Benyin Zeng’s research while affiliated with Beihang University and other places

The fatigue properties of composite materials are degraded seriously in hygrothermal environments, so taking into account their influence is very important when evaluating the fatigue life of composite structures. Tensile fatigue experiments of carbon fiber reinforced resin composite cross-ply laminates were conducted in room temperature/dry (RTD), cool temperature/dry (CTD) and elevated temperature/wet (ETW) conditions. The S-N curves and fatigue failure modes of the cross-ply laminates were obtained in three conditions. On this basis, a finite element model was established to discuss the influence of temperature and moisture content on the fatigue properties, as well as a method for determining environmental factors of fatigue life of cross-ply laminates was established. The results show that the saturation moisture absorption and temperature have a significant influence on the tensile fatigue properties of cross-ply laminates. The high-cycle fatigue property is weakened significantly by the saturation moisture absorption and high temperature, but the low-cycle fatigue properties were strengthened in cool temperature conditions. The delamination failure mode in ETW is the most severe, presenting with an obvious necking phenomenon. The influence of temperature has a greater effect than that of moisture content, but moisture absorption would play its affect obviously when temperature exceeds 40 °C.

To investigate the effect of notches on static and fatigue properties of T800 fabric carbon fibre reinforced epoxy, comparative tests were conducted between specimens of three depths of notches and un-notched specimens. Results demonstrate that the residual tensile strength of specimens is linearly decreased with the increasing depth of notches from 2mm to 4mm. The tensile strength on notched specimens dropped 31.5% comparing with no-damaged laminate. Due to the effect of notches, the fatigue limit was reduced to 55% of UTS and the slope of the S-N curve tends to be horizontal. In-situ damage expansion process was observed and the shape of proposed normalized S-N curves could be explained and concluded as two stages: the first stage accounts for 20% of the fatigue life, showing a dramatic decrease of fatigue strength; the remaining stage takes up the rest of the total life span, representing a steady decline in strength. It shows that no-damaged and notched laminates exhibit different behaviours in terms of damage evolution.

Low-velocity impact performance of scarf-repaired composite laminates was investigated by experimental and numerical methods. The numerical model was built using continuum damage mechanics and cohesive elements were used in all the interfaces of the adherends (i.e. the patch and the parent). The model was verified to be efficient. Based on it, damage profiles and damage mechanism were studied, and the effects of the scarf angle, the adhesive thickness and the patch off-axis angles were analyzed. Results show that the patch damage occurs in its center region through the thickness, while the damages in the parent and adhesive mainly occur around the back face. Damage area of the patch is obviously more serious than that of the parent, and the adhesive can protect the parent to some degree. The scarf angle, the adhesive thickness and the patch off-axis angles have effects on the impact properties. The scarf angles ranging from 7° to 8°, the adhesive thicknesses of 0.15 mm, the patch off-axis angles ranging from 135° to 160° are recommended for better impact performances.

Composite-metal joints with a metal insert are one kind of connecting structure. In this paper, tensile experimental tests were carried out to investigate tensile properties of a composite-metal joint with a novel metal insert design. Finite element models of the joint were established, and strain distribution and tensile strength were analyzed. The numerical results are in good agreement with the experimental results. Results show that the joint failure is dominated by shear properties of the resin layer. Increasing the resin layer thickness in a certain range will improve the tensile strength of the joint, while increasing the radius of the fillet on the ending side of the metal insert will decrease the joint strength. Increasing the resin layer plasticity will improve the joint strength. The effect of the embedded depth of the metal insert can be ignored.