Project Objective 1
Ultrasonic Thickness Measurements
November 1-6, 2004

Using a digital ultrasonic thickness instrument, Mr. Jay Schraan and Mr. Randy Jones, from Inspection Technologies, Inc., will work with Dr. Donald Johnson and NPS researchers to measure steel hull thickness in at least 12 locations on USS Arizona. This technique was chosen because, unlike other methods, this one does not damage the hull plates being tested.
 

A starboard profile of Arizona's hull, as mapped in the 1980s. The vertical red bars indicate the approximate location of frames 70 (right) and 90 (left), where hull thickness testing will be done in November 2004. Original drawing by Jerry L. Livingston. NPS photo.

In November 2003, the instrument, a Krautkramer Ultrasonic Systems DMS 2 A-Scan Digital Thickness Gauge, was tested at the locations of six hull coupons, or test samples, collected in two vertical sections (one port, one starboard) at frame 75 in August 2002.  Because NPS archaeologists know the exact hull thickness at each of these locations, they made ideal test sites for the instrument.  Additional hull thickness measurements will be taken this year in four vertical sections (two port, two starboard) at frames 70 and 90.  This is critical data for completing the preliminary Finite Element Analysis, which is focusing on an 80-foot cross section of Arizona’s hull between frames 70 and 90. This year’s thickness data will augment empirically measured hull thickness from the 2002 coupons and will give an indication if corrosion is uniform along Arizona’s outer hull.

SRC archeologist Matt Russell uses a digital ultrasonic thickness gauge on the the hull of Arizona. Photo by Brett Seymour.

Finite Element Analysis (FEA) will be the primary predictive tool used during USS Arizona research.   FEA is a method to calculate stresses and shape changes in a structure under load using experimental data. For historical shipwrecks such as USS Arizona, an FEA allows manipulation of multiple variables, such as corrosion rate and hull thickness, to analyze loads and stresses on hull structure for prediction of probable collapse rate, nature and sequence and consequent impact on structures containing fuel oil.  In addition, the FEA provides a fundamental tool to evaluate consequences of proposed management alternatives involving structural intervention or preservation strategies. 

The next development stage of the FEA will focus on incorporating structural effects of the blast and fire that sank the vessel.  Modeling the structural changes to Arizona resulting from the explosion and subsequent fire that sank the ship should be seen as a starting point for understanding the vessel’s present condition and projecting its future condition and rate of deterioration.

The final stage of FEA development will incorporate external and internal corrosion and thickness measurements to complete the model of Arizona’s present condition and to allow researchers to extend the model into the future.