Shun-shan Feng

A Hyper-Viscoelastic Constitutive Model for Polyurea under Uniaxial Compressive Loading

A hyper-viscoelastic constitutive model for polyurea by separating hyperelastic and viscoelastic behaviors has been put forward. Hyperelasticity represents the rate-independent responses at low strain rates, described by a three-parameter Mooney-Rivlin model and a third Ogden model. By fitting the quasi-static experimental data, the Ogden model is more appropriate to describe the hyperelastic behaviors for its better agreement at strain over 0.3. Meanwhile, viscoelasticity represents the rate-dependent responses at high strain rates, described by the Standard Linear Solids (SLS) model and the K-BKZ model. By fitting the experimental data of split Hopkinson pressure bar (SHPB), the SLS model is more appropriate to describe the viscoelastic behaviors at strain rates below 1600 s−1, but the K-BKZ model performs better at strain rates over 2100 s−1 because of the substantial increase of Young’s modulus and the state of polyurea transforming from rubbery to glassy. The K-BKZ model is chosen to describe the viscoelastic behavior, for its low Root Mean Square Error (RMSE) at strain rates below 1600 s−1. From the discussion above, the hyper-viscoelastic constitutive model is chosen to be the combination of the Ogden model and the K-BKZ model.

Static and Dynamic Properties of Semi-Crystalline Polyethylene

Properties of extruded polymers are strongly affected by molecular structure. For two different semi-crystalline polymers, low-density polyethylene (LDPE) and ultra-high molecular weight polyethylene (UHMWPE), this investigation measures the elastic modulus, plastic flow stress and strain-rate dependence of yield stress. Also, it examines the effect of molecular structure on post-necking tensile fracture. The static and dynamic material tests reveal that extruded UHMWPE has a somewhat larger yield stress and much larger strain to failure than LDPE. For both types of polyethylene, the strain at tensile failure decreases with increasing strain-rate. For strain-rates 0.001–3400 s−1, the yield stress variation is accurately represented by the Cowper–Symonds equation. These results indicate that, at high strain rates, UHMWPE is more energy absorbent than LDPE as a result of its long chain molecular structure with few branches.

Ballistic Impact Characteristics of Flat-nose Projectile Penetrating Concrete and Soil Compound Target

Abstract Ballistic impact characteristics on the flat-nose projectile penetrating the concrete and soil compound target are studied. The deformation process and failure zone in the target are described by numerical simulation with finite element software. The results show that penetration depth, residual velocity and deceleration amplitude of flat-nose projectile increase with initial velocity. The features of concrete target after impact are approximately in agreement with experimental results. And the cracks and the tensile crush zone formed during penetration could characterize the damage and failure of target. Meanwhile, terminal ballistic characteristics of flat-nose projectile into single soil layer are studied to compare with that of concrete compound target. The results show that the overload of projectile penetrating hard-soil is only one-third of that of concrete compound target with low velocity. Reversely, the duration of the former is more than five times as long as the latter, and the rebound velocity of projectile penetrating soil medium is greater than the concrete compound target.

Effect of Fragment Shape on the Impact Damage of Carbon/Aluminum Laminated Shell Structure

Carbon fiber-reinforced composite materials are widely used in the aerospace area as protective frame against collision. The effect of fragment shape on the impact damage in carbon fiber reinforced aluminum laminates (CRALL) fabricated by carbon fiber reinforced plastic (CFRP) layers combined with aluminum alloy 2024-T3 layers was studied. The 2.48 mm thickness CRALL 2/1 layup and 4.16 mm thickness CRALL 3/2 layup were designed and investigated. The 30 g mass fragments with a flat, hemispherical or conical shape were launched up to 68~238 m/s by the air gun to impact and penetrate these composite targets normally. Experimental results on the 0°/90° CFRP and CRALLs reveal that the fracture modes, ballistic limits and energy absorption are significantly affected by the fragment shape. With the with the increase thickness of CRALLs, the influence of the projectile shapes on fracture modes will be diminished. Due to the strain rate hardening effect, the 4.16 mm thickness CRALL3/2 targets performed higher ballistic limits and better specific energy absorption ability than 4 mm 0°/90° laminated CFRP in all three shapes fragments.

Penetration Mechanisms of Ultrahigh Molecular Weight Polyethylene Fiber Reinforced Laminates by a Conical Projectile Laminated Shell Structure

Ballistic tests were performed on four different-thickness ultrahigh molecular weight polyethylene fiber reinforced laminates (Dyneema® HB26) by a conical-nosed steel projectile. The laminates exhibits much weaker resistance to penetration compared with those impacted by a spherical projectile. Ballistic limit of the laminates increases in the order of thickness: 4, 6, 2 and 8 mm. Fewer than 60% of the fibers that lay within the path of the conical-nosed projectile failed; this was 13.2% ~ 31.1% less than the percent of broken fibers in laminates penetrated by a spherical projectile. It is responsible for poor ballistic performance of the HB26 laminates impacted by the conical-nosed projectile.