Destructive Testing Methods

There are four main standards of hardness testing:

1. Brinell Hardness Test:

   • Method: Uses a hard steel or carbide ball as an indenter, which is pressed into the material's surface under a specific load for a set duration.

   • Differentiator: Produces a relatively large, visible indentation.

   • Usage: Typically used for softer metals like aluminum, brass, and soft steels. Due to its large indentation, it's less suitable for fine or precise applications.

2. Rockwell Hardness Test:

   • Method: Employs a diamond cone (for harder materials) or a steel ball (for softer materials) as the indenter. The depth of indentation, rather than its size, determines hardness.

   • Differentiator: Quick and simple with minor surface damage, making it a popular method.

   • Usage: Widely used for steels, brass, and other metals. Its minimal damage to the sample allows for testing of finished parts.

3. Vickers Hardness Test:

   • Method: Uses a square-based diamond pyramid as the indenter, applying a load ranging from a few grams to a few kilograms.

   • Differentiator: Known for its accuracy and consistency, it leaves a small square indentation.

   • Usage: Suitable for thin materials, surface layers, or small parts. It's versatile and can be applied to both very soft and very hard materials.

4. Knoop Hardness Test:

   • Method: Uses a diamond with an elongated, pointed shape as the indenter, making a shallower and longer indentation than the Vickers test.

   • Differentiator: Produces a very small, elongated indentation.

   • Usage: Ideal for testing brittle materials or thin layers, as the indentation is smaller and more precise than other methods.

Impact testing determines a material's toughness—its ability to absorb energy during plastic deformation. This energy corresponds to the amount needed to fracture a material, making impact testing essential to understand the material's resistance to shocks or impacts.

Similar to Hardness Testing, there are multiple impact testing methods:

1. Charpy Impact Test:

   • How It Works: A notched specimen is supported at both ends and struck at its center by a pendulum. The energy absorbed by the specimen before fracturing is measured.

   • When Used: Charpy is one of the most commonly used methods, especially for metals, and is employed to measure the energy absorbed by a material during fracture.

2. Izod Impact Test:

   • How It Works: Similar to the Charpy test, but the sample is held vertically and struck just above the notch.

   • When Used: Izod testing is less common than Charpy but still widely used for certain plastics and metals.

3. Drop Weight Test:

   • How It Works: A weight is dropped from a predetermined height onto a specimen to see if it fractures.

   • When Used: This test is typically used for larger, thicker samples and can be applied to metals, plastics, and ceramics.

4. Falling Dart Impact Test (also called Gardner Impact Test):

   • How It Works: Commonly used for plastics, a weighted dart or pendulum falls from a specific height onto a clamped specimen.

   • When Used: This method is popular for assessing the impact resistance of plastic films and sheeting.

Fatigue refers to the weakening or failure of a material subjected to repeated loading and unloading. If a material is subjected to a cyclic stress, it may fail at a stress much lower than its tensile strength during a single application of load.

It's commonly cited that fatigue accounts for approximately 90% of all mechanical service failures in metals, especially those subjected to varying cyclic loads. This highlights the importance of understanding and predicting fatigue life in engineering components and structures.

Why is it Important?:

• Unexpected Failures: Fatigue failures can occur without any significant deformation or evident distress.

• Low-Stress Failures: Materials can fail due to fatigue even at stresses well below the yield strength.

• Critical Structures: Many structures, like bridges or aircraft, are subjected to repeated loads throughout their service lives, making them susceptible to fatigue.

Fatigue Testing:

• Rotating Bending Fatigue Test: A specimen is loaded at one end and rotated, subjecting it to bending. This is a simple and common method.

• Axial Fatigue Test: The sample is subjected to a repeated or cyclic load along its axis. This can be both compressive and tensile.

• Torsional Fatigue Test: The specimen is twisted back and forth to study the effect of repeated torsional loads.

• Fatigue Crack Growth Test: This test measures the rate at which an initial crack will grow under cyclic loading conditions.

• Vibration Tests: Especially used for components in industries like automotive and aerospace, they subject parts to vibrations to replicate real-world fatigue conditions.

• Thermal Fatigue Testing: Evaluates how thermal cycling (repeated heating and cooling) can induce fatigue in materials.

• Random Load/Block Cycle Testing: Instead of a simple repetitive load, real-world complex load patterns are applied to see how they affect the material's fatigue life.

Creep is the time-dependent deformation of materials when subjected to a constant stress, usually at high temperature for materials that are subjected to high levels of stress.

In industries like power generation, aerospace, and any industry involving high-temperature processes, creep-induced failures can be significant. While it might not be as dominant as fatigue, it is especially crucial in high-temperature applications. Some sources indicate that a significant fraction (though not the majority) of failures in superalloy turbine components, for instance, can be attributed to creep.

Why is it Important?:

• Temperature-Dependent Deformation: Many materials that are stable and durable at room temperature can deform under the same load at elevated temperatures.

• Long-Term Failures: Even if a material can bear a certain load without immediate deformation or failure, it might still gradually deform over time due to creep.

• Affect on Material Properties: Creep can lead to grain boundary cavitation, void formation, and other microscopic changes, affecting a material's other properties.

Creep Testing:

• Constant Load (or Stress) Test: The most straightforward method, where a sample is subjected to a constant load at a specific temperature, and deformation over time is monitored.

• Constant Strain (or Strain-Rate) Test: A constant rate of strain is applied, and the stress required to maintain that rate is monitored. This method is more complex but can be more informative.

• Stepped Stress Test: The test starts at a low stress, which then gets incremented at regular intervals until rupture. This can help derive the complete creep curve more quickly.

• Impression Creep Test: Used mainly for soft metals. A hard indenter is pressed into the material under constant load at elevated temperatures, and the impression depth over time is measured.