Experimental investigation of bond characteristics between CFRP fabrics and steel plate joints under impact tensile loads

In recent years, the use of adhesively-bonded fibre-reinforced composite materials has attracted widespread attention as a viable alternative for the retrofitting of civil infrastructure such as buildings and bridges. This has been particularly the case for concrete structures. The retrofitting of metallic bridges and buildings with FRP materials, however, is still in its early stages. In real life, these structures are subjected to dynamic loads. Therefore, it is necessary to understand the bond behaviour between steel and the strengthening materials for both static and dynamic loads. To examine the bond between steel plates and carbon fibre-reinforced polymers (CFRP) fabrics, this paper describes the experimental procedures and results of double strap steel joints loaded at different loading rates (2 mm/min, 3.35, 4.43 and 5 m/s). In this test program, ultimate load-carrying capacity, effective bond length, failure mechanism and strain distribution were examined for all loading rates. Different numbers of CFRP layers with different bond lengths were investigated. Experimental findings reveal that the maximum improvement in joint capacity occurs at a rate of 3.35 m/s. It was observed that the effective bond length is insensitive to loading rate for both joints. The failure modes and strain distributions, however, exhibit little difference between static and dynamic conditions.     

Dynamic bond strength between CFRP sheet and steel

The efficiency of the application of adhesively-bonded CFRP materials to rehabilitate and/or strengthen steel structures is completely dependent on the bond between the CFRP composites and the steel. As the repaired and/or upgraded structural elements are likely to be exposed to impact tensile loads during service, this necessitates the importance of understanding the bond strength between CFRP materials and steel under dynamic loadings. This paper presents the experimental procedure, the equipment and the test results for double strap joints at four speeds of loading to highlight the effect of loading rate on the bond strength. Two different CFRP layouts with various bond lengths were examined. It was found that the loading rate has significant influence on bond strength and failure modes, although it has little influence on the effective bond length.     

Fatigue behaviour of laser repairing welded joints

This paper presents a fatigue study in Nd-YAG laser surface repairing welded joints in specimens of two base materials used in mould production. The tests were carried out in a servo-hydraulic machine in tension, under constant amplitude loading, with two stress ratios R = 0 and R = 0.4. Welded specimens were prepared with U notches and filled with laser welding deposits. The fatigue results are presented in the form of S–N curves obtained in welded and non-welded conditions. Complementary measurements of hardness and residual stresses profiles were carried out along the surface of laser welded specimens to understand the observed fatigue behaviour. The melted material was the weaker region, with lower values of hardness and higher tensile residual stresses, presenting also a high number of defects that are potential failure sites. The presence of such defects can explain the relatively poor fatigue strength of the laser repairing joints in comparison to base materials.     

Fatigue damage of stainless steel diffusion-bonded joints

Vacuum diffusion bonding was carried out on 316L stainless steel. Metallographic inspections and micro-hardness testing were conducted near the interface of diffusion-bonded joints. Fatigue tests were performed to investigate the mechanical performance of diffusion-bonded joints under cyclic loading. Results indicate that, although the static strength of joints closes to that of base metal, fatigue life of the diffusion-bonded joint is markedly lower than that of the base metal. Increments of electrical resistance of specimens were monitored and recorded at different cycles. By taking the electrical resistance as a damage parameter, the evolution of fatigue damage of diffusion-bonded joints was interpreted and the relationship between fatigue life and increments of electrical resistance was established. Overall good agreement between the experimental results and predicted life was obtained which provides confidence in the use of the developed approach for life prediction.     

Mechanical characterization of steel/CFRP double strap joints at elevated temperatures

This paper examines the mechanical performance of steel/CFRP adhesively-bonded double strap joints at elevated temperatures around the glass transition temperature (T_g, 42 °C) of the adhesive. A series of joints with different bond lengths were tested to failure at temperatures between 20 °C and 60 °C. It was found that the joint failure mode changed from adherend failure to debonding failure as the temperature approached T_g. In addition, the ultimate load and joint stiffness decreased significantly at temperatures near to and greater than T_g, while the effective bond length increased with temperature. Based on the ultimate load prediction model developed by Hart-Smith for double lap joints and kinetic modelling of the mechanical degradation of the adhesive, a mechanism-based model is proposed to describe the change of effective bond length, stiffness and strength degradation for steel/CFRP double strap joints at elevated temperatures. The modelling results were validated by the corresponding experimental measurements.     

Fatigue behaviour of riveted Glare lap joints

Fibre metal laminates, such as Glare, are a family of materials with very interesting properties for fatigue-critical applications. This article describes the results of a research programme carried out to evaluate the fatigue and damage tolerance characteristics of riveted Glare lap joints, representative of fuselage longitudinal joints. The comparison with the behaviour of metallic joints shows that different contributions are made to the total fatigue life, with the crack propagation life being by far the longer one for the Glare material whereas the crack nucleation life covers almost the entire fatigue life for metallic joints. Design rules should take this peculiar behaviour into consideration, to achieve the maximum benefit in the use of this class of materials in fatigue-critical applications.     

Fatigue analysis of non-load-carrying fillet welded cruciform joints

The goal of this investigation was to study the effect of local geometrical variations of the weld on the fatigue strength. Therefore the fatigue behaviour of non-load-carrying cruciform fillet welded joint under tensile loading has been studied parametrically. Several two-dimensional (2D) finite element models of the joint were analysed using plane strain linear elastic fracture mechanics (LEFM) calculations in order to get the magnification function M_k. A maximum tangential stress criterion was used to predict the crack growth direction under mixed mode K_I-K_II conditions. The derived M_k solution was then applied both for continuous weld toe cracks and also for semi-elliptical toe cracks at the deepest point of the crack front. An experimental crack aspect ratio development curve was used for propagating semi-elliptical cracks. The as-welded condition was assumed with the result that no crack initiation period was considered and stress ranges were fully effective. The Paris crack growth law was used to predict the growth rate. The effects of weld toe radius, flank angle and weld size on the fatigue strength were systematically studied. Finally, predicted fatigue strength values corresponding to different assumed crack sizes were compared with the available test results.     

Time-dependent behaviour of steel/CFRP double strap joints subjected to combined thermal and mechanical loading

Degradation of structural adhesives at elevated temperatures makes the time-dependent behaviour of adhesively-bonded steel/CFRP joints a critical issue for safety considerations of CFRP strengthened steel structures. This paper reports the examination of specimens at different load levels (i.e. 80%, 50%, and 20% of their ultimate load measured at room temperature) and constant temperatures from 35 °C to 50 °C (i.e. temperatures below and above the glass transition temperature T_g, 42 °C of the adhesive). Furthermore, a scenario of cyclic thermal loading between 20 °C and 50 °C was included to represent more realistic exposure. Joint time-dependent behaviour was demonstrated by the stiffness and strength degradation as a function of not only temperature but also time. At the same temperature level close to or above T_g, a higher load level corresponded to a shorter time-to-failure. In addition, up to 47% of strength recovery was found for the specimens subjected to cyclic temperatures compared with those under constant 50 °C which failed at the same load level. Based on the proposed temperature and time-dependent material property models, the time-dependent failure time of steel/CFRP double strap joints was well described and validated by the experimental results.     

Fatigue behavior of externally bonded steel fiber reinforced polymer (SFRP) for retrofit of reinforced concrete

Steel fiber reinforced polymer (SFRP) strips comprised of multiple high-strength wires have been introduced into the repertoire of the structural engineer in recent years. The deleterious effects of fatigue loading on FRP-to-concrete bond have been identified in previous studies by the author; therefore the effect of fatigue loading on the bond behavior of SFRP is investigated. Four large-scale beam specimens (4.9 m long) having externally bonded SFRP retrofits are tested. These specimens are paired with unretrofit and CFRP-retrofit companion specimens allowing a number of direct comparisons to be made. Of the SFRP specimens, one is tested in monotonic loading to failure while the remaining three are tested at various fatigue load levels ranging from service load level to an extreme load level. Service load fatigue is cycled for 2 million cycles and the specimen is then tested monotonically to failure to assess the effects of fatigue conditioning on the ultimate performance of the beam. Extreme loading is selected to result in fatigue-induced failure of the internal reinforcing steel.     

Fatigue behaviour of clinched joints in a steel sheet

Clinch joining has been used in sheet metal work owing to its simplicity and because it facilitates the joining dissimilar metal sheets. In this study, monotonic and fatigue tests were conducted using coach‐peel and cross‐tension type specimens to evaluate the fatigue strength of clinch joints in a cold‐rolled mild steel sheet. The monotonic experimental results reveal that the coach‐peel specimen exhibits the lowest monotonic strength among the three specimen configurations. The coach‐peel and cross‐tension specimen geometries exhibit very low fatigue ratios, compared with the tensile‐shear specimen. The maximum von Mises and principal stresses at the fatigue endurance limit are much higher than the engineering tensile strengths of the steel sheet used to determine the three specimen geometries. Compared with the effective stress and maximum principal stress, the Smith–Watson–Topper fatigue parameter can be used for an appropriate prediction of the current experimental fatigue life. With regard to the coach‐peel specimen geometry, all samples exhibit pull‐out failure mode in the fatigue testing range. However, for the cross‐tension specimen geometry, mixed (pull‐out and interface) and interface failure modes occurred, depending on the number of cycles to failure.     

Fatigue strength of CFRP strengthened welded joints with corrugated steel plates

This paper presents an experimental and numerical research on the carbon fibre reinforced polymer (CFRP) strengthened welded joints with corrugated plates. The effectiveness of the strengthening in the improvement of fatigue strength has been examined experimentally on the test joints through varying the number and the layout of the CFRP laminates. The test results show that the joints with transition curvature region reinforcement and single side reinforcement produce slightly lower rigidity but longer fatigue life in contrast to those with full width reinforcement on the double side of the main plate. Furthermore, a simplified two dimensional analytical model which allows for the geometric characteristics of the joint has been proposed to investigate the stress intensity factor of mode I. The proposed analytical model has been simulated by finite element technique and its solution result is compared with previously reported theoretical calculation. Parametric studies have been performed to investigate the effects of the number of CFRP layers and the moduli of carbon fibre & adhesive on the stress intensity factor. The combined influence of the corrugation angle and crack depth has also been considered. It has been found that these effects on the stress intensity factor are more significant for the joints with smaller corrugation angle.     

Effects of ultraviolet radiation and associated elevated temperature on mechanical performance of steel/CFRP double strap joints

Although extensive studies of the bond between steel and CFRP under various environmental exposures have been carried out, the effects of UV have not been examined to date. This knowledge gap is addressed in this paper. Specimens (epoxy adhesive, CFRP laminates and steel/CFRP adhesively-bonded joints) were exposed to UV for various time periods and identical reference specimens were exposed to only thermal environments without UV. It was found that UV exposure does not influence the tensile strength of CFRP composites. The tensile strength of the adhesive reduced by 13.9% while modulus showed a significant increase by 105% after 744 h of exposure. The tensile modulus of adhesive exposed to only thermal environment also increased by 38%, considerably less than that induced by UV exposure. The UV exposure also led to a decrease in joint strength but an increase in stiffness, caused by the temperature effect rather than the UV rays.     

Prediction of debonding strength of tensile steel/CFRP joints using fracture mechanics and stress based criteria

In this paper, the debonding strength of axially loaded double shear lap specimens between steel plates and carbon fibre reinforced plastic plates is investigated from the analytical, numerical and experimental point of view. Two steel plates were joined together by two carbon fibre reinforced plastic (CFRP) plates and epoxy adhesive in order to realize double shear lap specimens of different length. Failure of the steel-adhesive interface was identified as the dominant failure mode and fracture mechanics and stress based approach are presented in order to estimate the relevant failure load. A good agreement between the analytical-numerical results and experimental data is achieved.     

Fatigue of high strength steel joints welded with low temperature transformation consumables

In the present paper constant (CA) and variable amplitude (VA) fatigue testing have been carried out on out-of plane gusset fillet welded high strength steel joints. The joints were welded with conventional weld filler material and martensitic low transformation temperature weld filler, LTT, in order to study the influence of the residual stress on the fatigue strength. Residual stress measurements were carried out close to the weld toe using X-ray diffraction technique in order to study the relaxation due to VA fatigue. The residual stress showed different level of relaxation depending on the VA spectrum loading used. The LTT joints show 40% increase in mean fatigue strength compared to the conventional joints in CA. The LTT joints show 12% increase in mean fatigue strength compared to the conventional joints. The LTT joints show 33% increase in mean fatigue strength in CA compared to VA testing. However, the improvement of the fatigue strength is less significant in variable amplitude testing mainly due to the relaxation of the compressive residual stresses.     

Fatigue behaviour of friction welded medium carbon steel and austenitic stainless steel dissimilar joints

This paper reports the fatigue behaviour of friction welded medium carbon steel–austenitic stainless steel (MCS–ASS) dissimilar joints. Commercial grade medium carbon steel rods of 12 mm diameter and AISI 304 grade austenitic stainless steel rods of 12 mm diameter were used to fabricate the joints. A constant speed, continuous drive friction welding machine was used to fabricate the joints. Fatigue life of the joints was evaluated conducting the experiments using rotary bending fatigue testing machine (R = −1). Applied stress vs. number of cycles to failure (S–N) curve was plotted for unnotched and notched specimens. Basquin constants, fatigue strength, fatigue notch factor and notch sensitivity factor were evaluated for the dissimilar joints. Fatigue strength of the joints is correlated with microstructure, microhardness and tensile properties of the joints.     

Effect of fatigue loading on the bond behaviour between UHM CFRP plates and steel plates

Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism.     

CFRP约束钢管-活性粉末混凝土短柱轴压性能

为研究圆碳纤维增强树脂复合材料(CFRP)约束钢管-活性粉末混凝土(RPC)短柱的轴压性能,以CFRP粘贴层数和钢管壁厚为参数进行了12根CFRP约束钢管-RPC短柱、4根钢管-RPC短柱及4根钢管短柱的轴压力学性能试验。通过荷载-位移曲线分析了CFRP层数和钢管壁厚对试件极限荷载和变形能力的影响,探讨了提高系数、CFRP应变效率和延性系数等相关性能指标,最后通过提高系数关联套箍率提出了CFRP约束钢管-RPC短柱承载力模型。结果表明:CFRP约束能有效地增强钢管-RPC短柱的承载能力和变形能力。与CFRP约束钢管-混凝土相比,CFRP约束钢管-RPC表现出CFRP应变效率的下降,并且其延性不如CFRP约束钢管-混凝土。在钢管-RPC承载力的基础上提出了实用的CFRP约束钢管-RPC短柱轴压承载力计算模型。      

A fatigue reliability analysis method including super long life regime

An affordable and feasible method with moderate accuracy is developed to realize fatigue reliability assessment and life prediction including super long life regime (SLLR) through series of experimental researches on a railway axle steel and real axles. A competition damage mechanism for fatigue crack initiation and growth in SLLR is revealed to fascinate an understanding on wide fatigue damage behavior and to provide a weigh and balance on material primary quality control and on-line inspection capacity. Affordable material probabilistic strength-life (S-N) curves including SLLR are presented by an extrapolation approach on a concurrent probability rule between the S-N relations in mid-long life regime and the fatigue limits with a specified life definition. And then, structural probabilistic S-N curves are deduced by considering scale-induced effect on the material curves. Random cyclic stress-strain (CSS) relations are depicted for constructing structural random stressing history. Reliability assessment and fatigue life prediction are conducted by an interference model of the applied stress deduced from the random CSS relations and the strength capacity derived from the structural probabilistic S-N curves. Availability and feasibility of the present method are indicated by a successful application on a railway axle steel.     

Fatigue strength analysis of welded joints in closed steel sections in rail vehicles

A great need exists for practicability and reliability analyses of the various dimensioning concepts in railway vehicle production, as currently parts are commonly sized according to the nominal stress concept. Although Finite Element Analysis is used for the dimensioning of parts according to the nominal stress concept, no use is made of the locally resolved information from these calculation methods. Concepts based on local stress are highly applicable, as the Finite Element Analysis allows detailed modelling of critical areas of parts.