PC physical forces

all done

2 ways physical force can damage objects + examples (4, 2)

directly: by causing rotation, deformation, stress and pressure
indirectly: by causing collision between objects or object parts

5 important force related effects + descr.

impact: result of something striking an object, object striking hard surface/objects striking each other, can be concentrated (and increase susceptibility of object to force) or spread out
shock: result of strong impact, induces large deformations/strains in objects/parts, important in shipment
vibration: oscillating motion of an object relative to fixed point of reference
pressure: force applied on unit area of material, result of gravity or handling, if objects stacked - compressive loads applied to lower items
abrasion: where movement between two surfaces in contact

unit for shock intensity

g units of acceleration (1 g is acceleration due to earths gravity)

4 types of vibration

free vibration: vibration prone object will vibrate freely if displaced from its equilibrium position and released, eg tuning fork
forced vibration: object vibrating in response to externally applied source of vibration eg packaged item inside a transport vehicle
harmonic motion: simplest form of continuous vibration - movement that repeats itself exactly
random vibration: complex combination of many vibration frequencies with randomly varying amplitudes, eg in moving vehicles

2 quantities for describing vibration

frequency: in cycles per second/Hertz
amplitude: as displacement, velocity or acceleration

What is construction vibration amplitude expressed as, what does it correlate with

particle velocity, correlates with development of cosmetic cracking in structures

what vibration type is this + descr.

free vibration, amplitude diminishes with time due to damping forces eg friction and air resistance that dissipate vibration energy

what vibration type + descr.

continuous vibration - sine curve, amplitude vs time

what vibration type + descr.

random vibration, info on frequency components helps assess its effect on structures

How to calculate pressure on flat surface

force divided by the area on which the force acts

abrasion effects - what will it vary according to, + more stuffq

surface durability, amount of pressure applied to the surfaces, profile of the surfaces, presence of abrasive material between surfaces can cause/accelerate it, damage may appear after long period of exposure or quickly if surface is fragile!!

direct effects of force (8)

incompression, punctures, dents, tears, cracks, chips, scratches, abrasion

issues with load

gravity makes a continuous load on all objects, improperly made supports may lead to distortion of objects bc of load

mechanical shock description

energetic response of an object, characterized by substantial displacement and strains

4 possible outcomes of mechanical shock

low levels of shock can be absorbed and dissipated in the object w/o damage - eg bell
impact may cause object or its parts to movie → collision between objects, object parts and their surroundings
high shock levels may cause movement and induce strains in excess of critical threshold resulting in fatigue damage
if shock magnitude high enough - damage occurs in a single event (stress fracture)

vibration effects on people

can percieve as low as amplitudes of 0.1 to 0.5mm/s, peak is 5 to 30 Hz

what frequencies can objects vibrate at

many diff ones, due to their geometry, mass and elasticity

natural frequency def

lowest vibration frequency of an object

resonant frequencies def

vibration tendencies higher than natural frequenceies

4 examples of vibration prone objects

vase in cushioning, 2 flexible elements, tightly stretched string, canvas painting

label the vibration types

transmission: if vibration frequency lower than (fv), vase will vibrate at the same frequency and amplitude as the vibration source
resonance: if source matches the resonant frequency, amplitude of the vase will be greater than the vibration source, why package designers usually avoid having object resonances in the same range as the cushion system
attenuation: if vibration frequency is greater - vase will appear almost motionless while the package oscillates

damaging effect of vibration

mechanical fatigue

what must take place for mechanical fatigue to happen

critical stress threshold must be attained or exceeded during each vibration cycle, damage then occurs after a time interval that depends on the number of vibration cycles and their peak values

cycles and stress levels

if stress levels are below a critical level - almost indefinite cycling is possible w/o damage, BUT if sufficiently high stress - stress fracture can occur in one cycle

is fatigue an issue for museums

not really

cosmetic cracking def

damage to buildings by construction vibration

what to do with low level vibration due to construction - 4 scenarios

if far enough away do nothing, relocation of objects near is advisable (if objects can be moved safely), if moving not feasible - if object is in contact with a surface that transmits vibration (eg floor) separate the object from the floor with cushioning material, sliding of objects can be a concern - restraining appropriate for the type would be good

are acoustic sources (like live music) a concern for museum objects?

no! very hard to accidentally damage objects acoustically

important vibration effect

the by-product of the movement it causes - eg unrestrained objects on a shelf that may fall

catastrophic forces (low incidence/high intensity) (5)

earthquakes (most damage from movement rather than the forces of the earthquake), war and vandalism, shipment disasters, extreme handling hazards, roof or floor collapse

Working forces (High Incidence, Moderate-to-High Intensity) descr

Forces encountered in daily activities with moderate-to-high magnitudes and incidence rates, force magnitudes are usually predictable, they may affect one, several, or many items at a time

Working forces (High Incidence, Moderate-to-High Intensity) 6 egs

handling, transit (in house), shipment, gravitational loads, construction vibration, excavation

Cumulative Forces (High Incidence, Low Intensity) descr

Low-intensity forces with a high (based on collection use rates) or continuous (e.g. gravity) incidence that inflicts ongoing damage to one or many items in a collection

Cumulative Forces (High Incidence, Low Intensity) egs

handling, shipping, gravity

Low-level Forces (Variable Incidence, Low Intensity) descr + 3 egs

no risk; building vibration, construction vibration, acoustic sources

6 commercial shock fragility ratings

extremely fragile, very delicate, delicate, moderately delicate, moderately rugged, rugged

what affects sensitivity of any given object to forces (5)

mass (bc F = ma, as mass inc. force also does), material weakness, geometry, flexibility (weakness cause has a tendency to vibrate- can have resonance), vibration susceptibility

2 kinds of forces

long range (eg gravity), contact forces (normal or tangential (sheer, along a surface)

newtons law of motion

how objects respond to motion, brings together long range and contact forces, Fnet = ma

how does acceleration change with bigger mass

acceleration gets smaller

inertial mass def

how the object resists acceleration when force is applied to it

free body diagram

used to help analyze the forces of a system

newtons 3rd law

Fab = -Fba - is body a exerts force on body b then b exerts a force backwards on body a

what are normal forces in a museum setting

pressure/crushing

tangential forces in a PC setting

abrasion

what does friction do

convert a normal to a tangential force