Question

Which combination gives highest strength-to-weight ratio?

• A. Polymer + fibres
• B. Metal + fibres
• C. Polymer + particles
• D. Ceramic + voids

Hint:

Fiber-reinforced polymers (FRP) are widely used in commercial aerospace designs (e.g., Boeing 787 fuselage) specifically because of their superior specific strength.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the structural combination of materials that yields the maximum strength-to-weight ratio (also known as specific strength).
This parameter is critical in structural, aerospace, and transport engineering, where maximizing structural efficiency while minimizing overall weight is essential.

Step 2: Key Formula or Approach:
The strength-to-weight ratio is expressed as:
\[ \text{Specific Strength} = \frac{\sigma_{ts}}{\rho} \] Where:
$\sigma_{ts}$ is the ultimate tensile strength of the composite.
$\rho$ is the density of the composite.
To maximize this ratio, we require high tensile strength and low density.

Step 3: Detailed Explanation:

• Polymers have very low density ($\rho \approx 1.0 - 1.4\text{ g/cm}^3$) but generally exhibit low to moderate mechanical strength.

• Reinforcing fibers (such as carbon, aramid, or glass) possess exceptionally high tensile strengths.

• Combining a polymer matrix with high-strength continuous fibers creates a Fiber-Reinforced Polymer (FRP) composite.

• These composites achieve a remarkably high tensile strength because the fibers carry the primary structural loads, while the polymer matrix keeps the density extremely low.

• Metal matrix composites (Metal + fibres) are strong but have a much higher overall density (e.g., aluminum has a density of $2.7\text{ g/cm}^3$, and titanium has $4.5\text{ g/cm}^3$), which lowers their strength-to-weight ratio.

• Polymer-particle composites (Polymer + particles) generally possess lower tensile strength compared to fiber-reinforced composites because particulates are not as effective as fibers at transmitting and bearing tensile loads.

• Ceramic-void combinations (Ceramic + voids) contain voids which act as stress concentrators, dramatically reducing structural strength.

Step 4: Final Answer:
The Polymer + fibres combination provides the highest strength-to-weight ratio, matching option (A).