1. In AJMM, abrasive particles are accelerated in a gas stream.
a) True
b) False
Explanation: Fine micro abrasive particles are accelerated in a gas stream (commonly air at a few times atmospheric pressure). The particles are directed towards the focus of machining (less than 1mm from the tip).
2. In AJMM, as the jet fracture off the surface after striking it.
a) True
b) False
Explanation: As the particles impact the surface, they fracture off the surface and create cavities. As the particle impacts the surface, it causes a small fracture, and the gas stream carries both the abrasive particles and the fractured (wear) particles away.
3. The choice of abrasive varies with the type of machining.
a) True
b) False
Explanation: The choice of abrasive particles depends on the type of machining operation. The abrasives should have a sharp and irregular shape for better performance.
4. In AJMM, material removal starts when the fracture strength is reached.
a) True
b) False
Explanation: During AJMM, plastically deformed surface layer is formed near the eroded surface, and therefore, the yield strength of the material increases. Upon further deformation, the yield strength at the surface of the material will eventually become equal to its fracture strength. The surface becomes brittle and its fragments may be removed by subsequent impacts
5. Brittle fracture can occur due to indentation rupture.
a) True
b) False
Explanation: Brittle fracture may take place due to:
1) Indentation ruptures
2) Elastic–plastic deformation
3) Critical plastic strain theory
4) Radial cracking and propagation or surface energy criterion.
6. The mass loss of workpiece is proportional to_____
a) power supplied
b) nozzle diameter
c) amount of abrasive
d) carrier gas
Explanation: The mass loss of workpiece is proportional to the amount of abrasive.
Mass loss = (Kρmv2)/(2H), where, K (> 2) is a dimensionless factor, m and v amount and velocity of
particles, and ρ and H are density and hardness of the eroded material, respectively. The above relation is true for brittle erosion but not for softer materials (elastomers and some metals) due to time variant erosion behaviour.
7. In AJMM, there is initial weight gain in the workpiece.
a) True
b) False
Explanation: Especially at normal impact angle, particles tend to embed in the material, resulting in an initial gain in weight of the specimen. After this incubation time steady-state erosion is established and mass loss from the eroded material is proportional to the amount of abrasive particles.
8. Erosion rate is directly proportional to velocity of the jet.
a) True
b) False
Explanation: The principal empirical relation between erosion rate Erate, expressed as the quotient of mass loss and amount of abrasive, and particle velocity is given as a power function by
Erate α vk , where k= velocity coefficient
9. For metal, velocity coefficient ranges from_____
a) 0.5—1
b) 1.8—3
c) 2.3—3
d) 2—4
Explanation: velocity coefficient k commonly reported for,
1) for metals – between 2.3 and 3,
2) for glasses – between 2 and 4 and
3) for elastomers – between 1.8 and 3.2.
10. AJMM can also machine poly methyl methacrylate (PMMA).
a) True
b) False
Explanation: AJMM can machine much steeper side walls and flatter bottom section in PMMA than those in glass. For PMMA, there is only a small probability of a particle rebounding from the steep side wall and hitting the opposite side