Materials

Define stress

Internal resistance materials develops in response to external forces

What are the types of stress?

Tensile, Compressive, and Shear

What is pressure?

An external force applied uniformly and perpendicularlly to a surface

Define strain

How much a material deforms relative to the orginial shape when a force is applied

What are the types of strain?

Axial (push/pull), Shear (layer slide), Volumetric, and True (=ln L/Lo)

How is strain measured?

With a strain gauge (thin wires glued to the piece, resistance changes as it lengthens)

What does too much strain cause?

Plastic deformation, buckling, fatigue cracking, and elastic insability

What are the types of loads?

Tensile (mooring), compressive (bulkhead), torsional (prop shaft), bending (keel), and combining

What is a safety factor?

A design margin that ensures something can handle more than the expected conditions

How is a safety factor calculated?

SF x max strength/stress allowed = failure stress/working stress

What is hooke’s law?

The extension of a spring in proportional to the applied force if it doesn’t exceed the limit of proportionality

What are the limits of hooke’s law?

Only valid in the elastic limit and assumes a homogeneous isotropic material

Define ultimate strength

Maximum stress a material can withstand before necking

What is young’s modulus?

A constant that measures stiffness/rigidity

What does a FoS do?

Ensures a structure preforms safely and withstands cylics loads, corrosion fatigue, and failure modes

Define vibration

The oscillatory motion around an equalibrium point

What does vibration result from?

The interactions of inertia and the elastic restoring force

What does excessive vibration cause?

Fatigue failure, reduced component lifespan, and structural damage

When does vibration happen?

Whena disturbing force is applied, the system is elastic, or damping is insufficient

What cause cyclic motion to vary?

Mass/Inertia distribution, Component stiffness, Damping ratio, Dampin co-efficient, and Excitation frequency

What are the different damping ratios?

0 = undampened (oscillates forever), 1 = critically dampened (fastest return), 0-1 = underdampened (decaying), and 1+ = overdampened (returns slowly with no oscillation)

What are the main sources of vibration?

Reciprocating machinery, Prop blades, Worn bearings, Uneven cylinder power distribution, Improperly balanced machinery, and Machinery becoming unbalanced

What can cause machinery to become unbalanced?

Mechanical damage, Material loss, and Foreign matter deposits

How do reciprocating components cause vibration?

The masses create inertial forces, even with counterweights residual forces occur. Large engines reduce the effect some

How do prop blades cause vibration?

They encounter water with non-uniform density + flow creating an unsteady thrust load on the blades, known as blade passing frequency excitation

What does blade passing frequency excitation cause?

Causes vibreation throguh the shaft to the gearbox and engine, worsens with a damaged prop (can lead to resonance)

How can improperly balanced machinery cause vibration?

Slight imbalances causes periodic loading on bearings + mounts, producing radial vibrations, worse with high speed

How does properly balancing about the axis help with vibration?

Prevents generating centrifugal forces during operation

How does unbalancing due to damage cause vibration?

It introduces mass imbalances and shifts the center of gravity

How do the types of imbalances happen?

Mechanical damage (bent shaft), Material loss (erosion/corrosion), and Foreign matter accumulation (scale/sludge)

How does uneven power distribution in cylinders cause vibration?

The crankshaft gets non-uniform forces and it causes cyclic torsional vibration from crackshaft flexing, this travels through couplings and shafts

How can uneven power distribution happen?

Injector faults, compression losses, and fuel quality

How can worn bearings cause vibration?

The shaft becomes misaligned, increasing radial and axial play

What does extra play in bearing cause?

Non-linear and erratic motion, misalignment, noise, and fatigue

What is the elastic nature of material essential for?

Torsional loading (shaft), flexural loading (girders), vibratory excitation (prop), and they can store/transmit mechanical energy

Why is steel a good conductor of vibration?

It has low internal damping and high stiffness

What is the role of interfaces and discontinuities in vibration transmission?

Joints/Connections/Cracks reflect, reduce, or modify the frequency/amplitude

What are flexible mounts and isolation pads?

Rubber pads/mounts between engines/compressors/pumps and their structural supports

How do flexible mounts and isolation pads work?

Use layers of damping material to absorb vibrations and prevent them from reaching the hull

What are resilient couplings?

They connect drive components and absorb small angular misalignment and torsional vibrations

What are vibration dampers?

Dampers used in engines and crankshafts to convert mech vibration into heat or to spread it across a larger frequency range

What are the effects of stress

Component vibration causes varying alternating stresses that don’t exceed yeild strength but over time cause fatigue

What are design limits important for?

Shafts, mounting brackets, and welded joints

What are ship machines and structures designed for?

Natural frequency, fatigue resistance, and operation below fatigue limits

What is resonance?

What occurs whena system is subject to a force at a frequency that matches one of the natural ones causing energy accumulation over time

How is nautral frequency calculated?

F = (Root of k/m)/2 pi

What does amplified stress cause?

Increased displacement resulting in larger strain and stress in localized areas like weld toes and HAZs, bearing seats, and piping supports

What can small vibrations cause when there’s resonance?

Rapid cracking, growth under cyclic loading, and failure in short time

What is critical speed?

A rotation speed that produces a force with a frequency the same as a nautral one

Can critical speeds be fully avoided?

No, reduction gears, couplings, and props pass through a lot. As long as they pass through the speeds quickly or have low vibratory torque it’s fine

What can happen when something stays at a critical speed too long?

Vibration building, lossening of securing devices, overheating/uneven bearing wear, and fatigue stress

Why is vibration control needed?

Stops loosening bolts, connection failure, equpiment degradation, and reduces noise, inefficiencies, and stress

What are ways of reducing/preventing vibration?

Balancing in manufacturing, aligning shafts + couplings, anti-vibrations mounts, regular inspections for erosing/scale/corrosion, and avoiding critical speeds

What are the types of joints?

Mechanical, thermal and chemical

What are the different securing of joints?

Permanent, adhesive, and self secured

What is a self secured joint?

A mechanical joint formed by deformation or interlocking, no fasteners/adhesives

Why are self-secured joints used?

Simple, cost-effective, accessible for enclosed spaces, corrosion reduction, and recyclable

What is a grooved seam?

A type of self-securing joint used for cylindrical/rectangular ducts + tanks

What are the properties of grooved seams?

High shear strength, moderate peel resistance, and good vibration resistance

What is a panned down joint?

A self-secured joint used for sheet metal covers, and joinery panels

What are the properties of panned down joints?

Increased rigidity, reinforced edges, and improved alignment

What is a knocked up joint?

A self-secured joint used for vent panels, ductwork, and where clean/secure joints are needed

What is a permanent joint?

A joint meant to last for the entire life of the component and not be ND disassembled

What are permanent joints used?

High strength, reliable, good resistance to vibration/stress, and good for integrity/long-term/safety

What is a welded joint?

A permanent joint that melts two pieces together with a filler metal

What are properties of a welded joint?

Storng and continuous, can withstand high loads, vibration and pressure

What are the types of welded joints?

SMAW, TIG< MIG, SAW, and Oxy-Acet

What is a brazed/soldered joint?

A permanent joint that uses filler metal to bond two pieces without melting the base

What are the uses of brazing/soldering?

Brazing = copper pipe, non-ferrous fittings, and heat exchangers Soldering = electrical components, controls panels, and wires to terminals

What is riveting?

A permanent mechanical joint formed by inserting a metal pin into a hole and deforming the ends

What are the advantages of riveting?

Strong mechanical bond, not heat distortion, reliable, and simple repair

What are the disadvantages of riveting?

Added weight, labour intensive, corrosion risk, and stress concentration

What is heat staking?

A permanent joint using a plastic component locally softened and deformed over a part

What are the advantages of heat staking?

No additional hardware, quick, light and low cost

What are bonding plastics?

A permanent joint formed by softening plastic and adhering to another surface

What are bonding plastics used for?

Mostly non-structural components like covers/guards, trays, pipe insultation, and ducting

What is adhesive bonding?

A method of joining pieces without altering the structure and distributes stress

What are the types of adhesive bonding?

Glue, tape, adhesive film, hot melt, epoxy, paste, and spray

What are the advantages of adhesives?

Bonds dissimilar metals, good stress distribution, lightweight joints, smooth surface, vibration damping, and good for thin/delicate materials

What are the disadvantages of adhesive bonding?

Limited temp service range, suseptible to chemicals, requires lots of prep, strength can degrade, and longer curing times

What are the principles of use for adhesive bonding?

Surface prep, application, clamping/fixturing, curing, and load transfer

What are the different joint configuations for adhesive bonding?

Butt, Scarf, Plain lap, Beveled lap, Double lap, Joggle lap, Strap, and Double strap

What are epoxies?

Adhesives with a resin and hardener that when mixed form a crosslinks + a thermo-set polymer

What are the properties of epoxies?

Strong and heat/chemical resistant when cured

What is steel prep?

A process done to remove scale/rust/oil/dirt before applying protective coatings to allow for bonding

What are the types of steel prep?

SP5 (critical areas), SP10 (shipbuilding), SP2/3 (maintenance/spot cleaning), and SP11 (confined/hazardous areas)

What are the SSPC codes?

SP1 (solvent), SP2 (hand tool), SP3 (power tool), SP5 (white metal), SP6 (commerical), SP10 (near white metal), and SP11 (power tool to bare)

What are sealants?

Pastes that fill in gaps and harden/stay felxible to stop leakage

What are gaskets?

Pre-formed seals of compressable material

What is packing?

A material that fills dynamic joints that stays flexible and withstands motion

Gaskets vs Packing

Gaskets are static and use compression, filling in surface irregularities

Packing is dynamic and in a stuffing box, allows motion

What are glands

A type of packing that is rope-like fibers used in pumps/valves/stern tubes. It is inexpensive and simple but need tightening and is high friction

What are o-rings?

Donut shaped elastomet rings used in pipe flanges/filters. They are compact and cheap but limited in temp and pressure

What are mechanical seals?

Two lapped spring loaded faces used in rotating machinery like pumps/prop shafts. They are reliable and good high temp/pressure but expensive and need precision alignment

What are oil seals?

Seals with flexible lips that press to the shaft in gearboxes/crankshafts. They keep lubricants in and contaminants out but the surface needs to be smooth

What are labyrinth seals?

A complex path of ridges and grooves that traps fluid in/out. They are long lasting with no friction wear but allow slight leakage

What are the O-ring shapes?

O (can roll or twist, in pumps/fanges), X (stability in dynamic better sela in low pressure, in hydraulics), D (resists rolling or twists, more stable, hgih pressure or pulsing systems), Square (large contact, resistant to extrusion, static)

What are the components of mechanical seals?

Rotating face on shaft, stationary face on housing, secondary seals between faces, spring, and gland/seal housing to hold in place

How do oil seals work?

Fit tightly to shaft and garter spring keeps constant pressure of lips on film of LO

What are the components of an oil seal?

Rigid outer casing, sealing lips, spring, and dust lips for dirt