Identification of Fracture Precursor Characteristics of Rock-like Materials with Pre-Existing Flaws

Transient vibration signals can reveal changes in fracture activity that are less apparent in macroscopic displacement records. This study applied variational mode decomposition (VMD) and modal energy entropy to uniaxial compression tests on transparent epoxy-resin specimens with pre-existing flaws. Nine specimens were tested, and three complete vibration records were retained after signal-integrity screening. VMD and Hilbert analysis were used to describe the frequency content and redistribution of signal energy, while the entropy histories were compared with displacement and visible crack development on a common experimental time axis. The retained records show that stepwise entropy decreases during continued monotonic displacement. The first sustained decreases occur at approximately 10.5–13 s, with later decreases around 20–22 s. The dominant energy-fraction component is distinct from the component making the largest entropy contribution. A 3DEC model provides a qualitative interpretation of flaw-controlled deformation and local force fluctuations. The contribution of the study is an application of established signal-processing methods to the joint interpretation of vibration, displacement and visible fracture in transparent specimens. The observed entropy levels are specific to the analyzed records and processing settings and are not proposed as universal warning thresholds.