Material Testing Methods: Hardness, Ultrasound, and More
Hardness Test
In metallurgy, hardness is measured using a durometer for the penetration test. Depending on the tip used and the range of applied loads, there are different levels, suitable for different ranges of hardness.
The scales of current industrial use are:
- Brinell Hardness: Employs a hardened steel or tungsten carbide ball. For hard materials, it is very accurate but easy to apply. Imprecise with plates of less than 6mm thick. Estimates tensile strength.
- Rockwell Hardness: Uses a conical diamond tip (in some cases a steel ball). It is the most widespread, since the hardness is obtained by direct measurement and is suitable for all types of materials. It is usually considered a non-destructive test due to the small size of the footprint.
- Rockwell Superficial: There is a variant of the test, called Rockwell superficial, for the characterization of very thin pieces, such as razor blades or layers of materials that have received a hardening treatment.
- Webster Hardness: Uses manual machines for measuring parts, being suitable for difficult-to-manage long extruded profiles. The value obtained is often converted to Rockwell values.
- Vickers Hardness: Uses a diamond indenter with a square-based pyramid shape. For soft materials, Vickers values match the Brinell scale. An improved test for Brinell hardness with sheets up to 2mm thick.
- Shore Hardness: Uses a scleroscope. An indenter is dropped on the material surface, and the rebound is measured. It is dimensionless but has several scales. A higher bounce indicates a harder material. Applicable for surface quality control. The method is flexible, with no penetration like the others.
Ultrasound Test
A material can, in turn, transmit and reflect elastic waves. An ultrasonic transducer made of quartz, barium titanate, or lithium sulfate uses the piezoelectric effect to introduce a series of high-frequency elastic pulses into the material, usually above 100,000 Hz. Each pulse creates a compression wave of deformation, which propagates through the material. The elastic wave is transmitted through the material at a rate that depends on the modulus of elasticity and density of it.
Tensile Test
The tensile test involves subjecting a standardized specimen made of the material to an increasing axial tensile stress until the specimen ruptures. In a tensile test, various characteristics of the elastic material can be determined:
- Elasticity: Modulus of Young’s Modulus, which quantifies the previous proportionality.
- Poisson’s Ratio: Quantifies the ratio of longitudinal lengthening and shortening of the length transversely to the direction of force.
- Limit of Proportionality: The voltage value below which the elongation is proportional to the applied load.
- Limit of Yield or Apparent Yield Strength: The stress value that the specimen supports during the yield strength and creep phenomenon. This phenomenon occurs in the transition zone between elastic and plastic deformation and is characterized by a rapid increase in deformation without an appreciable increase in the applied load.
- Limit Strength (Yield Strength or Practical): The value of the voltage at which there is a default in advance elongation (0.2%, 0.1%, etc.). According to the extensometer used.
- Breaking Load or Tensile Strength: The maximum load resisted by the specimen divided by the initial section of the specimen.
- Elongation: The increase in length that the specimen has experienced. It is measured between two points whose position is normalized and is expressed as a percentage.
- Striction: The reduction of the section that occurs in the area of the rupture.
Normally, the yield limit is usually determined as irrelevant for the calculations. Nor is Young’s modulus calculated, as this is characteristic of the material, so all steels have the same modulus of elasticity, but their resistance can be very different.
Resilience Test
Charpy Impact Test
In engineering, resilience is the amount of energy that a material can absorb before irreversible deformation begins, i.e., plastic deformation. It corresponds to the area under the curve of a tensile test between the null and the deformation strain corresponding to the yield stress. In the International System of Units, it is expressed in joules per cubic meter.
It is determined by the Izod test method or Charpy pendulum, being a value indicative of the brittleness or impact resistance of the tested material. A high degree of resilience is characteristic of austenitic steels, steels with a high content of austenite.
In physics, the term is used to express the ability of a material to recover its original shape after being subjected to high pressures. In these cases, it is related to the energy that the material is capable of storing when its volume is reduced.
Compression Test
Compressive stress is a pressure that tends to cause a reduction in volume. When a material is subjected to a force of bending, shearing, and torsion, forces act simultaneously, twisting and compressing it.
It is the force acting on a building material, assuming it is composed of parallel planes, which makes the force try to reconcile these planes while maintaining its parallel (characteristic of stone materials).
Tests performed to measure compressive stress are contrary to the voltage applied, with respect to the direction and orientation of the applied force.
It has several limitations:
- The difficulty of applying a concentric or axial load.
- A cylinder of circular section is preferable to other forms.
Fatigue Testing
In engineering, particularly in materials science, fatigue of materials refers to a phenomenon whereby the rupture of materials under dynamic cyclic loads occurs more easily than under static loads. An example of this is a wire: flexing it repeatedly breaks it easily. Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses (bridges, airplanes, etc.). It can occur at a voltage lower than the tensile strength or elastic limit for a static load. It is very important because it is the leading cause of rupture of metallic materials (approximately 90%), although it also occurs in polymers and ceramics.
Types of Nondestructive Testing (NDT)
Superficial NDT
These tests provide information about the health of inspected surface materials. Superficial NDT methods are:
- VT – Visual Inspection
- PT – Penetrant Testing
- MV – Magnetic Particle Testing
- ET – Electromagnetic Testing
VT and PT are limited to detecting only surface discontinuities (open at the surface), while MT and ET can detect both surface and sub-surface discontinuities (those found under the surface but very close to it).
Volumetric NDT
These tests provide information about the internal health of inspected materials. Volumetric NDT methods are:
- RT – Industrial Radiography
- UT – Industrial Ultrasound
- AE – Acoustic Emission
These methods allow the detection of internal and sub-surface discontinuities and, under certain conditions, the detection of surface discontinuities.
Leakage Nondestructive Testing
These tests provide information on the extent to which fluids can be contained in containers without escaping into the atmosphere or getting out of control. NDT methods for sealing are:
- LT – Leak Testing
- Pressure Testing for Change (Pneumatic or Hydrostatic)
- Bubble Testing
- Mass Spectrometer Testing
- Leak Tests with Halogen Trackers
Common Nondestructive Tests
- Ultrasound
- Analysis of Oil and Ferrography
- Analysis of Vibration and Noise Analysis
- Metallographic Analysis
- Induced Current
- Penetrant Inspection
- Magnetic Particle Inspection
- Inspection of Welds
- Ultrasonic Inspection
- Loss of Magnetic Flux
- Radiography
- Thermography
- ACFM (Alternative Current Field Measurement)