Study On Mechanical Properties Of Composite Materials' Interface Between CFRP And Wood

Carbon Fibre Reinforced Polymer/Plastic (CFRP) bonded to wood is an effective technicalmeans to the improvement of its mechanical properties. Effective bonding of interface betweenCFRP and wood is the key to ensuring the co-work of CFRP and wood in composite material.Based on the related research results of reinforced building structure of CFRP, systematic studyhas been done on the key interface's Composite technology, stripping bearing properties,hygrothermal effects, fracture toughness as well as endurance performance. By combining thecharacteristics of domestic fast-growing wood (Larix olgensis and Cunninghamia lanceolata),CFRP made in China is applied for this research. Main research results are as follows:1)Interface pressure and resin content are the main factors for interface's Compositetechnology between CFRP and wood. In certain pressure range, the higher the interfacepressure, the thinner the thickness of interface layer. Accordingly epoxy resin is liable topenetrate into wood and interface gains in strength. There are significant differences in bondperformance of composite wood's interface between two species. Larch's wood property isbetter, bond performance superior to China fir. The structure of composite wood has asignificant influence on bonding performance. The increased resin content promotes thebonding strength, but too much resin easily leads to thickening of interface layer, increase ofporosity and reduction in bonding strength. CFRP-wood interface's bonding strength dependson the wood property and bonding properties. For composite wood, the increase of interfacelayer is disadvantageous to shearing strength.2)While studying on the distribution of interface's stress, the process of strippingre-appears through the position curve indication of interface's shearing stress. When thebonding length is longer, the development of interface strain from loading to damage can bedivided into three stages: elastic stage when interface at a low loading level, strain developmentstage in higher loading level and stripping stage. There is effective bond length existingbetween CFRP and larch or China fir. Under the condition that actual bond length is longer thanthe effective bond length, ultimate bonding carrying capacity between CFRP and wood will nolonger increase but the extended length helps delay the interface's damaging process. Themodel formula mentioned in this article has a high precision, suitable for calculating effectivebond length of CFRP-larch and CFRP-fir's interfaces and carrying capacity in striping.3)After testing wet strain of larch and CFRP and solving their coefficients of moistureexpansion, CFRP's coefficients of moisture expansion in the horizontal or vertical are no lessthan1/8of the larch's. Their coefficients of moisture expansion in the vertical are far largerthan in the horizontal. When CFRP responses to moisture strain, its moisture absorption processis slow, obviously lagging than larch. When humidity is in the range of35-60%, coefficients of both are the minimum. After constant warming and cooling treatment on them in the relativehumidity95%, the result shows that CFRP's coefficient of thermal expansion is the joint workof heat and humid. CFRP's coefficient of thermal expansion is smaller, no less than1/5of thelarch.4)For symmetrical structure composite material, CFRP-larch interface is less affected bythe environment wet; while composited orthogonally, the result is the opposite. Thus,temperature has less obvious effect on the thermal expansion and contraction generated bysymmetrical composite material interface. CFRP with its low moisture expansion rate caneffectively restrain larch's expansion in horizontal or vertical. For symmetric composite wood,CFRP improves larch's hot and humid resistance performance. After hot and humid treatment,CFRP-larch composite wood's bending strength and elastic modulus decrease exponentiallyalong with the increase of treatment time, they decline greatly. CFRP-wood interface's end isthe key part for bending capacity attenuation. In application, CFRP-wood composite shouldavoid hot and humid environment or to be protected to reduce the effect of wet effect. As forCFRP-larch asymmetric structure composite wood, the bending capacity weakens less after heattreatment.5)CFRP-wood interface's fracture toughness is superior to wood-wood's for differentmaterials on both sides are helpful for improving instable propagation of cracks. Tip of crack ismainly influenced by stress and displacement field. Tip of interface's crack bears so huge stressthat propagation of crack mostly happens in the interface layer. Composite wood has a greateffect on the fracture toughness. It is suggested that composite wood in practical use shouldavoid damp on account of its great fracture toughness in dry condition. DCB experiment basedon the method of flexibility is recommended for measuring the fracture toughness ofCFRP-wood interface.6)CFRP-wood's short-term creep process can be divided into two stages: the first creep,increasing in a nonlinear way; the second creep, low growth rate in a linear way. Temperatureand relative humidity are two important factors on creep. Creep amount in hot and humidenvironment is significantly larger than that in adverse environment. Interface layer is liable tocrack in high humid environment and high stress level. Creep deformation increases with thestress level. Creep curve of each stress level shows the same creep rule, that is, creepdeformation is obvious in the early stage but the creep rate gradually slows down.