A Comparative Study into the Engineering Properties of Neem Wood from Ghana

A study into the mechanical properties of Neem wood was done to explore the possibility of its usage as a structural material. This paper presents the results of some investigations into the strength properties of Neem (Azadirach-ta indica A. Juss.) and compares them with those of Odum (Milicia excelsa) and European Oak (Quercus spp.) which are well known structural wood species in Ghana and Europe, respectively. The properties were determined using British (BS 373, 1957) and American timber testing specifications (ASTM D143, 1983) using testing methods for small, clear specimens of wood. The results showed that at 12% moisture content the wood has a density of 740 kg/m 3 with a mean modulus of rupture of 8317 N/mm 2 , compressive strength parallel to the grain of 318.8 N/mm 2 , modulus of elasticity of 8644 N/mm 2 , and shear strength parallel to the grain of 17 N/mm 2 . The bending strength (MOR) of Neem is about 30% lower than that of Odum and about 2% higher than Oak. The modulus of elasticity of Neem is about 39% lower than Odum and 43% lower than Oak. The strength properties in the present work suggest that the wood of Neem has a potential for utilisation in structural applications.


Introduction
The utilisation of well-known wood species for structural applications has gone on for ages. Ghana's tropical forest continues to face dangers and severe threats of deforestation and degradation. Ghana is currently experiencing alarming deforestation and forest degradation with an annual rate of 2% (about 135,000 hectares/year) of forest cover loss through anthropogenic causes [1]. Hence Materials Sciences and Applications there is an imperative need to assess the strength properties of lesser-known timbers to determine their usage in the wood industry and adopt species whose strength falls within the acceptable range for structural works. This will ultimately reduce the over-exploitation of the well-known species, ensuring sustainability whilst keeping the industry thriving. As opined by [2], the present state of Ghana's forest has resulted in the adoption of more strict control on timber exploitation, which has affected both the furniture and the construction industries. Falling volumes of timber in Ghana, coupled with a rise in illegal lumbering, have compelled the former net timber exporter to import timber to augment the demands of the industry [3].
In Ghana, timber is harvested from two main sources, namely: timber production forest reserves and areas outside forest reserves commonly referred to as off-reserve areas [4]. The first forest policy of Ghana, drawn in 1948, provided for a progressive utilization of the off-reserve timber resource, without replacement and assumed the confinement of forestry practices to the permanent forest estates. However, later developments showed that timber from off-reserve areas will be needed on a continuous basis to supplement production from the forest reserves which are capable of an annual production of only 500,000 m 3 of wood. Ghana's off-reserve area is represented by a total land area of about 5.482 million hectares in the High Forest Zone. This is made up of 1,618,738 ha in the Wet/Moist Evergreen, 1,559,236 ha in the Moist Semi-deciduous Southeast, 1,071,758 ha in the Moist Semi-deciduous Northwest and 1,232,446 ha in the Dry Semi-deciduous [5]. In 1996, off-reserves supported a total tree standing volume of 95 million m 3 which decreased to 37 million m 3 by 2005 [5]. In the late 1980s and early 1990s, 80% of recorded timber production was from the off-reserve areas. Presently, about 30% of the recorded harvest originates from the off reserves. The resource is, however, still important to the timber industry and the treasury of the constitutional beneficiaries. Between 2003 and 2006, the off-reserve [6] resource generated between 19% and 27% of the annual timber revenue in Ghana [7].
In the tropics, one of the abundant but under-utilised species is the Neem (Azadirachta indica A. Juss) found both in the forest and off-reserve areas. In the almost 82,000 km 2 forest zone representing about 34% of Ghana's total landmass, there exist over 420 hardwood species [8]. Besides, National volume estimates indicate that several hardwood species are being under-utilised, whilst the primary species such as Odum (Milicia excelsa) are over-exploited [8]. Neem (Azadirachta indica A. Juss) is an attractive broad-leaved evergreen tree that can grow up to 30 m tall and 2.5 m in girth. This species is abundant not only in the forest but is said to grow "almost anywhere" in the lowland tropics [9]. Neem often grows rapidly. It can be cut for timber after just 5 -7 years. Maximum yields reported from northern Nigeria (Samaru) amounted to 169 m 3 of fuelwood per hectare after a rotation of 8 years. Yields in Ghana were recorded between 108 and 137 m 3 per hectare at the same time [9].
The Neem tree (Azadirachta indica A. Juss.) is widely known for its medicinal properties [10]- [18]. It is also exceptionally durable due to the presence of a vae. By inhibiting moulting, it keeps the larvae from developing into pupae, and they die without producing a new generation. Also, azadirachtin is frequently so repugnant to insects that scores of different leaf-chewing species-even ones that normally strip everything living from plants-will starve to death rather than touch plants that carry traces of it [9]. Another Neem substance, salannin, is a similarly powerful repellent. It also stops many insects from touching even the plants they normally find most delectable when Neem is sprayed on them.
For constructional work, mechanical tests of timber are most important [19].
It assesses the various strength properties, such as compressive or crushing strength, shearing strength, bending strength, stiffness strength, etc. These properties inform practitioners of the timber's load-bearing capacity. According to [20], citing from [21], knowledge of the mechanical properties of wood allows for better optimisation and the minimal use of raw material. The study of the mechanical properties of timber species is therefore indispensable if the species are to be selected and used in the various domains of engineering [20]. The knowledge of the mechanical properties of timber species allows for the characterization of their behaviour under different applications [22]. The designer of a building or any structural product must consider the types of loads the individual structural members, or groups of members, must carry so that having selected timber possessing known strength properties, the required sizes of the members can be computed [23]. However, there is a paucity of technical data on its strength properties and possible utilisation in structural applications such as furniture. Furniture is usually constructed from short-length components with small cross-sections.
From an engineering point of view, the timber used for highly stressed furniture components (e.g., chair legs and rails, sofa rails, bed frame) should be defect-free.

Sample Preparation
Wood samples for the investigation were obtained from three trees felled in Beposo off forest reserve, a suburb in the Western Region of Ghana. The preparations of the test specimens were based on BS 373:1957 [24]. The boards were cut, planed, and carefully examined for visible defects. From these boards, test specimens were prepared for the determination of basic density, static bending  The mean value for each set of the specimens was recorded.

Determination of Strength Properties
Compression strength parallel to the grain: The compression strength parallel to the grain test was performed according to BS 373:1957 [24]. 300*50*50 mm specimens were used, and the test was performed on a compression universal machine. As in the case of the static bending test, the specimens were stored in a room of relative humidity of 50% and a temperature of 23˚ Celsius for three weeks to allow the moisture content to equilibrate to about 12%. The actual moisture content of the specimens was determined by an electrical moisture meter after testing. Specimen dimensions were measured to the nearest 0.001 mm before the testing. Data analysis: The data were analysed using descriptive statistics, one-Way Analysis of Variance (ANOVA) using Statistical Package for Social Sciences (SPSS).

Mechanical Properties of Neem
Mean values of the mechanical properties of Neem were calculated and are presented in Table 1. At a moisture content of 12%, it had a density of 740 kg/m 3 with a compression parallel to the grain of 39.82 Mpa. The MOR and MOE were 54.07 and 8317 Mpa respectively. The density of Neem was found to be 740 kg/m 3 which is within the range reported by [29]. This density is significant  Table 2) and depicted graphically in (Figure 1). Statistical analysis (One-Way ANOVA) indicated, however, that the differences among the properties of the wood are not statistically significant with the adjusted sums of squares (AdjSS) giving the p-value (p-value = 0.00) (  Figure 1).

Conclusion
The results show that the strength properties of Neem are within the strength values for wood for structural purposes. Neem wood also compares favourably with those of Odum and European oak and can be used for some structural applications where these two known kinds of wood are used. Since Neem is widely