Materials Test 2

Composite

A material made from two or more distinct materials that remain separate but work together to improve properties

Matrix

The continuous phase in a composite that surrounds and supports the reinforcement

Reinforcement

The dispersed phase in a composite that provides strength and stiffness

Purpose of composites

To combine materials to get better properties than each material alone

Historical composites

Examples include straw reinforced mud bricks and early concrete

Two phase composite

A composite with a matrix phase and a reinforcement phase

Rule of mixtures

A method to estimate composite properties based on volume fractions of components

Volume fraction

The proportion of each material in a composite (fiber vs matrix)

Fiber reinforced composite

A composite where fibers provide strength and stiffness within a matrix

Common fiber types

Glass, carbon, aramid (Kevlar)

Fiber orientation

The direction fibers are aligned, which affects strength and stiffness

Unidirectional fibers

Fibers aligned in one direction, strong in that direction only

Multidirectional fibers

Fibers arranged in multiple directions for more uniform strength

Random fiber orientation

Fibers distributed randomly, providing isotropic behavior but lower strength

FRP

Fiber Reinforced Polymer, a composite using polymer matrix with fiber reinforcement

FRP advantages

High strength to weight ratio, corrosion resistance, lightweight

FRP disadvantages

Higher cost, brittle behavior, sensitivity to temperature

FRP applications in civil

Bridges, strengthening beams, columns, and infrastructure repair

FRP wrapping

A method of strengthening structures by wrapping them with fiber composites

Matrix in FRP

Usually a polymer like epoxy, polyester, or vinyl ester

Role of matrix

Binds fibers together, transfers load, protects fibers

Fiber matrix bond

The interaction between fiber and matrix that allows load transfer

Good bonding

Essential for effective stress transfer and composite performance

Poor bonding

Leads to failure by fiber pull out or debonding

Composite failure modes

Fiber breakage, matrix cracking, fiber pull out, delamination

Delamination

Separation of layers in a composite material

FRP performance

Depends on fiber type, orientation, matrix, and bonding quality

Sustainability of FRP

Lightweight reduces transport energy but recycling is difficult

Key idea of composites

Properties can be tailored by changing fiber type, amount, and orientation

Wood

A natural composite material made of cellulose fibers in a lignin matrix

Why wood is used

Renewable, lightweight, strong, easy to work with, and sustainable

Wood as a sustainable material

Stores carbon, renewable resource, lower energy production than steel or concrete

Wood engineering issues

Variability, moisture sensitivity, shrinkage, and anisotropy

Wood microstructure

Composed of cells that provide strength and transport nutrients

Growth rings

Annual layers showing earlywood and latewood growth

Earlywood

Faster growth, lighter, less dense

Latewood

Slower growth, darker, denser, stronger

Wood composition

Cellulose (strength), hemicellulose, lignin (binding material)

Cellulose

Main structural component providing tensile strength

Lignin

Binds fibers together and provides compressive strength

Hardwood

From deciduous trees, more complex structure, typically denser

Softwood

From coniferous trees, simpler structure, commonly used in construction

Softwood use

Main structural material in construction

Hardwood use

Furniture, flooring, and finish work

Wood anisotropy

Properties vary depending on direction (longitudinal, radial, tangential)

Longitudinal direction

Along the grain, strongest direction

Radial direction

Perpendicular to growth rings

Tangential direction

Tangent to growth rings, most shrinkage occurs here

Moisture content

Amount of water in wood relative to dry weight

Green wood

Freshly cut wood with high moisture content

Equilibrium moisture content

Moisture level wood reaches based on environment

Wood shrinkage

Occurs as wood dries and loses moisture

Anisotropic shrinkage

Shrinkage differs in longitudinal, radial, and tangential directions

Tangential shrinkage

Greater than radial shrinkage

Dimensional change coefficient

Measures how much wood shrinks or swells with moisture change

Wood seasoning

Process of drying wood to reduce moisture content

Air drying

Natural drying method over time

Kiln drying

Controlled drying using heat for faster results

Pressure treated lumber

Wood treated with chemicals to resist decay and insects

Wood products

Processed forms like plywood, particleboard, fiberboard

Plywood

Layers of wood veneer glued with alternating grain directions

Fiberboard

Made from wood fibers bonded together

Particle board

Made from wood particles and resin

Glulam

Glued laminated timber used for beams and columns

CLT

Cross laminated timber with layers oriented perpendicular

Engineered wood products

Designed to improve strength, stability, and uniformity

Wood processing

Includes cutting, drying, and treating wood for use

Key issue with wood

Moisture affects strength, durability, and dimensions

Mechanical properties of wood

Include strength, stiffness, elasticity, and durability

Elastic modulus (E)

Measures stiffness of wood, varies with direction (anisotropic)

Elastic modulus direction dependence

Highest parallel to grain, much lower perpendicular

Poisson’s ratio

Ratio of lateral strain to axial strain, varies with grain direction

Compressive strength wood

Resistance to crushing, strongest parallel to grain

Compression parallel to grain

High strength, typical structural loading direction

Compression perpendicular to grain

Lower strength, controls bearing capacity

Tensile strength wood

Resistance to pulling forces, strongest parallel to grain

Tension perpendicular to grain

Very weak, common failure mode

Stress strain behavior wood

Nonlinear, especially in compression

Wood anisotropy in strength

Strength varies significantly with grain direction

Hardwood vs softwood strength

Hardwoods generally stronger and denser than softwoods

Moisture content effect on strength

Strength decreases as moisture content increases

Why strength decreases with moisture

Water weakens bonds between wood fibers

Design moisture content

Typically around 8 to 12 percent for structural use

Wood viscoelasticity

Wood exhibits both elastic and time dependent deformation

Creep in wood

Gradual deformation under sustained load

Temperature effects on wood

High temperatures reduce strength and stiffness

Reversible temperature effects

Temporary changes in properties due to temperature

Irreversible temperature effects

Permanent damage from high heat exposure

Wood durability

Affected by environmental exposure and biological attack

Fungal attack

Occurs in moist conditions, causes decay

Insect attack

Damage from termites and beetles

Marine organism damage

Occurs in submerged wood

Bacterial attack

Leads to degradation over time

Preventing wood decay

Keep wood dry or use chemical treatments

Pressure treatment

Adding preservatives to improve durability

Mechanical properties of engineered wood

More uniform and predictable than natural wood

Engineered wood panels

Includes plywood and oriented strand board

Engineered structural shapes

Includes glulam and CLT

Advantages of engineered wood

Improved strength, consistency, and size availability

Wood vs engineered wood

Natural wood is variable, engineered wood is more controlled