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Supervisor: Gary S. Schajer Renewable Resources Laboratory |
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Background
Large structures such as bridges and major buildings must be inspected periodically and maintained as needed to ensure a long and safe working life. Foundation settlement can cause significant redistribution of internal loads within a structure, which in the course of time can produce major damage, and in extreme cases, collapse. Thus, as part of a preventative maintenance program, it important to be able to measure the loads, dead+live, in key structural members. This can give valuable information about the present and ongoing trends of internal structural loads. Such measurements are similarly valuable at the end of the lifespan of a major structure. Large pre-stresses often exist, and these must be safely unloaded prior to dismantling. If not, substantial elastic strain energy will be released as the structure is dismantled, potentially with catastrophic consequences.
Typical stress measurements involve measuring material deformations as the load is applied or released, for example, using strain gauges. However, this approach is not practical for large structures because dead loads exist continuously and cannot be altered. A practical approach is to use techniques developed for residual stress measurement. Residual or “locked-in” stresses are self-equilibrating stresses in materials that are commonly created during manufacture, for example, during casting, welding or forming. These stresses share the same feature as large-structure stresses in that the stress source cannot be adjusted to allow material deformation measurements to be made.
A common approach used for residual stress measurement is to modify the “applied load” by removing some stressed material and measuring the resulting deformation in the surrounding material. This is the basis of the hole-drilling method. A small hole is drilled in the specimen and the resulting deformation of the material surrounding the hole is measured, traditionally using strain gauges, and more recently using optical methods such as Digital Image Correlation (DIC). The stresses originally existing within the hole can then be evaluated from the measured deformations using established computation techniques.
When doing hole drilling according to the ASTM Standard Test Procedure E837-13, the hole is quite small, typically 2mm diameter and 1mm deep. Sub-surface stresses beyond half a hole diameter cannot be identified reliably. A typical steel bridge plate could be 25mm thick, so a through-hole 50mm in diameter would be needed to evaluate the through-thickness stresses. Residual stresses induced during the original manufacturing process are self-equilibrating, and thus correspond to zero net load. They are therefore easily separated from the structural stresses, which correspond to a non-zero net load. Both axial and bending structural loads can be separately identified in this way.
The ASTM Standard Test Procedure E837-13 for hole-drilling residual stress measurements specifies the use of strain gauges to measure the hole-drilling deformations. This is a practical approach for measurements in a lab or a sheltered field site. However, it is much less practical in the exposed sites typical on large structures such as major bridges because of the detailed surface preparation, strain gauge adhesion and wiring that is needed. An optical measurement technique such as Digital Image Correlation provides an alternative measurement method with much less demanding surface preparation requirements.
Digital Image Correlation involves taking digital photographs both before and after hole drilling. By mathematical analysis of the two images, it is possible to track the deformation of the imaged surface. With modern techniques even very small motions down to 0.01 of a pixel can be detected. When using modern cameras with many mega pixels, a very fine resolution can be achieved. The technique fits well with the scale of the proposed measurements and it straightforward to image the area around a 50mm diameter hole reliably, even under exposed field conditions.
Progress
Work is presently underway to develop a portable device for field use to measure the stresses in large steel members in bridges and other large structures. The device will have a robust drilling head sufficient to cut up to 50mm diameter holes in a controlled way in structural steel. It will also include a precision camera system to image the material around the hole location both before and after hole drilling. Finally, a practical computer software will be developed to do the required DIC computations and to evaluate the corresponding local structural stresses and residual stresses.
Significance
When compete, the large-structure stress sensor will enable field measurements of internal stresses in large structures such as bridges and major buildings. The sensor will be a valuable diagnostic tool in support of preventative maintenace programs to monitor and respond to the internal dead load variations caused by foundation settlement. In addition, the sensor will be useful for measurements of internal loads in pre-stressed structures to allow safe final dismantling.