Mechanical Tests A

Question 1

two types of test

Answer 1

destructive and non destructive

Question 2

cite mechanical properties (3)

Answer 2

strength, hardness and ductility

Question 3

what is the size effect

Answer 3

The larger the test specimen, the more likely it is
have flaws of greater severity.
Hence, the more likely it is to exhibit a lower fracture strength.

Question 4

describe the tensile test

Answer 4

In this test, a sample is elongated at a constant rate and the load necessary to produce a certain elongation is measured as a dependent variable.

Question 5

formula of engineering stress

Answer 5

sigma=P/A, sigma=stress, P=ratio of load, and A=cross sectional area

Question 6

formula of engineering strain

Answer 6

espilon=change of length / original length

Question 7

describe Hookes law

Answer 7

The relation between stress and strain in the elastic region is linear for metals and ceramics
sigma=E*epsilon, with E being Young’s Modulus

Question 8

What is Young’s Modulus

Answer 8

Young’s modulus, also known as elastic modulus, can be determined from a stress-strain curve as
E = deltaSigma / deltaEpsilon

Question 9

Materials behave in one of two primary modes:

Answer 9

1) Fracture of Brittle Materials
2) Yielding of Ductile Materials

Question 10

describe strenghts of brittle materials

Answer 10

Their tensile strengths are thus low and usually vary from sample to sample.
These same materials can nevertheless exhibit high compressive strengths

Question 11

yielding of ductile materials

Question 12

Whether a material behaves in a brittle or ductile manner depends on: (2)

Answer 12

1. Atomic or molecular structure
2. Service conditions

Question 13

Primary service conditions affecting behavior: (3)

Answer 13

1. Temperature
2. Strain Rate
3. Degree of Triaxiality

Question 14

What happens during plastic deformation (describe necking)

Answer 14

There is no change in the volume of the specimen when measured in the unloaded state.
A*L=constant
The length may increase, but the cross-sectional area becomes smaller, keeping the above equation constant.

Question 15

When do ductile materials tend to fail in tension

Answer 15

Ductile materials tend to fail in tension by shearing on planes at approximately 45o to the direction of the applied tensile stress.
This generally leads to a local contraction or ‘necking’ of the materials

Question 16

how do brittle materials tend to fail

Answer 16

Brittle materials tend to fail due to crack propagation
perpendicular to the axis of loading. Because of their lack of ductility, they exhibit little or no necking.

Question 17

Describe the compression test

Answer 17

In the compression test, the cross- section of the specimen increases, therefore, necking does not occur.
Materials that are extremely ductile are seldom tested in compression because the sample is constrained by friction at the points of contact with the platens of the apparatus.

Question 18

Describe potential problems in bending tests (4)

Answer 18

• High localized stresses at loading points
• Frictional forces at loading points
• Torsional loading
• Shear effect

Question 19

Define hardness

Answer 19

“The ability of a material to resist permanent deformation of its surface, in the form of scratching, indentation, abrasion or cutting.”
All Hardness tests are entirely empirical.
Not a Fundamental Property

Question 20

Describe the hardness test

Answer 20

In materials testing, the hardness test measures the resistance of a material to an indenter or cutting tool.
The indenter is usually made from a material such as hardened steel, sintered tungsten carbide, or diamond, and takes the shape of a ball, pyramid, or cone.
In most standard hardness tests, a load is applied by slowly pressing the indenter at a right angle to the surface of the sample for a given period of time.
For more accurate results, these tests are never taken near or at the edge of a test specimen.

Question 21

Describe the impact test

Answer 21

An impact test measures a material’s resistance of damage during an impact event.
Since impact events are random by nature, standard impact tests are empirical in design.
Not a Fundamental Property

Question 22

Describe the Charpy Test

Answer 22

The Charpy Test uses a notched rectangular bar, which is subjected to an impulse load, giving a relative indication of the notch toughness of a material under shock loading.

Question 23

Define fracture energy

Answer 23

The amount of energy that is irreversibly absorbed in the process of fracturing

Question 24

What are the three factors that can induce transition from ductile to brittle behavior

Answer 24

• Temperature
• Strain Rate
• Triaxiality