What is interfacial shear strength (IFSS) and how is it measured?

What is interfacial shear strength (IFSS) and how is it measured?

• A. IFSS is the shear strength within the matrix away from fibers
• B. IFSS is the shear strength at the fiber–matrix interface; measured by microbond pull-out, single-fiber fragmentation, or microbond tests
• C. IFSS is the shear strength of the laminate as a whole under bending
• D. IFSS is the compressive strength of the fiber along its axis

Answer: (B)

Interfacial shear strength is the shear stress required to cause sliding between the fiber and the surrounding matrix at their boundary. It specifically characterizes how well the fiber and matrix stick together, not the properties of the matrix itself or the fiber alone. To measure this, tests are designed to isolate the fiber–matrix interface. In a microbond type test, a tiny resin droplet is bonded to a single fiber and then the fiber is pulled to debond the droplet. The force at debonding, divided by the contact area, gives the interfacial shear strength. For a round fiber, that contact area is approximately the surface strip π d l, where d is the fiber diameter and l is the embedded length, so τ ≈ F / (π d l). Other related tests, like the single-fiber fragmentation test, also probe how the interface fails along the embedded length to provide a related measure of IFSS. This interpretation differs from the matrix’s shear strength away from fibers, the laminate’s overall bending strength, or the fiber’s axial compressive strength, which are governed by different mechanisms.

Interfacial shear strength is the shear stress required to cause sliding between the fiber and the surrounding matrix at their boundary. It specifically characterizes how well the fiber and matrix stick together, not the properties of the matrix itself or the fiber alone.

To measure this, tests are designed to isolate the fiber–matrix interface. In a microbond type test, a tiny resin droplet is bonded to a single fiber and then the fiber is pulled to debond the droplet. The force at debonding, divided by the contact area, gives the interfacial shear strength. For a round fiber, that contact area is approximately the surface strip π d l, where d is the fiber diameter and l is the embedded length, so τ ≈ F / (π d l). Other related tests, like the single-fiber fragmentation test, also probe how the interface fails along the embedded length to provide a related measure of IFSS.

This interpretation differs from the matrix’s shear strength away from fibers, the laminate’s overall bending strength, or the fiber’s axial compressive strength, which are governed by different mechanisms.