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Two important concepts of drying theory must be known to understand stress development;

• Timber dries from the surface first.
• Wood shrinks as it dries below 30% moisture content.

Moisture is lost from the outside surface of a board first, and as the surface layers dry below 30% shrinkage occurs. The surface shrinkage is restrained by the inner part of the board which has not yet started to shrink. The surface fibres are thus held apart (in tension) and this imposes compression stresses on the rest of the cross section. If the tension forces in the surface layer become too large, surface checking can develop.

The drying proceeds to the inside of the board, which then shrinks but the inside shrinkage is restrained by the already dry and set outer layers. The centre of the board now becomes tensioned and the surface layers become compressed. The board is now in tension at the centre and the surface in compression, this is the stress state present at the end of drying.

Why is case hardened timber a problem ?

Drying stresses are not visible but if a stressed piece of timber is deep ripped or heavily machined, the balance of stress within the piece is disturbed and some distortion such as cup or spring, will occur. Also the high stresses developed during HT drying can cause significant internal checking if they are not relieved after drying.

How is stress relieved ?

A high temperature, high humidity treatment period at the end of drying will relieve stress. If the kiln charge has been dried at CT then the final steam conditioning should be at 5 degrees centigrade above the dry bulb setting used to dry the timber, and the wet bulb should be set to the dry bulb temperature, i.e. if drying at 70/60 degrees centigrade then steam condition at 75/75 degrees centigrade. If the kiln charge has been dried at either ACT or HT then the conditioning should be as near to 100/100 degrees centigrade as can be reached. It is not normally possible to achieve conditions of 100/100 degrees centigrade (influenced by atmospheric pressure) but conditions of 98/97 degrees centigrade should be able to be achieved, and the difference between the dry and wet bulb must be less than 3 degrees centigrade to provide good steaming conditions.

In the high humidity conditions present during conditioning the board will pickup moisture at the surface that will cause a slight compression set to offset the tension set formed early in drying. Steam conditioning also reduces moisture gradients.

A kiln can be equipped with either a sprayline or a waterbath to provide the extra humidification required for stress relief. Often a high temperature kiln will be associated with a separate steaming chamber and this will include a water trough for steam generation.

How do you measure stress ?

There are two main methods for assessing stress levels;

Prong test

• Cut a defect free cross section 20-30mm long at least 200mm from the end of a board.

• Using a bandsaw place about 5 saw cuts (for 50mm thick timber) from one edge parallel to the face, stopping 10mm from an edge. Take care pulling the cross section back through the saw cut as a stressed piece may pinch and pull the bandsaw blade of the bandsaw wheel.

• The central two prongs can be broken out to show cases of severe stress.

• Assess the level of stress. 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 which causes the prongs to bend outwards and remain bent outward.

Cup test

• Cut a defect free cross section 20-30mm long at least 200mm from the end of a board.

• Deep rip the cross section into two (25mm rippings for 50mm thick timber)

• Observe the amount of cupping.