Meaning
A mechanical engineering ratio measures the reduction in the fatigue strength of a material caused by the presence of a geometric stress concentration like a notch or hole. This value, known as fatigue notch factor, compares the fatigue limit of a smooth specimen to that of a notched specimen of the same material. Structural designers use this metric to evaluate the durability of machined battery brackets, casing joints, and terminal tabs under dynamic vibration.
The factor holds only for the specific notch geometry and material combination tested.
Geometric Impact
The presence of sharp corners, threads, or holes concentrates local stresses, making the component more susceptible to crack initiation. However, the fatigue notch factor is usually lower than the theoretical stress concentration factor because of localized plastic deformation at the notch tip. This plastic deformation redistributes the stress, which partially mitigates the local severity of the notch.
Materials with high ductility exhibit greater notch sensitivity, meaning their fatigue strength drops more severely when a notch is introduced.
Material Variation
High-strength alloys tend to be more sensitive to surface notches than lower-strength, more ductile metals. For example, high-strength aluminum alloys used in lightweight battery frames require careful radiusing of all internal corners to prevent premature fatigue failure. In contrast, cast irons and some composites are less affected by manufactured notches because they already contain internal microstructural defects.
This difference means that material selection must balance the desire for high static strength with the risk of notch-induced fatigue.
Design Rule
Engineers apply the fatigue notch factor when performing finite element analysis and life prediction calculations for structural components. By applying this factor, they can adjust the allowable stress limits for areas of the battery pack that contain bolt holes, weld joints, or sharp bends. This adjustment ensures that the components do not develop fatigue cracks during the vibration profiles specified by automotive safety standards.
Proper application of this design metric prevents field failures and extends the service life of the battery system.
