Okay so I'm confused. In my OCR AS textbook the diagrams for destructive interference and for a stationary wave are the same. Yet at the antinodes on the stationary wave there is constructive interference because the waves are 'in phase' and there is maximum displacement. Why on the destructive interference diagram are the same points destructive and the resultant displacement 0 not maximum? I don't know if any of this makes sense but I'm confused and hopefully someone understands.
The terms stationary wave and interference are not alternative phrases for the same thing. They are two different aspects of waves.
The top diagram is showing two waves that occupy the same point being added together. This is about interference.
The bottom diagram is showing the same stationary wave at two different points in time. The wave will have a time period 'T' and the dotted line is the wave half T before or after the solid line. This is the wave you have if you tie a washing line at the ends and vibrate it at its resonant frequency.
In second diagram, the waves are out of phase (theta=90) with same amplitude and frequency/wavelength. Such waves can only interfere destructively which leads to the output as zero displacement. Hope it help....
Although the users before have given good explanations, I would add another one of myself:
In terms of inferference, it depends on the phases, so the interaction between wave peaks (= amplitudes with a maximum) and wave troughs (amplitudes with a minimum) . If a wave peak faces a wave trough, it is destructive. In the contrary case (wave peak faces a wave peak and thus a wave trough a wave trough), it is constructive.
A stationary wave is just an one which comes into being, if an emitted wave faces a reflected one. They are going in two different directions and when they interact with each other. A stationary wave, which is oscillating via the nodes, is the result of it. narak99 named a good example. When a washing line begins to oscillate, it comes to an emitted wave first, when the wave hits a clothes horse, reflection begins what leads to an interaction of reflected and emitted wave. This is an appearance of two different directions (and two waves with the same frequency of course).
Waves of destructive interference have the same direction instead. Just thinking about the wave trough which is creating waves by double slits. As they are in the same direction, destructive interference would never lead to a stationary wave.