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 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 Methodology Prong test The procedure for a prong test is: Cut defect-free cross section about 30mm wide at least 400mm from the end of the board Number the pieces for identification Using a bandsaw, cut about 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 Take care pulling the cross section back through the saw cut as a stressed piece may pinch and pull the bandsaw blade Photograph and measure the initial movement in each sample The central two prongs can be broken out to show cases of severe stress 5 Assess the level of stress using Figure 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 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 and include your own interpretation Comment on the possible causes of what you see Example No Immediate result Next day result 1A Immediate result Next day result Bob Comment Unequal MC% , surface drier than core, possibly caused in early stages of drying Worksheet No Comment KDA 503 Timber, its origins and characteristics – Block Worksheet Figure 1: Results from prong test KDA 503 Timber, its origins and characteristics – Block Worksheet ... early stages of drying Worksheet No Comment KDA 5 03 Timber, its origins and characteristics – Block Worksheet Figure 1: Results from prong test KDA 5 03 Timber, its origins and characteristics – Block...5 Assess the level of stress using Figure For stress free drying the prongs should ideally remain straight or curve out slightly If the prongs pinch in, then stress is present Excessive