Stanley T. Rolfe

A Comparison of the J -lntegral and CTOD Parameters for Short Crack Specimen Testing

This study investigates the applicability of the J-integral test procedure to test short crack specimens in the temperature region below the initiation of ductile tearing where J 1 c cannot be measured. The current J-integral test procedure is restricted to determining the initiation of ductile tearing and requires that no specimen demonstrates brittle cleavage fracture. The J 1 c test specimen is also limited to crack-depth to specimen-width ratios (a/W) between 0.50 and 0.75. In contrast, the crack tip opening displacement (CTOD) test procedure can be used for testing throughout the entire temperature-toughness transition region from brittle to fully ductile behavior. Also, extensive research is being conducted to extend the CTOD test procedure to the testing of short crack specimens (alW ratios of approximately 0.15). The CTOD and J-integral fracture parameters are compared both analytically and experimentally using square (cross-section) three-point bend specimens of A36 steel with a/W ratios of 0.50 (deep crack) and 0.15 (short crack). Three-dimensional elastic-plastic finite element analyses are conducted on both the deep crack and the short crack specimens. The measured J-integral and CTOD results are compared at various levels of linear-elastic and elastic-plastic behavior. Experimental testing is conducted throughout the lower shelf and lower transition regions where stable crack growth does not occur. Very good agreement exists between the analytical and experimental results for both the short crack and deep crack specimens. Results of this study show that both the J-integral and the CTOD fracture parameters work well for testing in the lower shelf and lower transition regions where stable crack growth does not occur. A linear relationship is shown to exist between J-integral and CTOD throughout these regions for both the short and the deep crack specimens. These observations support the consideration to extend the J-integral test procedure into the temperature region of brittle fracture rather than limiting it to J 1 c at the initiation of ductile tearing. Also, analyzing short crack three-point bend specimen (a/W < 0.15) records using the load versus load-line displacement (LLD) record has great potential as an experimental technique. The problems of accurately measuring the CMOD of short crack specimens in the laboratory without affecting the crack tip behavior may be eliminated using the J-integral test procedure.

Effects of crack depth on elastic-plastic fracture toughness

Short crack test specimens (a/W ≪ 0.50) are frequently employed when conventional deep crack specimens are either inappropriate or impossible to obtain, for example, in testing of particular microstructures in weldments and in-service structures containing shallow surface flaws. Values of elastic-plastic fracture toughness, here characterized by the crack tip opening displacement (CTOD), are presented for square (cross-section) three-point bend specimens with a/W ratios of 0.15 and 0.50 throughout the lower-shelf and lower-transition regions. Three dimensional, finite-element analyses are employed to correlate the measured load and crack mouth opening displacement (CMOD) values to the corresponding CTOD values, thus eliminating a major source of experimental difficulty in previous studies of shallow crack specimens. In the lower-transition region, where extensive plasticity (but no ductile crack growth) precedes brittle fracture, critical CTOD values for short crack specimens are significantly larger (factor of 2–3) than the CTOD values for deep crack specimens at identical temperatures. Short crack specimens are shown to exhibit increased toughness at the initiation of ductile tearing and decreased brittle-to-ductile transition temperatures. Numerical analyses for the two a/W ratios reveal large differences in stress fields ahead of the crack tip at identical CTOD levels which verify the experimentally observed differences in critical CTOD values. Correlations of the predicted stresses with measured critical CTOD values demonstrate the limitations of single-parameter fracture mechanics (as currently developed) to characterize the response.

Use of CFRP Overlays to Strengthen Welded Connections under Fatigue Loading

This study evaluates the performance of various methods to prevent and repair fatigue damage in welded connections, a recurring problem that affects a significant number of steel bridges. Experimental tests and analytical simulations were carried out to investigate the fatigue performance of coverplate specimens in which the welded connections were reinforced with carbon-fiber reinforced polymer (CFRP) overlays. Specimens were loaded in three-point bending induced by a cyclic load to evaluate the change in fatigue-crack initiation life of the welded connections caused by the attachment of the CFRP overlays. Test results showed that when bond between the CFRP overlays and the steel was maintained, the reduction in stress demand was sufficient to extend the fatigue life of the welded connections from AASHTO fatigue-design Category E’ in the unreinforced configuration to the infinite fatigue life range. Test results also showed that the fatigue strength of the bond layer was drastically improved by introducing breather-cloth material within the bond layer.

Parametric Analysis of Cross-Frame Layout on Distortion-Induced Fatigue in Skewed Steel Bridges

The effects of skew angle, cross-frame spacing, cross-frame layout, cross-frame stiffness, and load placement on the potential for distortion-induced fatigue damage in steel bridges was investigated by performing a suite of more than 1,000 analysis jobs of high-resolution three-dimensional finite-element models. Susceptibility to fatigue damage was quantified in terms of computed stress demand in the web gap region of the girders. Bridge configurations with three different cross-frame layouts were evaluated, including configurations with cross-frames placed parallel to skew angle (skewed-parallel) and perpendicular to the girder line, both staggered (skewed-staggered) and unstaggered (skewed-unstaggered). Skew angles of configurations evaluated ranged between 0 and 50°, and cross-frame spacing ranged from 2.29 to 9.14 m (7.50 to 30.0 ft). Influence and envelope surfaces were constructed to show the relationship between load placement, location of the maximum web gap stress, and the magnitude of the maximum web gap stress. It was found that maximum web gap stress always occurred when loads were positioned directly above the intersection of a cross-frame and girder web. The parametric study showed that cross-frame stiffness and spacing had a significant effect on the susceptibility to distortion-induced fatigue damage; greater cross-frame stiffness resulted in higher web gap stresses, and increased cross-frame spacing resulted in increased web gap stresses. It was also found that the bridge configuration was key to determining the location of the web gaps where damage is most likely to occur. In skewed-parallel and skewed-unstaggered layouts, maximum web gap stresses were identified in top web gaps, whereas in skewed-staggered configurations, maximum stresses occurred in bottom web gaps. It was found that in configurations with staggered cross-frames, maximum web gap stresses tended to occur in regions of support, where cross-frames are often placed back-to-back along the skewed alignment.

Development of a Technique to Improve Fatigue Lives of Crack-Stop Holes in Steel Bridges

A common technique to prevent the propagation of fatigue cracks in bridge girders is to drill crack-stop holes at crack tips. Stress concentrations at the crack tips are reduced and fatigue life of the bridge is extended. The size of the crack-stop hole needed to prevent further crack growth is determined by using known material properties and relationships developed through experimentation. However, these equations often result in a crack-stop hole diameter larger than can be practically drilled; physical limitations force crack-stop holes to be undersized in the field. To improve effectiveness of undersized holes to that of full-sized holes, a method is needed to strengthen undersized crack-stop holes. This study investigated the potential of a technique to improve the fatigue life of undersized, crack-stop holes. It uses piezoelectric actuators operated at ultrasonic frequencies to convert electrical signals into mechanical work. The technique produced residual compressive stresses of the same order of magnitude as those produced by static cold expansion. A suite of finite element models was created to quantify and characterize the residual stresses surrounding the cold-expanded, undersized, crack-stop holes. Results were compared with analyses in the literature.

Effects of Cross-Frame Placement and Skew on Distortion-Induced Fatigue in Steel Bridges

Because of detailing practices common before the mid-1980s, many bridges are highly susceptible to distortion-induced fatigue. This research explored the influence of cross-frame placement and skew angle in bridges subject to distortion-induced fatigue. Forty high-resolution, three-dimensional finite element analyses of a bridge with multiple cross-frame and skew configurations were performed to examine the relationships between skew angle, cross-frame placement, and stresses on distortion-induced fatigue susceptibility. Bridges with skew angles of 0°, 20°, and 40° and cross-frames spaced at 4.58 m (15 ft) and 9.15 m (30 ft) were investigated. Cross-frame configurations examined included staggered perpendicular to the girder line as well as parallel to the support skew. The analyses found maximum stresses in the web-gap occurred in positive moment regions but not necessarily in regions of highest differential deflection. In configurations with cross-frames placed parallel to the skew angle, maximum stress demand was in the top web-gap but it was in the bottom web-gap region when cross-frames were staggered. Increased spacing between cross-frames correlated with slightly increased maximum web-gap stresses for parallel-to-skew cross-frame arrangements and decreased maximum web-gap stresses for perpendicular-to-girder line cross-frame arrangements. Skew angle had a minimal effect on web-gap stresses in the bridge studied.

An analytical comparison of short crack and deep crack CTOD fracture specimens of an A36 steel

The effect of crack-depth to specimen-width ratio on crack tip opening displacement (CTOD) fracture toughness is an important consideration in relating the results of laboratory tests to the behavior of actual structures.Deeply cracked three-point bend specimens with crack-depth to specimen-width ratios (a/W) of 0.50 are most often used in laboratory tests. However, to evaluate specific weld microstructures or the behavior of structures with shallow surface cracks, specimens with a/W ratios much less than 0.50 often are required. Laboratory tests reveal that three-point bend specimens with short cracks (a/W = 0.15) exhibit significantly larger critical CTOD values than specimens with deep cracks (a/W = 0.5) up to the point of ductile initiation. In this study, finite element analyses are employed to compare the elastic-plastic behavior of square (cross-section) three-point bend specimens with crack-depth to specimen-width ratios (a/W) ranging between 0.50 and 0.05. The two-dimensional analysis of the specimen with an a/W ratio of 0.15 reveals a fundamental change in the deformation pattern from the deep crack deformation pattern. The plastic zone extends to the free surface behind the crack concurrent with the development of a plastic hinge. For shorter cracks (a/W = 0.10 and 0.05), the plastic zone extends to the free surface behind the crack prior to the development of a plastic hinge. For longer cracks (a/W > 0.20), a plastic hinge develops before the plastic zone extends to the free surface behind the crack. These results prompted further study of specimens with an a/W ratio of 0.15 using three-dimensional, elastic-plastic finite element analyses. Results of the short crack (a/W = 0.15) analysis are compared to the results of the deep crack (a/W = 0.50) analysis reported previously by the authors. In the linear-elastic regime (characterized by small-scale plastic deformation) the relationship of stress ahead of the crack tip to CTOD is identical for the short crack and the deep crack specimens. At identical CTOD levels in the elastic-plastic regime (large-scale plasticity, hinge formation), the crack tip stress is significantly lower for specimens with a/W = 0.15 than for specimens with a/W = 0.50. Correspondingly, at equivalent stress levels, the CTOD for the short crack is approximately 2.5 times the CTOD for the deep crack. This observation has considerable significance in the application of CTOD results to failure analysis or specification development where the fracture mechanism is cleavage preceded by significant crack tip plasticity.

Distortion-Induced Fatigue in Steel Bridges: Causes, Parameters, and Fixes

Distortion-induced fatigue in steel bridges is a continuing problem for bridge engineers. Accordingly, a cumulative review and discussion of scholarly literature on this topic as well as focused analytical research aimed at studying bridge system performance when various retrofit techniques are applied to connection details are described herein. Driving forces behind distortion-induced fatigue are presented, including the effects of bridge system geometry and specific detail geometries. Retrofit techniques including positive attachement, back-up transverse stiffeners, slotted connection stiffeners, and removal of lateral brace elements were studied analytically to determine effectiveness. Results from finite element analyses are presented showing relative success of various retrofit techniques. Appropriateness of retrofit techniques under various conditions is disseminated.

Improved Method for Bonding CFRP Overlays to Steel for Fatigue Repair

Experiments and computer simulations were carried out at the University of Kansas to investigate the use of composite materials for strengthening and repair of fatigue-vulnerable steel bridges. Prefabricated CFRP overlays were attached to welded coverplate specimens using a layer of epoxy resin. The specimens were then subjected to three-point bending under cyclic loading at a constant stress range. Early tests showed that bonding CFRP overlays to steel was an effective method to reduce the stress demand in areas susceptible to fatigue damage, and that this strengthening method could lead to substantial increases in crack initiation life. Fatigue tests also showed that maintaining the bond between the CFRP overlays and the steel was the most critical factor in the effectiveness of this strengthening technique. It was observed that the presence of a layer of breather fabric embedded within the layer of epoxy resin was the most important parameter affecting bond behavior under fatigue loading. This paper describes monotonic and fatigue tests carried out to evaluate the effect of the presence of breather fabric and thickness of the resin layer on bond strength and on the performance of the resin layer under fatigue loading. It was found that the mode of failure of the resin under monotonic and fatigue loading changed significantly due to the presence of the breather cloth. Although the presence of breather cloth had a negligible effect on the bond strength of thin resin layers, its effect on strength increased with the thickness of the resin layer. It was also found that the presence of the breather fabric had a very significant effect on the post-peak behavior of monotonic tests, which partially explains its beneficial effect on behavior under fatigue loading. BACKGROUND Techniques to repair connections vulnerable to fatigue damage in steel bridges must be developed considering the specific geometry and other relevant characteristics of the particular connection detail. This is particularly the case for fatigue repairs using composite material overlays, in which the geometry and location of the overlays should be optimized for the shape of the connection detail. Nonetheless, several studies have found that regardless of geometry a critical factor in the implementation of this repair technique is maintaining the bond between the composite material and the steel for the intended fatigue life of the repair. There is a wealth of studies investigating bond between resins and metal. Two studies closely related to the type of repair discussed in this paper were carried out by Nozaka et al. (2005; 2005a), who investigated the effectiveness of carbon-fiber-reinforced polymer (CFRP) strips for increasing fatigue life of steel girders. Testing described in Nozaka et al. (2005) was

Innovative retrofit technique for distortion-induced fatigue cracks in steel girder web gaps

Cracking in web gaps of steel girder bridges is often difficult and expensive to repair, especially at the top web gap. A commonly-used retrofit technique involves creating new load path between a transverse connection plate and the top flange of the girder, by means of bolted angles on both sides of the transverse connection plate. This technique often requires removing portions of the bridge deck to create the bolted connection at the top flange, which is an approach that can incur significant expense and inconvenience to the traveling public. This study was aimed at evaluating the effectiveness of a newly-developed retrofit technique, in which connection is made between the transverse connection plate and the girder web, through use of bolted angles on both sides of the transverse connection plate and a backing plate on the opposing face of the girder web. The retrofit was evaluated through extensive structural testing and finite element modeling. Testing was performed on a 9.1-m [30-ft] long test bridge system, comprised of three 910-mm [36-in.] deep girders and a concrete deck. The system was loaded to produce distortion-induced fatigue cracking, and then the bridge was retrofitted with the newly-developed stiffener-to-web repair technique. Results of the testing were compared to findings from finite element analyses, as well as findings from structural tests performed on 2.7-m [9-ft.] long segments of similar girders tested under distortion-induced fatigue. Results have indicated that the newly-developed retrofit technique has significant potential for effectively controlling distortion-induced fatigue cracking in web gaps of steel girder bridges without requiring disruption to the concrete deck.