Ultra-fast X-ray imaging unveils the strain-rate dependency and failure mechanisms of brittle composites

In the world of materials science and engineering, the quest to understand and optimise the behaviour of advanced composites is an ongoing endeavour. Among these materials, carbon fibre-reinforced plastics (CFRPs) stand out for their exceptional properties, including high strength and low weight. However, CFRP laminates exhibit sudden failure with complex damage patterns when subjected to high-speed impacts in contexts such as automotive and aerospace safety scenarios. The intricacy of damage patterns is due to the inherent anisotropic nature of the materials. The impact on CFRPs induces complex internal stress states and multiple failure modes that occur simultaneously, leading to significant deviations from predictions based on classical steady-state and quasi-static theories. Under these scenarios, it becomes a great challenge to detect the critical bulk damage points and the failure phenomena that occur within reinforcement layers, which are inaccessible by traditional optical imaging techniques. Therefore, it is paramount to study CFRP laminates under impact loads at a very short time scale, gaining comprehensive insight into the damage inside the material.

This study explores the strain rate-induced failure modes in CFRP laminates under dynamic loading. Ultrafast X-ray imaging employed at beamline ID19 enabled in-situ investigation of damage progression within CFRPs under high-speed impact conditions, capturing representative material scales hitherto inaccessible. Dynamic experiments were performed using the Split-Hopkinson pressure bar and imaging rates up to 5.68 MHz frame rates, which is of great importance when the entire failure phenomena occur within only a few microseconds. Figure 1 illustrates the setup: X-ray pulses from the storage ring are delivered every 176 ns, impinging on the sample at a precise moment of failure. An ultra-fast imaging detector consisting of a Shimadzu HPV-X2 camera with 2X magnification provides a spatial sampling of 15.2 µm per pixel. Moreover, by synchronising XPCI radiographs with strain gauge measurements, loads and displacements were simultaneously monitored and correlated to the in-situ observations of the material damage. This unique data made it possible to discern the effects of strain rate and material response on the damage and failure modes inside laminates.
 

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Fig. 1: Schematic illustration of ultra-fast synchrotron XPCI system used to monitor the experiments at ID19 with in-situ damage initiation and propagation in CFRP specimens under dynamic compressive loading at a high strain rate.

A key achievement of this research was the ability to track crack propagation at an unprecedented temporal resolution of 100 µs. This capability provided invaluable insights into the rapid onset of damage, the subsequent crack propagation dynamics, and their intricate interplay with microstructural failure mechanisms. The results indicated the formation of diagonal shear cracks through the volume and the disintegration of specimens into two pieces with an in-plane transverse fracture, as shown in Figure 1. By correlating the XPCI results with other techniques, optical and infrared imaging, the occurrence of similar damage patterns inside and on the surface of specimens was revealed with a strong temperature localisation. The XPCI images of crack propagation show the local delamination between layers and in-plane shear due to lay-up, which has not been observed by the specimen surface images. The XPCI method offers unprecedented capabilities in revealing microscale damage mechanisms and tracking the evolution of local microcracks and filament-level failure phenomena during real-time dynamic loading, enabling observations that were previously unattainable.

In summary, ultra-fast XPCI is a cutting-edge method that not only aids researchers and manufacturers to better understand the dynamic failure of materials but also offers critical guidance in the design and development of damage-tolerant composite structures. Advanced composites are commonly used in aerospace, automotive and sports equipment applications, as well as hydrogen industries, particularly in applications such as hydrogen pressurised vessels and composite piping, where the composite components are subjected to impacts, hits or collisions. The MHz X-ray imaging at ID19 enables researchers to predict and analyse fractures and internal damage caused by impacts under extreme conditions, which is imperative for constructing safer, more durable and sustainable infrastructure.

Principal publication and authors
In situ damage characterization of CFRP under compression using high-speed optical, infrared and synchrotron X-ray phase-contrast imaging, N, Pournoori (a), G.C. Soares (a), B. Lukić (b), M. Isakov (a), M.C.L. Belone (a), M. Hokka (a), M. Kanerva (a), Compos. Part A. Appl. Sci. Manuf. 175, 107766 (2023); https://doi.org/10.1016/j.compositesa.2023.107766
(a) Tampere University, Tampere (Finland)
(b) ESRF