Define porosity and its effect on mechanical properties.

• A. The fraction of voids has no effect on density.
• B. The fraction of voids in a material; increases density.
• C. The fraction of voids in a material; reduces strength, stiffness, density, and can lower fracture toughness.
• D. The fraction of voids is equivalent to grain size.

Answer: (C)

Porosity is the fraction of void volume within a material relative to its total volume. As porosity increases, density decreases because there is more empty space and less solid material to carry load. This empty space also reduces the material’s ability to bear stresses—strength drops because load must pass through fewer solid pathways, and stiffness (the elastic response) typically decreases since there’s less solid material to resist deformation. Voids act as stress concentrators and crack initiators, so fracture toughness can diminish as porosity grows. The size, shape, and distribution of the pores matter: large or connected pores have a greater detrimental effect than tiny, isolated ones. Porosity is about void content, not grain size, so it doesn’t measure the size of the solid grains themselves.