Material Mechanical Property Testing
Shock Testing
Shock trials distort the fragility or ability of a material to absorb instantaneous loads. They check if a machine or structure fails or becomes brittle under working conditions.
Charpy Methods
Flexion by shock on simply supported bars. The specimens are placed flat on the machine with the notch opposite to the impact point.
Izod Methods
Flexion by shock on an embedded bar. The cantilever specimen is placed in a vertical position with the notch facing the impact.
Fatigue Testing
Fatigue is the breakage of a material produced by systematically repeated changes in stress, such as tension, compression, flexure, or torsion.
Fatigue Test Classification: Constant Amplitude
Evaluates fatigue performance under default load or deformation cycles, generally with sinusoidal amplitude and constant frequency.
Variable Amplitude Fatigue Testing
Assesses the effect of accumulated damage due to the variation of stress amplitude over time.
High Cycle Fatigue Testing
Load-time spectrum of simple continuous load at constant amplitude, usually sinusoidal.
Fatigue Cycle Parameters
Parameters defining the cycle:
- Mean Stress: Pm = (P1 + P2) / 2
- Stress Amplitude: Pa = (P1 – P2) / 2
- Stress Ratio: R = P1 / P2
Stress Cycle Types: Alternating
The stresses change sign alternately.
Stress Cycle Types: Pulsating (from Zero)
Stresses are always in the same direction, varying from 0 to a certain value.
Stress Cycle Types: Pulsating (Non-Zero)
When the stress varies from a maximum to a minimum value other than 0 within the same sign.
(+) tension (-) compression
Fatigue Test Frequency Ranges
- LOW: f < 5 Hz
- MEDIUM: 5 < f < 30 Hz
- HIGH: 30 < f < 150 Hz
- VERY HIGH: f > 150 Hz
- Very high frequency is often used to reduce test times.
Origin of Fatigue Failure in Metals
Fatigue failure in metals originates from plastic deformation within the structure. Atomic spaces are generated near the surface of the metal, separating grains.
Hardness Testing
Hardness testing provides important material characteristics. It measures the resistance of a body to being scratched or penetrated by another.
Hardness Testing Methods
- Static penetration test
- Rebound test
- Scratch test
- Abrasion or erosion test
Penetration Testing Principle
Measures the resistance to penetration or plastic deformation that a material offers when pressed by a given penetrator under predefined loads.
Brinell Hardness Test
To measure Brinell hardness, a very hard steel ball is compressed onto the surface of the material for a specific time, producing a spherical cap impression.
HB = P / S
HB = 2P / [π * D * (D – √(D² – d²))] Kgf/mm²
Brinell Test Parameters
- Ball Diameters: 10, 5, 2.5, 2, 1 mm
- Ball Material: Steel (up to 450 HB), Tungsten Carbide (up to 630 HB)
- Time: 10-15 seconds for steel, 30 seconds for soft metals
- Loads: 3000 kgf, 1500 kgf, 500 kgf (and others depending on material)
Rockwell Hardness Test
Rockwell hardness is estimated based on the depth of penetration using an initial load followed by an additional load.
Rockwell Test Loads
- Initial Load: 10 kgf
- Additional Loads: 50 kgf, 90 kgf, 140 kgf
Rockwell Penetrators
- Steel Balls: 1/16″, 1/8″, 1/4″, 1/2″
- Diamond Cone (120°)
Common Rockwell Scales (HRC, HRB)
- HRC: Uses a diamond cone and 150 kgf total load.
- HRB: Uses a 1/16″ ball and 100 kgf total load.
Vickers Hardness Test
Similar to Brinell, Vickers hardness depends on the applied load and the surface area of the impression.
Vickers Test Parameters
- Load: 1 to 120 kgf (and higher)
- Penetrator: Diamond pyramid (square base, 136° angle)
Tensile Testing
Simple tension occurs when normal loads are applied evenly distributed across the cross-section of a body, causing elongation.
Static Tensile Test Results
Results obtained include:
- Yield Strength
- Maximum Load (Tensile Strength)
- Static Resistance (Ultimate Tensile Strength)