Fracture Mechanics for Crane Components
- CP&A Staff
- 6 days ago
- 5 min read
Highlights
Fracture Mechanics studies the behavior of pre-existing flaws under various loading conditions and estimates crack propagation rates or if a stress cycle will cause immediate failure.
Within fracture mechanics, one methodology to manage cracks and other defects is by using the damage tolerance method.
The results of fracture mechanics are largely dependent on the assumptions made, (e.g., the values of the constants in the crack growth equation, the size of initial flaws, and the shape of the initial crack). This information is not typically available at the design stage so these techniques are rarely used.
The most common fracture specifications, API 579 and BS 7910, are written to be fracture assessment tools to evaluate fitness for service.
In the case of some crane components such as container twist-locks, we have historical data and the cracks tend to form in a few select areas so fracture mechanics can be useful.
Fracture mechanics is a topic studied by several engineering disciplines including material science, aerospace, structural, and mechanical engineering. While fracture mechanics is related to fatigue, fracture mechanics is its own distinct field. Fatigue studies damage initiated by dynamic/cyclical loading while fracture mechanics focuses on the behavior of pre-existing flaws under various loading conditions and estimates crack propagation rates or if a stress cycle will cause immediate failure.
The study of crack propagation is called “fracture mechanics”. Fracture mechanics combines analytical methods with experimental research to quantify a crack’s grow potential. The consequences of component failure also plays a part in determining the criticality of a crack. A member or joint that has no alternate load paths and whose failure would cause a crane to collapse is called a single point of failure, or fracture critical if it carries tension.
Within fracture mechanics, one methodology to manage cracks and other defects is by using the damage tolerance method. Pioneered by the aerospace industry, the idea behind damage tolerance is that the engineer assumes there is a crack of the smallest size with a given inspection method. From there, the engineer can calculate the crack growth rate that will occur during normal use. By calculating the crack damage tolerance of a crane and implementing the resulting inspection program, chances are much better that repair work can be identified and scheduled to minimize operational down time.
The results of fracture mechanics are largely dependent on the assumptions made, (e.g., the values of the constants in the crack growth equation, the size of initial flaws, and the shape of the initial crack). This information is not typically available at the design stage so these techniques are rarely used. In fact, the most common fracture design codes, API 579 and BS 7910, are written to be fracture assessment tools to evaluate fitness for service. Unlike fatigue, fracture mechanics performs a more sophisticated assessment of the existing conditions allowing for a more precise analysis of stress and strain to determine whether operating equipment is fit for its intended service or whether in-service deterioration threaten the integrity of the equipment.
However, in the case of wear items such as container twist-locks, we have historical data and the cracks tend to form in a few select areas. Fractography of the crack surface can provide enough insight to validate assumptions such as the shape of the initial crack. Depending on the inspection method we can assume an initial crack size consistent with the NDT method and minimum crack size detection.
Fracture mechanics does not replace the more typical fatigue assessment, instead it will supplement the initial analysis as the objective is to study the relative influence of crack growth to set inspection intervals. While the math is a bit more involved than typical engineering applications, the concepts can be simple to grasp yet challenging to master.
Fracture Assessment Diagram
The basic workflow is to create a fracture assessment diagram (FAD) to analyze and determine the largest tolerable crack size for the service conditions. The FAD diagram creates a curve which considers several potential failure mechanisms. More specifically, the FAD diagram spans the entire range from linear-elastic fracture mechanics (LEFM) to elastic-plastic fracture mechanics (EPFM) and fully plastic behavior. Such an analysis accounts for the two extremes of brittle fracture and plastic collapse. At low stresses, the analysis reduces to linear elastic fracture mechanics but can still predict failure if stresses are sufficiently high. At intermediate stresses, the analysis applies a plastic correction which reduces the allowable stress intensity factor. Alternatively, if the material is sufficiently tough or if crack-like flaws are small, the component will not fail unless it is loaded into the fully plastic region.
A FAD diagram is created by plotting the stress ratio on the x-axis and the stress intensity ratio on the y-axis. An assessment point is created by plotting the stress ratio and stress intensity ratio. If the assessment point is under the curve it is fit for service.
Problems can arise in the plastic region, a slight error in load or material properties can have dramatic effects. Most specifications suggest performing a Monte Carlo analysis to determine the sensitivity and probability of failure occurring for a range of values of important inputs such as material properties and loads.
Once the largest crack size that is fit for service is determined, Paris Law can be used to determine the number of stress cycles that will cause the crack to grow from its initial size to the critical size. Recall that the initial size was based on the smallest detectable crack size for a given inspection method. This means the analysis assumed a crack was present but not detected by an inspection and was returned to service. Paris law is used to determine how many cycles the damage part can tolerate before failure. The idea is to set the inspection interval so that the part doesn’t catastrophically fail before the next inspection.
Serializing and Tracking Twist-lock Use
CP&A performed a root cause investigation of a twist-lock failure. The twist-locks that hold the spreader to the headblock failed. The spreader dropped while engaged with a load, which greatly disrupted operations. The source of this failure was that the twist-lock was supposed to have been discarded but was accidentally put back into service. Upon checking the twist-lock serial number and tracing it back to the manufacturer, it had been manufactured 15 years prior to the incident. This accident would have been prevented by serialization and tracking of twist-lock cycles and/or age. This illustrates how inspection maintenance and design are all inter-related.
