Testing Methods in Education

Types of Microscopes Used in Microscopic Examination

• Upright microscopes:
  • Most commonly used in metallographic examination
  • Suitable for smaller metal samples
• Inverted microscopes:
  • Offer a valuable alternative for observing thick or bulky metal specimens
  • Feature a larger working distance and allow for easier focusing on samples with uneven surfaces
• Semiconductor microscopes:
  • Specialized optical instruments designed for inspecting and analyzing semiconductor wafers and other microelectronics components
  • Have a large stage for whole wafers

Hardness Testing

• Definition:
  • Evaluates the material's hardness, strength, and wear resistance
• Method:
  • Pressing a hard object into the surface of a sample material and measuring the depth of the indentation
• Types of Hardness Tests:
  • Rockwell hardness test
    • Fast hardness test method for production control with a direct readout
    • Mainly used for metallic materials
  • Vickers hardness test
    • Can assess all test materials and samples, regardless of their texture
    • Measures the full range of hardness values
  • Knoop hardness test
    • An alternative to the Vickers test in the microhardness testing range
    • Used to overcome cracking in brittle materials and to facilitate the hardness testing of thin layers
  • Brinell hardness test
    • Used for hardness testing larger samples in materials with a coarse or inhomogeneous grain structure
    • Leaves a relatively large impression using a tungsten carbide ball

Shear Testing

• Definition:
  • Designed to apply stress to a test sample so that it experiences a sliding failure along a plane parallel to the forces applied
• Uses:
  • To determine the shear strength of a material
  • To determine the important design characteristics of many types of fasteners
  • To provide valuable information on how the material behaves under shear stress
• Procedures:
  1. Select the test specimen based on the type of test to conduct and the standard guiding it
  2. Set up the test specimen in the universal testing machine
  3. Apply a load to the material at a set rate until it deforms
  4. Record the applied load and deformation information
  5. Analyze the results by plotting a load vs. stress vs. strain graph
  6. Document every piece of information about the tested material
• Types of Shear Tests:
  • Four-point bend fixture or modified three-point flexure fixture
  • Tapered ends inserted into grasp fixtures
• Types of Materials Tested in Shear:
  • Adhesives
  • Layered composites
  • Stiff substrates

Compression Testing

• Definition:
  • Evaluates the material's behavior under compressive loads
• Uses:
  • To determine the compressive strength of a material
  • To determine the important design characteristics of many types of materials
  • To provide valuable information on how the material behaves under compressive stress
• Procedures:
  1. Determine the nearest testing procedure for your material
  2. Prepare your test samples according to the form and size specified in the test standard
  3. Set up your Universal Test Machine
  4. Load the test sample onto the platens, carefully aligning it with the loading frame's center axis
  5. The test sample breaks after reaching its ultimate compressive strength### Metallographic Mounting
• Mounting is a crucial step in metallography, preparing the sample for subsequent grinding and polishing.
• Requirements for metallographic mounting compounds:
  • Hardness
  • Abrasion resistance
  • Shrinkage
  • Chemical resistance
  • Low shrinkage during solidification and good adhesion to the specimen
  • Bubble-free condition
  • Correct viscosity during casting
  • Identical metallographic grinding or polishing behavior as the mounted material
  • Inert behavior during metallographic sample preparation and analysis
  • No reaction with the sample
  • No changes by high temperatures or pressure during the process
  • Good electrical conductivity for further processing (if necessary)

Types of Mounting

• Metallographic Hot Mounting:
  • Uses granulated polymeric material
  • Softened, compressed, and cooled sequentially in a metallographic hot mounting press
  • Suitable for pressure-resistant samples with simple geometries
  • Can be carried out at 150-200°C and 100-300 bar pressures
  • Two types of materials used:
    • Thermosets (e.g. phenolic resins, melamine and epoxy resins with mineral fillers)
    • Thermoplastics (e.g. PMMA powders)
• Metallographic Cold Mounting:
  • Minimal technical requirements compared to hot mounting
  • Requires mold and cold material compound
  • Main selection criteria:
    • Hardness
    • Abrasion resistance
    • Shrinkage
    • Curing
    • Exothermic heat development
  • Four main types of resins used:
    • Acrylic resin
    • Polyester resin
    • Epoxy resin
    • Light curing materials

Selection of the Appropriate Mold

• Polyolefin-based molds (PE and PP):
  • In case of older molds, the removable base may not be flat
  • Different grinding planes can be created depending on the position of the individual sample
• Teflon (PTFE) based molds:
  • High dimensional accuracy
  • Ideally suited for automatic grinding and polishing
  • Expensive
• Silicone rubber-based molds:
  • After repeated use, silicone molds lose their roundness
  • Suitable for the hardening of UV-initiated compounds
  • May react with polyester resins

Coarse Grinding

• Purpose:
  • Establish an initial flat surface crucial for subsequent grinding and polishing processes
  • Prevent cold working and deformation of the material
• Execution:
  • Can be conducted either wet or dry
  • Uses electrically powered disks or belts with grit sizes ranging from 80 to 180
  • Care must be taken to prevent significant heating of the sample
• Tools used:
  • Angle Grinder
  • Bench Grinder

Metallographic Etching

• Purpose:
  • Reveal microstructural details not evident on the as-polished sample
• Properties revealed:
  • Shape and size of grain boundaries
  • Metallic phases
  • Inclusions
  • Integrity of solder points
  • Cracks and other issues in welds
  • Uniformity, quality, and thickness of coating materials
• Types of Etching:
  • Chemical Etching:
    • Most common method
    • Sample is immersed in an etching solution for a controlled time
  • Electrolytic Etching:
    • Electrical current is applied during the etching process
    • Allows for more precise control over the etching rate and features revealed

Microscopic Examination

• Metallurgical microscopes:

  • Specialized optical instruments for examining and analyzing opaque materials (metals and alloys)
  • Used in materials science, metallurgy, quality control, and research
  • Inspect the microstructure and properties of metallic samples### Disadvantages of Brinell's Hardness Testing

• Small items cannot be tested due to the large indenter impression.

• Thin items should not be used because the test causes deep penetration.

• The test process is slow, taking a long time to complete.

Impact Test

• A type of destructive test used to evaluate the resistance of materials to impact and shock.
• Involves striking a material sample with a pendulum or a falling weight and measuring the energy required to fracture it.
• Used to evaluate the toughness and durability of materials such as metals, plastics, and composites.

Uses of Impact Test

• Useful for measuring a material's toughness and capacity to withstand deformation caused by impact loads.
• Helps engineers and designers select acceptable materials for a specific application, resulting in more effective and safe products.
• Allows producers to control the quality of their products, especially in the fabrication of items that require a certain level of toughness.
• Directs the design and optimization of components and structures, and helps verify that components meet relevant requirements and are safe to use.
• Many industries have tight specifications for the materials used in their products, and impact testing helps producers comply with these standards and regulations.

Procedure of Impact Test

• Place the specimen on the holder with the notch facing the pendulum.
• Set the pendulum at the specified height and release it.
• Record the final height of the pendulum and calculate the energy absorbed by the specimen.

Fatigue Test

• Used to evaluate the durability and resistance to cyclic loading of materials.
• Involves subjecting a material sample to repeated loading and unloading until it fails.
• Commonly used to evaluate the properties of metals and polymers subjected to cyclic loading in service.

Uses of Fatigue Test

• Determine the lifespan that may be expected from a material subjected to cyclic loading.
• Determine the maximum load that a sample can withstand for a specified number of cycles.

Procedure of Fatigue Test

• Load the sample into a fatigue tester or fatigue test machine.
• Load the sample using the pre-determined test stress, then unload to either zero load or an opposite load.
• Repeat the cycle of loading and unloading until the end of the test is reached.

Spark Test

• Produces sparks by grinding the metal against a wheel.
• The length, color, and shape of the sparks can identify the metal family.

Uses of Spark Test

• To reveal characteristics of ferrous metals.
• Can be used as a quick and inexpensive method to identify different metals.

Procedure of Spark Test

• Turn the grinder on and bring the metal into light contact with the grinding wheel.
• Observe the color, pattern, and length of the sparks produced.

Non-Destructive Testing

• A collection of analytical methods used to assess the characteristics of materials, parts, or systems without damaging them.
• Non-destructive testing (NDT) techniques provide the non-invasive examination and assessment of defects, fissures, gaps, inclusions, and discontinuities.

Radiographic Testing

• A non-destructive testing (NDT) method that examines a component's internal structure using x-rays or gamma rays.
• Frequently used in the petrochemical industry to find defects in machinery, such as pressure vessels and valves.

Types of Radiographic Testing

• Conventional Radiography
• Digital Radiography
• Computed Radiography
• Direct Radiography
• Real-time Radiography
• Computed Tomography
• Automated Radiographic Testing

Advantages of Radiographic Testing

• Internal defect detection
• High sensitivity
• Permanent record
• Versatility
• Non-contact inspection

Disadvantages of Radiographic Testing

• Safety considerations
• Equipment and expertise
• Limited to accessible surfaces
• Time-consuming
• Cost

Magnetic Particles Testing

• A method that magnetizes a component to create lines of magnetic flux inside it.
• A flux leakage field forms at and slightly above the component surface when the flux lines hit a discontinuity that is oriented roughly perpendicular to the direction of flux flow.

Types of Magnetic Particles

• Dry Magnetic Particles
• Wet Magnetic Particles

Advantages of Magnetic Particles Testing

• Sensitivity to surface defects
• Rapid and cost-effective
• Portable equipment
• Immediate result
• Versatility

Disadvantages of Magnetic Particles Testing

• Limited to ferromagnetic materials
• Surface accessibility
• Surface preparation
• Limited depth of penetration

Ultrasonic Testing

• A variety of non-destructive testing (NDT) methods that use ultrasonic waves to pass through a substance or object.
• Used to assess the properties of materials or find flaws.

Types of Ultrasonic Testing

• Contact
• Immersion

Advantages of Ultrasonic Testing

• Versatility
• Depth of Penetration
• High Sensitivity
• Real-time results
• Precise defect sizing

Disadvantages of Ultrasonic Testing

• Surface preparation
• Skill and Training
• Limited to homogenous Materials
• Limited accessibility

Eddy Current Testing

• A process of running electronic probes through the length of various types of tubes or along the surfaces of materials to find flaws in them.

Eddy Current Testing Step by Step

• Generate Alternating Current (AC)
• Induce Eddy Currents
• Detection of Change

This quiz assesses knowledge on testing methods in education, covering various aspects of educational assessment.