Timber Sample Testing And Failure Classification Biology Essay

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Timber materials are consumed by a large range of industries, a widely used material in construction either structurally such as roof trusses, gear wheels, boats ,furniture, and non structurally such as signal poles ,doors and window frames etc.It is simply define as a low density, cellular, polymeric composite material. Timber is a product of nature that is cut and machined from trees .It cannot be manufactured to a particular specification rather the best use has to be made of the materials already produced though it is possible to select from wide range available with the most desirable range of properties (Dinwoodie j. M). This property comprises of physical and mechanical properties such as strength and moisture content, etc. Timber used for structural purposes needs to have reliable strength properties so that it can resist the range of forces, or 'stresses', that will act on it in the standing structure. Three point bending test of small clears (small and knot free) and test of 4 by 2 Structural batten(big with knots) will be used to determine the strength and failure rate of six selected samples of wood for each method. The research project is carried out using the information in BS373:1957 and BS EN 408:2003 for the test of timbers.

2 THREE POINT BENDING TEST OF SMALL CLEARS

This was originally used for the derivation of working stresses for timber but was later superseded by structural-size timber in mid-1970s.it is still valid for characterizing new timbers and for strict academic comparism of wood from different species because of its small knot-free straight grained perfect test pieces which represent the maximum quality of wood that can be obtained.

A Task 1: Process Results & Classify Failure Mode

From the information contained in the Excel spread sheet tables (downloadable from Web CT) determine in conjunction Appendix A and where appropriate information from BS373:1957:

1. The Modulus of Elasticity (MOE)

2. Contained in Figure 2.1 are a selection of photographs taken of the test specimens after testing classify their failure modes (Figure 2.2) and make any relevant comments considering the load displacement curves.

3. Comment on your results and where appropriate make reference to appropriate literature resources.

SOLUTION

The Modulus of Elasticity (MOE)

Using

Where:

Em, app = Apparent modulus of elasticity

Fmax = maximum force

F1, F2 = load values in excel sheet

W1, w2 = corresponding displacement values of f1, f2 in excel sheet

l = 21952000000 mm

I = 13333.33

b = 20 mm

d = 20 mm

SEE TABLE FOR RESULTS BELOW

S/N

Fmax(N)

F1(N)

F2(N)

W1(MM)

W2(MM)

F2-F1

W2-W1

Em,app

1

1.31E+03

1.31E+02

5.22E+02

6.45E+01

2.20E+00

3.92E+02

1.55E+00

3.71E+15

2

1.56E+03

1.56E+02

6.25E+02

6.15E+01

2.18E+00

4.69E+02

1.57E+00

1.02E+07

3

1.59E+03

1.59E+02

6.36E+02

6.52E+01

2.20E+00

4.77E+02

1.55E+00

1.06E+07

4

1.56E+03

1.56E+02

6.24E+02

5.79E+01

2.11E+00

4.68E+02

1.53E+00

1.05E+07

5

1.03E+03

1.03E+02

4.11E+02

6.04E+01

2.07E+00

3.08E+02

1.47E+00

7.20E+06

6

1.40E+03

1.40E+02

5.59E+02

7.38E+01

2.66E+00

4.19E+02

1.92E+00

7.48E+06

2(a) Modes Of failure for small clear in the samples pictures (see appendix)

Horizontal shear: This occurs most when there are abrupt changes in the growth zones.

Splintering tension: It is as a result of low moisture content

Splintering tension

Brash tension: This normally occur when there is compression or decay. It shows the presence of abnormal molecular structure.

Cross grain tension: This is as a result of grain orientation in wood and this result in tension failure.

Simple tension and horizontal shear

2(b) Comments considering the load displacement curve

SMALL CLEAR 1 GRAPH

SMALL CLEAR 2 GRAPHS

SMALL CLEAR 3 GRAPHS

SMALL CLEAR 4 GRAPHS

SMALL CLEAR 5 GRAPHS

SMALL CLEAR 6 GRAPHS

Notice that at the beginning of the test, the curve shows a slight reverse curvature .This is as a result of local crushing of unintentional high sports as the ends of the specimen becomes seated on the platen of the testing machine.

To correct for this, the straight line portion of the curve may be extrapolated downwards to locate an adjusted zero mark MTP Construction).

From the curve, samples has different ultimate strength and displacement at same load rate which may be as a result of internal constituents of the samples.

The materials exhibits different types of failure.

(3) Comment on your results

Each sample mean value is subject to some degree of error, and small differences between the mean values of species up to about 10% are usually within the possible range of error of the values and may have no practical significance. When making these comparism that under certain conditions where strength is one of the principal requirements, there may be other factors which will also influence the choice of a timber for a particular purpose. Factors such as ability to resist fungal attack, stability with changes in humidity and temperature have to be considered. (MTP construction, The strength properties of timber, 1974)

3 TEST OF 4 BY 2 STRUCUTRAL BATTENS

B Task 2: Process Results & Classify Failure Mode

From the information contained in the Excel spread sheet tables (downloadable from Web CT) determine in conjunction Appendix A and where appropriate information from BS373:1957:

1. The Local and Global Modulus of Elasticity (MoE)

2. The Modulus of Rupture (MoR)

3. Contained in Figure 3.2 are a selection of photographs taken of the test specimens after testing classify their failure modes (Figure 3.3) and make comments relative to the determined MoE, MoR and load displacement curves.

4. Comment on your results and where appropriate make reference to appropriate literature

resources.

SOLUTION

(1a)The local and Global Modulus of Elasticity (MOE)

Using the Equation for Local MOE

Where:

F2-F1 is an increment of load in the linear region of the load-deflection curve (N),

wc2-wc1 is an increment of deflection corresponding to F2-F1 (mm),

I is the second moment of area of the batten (I=bd3/12),

a and c are the span distances shown in Figure 3.1

l1 is the clear span (mm) =5h

s = 1800

b = 50

d = 100

L = 600

c = 500

h = 100

t = 0.003hm/s

a = 600

I = 4166666.67

(1b)Global Modulus of elasticity

Where:

s is the span distance between supports,

ws2-ws1 is an increment of deflection corresponding to F2-F1 (mm).

Bending strength (MOR) is calculated using the following equation:

(2)MOR

Using the Equation

Where:

Fmax is the maximum force applied to the batten.

SEE TABLE FOR RESULTS BELOW

S/N

Fmax

F1(N)

F2(N)

W1(MM)

W2(MM)

F2-F1

W2-W1

Bat 1

1.73E+04

1.73E+03

6.92E+03

3.32E+00

1.32E+01

5.19E+03

9.92E+00

Bat 2

1.52E+04

1.52E+03

6.09E+03

3.21E+00

1.30E+01

4.57E+03

9.77E+00

Bat 3

1.48E+04

1.48E+03

5.93E+03

2.74E+00

1.09E+01

4.45E+03

8.14E+00

Bat 4

1.05E+04

1.05E+03

4.21E+03

2.38E+00

9.74E+00

3.16E+03

7.36E+00

Bat 5

1.81E+04

1.81E+03

7.23E+03

3.85E+00

1.47E+01

5.42E+03

1.09E+01

Bat 6

1.48E+04

1.48E+03

5.92E+03

2.62E+00

1.04E+01

4.44E+03

7.76E+00

S/N

MOEL

MOEG

MOR

Batten 1

2.35E+03

1.24E+04

1.25E+02

Batten 2

2.11E+03

1.11E+04

1.10E+02

Batten 3

2.46E+03

1.30E+04

1.07E+02

Batten 4

1.93E+03

1.02E+04

7.59E+01

Batten 5

2.25E+03

1.19E+04

1.30E+02

Batten 6

2.57E+03

1.36E+04

1.06E+02

(3a)Modes of failure for Batten in the sample pictures (see appendix)

a) Compression near a knot

b) Diagonal tension

c) Diagonal tension

d) Compression near a knot

e) Localised cross-grain tension

f) Compression near a knot

This failure modes in batten is always as a result of defects in the wood like knots

(3b)BATTEN 1 GRAPH

BATTEN 2 GRAPH

BATTEN 3 GRAPH

BATTEN 4 GRAPH

BATTEN 5 GRAPH

BATTEN 6 GRAPH

Modulus of elasticity and modulus of rupture in the displacement curve are almost the same.

Modulus of elasticity: Initial straight line part of the load - displacement curve. MOE can be found from the bending test, but its value will differ slightly from that found from the compression test. The reason is that the measured beam deflection are affected by shear strains as well as by flexural strains . The linear relationship between stress and strain within the elastic range of a material, providing an indication of stiffness.

Modulus of Rupture: The highest stresses in the outermost fibres of the wood when the beam breaks under the influence of a load.

The modulus of rupture is the value of mc/I which would be compute using the bending moment caused by the ultimate load. Modulus of rupture is not the value of extreme-fibre bending stress at failure. Bending stresses under elastic conditions vary as a straight line.

(4) Comment on results

TheMean value derived from structural size data is low .

4 CONCLUSIONS

The highest strength of clear strength grained timber is in tension along the grain, whereas the lowest is in tension perpendicular to the grain.Tensile strength of structural size is lower than the compression strength while in small clear tensile strength is higher than the compression strength, this is as a result of knots and associated distorted grains in structural size. Normal distribution curve fits the data from small clear test piece better than data from structural size.

Stiffness (Modulus of Elasticity) The values obtained in any determination of the properties of timber depend upon the test methods used. It is

therefore desirable that these methods be standardized so that results from different test centres can be correlated .

Moreover, with the adoption of limit state design and with the development of both visual and machine stress grading, attention will be increasingly centred on the determination and monitoring of the strength properties and variability of timber in structural sizes. Again, this can be more effectively undertaken if the basic data are defined and obtained under the same conditions.

Timber type and grain formation etc, are very important factor in both test method because the have effects to the general outcome of the test result.

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