Objective 01: Destructive vs. Non-Destructive Testing and Tensile, Hardness, & Impact Testing Flashcards
Why must metals for manufacturing pressure equipment be tested?
- To establish and document their mechanical properties under various temperature-dependent, time-dependent, or cyclical loading service conditions
- After construction or repair to determine whether welding, forming or cutting procedures have maintained, improved, or diminished the mechanical properties of the material
- To determine whether a vessel material’s mechanical properties, over its normal service life, have diminished due to corrosion and cracking
- To locate defects in the parent material, weld joints, and the areas adjacent to weld joints that may cause premature or drastic failure of the vessel pressure boundary
What are the 2 main categories of material testing?
- Destructive Testing
2. Non-Destructive Testing/Non-Destructive Examination (NDE)
Destructive Testing
Test procedures that purposely cause damage to the tested metal, thereby destroying its integrity and rendering it unusable
Non-Destructive Testing/NDE
Test procedures that do no damage to the tested metal, allowing it to retain its integrity and its usefulness
What are 4 methods of Destructive Testing?
- Tensile Tests
- Hardness Test
- Impact Tests
- Proof Tests
What are 7 methods of Non-Destructive Testing?
- Visual
- Magnetic Particle
- Liquid Penetrant
- Ultrasonic
- Radiographic
- Acoustic Emission
- Hydrostatic and Pneumatic Leak Testing
Tensile Test/Tension Test
Test used to determine how a metal will react when tension forces are applied
What is the procedure of the Tension Test?
- Tension is applied to a test piece, thereby producing tensile stress in the metal
- As the material is stressed, the elongation and stress of the material are determined
How are valid results ensured from a Tension Test?
Consideration is given to the specimen’s shape and dimensions plus the choice of grips and faces
Specimen
Piece of metal that is tested
How is the shape of a specimen defined?
By the standard or specification being utilized in the test
Why is the shape of a specimen important to be considered?
It ensures the break occurs within the “gauge length” section (which is the cross-sectional area or diameter of the specimen that is reduced)
What happens to the stress on the specimen in the Tensile Test?
As the diameter is reduced within the gauge length, the stress becomes greater because stress is inversely proportional to the cross-sectional area under load
What basically happens to a specimen in the Tensile Test?
It is extended at constant rate while the load needed to maintain that is measured
Proportional Limit
The stress point, which may or may not be well-defined depending on the material being tested, at which slip (or glide) due to dislocation movement occurs. Above this point the transition from elastic to plastic deformation begins
Yield Stress
Stress at which yielding occurs across the whole specimen and deformation is purely plastic (non-reversible)
What determines the amount of stress required for slip in a particular metal grain?
The orientation of the grain
Why is the proportional limit and yield stress points generally coincident in a polycrystalline sample?
Polycrystalline material is composed of variously oriented, small individual crystals
Proof Stress
The point at which the specimen has undergone a certain (arbitrary) value of permanent strain, usually 0.2%
Why is Proof Stress used?
The precise positions of Proportional Limit and Yield Stress are often difficult to define and depend to some extent on the accuracy of the testing machine
What point defines the Ultimate Tensile Strength (UTS)?
The point at which plastic deformation becomes unstable and a narrow region (a neck) forms in the specimen. It is the peak (maximum) value of nominal stress during the test and deformation will continue in the necked region until fracture occurs.
Final Instability Point
The failure point at which fracture occurs
What are the 2 main types of strain?
- Elastic Strain
2. Plastic Strain
Elastic Strain
Stretching of material within its elastic limit, i.e. it will return to original dimenions when stress is removed
Hooke’s Law
Mathematical relationship that shows proportional relationship between stress and Elastic Strain via Young’s Modulus
Plastic Strain
Irreversible deformation of a material when it has exceeded its elastic limit
What is the equation that relates Plastic Strain to Stress?
There is none
Engineering Strain
The ratio of the change in length to the original length
True Strain
The ratio of the instantaneous length of the specimen as the test progresses to the original length
Ultimate Tensile Strength (UTS)
The maximum load the specimen sustains during a tensile test which may or may not equate to the strength at fracture
What determines whether the UTS equates to the strength at fracture?
If the tested metal is brittle, ductile, or exhibits both properties
Hardness Testing
Measurement of the resistance of a material to surface indentation
What are 2 ways of testing Indentation Hardness?
- Use a specfic force and measure either the penetration depth of the indentation or the area of the indentation
- Measure the load (i.e. force) that is applied to the indentor
What is the most important criterion for an Indentor?
Its ability to provide indentations that are geometrically similar and well-defined
What are 4 common Hardness Tests?
- Brinell
- Rockwell
- Vickers
- Knoop
What are 5 different methods to apply test loads in a Brinell Hardness Test?
- Dead weight
- Pneumatic
- Spring
- Hydraulic
- Impact
What is the procedure for a typical hydraulic Brinell hardness tester?
- The specimen is placed on the anvil
- The load is applied
- The indentor penetrates the specimen for 10 to 30 seconds
- The diameter of the round indentation is measured twice, with the second measurement taken 90 degrees from the first
- The mean diameter of the indentation is determined
- The Brinell hardness number is either calculated or determined from an appropriate table
What tool is used to measure the indentation for a Brinell Hardness Test?
Microscope
What causes errors in determining the actual diameter of indentation for a Brinell Hardness Test?
Incorrect instrument readings, a poorly defined indentation boundary, or both
What is the max acceptable error in reading a Brinell microscope?
0.01 mm over the entire 7 mm scale
What does the definition (clarity) of the indentation boundary depend on?
The characteristic of the tested material
What is the most widely used method for determinig hardness?
Rockwell Hardness Test
Why is the Rockwell Hardness Test the most widely used method for determining hardness?
Its simplicity
What are the 2 types of indentors used in the Rockwell Hardness Test?
- 120º diamond cone for Rockwell C Test
2. 1.6 mm diameter ball for Rockwell B Test
Rockwell Hardness Test
Hardness Test that applies 2 loads to a specimen and measures the difference in depth of penetration between the light (minor) load and the heavy (major) load
How heavy is the minor load used in the Rockwell Hardness Test?
10 kg
How heavy is the major load used in the Rockwell Hardness Test?
60, 100, or 150 kg
What is the Rockwell Hardness Test procedure
- The minor load is applied first to zero the setting on the dial depth gauge
- The major load is then applied for a specific time and then released
- The hardness number is indicated on a dial gauge
What kind of indentor is used for the Vickers Hardness Test?
136º diamond pyramid
How long is a load applied to the indentor for the Vickers Hardness Test?
10-15 seconds
How much load is applied to the indentor for Vickers Hardness Test?
1-120 kg with standard loads of:
1) 5 kg
2) 10 kg
3) 20 kg
4) 30 kg
5) 50 kg
6) 100 kg
7) 120 kg
How is the Vickers Hardness (HV) Number derived?
Ratio of Load to Indented Area
How is the indented area measured for the Vickers Hardness Test?
The diagonals of the indentation are measured using a micrometer microscope and the average reading of both diagonals used
Can a table be used to determine the Vickers Hardness Number?
Yes
Interpret 820HV50
Vickers Hardness of 820 derived from a 50 kg load
What are the advantages of the Vickers Hardness Test over other hardness tests?
- Method is accepted worldwide
- Indentor is accurate and shape is not distorted under loads
- Impressions made are small and thus limit damage to finished products
- Light loads of 1 kg allow for testing of thin materials
- There is only one scale for the hardness of all materials
- Damage to indentors is readily apparent as viewed under the microscope
- The diagonal of a square impression can be measured more accurately than the diameter of a ball impression
What are 4 guidelines that a Vickers Hardness Test should adhere to?
- The full test load must be applied for a sufficient time; 15 seconds is recommended
- The spacing of indentations is important; an important rule of thumb is that the distance between indentations or from the edge of the test specimen should be at least 2.5 times the length of the diagonal of the indentation
- Diagonals of indentations should be measured with the indentation perfectly centered within the viewing field of the microscope
- The micrometer microscope should be calibrated against a stage micrometer
Describe the indentation made by Knoop Microhardness Testing
A rhombic-based pyramidal diamond is used that produces a diamond shape with a long diagonal of 7 times the short diagonal. The expected depth of the indentations is about 1/30th of the long diagonal axis
What are the typical testing loads used by Knoop Microhardness Testing?
<500 g
Where is Knoop Microhardness Testing used?
Strictly confined to laboratory applications
What are the 3 categories of Hardness Testing?
- Microhardness Testing
- Low-Load Hardness Testing
- Macrohardness Testing
Microhardness Testing
Hardness Testing that uses up to 200g of load
Low-Load Hardness Testing
Hardness Testing that uses 200g-3kg of load
Macrohardness Testing
Hardness Testing that uses more than 3 kg of load
What category does Brinell Hardness Testing fall under?
Macrohardness
What category does Rockwell Hardness Testing fall under?
Macrohardness
Why can Brinell HT also be described as Bulk Hardness Testing?
It averages out small imperfections
How does the diameter of the indentations made in a Rockwell HT compare to Brinell Indentor?
They are significantly smaller
What category does Vickers Hardness Testing fall under?
Either Low-Load Hardness Test or Macrohardness Test
What kind of materials is the Vickers HT used on?
Because of the diamond test point:
1) hard materials with high loads; and
2) the hardness of small areas
What category does Knoop HT fall under?
Microhardness
What are the applications of Knoop HT?
- Hardness of micro-constituents of a matrix
- Failure analysis of components
- Testing extremely brittle materials
Why is Hardness Testing used during production?
It’s a simple and inexpensive test for evaluating material properties
During manufacturing why and where is Hardness Testing performed?
Hardness Testing ensures uniformity of heat treated components and are used to determine the suitability of pressure component materials for their intended use. It is done on materials that have been cold worked, quenched and tempered, or precipitation hardened
What is Indentation Hardness Testing used to identify in the fabrication and repair of pressure components?
- Misuse of welding consumables
2. Fabrication techniques that require post-weld heat treatment
In welding applications, what is hardness testing used to determine?
- If the strengths of the base and weld metals are matched
2. Provide indications of any effects the welding process may have on the heat affected zone (HAZ)
How is the hardness of a weld most effectively tested?
The Welding Procedure Specification (WPS) is supported by a Procedure Qualification Report (PQR) that has had hardness tests completed by the Rockwell Superficial Hardness or Vickers Diamond Pyramid Hardness testing technique
How should butt welds be tested for hardness?
Traverse hardness testing from the base metal through the HAZ, weld metal, and ending in the adjoining base metal. A number of traverses across the cross-section of the weld at different elevations are necessary to provide a representative sample of the weld
Impact Toughness
The ability of a metal to resist fracture when subjected to shock loading
What is the standard measure associated with Impact Testing?
The energy required to break a piece of metal of standardized shape with a cross-sectional area of 1 cm².
What are 2 common tests of Toughness?
- Charpy Test
2. Izod Test
What is the procedure for Impact Toughness?
- The metal to be tested is formed into a rectangular bar, with a 45 degree V-shaped notch removed
- The specimen is carefully placed on the apparatus’ anvils with precision tongs
- Then the bar is struck with a striker, mounted on a pendulum, causing the specimen to break
- The height to which the pendulum rises after the strike is compared to the height from which the pendulum originally fell
- The difference between the potential energy before and after the strike is a measure of the energy absorbed in the fracture
What is the difference between the Charpy and Izod tests?
- In the Charpy test the specimen is held at each end and is struck in the middle at the notch location.
- In the Izod test the specimen is held on one side of the notch and is struck on the other side of the notch
How many different standards are in use for Impact Testing?
7
How do the different standards differ from each other for Impact Testing?
The parameters of the striker, machine, and test pieces are slightly different which makes the results different and difficult to compare
How can the notch behaviour of FCC metals and alloys, a large group of nonferrous materials and the austenitic steels be estimated?
From their common tensile properties
Can the behaviour of ferritic steels under notch conditions be predicted from their properties?
No
