Prevent Fatigue Crack Research Articles

Repairing Distortion-Induced Fatigue Cracks in Steel Bridge Girders Using Angles-with-Plate Retrofit Technique. II: Computer Simulations

This paper presents the results from computer simulations of 914 mm (36 in.) deep girder-cross frame subassemblies to study the effects of distortion-induced fatigue and to evaluate the effectiveness of a newly proposed cost-effective retrofit measure. The proposed retrofit measure consists of adding steel angles connecting the girder web and the transverse connection plate (CP), and a steel plate on the back side of the girder web. The retrofit measure is intended to reduce stress demand at the welds, to restrain the web-gap region from deforming in the out-of-plane direction, and to distribute lateral forces transferred by cross-frames over a wider region of the web. Parametric studies were carried out to determine the optimal configuration to prevent growth of fatigue cracks of various lengths in the web-gap region. It was found that the proposed retrofit measure reduced peak stress demands with respect to the unretrofitted configuration by 50% or more, making it likely that it will prevent fatigue crack reinitiation. The parametric studies showed that the proposed retrofit measure became more effective in reducing the peak stress demand as the stiffness of the elements increased, with the lowest average stress demand occurring when the angles and backing plate were assigned thicknesses equal to 2.67 times the thickness of the web. Experimental verification of this study is presented in a companion paper.

Fatigue Behaviour of Steel Bridge Joints Strenghtened with FRP Laminates

One of the most relevant concerns about steel and metal bridges stands on the repair and rehabilitation of existing structures. In fact, the remaining service life of steel bridges is limited by fatigue damage and in order to ensure the safety of these bridges it is often necessary to inspect the structure in order to discover the presence of fatigue cracks. New reinforcement techniques are needed, in order to prevent fatigue cracking and to increase bridge safety at the same time. According to the contemporary knowledge on the matter, two alternatives of rehabilitation are given: the traditional interventions and the use of carbon fibre composites. In this paper, in order to explain the appropriate use of these techniques, the most common knowledge on the matter are presented and explained, focusing in particular on the real scale testing results coming from the last decades research applications related to steel bridge engineering. This study should in this way be useful for future research applications and for bridge engineering real case rehabilitations.

Experimental study of fatigue crack growth behaviour in adhesively reinforced steel structures

The use of adhesively-bonded composite materials has lead to great advances in repairing and strengthening cracked or aged metallic structures. However, less is known about the fatigue performance of the bonding between the composites and the metallic substrate, which is the deciding factor when designing the strengthening. In this study, a series of experiments is performed to investigate the effectiveness of the composites on preventing fatigue crack propagation and extending the fatigue life of steel plates. The composite material of CFRP sheet is adopted. Both single-sided and double-sided repairs are studied. Experimental results show that the application of composite patches substantially reduces crack growth rate and prolong fatigue life. The double-sided repair scheme increased the fatigue life by 2.2–2.7 times over un-patched steel plates when normal modulus CFRP sheets were used, and by 4.7–7.9 times when high modulus CFRP sheets were used. The CFRP sheets with high modulus were found to be much more efficient. Bond width has considerable influence on the crack growth life. These two findings were especially true for double-sided repairs. The parameters of patch thickness , patch length and patch configuration, had an influence on fatigue life increase of over 20% for both single-sided and double-sided repairs, except that patch configuration had only a 6% influence on fatigue life increase for double-sided repairs. Furthermore these three parameters had slightly more influence on single-sided repairs than on double-sided repairs.

Twin boundaries: Strong or weak?

Fatigue cracking along twin boundaries (TBs) has significantly different features in Cu and Cu–Al alloys. In Cu and Cu–5 at.% Al alloy, the TBs are intrinsically strong and resist fatigue cracking. With the decrease in stacking fault energy (SFE), some fatigue cracks were observed along or near the TBs in Cu–16 at.% Al alloys. The current findings confirm that the TBs are not always strong enough to prevent fatigue cracking, which is also affected by SFE or slip mode.

Effect of Fatigue Crack Propagation in the Shape Memory Alloy Fiber Reinforced Smart Composite

In this paper, the TiNi fiber reinforced / Polycarbonate(PC) composite material is developed, and its properties is studied. Conducting fatigue experiments, shape memory effect of the material for preventing fatigue crack growth are investigated. The fatigue behavior and crack propagation are observed under increasing temperature with a SEM servo-pulser, which is a fatigue testing instrument with scanning electron microscope. As the results, the effectiveness of fatigue resistance is confirmed. The shape memory effect and expansion behavior of the matrix caused by increasing temperature create the effect of the fatigue crack propagation control. It is verified that the controlling of fatigue crack growth is attributed to the compressive stress field in the matrix which is caused by shrinkage of the TiNi fibers above austenitic finishing temperature (Af).

Deformation and fatigue behavior of hot dip galvanized coatings

This paper reports on the results of a study of the effect of static and dynamic stresses on hot dip galvanized coatings on SAE 1020 steel substrates. Galvanizing was performed using baths maintained at 450 °C, the zinc containing 0.16% Ti and 0.02% Fe and with Al and Ni in the ranges 0–0.20% and 0–0.30%, respectively. Static three-point bend tests were conducted with applied stresses in the range 428–790 MPa. Dynamic bend-fatigue tests involved stresses in the range 228–578 MPa at a cyclic frequency of 0.25 Hz for up to 700 cycles. The total crack density in the coatings was measured before and after the tests using light optical and electron microscopy. The results showed that the crack density increased as the applied stress increased and crack propagation was promoted perpendicular to the substrate. The number of cycles had no effect on the crack density and propagation at stresses lower than 386 MPa. At higher stresses the number of applied cycles contributed only to crack propagation. It was concluded that the best bath composition for preventing fatigue crack propagation is one that minimized the formation of thinner brittle layers in the galvanized coatings.

Crack growth induced delamination on steel members reinforced by prestressed composite patch

ABSTRACT Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce a crack closure effect. Moreover, it modifies the crack geometry by bridging the crack faces and so reduces the stress intensity range at the crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A crack growth induced debonded region in the adhesive‐plate interface was observed using an optical technique. Moreover, the size of the debonded region significantly influences the efficiency of the crack repair. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique (MVCCT). A parametric analysis is performed to investigate the influence of some design parameters such as the composite patch Young's modulus, the adhesive thickness and the pretension level on the adhesive‐plate interface debond.

低合金鋼の高温疲労におけるフィッシュアイを伴う内部破壊と動的ひずみ時効による表面改質

Stepwise S-N curves were obtained under rotating bending fatigue tests at high temperatures for an low alloy steel, as well as fatigue tests at room temperature for surface hardened specimens by shot peening or carburizing treatment. In the high cycle region, internal fracture with fish-eye pattern was dominant. As for mechanisms of internal fatigue fracture for 1Cr-0.5Mo steel, under high cycle region at high temperatures, following results can be obtained.(1) Oxidized surface layer itself has no effect on preventing fatigue crack from initiating at the specimen surface.(2) Specimen surface hardens during rotating bending fatigue test at 673K due to dynamic strain aging. Because of this phenomenon, it is difficult for Stage I cracks to initiate at specimen surface.(3) Effects of surface improvement by dynamic strain aging on fatigue strength can be recognized.

Delamination effects on cracked steel members reinforced by prestressed composite patch

Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce crack closure effect. Moreover, it modifies the crack geometry by bridging the crack lips and reduces the stress range at crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A debond crack in the adhesive–plate interface was observed by optical technique. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique. A parametric analysis is performed in order to investigate the influence of some design parameters such as the composite patch Young’s modulus, the adhesive thickness and the pretension level on the adhesive–plate interface debond.

EVALUATION OF FATIGUE CRACK PROPAGATION PROPERTIES UNDER RANDOM LOADING AVOIDING CRACK CLOSURE

Abstract— Fatigue crack propagation rates and the fatigue threshold of HT80 steel were measured by maintaining the maximum load during the whole period of random loading in order to prevent fatigue crack closure. The random loading pattern involved 62 level block loadings in which the waveform was approximated to the Rayleigh distribution of peaks. The fatigue crack propagation rates under random loading were well predicted from those obtained from constant amplitude loading and assuming a linear cumulative damage law. That is, da/dn =C{ΔKmeq−ΔKmth} where the equivalent stress intensity factor, ΔKeq={=niΔKmi/dni}1/m, where ni= 0 for ΔKi≤ΔKth, or ni=ni for ΔKi > ΔKth.

Fatigue properties of hot-dip galvanized steels: Nilsson, T., Engberg, G. and Trogen, H. Scand. J. Metall. 1989 18, (4), 166–175

This paper reports on the results of a study of the effect of static and dynamic stresses on hot dip galvanized coatings on SAE 1020 steel substrates. Galvanizing was performed using baths maintained at 450 °C, the zinc containing 0.16% Ti and 0.02% Fe and with Al and Ni in the ranges 0–0.20% and 0–0.30%, respectively. Static three-point bend tests were conducted with applied stresses in the range 428–790 MPa. Dynamic bend-fatigue tests involved stresses in the range 228–578 MPa at a cyclic frequency of 0.25 Hz for up to 700 cycles. The total crack density in the coatings was measured before and after the tests using light optical and electron microscopy. The results showed that the crack density increased as the applied stress increased and crack propagation was promoted perpendicular to the substrate. The number of cycles had no effect on the crack density and propagation at stresses lower than 386 MPa. At higher stresses the number of applied cycles contributed only to crack propagation. It was concluded that the best bath composition for preventing fatigue crack propagation is one that minimized the formation of thinner brittle layers in the galvanized coatings.

Code rules for fatigue analysis of crack-like discontinuities and small cracks

Practical application of Code rules for preventing fatigue crack initiation in geometrical discontinuities having the appearance of cracks is discussed. Attention is given to the RCC-MR specific rules for fatigue analysis of crack-like discontinuities. These rules are based on the ‘d-concept’, that is to say that the strain range is computed at some finite distance d below the surface, this distance d being characteristic of the material. This strain range and the conventional fatigue design curves are then utilized as usual for fatigue analysis. Experimental validation of these rules is discussed and several examples of practical application are given. In addition it is shown this rule can be applied to avoid fatigue crack initiation from small cracks.