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