68 Cronin St Annerley QLD 4103 Telephone: 0414416606 E-mail:
[email protected] 29 April 2012
Dr Peter Ho Queensland University of Technology Brisbane QLD 4000 Dear Doctor I am pleased to present you Timber Practical Report: Flexural Properties of Timber Members. This report presents the results of the flexural properties of four types of timber products that are of interest to Engineers and Architects. I thank you for giving me the opportunity to work on such a report and look lo ok forward to receiving your feedback. Yours Sincerely
Trent Paschkow Engineering Student
Executive Summery A testing rig was used to analyze the flexural properties of four different timber samples. Flexural stiffness and ultimate fiber strength capacity was calculated to aid in design for both strength and deflection. A three-point flexure test approach was taken. Included in this report are particulars on testing methods, calculations, and results. In this report will be found discussions on the elastic modulus, modulus of rupture and density as well as relationships between these. An investigation on how moisture correlates to that of strength and stiffness and look into the application of these different timber products will be reviewed. The overall objectives of the study were met, relationships between density and elasticity were found, as density increases elasticity decreased. Additionally the two composite materials were found to have fallen within the Australian Standard for these materials.
Table of Contents Executive Summery ........................................................................................................... 3 Introduction ......................................................................................................................... 5 1
Testing Methods ......................................................................................................... 5 1.1 1.2
Visual Strength Grading .............................................................................................. 5 Three-Point Flexure Test ........................................................ .................................... 5
2
Results............................................................................................................................ 6
2
Load vs. Deflection ..................................................................................................... 7
3
Calculations: ................................................................................................................ 8 3.1 Density .............................................................................................................................. 8 3.2 Moment of Inertia (mm^4) ........................................................................................ 8 3.3 Modulus of Elasticity .................................................................................................... 8 Softwood .................................................................................................................................................... 8 Hardwood .................................................................................................................................................. 9 Chipboard .................................................................................................................................................. 9 Plywood...................................................................................................................................................... 9 3.4 Modulus of Rupture ....................................................................................................10 Softwood ................................................................................................................................................. 10 Hardwood ............................................................................................................................................... 10 Chipboard ............................................................................................................................................... 10 Plywood................................................................................................................................................... 11
4
Evaluation .................................................................................................................. 11
5
The Effects of Moisture on E and MOR ............................................................. 12
6
Applications in Building ....................................................................................... 13 6.1 6.2 6.3 6.4
Softwood .........................................................................................................................13 Hardwood......................................................... ....................................................... .......13 Chipboard ......................................................... ....................................................... .......13 Plywood ..........................................................................................................................13
7
Conclusion ................................................................................................................. 14
8
References ................................................................................................................. 15
Appendix ............................................................................................................................. 16
Introduction Timber has always been one of the more plentiful natural resources available and consequently is one of the oldest know materials used in construction (Kermani 1999, 1). As walker (2006, 1) points out commercial timber falls into two categories, softwood and hardwood. Softwoods are the timbers of needle like trees i.e. Pines, were hardwoods on the other hand are a product of broader leafed trees such as oaks. Timber is a material that is used for a variety of structural forms, such as beams, columns , trusses and girders (Porteous 2007, 1) This report will provide information and characteristics of four types of timber samples, namely, softwood, hardwood, chipboard and plywood. All of which are of interest to engineers and architects alike. Included in this report are particulars on testing methods, calculations, and results. In this report will be found discussions on the elastic modulus, modulus of rupture and density as well as relationships between these. An investigation on how moisture correlates to that of strength and stiffness and look into the application of these different timber products will be reviewed.
1
Testing Methods
1.1
Visual Strength Grading
Defects in timber, whether natural or caused during conversion or seasoning, will have an effect on structural strength as well as on fixing, stability, durability and finished appearance of timber (Kermani 1999, 7). The visual inspection of timber for grading purposes is quite subjective, as experience and knowledge plays a key role. Timber samples were inspected in this way before applying the three-point flexure test approach. The four timber samples were scrutinized for characteristics such as a bow, spring, cup or twist, knots, slope of grain, wane and shakes or distortion. The results of the inspection can be seen: Table 1 – Sample Characteristics
Sample Softwood Hardwood Chipboard Plywood
1.2
Visual Characteristics Edge Grain, No Defects Edge Grain, Slight Bow Particles Visible, No Defects 3 Layers, No Defects
Three-Point Flexure Test
For each test sample provided (softwood, hardwood, chipboard, plywood) the average cross section dimensions, length and mass was recorded. The test span (distance between the supports) was also recorded. The test specimen was placed in the testing rig with the dial gauge beneath the load point ensuring it just slightly touches the specimen. The gauge was then zeroed. The loading arm with a mass of 0.5kg was then placed on the specimen and the deflection was recorded. In increments of 0.5kg, masses were added to the loading arm and the corresponding deflection was recorded. Weights were added until rupture occurred.
Figure 1 – 3 Point Flexure Test
2
Results
Table 2 – Timber Sample Measurements
Sample
Mass (g)
Length (mm)
Width (mm)
Depth (mm)
Softwood Hardwood Chipboard Plywood
46.3
998
12
12
35.35
610
9.3
9.1
176.16
751
20.1
16.3
47.88
749
20.4
6.75
Test Span
550mm
Table 3 – Load Deflection
Load Load (kg) 0 1.4 1.9 2.4 2.9 3.4 3.9 4.4 4.9 5.4 Failure
Deflection (mm) Newtons
Softwood
Hardwood
Chipboard
Plywood
0
0
0
0
0
13.72
2.96
3.45
1.61
14.82
18.62
3.71
4.94
2.28
20.07
23.52
4.51
6.73
2.88
25.12
28.42
5.31
7.9
3.55
29.43
33.32
6.18
9.74
4.24
35.42
38.22
6.93
10.91
4.94
42.25
43.12
7.83
11.43
5.4
47.16
48.02
8.44
13.76
6.26
-
52.92
9.28
14.3
7.05
-
10.4 kg (101.92 N)
5.4 kg (52.92 N)
22.4 kg (219.5 N)
11.4 kg (111.72 N)
2
Load vs. Deflection
The data was plotted on a graph for all samples (softwood, hardwood, chipboard, plywood) and a line of best fit was drawn. The line of best fit was drawn from the origin and the line equation was calculated.
() () From this, the gradient (m) was obtained and further analysis could be achieved. Figure 2 – Load vs. Deflection Graph
Load vs Deflection 50 y = 1.0801x 45
40
35
) 30 m m ( n o25 i t c e l f e D
Softwood Hardwood Chipboard
20
Plywood y = 0.2776x
15
y = 0.1812x
10
y = 0.1286x 5
0 0
10
20
30 Load (N)
40
50
60
3
Calculations:
3.1
Density
Table 4 – Sample Density
Sample
Mass (g)
Length (mm)
Width (mm)
Depth (mm)
Volume (mm^3)
Density (g/mm^3)
Softwood Hardwood Chipboard Plywood
46.3
998
12
12
143712
35.35
610
9.3
9.1
51624.3
176.16
751
20.1
16.3
246050.13
47.88
749
20.4
6.75
103137.3
3.222E-04 6.848E-04 7.160E-04 4.642E-04
3.2
Moment of Inertia (mm^4)
Were: I = Moment of Inertia (mm^4) b = Breadth (mm) d = Depth (mm) Table 5 – Moment of Inertia
Sample Softwood Hardwood Chipboard Plywood
3.3
Width (mm)
Depth (mm)
I (mm^4)
12
12
1.73E+03
9.3
9.1
5.84E+02
20.1
16.3
7.25E+03
20.4
6.75
5.23E+02
Modulus of Elasticity
Softwood
() ()
()
()( )
Hardwood
() ()
()
()( )
Chipboard
() ()
() ()()
Plywood
() ()
()
()( )
3.4
Modulus of Rupture
Table 6 - Rupture
Load Failure
Softwood 22.4 kg (219.52N)
Hardwood 11.4 kg (111.42N)
Chipboard 10.4 kg (101.92N)
Plywood 5.4 kg* (52.92N)
Plywood sample did not rupture, deflection became too great and sample was deemed failed.
Were: M = Bending Moment at Failure = PL/4 P = Load at Failure y = Distance from Neutral Axis to Extreme Fibers = d/2 I = Moment of Inertia Softwood
Hardwood
Chipboard
Plywood
4
Evaluation
Density is the best single indicator of the properties of timber and is a major factor determining its strength (Kermani 1999, 7). As Cardarelli (2008, 987) describes, the higher the density, the higher the tensile and compressive strength will be. The following table shows the flexural characteristics of the four samples. Table 7 – Density, E and MOR
Samples
Softwood Hardwood Chipboard Plywood
Density (ρ) (g/mm^3) 3.222E-4 6.848E-4 7.160E-4 4.642E-4
Moment of Inertia (mm^4) 1728.00 584.02 7254.00 522.83
Elastic Modulus (E) (Mpa) 11069.93 21379.69 3715.59 6137.94
E/ρ Mpa/(g/mm^3) (10^6) 34.36 31.22 5.19 13.22
Modulus of Rupture (MOR) (Mpa) 104.81 119.68 15.74 46.74**
MOR/ρ Mpa/(g/mm^3) (10^4) 32.53 17.48 2.20 10.12
** Plywood Sample did not rupture, deflection became to great and was deemed failed
As can be seen from table 7 hardwood contains the highest density when compared to the softwood sample (chipboard and plywood left out of relationship as these are composite materials). This is distinctive of hardwoods as they grow at a slower rate when compared to softwood. This generally results in a timber of high density and strength, which takes time to mature, over 100 years in some instances (Kermani 1999, 5). Generally speaking density and elasticity go hand in hand. In a study conducted in japan a direct correlation was found between density and elasticity, that being, the more dense a sample the higher the elastic modulus would be and vice versa (Ayarkwa 1999). These results are backed up by data seen in this report. The chipboard or particleboard is nearly double in density when compared to that of the plywood. Chipboard is a composite material; principally softwood cut into flakes, dried and then sprayed with an adhesive and then pressed (Dinwoodie 2000, 30) thus allowing it to have such great density. The Modulus of Rupture is very low when compared to the other materials and also falls quite short of the national standard of 15Mpa. The elastic modulus (6137Mpa) is also quite low which seems to suggest that this material is not used for its structural properties.
Different from particleboard, plywood’s are ma de from either hardwood or softwood. Logs are peeled from rotation against a slowly advancing knife to give a continuous strip. After drying, sheets of veneer for plywood manufacture are cooled with adhesive and are laid up and then pressed with the grain direction at right angles in alternate layers (Dinwoodie 2000, 7). In this experiment the modulus of elasticity and modulus of rupture cannot be accurately compared to that of the other specimens, as the sample did not actually rupture. Instead the sample was deemed to have failed due to the immeasurable deflection after 43.12N. Generally speaking the industry standard of plywood has a modulus of elasticity of 6900Mpa which is very similar to the achieved value of 6137.94Mpa.
5
The Effects of Moisture on E and MOR
Unlike other materials, the strength of timber is dependent on its moisture content (Kermaini 1999, 6). Figure 3 – Strength/Stiffness and Moisture Content
Kermani. A, 1999, Structural Timber Design, Chapter1, Pg 8
Figure 3 shows the relationship between moisture content and strength or stiffness characteristics. Looking at the graph it can be identified that there is linear loss in strength up to 30% of saturation. Saturation after this point has no effect on strength or stiffness. Most timber is air dried to between 17% and 23% and further reduction results in shrinkage of then fiber (Kermaini 200,6). Moisture content depends on the humidity of the surrounding, the higher the humidity the higher the moisture content will be. As shown by Cardarelli (2008, 986) Wood is categorized into five classes:
Green – Fresh wood, has received no formal drying. Air-Dried – wood having a moisture content of
