A study of the effect of filler metal thickness on tensile strength for a stainless steel plate-fin structure by experiment and finite element method

This paper presents a study of the effect of filler metal thickness on tensile strength for a stainless steel plate-fin structure by finite element method and experiment. The results show that the filler metal thickness has a great effect on tensile strength. The tensile strength is increased with the filler metal thickness increase, then it keeps stable when the filler metal thickness is 105–140 μm. But it decreases rapidly when the filler metal thickness is larger than 140 μm. The fracture location is shown at the end of vertical fin when the filler metal thickness is 105–140 μm. Specimens with filler metal thickness smaller or larger than 105–140 μm rupture in the brazed filler metal. The optimal filler metal thickness is 105 μm, using which can get higher strength for 304 stainless steel plate-fin structures.     

Effect of brazing temperature on tensile strength and microstructure for a stainless steel plate-fin structure

This paper presented a vacuum brazing technology for 304 stainless steel plate-fin structures with BNi2 filler metal. The effect of brazing temperature on tensile strength and microstructure has been investigated. The tensile strength is increased along with the increasing of brazing temperature. The microstructure is very complex and some Boride compounds are generated in the brazed joint. Full solid solution can be generated in the middle zone of joint when the brazing temperature is increased to 1100 °C. The brittle phases always exist in the fillet no matter how the brazing temperature changes, but the microstructure in fillet becomes more uniform and the tensile strength is increased with the brazing temperature increasing. In total, the brittle Boride compounds are decreased with the brazing temperature increase. Brazing with a filler metal thickness 105 μm and 25 min holding time, 1100 °C is the best suitable brazing temperature and a tensile strength of 82.1 MPa has been achieved for 304 stainless steel plate-fin structure.     

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.     

Fatigue life prediction of carbon fibre reinforced laminates by using cycle-dependent classical laminate theory

In this work a study about the adaption of the classical laminate theory for fatigue loads is presented. Cycle dependent stiffnesses of single UD 0°, UD 45° and UD 90° plies are implemented in order to calculate the fatigue-induced stiffness decrease of a multidirectional lay-up with the stacking sequence [0°/+45°/−45°/90°/90°/−45°/+45°/0°]. As second input alternative, UD 0°, UD 90° and ±45° plies are used. The calculated cycle-dependent stiffness parameters are compared to experimentally measured fatigue data of the multidirectional lay-up. The experimental test procedure used for the measurement of cycle-dependent stiffness parameters has been published previously. Results show that the experimentally measured stiffness decreases of the multidirectional lay-up can be estimated accurately based on the cyclic unidirectional input parameters.     

On the fatigue life prediction of CFRP laminates using the Electrical Resistance Change method

The electro-mechanical response (Electrical Resistance Change method) as a damage index of quasi-isotropic Carbon Fiber Reinforced (CFRPs) laminates under fatigue loading was investigated. The effect of dispersed Multi-Wall Carbon Nanotubes (MWCNT) into the epoxy matrix was additionally evaluated and compared with neat epoxy CFRPs. The longitudinal resistance change of the specimens was monitored throughout the fatigue experiment. Three different stress levels were tested. The frequency and the ratio (R) of the minimum applied load (stress) to the maximum applied load (stress) were kept constant for the different stress levels. The temperature of the specimen was also monitored throughout the process in order to deduce its effect on the electrical resistance of the specimen. The electrical behavior of the quasi-isotropic CFRP deviated from the commonly observed electrical response of unidirectional or cross-ply CFRPs due to the presence of the 45° layers. During initial stages of loading the resistance drops and afterwards it follows a positive slope up to final fracture. This repeatable pattern was observed for both the neat and the CNT-doped specimens, with the latter having smoother electrical recordings. The effect of temperature was calculated to be limited for the specific material and test/measurement configuration. The electro-mechanical response was correlated to stiffness degradation and acoustic emission findings enabling the identification of the specific regions during the fatigue life referring to specific mechanisms of damage accumulation. More specifically the experimental results revealed that the occurrence of the initial drop of the electrical resistance is linked with the occurrence of the Characteristic Damage State (CDS), associated with a specific percentage of stiffness reduction. This finding was used in order to predict the remaining life independently from the applied stress level with a high degree of confidence, assuming a constant stress level throughout the whole lifetime. The remaining life prediction for the CNT-doped specimens had higher coefficient of confidence (R²).     

Fatigue life prediction under variable loading based on a new damage model

To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T 6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This paper contains the results of mono-axial random load fatigue tests with different mean and amplitude values performed on 6082 T 6 aluminium alloy specimens. Cycles were counted with rainflow algorithm and damage was cumulated with a new model proposed in this paper and with the Palmgren–Miner model. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.     

Computational fatigue life prediction of continuously fibre reinforced multiaxial composites

The potential of a fatigue-life prediction method for continuously fibre reinforced carbon/epoxy laminates has been investigated. Stress analysis conducted with a finite element solver in combination with the experimentally measured anisotropic S–N curves was used as input parameters. Subsequently, lifetime of a unidirectional and a multidirectional composite was calculated for a cyclic tension–tension load case and validated with experimental fatigue tests. The predicted lifetime of the unidirectional laminate correlated well to the experimental results. For the fatigue-life calculation of multidirectional composites, the software underestimated the experimental data. Results and possible improvements based on the presented calculations are discussed in detail.     

In-situ repair of composite sandwich structures using cyanoacrylates

A novel method for the in-situ repair of composite sandwich structures using microvascular networks and cyanoacrylate (CA) adhesive systems has been presented. Upon a damage event, the vascules become ruptured, providing a route for the introduction of adhesive directly into the damage site. The efficacy of the two repair agents was first assessed under static and fatigue conditions using a modified double cantilever beam (DCB) method. Once baseline fracture behaviour of the cyanoacrylates has been established, they were further assessed by injection into a series of pre-damaged T-joint specimens. The presence of the vasculature was shown to have no detrimental impact on mechanical performance, whilst both of the cyanoacrylates were shown to be highly effective in the recovery of stiffness and ultimate strength of the T-joint specimens.     

Fatigue life prediction for stainless steel welded plate CCT geometry based on Lawrence''s local-stress approach

In the present study, the effect of welding process and procedure on fatigue crack initiation from notches and fatigue crack propagation in AISI 304L stainless steel welds was experimentally investigated. Full penetration, double-vee butt welds have been fabricated and CCT type specimens were used. Lawrence's local-stress approach (a two-stage model) is used to predict the fatigue life. The notch-root stress method was applied to calculate the fatigue crack initiation life, while the fatigue crack propagation life was estimated using fracture mechanics concepts. The fatigue notch factor is calculated using Lawrence's approach. Constant amplitude fatigue tests with stress ratio, R=0 were carried out using 100 kN servo-hydraulic DARTEC universal testing machine with a frequency of 30 Hz. The predicted lives were compared with the experimental values. A good agreement has been reached. It is found that the weld procedure has a stronger effect on lives to initiation than on propagation lives.     

Fatigue crack growth at the face sheet-core interface in a discontinuous ceramic-tile cored sandwich structure

The fatigue failure mechanism of a sandwich structure with discontinuous ceramic tile core is characterized. The sandwich structure in consideration comprises ceramic core tiles bonded to composite face sheet with a compliant adhesive layer. The discontinuous nature of the core results in a non-uniform stress field under in-plane loading of the sandwich. Static tensile tests performed on sandwich coupons revealed first damage as debonding at the gaps between adjacent tiles in the core. Tension–tension fatigue tests caused debonding at the gaps followed by initiation of cracks in the adhesive layer between the face sheet and core. Experimental data for crack length versus number of cycles is collected at various load levels. Crack growth rates (da/dN) are determined based on the experimental data acquired. The energy release rate available for crack propagation is computed using an analytical model and finite element analysis. Mode separation performed using the Virtual Crack Closure Technique (VCCT) revealed that crack propagation is completely dominated by shear (mode II). Fatigue crack growth behavior for the discontinuous sandwich structure is quantified by correlating the cyclic energy release rate with the rate of crack propagation. The loss of specimen stiffness with crack propagation is quantified using an analytical model.     

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.     

Experimental validation of modal strain energies based damage identification method for a composite sandwich beam

A vibration-based damage identification method, based on changes in modal strain energies before and after occurrence of damage, is presented for a composite sandwich beam. Experiments were performed to obtain the natural frequencies and mode shapes for validation of the presented method. The observed changes in modal strain energies were used for the prediction of existence and location of damage in a composite sandwich beam. Subsequently, these changes were also used to predict damage extents in the two stages. In the first stage, the proposed method is used to approximate the damage extents in the face and core. These were updated to obtain more accurate values of damage extent at the second stage by using the model updating theory. Experimental and numerical results were presented to demonstrate and verify the effectiveness of this method for several single and multiple damage cases. These damage cases also include interactive damage modes. Results indicate that the proposed method is capable of identifying the location and extent of damage in the faces and core of a composite sandwich beam.     

Fatigue failure of sandwich beams with face sheet wrinkle defects

This paper presents experimental fatigue results for GFRP face sheet/balsa core sandwich beams with face sheet wrinkle defects, subjected to fully reversed in-plane fatigue loading. An estimate of the fatigue design limit is presented, based on static test results, finite element analyses and application of the Northwestern University failure criteria. The presence of a wrinkle defect reduced the fatigue life by approximately 66%, compared to that of an unnotched reference laminate. Furthermore, the results from the fatigue tests revealed that the design limit was initially overestimated, as the specimens loaded close to the predicted design limit typically failed before reaching the target life, or reached test run-out with visible face sheet damage indicating imminent final failure in the worst case. It was found that specimens would reach target life with no visible or otherwise detectable damage by lowering the fatigue load amplitude below 80% of the predicted design limit. By extrapolating the test results it appears that the undamaged specimens would reach a fatigue life of 10⁷–10⁸ load cycles and would thus be safe for design of wind turbine blades.     

Fatigue life prediction at a ship deck/superstructure intersection

Difficulties arise in identifying an appropriate method to predict fatigue life for the weld joint at the intersection of a ship's deck and superstructure. The problem relates to the gross stress concentration and rapidly changing stress at this location. Variations on the 'hot spot' stress approach to fatigue life analysis are reviewed, and applied to results from an experimental programme of fatigue tests on welded joints. Finally tentative recommendations are made for a procedure to predict fatigue life of the structure.     

Tension,compression and shear fatigue of a closed cell polymer foam

A closed cell foam of polymetacrylimide (Rohacell) with three different densities is studied. The foam is tested quasistatically in tension, compression and shear. The tensile properties scale very well with the relative density of the foam, but the compression and shear properties do not scale the same way. It is believed to be due to cell edge and cell wall buckling being the dominated deformation mechanism in compression and shear for lower densities that does not occur for higher densities. Fatigue testing is then performed in tension, compression and shear. It is seen that for all load cases and densities, the fatigue life can be plotted using Basquin’s law. The results also show that the different failure mechanisms found in the static tests are the same in fatigue. This means that the fatigue life for different load types exhibit different failure mechanisms. This shows not only as a clear difference in the stress levels for fatigue failure, but also on the slope in the fatigue life relation.     

Fatigue life prediction of copper single crystals using a critical plane approach

A critical plane multiaxial fatigue criterion was employed to predict the fatigue life of copper single crystals. The detailed stress-strain response was obtained through the constitutive modeling using a newly developed crystal plasticity theory. The constitutive model was capable of capturing the major deformation features of copper single crystals under cyclic loading including the cyclic stress-strain curves, cyclic hardening behavior, and the evolution of the hysteresis loops with increasing number of loading cycles. Fatigue life prediction of the single crystal copper was conducted based upon the stress-strain response obtained from the cyclic plasticity model. The fatigue criterion takes into account the plastic strain localization within a single crystal. The critical plane (cracking plane) was identified as the material plane where the fatigue damage accumulation first reached a critical value. For copper single crystals with the crystal orientations being within the standard crystallographic triangle, the fatigue criterion can predict both fatigue life and cracking direction consistent with the experimental observations. More importantly, the constants used in the fatigue criterion were found to be identical to those used for the pure polycrystalline copper with different grain sizes and texture.     

Fatigue life prediction of cracked attachment lugs using XFEM

In the present paper, a three dimensional finite element method (FEM) is used to compute the stress intensity factor (SIF) in straight lugs of Aluminum 7075-T6. Extended finite element method (XFEM) capability available in ABAQUS is used to calculate the stress intensity factor. Crack growth and fatigue life of single through-thickness and single quarter elliptical corner cracks in attachment lug are estimated and then compared with the available experimental data for two different load ratios equal to 0.1 and 0.5. The SIF calculated from XFEM shows that the introduction of different loading boundary conditions significantly affect the estimated fatigue life.     

Fatigue life prediction of a structural steel under service loading

Fatigue life prediction techniques for variable amplitude load histories are reviewed. The fatigue crack growth rate and crack closure responses of BS4360 50B steel are determined for a service load history experienced by a gas storage vessel. Crack propagation rates are found to be independent of specimen thickness. Crack growth is successfully predicted by linear summation using the Paris law; no significant improvement is achieved by incorporating crack closure into the analysis. The particular choice of cycle counting technique is also found to have an insignificant effect on the predicted fatigue life. The load-interaction model proposed by Willenborg et al correctly indicates the absence of retarded growth, whilst the Wheeler and Führing models erroneously predict retarded crack growth.     

Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds

The effects of weld microstructure and residual stress distribution on the fatigue crack growth rate of stainless steel narrow gap welds were investigated. Stainless steel pipes were joined by the automated narrow gap welding process typical to nuclear piping systems. The weld fusion zone showed cellular–dendritic structures with ferrite islands in an austenitic matrix. Residual stress analysis showed large tensile stress in the inner-weld region and compressive stress in the middle of the weld. Tensile properties and the fatigue crack growth rate were measured along and across the weld thickness direction. Tensile tests showed higher strength in the weld fusion zone and the heat affected zone compared to the base metal. Within the weld fusion zone, strength was greater in the inner weld than outer weld region. Fatigue crack growth rates were several times greater in the inner weld than the outer weld region. The spatial variation of the mechanical properties is discussed in view of weld microstructure, especially dendrite orientation, and in view of the residual stress variation within the weld fusion zone. It is thought that the higher crack growth rate in the inner-weld region could be related to the large tensile residual stress despite the tortuous fatigue crack growth path.     

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.