503 335 3 residual stress assessment

Residual stress Workshop session: Timber properties KDA 503-335: Timber, its origin and characteristics Student Aim This exercise aims to assess the stress that may remain in the se

3 Residual stress

Workshop session: Timber properties

KDA 503-335: Timber, its origin and characteristics

Student

Aim

This exercise aims to assess the stress that may remain in the section of dry timber test samples

To do this you will select test samples, cut a cross section, and cut these for the prong test, evaluate and record movement initially and after a set period, and then evaluate these movements to access stress

Background

During the initial stages of timber drying, the outer case of the timber often becomes much drier than the core This causes stresses to develop Initially, stress is generated by the outside case shrinking over the unshrinking core This prevents the case from shrinking normally and its goes into tension This compresses the core and stretches the timber in the case

As drying continues, the core dries and the process reverses The shrinkage in the core is now restrained by the set oversized outer case At the conclusion of drying, a stress reversal develops where the case is now in compression and the core is in tension

This results in pieces of timber that have a relatively even moisture content throughout the piece and are stable in the full section However, as residual drying stress exist across the section, movement occurs if it ripped or moulded in later processing This results in twisted, cupped or bowed products

A prong test indicates the amount of elastic stress in the wood at the time of cutting The prongs may change their shape after cutting if the moisture content changes as detailed in Figure 1

Methodology

Prong test

The procedure for a prong test is:

1 Cut 4 defect-free cross section about 30mm wide at least 400mm from the end of the board

2 Number the pieces for identification

3 Using a bandsaw, cut about 5 saw cuts (for 50mm thick timber) from one edge parallel

to the face, stopping 10mm from an edge for each sample See the sketch below

5 Assess the level of stress using Figure 1

For stress free drying the prongs should ideally remain straight or curve out slightly

If the prongs pinch in, then stress is present Excessive steaming may cause reverse casehardening This causes the prongs to bend outwards and remain bent outward

6 Leave the sample standing till the next day and examine it again Photograph, measure and assess movement in each sample

Results

Using the sheet below, list the immediate and next day results observed Use the reference numbers from Figure 1 and include your own interpretation Comment on the possible causes

of what you see

Example

No Immediate

result Next day result Comment

Bob 1 1 1A Unequal MC% , surface drier than core, possibly caused in

early stages of drying

Worksheet

No Immediate

result Next day result Comment

Figure 1: Results from prong test