Specimen Configuration Research Articles

Three-Sheet Spot Welds of 5052 Aluminum Alloy: Weld Growth and Failure Behavior

Resistance spot welding is used extensively in auto industry. Every commercial vehicle has 4000-5000 spot welds. The weld ability, performance, and reliability are therefore important issues in design. In this paper, we studied weld nugget formation and failure behavior of three-sheet 5052 aluminum alloy resistance spot welds. The Peltier effect between the Cu-Al (the electrode and the Al worksheet) to the nugget formation was noticed. The mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were studied using tensile shear specimen configuration. Three failure modes were identified, namely, interfacial, mixed, and pull-out. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

The Climbing Drum Peel Test: An alternative to the Double Cantilever Beam for the determination of fracture toughness of monolithic laminates

The experimental determination of the fracture toughness of inter-ply interfaces in monolithic composite specimens is far from trivial: even in standard test methods such as the Double Cantilever Beam (DCB), some precautions must be taken in the choice of the test configurations and in the post-treatment of the experimental results. Furthermore, non standard measurements such as the crack tip position during propagation are generally required. In this paper, we investigate an alternative test configuration, the Climbing Drum Peel (CDP) test, classically used in the ‘adhesives’ community. The adaptation of the CDP specimen configurations to the testing of monolithic composites is discussed and a systematic comparison is carried out between the CDP and the DCB tests in terms of global and local indicators of the crack propagation behavior.

A linear least squares approach for evaluation of crack tip stress field parameters using DIC

In the present work, an experimental study is carried out to estimate the mixed-mode stress intensity factors (SIF) for different cracked specimen configurations using digital image correlation (DIC) technique. For the estimation of mixed-mode SIF׳s using DIC, a new algorithm is proposed for the extraction of crack tip location and coefficients in the multi-parameter displacement field equations. From those estimated coefficients, SIF could be extracted. The required displacement data surrounding the crack tip has been obtained using 2D-DIC technique. An open source 2D DIC software Ncorr is used for the displacement field extraction. The presented methodology has been used to extract mixed-mode SIF׳s for specimen configurations like single edge notch (SEN) specimen and centre slant crack (CSC) specimens made out of Al 2014-T6 alloy. The experimental results have been compared with the analytical values and they are found to be in good agreement, thereby confirming the accuracy of the algorithm being proposed.

Investigation on Flexural Response of GFRP Composite Laminate Subjected to Low Velocity Cyclic Loading

The suspension systems in automobiles are to provide adequate flexure modulus and damping in response to the road tyre interaction. This will ensure adequate driving comfort. In this context, as a weight reduction measure and to provide sufficient response to the vehicle dynamics leaf springs made of composite materials and can be a replacement for traditional metal leaf springs. Hence the present study on the GFRP composite laminate is to experimentally simulate the loading environment in the vehicle and the response of spring leaf made of composite material. GFRP composite laminate specimen of configurations such as Unidirectional Laminate (UD-0), angle ply laminate (0/30/60/0 and 0/45/0/-45) and symmetric and non-symmetric cross ply laminates (0/90/90/0 and 0/90/0/90) are prepared by hand lay-up technique. The laminates are exposed to low cycle (4.6 Hz and 8.6 Hz), constant amplitude cyclic loading in the laboratory setup. The flexural modulus of the virgin and pre-cyclic loaded specimen is measured by three point bend test using digital UTM as per ASTM D 790. The flexural modulus of the pre-cyclic loaded laminate specimen presents a positive improvement up to certain loading cycles at low frequency (4.6Hz). At high frequency (8.6 Hz) loading cycles there existdegradation in the flexural modulus of the pre-cyclic loaded specimen. The enhancement and degradation of flexural modulus on exposure to cyclic loading signify the influence of loading cycle frequency and laminate configuration. Any variation in the specimen configuration apart from UD-0 ply, record a drop in the flexural modulus. The optimum flexural responses characteristics in composite material are attributed to the selection of lay-up configuration and loading condition.

Fatigue strength evaluation of self-piercing riveted Al-5052 joints under different specimen configurations

In this study, static and fatigue tests were conducted using coach-peel, cross-tension and tensile–shear specimens with Al-5052 plates for evaluation of the fatigue strength of the SPR joints. For the coach-peel, cross-tension and tensile–shear geometries, the ratios of the fatigue endurance limit to static strength were 11%, 14% and 34%, respectively, assuming fatigue cycles of 106 for an infinite lifetime. The equivalent stress intensity factor range can properly predict the current experimental fatigue lifetime. Fatigue crack initiation occurred due to fretting damage between the upper and lower sheets and between the rivet and these sheets.

Influence of the resin cement thickness on the fatigue failure loads of CAD/CAM feldspathic crowns

Objectivesto evaluate the influence of the occlusal resin cement thickness on the cyclic loads-to-failure of feldspathic crowns and to compare the results to data from monotonic tests. A large range of cement thickness (50μm and 500μm) was tested, in order to better measure the influence of this variable. MethodsFeldspathic ceramic crowns (Vita Mark II blocks, Vita Zahnfabrik) were bonded to dentin analog dies (G10 (NEMA grade G10, International Paper), with occlusal resin cement thicknesses of 50μm and 500μm (Multilink Automix, Ivoclar). The dies were prepared with microchannels for water transport to the cement layer. After 96-h water storage, the specimens (n=20) were submitted to cyclic loads (500,000 cycles at 20Hz; initial maximum load=40% of monotonic load, from previous data) following a staircase sensitivity design (step size=25N). Failure loads at 500,000 cycles were compared to monotonic failure loads (from a previous study with specimens produced by the same author, using the same materials, specimen configuration and cementation protocol). ResultsCrowns with an occlusal cement layer of 50μm were more resistant than those cemented with 500μm (246.4±22.9N vs. 158.9±22.9N), under wet cyclic testing conditions (p<0.001). The fatigue failure loads were reduced compared to monotonic loads: to 37% of monotonic for 50μm; to 53% of monotonic for 500μm. SignificanceAn occlusal cement thickness of 50μm was more favorable for the structural performance of feldspathic crowns than was 500μm. Cyclic fatigue reduced failure loads well below those found under monotonic loading.

Estimation of J-resistance Curves for CT Specimen Based on Unloading Compliance Method and CMOD Data

Abstract The unloading compliance method is one of the most widely used methods in laboratory testing for J-resistance estimation. A specimen configuration including compact tension (CT) is recommended by almost all the standard test procedures for evaluating fracture toughness, such as ASTM E1820-11, ISO 12135-2002, and GB/T 21143-2007. In the unloading compliance method, key techniques including instantaneous crack size measurement, correction of specimen rotation, and J-integral computation can be used for a CT specimen when using load line displacement (LLD) but not when using crack mouth opening displacement (CMOD) from those standard test procedures. However, a CT specimen with CMOD measurement is also used for J-resistance estimation under certain severe environments because of the convenience of measuring displacement. This study proposes a simplified linear formula for crack size calculation for a CT specimen with CMOD data, discusses the rotation correction in detail based on the equivalence of work, and develops reasonable incremental expressions for J-integral computation and a CMOD-to-LLD conversion equation. By measuring the LLD and CMOD data of a CT specimen simultaneously, we carried out experiments on J-resistance curve estimation for four different types of materials. According to the developed procedure of the unloading compliance method in this study, J-resistance curves for all materials determined from LLD and CMOD data of the CT specimen are clearly exhibited.

On geometry dependent R-curve from size effect law for concrete-like quasibrittle materials

R-curve based on the size effect law previously developed for geometrically similar specimens (geometry type III) is extended to geometries with variable depth (geometry type I) as well as with variable notch (geometry type II), where the R-curve is defined as the envelope of the family of critical strain energy release rates from specimens of different sizes. The results show that the extended R-curve for type I tends to be the same for different specimen configurations, while it is greatly dependent on specimen geometry in terms of the initial crack length. Furthermore, the predicted load-deflection responses from the suggested R-curve are found to agree well with the testing results on concrete and rock materials. Besides, maximum loads for type II specimen are predicted well from the extended R-curve.

Weld Growth Mechanisms and Failure Behavior of Three-Sheet Resistance Spot Welds Made of 5052 Aluminum Alloy

This paper investigates the weld nugget formation in three-sheet aluminum alloy resistance spot welding. The nugget formation process in three equal thickness sheets and three unequal thickness sheets of 5052 aluminum alloy were studied. The results showed that the nugget was initially formed at the workpiece/workpiece interfaces (i.e., both upper interface and lower interface). The two small nuggets then grew along the radial direction and axial direction (welding direction) as the welding time increased. Eventually, the two nuggets fused into one large nugget. During the welding process, the Peltier effect between the Cu-Al caused the shift of the nugget in the welding direction. In addition, the mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were also studied using tensile shear specimen configuration. Three failure modes were identified, namely interfacial, mixed, and pullout. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

Experimental Seismic Performance of a Hybrid Sliding–Rocking Bridge for Various Specimen Configurations and Seismic Loading Conditions

AbstractThis paper presents the major findings of a shake table testing program on a large-scale (1:2.39) novel segmental concrete single-span bridge specimen. Emphasis is given on various specimen configurations and seismic loading conditions. The bridge specimen, termed hybrid sliding–rocking bridge, incorporated a box-girder superstructure with rocking joints and internal unbonded posttensioning (PT), and two single-column piers with internal unbonded PT. The pier columns included end rocking joints and intermediate sliding joints along the column height. Various configurations of the bridge specimen were considered with respect to the seismic mass and the superstructure-to-substructure connectivity. These configurations were subjected to far-field (F-F) and near-fault (N-F) ground motion ensembles scaled to various seismic hazard intensities. Asynchronous support excitation was also considered. The testing program included approximately 145 seismic tests. The dynamic response of the specimen was found...

Influence of interfacial friction and specimen configuration in Split Hopkinson Pressure Bar system

Influences of interface friction and specimen configuration on the material dynamic response using split Hopkinson pressure bar (SHPB) experiment are evaluated using nonlinear finite element (FE) analysis. The effect of various friction conditions between specimen and the transmitted/incident bars in SHPB system is investigated for different specimen geometries. Cylindrical and cuboid specimens with one- and four-layered configurations are adopted and the stress states along the specimen are analyzed. Results indicate that the transmitted signal decreases and the reflected signal increases with friction coefficient increasing. Interface friction brings great variation in stress triaxiality and Lode parameters in the SHPB specimen. Experimental tests are also conducted in this study to verify the conclusions made through FE simulations.

Spring-in of curved CFRP/foam-core sandwich structures

This paper reports on a comprehensive study on process-induced distortions (PID) of curved CFRP/foam-core sandwich panels. The study’s aim is to validate the prediction quality of two existing analytical models and a novel straightforward simulation technique, which utilizes measured spring-in distortions of representative monolithic samples as input. Sandwich specimens with a radius of approximately 350mm are manufactured with three different core thicknesses and two different core run-out geometries.The manufactured parts show a significant radius reduction which declines for increasing core thickness which is satisfactorily predicted by both, the analytical models and the simulation. The effect of the run-out shape turned out to be of inferior relevance for the investigated specimen configurations. However, it is found to be essential to use plane-stress assumptions and to capture the stiffness ratio between the skins and the core. Therefore, an extension of the Fernlund model is proposed whose predictions match excellently with the measured distortion of the specimens. Given the simplicity of the simulation approach in combination with the limited number of inputs makes it a tool of great value, particularly early on in the design cycle.

Numerical investigation of compliance equations used in the R‐curve testing for clamped SE(T) specimens

AbstractThree‐dimensional finite element analysis of clamped single‐edge tension specimen is performed to examine the accuracy of the crack‐mouth opening displacement compliance equations reported in the literature. The analysis covered both plane‐sided and side‐grooved specimens with a wide range of specimen configurations including nine relative crack lengths (a/W) ranging from 0.1 to 0.9 and seven relative thicknesses (B/W) ranging from 0.25 to 4. Based on the finite element analysis results, the crack length/compliance modification factors (ρ(a/W) and ρ(BCE)) are developed to improve the accuracy of the compliance equations. The results of this study can improve the accuracy of the experimentally determined J‐R and CTOD–R curves using the single‐specimen technique and single‐edge tension specimens.

Revised η-Factors andJ-CTOD Relationships for SE(B) Fracture Specimens Including 3D Effects and Implications for Fracture Toughness Measurements

AbstractThis work provides revised η-factors derived from CMOD records and J-CTOD relationships applicable to estimate the J-integral and CTOD in SE(B) specimens with varying crack size and specimen configuration. Very detailed non-linear finite element analyses for plane-strain and 3D models provide the evolution of load with increased crack mouth opening displacement, which is required for the estimation procedure. The analysis matrix considers SE(B) specimens with W=2B and W=B configurations with and without side grooves covering a wide range of specimen thickness, including a precracked Charpy (PCVN) specimen. Overall, the present results provide validation of the J and CTOD evaluation procedure currently adopted by ASTM E1820-11, while at the same time giving improved estimation equations for J and CTOD incorporating 3D effects which enter directly into more accurate testing protocols for experimental measurements of fracture toughness values using 3P SE(B) specimens.

Demonstration of pseudo-ductility in unidirectional hybrid composites made of discontinuous carbon/epoxy and continuous glass/epoxy plies

A new, partially discontinuous architecture is proposed to improve the mechanical performance of pseudo-ductile, unidirectional (UD) interlayer carbon/glass hybrid composites. The concept was successfully demonstrated in different laminates with high strength and high modulus carbon and S-glass epoxy UD prepregs. The novel hybrid architecture provided pseudo-ductile tensile stress–strain responses with a linear initial part followed by a wide plateau and a second linear part, all connected by smooth transitions. The best hybrid configuration showed 60% improvement in modulus compared to pure glass, 860MPa plateau stress and 2% pseudo-ductile strain. The initial modulus, the plateau stress and the overall tensile stress–strain response of each specimen configuration were predicted accurately.

Performance of structural insulated panels with rigid skins subjected to windborne debris impacts – Experimental investigations

Structural insulated panels (SIPs) are high-performance building materials used as building envelope for residential, industrial and light commercial construction. When the building envelope is impacted by windborne debris in cyclonic area, the debris might perforate the panel resulting in dominant openings. Dominant openings in the building envelope might cause differential internal pressurization and result in the damage to the whole building structure. In the present study, the dynamic response of SIPs with Extended Polystyrene (i.e., EPS) foam core sandwiched by two rigid skins of Oriented Strand Board (i.e., OSB) or fiber cement board were experimentally investigated by using a pneumatic cannon system for windborne debris impact. The failure and damage modes under various projectile impact scenarios were observed and compared. The performances of the SIPs were examined quantitatively in terms of the projectile penetration length, and deformation and strain time histories of the back skin. The effects of various specimen configurations, impact locations, projectile impact velocities and boundary conditions on their performance were studied. The penetration resistance capacity of the SIPs against windborne debris impact was analyzed.

An Appropriate Specimen Configuration for Evaluating the Perforation Corrosion Resistance of Automotive Coated Steel Sheets in Accelerated Corrosion Tests

A variety of metallic coatings have been developed for automotive steel sheet panels to achieve better corrosion resistance. Accelerated corrosion tests are used to develop and select coated steel sheets in a short period. In addition to the test conditions, a specimen configuration simulating the lapped portion of vehicles is another important issue when evaluating resistance to perforation corrosion in the laboratory since the specimen must reproduce the unique environment inside the lapped portion. In this study, lapped specimens with different configurations were prepared and the influence of a bare area in a large crevice on the corrosion behavior of zinc coatings was systematically analyzed by comparing the results of laboratory tests with the corrosion observed in vehicles in service. It was found that the controlling factor of resistance to perforation corrosion was influenced by the configuration of the test specimen. A large crevice gap and bare area in the crevice of the lapped portion changed ...

Fatigue behaviour of clinched joints in a steel sheet

AbstractClinch joining has been used in sheet metal work owing to its simplicity and because it facilitates the joining dissimilar metal sheets. In this study, monotonic and fatigue tests were conducted using coach‐peel and cross‐tension type specimens to evaluate the fatigue strength of clinch joints in a cold‐rolled mild steel sheet. The monotonic experimental results reveal that the coach‐peel specimen exhibits the lowest monotonic strength among the three specimen configurations. The coach‐peel and cross‐tension specimen geometries exhibit very low fatigue ratios, compared with the tensile‐shear specimen. The maximum von Mises and principal stresses at the fatigue endurance limit are much higher than the engineering tensile strengths of the steel sheet used to determine the three specimen geometries. Compared with the effective stress and maximum principal stress, the Smith–Watson–Topper fatigue parameter can be used for an appropriate prediction of the current experimental fatigue life. With regard to the coach‐peel specimen geometry, all samples exhibit pull‐out failure mode in the fatigue testing range. However, for the cross‐tension specimen geometry, mixed (pull‐out and interface) and interface failure modes occurred, depending on the number of cycles to failure.

Investigation of plastic eta factors for clamped SE(T) specimens based on three-dimensional finite element analyses

Three-dimensional finite element analyses of clamped single-edge tension (SE(T)) specimens are performed to evaluate the plastic eta factors (i.e. ηpl) for computing the J-integral. The analysis covers both plane-sided and side-grooved specimens with a wide range of specimen configurations (a/W from 0.2 to 0.7 and B/W equal to 1 and 2) and strain hardening exponents (n=5, 8.5, 10, 15 and 20). Based on the analysis results, a set of expressions for ηpl are proposed.

재료의 취성과 연성이 균열의 진전에 미치는 영향

In this paper, the effect of material properties on fracture behavior was studied using cohesive zone model and extended finite element method. The rectangular tensile specimen with a central inclined initial crack was modeled by plane stress elements. In the CZM modeling, cohesive elements were inserted between every bulk elements in the predicted crack propagation region before analysis, while in the XFEM the enrichment to the elements was added as needed during analysis. The crack propagation behavior was examined for brittle and ductile materials. For thin specimen configuration, wrinkle deformation was accounted for by geometrically nonlinear post-buckling analysis and the effect of wrinkling on the crack propagation was investigated.