Peel Stresses Research Articles

Influence of Directional Random Vibration on the Fatigue Life of Solder Joints in a Power Module

In this paper, fatigue life of solder joints of the power module and the printed circuit board in a power inverter under directional random vibration using the finite element method (FEM) was evaluated. Five angles of 0°, 30°, 45°, 60°, and 90° were selected for direction of vibration loading. The simulation results showed that the outermost corner of the solder layer undergoes the maximum stress during the vibration frequencies in all the situations. In general, with the increase of input frequency, the solder joints got more susceptible to the catastrophic failure. Moreover, it was found that in lower angles of loading direction, the peeling stress is dramatically increased in the solder layer. This event can be attributed to the considerable portion of shear stress in loading directions with lower angles. The experimental results also proved the FEM simulation and showed that the void growth and coalescence, resulted by fatigue, are intensified in the lower angles of vibration loading. Overall, this paper indicates that the reliability of a power module under a normal loading (90°) is the highest among all other conditions.

Geometrical and material optimization of tensile loaded tubular adhesive joints using cohesive zone modelling

ABSTRACTBonding with adhesives is increasingly being used in the design of mechanical structures, because of the significant advantages of this technique compared to traditional joints. Different joint configurations are available, depending on the desired bond strength to be achieved and geometry of the parent structures. Tubular joints find applications in the piping industry, vehicle frames or thin-walled tubes, for instance, but they are seldom studied in the literature. This work numerically compares the performance of three structural adhesives on the tensile strength of aluminium tubular joints, after validation of the numerical tool with experiments. The numerical analysis consisted of using the Finite Element (FE) method to analyse peel (σy) and shear stresses (τxy) in the adhesive layer and cohesive zone models (CZM) to predict the maximum load (Pm). Numerically, the effect of the overlap length (LO) and the thickness of the inner and outer tubes (tSI and tSE, respectively) is addressed. Additionally, outer and inner chamfering of the tubes was tested to reduce peak stresses and increase Pm. The CZM technique was positively validated for the strength analysis of tubular joints. It was also shown that the joints’ geometry and type of adhesive highly influence the joints’ behaviour.

Extended finite element modelling of aluminium stepped-adhesive joints

ABSTRACTFor the joint strength prediction of bonded joints, Fracture Mechanics-based techniques are often used. In this context, the tensile and shear toughness of the adhesives are two of the most important parameters to predict the joint behaviour. The Finite Element Method has been used for strength prediction in the last decades. Cohesive zone modelling coupled to a FEM analysis is generally accepted as an accurate method. More recently, the Extended Finite Element Method (XFEM) has emerged. This work aims to validate the XFEM to predict the behaviour of stepped-lap joints with different overlap lengths. Three adhesives, Araldite® AV138, Araldite® 2015 and Sikaforce® 7752, whose properties are quite different, were tested. Peel and shear stresses in the bondline were analysed, which allows an analysis of the behaviour of the three adhesives under different geometrical conditions. For the XFEM strength prediction, different damage initiation criteria were used, based either on stresses or strains. The damage law shape was also evaluated, namely the linear and exponential damage propagation laws. The XFEM was found to be adequate to predict the joint strength using the Quadratic Stress and Maximum Stress damage initiation criteria.

Analysis of mortars used in the rehabilitation of wall coating

Two types of lime-based mortars were used in the rehabilitation of wall coverings, and the need to study their mechanical behavior and durability arose.About the mechanical behavior were analyzed flexural strength, compressive strength and the peeling stress at the "pull-off" test.As to durability, particularly in view of the rising damp and infiltration, this study was performed by capillary water absorption test and immersion due to water absorption at 48 (atmospheric pressure) for the determination of water content, focusing on its state of degradation, in order to mitigate the risks of choice.In this study, the results obtained in the tests of a traditional lime mortar with a 1: 3 volume trace with the samples of a prefabricated mortar of hydrated lime were compared, and in both mortars obtained values close to those recommended.However, the traditional hydraulic lime mortar has a peel force resistance well above the prefabricated mortar, and the recommended amount in domestic articles.

Static strength improvement of tubular aluminium adhesive joints by the outer chamfering technique

Tubular adhesive joints find applications in the piping industry, vehicle frames or thin-walled tubes, for instance, but they are seldom studied in the literature. This work numerically assesses the maximum load (Pm) of aluminium tubular joints, by using an outer chamfer in the tubes in the overlap region, after validation of the numerical tool with experiments. The numerical analysis consisted of using the Finite Element (FE) method to analyse peel (σy) and shear stresses (τxy) in the adhesive layer and cohesive zone models (CZM) to predict Pm. The CZM technique was positively validated for the strength analysis of tubular joints. It was also shown that the chamfer highly affects the joints’ behaviour, and that an optimal configuration exists that enables maximum Pm results.

Modelling of tubular adhesively-bonded joints by the Extended Finite Element Method

This work aims at a parametric study of tubular adhesive joints with different adhesives and different overlap lengths (Lo) by the eXtended Finite Elements Method (XFEM). The main objective is to evaluate the strength prediction abilities of the XFEM. The parametric numerical study also allowed a critical evaluation of the distribution of peel (σ) and shear stresses (τ) for Lo=20 and 40 mm. It was concluded that, with a correct choice of damage initiation and propagation criteria, the XFEM is a viable and accurate method for the strength prediction of tubular joints.

Effect of material hybridization on the strength of scarf adhesive joints

Adhesively-bonded joints have become more efficient due to the improvement of adhesives’ characteristics. On the other hand, with the use of composites in structures it is possible to reduce weight. Due to this, new techniques are being explored, including adhesively-bonding different materials. Nowadays, in many high performance structures, it is necessary to combine composite materials with other light-weighted metals such as aluminium or titanium. This work reports on an experimental and numerical study for hybrid scarf joints between composite and aluminium adherends, and considering different values of the scarf angle (α). The numerical analysis by Finite Elements (FE), using the software Abaqus®, enabled the obtainment of peel (σy) and shear stresses (τxy), which are then used to discuss the strength between different joint configurations. Cohesive zone modelling (CZM) was used to predict the joint strength and the results were compared to the experiments for validation. The joints’ behaviour was highly dependent on α, and CZM were validated for the design process of hybrid scarf joints.

Cohesive Zone Analysis of Tubular Adhesively-Bonded Joints

Bonded joints are also widely used to join tubular components in the pipeline industry, in vehicle frames and in space structures. This work performs an experimental and numerical study of axially-loaded tubular joints between aluminium adherends and bonded with three different adhesives. The effect of the overlap length between inner and outer tubes (LO) was addressed in the experiments and numerical study. A Finite Element Method (FEM) analysis was undertaken to analyse peel (σ) and shear stresses (τ) in the adhesive layer. Cohesive zone models (CZM) were employed to predict the joint strength. The CZM technique was positively validated for the strength analysis of tubular joints.

Near free-edge stresses in FRP-to-concrete bonded joint due to mechanical and thermal loads

Over the last few decades, a considerable amount of theoretical and experimental investigations have been conducted on the mechanical strength of composite bonded joints. Nevertheless, many issues regarding the debonding behavior of such joints still remain uncertain. The high near free-edge stress fields in most of these joints are the cause of their debonding failure. In this study, the performance of an externally bonded fiber-reinforced polymer (FRP) fibrous composite to a concrete substrate prism joint subjected to mechanical and thermomechanical loadings is evaluated through employing the principles of lamination theory. An inclusive Matlab code is generated to perform the computations. The bond strength is estimated to take place in a region- also termed the boundary layer- where the peak interfacial shearing and transverse peeling stresses occur; whereas the preceding stress field is observed to be the main failure mode of the joint. The proposed features are validated through the existing experimental data points as well as the commercial finite element (FE) modeling software Abaqus. Comparison between the calculated and experimental results demonstrates favorable accord, producing quite a high average ratio. The current approach is advantageous to failure modeling analysis, optimal design of bonded joints, and scaling analyses among others.

Spring Contact Model of Tape Peeling: A Combination of the Peel-Zone Approach and the Kendall Approach

Energy-based and force-based approaches are two basic ways to establish an adhesion model. For the adhesion of tape-like thin films, the Kendall equation considers the overall energy balance but inherently contains little information of the peel zone geometry and stress distribution. The peel zone model provides an empirical approximate of the peel front from the approach of a force description and coincides well with experimental results for a wide range of peel angles. However, the peel-zone model itself has not been unified with the Kendall equation yet. We propose a two-layer spring contact tape peeling model which considers the balance between the stretching force of the backing layer and the adhesive force transferred through the adhesive layer. The model provides an analytic shape description of the curved bifurcation region of the peel front. An approximate analytic solution of the peel force reduces to the Kendall equation by considering a Kendall-like energy conservation critical criterion, which further supports the proposed model. Further analysis of the relationship between the length of the peel zone and the adhesive force provides insight into the validity of the peel zone model. The proposed model provides a new insight in the tape peeling process and mathematically builds a potential bridge between the Kendall model and the peel-zone model.

Surface toughening – a concept to decrease stress peaks in bonded joints

ABSTRACTBy using fiber-reinforced plastics as the material for lightweight structures, adhesive bonding becomes an increasingly focused solution for joining these components. In comparison to bolted joints, adhesive bonding has a lot of known benefits, e.g. sealing, no weakening of adherends and a homogeneous load transfer. However, the stress distributions in single-lap joints are not homogeneous all over. Both the shear and peel stress distribution have a peak at the overlap ends of the joint and a lower loaded part in the middle, as shown by Volkersen and Goland and Reissner. In order to reduce these stress concentrations, there are known concepts for the modification of the joint. In this paper, adherend chamfering, different adhesive spew fillet geometries, and the mixed adhesive joint are investigated and compared to a novel local adherend surface toughening concept by using a thermoplastic PVDF layer. A high-resolution digital image correlation is used to visualize the deformation in the bondline. While the state-of-the-art used concepts do not show a high increase of joint strength for single lap joints, the joint strength for the surface toughening specimens can be increased by up to 84%.

An analytical approach to characterize uniaxial in-plane responses of commercial hexagonal honeycomb core under large deformations

In this article, an analytical model is derived to investigate the in-plane homogenized stress-strain relationship of an adhesively bonded commercial hexagonal honeycomb core under large deformation. The model incorporates some advanced features of the core cell such as cell wall curvatures, node bond adhesive layers, and adhesive fillets at the cell wall intersections in the analysis. Nonlinear homogenized results of the fiberglass/phenolic honeycomb core predicted by the analytical model are compared with test data as well as predictions from finite element models (FEMs) of both real and idealized (without modeling the node bond adhesive) core cells to investigate the effects of the node bond adhesive. Predictions of the analytical model are in good agreement with the test data and predictions of the FEM of the real core cell. Homogenized properties of the honeycomb core obtained from the analytical model at infinitesimal strains are compared with several analytical models from the literature. Adhesive peel stress is also calculated by the analytical model and compared with the FEM.

Application a direct/cohesive zone method for the evaluation of scarf adhesive joints

With the increasing use of structures with adhesive bonds at the industrial level, several authors in the last decades have been conducting studies concerning the behaviour and strength of adhesive joints. Between the available strength prediction methods, cohesive zone models, which have shown good results, are particularly relevant. This work consists of a validation of cohesive laws in traction and shear, estimated by the application of the direct method, in the strength prediction of joints under a mixed-mode loading. In this context, scarf joints with different scarf angles (α) and adhesives of different ductility were tested. Pure-mode cohesive laws served as the basis for the creation of simplified triangular, trapezoidal and exponential laws for all adhesives. Their validation was accomplished by comparing the numerical maximum load (Pm) predictions with the experimental results. An analysis of peel (σ) and shear (τ) stresses in the adhesive layer was also performed to understand the influence of stresses on Pm. The use of the direct method allowed obtaining very precise Pm predictions. For the geometric and material conditions considered, this study has led to the conclusion that no significant Pm errors are incurred by the choice of a less appropriate law or by uncoupling the loading modes.

A review on stress distribution, strength and failure of bolted composite joints

In this study, analytical models considering different material and geometry for ‎both single and double-lap bolted joints were reviewed for better understand how to ‎select the proper model for a particular application. The survey indicades that the ‎analytic models selected for the adhesively single or double bolted lap joints, as well ‎as T, scarf, and stepped joints, with linear material properties are mostly two ‎dimensional and the studies on stress distribution and/or failure of the joint are ‎performed either experimentally, analytically or by finite element method. The results ‎seem to be generally accurate and adequate. Additionally, it was shown that any ‎increase in the bolt-hole clearance leads to an increase in bolt rotation, as well as a ‎decrease in bolt-hole contact area, and hence, a reduction in joint stiffness. Moreover, ‎studies on hybrid joints have revealed that the proper choice of adhesive material in ‎conjunction with bolts or rivets in a joint, allows for significant increase in the static ‎and fatigue strength compared to similar pure bonded joints. Additionally, the results ‎on hybrid scarf joints showed that it is vital to place fasteners closer to the ends of the ‎overlap to suppress the peak peeling stresses and hence, delay the effects of early ‎crack initiation in the adhesive layer...

Effects of Thermo-Mechanical Fatigue and Low Cycle Fatigue Interaction on Performance of Solder Joints

This paper demonstrates the feasibility of low cycle fatigue effects on the thermo-mechanical fatigue evolution in the solder joints of a power module. To achieve this goal, finite-element method (FEM) simulation and experimental studies have been carried out. One of the prepared samples was exposed to the sole thermal cycling while another one underwent frequent drop impacts, as an indicator of low cycle fatigue event, during the thermal cycling. The FEM results indicate that the thermal cycling leads to the accumulated creep strain in the solder joints. The amount of creep strain considerably increases when the drop impacts are coupled to the thermal cycling. This phenomenon is due to the additional peeling stress applied by drop impacts to the solder joints. It is also revealed that with an increase in the number of drop impacts, the rate of strain accumulation declines which may be due to the restriction of changes led by stress triaxiality. The fractography also validates that the sample exposed to both drop and thermal loadings shows an entire brittle fracture mode. Moreover, the EDS map shows that the elemental distribution heterogeneity and intermetallic growth were created under mutual effects of drop impacts and thermal cycle fatigue.

Static strength of RTM composite joint with I-fiber stitching process

Resin transfer molding (RTM) is a mass production process that can replace autoclave processes, and composite lap joints are extensively used in composite structures. When a tensile load is applied to a single-lap joint, both shear and tensile peel stresses are generated owing to the eccentric load effect. Reinforcing the composite joint in the thickness direction can reduce the shear and peel stresses generated in the joint, which can contribute considerably to the increase in the strength of the composite joint. Several reinforcing methods have been developed to improve the directional properties along the material’s thickness. Accordingly, a stitching process is typically used. However, the conventional stitching process has disadvantages because a) it requires complex equipment, and b) it cannot use highly elastic brittle fibers, such as carbon fibers. Recently, we proposed an I-fiber stitching process to minimize the bending of carbon fibers to prevent their fracture.In this study, composite, single-lap joint specimens were fabricated with RTM using an I-fiber stitching process, and their strengths were evaluated. The strengths of composite joint specimens fabricated at different stitching intervals and with the use of different patterns were compared with those of specimens fabricated without the use of the stitching process.

Improving the reliability of ball grid arrays under random vibration by optimization of module design

In this work, three different arrangements of circuit board in an electronic device were designed and the effects of random vibration frequencies on the reliability of ball grid arrays (BGA) in these arrangements were evaluated. The failure criterion in solder balls was the root mean square of peeling stress exerted during the dynamical loadings. According to the finite element method (FEM) results, the uttermost stress concentration was generated at the interface of printed circuit board (PCB) and the solder balls. It was also revealed that the increase of input power spectral density (PSD) decreased the fatigue life of solder joint in all the arrangements. Considering the arrangements of circuit boards, it was found that the maximum domain of peeling stress had a minimum effect on the solder balls when the heat sinks were away from the packages. The microstructural characterization of critical zone in solder balls indicated that with the increase of maximum peeling stress, the crack initiated and propagated along the interface.

Investigation of Defect Effects on Adhesively Bonded Joint Strength Using Cohesive Zone Modeling

Abstract In this paper, effects of the defect in an adhesively bonded joint have been investigated using cohesive zone modeling. Consequently, a 3D finite element model of a single lap-joint is constructed and validated with experiments. Strength prediction of current model is found desirable. Accordingly, different sizes of square shape defects are imported to model in the form of changing (raised or degraded) material properties (heterogeneity) and locally delaminated areas (as inclusion/void), respectively. Joint strength is investigated and a stress analysis is carried out for adhesive layer and adherends. Obtained Results show that, defect has significant impact on the results. It is found that at constant size of defect, local delamination has more impact on bonded joint strength than the heterogeneity. Furthermore, stress analyses demonstrate that the stress field does not change in adherends by taking defects into account. However, stress values decrease with degraded material properties and joint’s strength. Through evaluation of peel and transverse shear stresses in adhesive layer it is found that there is a change of stress distribution for both types of defects. Whereas, there is a considerable stress concentration in the delaminated adhesive layer.

A strategy to reduce delamination of adhesive joints with composite substrates

The use of bonding for joining composite materials in high-performance structures has increased significantly, as this joining method offers improved stress distributions and capability of joining dissimilar materials. However, the use of adhesive bonding for this purpose might lead to delamination failure, caused by peel stresses acting on the generally weaker transverse direction of the composite adherends. This work focused on improving the resistance to delamination of composite adhesive joints by using a novel composite with a reinforced high toughness resin on the surfaces. Single-lap joints using the novel composite material as adherends, were found to have 22% higher failure loads when compared with the specimens using carbon fiber reinforced polymer only adherends, with the failure mode changing from delamination of the adherends to cohesive failure in the adhesive. The lap shear strength was also close to that attained when using high strength steel adherends. A finite element analysis, using cohesive elements, was performed with the objective of reproducing the experimental results and better understanding the failure mechanism. Using this model, it has been determined that the change of failure mode and the plasticity on the surface layers are the two key factors underlying the increase in strength obtained with the novel adherends.

Thermal Stress Analysis of Chip Scale Packaging by Using Novel Multiscale Interface Element Method

In this study, a multiscale interface element method (MIEM) is developed to evaluate the interfacial peel and shear stress distributions in multilayer packaging structures. A newly developed polygon interface element contained many sides that could easily connect with many other polygon elements of much smaller scale. Simple model and less computation can obtain accurate stress distributions. After verification of the validity of the developed model, MIEM is applied to analyze thermal stress distribution of the chip scale package (CSP). In general, a good qualitative agreement has been observed between the various results. Moreover, MIEM can directly obtain the stress value at the interface, which is also one of the advantages of this method in dealing with multilayer structures.