In structural and mechanical engineering, materials rarely fail under slow, gradual static loads alone. Catastrophic failures often occur when components are subjected to sudden, dynamic shock or impact loads. To evaluate the capacity of a metal to absorb shock energy without fracturing, engineers rely on the Notched Impact Test.
The two universal standard notched impact testing methods tested extensively in competitive exams (APPSC AEE, TNPSC AE/JDO, SSC JE, GATE, and ESE) are:
- Charpy Impact Test: Specimen supported as a Simply Supported Beam.
- Izod Impact Test: Specimen clamped as a Cantilever Beam.
📌 Key Exam Takeaways at a Glance
- 🔨 Primary Purpose: Measures dynamic Toughness (energy absorbed up to fracture) and detects brittleness under shock.
- 🌉 Charpy Method: Specimen is Simply Supported horizontally; Striker hits behind the notch (opposite face).
- 🏗️ Izod Method: Specimen is clamped vertically as a Cantilever; Striker hits on the notched side (22 mm above notch).
- ⚡ Energy Capacities: Charpy typically 300 Joules | Izod typically 168 – 170 Joules.
- 📐 Specimen Dimensions: Charpy: 10 × 10 × 55 mm | Izod: 10 × 10 × 75 mm.
1. Fundamentals: Toughness, Resilience & The Role of Notches
Before comparing the tests, it is essential to distinguish between fundamental mechanical properties:
Toughness
The ability of a material to absorb energy and deform plastically before fracturing. In a stress-strain diagram, it equals the total area under the stress-strain curve up to fracture. Impact tests measure this dynamic toughness directly in Joules (J).
Resilience & Modulus of Resilience
The capacity of a material to absorb energy when deformed elastically and release that energy upon unloading. Modulus of Resilience (Ur) is the strain energy per unit volume up to the elastic limit:
❓ Why is a "Notch" Introduced in the Specimen?
Without a notch, a ductile material undergoes severe plastic deformation across its entire gauge length, absorbing huge amounts of energy. A sharp notch creates severe stress concentration and introduces a triaxial state of tensile stress at the root of the notch. This triaxial tension restricts plastic slip (yielding), forcing the metal to exhibit its tendency toward sudden brittle fracture under high strain rates.
2. Comprehensive Comparison: Charpy vs. Izod Test
This table summarizes all direct questions frequently asked in competitive engineering exams:
| Parameter | Charpy Impact Test | Izod Impact Test |
|---|---|---|
| Support Condition | Simply Supported Beam (supported at both ends across two anvils) | Cantilever Beam (firmly clamped at the base in a heavy vice) |
| Specimen Orientation | Horizontal | Vertical |
| Standard Specimen Dimensions | 10 mm × 10 mm × 55 mm | 10 mm × 10 mm × 75 mm (or round 11.4 mm dia) |
| Notch Location | At the center of the span (27.5 mm from ends) | 28 mm from one end (clamped section) |
| Position of Striker vs. Notch | Striker strikes at the midpoint on the face opposite to the notch (notch faces away from striker) | Striker strikes on the same side as the notch, at a fixed height of 22 mm above the notch centerline |
| State of Stress at Notch | Notch is placed in tension (bending puts opposite side in tension) | Notch is placed in tension (cantilever bending puts struck side in tension) |
| Pendulum Maximum Energy | Typically 300 Joules | Typically 168 to 170 Joules (approx. 120 ft-lb) |
| Striking Velocity | Approximately 5.0 to 5.5 m/s | Approximately 3.5 to 3.8 m/s |
| Standard Notch Types | V-notch (45°, 2 mm deep, 0.25 mm root radius) or U-notch / Keyhole notch | Standard V-notch (45°, 2 mm deep, 0.25 mm root radius) |
| Primary Application | Universal standard for steels, low-temperature DBTT transition testing | Widely adopted in British standards, plastics, and room-temperature metal checks |
3. Working Principle & Energy Calculation
Both machines operate on the pendulum principle based on the law of conservation of energy:
1. Initial Energy: A heavy pendulum hammer of weight W (or mass m) is raised to an initial height H1, storing initial potential energy:
2. Impact: The pendulum is released, swings down under gravity, and strikes the notched specimen at the lowest point of its swing with maximum velocity.
3. Residual Energy: After fracturing the specimen, the hammer continues its upward swing to a reduced height H2 (angle θ2), retaining residual potential energy:
4. Absorbed Impact Energy: The Impact Energy Absorbed (KV) by the specimen to cause fracture is the difference between initial and residual energies:
*Note: The machine's calibrated dial pointer directly measures and displays this absorbed energy (KV) in Joules (J).
4. Temperature Effect: Ductile-to-Brittle Transition (DBTT)
One of the most vital engineering insights derived from Charpy impact testing is how impact toughness changes with temperature:
- BCC Metals (Ferritic steels, Mild steel): Exhibit a distinct Ductile-to-Brittle Transition Temperature (DBTT). At elevated temperatures, energy absorption is high (ductile fibrous fracture). Below DBTT, energy absorption plummets drastically (brittle cleavage fracture). Famous historical ship fractures (like the Titanic and WWII Liberty Ships) were caused by steels operating below their DBTT in icy sea waters!
- FCC Metals (Aluminium, Copper, Nickel, Austenitic Stainless Steel): Do NOT exhibit a sharp DBTT. They maintain high impact toughness even at extremely low cryogenic temperatures (-196°C) due to numerous slip systems.
- HCP Metals (Titanium, Magnesium, Zinc): Generally show lower toughness and a less pronounced transition.
📝 12 High-Yield Practice MCQs (With Detailed Explanations)
These questions reflect the most frequent patterns in APPSC AEE, TNPSC AE, SSC JE, and GATE examinations.
Q1. In the Charpy impact test, the test specimen is supported as a:
(A) Fixed beam
(B) Cantilever beam
(C) Simply supported beam
(D) Overhanging beam
View Answer & Explanation
Correct Answer: (C) Simply supported beam
Explanation: In the Charpy test, the specimen is mounted horizontally as a simply supported beam across two anvils (span of 40 mm). In contrast, the Izod test supports the specimen vertically as a cantilever.
Q2. In an Izod impact test, the hammer strikes the specimen:
(A) On the side opposite to the notch
(B) On the face containing the notch, above the notch
(C) Directly inside the notch groove
(D) Simultaneously from both sides
View Answer & Explanation
Correct Answer: (B) On the face containing the notch, above the notch
Explanation: In the Izod test, the specimen is clamped vertically as a cantilever with the notch facing the striker. The hammer strikes 22 mm above the notch on the same notched face, inducing bending tension at the root of the notch.
Q3. What are the standard cross-sectional dimensions and length of a Charpy impact test specimen?
(A) 10 mm × 10 mm × 75 mm
(B) 10 mm × 10 mm × 55 mm
(C) 12 mm × 12 mm × 60 mm
(D) 15 mm × 15 mm × 55 mm
View Answer & Explanation
Correct Answer: (B) 10 mm × 10 mm × 55 mm
Explanation: The standard Charpy specimen is 10 mm × 10 mm × 55 mm with a 45° V-notch (depth 2 mm) at the center. The standard Izod square specimen is 10 mm × 10 mm × 75 mm.
Q4. The mechanical property determined by a notched bar impact test is:
(A) Hardness
(B) Malleability
(C) Toughness
(D) Elasticity
View Answer & Explanation
Correct Answer: (C) Toughness
Explanation: The impact test determines dynamic toughness (or notch toughness), which is the energy required to fracture a notched specimen under high-speed shock loading.
Q5. In the Charpy test, the notch is located:
(A) On the compression side during impact
(B) On the tension side, facing away from the striking edge
(C) Directly facing the pendulum blow
(D) At either end of the span
View Answer & Explanation
Correct Answer: (B) On the tension side, facing away from the striking edge
Explanation: The hammer strikes the center of the unnotched rear face. This three-point bending creates tensile stress on the opposite notched face, triggering fracture initiation at the root of the notch.
Q6. Which of the following crystal structures does NOT exhibit a sharp Ductile-to-Brittle Transition Temperature (DBTT)?
(A) Body-Centered Cubic (BCC)
(B) Face-Centered Cubic (FCC)
(C) Hexagonal Close-Packed (HCP)
(D) Body-Centered Tetragonal (BCT)
View Answer & Explanation
Correct Answer: (B) Face-Centered Cubic (FCC)
Explanation: FCC metals (such as Aluminium, Copper, Nickel, and Austenitic Stainless Steel) possess 12 close-packed slip systems that remain active at low temperatures. Therefore, they remain ductile and do not exhibit a sharp DBTT.
Q7. The energy capacity of a standard pendulum hammer in the Charpy impact test is usually around:
(A) 50 J
(B) 100 J
(C) 168 J
(D) 300 J
View Answer & Explanation
Correct Answer: (D) 300 J
Explanation: Standard Charpy impact testing machines are calibrated for a maximum pendulum energy of 300 Joules (or 300 to 360 J). Izod machines are typically rated for 168 – 170 Joules.
Q8. In an Izod impact test, the distance of the striking edge from the notch root is:
(A) 10 mm
(B) 22 mm
(C) 28 mm
(D) 40 mm
View Answer & Explanation
Correct Answer: (B) 22 mm
Explanation: According to BS 131 and standard test specifications, the striking knife-edge impacts the cantilever specimen at a fixed distance of exactly 22 mm above the root of the notch.
Q9. What is the standard angle and depth of the standard V-notch used in impact testing?
(A) 30°, depth 1 mm
(B) 45°, depth 2 mm
(C) 60°, depth 2 mm
(D) 90°, depth 3 mm
View Answer & Explanation
Correct Answer: (B) 45°, depth 2 mm
Explanation: The standard V-notch has an included angle of 45°, a depth of 2 mm, and a root radius of 0.25 mm. This leaves an effective net fracture ligament area of 10 mm × 8 mm = 80 mm².
Q10. The state of stress produced at the root of a sharp notch during an impact test is:
(A) Uniaxial tension
(B) Pure shear
(C) Triaxial tension
(D) Biaxial compression
View Answer & Explanation
Correct Answer: (C) Triaxial tension
Explanation: The geometry of the notch restrains lateral contraction, creating tensile stresses in all three orthogonal directions (triaxial tension). This severely inhibits plastic shear deformation (τmax ≈ 0), leading to brittle cleavage fracture.
Q11. If a pendulum of weight 200 N is released from a height of 1.5 m and swings up to 0.4 m after breaking a Charpy specimen (neglecting friction), what is the energy absorbed?
(A) 80 J
(B) 220 J
(C) 300 J
(D) 380 J
View Answer & Explanation
Correct Answer: (B) 220 J
Explanation: Energy absorbed = Initial Potential Energy − Final Potential Energy = W · (H1 − H2) = 200 × (1.5 − 0.4) = 200 × 1.1 = 220 Joules.
Q12. In a ductile fracture, the fractured surface typically appears:
(A) Shiny and crystalline with flat facets
(B) Dull and fibrous with shear lips
(C) Completely glassy and smooth
(D) Striated without any plastic deformation
View Answer & Explanation
Correct Answer: (B) Dull and fibrous with shear lips
Explanation: Ductile fractures absorb high energy and show a dull, grey, fibrous surface accompanied by lateral contraction (shear lips). Brittle fractures (cleavage) exhibit bright, granular, crystalline surfaces with zero lateral expansion.
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