Cutoff Wheel Failure

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Engineering design requires creativity and an astute attention to detail. Each choice an engineer makes affects functionality, cost, and longevity of the part. Customers expect quality and therefore demand products to function well and last as long as possible. To ensure the strength and longevity of design, engineers consider a plethora of failure modes and causes. One part can fail from any number of stresses or manufacturing defects. Specifically, AKE has examined two cases in which circular saw blades failed causing personal injury. In each case, otherwise very similar blades failed due to completely different sources of stress. The first failed while spinning at idle, due to a manufacturing defect. The second failure occurred as the blade bent under applied side force.

As demonstrated in the following figure, each failure produced distinctly different patterns.

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Figure 1: Overspun Blade with Defect (left) Bent Blade (right)

Interestingly, the blade on the left split into three very similar pieces, injuring the operator. While one might expect the failure to occur in a more or less random pattern, the failure produced seemingly predictable fragments. This relates to a known phenomenon called a “tri-hub-burst”. It relates to the propensity of circular objects to fail at high rotational speed, producing three, almost identical pieces. Fractures occur along the hub and radially at approximately 120  from each other.

Fracture mechanics naturally follow the most efficient failure mode to alleviate a buildup of energy within a stressed part. To clarify, objects in nature will fail in such a way that they relieve the most possible stress. Consequently, each fractured piece of the disk will maintain a portion of the energy the cutting wheel had formed from spinning. This kinetic energy is proportional to the mass of the fragments and exponentially proportional to the velocity of each. Both the mass and velocity of the fragments are based on the number of fragments created. As the number of pieces increases, the mass decreases. However the center of mass of each fragment moves further from the center of the disk. This increase in radial distance to the centroid results in higher velocity. The following plot displays the kinetic energy of each fragment as a function of the angle between the radial fracture lines.

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Figure 2: Plot of Kinetic Energy per Blade Mass and Radius with the Angle between Radial Fracture Lines

As the plot demonstrates, the maximum energy occurs at approximately 130 degrees between fractures. The closest value to go evenly into 360 is 120.

Therefore when the blade splits into 3 pieces it releases the maximum possible energy. The deviation from the theorized radial splits in the subject blade is due to a manufacturing defect. This caused the blade to begin fracturing at approximately a 55  angle from the radial direction.

Conversely, the blade on the right in Figure 1 does not display even, radial failures. Instead, it fractured into three inequivalent segments, injuring a bystander. This failure mode relates to the uneven loading the blade underwent. In this case, the blade cut into a pipe which was then angled to produce a bending moment on the blade. Additionally, this blade was intended for a slower saw than the subject saw. This combination of bending and overspeed of the blade caused it to fail. Because the blade underwent stresses from bending and not overspeed alone, the failure pattern was significantly different from any blade experiencing just overspeed. This demonstrates the potential for products to fail from any number of stresses induced and the importance of utilizing equipment properly.

The two cases described represent only two modes and causes of failure for these cutting wheels. In reality engineers must account for many more possibilities on any design. Stresses can be induced from vibration, cyclic loading, bending, tension, compression, torsion, and other sources. Additionally engineers must account for the materials they choose and physical characteristics such as geometry and the overall purpose of their design. Each of these factors allows for a certain freedom of choice and creativity to engineers however they must balance function, form, and cost all while avoiding product failure.