Strain-Rate Dependence of Mode II Delamination Initiation in a Polyester-Fiberglass Composite Using Dynamic ENF Testing

When engineers design structures that must withstand sudden impacts—like the fuselage of an aircraft or the hull of a ship—they rely heavily on composite materials. These are not single blocks of metal or plastic, but rather layers of fabric, such as fiberglass, held together by a sticky resin. The strength of these materials often depends less on the individual layers and more on how well those layers stick to one another. If the layers separate, a process known as delamination, the entire structure can fail catastrophically. This separation can happen in different ways: the layers might be pulled apart like pages in a book, or they might be forced to slide past each other, like two cards being pushed in opposite directions. The latter is called shear, and understanding how much energy it takes to start this sliding separation is critical for safety. While scientists have long studied how these materials behave under slow, steady pressure, the real world often delivers sudden, violent shocks. The question remains: does the speed at which a force is applied change how easily these layers slide apart?

A team of researchers at the Institute of Space Technology in Islamabad set out to answer this question for a specific type of material: fiberglass reinforced with polyester resin. They focused on the sliding separation, or shear, because it is a common failure mode in many structural applications. To do this, they needed to test the material not just slowly, but at high speeds that mimic real-world impacts. They used a specialized machine called a Split Hopkinson Pressure Bar, which acts like a high-speed hammer. In this setup, a metal striker is fired by compressed gas to hit a long metal bar. This impact sends a powerful wave of pressure through the bar and into a small sample of the fiberglass material placed in the middle. By adjusting the gas pressure, the researchers could control exactly how fast the material was deformed, testing it at rates ranging from 70 to 150 times per second. They prepared their samples as small, square blocks with a tiny, pre-made crack in the middle, designed to force the layers to slide apart when bent.

The researchers placed these samples in a three-point bending fixture, essentially setting them up like a bridge supported at both ends with a load applied in the center. As the pressure wave hit the sample, it bent the material, forcing the pre-existing crack to open and slide. To capture what happened in the split second of the impact, they used a high-speed camera capable of taking 100,000 pictures every second. This allowed them to watch the crack grow and the material deform in real time. By measuring the force applied and the distance the material bent, they could calculate the energy required to start the crack moving. This value, known as the strain energy release rate, tells engineers how much energy the material can absorb before it begins to fail.

The results revealed a clear and significant trend: the faster the material was loaded, the more energy it took to start the crack sliding. Under slow, static conditions, previous tests on similar materials showed an energy requirement of about 1.2 units. In these high-speed tests, that number jumped dramatically. The energy needed to initiate the crack ranged from 0.64 to 7.19 units, with the higher values appearing at the fastest strain rates. This suggests that the material becomes tougher against sliding separation when hit quickly. However, the data was not perfectly smooth. The researchers observed that the cracks did not always travel in a straight, flat line between the layers. Instead, the cracks sometimes zigzagged or caused fibers to bridge across the gap, pulling on the layers and making the failure process more complex. These irregularities caused some variation in the numbers, with one test at the highest speed showing a particularly large spike in energy absorption.

Despite the variability, the overall message from the experiment is that speed matters. The study confirms that the resistance to sliding separation in polyester-fiberglass composites is strongly dependent on how fast the force is applied. The researchers noted that their samples were relatively short, which created high shearing forces that might have influenced the results. They suggest that future tests should use longer samples to ensure the cracks grow more steadily and to isolate the pure sliding behavior from other effects. For now, the work provides a vital piece of the puzzle for engineers designing structures that must survive dynamic impacts, showing that materials which might seem fragile under slow pressure can actually resist sudden, violent sliding forces much better than previously thought.