Fractures

Some fractures were dramatic and severe:
 


John P. Gaines, November 1943. 
Vessel broke in two off Shumagin Aleutians with the loss of ten lives.







 

Schenectady

On 16 January 1943, Schenectady, a Liberty tanker, split in two while moored in calm water at the outfitting dock at Swan Island, Oregon. A US Coast Guard report described the incident:
 

Without warning and with a report which was heard for at least a mile, the deck and sides of the vessel fractured just aft of the bridge superstructure. The fracture extended almost instantaneously to the turn of the bilge port and starboard. The deck side shell, longitudinal bulkhead and bottom girders fractured. Only the bottom plating held. The vessel jack-knifed and the center portion rose so that no water entered. The bow and stern settled into the silt of the river bottom.
 

The ship was twenty-four hours old. 
 

The official Coast Guard report on the Schenectady incident attributed the fracture to welds in critical seams that "were found to be defective." 

Esso Manhattan, 29 March 1943, 
at the entrance to New York Harbor. 

But while most fractures were were not so dramatic, there were many of them:
 

Observed Fracture Rates. The figure was constructed by ordering all ships, irrespective of yard, by date of keel laying. The fracture rate is a moving 100-ship window measuring the fraction of all the ships within the window that eventually produced Note the steady rise in the fracture rate prior to February 1943, and the dramatic decline therafter. Despite uncertainty about the causes of the fractures, a major research effort funded by the Board generated numerous important design changes between February and May 1943. The effect of the design changes was a decline in the fracture rate from thirty percent for ships with keels laid in February 1943 to about five percent only four months later.
 
 
 
 
 
 
 
 
 
 
 
 


Fracture rates varied significantly across yards. This variation could be accounted for by systematic variations in steel quality in the mills supplying the yards, an issue that received much attention in 1943 and 1944. However, steel quality cannot be all the explanation, because yard differences in fracture rates were clearly related to productivity differences. This figure plots  two available measures of fracture rates against labor requirements for the first ship built in the eighth round of the ways. The correlation between productivity and fracture rates is clearly visible, and is statistically significant at conventional levels. Moreover, fracture rates exhibited a marked tendency to rise during the first two years of the program. In fact, the first figure, which pools data across yards, understates the extent to which fracture rates increased over time within some of the larger yards. These features of the data strongly suggest that, even though design and steel quality were contributing factors, production practices were related to the fracture problems.
    [Whether or not a defect in a weld leads to a fracture depends on the size of the defect, the stress, and the toughness of the material. For a given stress, tougher steel can withstand larger defects without fracturing. Modern methods of fracture mechanics can use these parameters to calculate the critical defect size above which a fracture is expected to occur. Most descriptions of Liberty fractures indicate that the cracks were accompanied by loud bangs which is characteristic of brittle fractures (i.e. failures because of insufficiently tough steel). I am indebted to Dr. Peter Pumphrey for this information.]