Direct Testing of Gradual PostPeak Softening of Notched Specimens of Fiber Composites Stabilized by Enhanced Stiffness and Mass

TL;DR

This paper demonstrates that using stiffer and heavier grips in fiber composite fracture tests enables stable postpeak softening observation, validating fracture mechanics applicability and resolving previous doubts about dynamic snapback.

Contribution

It introduces a method of using enhanced grip stiffness and mass to observe stable postpeak softening in fiber composite fracture tests.

Findings

Stiffer, heavier grips enable stable postpeak observation.

Fracture energy from load-deflection matches size effect tests.

Dynamic snapback suspicions are dispelled.

Abstract

Static and dynamic analysis of the fracture tests of fiber composites in hydraulically servo-controlled testing machines currently in use shows that their grips are much too soft and light for observing the postpeak softening. Based on static and dynamic analysis of the test setup, far stiffer and heavier grips are proposed. Tests of compact-tension fracture specimens of woven carbon-epoxy laminates prove this theoretical conclusion. Sufficiently stiff grips allow observation of a stable postpeak, even under load-point displacement control. Dynamic stability analysis further indicates that stable postpeak can be observed under CMOD control provided that a large mass is rigidly attached to the current soft grips. The fracture energy deduced from the area under the measured complete load-deflection curve with stable postpeak agrees closely with the fracture energy deduced from the size effect tests of the same composite. Previous suspicions of dynamic snapback in the testing of composites are dispelled. So is the previous view that fracture mechanics was inapplicable to the fiber-polymer composites.