Fatigue Strength of Mortise and Tenon Furniture Joints Made from Oil Palm Lumber and Some Malaysian Timbers
A study was undertaken to evaluate comparative bending and fatigue strengths of rectangular mortise and tenon furniture joints made from oil palm (Eleais guineensis) lumber against a few established furniture wood materials. This is owing to the fact that, although the rectangular mortise and tenon joints are the most commonly joints used in chair construction, studies on its strength under cyclic loading is very limited. The fact that furniture joints are the weakest link in the furniture structure, it is necessary to establish its strength performance especially under repeated loading as experienced by the chair in service. The results showed that the bending strength of the oil palm lumber joints were half the strength of the wooden joints. However, in terms of fatigue strength, joints made from oil palm lumber showed comparable performance with the other wood materials. The results also showed that the allowable design stresses for rectangular mortise and tenon joints could be set at 20% of its bending strength. This study has far reaching implications on furniture engineering as it suggest that proper joint design is mandatory not only to ensure the furnitures reliability in service, but also improve its aesthetic and production imperatives.
Received: July 24, 2010;
Accepted: September 18, 2010;
Published: October 11, 2010
Furniture is considered a semi-rigid structure and is subjected to both static
and cyclic loadings in service. In furniture engineering, it is common practice
to size the joints to meet the aesthetic requirements of the furniture, although
such joints may not necessarily be of optimal dimensions (Cai
and Wang, 1993; Bao and Eckelman, 1996; Ratnasingam
et al., 1997). However, in many instances these joints are over-sized,
which has cost implications to the furniture manufacturer. Nevertheless, such
over-sized joints are often overlooked as the joints are regarded to be the
weakest link the furniture structure (Eckelman, 1999).
Therefore, any under-sized joints are perceived to be counter-productive to
the furniture engineering practice, as it may impair the final strength and
reliability of the furniture while in service. Inevitably, the strength performance
of furniture joints remains not well researched (Ratnasingam
et al., 1997; Ratnasingam and Ioras, 2010).
Furniture is often produced using rectangular mortise and tenon joints, especially
for joining the back leg and the side rail of chair frame construction (Eckelman,
1999). Factors such as the tightness of fit between the mortise and tenon,
the density and moisture content of the material as well as the dimensions of
the joints have been shown to have strong positive influence on the overall
strength of such joints (Eckelman and Zhang, 1995; Erdil
et al., 2005). Previous researches have shown that large dimensioned
joints of tighter fitness, made from higher density wood material with lower
moisture content, improved the overall strength of the joints significantly
(Erdil et al., 2005). Although, the round mortise
and tenon joints are efficient load bearers and are highly resistant to cyclic
loading, Tankut and Tankut (2005) found that rectangular
mortise and tenon joints to be 15% stronger due to the greater surface area
for load distribution. Strong joints are essential in furniture as furniture
failure often occurs at joints, which are subjected to both static and cyclic
loads (Ratnasingam et al., 1997). Consequently,
information on the strength performance of joints is important if a particular
material is to be promoted as a furniture material.
In recent years, oil palm (Eleais guineensis) lumber has emerged as
a potential raw material for the manufacture of educational school furniture
in the South East Asian region, on the basis of its acceptable strength and
working properties (Ratnasingam et al., 2007, 2008;
Ratnasingam and Wagner, 2009). Although the static and
fatigue strengths of furniture components made from oil palm lumber and other
wood materials have been studied (Ratnasingam and Ioras,
2010), the strength performance of joints made from oil palm lumber remains
unknown and the lack of information restricts its use in furniture frame construction
(Haviarova et al., 2001; Ramirez-Coretti
et al., 2009; Ratnasingam and McNulty, 2009).
Therefore, a study was undertaken to evaluate the bending and fatigue strengths
of rectangular mortise and tenon furniture joints made from oil palm lumber
in comparison to the joints made other commonly used furniture wood materials
such Rubberwood (Hevea brasiliensis), Nyatoh (Pallaquim sp.),
Meranti (Shorea sp.) and Sepetir (Sindora sp.). The results of
this study will provide fundamental information on the strength behaviour of
furniture joints, which will in turn help optimize wooden furniture engineering
design and construction.
MATERIALS AND METHODS
In this study, T-joints were assembled using the five different experimental
materials (i.e., oil palm lumber, Rubberwood, Nyatoh, Meranti and Sepetir),
glued together with a 40% solid content, two-part poly-vinyl acetate (PVAc)
adhesive. All the experimental materials were sourced from a local wood supplier,
while the adhesive was supplied by the Casco Nobel Adhesive Company in Malaysia.
The joint components consisted of a 450 mmx30 mmx25 mm leg member and a 350
mmx 55 mmx25 mm rail member. A total of 50 replicates were produced for each
of the experimental material. The rectangular mortise were cut using a mortising
machine and had a dimension of 40 mm in length, 20 mm in width and 12 mm in
thickness. The tenons were cut using a single-end tenon machine to the dimension
of 39.75 mm in length, 19.85 mm in width and 11.85 mm in thickness, to ensure
a tight-fit of the joint. The components were stored in a conditioning room
at 20°C and 65% relative humidity for a month, to achieve the target moisture
content of 12%. The T-joints were then assembled, using the PVAc adhesive, which
was applied liberally on to the surfaces of the tenon and mortise. The assemblies
were pressed under a pressure of 3 N mm-2 for a period of 5 min,
using pneumatic clamps. The excessive adhesive was wiped off from the surfaces
of assemblies, which were then left in a conditioning room at 65% relative humidity
and 20°C in temperature, for another month before testing. All testing were
carried out at the Materials Testing Laboratory of Bio-Resources Engineering
Inc. in Singapore.
25 T-joint specimens from each experimental material were tested in an INSTRON universal testing machine fitted with an aluminium alloy angle plate which held the leg section, while the rail section was loaded by means of a stirrup attached to the machine cross head which moved at 12 mm min-1 during the test (Fig. 1).
|| Experimental set-up for bending test of joints
|| Experimental set-up for fatigue strength test of joints
In this study, the ultimate moment capacity of the joint was calculated as
the product of the joint failure load and the distance between the points of
application of the load on the rail member (i.e., 310 mm) The ultimate moment
capacity is taken to be equal to the bending moment or bending strength, as
described in Tankut (2007) and Tankut
and Tankut (2010). Therefore, in this study the load which produced a sudden
discontinuity in the load/deflection curve during the testing, corresponding
to the joint failure was noted and used to determine the bending strength in
The other 25 specimens of the T- joint from each of the experimental material were tested in a specially designed fatigue strength test rig, which applied a cyclic load by means of an air cylinder at a rate of 20 cycles per minute on to the rail section of the T-joint (Fig. 2). Air regulators were used to adjust the applied load, while mechanical counters kept track of the number of cycles the specimens survived during the test. A limit switch was attached to turn off the test when the dial gauge attached to the specimen recorded a deflection exceeding 5 mm at the joint. The fatigue load applied were 10, 15, 20, 25 and 30% of the bending strengths determined previously from the earlier tests.
The results of this study are presented in three parts.
Part I: Bending strength of joints: Table 1 shows
the average bending strength of the joints made from the five different experimental
materials used in this study. Generally, the bending strength of joints made
from oil palm lumber was about half the strength of the joints made from the
other wood materials. However, bending strength showed a linear relationship
with density of the material and the bending strength of the experimental joints
increased in the order of Sepetir>Nyatoh>Meranti >Rubberwood>OPL.
This increasing trend in bending strength is possibly attributed to the correspondingly
higher shear strength possessed by the materials, as suggested by Huber
and Eckelman (1999) and Tunkut and Tunkut (2005).
This argument is further supported by the fact that failure in the oil palm
lumber joints were predominantly splits in the leg member, while in the other
wood material joints, failure occurred primarily by tenon breakage. As suggested
by Ratnasingam et al. (1997), failure by splitting
in furniture joints is often attributed to the inherently low shear strength
of the material, which cannot overcome the shearing forces acting on the joints.
|| Fatigue strength of furniture joints
|| Comparative bending strength of joints
|*Shear strength values based on Ratnasingam
and McNulty (2009). Average density determined on the basis of 25 samples.
Values in parentheses indicate the standard deviation
The observed low shear strength of the oil palm lumber could be explained by
the anatomical structure of the oil palm lumber, which impairs its shear strength
and splitting resistance (Ratnasingam and Ioras, 2010).
Part II: Fatigue strength of joints: In terms of the fatigue strength,
the results showed that the oil palm lumber specimens have similar strength
performance compared to the solid Rubberwood specimens at the 10% load level,
although it was not comparable to the other wood materials used in this study.
Nevertheless, at the higher load levels, the specimens made from the other wood
materials out-performed the oil palm lumber specimens (Fig. 3).
Further, the result also reaffirms the previous findings that the fatigue strength
of joints cannot be linearly related to its bending strength (Tsai
and Ansell, 1990; Ratnasingam et al., 1997).
Fatigue failure is a progressive, yet accumulative phenomenon (Tsai
and Ansell, 1990; Thompson et al., 2002,
2005) and in the case of furniture joints, the fatigue
strength reduction in joints shows a steady trend until a point, when abrupt
failure occurs. Under such behaviour, as long as the allowable design stress
is not exceeded, the joints can be expected to perform throughout the service
life of the product of stipulated in the standard (Eckelman,
1999). Further, it seem that the density and the anatomical structure of
the material has a significant influence on the fatigue property of the material,
as shown by the different fatigue strengths of joints made from the different
materials (Eckelman, 1999).
|| Determination of allowable design stress for joints
Part III: Allowable design stress for joints: Based on the fatigue strength
of the mortise and tenon joints from this study, the recommended allowable design
stress for furniture joints could be set at 20% of its bending strength, which
could ensure its compliance with the minimum 200,000 cycles of load (Fig.
4), required in most furniture performance testing standards (Eckelman,
Although the allowable design stress for furniture components could be set
at 40% of its bending strength (Ratnasingam and Ioras, 2010),
the allowable design stress for furniture joints are expectedly lower as joints
are usually the weakest link in the furniture structure (Ratnasingam
et al., 1997).
Considering the fact that reports on the fatigue strength of furniture joints
are sparse, the results of this study supports the findings of the few previous
studies by Erdil et al. (2005), Tankut
and Tankut (2005) and Tankut (2007), that reported
that the allowable design stress for rectangular mortise and tenon furniture
joints could be set between 15 to 30% of the bending strength. In fact, the
results of this study provides evidence to support the recommendation that the
allowable design stress for joints could be set at 20% of its bending strength,
which narrows the working range provided in previous studies. Nevertheless,
it must emphasized that in furniture engineering, joint design and dimensioning
is often treated secondary to the aesthetic need of the furniture, which in
turn leads to inappropriately engineered joints (Ratnasingam
et al., 1997). Therefore, this study provides fundamental information
on the optimal design and dimensioning of rectangular mortise and tenon joints
and recommends that the allowable design stress for rectangular mortise and
tenon joint be set at 20% of its bending strength, to ensure better furniture
engineering and construction.
Industrial implications: The current practice of furniture engineering
focuses on product aesthetics rather that strength performance, often resulting
in over-sized components and joints (Ratnasingam and McNulty,
2009). This is not only wasteful but is also not cost-effective manufacturing.
Further, fatigue failure is the most common cause of failure in furniture, especially
in chairs and therefore efforts to ensure the fatigue strength requirements
of furniture are taken into consideration in its design and construction is
of great importance (Kyanka, 1980). Against this background,
this study shows that the recommended allowable design stresses for rectangular
mortise and tenon furniture joints could be set at 20% of its bending strength
to ensure the compliance of the furniture in terms of its strength requirements.
Further, the study also shows that oil palm lumber can be used as a furniture
material provided the allowable design stresses are taken into consideration
in the furniture engineering and construction stages. Considering the fact that
studies on the fatigue strength of furniture components and joints are very
limited (Eckelman, 1974, 1988;
Ratnasingam et al., 2010), the results of this
study make important contributions towards a better understanding and design
of furniture frames.
The results of the study shows that the fatigue strength of furniture joints can be taken as 20% of its bending strength and under such loading circumstances, the joints can be expected to withstand the 200,000 load cycles stipulated in furniture performance standards. Further, the study also shows that oil palm lumber can be used as a furniture material, provided the recommended allowable design stresses are adhered to, during furniture engineering and construction.
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