Polyether-ether-ketone (PEEK) is commonly used in engineering and biomedical applications that require miniature components and thin structures. The effect of specimen size on the deformation and fracture behaviour of PEEK is not fully understood. The current work investigates the size effect on the deformation behaviour, damage evolution, and fracture mechanisms of PEEK at different strain rates, temperatures, and stress triaxialities. Experimental tensile tests were conducted on in-situ and bulk smooth and notched tensile samples at different strain rates and temperature range. Interrupted tensile tests of bulk PEEK samples and scanning electron microscopy (SEM) were used to characterise the void nucleation, growth, and coalescence, as well as the fracture surface morphology. A coupled constitutive–damage framework combining the Zhu–Ou–Popov (ZOP) constitutive model and the Gurson–Tvergaard–Needleman (GTN) damage model was developed to capture the deformation and fracture behaviour of PEEK at different testing conditions. The findings show that specimen thickness has a major effect on fracture behaviour, where thin specimens exhibit earlier strain localisation, lower fracture strain, and reduced ductile fracture regions as compared with the bulk specimens. Increasing strain rate reduced the critical and fracture void volume fractions, whereas increasing temperature enhanced ductility of PEEK and promoted void growth prior to fracture. The fracture mechanism shifted from dominant ductile–brittle mixed fracture at low stress triaxiality to accelerated multi-site crack initiation under high stress triaxiality. The proposed ZOP–GTN framework accurately predicted the stress–strain response, void evolution, and fracture under all investigated conditions without recalibration of damage parameters. The study shows that specimen size has a significant impact on PEEK deformation and fracture behaviour and should be taken into account when modelling constitutive damage in miniature thermoplastic components.
An improved Split Hopkinson Pressure Bar setup (SHPB) is developed to establish an exact three-dimensional (3D) finite element (FE) model of threaded connection. Based on this model, a series of numerical tests of axial compressive impact is conducted within a velocity range of 10–40 m/s. The influences of impact velocity, thread length, thread geometry and preload, etc., on the impact response characteristic of threaded connections are investigated systematically. Related results have demonstrated that as the impact velocity increases, the deformation of the bolt gradually intensifies, while the load transfer rate progressively decreases. The thread engagement length has a significant influence on the failure mode of the bolt and the impact load transfer behavior. A shorter thread engagement length (5 mm) causes stress concentrations and local crushing, and consequently reduces the load transfer rate; comparatively, a longer thread engagement length (10 mm) allows more threads to share the impact load and maintains a higher load transfer rate. Thread geometry also affects the stress distribution, deformation behavior, and load transfer characteristics of threaded connection. Under low impact velocities (≤ 20 m/s), different thread-pitch configurations show similar load transfer rates; under high impact velocities (20–30 m/s), coarse-threaded bolts exhibit more severe stress concentration at the thread roots and local failure, resulting in a lower load transfer rate; under ultra-high impact velocities (≥ 35 m/s), the medium-pitch structure (pitch = 1 mm) provides a better balance between load-bearing capacity and multi-thread load sharing, and therefore achieves the highest load transfer rate. Comparatively, preload mainly modifies the initial contact pressure and frictional energy dissipation at the threaded interface. However, under high-amplitude compressive impact loading, its influence on the load transfer rate remains limited. The related research quantitatively reveals the intrinsic mechanisms of various influencing factors and provides essential data support for the performance optimization of threaded connections in engineering applications.
Solid propellant constitutes the core component of solid rocket motors. Aging-induced degradation of its mechanical properties significantly compromises the structural integrity of the propellant grain and the operational safety of the motor. In this paper, a novel viscoelastic phase‑field fracture model under finite deformation is developed to characterize the mechanical property degradation and fracture behavior of thermally accelerated aged solid propellants. The proposed model integrates finite‑deformation viscoelastic theory with the phase‑field fracture framework, enabling the capture of damage and fracture behavior in solid propellants under large deformations. The elastic strain energy driving fracture evolution is contributed by both the equilibrium and non‑equilibrium branches of the generalized Maxwell model within the viscoelastic constitutive formulation. The contributions of elastic and viscous components to the total energy are decoupled via deformation gradient decomposition. A volumetric–isochoric energy decomposition is adopted to distinguish the different driving forces under tension and compression. The coupled phase‑field and displacement fields are solved using a staggered iterative finite element scheme. Thermally accelerated aging tests are conducted on solid propellant specimens to simulate long‑term storage aging. Comparison between the experimental data and numerical simulation results yields prediction accuracies of 7.36% for the unaged condition and 6.6% for the aged condition, fully demonstrating the predictive capability of the proposed model.
