Peel Stresses Research Articles

Modified hinged beam test on steel fabric reinforced cementitious matrix (SFRCM)

Abstract An experimental campaign based on modified hinged beam test (MhBT) set-up has been reported in the present study. The samples consist of two concrete blocks coupled by a proper hinge device and laminated with steel wire fabrics embedded in a cementitious mortar layer. Two kinds of fabrics, made of galvanized steel strands with different mesh spacing, have been used to reinforce the concrete joists. With the aid of a DIC monitoring system, slippage profile at the interface between the concrete support and the mortar laminate along the contact region has been assessed, together with the fracture opening. Force vs slippage at the interface has been retrieved for the samples tested according to the MhBT set-up. With the aim to obtain predictive ultimate load design formulas, a novel classification of laminate here proposed will be argued and related to a MhBT design formula. The influence of peel and shear stresses interaction on the ultimate strength of the system has been discussed in detail.

Performance enhancement of tubular multilayers via compliance-tailoring: 3D printing, testing and modeling

Material-tailoring of bondlayer has been demonstrated to be an effective route for the design of high performance adhesively bonded multilayers. Herein, enhanced performance of tubular bonded joints (TBJs) with compliance-tailored bondlayer is demonstrated both via experiments utilizing multimaterial additive manufacturing (AM) and modeling. An edge-compliance is imparted to end regions of the bondlayer mimicking the cortical bone microstructure via AM. For the ease of fabrication, TBJs with bi-modulus bondlayer having a single-step change in compliance is also 3D printed. The experimental tests performed on AM-enabled TBJs under axial tension show 100% and 70% increase in load carrying capacity and toughness respectively, compared to joints with homogeneous bondlayer. Finite element (FE) analyses performed on 3D printed joints under the same loading conditions reveal that the peak shear and peel stresses in the bondlayer are lower due to its increased edge-compliance. To identify optimal compliance-tailoring schemes and to characterize stress transfer through such bondlayer, an analytical model validated by an analogous FE model is proposed within the context of linear elastostatics. The effect of linearly-tailoring the compliance of the bondlayer with respect to mid-bondlength either partially or fully over the bondlength, on the stress-state is evaluated. A parametric study is conducted by perturbing the geometric and material properties so as to identify parameters for the optimal design of compliance-tailored multilayers.

Conductive-paste-based high-yielding interconnection process for c-Si photovoltaic modules with 50 µm thin cells

Thin crystalline silicon (c-Si) photovoltaic (PV) cells (< 100 µm) have the potential to curtail manufacturing costs by reducing the amount of Si needed per wafer. However, thermo-mechanical stress induced by high-temperature (> 200 °C) soldering causes frequent wafer breakage in thin c-Si-based modules. Hence, in this work, we proposed low-temperature interconnection method using conductive paste (CP) for thin c-Si PV modules and systematically studied the modules’ electrical and mechanical properties as a function of annealing temperature of CP. The potential advantage of this method is significantly reduced wafer bowing due to the low-temperature tabbing (< 150 °C) of CP dispensed cells to ribbons using heat and pressure during lamination. Module degradation and peel stress tests indicated that CP cured above its melting point provides stable (degraded 3.0% after 500 h damp heat test) and efficient current flow paths. By contrast, CP annealed below the melting point is vulnerable to thermal and humidity stress, leading to 7.8% degraded output after the test. Given these features, stable, large modules with thin c-Si cells integrated using a CP approach (laminated at 150 °C) were successfully realized without cell breakage.

Digital Image Correlation Analysis Of The Effects Of The Overlap Length, Adhesive Thickness And Adherends Yield Strength Over Similar And Dissimilar Joints Of High Strength Steel And Aluminum Alloys

This work encompasses the study of the mechanical behavior of single lap joints (SLJ) with similar and dissimilar materials, joined by epoxy adhesives. The objective was to analyze the effect of the variables overlap length (OL), adhesive thickness (AT) and adherends yield strength (YS) over the SLJ failure load (FL). The stress state of the joints was analyzed through an optical technique. Tensile tests were performed with samples containing three levels of the factors OL, AT and YS. This last variable analysis was performed using aluminum (AA5083 H111) and high strength steel (DP600) adherends, combined in three different levels – similar AA, similar HSS and dissimilar. The digital image correlation (DIC) technique was used in order to analyze and quantify the incidence of deformation on the adherends and adhesive during the tensile tests, in order to verify how the tensile load was distributed through the SLJ of this work. All the variables had significance over the SLJ. The OL increased the SLJ FL, but the effect reduced as the length is increased. The adherends YS also increased the SLJ FL, but not as much as the OL. Plastic deformation of the adherends leaded to the onset of high peeling stress, which reduced considerably the joints FL. The AT had a negative effect over the SLJ FL. This relationship varied when the adherends faced remarkable plastic strain.

Reliability analysis of different structure parameters of PCBA under drop impact

The establishing process of PCBA is modelled by finite element analysis software ABAQUS. Firstly, introduce the Input-G method and the fatigue life under drop impact are introduced and the mechanism of the solder joint failure in the process of drop is analysed. The main reason of solder joint failure is that the PCB component is suffering repeated tension and compression stress during the drop impact. Finally, the equivalent stress and peel stress of different solder joint and plate-level components under different impact acceleration are also analysed. The results show that the reliability of tin-silver copper joint is better than that of tin- lead solder joint, and the fatigue life of solder joint expectancy decrease as the impact pulse amplitude increases.

Bond Layer Properties and Geometry Effect on Interfacial Thermo-mechanical Stresses in Bi-material Electronic Packaging Assembly

Thermo-mechanical mismatch stress is one of the reasons for mechanical as well as functional failure between two or more connected devices. In electronic packaging, two or more plates or layers are bonded together by an extremely thin layer. This thin bonding layer works as an interfacial stress compliance which is expected to alleviate the interfacial stresses between the layers. Therefore, it is very important to identify the suitable interfacial bonding characteristics for reducing the interfacial thermal mismatch stresses to maintain the structural integrity. This research work examines the influences of bond layer properties and geometry on the interfacial shearing and peeling stresses in a bi-material assembly. In this study a closed form model of bi-layered assembly is used with the up-to-date bond layer shear stress compliance expression. The key bond layer properties namely Young’s modulus, coefficient of thermal expansion, Poisson’s ratio, and physical parameters like temperature and thickness are considered for interfacial stress evaluation. It is observed that the Young’s modulus, the thickness and the temperature of the bond layer have significant influence on the interfacial shearing and peeling stress. The results obtained are likely to be useful in designing bond layer properties in microelectronics and photonics applications.

Material Selection for Interfacial Bond Layer in Electronic Packaging

In electronic packaging, typically two or more thin dissimilar plates or layers are bonded together by an extremely thin adhesive bond layer. Electronic assemblies are usually operated under high power conditions which predictably produces a high temperature environment in the electronic devices. Therefore, thermal mismatch shear and peeling stress inevitably arise at the interfaces of the bonded dissimilar materials due to differences in Coefficient of Thermal Expansion (CTE) typically during the high temperature change in the bond process. As a result, delamination failure may occur during manufacturing, machining, and field use. As such, these thermo-mechanical stresses play a very significant role in the design and reliability of the electronic packaging assembly. Consequently, critical investigations of interfacial stresses under variable load conditions in composite structure can result in a better design of electronic packaging with higher reliability and minimize or eliminate the risk of functional failure. In order to formulize bond material selection, analytical studies are carried out in order to study the influence of bond layer parameters on interfacial thermal stresses of a given package. These parameters include Coefficient of thermal expansion (CTE), poison’s ratio, temperature, thickness, and stiffness (compliant and stiff) of the bond layer. From the study, stiffness and bond layer thickness are identified as the key parameters influencing interfacial shearing and peeling stresses. The other parameters namely CTE, poisons ratio has shown insignificant influence on interfacial stresses due to the very thin section of bond layer compared to the top and bottom layers. The results also show that the interfacial stresses increases proportionally with the increase of temperature in the layers. Therefore, it is very important that the temperature is maintained as low as possible during the chip manufacturing and operating stages. Since only two parameters namely stiffness and bond layer thickness are identified as the key parameters, the interface thermal mismatch stresses can be reduced or eliminated by controlling these two parameters only. Therefore the identification of suitable bond layer parameters selection with reasonable accuracy is possible even without performing optimization process. Finally, this paper proposes a Metal Matrix Composite (MMC) bond material selection approach using rule of mixture material design. The outcome of this research can be seen in the forms of practical and beneficial tools for interfacial stress evaluation and physical design and fabrication of layered assemblies. The Engineers can utilize this research outcome in conjunction with guidelines for electronic packaging under variable thermal properties of layered composites.

Strength and Failure Mechanism of Composite‐Steel Adhesive Bond Single Lap Joints

Carbon fiber‐reinforced plastics‐ (CFRP‐) steel single lap joints with regard to tensile loading with two levels of adhesives and four levels of overlap lengths were experimentally analyzed and numerically simulated. Both joint strength and failure mechanism were found to be highly dependent on adhesive type and overlap length. Joints with 7779 structural adhesive were more ductile and produced about 2‐3 kN higher failure load than MA830 structural adhesive. Failure load with the two adhesives increased about 147 N and 176 N, respectively, with increasing 1 mm of the overlap length. Cohesion failure was observed in both types of adhesive joints. As the overlap length increased, interface failure appeared solely on the edge of the overlap in 7779 adhesive joints. Finite element analysis (FEA) results revealed that peel and shear stress distributions were nonuniform, which were less severe as overlap length increased. Severe stress concentration was observed on the overlap edge, and shear failure of the adhesive was the main reason for the adhesive failure.

Stress analysis of composite adhesive bonded joints under incipient failure conditions

Composite structures with their remarkable properties often require assembling different components and repairing the damaged regions by employing mechanical or adhesive bonding joints. Among the bonding techniques, adhesive bonding is considered the most diffused and efficient method.Analysis of stress distribution that develops in the joint is a crucial field of research to put effort with the aim to develop engineering strength criteria. Once an adhesive bonded joint is subjected to the axial tensile load, shear and peel stress intensification takes place at the ends of the overlap. With regard to the literature, it can be concluded that there are few reliable predicting tools for brittle adhesives, for which failure usually initiates at the edge of the bonding area and propagates through the interface region. The aim of this paper is an investigation on stress field in single-lap joint (SLJ) and scarf-lap joint (ScJ) under the tensile loading. A two-dimensional numerical analysis by employing Finite Element Method (FEM) and a developed analytical solution were implemented to realize the correlation of the results. Furthermore, a comparison between possible failure criteria was performed by using FE results. Adherend thickness, overlap length, and scarf angle were considered as the joint geometry parameters. Effects of these parameters on the fracture behaviour of bonded joints were evaluated.

Design Analysis of Adhesively Bonded Structures.

The existing analytical solutions for the peeling and shearing stresses in polymeric adhesively bonded structures are either too inaccurate or too complex for adoption by practicing engineers. This manuscript presents a closed-form solution that is reasonably accurate yet simple and concise enough to be adopted by practicing engineers for design analysis and exploration. Analysis of these concise solutions have yielded insightful design guidelines: (i) the magnitude of peeling stress is generally higher than that of shearing stress; (ii) the peeling stress in a balanced structure may be reduced most effectively by reducing the elastic modulus of the adherends or by increasing the adhesive-to-adherend thickness ratio and less effectively by reducing the elastic modulus of the adhesive; and (iii) the peeling stress in an unbalanced structure may be reduced by increasing the in-plane compliance of the structure, which may be implemented most effectively by reducing the thicknesses of the adherends and less effectively by reducing the elastic modulus of the adherends.

Laminated process effect of high-density redistributed trace lines on the risk estimation of induced-stress failure for 3D-IC embedded interposer

The embedded interposer carrier (EIC) technology for 3D-integrated circuits (3D-ICs) play an important role as a promising candidate to deal with the assembly/bonding processes of thin stacked chip and substrate. However, peeling stress of the interface between stacked via holes and copper-filled through silicon via silicon-based interposer may cause a thermomechanical reliability issue among multi-stacked film EIC architectures. For dealing with this situation, we propose a process-oriented finite element analysis method considering material nonlinear behaviors as well as process flows including process temperature and steps. This method is able to analyze systematically the effect of cycling temperature loads induced during the bonding process. From the result of simulations, it indicates that the curing process of laminated material is the critical step among the entire bonding process. In addition, the material about a compliant interposer is recommend to be a suitable solution for EIC architecture enhancing operational reliability. Furthermore, the thermal stress of each thin film stacked inside EIC structure can be relieved by means of the flexible of the carrier laminate design.

Use of a resilient bond line to increase strength of long adhesive lap joints

The load capacity of long adhesive lap joints is often governed by stress concentrations at the lap ends. This paper investigates a method to minimize these stress concentrations by using a bond line with low shear stiffness and sufficient strength, here denoted as a resilient bond line. The resilient bond line is intended to increase the load carrying capacity of long lap joints by achieving a more uniform shear stress distribution while maintaining an elastic joint behaviour without damage or plastic deformation. The study comprises analytical, numerical and full-sized experimental work on double lap joints with lengths 200–700 mm comparing conventional stiff bond lines to resilient bond lines. Different resilient bonds lines were obtained by using rubber-like adhesives and by having a rubber mat within the bond line. An analytical definition of a ‘long’ lap joint is suggested and a study of adhesive-rubber bonding is also presented. The numerical analysis clearly indicates that an increase in load carrying capacity is made possible using resilient bond lines. A good agreement is also found between the numerical results and the analytical Volkersen theory, indicating that reasonable strength predictions can be obtained by hand calculations if the joint is designed in order to minimize the influence of peel stress. The experimental results of the resilient bond line verify the numerical findings, although production difficulties decrease the statistical significance of the result. On the contrary, the experimental results of the conventional bond lines significantly exceeded the numerical predictions, showing similar load carrying capacities to the resilient bond line. This is probably due to the specific boundary conditions used in the test setup. Despite some contradictory experimental results, the conclusion of this study is that the efficiency of long lap joints can be increased by the use of a bond line with low shear stiffness and sufficient strength.

Stress Reduction of 3D Printed Compliance‐Tailored Multilayers

Multilayered multi‐material interfaces are encountered in an array of fields. Here, enhanced mechanical performance of such multi‐material interfaces is demonstrated, focusing on strength and stiffness, by employing bondlayers with spatially‐tuned elastic properties realized via 3D printing. Compliance of the bondlayer is varied along the bondlength with increased compliance at the ends to relieve stress concentrations. Experimental testing to failure of a tri‐layered assembly in a single‐lap joint configuration, including optical strain mapping, reveals that the stress and strain redistribution of the compliance‐tailored bondlayer increases strength by 100% and toughness by 60%, compared to a constant modulus bondlayer, while maintaining the stiffness of the joint with the homogeneous stiff bondlayer. Analyses show that the stress concentrations for both peel and shear stress in the bondlayer have a global minimum when the compliant bond at the lap end comprises ≈10% of the bondlength, and further that increased multilayer performance also holds for long (relative to critical shear transfer length) bondlengths. Damage and failure resistance of multi‐material interfaces can be improved substantially via the compliance‐tailoring demonstrated here, with immediate relevance in additive manufacturing joining applications, and shows promise for generalized joining applications including adhesive bonding.

A closed-form solution of the interfacial stresses and strains in steel beams strengthened with externally bonded plates using ductile adhesives

Ductile adhesives are known to be beneficial in enhancing the capacity and ductility of bonded joints. However, there is a lack of closed-form analytical solutions for plated metallic beams that account for adhesive plasticity. This paper presents a first order, elastic-plastic bond analysis for beams strengthened with externally bonded fiber reinforced polymer (FRP) plates using ductile adhesives, based on a shear-lag formulation. This model is able to analyze arbitrary mechanical and thermal loading conditions and closed-form solutions under shearing and peeling are given. Following a review of the existing stress-based analytical solutions, a comparison between the existing and proposed analyses is also presented. The shear solution was validated by comparing with the experimental results of carbon fiber reinforced polymer (CFRP) strengthened steel beams under moderately elevated temperatures. A comprehensive parametric study has been conducted to illustrate the effects of different design parameters on the bond behavior. It is found that the adhesive shear toughness is the most critical parameter in determining the debonding failure load while the adhesive Young’s modulus does not significantly affect the bond stresses in the elastic-plastic regime. Further, the magnitude of the peak peeling stresses is self-limiting after shear yielding, and the use of a thinner plate with higher Young’s modulus is beneficial in further reducing the peeling stress.

Bond Characteristics of CFRP/Steel Interface End-anchored with G-shaped Clamps

In order to prevent the debonding failure of CFRP laminate/steel plate joints, this study attempted to use a G-shaped clamp anchorage system to eliminate the peeling stress at the end of joints which causes the debonding of interfaces. Bond characteristics of CFRP/steel interface with and without end anchorage were obtained by conducting double strap tensile tests, and the effectiveness of end anchorage was verified as well. By anchoring on the weak points, the bond strength of the composite plates can be increased. However, for completely preventing debonding of the composites, a more advanced anchorage system which could against the intensive shear stress still needs further investigation.

Experimental and numerical study on the composite adhesive joint reinforcement using wavy edge

The joints are usually the weakest part of the engineering structures. In this study, the employment of wavy edges for increasing the adhesive joint load-bearing capacity is considered. The effects of geometric parameters of the wavy edges on the strength of the adhesive joints were investigated, experimentally. Two different adhesives, Araldite 2015 and Epoxy RL440/HY441 as ductile and brittle adhesives were used, respectively. The finite element model was also developed for more investigation. The joint stress distributions were used successfully to explain the experimental observations. For the appropriate wavy joint configuration, compressive peel stress on the both ends of the adhesive led to a considerable delay in damage initiation and consequently increased the joint strength. The effects of geometrical parameters of the wavy edge on the joint strength were also examined. For the optimum configuration, the joint with wavy edge offered 32% more strength than the flat single lap joint.

Separating Temporary Carrier from Large Thin Panel with Air Jetting

Abstract As electronic components becomes thinner and more powerful than ever, substrates are being pushed to be so thin that they are flexible. Furthermore, both passive and active components are being embedded into substrates to further reduce packaging thickness. To fabricate a substrate that is too thin to be rigid and flat, a temporary carrier is applied onto the substrate to support the fabrication. However, but it must be removed upon completion, and the substrate and carrier must be bonded with very high adhesion strength during solder reflow cycles, due to a mismatch in the coefficient of thermal expansion (CTE). Such high adhesion strength typically creates more problems in the debonding process. Unlike mechanical debonding, which generates significant peeling stress on a device's surface, air jet debonding introduces air streams between the carrier and the substrate, thus lifting the carrier from underneath and compressing the substrate surface downward. This results in the most uniform stress distribution, which is best suited for larger-area debonding processes. In this paper, the air-jet debonding of a glass carrier from a 300 x 300 mm thick panel is demonstrated. The bonding adhesive Z-Coat 421G is developed to temporarily bond with panel substrate for lead-free assembly.

A Coupled Peel and Shear Stress-Diffusion Model for Adhesively Bonded Single Lap Joints

In this study, the Fickian diffusion formulation is extended to the adhesive layer of a single lap joint (SLJ) model, in order to develop a coupled peel and shear stress-diffusion model. Constitutive equations are formulated for shear and peel stresses in terms of adhesive material properties that are time- and location-dependent. Numerical solution is provided for the effect of diffusion on shear and peel stresses distribution. Detailed discussion of the results is presented.

The mechanics of bondline thickness in balanced sandwich structures

Assuming negligible in-plane stress, non-varying shear stress, and linearly varying transverse stress along the thickness of the adhesive, the closed-form solutions for the interfacial shear and the interfacial peeling stresses in balanced bonded sandwich structures was derived. The nil-shear-stress condition at the free edge of the adhesive was enforced through the use of a decay function. The solutions for the shear and the peel stresses along the interfaces right up to the free edge were validated with the finite element analysis. The solutions were used to investigate (i) the observed increase susceptibility of single lap joints with increase bondline thickness and (ii) the nil report of such susceptibility for electronic assemblies experiencing differential thermal strain. The first investigation was compromised by the inability of the solutions to model the singular stress/strain field at the free-edge of the interfaces. The second investigation revealed a far higher rate of reduction in the magnitudes of the interfacial shear and peeling stresses in structures (electronic assemblies) that experience differential thermal strain than in structures (single lap joints) that experience bending strain.

Simplified stress analysis of hybrid (bolted/bonded) joints

The load transfer in hybrid (bolted/bonded) – denoted HBB – single-lap joints is complicated due to the association of two different transfer modes (discrete and continuous) through elements with different stiffnesses. The Finite Element (FE) method can be used to address the stress analysis of those joints. However, analyses based on FE models are computationally expensive and it would be profitable to use simplified approaches enabling extensive parametric studies. Two among the authors of this paper participated in the development of a dedicated 1D-beam approach [25]. This paper presents an extension of this framework enabling (i) the analysis of HBB joints made of dissimilar laminated or monolithic adherends, and (ii) the introduction of non linear material behaviour for both the adhesive layer and the fasteners. The output data are the distributions of displacements and forces in the adherends and fasteners, as well as those of adhesive shear and peeling stresses, allowing for a fast assessment of the material behaviour and strength prediction of HBB joints. The use of this model is illustrated in the identification of the failure mechanisms of HBB joints under quasistatic loadings, based on experimental and numerical tests on single-lap HBB joint. It is worth mentioning that the model can support pure bonded and pure bolted configurations. It can be used during the presizing phase at the design office (possibly independently on commercial software), to obtain quickly mechanical performances and to help in decision making. Moreover, it was shown that the judicious choice of the adhesive material allows for a significant increase of the static and fatigue strength compared to pure bolted or bonded corresponding configurations [17,24]. The model can then be used to formulate at best adhesive materials to optimize the mechanical performance of HBB joints according to work specifications.