Lap-shear Specimens Research Articles

Modeling of failure near spot welds in lap-shear specimens based on a plane stress rigid inclusion analysis

In this paper, the failure mechanism of resistance spot welds in dual-phase steel lap-shear specimens is investigated based on experimental observations, two-dimensional elasticity theories and two-dimensional finite element analyses. Optical micrographs of the cross sections of spot welds in lap-shear specimens of a dual-phase steel before and after failure are first examined to understand the failure mechanism. The experimental results suggest that under lap-shear loading conditions, a necking failure is initiated near the middle of the nugget circumference in the base metal and then the failure propagates along the nugget circumference in the sheet to final fracture. Based on the stress function approach of the elasticity theory, an analytic solution for an infinite plate containing a rigid inclusion subjected to a resultant shear force is developed and used to investigate the stress and strain distributions near the nugget in lap-shear specimens. The results of the elastic analytic solution and those of a two-dimensional elastic finite element analysis indicate that the initial yielding starts on the two side edges of the inclusion in the sheet. However, the results of a two-dimensional elastic–plastic finite element analysis indicate that as the applied displacement increases, the maximum equivalent plastic strain shifts from the two side edges of the inclusion to the middle of the inclusion along the inclusion circumference in the sheet. The computational results suggest that the location of the initial necking failure should occur near the middle of the nugget circumference in the sheet as observed in experiments based on the forming limit diagram (FLD) for ductile sheet metals.

Mode I stress intensity factor solutions for spot welds in lap-shear specimens

The analytical solutions of the mode I stress intensity factor for spot welds in lap-shear specimens are investigated based on the classical Kirchhoff plate theory for linear elastic materials. First, closed-form solutions for an infinite plate containing a rigid inclusion under counter bending conditions are derived. The development of the closed-form solutions is then used as a guide to develop approximate closed-form solutions for a finite square plate containing a rigid inclusion under counter bending conditions. Based on the J integral, the closed-form solutions are used to develop the analytical solutions of the mode I stress intensity factor for spot welds in lap-shear specimens of large and finite sizes. The analytical solutions of the mode I stress intensity factor based on the solutions for infinite and finite square plates with an inclusion are compared with the results of the three-dimensional finite element computations of lap-shear specimens with various ratios of the specimen half width to the nugget radius. The results indicate that the mode I stress intensity factor solution based on the finite square plate model with an inclusion agrees well with the computational results for lap-shear specimens for the ratio of the half specimen width to the nugget radius between 4 and 15. Finally, a set of the closed-form stress intensity factor solutions for lap-shear specimens at the critical locations are proposed for future applications.

Mechanical and microstructural properties of SnAgCu solder joints

Mechanical and microstructural properties of SnAgCu solder joints with hypoeutectic, eutectic and hypereutectic compositions were studied. Eutectic SnPb joints were used as the reference. Reflowed lap shear specimens made of FR-4 glass epoxy printed circuit boards with OSP and NiAu surface finishes were used in the tests. Mechanical properties and microstructural features of the joints were examined in the as-reflowed condition and after isothermal aging at 85°C for 1000h. Both the composition and PCB surface finish had a notable effect on the mechanical behaviour of the SnAgCu solder joints. The shear strength value of SnAgCu solder joints was mainly dependent on the size and distribution of Ag3Sn dispersions. The coarseness of the dispersions depends strongly on the amount of Ag in the solder alloy, the cooling rate after the reflow and the aging history of the solder joints.

A fatigue crack growth model for spot welds under cyclic loading conditions

Based on the experimental results for spot welds in square-cup, lap-shear and coach-peel specimens of dual phase, low carbon and high strength steels, kinked fatigue crack paths near spot welds are first examined. A fatigue crack growth model is then developed for spot welds based on the local stress intensity factor solutions for kinked cracks. Various global stress intensity factor solutions for spot welds in the three types of specimens are used to obtain the local stress intensity factor solutions with the experimentally determined kink angles. The predicted fatigue lives based on the fatigue crack growth model are then compared with the experimental data. The results indicate that the fatigue crack growth model can predict reasonably well the fatigue lives of spot welds based on the global stress intensity factor solutions and the assumed material constants in the Paris law for fatigue crack growth.

Geometric functions of stress intensity factor solutions for spot welds in lap-shear specimens

In this paper, the stress intensity factor solutions for spot welds in lap-shear specimens are investigated by finite element analyses. Three-dimensional finite element models are developed for lap-shear specimens to obtain accurate stress intensity factor solutions. In contrast to the existing investigations of the stress intensity factor solutions based on the finite element analyses, various ratios of the sheet thickness, the half specimen width, the overlap length, and the specimen length to the nugget radius are considered in this investigation. The computational results confirm the functional dependence on the nugget radius and sheet thickness of the stress intensity factor solutions of [Zhang, S., 1997. Stress intensities at spot welds. International Journal of Fracture 88, 167–185; Zhang, S., 1999. Approximate stress intensity factors and notch stresses for common spot-welded specimens. Welding Journal 78, 173s–179s]. The computational results provide some geometric functions in terms of the normalized specimen width, the normalized overlap length, and the normalized specimen length to the stress intensity factor solutions of [Zhang, S., 1997. Stress intensities at spot welds. International Journal of Fracture 88, 167–185; Zhang, S., 1999. Approximate stress intensity factors and notch stresses for common spot-welded specimens. Welding Journal 78, 173s–179s] for lap-shear specimens. The computational results also indicate that when the spacing between spot welds decreases, the mode I stress intensity factor solution at the critical locations increases and the mode mixture of the stress intensity factors changes consequently. Finally, based on the analytical and computational results, the dimensions of lap-shear specimens and the corresponding approximate stress intensity factor solutions are suggested.

A computational study of local stress intensity factor solutions for kinked cracks near spot welds in lap-shear specimens

In this paper, the local stress intensity factor solutions for kinked cracks near spot welds in lap-shear specimens are investigated by finite element analyses. Based on the experimental observations of kinked crack growth mechanisms in lap-shear specimens under cyclic loading conditions, three-dimensional and two-dimensional plane-strain finite element models are established to investigate the local stress intensity factor solutions for kinked cracks emanating from the main crack. Semi-elliptical cracks with various kink depths are assumed in the three-dimensional finite element analysis. The local stress intensity factor solutions at the critical locations or at the maximum depths of the kinked cracks are obtained. The computational local stress intensity factor solutions at the critical locations of the kinked cracks of finite depths are expressed in terms of those for vanishing kink depth based on the global stress intensity factor solutions and the analytical kinked crack solutions for vanishing kink depth. The three-dimensional finite element computational results show that the critical local mode I stress intensity factor solution increases and then decreases as the kink depth increases. When the kink depth approaches to 0, the critical local mode I stress intensity factor solution appears to approach to that for vanishing kink depth based on the global stress intensity factor solutions and the analytical kinked crack solutions for vanishing kink depth. The two-dimensional plane-strain computational results indicate that the critical local mode I stress intensity factor solution increases monotonically and increases substantially more than that based on the three-dimensional computational results as the kink depth increases. The local stress intensity factor solutions of the kinked cracks of finite depths are also presented in terms of those for vanishing kink depth based on the global stress intensity factor solutions and the analytical kinked crack solutions for vanishing kink depth. Finally, the implications of the local stress intensity factor solutions for kinked cracks on fatigue life prediction are discussed.

The lap-shear strength of bonds between Oriented Strand Board (OSB) like strands coated with pMDI resin

Subject. It is well known that resin is applied to wood strands as small discrete droplets during the manufacture of OSB. The quantity of resin applied is low and coverage of strands is incomplete. During pressing strands come into close proximity and bonding between strands may occur as a result of resin to wood contact, resin to resin contact, or a combination of the two. Strong bonds are routinely made during the manufacture of plywood where resin is applied to only one side of the veneer sheet and transferred to the adjacent veneer during pressing. For OSB it is not known which of the bond types produce stronger bonds: resin-to-wood or resin-to-resin contacts. This note investigated the bond strength of OSB-like lap-shear specimens in which pMDI-resin was applied to either one or both surfaces of the strands prior to pressing. Two resin levels were employed, 2 or 10%. The aim was to determine which bond type, resin-to-resin contact or resin-to-wood contact, results in higher bond strength between OSB strands.

Mixed Mode Fracture Analysis of the Lap Shear Specimen Test Per ASTM D1002

ASTM D1002 is a standard test method used to evaluate the shear strength of adhesives. This method is popular due to the simplicity of the test, ease to replicate, and the consistency of the testing results. However, this test is also the most criticized one, as the results can be too misleading to be used as design criteria.Due to the material elastic mismatch and the eccentricity of the test setup, a mixed mode loading is introduced to the adhesive joint. This paper presents a detailed study on the behavior of the adhesive joint under the shear loading, tested by the ASTM D1002 method. The conventional fracture mechanics theory is utilized.A crack embedded in the adhesive joint specimens, including both the single thickness and the double thickness, are used for the analysis. The specimen is loaded on a tensile test machine until failure, and the load and displacement are recorded for each specimen. The result from the experiment is used as the data for the stress and fracture mechanics analysis.From the experimental results, it is seen that the specimen also experiences out-of-plane tensile stress. There is an indication that the final failure might be due to cleavage loading, for the double thickness. The eccentricity of the loading path cannot be eliminated completely. The results of the experiment and the effect of the mixed mode loading are studied in detail.This paper provides a better understanding of the behavior of adhesives under a mixed-mode fracture. This knowledge is beneficial to understand and design reliable adhesive bond joints.

Electrochemical impedance evaluation of aluminum/boron–epoxy composite bonds

Al/boron-epoxy double lap-shear specimens with five different surface preparations were exposed to hot, humid conditions for up to 3 years. Electrochemical impedance spectroscopy (EIS) measurements were taken using an in situ sensor throughout the exposure period. Periodically, some specimens were pulled to determine bond strength degradation. Aside from a set of deliberately poorly prepared specimens, all specimens retained at least 80% of their initial bond strength with the best specimens showing only a little degradation of strength. The low-frequency EIS impedance for measurements taken with sensor electrodes on the same composite surface decreased exponentially by two orders of magnitude during the exposures as the composite absorbed moisture. This decrease happens well before significant bond degradation, thus allowing the measurements to be used as a warning of moisture intrusion before the moisture has weakened the interface and resulted in a reduction of bond properties. One application of this bond health monitoring technology would be in bonded repair patches.

이차원 하이브리드 요소를 이용한 균열을 내포하는 용접점의 유한요소 파단해석

This paper employed a systematic analysis using a 2-D hybrid special finite element containing an edge crack in order to describe the fracture behavior of spot-welds in automotive structures. The 2-D hybrid special finite element is derived form a mixed formulation with a complex potential function with the description of the singularity of a stress field. The hybrid special finite element containing an edge crack can give a better description of its singularity with only one hybrid element surrounding one crack. The advantage of this special element is that it can greatly simplify the numerical modeling of the spot welds. Some numerical examples demonstrate the validity and versatility of the present analysis method. The lap-shear, lap-tension and angle-clip specimens are analyzed and some useful fracture parameters such as the stress intensity factor and the initial direction of crack growth are obtained simultaneously.

차체 셀프-피어싱 리벳 접합의 구조강성 및 피로수명 평가

Recently, Self Piercing Rivet(SPR) has been spotlighted in the automotive industry as a substitutive resort of spot welding and has also been watched by the designer as lightening a car body due to their superior assembly processes. Fatigue behavior of SPR joint needs to be investigated experimentally and numerically to predict its structural stiffness and fatigue life. Testing of lap-shear specimens with various material combinations is performed to obtain the joining strength and the fatigue life of SPR connections. The simulation of SPR lap-shear specimens is also conducted to obtain the structural stiffness of SPR connections under different material combinations. A Finite element model of the SPR lap-shear specimen is developed using a FEMF A T SPR pre-processor. The fatigue life of SPR specimen is predicted using a FEMFAT 4.4e based on the liner finite element analysis.<br/>

Vibration Welding of Continuous-Fiber Thermoplastic Matrix Composites

In this paper, we discuss vibration welding of continuous-fiberreinforced thermoplastic matrix composites. Several parameters affecting the lap shear strength of vibration-welded joints of glass and carbon fiber-reinforced polypropylene and polyamide-6 matrix composites have been experimentally investigated. These parameters were clamping pressure, vibration time, vibration direction, and holding time. Weld strength at elevated temperatures was also investigated. The results showed that the combination of clamping pressure and vibration time had a significant effect on the lap shear strength. Holding time also influenced lap shear strength, but the effect of vibration direction was not significant. In addition to lap shear specimens, ring specimens were designed and tested to determine the crosstension strength of these composites under tensile stress normal to the weld plane.

Fatigue life prediction for spot welds in coach-peel and lap-shear specimens with consideration of kinked crack behaviour

A fracture mechanics approach is adopted to examine the fatigue lives of spot welds in coach-peel and lap-shear specimens with consideration of kinked crack behaviour. The paths of fatigue cracks near the spot welds in these two types of specimens are first discussed. A fatigue crack growth model is then developed for spot welds in these two types of specimens based on the local stress intensity factor solutions for kinked cracks and the Paris law for crack propagation. Various global stress intensity factor solutions for spot welds in these two types of specimens are used to obtain the local stress intensity factor solutions with the experimentally determined kink angles. The results indicate that the fatigue life predictions based on accurate global stress intensity factor solutions can characterise the general trend of the experimental results. The effects of the kink angle on the fatigue life predictions are also discussed.

Treatment of CFRP by IAR method and its effect on the fracture behavior of adhesive bonded CFRP/aluminum composites

It was shown in the previous studies that adhesive shear strength of carbon fiber reinforced plastics (CFRP) to aluminum composites could be improved by the surface treatment of CFRP using Ar + ion irradiation. In the present work, the effect of CFRP treatment by Ar + ion irradiation on the fracture behavior of CFRP/aluminum joint was studied. The aluminum used was 7075-T6 and the CFRP used was multi-directional graphite/epoxy composites whose stacking sequence was [0°/±45°/0°] 3s. The surface of CFRP was treated using Ar + ion irradiation in an oxygen environment. The Ar + ion dose used was 1×10 16 ions cm −2. Fracture toughness of CFRP/aluminum joint was determined from cracked lap shear specimens using work factor approach. Then, the fracture toughness of ion beam-treated CFRP/aluminum joint was compared with that of untreated CFRP/aluminum joint. The results showed that the fracture toughness of ion beam-treated CFRP/aluminum case was about 72% higher than that of untreated CFRP/aluminum case. X-ray photoelectron spectrometer analysis showed that intensity of hydrophilic bonds, CO (carbonyl group) and OCO (carboxyl group) was increased by the Ar + ion-irradiation in an oxygen environment. Scanning electron microscope examination showed that cohesive failure occurred for ion beam-treated CFRP/aluminum joint while adhesive failure occurred for untreated CFRP/aluminum joint.

표면처리된 CFRP와 알루미늄 복합재료의 파괴인성 향상에 대한 연구

In this study, the effect of surface treatment of CFRP and aluminum on the fracture toughness of CFRP/aluminum composites was investigated. CFRP was surface-treated by Ar^+ ion beam under oxygen environment, and the aluminum was surface-treated by DC plasma. CFRP was adhesively bonded to aluminum using the secondary bonding procedure. Cracked lap shear specimens were used to determine fracture toughness. Three cases of cracked lap shear specimens were made depending on the surface treatment. The values of fracture toughness of three cases were compared to each other. It was found that the fracture toughness of ion beam-treated CFRP/aluminum composites was almost 72 % higher than that of untreated CFRP/aluminum composites. The fracture toughness of CFRP/plasma-treated aluminum composites was 50 % higher than that of untreated CFRP/aluminum composites.

Surface Analysis of Disbonded Titanium/Sol-Gel/Polyimide Joints

Disbonded lap-shear specimens were analyzed to determine the locus of failure within bonded titanium (Ti) sol-gel polyimide joints. Bonded Ti alloys are being evaluated for use at an operating temperature of 175 °C. Determining the locus of failure for bonded Ti lap-shear specimens is part of a larger effort to develop durable, environmentally safe surface treatments for Ti alloys. Surface-treated Ti alloy (Ti-6Al-4V) plates are bonded in a standard lap-shear specimen configuration and exposed to temperature for specified intervals. The lap-shear bond joint consists of two etched Ti panels coated with a silicon and zirconium containing sol-gel, primed with a polyimide, and then bonded together with adhesive and supporting scrim material. The lap-shear specimens are tested for overall strength and failure modes. Specimens with adhesive failure modes were examined with x-ray photoelectron spectroscopy (XPS) to determine the composition of the bond joint failure layer. Analysis shows that the failure was located closer to the sol-gel/polyimide interface than to the Ti/sol-gel interface. Transmission electron micrographs (TEM) of the cross-sectioned joint confirmed the chemical distribution determined from the XPS data.

Fatigue behaviour and life prediction of fibre reinforced metal laminates under constant and variable amplitude loading

Fatigue crack growth of fibre reinforced metal laminates (FRMLs) under constant and variable amplitude loading was studied through analysis and experiments. The distribution of the bridging stress along the crackline in centre-cracked tension (CCT) specimen of FRMLs was modelled numerically, and the main factors affecting the bridging stress were identified. A test method for determining the delamination growth rates in a modified double cracked lap shear (DCLS) specimen was presented. Two models, one being fatigue-mechanism-based and the other phenomenological, were developed for predicting the fatigue life under constant amplitude loading. The fatigue behaviour, including crack growth and delamination growth, of glass fibre reinforced aluminium laminates (GLARE) under constant amplitude loading following a single overload was investigated experimentally, and the mechanisms for the effect of a single overload on the crack growth rates and the delamination growth rates were identified. An equivalent closure model for predicting crack-growth in FRMLs under variable amplitude loading and spectrum loading was presented. All the models presented in this paper were verified by applying to GLARE under constant amplitude loading and Mini-transport aircraft wing structures (TWIST) load sequence. The predicted crack growth rates are in good agreement with test results.

Testing of adhesives for bonding of polymer composites

The aim of this project was to select the most suitable epoxy-based adhesive, from three tested, for bonding glass fibre/polyester composites exposed to air at room temperature and 60°C, and cool water. Lap shear specimens (many adhesive-bonded composite assemblies are designated to shear stresses) were tested at room temperature within 1 year. Most commonly used adhesive EPOXY 371 has maintained the shear properties without great changes in all environments, also with other type of primer (P11), and exhibited the best shear strength in cool water. It is recommended for universal usage. Mixed adhesive–cohesive failure in the specimens with increasing part of cohesive failure for joints stored at room and elevated temperature was observed. Adherend was torn up at the joints with LETOXIT LX 012. Joints with this adhesive exhibited the best shear properties at room temperature and 60°C, where damaged adherend area reached up to 80% of the whole bonded area.

Use of an electrochemical probe for predictive assessment of bonded repair

Boron/epoxy-Al double lap shear specimens were prepared and exposed to high humidity and temperature. Half of these was prepared using the grit blast/silane process used for bonded repair of aircraft. The other half was prepared without the silane to reduce durability. Electrochemical impedance spectroscopy (EIS) measurements were made on these specimens with an electrode probe. A set of these specimens was mechanically tested periodically to determine pull strength. The EIS impedance spectra were analysed by means of equivalent circuit analysis. Several circuit parameters exhibited changes upon exposure to moisture. Moisture uptake was calculated from changes in the capacitance of the equivalent circuit. When sensor electrodes are placed in different locations, variations in moisture concentration in the bonded system can be determined. The approach provides advance warning of significant reduction of bond strength and has the potential to monitor adhesively bonded repair patches and indicate the need for preventative maintenance prior to any degradation of bond performance.

Determination of Fracture Toughness, GC of Graphite/Epoxy Composites from a Cracked Lap Shear (CLS) Specimen

Previously, the work factor approach was applied to determine fracture toughness, Gc of unidirectional graphite/epoxy CLS specimen, and it is required to apply the approach to determine Gc of multi-directional CLS specimen. In this work, the elastic work factor, η el form of multi-directional CLS specimen was calculated numerically as a function of delamination length in order to investigate stacking sequence effect on η el. The calculated η el was applied experimentally to determine fracture toughness, G c of multi-directional ([0/±45/0] s/[02/±452/02] s, [0/±45/0] s/[02±452/902] s, and [902/02] s/[04/904] s for the lap and the strap) graphite/epoxy composites. For each case, the Gc values determined from the η el approach were compared with those determined from the compliance method. Numerical results show that η el is not affected by the fiber angles of plies, and η el is linearly related to delamination length. It was found from the experimental results that Gc determined from the η el approach is in a maximum 15% range of difference with that determined from the compliance method for each case. It was also found from the SEM examination that delamination propagated irregularly through interfaces of lap and strap (8th and 9th plies) for [902/02] s/[04/904] s case while no delamination transfer was observed for [0/±45/0] s/[02/±452/02] s and [0/±45/0] s/[02/±452/902] s cases.