V. COMPRESSION OF WOOD. GENERAL: The results of mechanical tests performed on wood specimens frequently show large scatter because wood is neither homogeneous nor isotropic. It is extremely difficult to perform tension tests on wood due to physical problems of gripping the specimen in a testing device. For this reason, mechanical tests on wood are usually limited to compression and bending tests.

plantcalicobeansΠολεοδομικά Έργα

29 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

94 εμφανίσεις



CE 305L



Civil Engineering Materials Lab



305
-
Wood Compression.doc


1

V. COMPRESSION OF WOOD.


GENERAL: The results of mechanical tests performed on wood specimens frequently show
large scatter because wood is neither homogeneous nor isotropic. It is extremely difficult to
perform tension tests on wood due to physical pro
blems of gripping the specimen in a testing
device. For this reason, mechanical tests on wood are usually limited to compression and
bending tests.


Compression tests of wood are generally performed with the load applied both parallel to,
and perpendicula
r to the grain. The parallel
-
load tests may be used to determine a modulus of
elasticity, a highly variable property for construction lumber, with an upper range of
approximately 1.8

x

10
6

psi, which is about six percent that of ferrous metals. Wood memb
ers
loaded in the direction perpendicular to the grain include joists bearing on beam and other
similar situations), hence, this test is performed mainly for the purpose of establishing minimum
standards and specifications for this type of loading. The te
st, also, demonstrates the very small
tolerance to loading in this direction compared to that parallel to the grain.




The modulus of elasticity determination from the parallel compression tests is done by a
successive accumulation of data of simultaneous

load and deformation in the same manner it is
done for the tensile test of steel. Other mechanical properties such as the yield stress and the
ultimate stress are selected from the stress
-
strain curve, although this may not provide a clearly
defined yiel
d point.




One of the most important mechanical properties to be determined is the ultimate stress
of wood in compression, because working stresses defined by building codes and other standards
usually employ a factor of safety based on this ultimate stre
ss. This results in working stresses
that are fractional values of the ultimate stress, and not the yield stress. The yield stress,
therefore, is not a highly important property.


OBJECTIVES:
In these tests, the behavior of wood under axial compression
parallel to and
perpendicular to the grain will be evaluated with the determination of certain mechanical
properties of wood.


EQUIPMENT:

1. Universal Testing Machine (UTM)

2.

A mechanical compressometer (parallel test)

3.

1
-
inch dial gage
(perpendicular test)

4.

Mechanical calipers

5.

Tape measure


SPECIMEN:

1. Nominal 2” x 2” x 8” clear wood specimen for compression parallel to
the grain.


2. Nominal 2” x 2” x 8” clear wood specimen for compression
perpendicular to the grain.

PROCEDURE:


1.

Measu
re and record the actual

dimensions of the specimens.



CE 305L



Civil Engineering Materials Lab



305
-
Wood Compression.doc


2


2.

Attach the compressometer to the specimen for parallel testing. Place the specimen in
the testing machine with its long dimension parallel to the direction of the load. Be
careful to position the spe
cimen in such a way that the load will be a truly centric
load.


3.

Select a load range on the testing machine that will supply a minimum of 20 kips.
Apply the load
slowly

until failure occurs, obtaining simultaneous readings of load
and axial deformation fo
r every 0.001

inch of specimen deformation. Describe the
type of failure (see Figure 1).


4.

Place the perpendicular
-
to
-
the
-
grain specimen in the testing machine with its long
dimension perpendicular to the direction of the load. See Figure 2 for the orient
ation
of the cross grain with respect to the direction of the load.


5.

Place a two
-
inch wide steel plate on top of the specimen to concentrate the applied
load to the center two inches of the specimen.


6.

Apply the load
slowly

until a total deformation of 0.10

in. is recorded on the dial
gage. Record load and deformation every 0.001

inch to 0.040

inch and every
0.010

inch thereafter till at total deformation of 0.10

inch is obtained.


REQUIRED:



1a.

Plot the stress
-
strain curve for the parallel test.



1b.

P
lot the load
-
deformation curve for the perpendicular test.


2.

Calculate the following for the parallel test:

a.

Proportional limit stress,

b.

Modulus of elasticity,

c.

Ultimate compressive stress.


3.

Calculate the following for the perpendicular test:

a.

Proportional limi
t stress,

b.

Compressive stress at 0.04

inch deformation,

c.

Ultimate compressive stress (at 0.1

inch deformation).


4.

Compare the experimentally observed values above with established
values for clear wood. Calculate percent differences.


REFERENCES:



1.

ASTM D143
,
Standard Methods of Testing Small Clear Specimens of Timber
, Vol

4.10.


2.

ASTM D2555,
Standard Test Methods for Establishing Clear Wood Strength Values
,
Vol

4.10.



CE 305L



Civil Engineering Materials Lab



305
-
Wood Compression.doc


3


Figure 1. Types of Failure in Compression Parallel to the Grain.




CE 305L



Civil Engineering Materials Lab



305
-
Wood Compression.doc


4




Figure 2. Compr
ession Perpendicular to the Grain (note orientation of grain parallel to load).