Oh my goddness, it's a snowscooter.
Analysis
First off let's start with some definitions. Material strength is usually described as its Ultimate Tensile Strength or UTS. This is the maximum stress a material can withstand before failure. Sometimes Yield Strength is given for a material. This is the stress at which a material deviates from the proportional zone of the stress-strain curve(see fig.1). In other words, this is where plastic deformation begins. The Modulus of Elasticity (also referred to as Young's Modulus when in tension or compression) is a measure of the rigidity of the material. It is the ratio of stress to strain within the proportional area of the stress stain curve (see fig 1). For a complete explanation of all these terms refer to a reference text such as the ASM handbook or a Mechanics of Materials text.
This analysis was used to calculate for a impact such as a crash at 35 mph with a .5 sec impulse.
This analysis was used to calculate the impact from a 7.5ft vertical drop onto firm snow.
Here is the FEA (Finite Element Analysis) model I created to show deflection for each tube. All models are of actual tube dimensions as given by the manufacturer. All tube models are fixed at one end, and have a 100 Lbf load applied downward to the other end.
Fatigue is a concept that is hard for many folks to grasp, but it's a very common failure mode in this type of frame. Fatigue occurs when a material fails under a cyclical stress which is below the yield stress of the material. What is happening here is the material is repeatedly subject to an average stress which is lower than the material's yield stress. However there are localized regions of the body which are seeing localized stresses which exceed the yield stress. This can be due to surface irregularities, part geometry, or microscopic material defects, or even residual stress in the part left from manufacturing. At these regions of localized high stress, a micro crack develops because the material has exceeded its yield strength locally. This micro-crack now forms a stress riser in the material, and on each repeated stress cycle, the crack grows. This will continue until the material fails and a viable crack forms.