Engineer’s Edge: Which Strength Tests Are Most Misunderstood? Our Top 3

Not all strength tests mean what people think they do. Here are three that cause the most confusion:

1. Tensile Strength
Often mistaken as “how tough a material is,” but tensile strength only measures the maximum stress a material can withstand before breaking in tension. It says little about long-term durability, fatigue life, or resistance to crack growth. For composites, fiber orientation heavily influences tensile strength results, so values can be misleading if taken out of context.

2. Compressive Strength
Commonly assumed to be the inverse of tensile, but compression involves very different failure modes. In composites, compressive failure often comes from fiber micro-buckling, shear instability, or matrix cracking—phenomena not present in tensile tests. That’s why compressive strength can be disproportionately lower than tensile strength, even in the same material.

3. Impact Strength
Many assume it predicts a material’s toughness or fatigue resistance, but it measures performance under a single sudden load (like a drop weight or Charpy/Izod strike). It doesn’t capture long-term durability or performance under repeated impacts. For composites, the test may not even show hidden internal delamination damage, making it a poor stand-alone predictor of real-world resilience.

What to Use Instead

To get a more complete picture of performance, engineers often combine or substitute the following tests:

• Fatigue Testing – Evaluates how a material performs under repeated cyclic loading, a far better predictor of long-term service life.

• Fracture Toughness (K_IC / G_IC) – Measures resistance to crack initiation and growth, essential for damage-tolerant designs.

• Interlaminar Shear Strength (ILSS) – Critical for composites, it shows how well layers resist sliding or delamination under stress.

• Compression After Impact (CAI) – Especially important for aerospace and defense, it reveals hidden internal damage from impacts and how it affects residual strength.

Why This Matters

Understanding what these tests really reveal—and what they don’t—is critical to designing reliable composite parts. At GP&C, we cut through the confusion with the right combination of tests, fiber architectures, and resin systems to deliver predictable performance under real-world conditions.