Mechanical Property Testing

Question 1

When and why did materials testing begin?

Answer 1

1830’s when early steam boilers started to explode without warning

Question 2

The tensile test was developed to determine a materials strength , describe the process

Answer 2

• sample (standard dimension) held between 2 grips pulled apart until breaks
• detail can be defined precisely e.g. dimensions diameter, gauge length, surface finish… all need to be controlled
• strain rate, precise direction of application of load and slack in machine all need to be controlled

Question 3

Why do we have to careful about how a standard specimen represents the actual material

Answer 3

Mechanical test, only represents the properties of the specimen

e.g. if contains a defect the info gained is valueless

Question 4

What information can be gained if the Tensile test is carried out correctly?

Answer 4

UTS: Ultimate tensile strength: max strength supported by the sample

yield stress: stress at which material begins to behave plastically

elongation at failure: represents ductility of the sample

stiffness

toughness

Question 5

What is the tensile stress (engineering stress) defined as?

Answer 5

sigma = Force (N) / CSA

CSA = original cross- section area of the gauge length

Question 6

What is Strain (engineering strain) defined as?

Answer 6

epsilon (e) = change in gauge length / original gauge length

Question 7

What id the Youngs modulus (Elastic Modulud, E)?

Answer 7

Describes the behaviour of a material during the elastic regime

E = stress/ strain

describes stiffness of a material, its (recoverable) deformation under an applied load

Question 8

What is toughness and how is it determined

Answer 8

Determined from area under the stress-strain curve

energy absorbed by the material unto failure is caused

Ductile material can often absorb more energy than a brittle material and therefore = tougher

Question 9

Cross sectional area changes through a tensile test, as gauge length increases and CSA decreases (particularly during plastic deformation) what reflects theses changes

Answer 9

True stress and true strain

True stress = applied load / instantaneous CSA

True strain = ln( new gauge length (li)/ original gauge length(lo))

since no volume change during deformation:
Ai.li = Aolo

Question 10

How are true stress and strain related to engineering stress and strain

Answer 10

true stress = stress( 1 + strain)

true strain = ln(1 +strain)

Question 11

What is shown when the true stress and strain are plotted, rather than engineering values

Answer 11

shows that the metal is strengthened by deformation

true stress-true strain curve shows the metal to have work-hardened, to have increased its yield strength by deformation