mechanical testing our materials respond to forces it's essential data for design

defining the limiting data that a structure can withstand without failure

mechanical tests are broadly divided into two types quantitative and

qualitative quantitative tests give numerical results that are used for

design whereas qualitative tests are used for making comparisons well in this

short introduction to mechanical testing we'll be looking at tensile testing Bend

testing sharpen and ctod testing hardness testing and lastly the world

brake test so off we go tensile testing both a qualitative and

quantitative text in a nutshell a suitably prepared specimen like this

machined two dimensions dictated by the codes is pulled apart uniaxial ache at a

specified rate both the load and the test piece extension are continuously

measured and recorded so from this test we can create a stress strain curve

defining the ultimate tensile strength and the yield point in other words

measures of the material strength as well as the percentage elongation and

the percentage reduction in area both measures of the materials ductility and

of the slope of the elastic portion of the curve essentially a straight line

gives us the Youngs modulus unlike the tensile test the bend test provides a

purely qualitative result that can be used to evaluate both the ductility and

the soundness of the material usually a weld section it's often used for quality

control of but welded joints both the test piece and the test equipment are

simple the test uses a coupon that is bent in three point bending to a

specified angle since the outside of the bend is extensively plastically deformed

any defects or signs of embrittlement are quickly revealed the test kit itself

varies from the popular male-female former to the wraparound bender

rather like a plumbers pipe former the strain applied to the specimen depends

on the diameter of the former which is specified in the test standard and is

related to the coupon thickness the angle may be 90 120 or 180 degrees and

may be face side or root bends now measuring the toughness of a material

demands different equipment altogether is broadly a measure of the amount of

energy required to cause an item to fracture and fail to measure this we

most commonly use the sharpie v-notch test it involves striking a standard

notched specimen with a controlled weight pendulum traveling at a set speed

only a local material is required for each specimen they're 55 millimetres

long 10 millimeters square and have a two millimeter deep notch but can be

smaller for thin materials the specimen is struck on the opposite face to the

notch it's fractured and the pendulum swings through by a specific amount

giving a measure of the energy required to break the material the amount of

energy absorbed in fracturing the piece gives an indication of the knotch

toughness of the test material if a small amount of energy is absorbed

the material is brittle ductile materials absorb a great deal of energy

experience indicates that a specified energy absorbed is required to avoid

brittle fracture measuring fracture toughness accurately in a way that can

be related to a tolerable floor size is a job for the crack-tip opening

displacement test or ctod as we call it unlike the small inexpensive Sharpie

specimen the ctod specimen may be the full thickness of the material it

contains a genuine fatigue crack and will be loaded at a rate representative

of the service conditions as the name suggests the crack tip opening is

measured throughout the test and may be used in fitness for purpose assessments

and his bending proceeds the crack-tip plastically deformed until a critical

point is reached when the crack has opened sufficiently to initiate a

fracture event and either partial or complete failure of the specimen many

specimen geometries may be used and this type of test piece is known as the

single edged notched Bend Sen bee specimen the compact tension or

CT specimen may also be used to characterize fracture behavior it's

similar to the Sen bee specimen in that it's our proportional specimen of full

plate thickness containing a deliberately introduced fatigue Creek CT

specimens require much less material to manufacture but are considerably more

expensive to machine ctod tests enable critical floor sizes to be calculated

and decisions made about fitness for service or the required level and

sensitivity of non-destructive examination so they are quantitative

tests there are however a number of qualitative tests which dictate a go or

no-go result typical of these is the nil ductility temperature test

it's confusingly dubbed the NDT test the sample size is standardized depending

upon plate thickness a brittle crack is introduced on one side and the simply

supported specimen is struck in a standard way the test is carried out on

a number of specimens at progressively lower temperatures until the test piece

breaks in a brittle fashion this is known as the nil ductility temperature

or in DT now hardness testing is something different altogether the term

hardness has several meanings but it broadly means resistance to indentation

and is a measure of tensile strength the Vickers hardness test uses a

pyramid-shaped Dimond indenter which of course doesn't deform at high loads by

lowering the indenter under a standard load for 30 seconds onto the surface of

the sample it's possible to calculate the hardness by measuring the impression

it leaves needless to say the indenter dimensions and the loads applied are

standardized in the codes the rockwell hardness test named after the american

Stanley Pickett Rockwell works on a similar principle with a conical

indenter but uses two applied loads the permanent increase in penetration depth

is used to calculate the Rockwell hardness number different loads are used

for materials with different hardnesses the most common scales are Rockwell B

and Rockwell C harness now to specialist hardness testing for determining

hardness in cramped environments in service components too large for the

test laboratory or indeed on very small samples there are three common methods

the noop test the Vickers test and the micro hardness test both the

noop test and the Vickers test use a pyramid-shaped indenter the loads are

scaled down appropriately to between a gram and a kilogram and produce

impressions that require a hundred times magnification microscope to see and

measure finally the fill it fracture or weld break test a simple and very crude

test to examine a welds soundness often in a production environment on the shop

floor two plates are welded together on one side only in a t configuration of

course the base metal weld metal and welding parameters must adhere strictly

to those prescribed in the welding procedure under scrutiny the Filat world

is not with the sort cut along its centre line

force is then applied to the specimen to bend open the world using a testing

machine a press or blows from the camera the fractured surfaces can then be

examined for internal defects such as porosity incomplete root penetration or

sidewall fusion and slag inclusions well that about wraps up this brief

introduction to mechanical testing to learn more about any of the tests we've

covered why not contact Twi on 4 or if you're outside the UK one two two three

eight nine nine triple zero oh why not visit the website that's www.wymt.com