Fatigue testing is used to determine how many load cycles a material can sustain or the failure load level for a given number of cycles. The results of fatigue testing vary dramatically depending on the material. For example, most steels and aluminum alloys behave very differently under fatigue. Steel typically has a fatigue threshold, which means that if it is tested at loads lower than the threshold, it will never break. Most aluminum alloys do not have a fatigue limit, so it is more difficult to judge when they will break.  Even at a small load, most aluminum alloys will fail after a sufficient number of cycles. Plastics (polymers) are very sensitive to strain rate, or the speed of testing. Testing plastics at a higher rate will lead to different results than testing them at a low speed. Similarly, plastics are temperature sensitive, meaning that they behave very differently at high temperatures than at low temperatures. This testing is very common in the automotive and aerospace industries. This type of mechanical testing is performed using very simple sinusoidal load cycles, or may include very complex reproductions of actual service life load profiles. This testing gives much better data to predict the in-service life of materials.