Acoustic Properties of Micro-Perforated Panels Made from Oil Palm Empty Fruit Bunch Fiber Reinforced Polylactic Acid

This paper presents the development and performance of micro-perforated panels(MPP)from natural fiber reinforced composites.The MPP is made of Polylactic Acid(PLA)reinforced with Oil Palm Empty Fruit Bunch Fiber(OPEFBF).The investigation was made by varying the fiber density,air gap,and perforation ratio to observe the effect on the Sound Absorption Coefficient(SAC)through the experiment in an impedance tube.It is found that the peak level of SAC is not affected,but the peak frequency shifts to lower frequency when the fiber density is increased.This phenomenon might be due to the presence of porosity in the inner wall of the holes.Increasing or decreasing the air gap and perforation ratio shifts the peaks of acoustic absorption either way.

The Influence of Two Natural Reinforcement Fibers on the Hygrothermal Properties of Earthen Plasters in Mogao Grottoes of China

Murals in Mogao Grottoes consist of three parts:support layer,earthen plasters and paint layer.The earthen plasters play a key role in the preservation of murals.It is a mixture of Dengban soil,sand,and plant fiber.Two different natural fibers,hemp fiber and cotton fiber,were reinforced to earthen plasters in the same percentage to evaluate the influence on hygrothermal performance.The two types of earthen plasters were studied:one containing hemp fiber in the fine plaster(HFP)and the other containing cotton fiber in the fine plaster(CFP).Specific heat capacity,dry thermal conductivity,water vapor permeability,and sorption isotherms were investigated.The results showed that the difference between two natural fibers has much more impact on the hygric properties(water vapor permeability and sorption isotherms)of earthen plasters than on their thermal performance(specific heat capacity and dry thermal conductivity).The CFP with higher density has higher thermal conductivity than the HFP with lower density.But no significant differences of specific heat capacity were observed.Compared with HFP,CFP used in murals can reduce the rate of water transfer and prevent salt from transferring water to the mural surface.The overall findings highlight that all these features of CFP are beneficial to the long-term preservation of murals.The study of the earthen plasters in Mogao Grottoes is of general significance,and the measured properties can be used to obtain coupled heat and moisture analysis of the earthen plasters and to dissect the degradation mechanism of murals.

Sound absorption performance of natural fibers and their composites

This research aimed to study the sound absorption properties of natural fibers and their reinforced composites.Sound absorption coefficients of three types of natural fibers,i.e.,ramie,flax and jute fibers and their composites were measured by the two-microphone transfer function technique in the impedance tube.The results were compared with synthetic fibers and their composites.It was found that both natural fibers and their composites had superior capability of noise reduction.The multi-scale and hollow lumen structures of natural fibers contributed to the high sound absorption performance.Moreover,the sound absorption properties of these natural fibers were also calculated by the Delany-Bazley and Garai-Pompoli models.They showed good agreement with the experimental data.It was concluded that multi-functional composite materials can be made by natural fibers so that both the mechanical and acoustical functions can be achieved.

Humidity Diffusion on Jute/Polyester Laminated Composites and Its Effects on Mechanical Properties

Jute is a natural fiber widely used as reinforcement in composites due to its high tensile strength and stiffness,but they can easily absorb water and have their physical properties compromised.The water absorption properties of jute/polyester composites are evaluated according to ASTM D 570 and the effect of humidity in the composite mechanical behavior is also analyzed.The composite showed a pseudo-Fickian behavior and gained 13.37%in weight after the test.It also lost tensile strength and elasticity modulus,and increased its specific deformation.Scanning electron microscope images showed that wet specimens were more subject to cracks,voids and fiber pullout than dry specimens.Failures produced by water diffusion in composite and polymer plasticization,added to breakdown in the fibers’cellulosic structures,justify the change in mechanical properties due to water absorption.

Improving interlaminar fracture toughness of flax fiber/epoxy composites with chopped flax yarn interleaving

In this research, unidirectional flax fabrics reinforced epoxy laminates were intedeaved with randomly oriented chopped flax yarns at various yarn lengths and contents. Mode I interlaminar fracture toughness of the laminates was evaluated via Double Cantilever Beam （DCB） tests. The results showed that Mode I interlaminar fracture toughness increased with the introduction of the chopped yarns. With moderate yarn length and content, the best toughening effect （31% improvement in Mode I inter- laminar fracture toughness） was achieved. It was observed with the aid of Scanning Electronic Microscopy （SEM） that the in- troduction of the chopped yarns resulted in more tortuous in-plane crack propagation paths as well as the ＂trans-layer＂ phe- nomenon and fiber bridging effect of both the unidirectional yams and the chopped yams. These hindered the growth of the crack and led to more energy dissipation during delamination progress. Excessive yam length or content would induce unstable crack propagation and thus weakened the toughening improvement. No remarkable change was found in the tensile properties and the Charpy impact strength for the interleaved laminates, which indicated that this interleaving method was effective on interlaminar toughening without sacrificing the comprehensive mechanical properties of the laminates.