Impact Test and Torsion Test: Procedure, Observation Table and Calculations

Impact Testing Machine and Torsion Testing Machine: Experiments, Procedure and Calculations

The impact test and torsion test are two essential experiments in the Strength of Materials and Materials Science laboratory, complementing the UTM tensile test. They measure material properties under dynamic loading (impact) and twisting loads (torsion) that cannot be determined from a static tensile test alone. This guide covers both experiments with complete theory, procedure, observation tables, and calculations for engineering lab reports.

Part A: Impact Test — Charpy and Izod

Objective

• To determine the impact energy absorbed by a standard notched specimen during fracture using the Charpy or Izod test.
• To assess material toughness (energy absorbed per unit cross-sectional area of notch).
• To compare impact toughness of different materials (mild steel, cast iron, aluminium, brass, polymers).

Theory

Impact energy E = mgh1 – mgh2 = mg(h1 – h2), where m = pendulum mass, h1 = initial height, h2 = final height after fracture, g = 9.81 m/s squared.

Charpy test: specimen supported as a beam at both ends (simply supported), struck at mid-span. Notch faces away from the striker. Izod test: specimen clamped vertically (cantilever), struck 22 mm above the notch. Notch faces the striker.

Impact toughness = Energy absorbed / Cross-sectional area at notch (J/m squared or kJ/m squared)

Procedure

1. Measure specimen dimensions at the notch cross-section: width b (mm) and height at notch d (mm). Calculate notch cross-section area A = b x d.
2. For Charpy test: position the specimen on the anvil supports, notch centred and facing away from the hammer. For Izod: clamp specimen vertically in the vice, notch at the specified height, facing the hammer.
3. Raise the pendulum to the initial angle and latch it. Read and record the initial angle alpha1 on the scale.
4. Release the pendulum. It strikes the specimen and swings through to angle alpha2 on the other side.
5. Read the final angle alpha2 from the scale pointer.
6. Calculate: h1 = R(1 – cos alpha1), h2 = R(1 – cos alpha2), where R = pendulum radius. Energy absorbed = mg(h1 – h2). (Most machines show energy directly on a calibrated scale.)
7. Repeat with different materials. Note fracture appearance: ductile (fibrous, dull grey) or brittle (crystalline, bright).

Observation Table — Impact Test

Material	Notch Width b (mm)	Notch Height d (mm)	Area A (mm squared)	Initial angle (deg)	Final angle (deg)	Energy absorbed (J)	Toughness (kJ/m squared)	Fracture type
Mild steel
Cast iron
Aluminium

Part B: Torsion Test

Objective

• To determine the modulus of rigidity G (shear modulus) of a given material.
• To determine the shear yield stress and ultimate shear strength.
• To plot the torque-twist (T-theta) diagram for the specimen.

Theory

Torsion formula: T/J = G x theta / L = tau / r

Where T = applied torque (N.m), J = polar second moment of area = pi x d^4 / 32 for solid circular section, G = modulus of rigidity (GPa), theta = angle of twist (radians), L = gauge length (m), tau = shear stress, r = radius.

Modulus of rigidity: G = (T x L) / (J x theta)

Procedure

1. Measure specimen diameter d at three points along gauge length. Use minimum value. Calculate J = pi x d^4 / 32.
2. Mark the gauge length L with punch marks. Record L.
3. Mount the specimen in the hexagonal chucks of the torsion machine. Ensure firm grip with no slip.
4. Set the torque and angle indicators to zero.
5. Apply torque in increments (5-10 N.m steps for 10 mm diameter specimens). At each step, record torque T and angle of twist theta.
6. Continue until fracture. Note the maximum torque (ultimate shear), the torque at which the T-theta curve deviates from linear (shear yield), and the fracture angle.
7. Calculate G from the linear (elastic) region of the T-theta plot: G = (T x L) / (J x theta).

Observation Table — Torsion Test

Material: _____ | Diameter d = _____ mm | J = _____ mm^4 | Gauge length L = _____ mm

Obs.	Torque T (N.m)	Angle of Twist theta (degrees)	theta (radians)	Shear Stress tau (MPa)	Modulus G (GPa)
1
2
3
4
5

Related Experiments and Equipment

Perform alongside the UTM tensile test. Browse Scientico impact testing machine and torsion testing machine product pages. Full Strength of Materials lab equipment range. Country pages: UAE, Kenya, Bangladesh.

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