1. Macroscopic fracture characteristics
Macroscopically, the fatigue fracture surface of a spline shaft can usually be clearly divided into three characteristic areas:
1. Fatigue source area
This is the initiation point of fatigue cracks. For spline shafts, due to the sudden change in geometry at the tooth root, severe stress concentration will occur, so the fatigue source is usually located on the tension side surface of the key tooth root. On the macroscopic fracture surface, this area usually appears as a relatively smooth and flat "bright spot" or "bright area".
2. Fatigue expansion zone
This is the area where cracks expand stably under the action of alternating stress, and is also the core macroscopic feature of fatigue fractures. This area will show alternating light and dark shell-like or beach-like stripes, also known as "fatigue arcs" or "shell lines". The centers of these arcs point to the fatigue source, and their spacing can reflect the stress amplitude to a certain extent.
3. Instantaneous interruption zone
When fatigue cracks expand to a critical size and the remaining effective section cannot bear the load, rapid instantaneous fracture occurs. The morphology of this area is similar to overload fracture under static load, and the surface is rough. For ductile materials, the fracture surface is often dark gray and fibrous; for brittle materials, it is a crystalline flat fracture surface. The area of the instant rupture zone is related to the load level before fracture. The greater the load, the larger the area of the instant rupture zone.
2. Microscopic fracture characteristics
Observed under a scanning electron microscope (SEM), the micromorphology of the fatigue fracture can provide more precise failure information:
Fatigue banding (fatigue striations): This is the most critical microscopic criterion for fatigue fracture. In the fatigue growth zone, a series of light and dark strip patterns that are parallel to each other and perpendicular to the crack propagation direction can be observed. Each fatigue band usually corresponds to one stress cycle.
Secondary cracks: Between or near fatigue bands, small secondary cracks parallel to the direction of main crack propagation are sometimes observed.
Microscopic morphology of the instantaneous fracture zone: In the final instantaneous fracture zone, features such as dimples (ductile materials) or cleavage steps, river patterns (brittle materials), etc. can usually be seen microscopically.
3. Special instructions for spline shafts
Since the spline shaft mainly bears torsional load when transmitting torque, its fatigue fracture may also show some special morphology related to torsion:
Spiral expansion: Under torsional load, fatigue cracks may expand along the maximum normal stress plane at a certain angle (such as 45°) to the axis, making the fatigue arc on the fracture surface appear spiral.
Ratchet or sawtooth shape: If the spline shaft has multiple fatigue sources initiating and expanding at the same time at the tooth root where stress is concentrated, these cracks may form steps similar to a ratchet or starburst shape on the fracture surface during connection.
In summary, by analyzing the fatigue source location, shell-like arc, instantaneous fracture zone morphology and microscopic fatigue strips on the fracture surface of the ZSY reducer spline shaft, we can accurately determine that the failure mode is fatigue fracture, and further trace the origin and expansion history of the crack.
