Fatigue Lifetime Of Joints Research Articles

Experimental study of the impact of chamfer and fillet in the frontal edge of adherends on the fatigue properties of adhesive joints subjected to peel

The paper presents the influence of simple structural modifications of the adherend on the fatigue properties of adhesive joints subjected to peel. The considered modifications consisted in making a chamfer and fillet on the front edge of the adherend. The purpose of such modifications was to locally increase the thickness of the adhesive layer in the area of stress concentration. Fatigue strength tests were carried out using an electrodynamic shaker at the resonant frequency of the flexible adherend. On the basis of fatigue strength tests carried out at the limited number of cycles equal to 2×106, it was shown that a local increase in the thickness of the adhesive layer in the front part of the joint allows a significant increase in the joint's fatigue lifetime and fatigue strength. The greatest effect was shown for the variant with the fillet R2. In this case, an increase in fatigue strength of 33.1% compared to the base variant was demonstrated. For the fatigue stress level of 20.25 MPa, an increase in fatigue lifetime of 337.7% was also demonstrated. Based on the conducted research, it was shown that the reason for the improvement of the fatigue properties of the joints due to the local increase in the thickness of the adhesive layer is the phenomenon of energy absorption in the frontal area of the joint. Absorption of energy that inhibits the process of fatigue results from, among others, local flexibility of the joint, as well as nucleation of cracks in the locally increased volume of the adhesive.

Multiaxial static and fatigue behaviour of elastic and structural adhesives for railway applications

Adhesive bonding has become a fundamental manufacturing technology in the railway industry by allowing lightweight design of structures, which is beneficial in terms of energy, environmental and cost efficiency. Due to the polymeric nature of adhesive materials, the presence of stress multiaxiality has a strong effect on their mechanical behaviour under static and fatigue conditions. The present work deals with an investigation of the multiaxial behaviour of an elastic (polyurethane-based) and a structural (epoxy-based) adhesive for railway applications. Samples with different stress multiaxialities (butt joint, scarf joint, and thick adherend shear test joint) were tested under static and fatigue conditions. The stress multiaxiality was defined as the ratio between the hydrostatic stress (p) and the von Mises equivalent stress (q). Finite Element Analysis showed that the mechanical properties of adhesives have a strong influence of the stress multiaxiality distribution of joints with elastic adhesive reaching higher levels of stress multiaxiality. Static tests revealed that elastic adhesive joints are more sensitive to multiaxiality (i.e. higher hydrostatic stresses) than their structural counterparts, especially in samples with larger hindering of lateral contraction. From fatigue test results of both adhesives, namely SN curves, it was demonstrated how multiaxial p-q fatigue diagrams can be constructed. One of the main advantages of this approach is the possibility of predicting the fatigue lifetime of joints regardless of their multiaxial stress state.

The influence of heat treatment on the behavior of fatigue crack growth in welded joints made of S355 under bending loading

The work concerns on the results of the tests of the growth of the fatigue crack experiment. All specimens were subjected to bending at constant load using S355 steel. Rectangular cross-section specimens were weakened by external notches and fillet welded joints. The tests were carried out under cyclic bending without and with mean load values. Additionally, the impact of heat treatment on fatigue lifetime was investigated. Crack initiation and propagation stages were observed on the specimens’ surface using a light microscope. This research also shows microstructures with cracks of welded connectors and with changes in the material after heat treatment as well as their effect on the fatigue lifetime of joints. The highest durability was demonstrated by specimens without heat treatment. On the other hand, a similar fatigue crack growth mechanism was observed in different weld types. The numerical analysis also explains similar fatigue crack growth mechanism by equivalent stress intensity factors values. The fatigue life of welded specimens without heat treatment for both stress R-ratios was higher than for specimens after heat treatment.

Numerical study on the ideal interference of aircraft riveting joints

The interference is the main parameter affecting the fatigue lifetime of the interference-fit joints. In this paper, the numerical simulation method of the interference-fit riveting process was studied for the object of the aluminium alloy joint commonly used in aircraft. On this basis, the influence of different interferences on the joint life was calculated and determined, which can provide a basis for determining the ideal riveting interference. The numerical simulation results show that the model built in this paper can well describe the whole forming process of the rivet including the overall upsetting and local upsetting. With the increase of the interference, the fatigue lifetime of the joint increases first and then decreases. For the joint structure given in this paper, when the interference is between 2.9% and 3.23%, a higher life increment can be obtained, which is basically consistent with the ideal interference obtained by experiments.

Estimation of the Lifetime of Bonded Joints under Cyclic Loads at Different Temperatures

The fatigue lifetime of joints bonded with a toughened epoxy adhesive has been estimated using the stress-life method, based on experimental SN curves. Five temperatures (35°C, −10°C, RT, +50°C, +80°C) were considered. The estimations were performed using a direct approach, for joints having near-homogeneous stress distributions (scarf joints), and; an optimised approach, for joints having inhomogeneous stress distributions (single lap joints). The stress distributions were calculated considering linear-elastic material behaviour using analytical methods and finite element analysis. For inhomogeneous stress distributions the effective stress was defined in terms of the theory of critical distances. The calculations were performed considering: (i) uniaxial stress, evaluating the maximum principal stress, and considering (ii) multiaxial stress, for the influence of the relationship between hydrostatic and deviatoric stresses. The estimations were validated with fatigue experiments under tension–tension loading at a stress ratio of 0.1 and a frequency of 7 Hz. The experimental results showed a reduction of fatigue lifetime with increasing temperature. The estimated values indicate a better quality of prediction for the multiaxial stress and that it is more preferable to use a base SN curve having a stress distribution as similar as possible to the stress distributions of the components to be designed.

IMC Growth Mechanisms of SAC305 Lead-Free Solder on Different Surface Finishes and Their Effects on Solder Joint Reliability of FCBGA Packages

Intermetallic compound (IMC) growth behavior of lead-free solder plays an important role in ball grid array (BGA) solder joint reliability for flip chip BGA (FCBGA) packaging applications. The growth mechanism of IMC is reported based on a diffusion model. Thermal treatment such as accelerated thermal cycling (ATC) and isothermal aging exposure also contribute to the growth rate and morphology of lead-free solder IMC. Among the lead-free solder alloys, Sn-3.0wt.%Ag-0.5wt.%Cu (SAC305) solder is a promising substitute for Sn-Pb because of its good mechanical properties and wettability with current surface finishes. After thermal exposure, BGA solder joint reliability is degraded due to IMC formation and growth. In this study, two different thermal treatments, ATC and isothermal aging, and two different pad surface finishes, solder on pad (SOP) and electroless Ni immersion gold (ENIG), are considered in terms of IMC growth rate and mechanical solder joint reliability. An SOP finished interface forms a thin ε-phase Cu3Sn layer and a scallop-like η-phase Cu6Sn5 layer. In contrast, the ENIG finished interface forms a thick (Cu,Ni)6Sn5 IMC layer and prevents overall IMC growth. Different surface finished test vehicles are evaluated in an ATC test in a 0°C to 100°C temperature range and the Ni diffusion layer shows a longer solder joint fatigue lifetime than the nondiffusion barrier interface based on the micro cross-section and dye penetration analysis results. In an isothermal aging test at 100°C and 150°C, the aging temperature and time are valid factors to decide mechanical shock reliability. Interfacial fractures are found in the 100°C aged test vehicle due to easier crack propagation at the interface between the thin Cu3Sn layer and the scallop-like Cu6Sn5 layer based on SEM microstructure analysis results. Finally, this investigation proposes how to improve solder joint reliability and prevent interfacial fracture for SAC305 lead-free application.

Effects of solder joint shape and height on thermal fatigue lifetime

Solder joint thermal fatigue failure is a major concern for area array technologies such as flip chip and ball grid array technologies. Solder joint geometry is an important factor influencing thermal fatigue lifetime. In this paper, the effects of solder joint shape and height on thermal fatigue lifetime are studied. Solder joint fatigue lifetime was evaluated using accelerated temperate cycling and adhesion test. Scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), scanning acoustic microscopy (nondestructive evaluation) and optical microscopy were utilized to examine the integrity of the joint and to detect cracks and other defects before and during accelerated fatigue tests. Our accelerated temperature cycling test clearly shows that solder joint fatigue failure process consists of three phases: crack initiation, crack propagation and catastrophic failure. Experimental results indicated that both hourglass shape and great standoff height could improve solder joint fatigue lifetime, with standoff height being the more effective factor. Experimental data suggested that shape is the dominant factor affecting crack initiation time while standoff height is the major factor influencing crack propagation time.

Effect of intermetallic compounds on the shear fatigue of Cu/63Sn-37Pb solder joints

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints has been studied by means of shear cycling. The samples consist of leadless ceramic chip carriers (LCCC) soldered onto FR-4 printed circuit boards (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste and then aged at 150/spl deg/C for 1, 4, 9, 16, 25, 36, and 49 days. The specimens are subjected to low cycle fatigue shear tests controlled by the displacement. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of the LMC layer between the Cu-pad and bulk solder, and the quantitative relationship between the lifetime and thickness can be described as a monotonically decreasing curve. The greatest decrease is over the thickness range up to 2.8 /spl mu/m, when the IMC/bulk solder interface becomes flat, corresponding to a lifetime decrease to 62% of the as assembled value. For further increase in IMC thickness the lifetime decreases more slowly. Evidently, the effect of the Cu-Sn intermetallic compounds on the joint fatigue lifetime is not only concerned with the IMC thickness hut also the interface morphology. A thick and flat LMC layer has a deleterious effect. The results of X-ray diffraction and metallographic analysis show that cracks initiate underneath the component metallization, and propagate along the IMC/solder interface, then toward the fillet. The Cu/sub 3/Sn (/spl epsiv/-phase) is formed between the Cu-pad and p-phase, and grows more quickly than the /spl eta/-phase during storage and long term operation or aging tests. However, the Cu/sub 3/Sn makes only a small direct contribution toward fatigue failure.