self-weight?
Timber has lower density (5 kN/m³ vs. 22 kN/m³ for concrete), reducing self-weight stress.
Why is steel reinforcement used in concrete beams?
Concrete is weak in tension, steel resists tensile stresses in bending.
What is fatigue failure, and where is it relevant?
Progressive failure under cyclic stresses (e.g., bridges, parking decks). Fatigue strength is the stress level a material can endure for a given number of cycles before failing.
Name two factors affecting fatigue life.
Magnitude of cyclic stress.
Material defects/microstructure.
What causes hygral deformations in materials?
Moisture absorption/desorption (e.g., timber swelling when wet, concrete shrinking when drying).
How do hygral deformations impact design?
Restrained hygral movements can induce cracking (e.g., in concrete slabs or timber joints).
