Orthotropy of tensile elasticity

Experimental material consisted of 3.2 mm (1/8 in) thick structural veneer sheets made of the three Appalachian hardwood species. The sheets were peeled and dried in a commercial structural composite plant.

The equipment used for dynamic MOE determination was an ultrasonic device, developed by Hungarian researchers (Divos, 2000). The device consists of an ultrasonic timer and two piezoelectric accelerometers. The transducers use a 127 V, 45 kHz impulse

0 0 6 7

ULTRASONIC TIMER

t = µs ^{PROBES SCOPE}

POWER

Distance

Accelerometer Rubber pad

Rubber sheet Accelerometer

Rubber pad Pressure

application bar

Specimen

b.

a.

Figure 5.6 – Schematic of the ultrasonic testing equipment

Time (

µµµµ

10 20 30 40 50 60 70 80 90 100

Response signal (V)

-3 -2 -1 0 1 2 3

Response signal Threshold Measured Propagation Time ( The excitation signal is sent at 0 s )

that lasts for 30 µs. The impulses follow at 1s intervals. 3-4 MPa surface pressure between the transducers and the veneer sheets provides adequate contact, using sandpaper as coupling material.

Figure 5.6 shows the schematic of the ultrasonic equipment and a picture of the experimental setup. The distance between the transducer and the receiver (i.e.

measurement span) was 160 mm. The material was clamped on a special table that was covered with a rubber sheet to avoid bridging of the signal between the transducer and the receiver. A clamp provided adequate surface pressure between the transducers and the material.

Figure 5.7 – Operation principle of the ultrasonic timer

Figure 5.7 demonstrates the operation principle of the ultrasonic timer. Timing starts when the excitation impulse rises, and stops when the received signal reaches a threshold value, above the noise level. The advantage of this method is that there is minimal delay between the reception of the signal and the stoppage of the timer. The measured time must be corrected to account for travelling time in the transducer house.

Preliminary measurements were executed on veneer sheets, using various grain orientations. The exact location of the accelerometers was marked, and thin veneer strips were cut from the sheet, to include the marked measurement locations. Propagation times measured on veneer strips were no different from those obtained by measuring large sheets. This led to the conclusion that Equation 4.15 is sufficient for dynamic MOE determination.

Specimen manufacturing and testing included the following steps:

• Cutting structural veneer sheets to target dimensions of approx. 200 x 200 mm;

• Specimen conditioning (see section 5.1.2);

• Taking two width and length and four thickness measurements to 0.01 mm accuracy;

• Measuring weight to the nearest 0.01 g;

• Marking both sides of the sheets. Drawing lines at 0° to 90° angle to the grain, with 15° increment, to designate the measurement directions;

• Measuring the propagation time of ultrasound in the directions marked.

Data analysis involved calculating the density of the specimens, and the propagation time (average of the corresponding measurements on the two sides) in each

Figure 5.8 – Schematic of the static tension test and the experimental setup

a.

Specimen Extensiometer

Tension grips

b.

transducer distance with the propagation time. Finally, Equation 4.15 provided the ED

values. Since every specimen was tested in each of the seven measurement directions, this situation corresponds to a randomized complete block (RCB) design.

Relationship between the dynamic and static MOE was established by testing several veneer and solid wood specimens of each species (conditioned according to section 5.1.2), at four grain angle levels (0°,15°, 30° and 45°). The target dimensions were 300 mm in length and 25 mm in width. Solid wood specimens were 12 mm thick.

Dynamic MOE was measured on these specimens in a similar manner as described above. Static MOE was measured using the MTS servo-hydraulic testing machine, equipped with mechanical tension grips. Load and strain values were collected using a computerized data acquisition system, with a data collection frequency of 1s. Figure 5.8 shows the measurement setup of the static tension test. Testing speed and other testing parameters were in accordance with the standard ASTM 143 – 94.

5.2 Densification

The effect of the density increase on the MOE of veneer was tested using the stress-wave timer, and the same specimens that had been used to assess the orthotropic tensile MOE. Preparation of the specimens included densification of the veneer sheets in a small scale laboratory press for 15 minutes. The temperature of the heated platens was 120°C (250°F). Densification pressures varied so as to cause different levels of

Specimen testing procedure was similar to that described in section 5.1.5. Length and width measurements were omitted; these dimensions were assumed unchanged.

Stress-wave propagation directions corresponded to the previously marked lines, used for orthotropic tensile MOE determination. Because the original dynamic MOE values in these directions were already available, this made the calculation of a percent MOE increase possible, for every direction in each veneer sheet, individually.

A small-scale static densification study was also conducted, to verify the capacity of the dynamic test to estimate the effect of densification on the static MOE of veneers.

This study involved ten, 300 by 300 mm² yellow-poplar veneer sheets, that were cut, parallel to the grain, into 25 mm wide strips, and marked for the identification of the original sheet. Strands were randomly assigned into groups, containing one strand from each sheet. Each group went through densification, using the same pressing parameters as described above, and different densification pressures, to achieve different levels of compaction for each group. One group was retained as an undensified control.

After a conditioning period of several days (21°C, 65% RH), a static testing procedure, as described in section 5.1.5, established the tensile MOE of each veneer strip.

The level of densification was calculated from the original and densified thickness of each veneer strand. The MOE of the undensified control specimen originating from the same veneer sheet provided a basis for computing the MOE increase of each densified specimen.

5.3 Mechanical properties of the composites