Observation of Acrylic Block Fracture during Impact Compression Using the Hopkinson Bar Method

Significance of the topic

Accurate characterization of material behavior under dynamic loading is essential for safe and efficient design in sectors such as transportation, defense and industrial manufacturing. Impact events can produce stress and strain conditions that differ markedly from quasi-static loading, so high-speed, high-resolution observation methods are necessary to quantify wave propagation, crack initiation and fast fracture processes. High-frame-rate optical recording combined with controlled impact methods enables direct visualization of these transient phenomena, improving mechanistic understanding and informing material selection and structural design.

Objectives and overview of the study

This work demonstrates the application of a high-speed video camera (HyperVision HPV-X3) to record fracture processes in an acrylic specimen subjected to impact compression using the Hopkinson bar method. The goals were to capture stress-wave propagation immediately after loading, to observe crack initiation and propagation from a central geometrical defect (a through-hole), and to assess the capability of the HPV-X3 to resolve fast fracture dynamics at multi-megahertz frame rates.

Methodology

The study combined a classical split-Hopkinson-bar-style impact compression arrangement with laser back-illumination and macro optical imaging. Key experimental features:

• Specimen: 10 mm acrylic cube with a central through-hole, 2 mm diameter.
• Loading: Hopkinson bar (bar launcher, input and output bars, strike and absorption bars) to produce a sudden compressive pulse.
• Imaging system: HyperVision HPV-X3 high-speed camera equipped with a close-up ring, bellows and a 105 mm macro lens; laser illumination transmitted from the specimen rear toward the lens to enhance contrast of transient features.
• Recording conditions: Two primary imaging runs — imaging 1 at 5 Mfps (to capture stress-wave onset and early crack propagation) and imaging 2 at 10 Mfps (to capture later-stage fracture evolution).

Instrumentation

• High-speed camera: HyperVision HPV-X3 (Shimadzu), up to 20 million frames per second and higher spatial resolution (three times that of the HPV-X2).
• Optics: Close-up ring, bellows and 105 mm macro lens for high-magnification imaging of the small specimen.
• Illumination: Laser back-illumination to generate high-contrast images of wavefronts and cracks.
• Mechanical: Hopkinson-bar apparatus including input/output bars, strike bar and absorption bar to generate controlled impact compression pulses.

Main results and discussion

Imaging at 5 Mfps revealed a traveling wavefront inside the compressed acrylic block immediately after impact. The wavefront was first observed near one corner and propagated across the specimen face; from sequential frames the apparent propagation speed was estimated at approximately 2.7 km/s. Within 16 microseconds from the initial frames, a crack initiated at the edge of the central 2 mm hole and propagated laterally toward the specimen sides.

Imaging at 10 Mfps captured the later stages of failure with higher temporal resolution. The central hole deformed into an elliptical shape under compression prior to crack nucleation. Crack growth progressed both laterally and along diagonals, with multiple crack fronts developing and coalescing to produce complete fracture. The observed fracture sequence—initial stress wave, hole-edge crack initiation, lateral propagation and diagonal branching to full separation—aligns with prior observations of hole-influenced fracture under dynamic compression.

These image sequences allowed qualitative and semi-quantitative analysis of wave and fracture dynamics: direct visualization of stress-wave arrival and travel, timing of crack initiation relative to the loading pulse, and the multi-directional propagation paths that lead to final failure. The increased spatial resolution and high frame rates of the HPV-X3 were instrumental for resolving these transient features.

Benefits and practical applications

• High temporal and spatial resolution imaging enables direct observation of stress-wave propagation and crack nucleation, improving validation of dynamic material models and constitutive laws used in design simulations.
• Visualization of failure mechanisms around geometric defects (here, a central hole) supports improved component design, defect-tolerance assessment and safety margins in impact-prone applications.
• Data from synchronized high-speed imaging and Hopkinson-bar tests can support calibration of computational fracture and wave-propagation models, and inform material selection for automotive, aerospace and protective-structure applications.
• The HPV-X3’s increased resolution and up-to 20 Mfps capability make it suitable for a broad range of high-speed experimental mechanics studies, including ballistic and explosion-related testing where extremely short timescales govern failure.

Future trends and potential applications

• Combining ultrahigh-speed imaging with full-field measurement methods (e.g., high-rate digital image correlation) to obtain quantitative strain fields during impact events.
• Integration with multi-sensor Hopkinson-bar diagnostics (strain gauges, laser interferometry) and automated image analysis to derive synchronized force, displacement and fracture metrics for model validation.
• Use of even higher-resolution sensors and computational imaging techniques to resolve micro-scale crack nucleation sites and energy dissipation pathways in heterogeneous materials and composites.
• Application to additive-manufactured and functionally graded materials to assess dynamic performance and defect sensitivity specific to emerging manufacturing routes.

Conclusion

The study demonstrated that high-frame-rate optical imaging with the HPV-X3 effectively captures the rapid sequence of events produced by Hopkinson-bar impact compression in a small acrylic specimen containing a central hole. The experiments visualized stress-wave travel (estimated speed ~2.7 km/s), crack initiation at the hole edge, and subsequent lateral and diagonal crack propagation leading to complete fracture. The combination of controlled impact loading and high-speed, high-resolution imaging provides valuable mechanistic insight into dynamic fracture processes and supports improved experimental characterization and modeling of materials under impact.

References

1. Hopkinson B.: A Method of Measuring the Pressure Produced in the Detonation of High Explosives or by the Impact of Bullets, Philosophical Transactions of the Royal Society of London A, 213, pp. 437–456 (1914).
2. Related application summaries (as cited in the original report): Observations of fracture behavior and strain distribution in impact compression tests using Hopkinson-bar methods and high-speed imaging.