Why no thermal expansion for camping tents? I know that solid materials expand when heated, which is called thermal expansion.
But what is happening to tent square or tent canvas of (plastic?) camping tents? I have noticed, that during the day, when it’s hot, there is a lot of tension in the skin of a tent. But when it gets colder (when raining or during the night), the tension is less and the fabric gets sloppy.
What is the reason?
 A: The thermal expansion coefficient for the frame poles must be larger than the thermal expansion coefficient for the fabric.
This is a factor to keep in mind whenever you're considering thermal expansion for two materials at once. For instance you could have a container full of fluid which can be filled to the brim at one temperature, then spill when heated (or cooled). If not for the happy coincidence that concrete and steel have very similar coefficients of expansion, rebar-reinforced concrete would be unstable and the last half-century of architecture and bridge and highway construction would be very different!
A: Without doing the experiment I think it would be hard to pin down the precise reason. However my guess is that this is because most fabrics become more elastic, i.e. less stiff, in the presence of water.
In the heat of the day when you pitch the tent the fabric is dry and relatively stiff, but if it rains or dew forms overnight the fabric becomes less stiff. This means it stretches more under the tension you created when pitching the tent, so the tent fabric expands and sags.
If you're using a canvas tent this will be particularly marked because canvas is mostly cellulose and much of its strength comes from internal hydrogen bonds. In the presence of water these bonds are weakened so the stiffness of the fabric is reduced. This is basically the same chemistry that causes paper to become weak when wet. With a synthetic material like nylon the effect will be reduced because hydrogen bonds are less important to the fabric strength. However a quick Google suggests that even for nylon there is a significant loss of strength in the presence of water.
A quick experiment would be to pitch the tent tightly in the heat day then spray it with water heated to be the same as the ambient temperature - i.e. no temperature change. If the sagging is due to the effect of water you should see the tent sag even though the temperature hasn't changed.
