Bruce D. Westermo

New strain measurement technology for material damage assessment

A new strain sensing methodology for the measurement of peak strain in engineering materials has been developed. The approach involves the correlation of change in the magnetic susceptibility attendant with the strain-dependent, solid-state phase transformation in a sensing element. The sensing materials are metastable steel alloys that irreversibly transform from nonferromagnetic to ferromagnetic behavior as a function of the peak, applied normal strain. The technology makes available a reliable method for passive, semi-active, or active monitoring of strains or deflections and is applicable as either embedded sensors or attached strain gages, i.e., monitors. Strain assessment devices of various types have been conceptualized to meet a variety of needs. The history, principles, and test data on the development are presented. Discussion and results of a prototype system installed on the I-95 Savannah River bridge are presented. Examples of current projects and future applications of the technology are presented and discussed. A range of applications is discussed that illustrates the versatility of the approach and the value of such systems in materials and structural safety assessment.

Design and evaluation of passive and active structural health monitoring systems for bridges and buildings

Structural health monitoring systems have been designed and evaluated in the laboratory for installation in several bridges and commercial buildings. The systems employ solid-state sensor elements which experience a strain-dependent phase transformation from a metastable, nonmagnetic, austenitic phase to the stable, ferromagnetic, martensitic phase. The irreversible phase transformation is useful for indicating the level of peak structural strain experienced in a particular monitored location. Some of the sensor material characteristics and details related to the phase transformation are discussed as applied to structural health monitoring. The design of representative systems for bridges and commercial buildings is included. Important system(s) features and design capabilities are discussed. Finally, the evaluation of passive and active systems in the laboratory is discussed. The results of experiments detailing the behavior of these systems under uniaxial tension and cyclic loading conditions are presented.

Development of smart structural attachment fixtures

This paper discusses efforts related to the development of smart bolts to be used as structural attachment fixtures. The work has been directed at meeting the high-strength application requirements for the aircraft industry and selected applications within the construction industry. The bolts are fabricated from metastable austenitic steel materials which progressively and irreversibly transform from a nonferromagnetic, austenitic parent phase to a thermodynamically stable, ferromagnetic martensitic phase as a function of applied strain. Thus, the ferromagnetic response of a bolt in service can be used to indicate the degree of inelastic deformation, i.e., the peak strain, that the bolt has experienced up to that particular point. A combination of bolt alloy chemistry and thermomechanical treatment has been utilized to produce a sharp ferromagnetic response with respect to small plastic deformation strains. The extreme strength of the materials allows for a centrally drilled access hole for the placement of sensitive Hall sensor probes to detect any inelastic material behavior along the length of the bolt without removal from the structure. A discussion of the smart materials behavior of the bolts will be followed by a presentation of recent test results which illustrate the structural bolt monitoring technique with some possible applications.

Passive monitoring systems for structural damage assessment

Engineering structures are designed to function within the elastic domain and designs are chosen based on their relative merits and capabilities to suitably address the specific requirements. The decision to monitor a structure to determine the long-term performance characteristics and operational stability involves many factors including, but not limited to, the objectives of the monitoring and the available resources to do so. One must choose between active monitoring systems, i.e., those that require real-time power supplies, and ancillary data acquisition, storage and monitoring systems and passive systems which, as the name implies, operate without any of these constraints. Passive techniques for structural health monitoring and materials damage assessment are reviewed. Data are presented to illustrate their application as related to structural health monitoring, crack detection and crack growth monitoring, and damage assessment in composite materials.

Passive and active structural monitoring experience: Civil engineering applications

State Departments of Transportation and regional city government officials are beginning to view the long-term monitoring of infrastructure as being beneficial for structural damage accumulation assessment, condition based maintenance, life extension, and post-earthquake or –hurricane (-tornado, -typhoon, etc.) damage assessment. Active and passive structural monitoring systems were installed over the last few years to monitor concerns in a wide range of civil infrastructure applications. This paper describes the monitoring technologies and systems employed for such applications. Bridge system applications were directed at monitoring corrosion damage accumulation, composite reinforcements for life extension, general service cracking damage related to fatigue and overloads, and post-earthquake damage. Residential system applications were directed primarily at identifying damage accumulation and post-earthquake damage assessment. A professional sports stadium was monitored for isolated ground instability pr...

Development of passive smart structural attachment fixtures

This paper discusses efforts related to the development and evaluation of smart bolts and fasteners. The work has been directed at meeting the high-strength application requirements for the aircraft industry and selected applications within the construction industry. The fasteners are fabricated from metastable austenitic steel materials which progressively and irreversibly transform from a non- ferromagnetic, austenitic parent phase to a thermodynamically stable, ferromagnetic martensitic phase as a function of applied strain. Thus, the ferromagnetic response of an in-place fastener can be used to indicate the degree of inelastic deformation, i.e., the peak, post-yield, strain, that it has experienced up to that time. A combination of alloy chemistry and thermomechanical treatment was utilized to produce the desired martensitic transformation vs. peak strain behavior. A discussion of the smart materials behavior of the fasteners will be followed by a presentation of recent test results which illustrate the structural bolt monitoring technique with some possible applications.

Health monitoring in composite materials via peak strain sensing

Fiber-reinforced composite materials are beginning to be employed in applications related to retrofit and repair of large-scale civil structures. This paper discusses the utilization of a passive, pea, strain monitoring technology to the damage and health assessment of composite structures. Applications considered include epoxy-matrix composite materials reinforced with chopped glass, continuous glass fibers, carbon-fiber mat as well as continuous carbon-fiber. The advantages of the various material applications are discussed as they apply to large civil structures with peak strain monitoring data presented to illustrate how the systems can be field monitored. Full-scale structural component testing as well as subscale laboratory testing results will be presented and discussed. Recommendations are provided to guide the engineering community in such composite applications and to provide a design framework for the inclusion of simple and reliable sensor systems to detect both short-term and long-term damage.

Applications of a new solid state structural health monitoring technology

A new methodology has been developed for structural damage assessment and monitoring based on smart material sensors. The sensors transform to a ferromagnetic phase progressively as a function of strain. The solid-state phase transformation is useful in determining the local structural peak strains in monitored high stress sites. The technology is discussed with applications provided to illustrate the utility of the approach in addressing a range of engineering problems