SEM

RESUME

C. Kendall Clarke
Ph.D., P.E.

President and Principal Engineer

Don Halimunanda
M.S., P.E.

Mechanical Engineer

FRACTURE MECHANICS

Crack-like indication in cast iron dryer drum in paper machine.

Fracture mechanics is a relatively new field that enabled the safe use of high strength alloys in the Apollo program, jet aircraft and other sophisticated applications. This is because high strength alloys are susceptible to serious strength reduction in the presence of small cracks and flaws. Fracture mechanics provides the engineering basis to quantitatively predict the effects of these cracks. How much a given crack size will affect strength and how long it will take for a flaw to grow to a crack size that is dangerous for operation can be answered for many materials. This approach works best for high strength, brittle alloys, ceramics, and some non-metals. Different techniques can be used for more ductile materials.

Fracture mechanics is a powerful tool in root cause failure analysis. Notches, flaws and specimen geometry features that concentrate stresses are frequently cited as the sole cause of fatigue failures. Stress levels can be calculated for defects and compared with fatigue crack propagation life estimates. In some cases, measurements of fatigue striation spacing in conjunction with fatigue crack propagation data can yield estimates of operating stress levels. This type of analysis frequently leads to recognition of abnormal overloads, which cause the fatigue. The ability to estimate stresses at failure is also important in root cause failure analysis. This technique has been used often in expert witness cases.

Low stress axle fracture from point origin

Fracture mechanics can also be used in design. Both crack growth and cumulative damage techniques can be used to predict structural lives and set inspection intervals. Knowledge of the material fracture toughness can be used to estimate critical crack sizes for equipment. Equipment whose critical crack size for failure at operational stresses is below that which can be reliably detected by inspection can be dangerous.

The following examples highlight some of these techniques:

• Crack like indications in cast iron paper machine dryer heads have been evaluated for the size at which they could cause rupture. Acoustic emission was used to validate that the indications were not growing.

• Automotive axles with preexisting cracks have been discovered in full scale tests. Measurements of the preexisting crack sizes has provided data to interpret axle failures.

Striations such as these in a Ti alloy shaft can be measured for stress estimates.

• Striation measurements from fatigue failures have been successfully used to estimate operating stress levels. A full knowledge of the operating conditions and a well preserved fracture surface is required to obtain good stress estimates.

• Large high strength steel castings for an elevator assembly were reviewed for possible crack growth and catastrophic failure in service. The first part of the analysis required development of a fracture control plan for the castings. A plan utilizing Charpy impact test requirements based on the AASHTO approach for bridges was developed. Charpy energy values were specified at a given temperature in order to insure against brittle fracture at the lowest service temperature and maximum service loading rate. The second part of the project involved finite element stress analysis, cumulative damage fatigue life estimates and a finite element crack stress intensity analysis. The finite element analyses were contracted from outside sources. Evaluation of all data yielded a conclusion that the castings were reasonably certain to provide a successful service life for the projected conditions of service.