Fracture Mechanics & Stress Analysis of Cracks

(click on thumbnail to enlarge)

The size of the dark, preexisting crack was used to estimate stresses at failure

The size of the dark, preexisting crack was used to estimate stresses at failure

Crack length vs # of flights curve was developed from fractographic stress estimates and crack growth data

Crack length vs # of flights curve was developed from fractographic stress estimates and crack growth data

Brittle fracture in ammonia condensate line

Brittle fracture in ammonia condensate line

Fracture mechanics is a relatively new field of stress analysis which permits a quantitative analysis of cracks and defects. Linear elastic fracture mechanics (LEFM) 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 size that is dangerous for operation can be answered for many materials. Linear elastic fracture mechanics works best for high strength, brittle alloys, ceramics, and some non-metals (different techniques are used for more ductile materials).

Linear elastic fracture mechanics is used heavily in the analysis and prediction of fatigue crack growth in aircraft and pressure vessels. If an initial flaw or defect size and load spectra are known, fatigue life for a growing crack to a critical size can be estimated. Conversely, actual operating stress ranges can often be estimated by fatigue fracture striation surface measurements.

Linear elastic fracture mechanics analysis of fracture works best for high strength alloys. Tearing instability can be used to evaluate seemingly brittle failure of lower strength, high toughness structures where stored elastic energy becomes important. Low strength, high toughness steel failures can also be evaluated using ideal plastic analysis. These two additional approaches extend the use of fracture mechanics to the more common structural steels of high toughness.

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.

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.

Fracture Mechanics & Stress Analysis Capabilities

Back to top