Timber Practical
Timber Practical ENB273 – Civil Materials Tim Starkey n8588163
Table of Contents Introduction ....................................................................................................................................... 3 Aim.................................................................................................................................................................... 3 Calculations ................................................................................................................................................... 3 Specimens ...................................................................................................................................................... 3 Softwood ..................................................................................................................................................................... 4 Hardwood ................................................................................................................................................................... 4 Chipboard ................................................................................................................................................................... 4 Plywood ....................................................................................................................................................................... 4
Method.................................................................................................................................................. 5 Load Deflection ............................................................................................................................................ 6
Discussion ........................................................................................................................................... 6 Engineering Characteristics .................................................................................................................... 6 Analysis ........................................................................................................................................................... 7 Errors............................................................................................................................................................ 10
Conclusion ........................................................................................................................................ 10 References ........................................................................................................................................ 11 Appendices....................................................................................................................................... 12
2
Introduction The design of timber structures frequently relies on the use of flexural properties of the timber members. These flexing properties may be a designed feature or an unexpected failure in a structure. It is then important to know the flexural stiffness and ultimate fibre strength capacity of the timber under consideration to enable adequate design for both strength and deflection.
Aim The aim of this practical is to examine the properties of some typical timber materials. Although the test methods adopted in this investigation are not standard, they provide an indication as to the relative performance of various timbers and timber products.
Calculations Below is a list of calculations and their variables required to calculate the specific quantities. Moment of Inertia: Where:
b = Cross-sectional width of test specimen d = Cross-sectional depth of test specimen
Deflection: Where:
P = Load applied to test specimen L = Span length between supports E = Modulus of elasticity I = Moment of inertia of section
Failure Bending Moment: Where:
P = Load at Failure
Modulus of Rupture: Where:
M = Bending moment at Failure y = Distance from neutral axis to the extreme fibre
Specimens The investigation of the flexure looks at four typical timbers and they’re ability to withstand deflection. The most common timbers used in construction are softwoods, hardwoods and man made timbers for both their properties and cost.
3
For this reason, this study looks at the deflection in a hardwood, softwood, plywood and chipboard specimen.
Softwood Softwood is a timber from a conifer such as a pine or fir tree. Conifers, or gymnosperm, trees usually have needle like leaves which remain on the tree all year round. This type of tree tends to be much faster growing than hardwood, creating large, distinct growth rings in the timber. These larger growth rings make the wood a lot less dense and less durable than most hardwoods but higher calorific values. Because of these different properties, softwood tends to be a lot more plentiful and cheaper than hardwood and used in the majority of small construction (houses etc). Softwoods have a relatively simple, microstructure made up of large, open longitudinal cells. Adhesives, finishes and moisture are absorbed into softwood better than hardwoods because of these open cells. Hardwood The timber from a dicotyledonous tree is known as hardwood. These timbers are usually more dense and durable than softwoods due to thicker cell walls. Tending to take a longer period of time to mature, hardwoods are usually more expensive and denser with tighter growth rings. The main difference between softwood and hardwood is the microstructures. Hardwood has a much more complex microstructure than softwood. Made up with two types of longitudinal cells along with vessels. These vessels create large tube like voids in the timber that deliver nutrients throughout the tree.
Chipboard Manufacture from small off cuts, chipboard is produced from the left overs at a sawmill bound together with resin and glue to produce a flat rectangular sheet. Used largely as a base material, chipboard is commonly used as an underlayment panel creating a resilient floor. Plywood Plywood is another man made timber that is constructed specifically for its high strength but low weight features. Built out of thin layers of hardwood, for each new layer of veneer, the grain is laid perpendicular to the previous layer to maximise strength and minimise warping. This creates a crossing over of grains throughout the timber, preventing a line of weakness from occurring. However, although the outer layers of plywood are aesthetically pleasing, the inner layers are usually made from cheaper timbers. This can weaken the structure as knots and other defects within the veneers create weak pockets in the sheet.
4
Method For each of the four timber specimens provided: (a) Measure and record the average cross section dimensions and the length of the sample. Determine the mass of each sample. (b) Measure and record the test span for the three-point flexure testing (that is how far between the supports). (c) Place test specimen in testing rig and place dial gauge beneath the load point of the sample, ensuring it just touches the underside of the sample. Zero the dial gauge. Do not place leading arm on sample prior to zeroing. (d) Place loading arm on the sample and measure the deflection. Given that the loading arm has a mass of 1.4kg, record this load and its corresponding deflection. (e) In increments of 0.5kg, place masses on the loading arm, record the sample deflection for each increase mass. Stop when the total mass supported by the sample (including the 1.4kg mass of the loading arm) reaches 5.4kg. Please note the chip wood may break at 3.0kg. (f) Remove the dial gauge. (g) Continue loading the test specimen until failure. supported by the samples.
Record the ultimate load
(h) Using your results determine the Elastic Modulus (E) and Maximum Tensile Stress (or Modulus of Rupture MOR) for each samples.
Results Prior to any testing of the materials, each specimen was examined for any defects, as these would affect the outcome of the testing. Along with these flaws, the type of grain of the specimen was recorded due to the relationship between grain direction and strength (T.O'Hara & D.Grant, 1985). After inspecting the specimens, dimensions and weights were recorded so that the density could be calculated. The width and depth of the specimen is a crucial factor of the deflection for member, defining the Moment of Inertia. For this reason, a dedicated testing top and bottom was identified prior to measuring. The results are as follows: Sample
Description Mass (g)
Softwood
Edge Grain, No visible defects Edge Grain, Knot and wane at opposite ends
Hardwood
51.2
Length (mm) 606
Width (mm) 11.9
Depth (mm) 11.6
38.3
599
9.1
8.8
5
Chipboard Plywood
109 55.0
597 600
20.4 20.9
12.4 9.0
Because Chipboard and Plywood are man-produced timbers with no continuous growth ring throughout the timber, any observation made from the surface of the specimen would not be correct internally. However, Softwood and Hardwood may still have internal flaws that cannot be seen by observations but these were considered to be negligent as the majority of the timber was of continuous grain.
Load Deflection The specimens were tested through a device to measure the amount of deflection under particular loads. After recording the amount of deflection to a particular load, the specimens were loaded until failure so that the ultimate loading strength was identified. The results are as follows: Load Deflection (mm) (kg) (N) Softwood Hardwood Chipboard Plywood 0 0 0 0 0 0 1.4 13.72 1.90 3.43 3.39 N/A 1.9 18.62 2.25 4.59 5.10 N/A 2.4 23.52 2.85 5.87 6.65 N/A 2.9 28.42 3.52 7.07 8.07 N/A 3.4 33.32 4.07 8.51 9.69 N/A 3.9 38.22 4.97 9.51 11.17 N/A 4.4 43.12 5.47 10.29 13.17 N/A 4.9 48.02 6.00 12.42 14.94 N/A 5.4 52.92 6.75 13.28 N/A Fail Load (N) 219.52 136.22 52.92 Load versus Deflection Graph Appendix 1
Due to the properties of Plywood, under the first load the device had passed its maximum-recorded deflection so no load data could be collected for this specimen.
Discussion Engineering Characteristics The recorded data can then been used to calculate certain Engineering Characteristics for each of the timbers. By knowing these qualities of the specimens, appropriate applications for each timber can be determined. The characteristics for each specimen are as follows: Sample Softwood
Density (kg/m3) Moment of Inertia (mm4) E (MPa) MOR (MPa) 612.07
1547.88
6
3285.32
113.1
Hardwood
798.45
516.78
4955.79
159.47
Chipboard
721.77
3241.26
615.29
13.91
Plywood
487.33
1269.68
N/A
N/A
Looking at these Engineering Characteristics, the workability of each sample has been identified. These limits demonstrate the maximum stresses that the timbers of each sample could be placed under.
Analysis The results for the three samples that were successfully tested were plotted in a ‘Load Versus Deflection’ graph in Appendix 1. This graph reflects the samples ability to deflect under particular loads. The stiffer the sample was, the less it deflected. However, the stiffness of each sample cannot be directly related due to the different dimensions, instead the gradient, Young’s Modulus, is compared. Looking at the Modulus of Elasticity and Modulus of Rupture in the four samples, there is quite a range in the data. Softwood has a very low Modulus of Elasticity
Softwood Load Versus Deflection 8 7
y = 0.6816x + 0.0293
Deflection (mm)
6 5 4 3 2 1 0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
4.9
5.4
when compared to the rest of the results. This indicates that the softwood sample used was very flexible material in relation to the other samples. Looking at the data recorded for softwood, there was very little deflection compared to the other samples. Despite having a Modulus of Elasticity that 7
depicts the timber to be flexible, there was very little flex when the sample was tested. This anomaly between the flex in the test and the Modulus of Elasticity could be due to the sample that was tested. Rather than a more blank like geometry, the softwood sample had a squarer cross-section. This created a larger Moment of Inertia than the hardwood sample. During testing however, softwood had the largest breaking load compared to the other samples. Holding more than 20kg before failing, the sampled softwood was much stronger than previous testings. When viewing the results for softwood’s Modulus of Elasticity and Modulus of Rupture it can be seen that softwood is stronger in tension than deflection. With quite a large Modulus of Elasticity, which runs parallel to the sample, this sample of softwood would be very strong when used as a member in tension. However, the Modulus of Rupture shows that in deflection or under a perpendicular, softwood does not have anywhere near as much strength as along its grain.
Hardwood Load Versus Deflection 16 14
y = 1.3534x + 0.0533
Deflection (mm)
12 10 8 6 4 2 0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
4.9
5.4
Hardwood’s Modulus of Elasticity showed that it was a very flexible timber. Looking at the Load versus Deflection graph somewhat depicts that. The steeper gradient shows that the timber deflected under the loads a lot more than softwood. This is characteristically incorrect, as hardwoods tend to be stiffer than softwoods. However, this would be due to the different cross-sectional areas across the samples.
8
The results calculated from the data show that hardwood was the strongest sample in both tension and deflection. Having the highest values for Modulus of Elasticity and Modulus of Rupture show that this hardwood sample would withstand the largest force in tension and perpendicular to the grain than any of the samples. This would be due to the timbers density, with more growth rings and finer grain, the load is displaced through more particulars, spreading the load throughout the timber better than the other samples.
Chipboard Load Versus Deflection 18 y = 1.738x - 0.67
16
Deflection (mm)
14 12 10 8 6 4 2 0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
4.9
5.4
Being a man made timber, chipboard doesn’t consist of a continuous natural grain. This prevents loads being transferred through the member efficiently. Without this transfer of force, chipboard was very stiff when tester. Rather than deflecting in an arc, it hinged where the load was applied, creating a ‘V’ shape as the load was applied. Not only is there no continuous grain, chipboard is constructed from multiple, small pieces of different timbers glued together, creating endless lines of weaknesses within the structure, making the sample very brittle. At point of failure, rather than splintering off like softwood and hardwood, chipboard had a very straight fracture, perpendicular to the length of the sample. Unlike the softwood and hardwood samples, chipboard failed before the end of the test, failing at a much smaller load compared to softwood and hardwood. This is present in the timbers Modulus of Elasticity and Modulus of Rupture. Despite
9
being much stronger in tension than under a moment force, chipboard’s overall strength compared to the other timbers was dramatically weaker. As a structural timber, chipboard would not be appropriate under any load. Due to the properties of plywood, it could not be tested under the same circumstances as the other samples. Because of the alternating grain directions within this man made timber, plywood is a very ductile, yet strong material. During testing, plywood deflected further than the rig could measure under the first load. It was then able to take the second load before deflecting more than the rig allowed. The sample bowed more than 15cm under the load then returned to its original state when the load was removed. This demonstrated the ductility of plywood under a load in this direction compared to its grain. Plywood veneers are laminated horizontally to each other, effectively creating multiple small depth versus large breath Moments of Inertias, making the timber very ductile in that direction of loading. However, if the testing was performed with the material sitting with vertical veneers, the sample wouldn’t have been so ductile and results could have been recorded. Overall, looking at the results calculated and recorded, hardwood was the best performing sample. Strong in tension and under a moment force load, out of the samples tested, hardwood was the ideal material under these types of loads.
Errors When performing the testing of the materials, a few errors could have occurred which affected the data recorded. The first error that would affect the data was the recording of the original position. The only way that the datum could’ve been recorded on the rig was to position the loading bar to where it just touched the sample. This was performed by eye and would give a slight error in the data’s original position. Another error that occurred while testing the material was a continuous load. Due to the limited range of weights available to load the rig with, the load was sometimes removed to add a larger weight. This lead to the sample being testing through varying loads, affecting the yield point for the material. The moisture content of each sample would also affect the strength and ductility of the samples. With more moisture in the timber, the sample would be more ductile than it would be naturally. This then affects the results for density and Moment of Rupture.
Conclusion After testing the materials, recording data, analysing the results and discussing possible errors in the data, the test of flexural properties in timbers resulted in data useful for use in real world situations. The limits in strength and deflection of the four materials tested were calculated so that these could be considered in design.
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References Woodwork Basics. (n.d.). Plywood. Retrieved 4 11, 13 from WOODWORKBASICS.COM: http://www.woodworkbasics.com/ Woodwork Basics. (13, 4 22). Softwood. From WOODWORK BASICS: http://www.woodworkbasics.com/softwood.html About.com. (13, 4 22). What is the difference between a hardwood and softwood? From About.com Forestry : http://forestry.about.com/cs/treeid/f/Tree_ID_wood.htm Allen, E., & J.Iano. (2008). Fundamentals of Building Construction, Materials & Methods (Fifth Edition ed.). New Jersey: John Wiley & Sons. Diffen. (13, 4 22). Hardwood vs Softwood. From Diffen: http://www.diffen.com/difference/Hardwood_vs_Softwood How Stuff Works. (13, 4 22). What is the difference between a hardwood and a softwood? From How Stuff Works: http://science.howstuffworks.com/life/genetic/question598.htm T.O'Hara, & D.Grant. (1985). Sloping Grain and the Strength of Structural Timber. Sydney: NSW Department of Primary Industries.
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Appendices Appendix 1: Load Versus Deflection Graph
Load Versus Deflection Softwood
Hardwood
Chipboard
16
14
Deflection (mm)
12
10
8
6
4
2
0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
Appendix 2: Calculations Density Sample Softwood
Equation
Results 5.998165739 gm/cm3
Hardwood
7.824812917 gm/cm3
Chipboard
7.073370225 gm/cm3
Plywood
4.77582846 gm/cm3
12
4.9
5.4
Moment of Inertia Sample Softwood
Equation
Results 1547.888533 mm4
Hardwood
516.7829333 mm4
Chipboard
3241.2608 mm4
Plywood
1269.675 mm4
Modulus of Elasticity Rearrange To (equivalent to y=mx) Therefore: Rearranged to: Sample Softwood
Equation
Result 3285.320 MPa
Hardwood
4955.7944 MPa
Chipboard
615.2947214 MPa
Modulus of Rupture Sample M Equation Softwood
Equation
Result 113.101274 MPa
Hardwood
159.4742444 MPa
Chipboard
13.91875382 MPa
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Table of Contents Introduction ....................................................................................................................................... 3 Aim.................................................................................................................................................................... 3 Calculations ................................................................................................................................................... 3 Specimens ...................................................................................................................................................... 3 Softwood ..................................................................................................................................................................... 4 Hardwood ................................................................................................................................................................... 4 Chipboard ................................................................................................................................................................... 4 Plywood ....................................................................................................................................................................... 4
Method.................................................................................................................................................. 5 Load Deflection ............................................................................................................................................ 6
Discussion ........................................................................................................................................... 6 Engineering Characteristics .................................................................................................................... 6 Analysis ........................................................................................................................................................... 7 Errors............................................................................................................................................................ 10
Conclusion ........................................................................................................................................ 10 References ........................................................................................................................................ 11 Appendices....................................................................................................................................... 12
2
Introduction The design of timber structures frequently relies on the use of flexural properties of the timber members. These flexing properties may be a designed feature or an unexpected failure in a structure. It is then important to know the flexural stiffness and ultimate fibre strength capacity of the timber under consideration to enable adequate design for both strength and deflection.
Aim The aim of this practical is to examine the properties of some typical timber materials. Although the test methods adopted in this investigation are not standard, they provide an indication as to the relative performance of various timbers and timber products.
Calculations Below is a list of calculations and their variables required to calculate the specific quantities. Moment of Inertia: Where:
b = Cross-sectional width of test specimen d = Cross-sectional depth of test specimen
Deflection: Where:
P = Load applied to test specimen L = Span length between supports E = Modulus of elasticity I = Moment of inertia of section
Failure Bending Moment: Where:
P = Load at Failure
Modulus of Rupture: Where:
M = Bending moment at Failure y = Distance from neutral axis to the extreme fibre
Specimens The investigation of the flexure looks at four typical timbers and they’re ability to withstand deflection. The most common timbers used in construction are softwoods, hardwoods and man made timbers for both their properties and cost.
3
For this reason, this study looks at the deflection in a hardwood, softwood, plywood and chipboard specimen.
Softwood Softwood is a timber from a conifer such as a pine or fir tree. Conifers, or gymnosperm, trees usually have needle like leaves which remain on the tree all year round. This type of tree tends to be much faster growing than hardwood, creating large, distinct growth rings in the timber. These larger growth rings make the wood a lot less dense and less durable than most hardwoods but higher calorific values. Because of these different properties, softwood tends to be a lot more plentiful and cheaper than hardwood and used in the majority of small construction (houses etc). Softwoods have a relatively simple, microstructure made up of large, open longitudinal cells. Adhesives, finishes and moisture are absorbed into softwood better than hardwoods because of these open cells. Hardwood The timber from a dicotyledonous tree is known as hardwood. These timbers are usually more dense and durable than softwoods due to thicker cell walls. Tending to take a longer period of time to mature, hardwoods are usually more expensive and denser with tighter growth rings. The main difference between softwood and hardwood is the microstructures. Hardwood has a much more complex microstructure than softwood. Made up with two types of longitudinal cells along with vessels. These vessels create large tube like voids in the timber that deliver nutrients throughout the tree.
Chipboard Manufacture from small off cuts, chipboard is produced from the left overs at a sawmill bound together with resin and glue to produce a flat rectangular sheet. Used largely as a base material, chipboard is commonly used as an underlayment panel creating a resilient floor. Plywood Plywood is another man made timber that is constructed specifically for its high strength but low weight features. Built out of thin layers of hardwood, for each new layer of veneer, the grain is laid perpendicular to the previous layer to maximise strength and minimise warping. This creates a crossing over of grains throughout the timber, preventing a line of weakness from occurring. However, although the outer layers of plywood are aesthetically pleasing, the inner layers are usually made from cheaper timbers. This can weaken the structure as knots and other defects within the veneers create weak pockets in the sheet.
4
Method For each of the four timber specimens provided: (a) Measure and record the average cross section dimensions and the length of the sample. Determine the mass of each sample. (b) Measure and record the test span for the three-point flexure testing (that is how far between the supports). (c) Place test specimen in testing rig and place dial gauge beneath the load point of the sample, ensuring it just touches the underside of the sample. Zero the dial gauge. Do not place leading arm on sample prior to zeroing. (d) Place loading arm on the sample and measure the deflection. Given that the loading arm has a mass of 1.4kg, record this load and its corresponding deflection. (e) In increments of 0.5kg, place masses on the loading arm, record the sample deflection for each increase mass. Stop when the total mass supported by the sample (including the 1.4kg mass of the loading arm) reaches 5.4kg. Please note the chip wood may break at 3.0kg. (f) Remove the dial gauge. (g) Continue loading the test specimen until failure. supported by the samples.
Record the ultimate load
(h) Using your results determine the Elastic Modulus (E) and Maximum Tensile Stress (or Modulus of Rupture MOR) for each samples.
Results Prior to any testing of the materials, each specimen was examined for any defects, as these would affect the outcome of the testing. Along with these flaws, the type of grain of the specimen was recorded due to the relationship between grain direction and strength (T.O'Hara & D.Grant, 1985). After inspecting the specimens, dimensions and weights were recorded so that the density could be calculated. The width and depth of the specimen is a crucial factor of the deflection for member, defining the Moment of Inertia. For this reason, a dedicated testing top and bottom was identified prior to measuring. The results are as follows: Sample
Description Mass (g)
Softwood
Edge Grain, No visible defects Edge Grain, Knot and wane at opposite ends
Hardwood
51.2
Length (mm) 606
Width (mm) 11.9
Depth (mm) 11.6
38.3
599
9.1
8.8
5
Chipboard Plywood
109 55.0
597 600
20.4 20.9
12.4 9.0
Because Chipboard and Plywood are man-produced timbers with no continuous growth ring throughout the timber, any observation made from the surface of the specimen would not be correct internally. However, Softwood and Hardwood may still have internal flaws that cannot be seen by observations but these were considered to be negligent as the majority of the timber was of continuous grain.
Load Deflection The specimens were tested through a device to measure the amount of deflection under particular loads. After recording the amount of deflection to a particular load, the specimens were loaded until failure so that the ultimate loading strength was identified. The results are as follows: Load Deflection (mm) (kg) (N) Softwood Hardwood Chipboard Plywood 0 0 0 0 0 0 1.4 13.72 1.90 3.43 3.39 N/A 1.9 18.62 2.25 4.59 5.10 N/A 2.4 23.52 2.85 5.87 6.65 N/A 2.9 28.42 3.52 7.07 8.07 N/A 3.4 33.32 4.07 8.51 9.69 N/A 3.9 38.22 4.97 9.51 11.17 N/A 4.4 43.12 5.47 10.29 13.17 N/A 4.9 48.02 6.00 12.42 14.94 N/A 5.4 52.92 6.75 13.28 N/A Fail Load (N) 219.52 136.22 52.92 Load versus Deflection Graph Appendix 1
Due to the properties of Plywood, under the first load the device had passed its maximum-recorded deflection so no load data could be collected for this specimen.
Discussion Engineering Characteristics The recorded data can then been used to calculate certain Engineering Characteristics for each of the timbers. By knowing these qualities of the specimens, appropriate applications for each timber can be determined. The characteristics for each specimen are as follows: Sample Softwood
Density (kg/m3) Moment of Inertia (mm4) E (MPa) MOR (MPa) 612.07
1547.88
6
3285.32
113.1
Hardwood
798.45
516.78
4955.79
159.47
Chipboard
721.77
3241.26
615.29
13.91
Plywood
487.33
1269.68
N/A
N/A
Looking at these Engineering Characteristics, the workability of each sample has been identified. These limits demonstrate the maximum stresses that the timbers of each sample could be placed under.
Analysis The results for the three samples that were successfully tested were plotted in a ‘Load Versus Deflection’ graph in Appendix 1. This graph reflects the samples ability to deflect under particular loads. The stiffer the sample was, the less it deflected. However, the stiffness of each sample cannot be directly related due to the different dimensions, instead the gradient, Young’s Modulus, is compared. Looking at the Modulus of Elasticity and Modulus of Rupture in the four samples, there is quite a range in the data. Softwood has a very low Modulus of Elasticity
Softwood Load Versus Deflection 8 7
y = 0.6816x + 0.0293
Deflection (mm)
6 5 4 3 2 1 0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
4.9
5.4
when compared to the rest of the results. This indicates that the softwood sample used was very flexible material in relation to the other samples. Looking at the data recorded for softwood, there was very little deflection compared to the other samples. Despite having a Modulus of Elasticity that 7
depicts the timber to be flexible, there was very little flex when the sample was tested. This anomaly between the flex in the test and the Modulus of Elasticity could be due to the sample that was tested. Rather than a more blank like geometry, the softwood sample had a squarer cross-section. This created a larger Moment of Inertia than the hardwood sample. During testing however, softwood had the largest breaking load compared to the other samples. Holding more than 20kg before failing, the sampled softwood was much stronger than previous testings. When viewing the results for softwood’s Modulus of Elasticity and Modulus of Rupture it can be seen that softwood is stronger in tension than deflection. With quite a large Modulus of Elasticity, which runs parallel to the sample, this sample of softwood would be very strong when used as a member in tension. However, the Modulus of Rupture shows that in deflection or under a perpendicular, softwood does not have anywhere near as much strength as along its grain.
Hardwood Load Versus Deflection 16 14
y = 1.3534x + 0.0533
Deflection (mm)
12 10 8 6 4 2 0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
4.9
5.4
Hardwood’s Modulus of Elasticity showed that it was a very flexible timber. Looking at the Load versus Deflection graph somewhat depicts that. The steeper gradient shows that the timber deflected under the loads a lot more than softwood. This is characteristically incorrect, as hardwoods tend to be stiffer than softwoods. However, this would be due to the different cross-sectional areas across the samples.
8
The results calculated from the data show that hardwood was the strongest sample in both tension and deflection. Having the highest values for Modulus of Elasticity and Modulus of Rupture show that this hardwood sample would withstand the largest force in tension and perpendicular to the grain than any of the samples. This would be due to the timbers density, with more growth rings and finer grain, the load is displaced through more particulars, spreading the load throughout the timber better than the other samples.
Chipboard Load Versus Deflection 18 y = 1.738x - 0.67
16
Deflection (mm)
14 12 10 8 6 4 2 0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
4.9
5.4
Being a man made timber, chipboard doesn’t consist of a continuous natural grain. This prevents loads being transferred through the member efficiently. Without this transfer of force, chipboard was very stiff when tester. Rather than deflecting in an arc, it hinged where the load was applied, creating a ‘V’ shape as the load was applied. Not only is there no continuous grain, chipboard is constructed from multiple, small pieces of different timbers glued together, creating endless lines of weaknesses within the structure, making the sample very brittle. At point of failure, rather than splintering off like softwood and hardwood, chipboard had a very straight fracture, perpendicular to the length of the sample. Unlike the softwood and hardwood samples, chipboard failed before the end of the test, failing at a much smaller load compared to softwood and hardwood. This is present in the timbers Modulus of Elasticity and Modulus of Rupture. Despite
9
being much stronger in tension than under a moment force, chipboard’s overall strength compared to the other timbers was dramatically weaker. As a structural timber, chipboard would not be appropriate under any load. Due to the properties of plywood, it could not be tested under the same circumstances as the other samples. Because of the alternating grain directions within this man made timber, plywood is a very ductile, yet strong material. During testing, plywood deflected further than the rig could measure under the first load. It was then able to take the second load before deflecting more than the rig allowed. The sample bowed more than 15cm under the load then returned to its original state when the load was removed. This demonstrated the ductility of plywood under a load in this direction compared to its grain. Plywood veneers are laminated horizontally to each other, effectively creating multiple small depth versus large breath Moments of Inertias, making the timber very ductile in that direction of loading. However, if the testing was performed with the material sitting with vertical veneers, the sample wouldn’t have been so ductile and results could have been recorded. Overall, looking at the results calculated and recorded, hardwood was the best performing sample. Strong in tension and under a moment force load, out of the samples tested, hardwood was the ideal material under these types of loads.
Errors When performing the testing of the materials, a few errors could have occurred which affected the data recorded. The first error that would affect the data was the recording of the original position. The only way that the datum could’ve been recorded on the rig was to position the loading bar to where it just touched the sample. This was performed by eye and would give a slight error in the data’s original position. Another error that occurred while testing the material was a continuous load. Due to the limited range of weights available to load the rig with, the load was sometimes removed to add a larger weight. This lead to the sample being testing through varying loads, affecting the yield point for the material. The moisture content of each sample would also affect the strength and ductility of the samples. With more moisture in the timber, the sample would be more ductile than it would be naturally. This then affects the results for density and Moment of Rupture.
Conclusion After testing the materials, recording data, analysing the results and discussing possible errors in the data, the test of flexural properties in timbers resulted in data useful for use in real world situations. The limits in strength and deflection of the four materials tested were calculated so that these could be considered in design.
10
References Woodwork Basics. (n.d.). Plywood. Retrieved 4 11, 13 from WOODWORKBASICS.COM: http://www.woodworkbasics.com/ Woodwork Basics. (13, 4 22). Softwood. From WOODWORK BASICS: http://www.woodworkbasics.com/softwood.html About.com. (13, 4 22). What is the difference between a hardwood and softwood? From About.com Forestry : http://forestry.about.com/cs/treeid/f/Tree_ID_wood.htm Allen, E., & J.Iano. (2008). Fundamentals of Building Construction, Materials & Methods (Fifth Edition ed.). New Jersey: John Wiley & Sons. Diffen. (13, 4 22). Hardwood vs Softwood. From Diffen: http://www.diffen.com/difference/Hardwood_vs_Softwood How Stuff Works. (13, 4 22). What is the difference between a hardwood and a softwood? From How Stuff Works: http://science.howstuffworks.com/life/genetic/question598.htm T.O'Hara, & D.Grant. (1985). Sloping Grain and the Strength of Structural Timber. Sydney: NSW Department of Primary Industries.
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Appendices Appendix 1: Load Versus Deflection Graph
Load Versus Deflection Softwood
Hardwood
Chipboard
16
14
Deflection (mm)
12
10
8
6
4
2
0 0
1.4
1.9
2.4
2.9 3.4 Load (kg)
3.9
4.4
Appendix 2: Calculations Density Sample Softwood
Equation
Results 5.998165739 gm/cm3
Hardwood
7.824812917 gm/cm3
Chipboard
7.073370225 gm/cm3
Plywood
4.77582846 gm/cm3
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4.9
5.4
Moment of Inertia Sample Softwood
Equation
Results 1547.888533 mm4
Hardwood
516.7829333 mm4
Chipboard
3241.2608 mm4
Plywood
1269.675 mm4
Modulus of Elasticity Rearrange To (equivalent to y=mx) Therefore: Rearranged to: Sample Softwood
Equation
Result 3285.320 MPa
Hardwood
4955.7944 MPa
Chipboard
615.2947214 MPa
Modulus of Rupture Sample M Equation Softwood
Equation
Result 113.101274 MPa
Hardwood
159.4742444 MPa
Chipboard
13.91875382 MPa
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