Specimen Geometry Research Articles

Accelerating environmental ageing effects of adhesively bonded aluminium joints by using smaller bonding areas

ABSTRACT In all areas of application, adhesive joints are exposed to environmental conditions, which can weaken the joint. To verify the functionality of the joints for a certain service life, accelerated ageing tests are used. To assess the mechanical performance of aged joints, single lap joints (SLJ) are often used. The influence of various geometries of SLJ on the strength and stress behaviour is known. The various ageing processes in bonded joints are not yet sufficiently known. Thus, the influence of the specimen geometry on the ageing behaviour cannot be reliably estimated and is therefore investigated in this work. SLJ with different geometries made of an AlMgSi alloy and a one-component epoxy-hybrid adhesive were artificially aged. T dimensions of the SLJ were varied in width, overlap length, adhesive thickness or joint design. Similar ageing results were seen in a significant shorter time with narrower samples or shorter overlap length. A SLJ with a bonding area of 6.3 mm x 6.3 mm was derived from these results. After 4–5 weeks of ageing, this shows comparable results to the standard test SLJ after 8 weeks. This allows to speed up industrial ageing tests nearly by factor two without changing ageing mechanisms.

Comparative analysis of specimen geometries in material extrusion 3D printing: Implications for mechanical property evaluation

AbstractThis study investigated the influence of specimen geometry on the mechanical properties of 3D‐printed parts by material extrusion (MEX). Four widely used technical standards were evaluated (ASTM D638, ASTM D3039/D3039M‐14, ISO 527/2, and ISO 37), and all possible geometries were tested. The specimens were subjected to uniaxial tensile loading using a universal testing machine to obtain the mechanical properties of ultimate tensile strength, yield strength, and Young modulus. Mean, , and values, as well as (number of specimens tested to reach at least 10 valid data) are geometry‐dependent. A critical analysis of these data allowed proposing the standard evaluation index () of the specimen geometry (standard) based on data variance and test efficiency to achieve valid data. The results indicate that ASTM D638 performed better than any other standard. For investigations involving the mechanical properties of tensile strength Types I and II (ASTM D638) are the most recommended specimens geometry (standard) for 3D MEX printing because it combines the lowest with the minimum . By following it, researchers and material suppliers can ensure accurate and reliable assessments of the mechanical properties of 3D‐printed parts, contributing to increased safety, reliability, and quality in additive manufacturing. Notes: MEX – Material Extrusion, ASTM – American Society for Testing and Materials, ISO – International Organization for Standardization, SD – Standard Deviation, CV – Coefficient of Variation, – Number of specimens tested to reach at least 10 valid data, – Standard Evaluation Index.

Two Contributions to Rolling Contact Fatigue Testing Considering Different Diameters of Rail and Wheel Discs

Scaled rolling contact fatigue tests, used to practically simulate the wear of the wheel and rail material under laboratory conditions, are typically classified into two categories. Tests in the first category use twin-disc stands, while the second group of test rigs use two discs of different diameters considering the rail disc as the larger one. The latter setup is closer to the real situation, but problems can occur with high contact pressures and tractions. The focus of this paper is on two main contributions. Firstly, a case study based on finite element analysis is presented, allowing the optimization of the specimen geometry for high contact pressures. Accumulated plastic deformation caused by cycling is responsible for abrupt lateral deformation, which requires the use of an appropriate cyclic plasticity model in the finite element analysis. In the second part of the study, two laser profilers are used to measure the dimensions of the specimen in real time during the rolling contact fatigue test. The proposed technique allows the changes in the specimen dimensions to be characterized during the test itself, and therefore does not require the test to be interrupted. By using real-time values of the specimen’s dimensional contours, it is possible to calculate an instantaneous value of the slip ratio or the contact path width.

A comparison of high strain rate response and adiabatic shear band formation in additively and traditionally manufactured Inconel 718

3D printing of Inconel 718 has been increasingly commercialized, as such research comparing the mechanical properties of additively manufactured (AM) and traditionally manufactured (TM) components in both quasi-static and high rate regimes should be conducted. Herein, the high strain rate material response, as well as the formation of adiabatic shear bands (ASBs) in AM and TM Inconel 718 are studied using a split Hopkinson pressure bar (SHPB) system. A top-hat cylindrical specimen geometry was used to convert the compressive load to shear and conduct high rate shear tests in the SHPB. The quasi-static and high rate stress-strain data reported herein indicates that TM components have at least a 70 MPa higher compressive yield strength. Interestingly, the AM specimens were more susceptible to ASB formation than their TM counterparts. The critical shear strain at which an ASB forms was higher for the TM specimens compared to the AM specimens. This was attributed to the TM material having a higher critical stress for dynamic recrystallization initiation, which is required to ASB formation. The microstructures of the shear zone in the top-hat specimens were examined using an optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). Dynamic recrystallization was observed at the center of the shear affected zone (SAZ) using TEM from material lifted out from the ASB. Due to the plastic deformation in the shear region, the material strain hardens and therefore has a higher hardness than the bulk material. At the center of the shear region, a peak in nano-hardness of 585 HV was recorded on the ASB. The outcome of this research will lead to the prevention, containment, and control of shear instability in AM structures.

Modeling the anisotropic squeeze flow during hot press consolidation of thermoplastic unidirectional fiber-reinforced tapes

The anisotropic material behavior of continuous-fiber-reinforced composites that is evident in their mechanical properties should also be considered in their processing. An important step in the processing of thermoplastic unidirectional (UD) fiber-reinforced tapes is consolidation, where a layup consisting of locally welded UD tape layers is firmly bonded. Compression of the molten thermoplastic matrix material during consolidation leads to a squeeze flow, the direction of which is determined by the fibers. This work presents a model that describes the influence of fiber direction on compression and flow behavior, implemented in the computational fluid dynamics (CFD) software tool OpenFOAM®. To validate the simulation results, we performed experiments in a laboratory consolidation unit, capturing the squeeze flow with cameras and then quantifying it by gray-scale analysis. The specimens used were UD polycarbonate tapes (44% carbon fibers by volume) of various sizes and with various fiber directions. The simulation allows prediction of the changes in specimen geometry during consolidation and is a first step towards optimizing the process by avoiding extensive squeeze flow.

Cohesive Zone Modeling of the Interface Fracture in Full-Thermoplastic Hybrid Composites for Lightweight Application.

With the increasing demand for lightweight and high-performance materials in the automotive and aerospace industries, full-thermoplastic hybrid composites have emerged as a pivotal solution, offering enhanced mechanical properties and design flexibility. This work aims to numerically model the fracture strength in full-thermoplastic hybrid composites made by forming and overmolding organosheets. The mode I fracture was investigated by modeling the behavior of T-joint specimens under a tensile test following the cohesive zone modeling (CZM) approach. The sample was designed to replicate the connection between the laminate and the overmolded part. Double cantilever beam (DCB) specimens were manufactured with organosheets and tested to mode I opening to determine the interlaminar fracture toughness. The fracture toughness out of the mode I test with DCB specimens was used to define the CZM parameters that describe the traction-separation law. Later, due to the particular geometry of the T-join specimens that under tensile load work close to pure mode I, the cohesive parameters were determined by inverse analysis, i.e., calibrating the theoretical models to match experimental results. The fracture resistance T-joint specimens appeared dependent on the fiber-bridging phenomenon during the delamination. In particular, the presence of fiber-bridging visible from the experimental results has been replicated by virtual analyses, and it is observed that it leads to a higher energy value before the interface's complete breakage. Moreover, a correspondence between the mode I fracture toughness of the DCB specimen and T-joint specimens was observed.

Integrating multiple samples into full-field optimization of yield criteria

Describing the anisotropic yielding behavior of sheet metal requires the determination of a yield criterion along with its model parameters. A method often used for this task is inverse full-field optimization. The method is typically combined with either multiaxial experiments or complex-shaped specimens to cover various stress states, over which the parameters of the yield criterion are optimized. This study investigates the feasibility of replacing the complex experiments with multiple simpler ones simultaneously. As material, the aluminum alloy AA6014-T4 is used. The yield criterion considered is YLD2000 2D, assuming both the associated and non-associated flow rules. Firstly, it is demonstrated through identifiability analysis that the model parameters, although often attempted in the literature, cannot be uniquely identified using only conventional experiments if non-associated flow is assumed. Secondly, it is shown that parameter identifiability in the latter case is achieved by combining multiple specimen geometries and orientations to rolling direction simultaneously. Thirdly, it is quantitatively shown that the use of multiple specimens simultaneously generally enhances the identifiability of model parameters due to an increase in the aggregated stress heterogeneity. Furthermore, the result indicates that while the full-field optimization of yield criteria assuming the associated flow rule can rely on force–displacement or direction of plastic strain increment alone, yield criteria assuming the non-associated flow rule require both. All yield criteria are benchmarked based on the strain distribution of Nakazima experiments, demonstrating comparable or higher accuracy compared to conventionally derived models.

Crack propagation mechanism in bedded rock with parallel flaws: Insights from moment tensor inversion

Studying defects’ effect on bedded rock mechanical properties and failure mechanisms is essential for slope and underground excavation engineering. Acoustic emission helps analyze microcrack generation in rocks, shedding light on quasi-brittle material failure. However, previous studies have shown that the inhomogeneous geometry of defect-containing specimens complicates the localization of acoustic emission events, and some events may go undetected by sensors. This study uses discrete element modeling to examine bedded rock failure with parallel defects. We differentiate damage between acoustic emission events and macro fractures by assessing forces on failure sources. Our findings are as follows: (1) Bedding influences crack propagation direction but has limited impact on final failure. Cracks merging and penetrating in the bridge area are critical for catastrophic failure of samples. (2) Shear failure accumulates at the bridge area and defect tip, resulting in fragmented macroscopic failure. Tensile failure occurs during nucleation region development, leading to a more regular macroscopic failure. (3) High-magnitude events primarily arise from microcrack merging and connections, while crack propagation struggles to generate high-magnitude events. Most acoustic emission events involve 1∼4 microcracks, regardless of bedding angles or defects. The acoustic emission simulation method based on moment tensor inversion presents a novel approach for quantitatively determining the crack propagation law and failure mechanism of rocks with defects.

Very High Cycle Fatigue of Welds: A Review

The design life of welded structures and components extends into the very high cycle fatigue (VHCF) regime across various applications. However, the availability of data on the fatigue behaviour of welded joints in the VHCF regime is limited, particularly when compared to the low and high cycle fatigue regimes. The development of ultrasonic fatigue testing equipment has accelerated fatigue testing and allowed for the VHCF properties of welds to be investigated in a feasible timeframe. In the present review, the emerging research concerning the VHCF behaviour of welds of various steels and non-ferrous alloys are individually explored. Overall, it is observed that welded joints have significantly lower fatigue strength than the base metal in the VHCF regime and that welding defects have a considerable influence on fatigue strength. Through the discussion of the relevant literature, important findings concerning the effects of specimen geometry and fatigue improvement methods are underlined. Furthermore, the guidance provided within design standards is compared, and some examples of VHCF failures of in-service components are highlighted. Finally, perspectives on future directions of investigation are put forward with the aim of encouraging further research in the field of VHCF of welds.

Fracture toughness evaluation of a nuclear graphite with non-linear elastic properties by 3D imaging and inverse finite element analysis

Effective small specimen tests are needed to obtain fracture toughness and elastic properties as the limited availability of irradiated graphite restricts the quantity and dimensions of test specimens. Both properties have evaluated simultaneously in a crack propagation test with the double cleavage drilled compression (DCDC) specimen geometry of a fine-grained graphite (SNG742) that has non-linear elastic properties in the unirradiated condition. Three-dimensional displacement fields were obtained by digital volume correlation of in situ laboratory X-ray computed tomographs, and the 3D crack geometry (crack tip position, crack opening displacements and angle) was determined objectively by a wavelet variance method. The tensile softening of the Young modulus was determined by inverse analysis of the strain field using the finite element model updating (FEMU) method. The strain energy release rate of the quasi-static propagating crack was calculated using the contour integral method in a finite element model with the derived non-linear elastic properties and the measured displacements as boundary conditions. The critical strain energy release rate was constant with crack length (118 ± 14 J m−2) and equivalent to a fracture toughness of 1.13 ± 0.07 MPa m1/2.

Influence of specimen geometry on the energy release rate in concrete

The energy release rate during the crack propagation in concrete was studied using three-point bending (TPB) beams with different geometries. Expressions for determining the energy release rate were established. A total of 44 beams, i.e., TPB specimens with the height of 150 mm and various spans and the same span of 800 m but different heights, named as Series-T and Series-TH, respectively, were tested. The obtained results reveal that (1) with the crack propagation, the energy release rate curves (GR-curves) monotonically increased and convergence to the fracture energy of each specimen, and the average values were 161.136 N/m and 112.343 N/m for Series-T and Series-TH. (2) For Series-T, with increasing the specimen span from 300 mm to 900 mm, the maximum fracture process zone (FPZ) length kept constant of 68.317 mm, and for Series-TH, the result increased by 187.6 % with increasing the specimen height from 100 mm to 300 mm. Moreover, the relative FPZ length was not affected by the spans and depths of the specimens. (3) The evolution of crack tip opening displacement was only affected by the specimen ligament length, i.e. for the same crack extension length, a larger value was noticed for longer ligament length. (4) The stability during crack propagation can be analyzed using GR-curves.

Mechanical Behavior of Seamless Pipes Using Ring Expansion Technique and Novel Hoop Stress Correlation Factor (K)

This study investigated the stress–strain behavior of seamless pipes in the hoop direction using the ring expansion test, which is a non-standardized mechanical testing technique used for evaluating the mechanical properties of round tubes. However, this technique has limitations, such as unidentified specimen geometry, strain measurement, and the estimation of friction coefficients. The study employed experimental, numerical, and analytical methodologies to address these limitations and throughout the study, a novel hoop stress correlation factor (K) was identified to be multiplied by the hoop stress derived equation for reduced section ring specimens. The experimental strain was measured using a newly derived analytical equation, and a mathematical predictive model was developed to estimate the K-factor using the Design of Experiment (DoE) and Design-Expert statistical software. The study concluded that the ring expansion test is a promising technique for evaluating the mechanical properties of seamless pipes similar to the unified axial tensile stress–strain behavior. However, future research is needed to estimate the hoop stress correlation value (K) for all ring geometries. The study's finding of the novel hoop stress correlation factor (K) in the case of a reduced section ring specimen is particularly noteworthy, as it addresses a significant research gap in the field.

Characterization of a short fiber-reinforced adhesive designed for wind turbine rotor blades

Wind turbine rotor blades are usually made of two half-shells and shear webs that are joined together by structural adhesives. The adhesive joints suffer from longitudinal strains from blade bending combined with shear strains from torsion and shear forces acting on the blade. It is thus important to provide an accurate and reliable characterization of the adhesive as a bulk material to account for cohesive failure.This work focusses on the characterization of a short fiber-reinforced adhesive used in wind turbine rotor blades under uniaxial tension, compression, and shear as well as biaxial tension-compression/shear for both static and fatigue loading. To obtain small scatter in measurement results, a specimen geometry was designed that ensures a clear maximum stress in the test section while minimizing stress concentrations in the transition between the load introduction regions and the test section. The manufacturing process was then designed in such a way that an optimum mixture of resin and hardener was obtained without the inclusion of voids. The latter was verified after manufacturing by means of micro-CT scanning for all specimens. A very extensive test campaign was then carried out in order to quantify stiffness, static strength, fatigue strength, and fatigue-related stiffness degradation. Excerpts of this test campaign are presented in this paper.

Cohesive zone modelling to predict crack growth under fatigue loading

This manuscript presents the partial results in the development of numerical models based on Cohesive Zone Modelling to analyse quasi-static and fatigue crack propagation in adhesive interfaces. This work applied to adhesive interface between fibre laminates and structural epoxy adhesive of a wind turbine blade 3rd shear-web connection to the trailing edge panels. Fracture characterization experiments using the Double Cantilever Beam with Unequal Bending Moments (DCB-UBM) specimen are performed, and results are used to determine different modelling parameters, such as the mixed-mode cohesive laws and fatigue damage parameters for cohesive crack propagation. The developed numerical model is compared and validated against sub-component testing on two different scale levels, using a Modified Tear Test (MTT) specimen geometry with simplified geometry and loading, and a more complex geometry sub-component specimen with a simplified Shear Web Disbond Test (SBDT). The work is carried out as part of the CORTIR-II project, funded by the Danish Ministry of Energy.

Fracture toughness of freestanding plasma sprayed yttria stabilized zirconia coatings via in situ tensile experiments

Tensile response and fracture toughness of the freestanding atmospheric plasma spray 8YSZ coatings was investigated using in situ high resolution optical full-field strain measurement. The elastic properties obtained directly from the tensile experiments were employed in computing the critical mode-I stress intensity factor. Comparison between the fracture toughness values computed from specimen geometry and J-integral via displacement field around the crack tip showed small differences on single edge notched tensile (SENT) specimens to extract their intrinsic mode-I fracture toughness. Two kinds of coatings having different porosity were studied to obtain stress-strain response and compute effect of porosity on mode-I fracture toughness. The samples were fabricated by depositing the coatings on, un-notched and pre-notched substrate templates, and then carefully separating the coatings by acidic dissolution of the substrate. High resolution in situ deformation of the coatings was captured for both tensile and SENT specimens using digital image correlation method combined with an in-house designed microtensile stage. The deformation from the specimen surface was used to obtain elastic material properties under uniaxial tension, which are then used to compute the energy release rate by the path independent J-integral. The critical stress intensity factors calculated from SENT geometry for the low and high porosity coatings were found to be 1.23 ± 0.21 MPa-m0.5 and 0.67 ± 0.12 MPa-m0.5, respectively. In addition, considering the experimental observation of nucleation and propagation of a crack from the notch tip before catastrophic failure, the J-integral based calculation of the critical fracture toughness values were 1.37 ± 0.19 MPa-m0.5 and 0.5 ± 0.1 MPa-m0.5, for the low and high porosity coatings, respectively. As the crack always nucleates from the notch before complete fracture, it can be concluded that the J-integral based method is accurate and suitable for finding the fracture toughness in porous coatings.

Orientation-dependent extreme shear strain in single-crystalline silicon - from elasticity to fracture

Measuring strain accurately at small length scales poses a significant challenge, making it difficult to obtain precise elastic properties of small materials. This becomes particularly pronounced for test geometries beyond micro-pillars and for materials with high elastic limits and high Peierls stresses. This study investigates the elastic strain limits and strain distribution in micro double shear tests conducted on single-crystalline silicon with different crystallographic orientations. In situ scanning electron microscopy images were used to obtain full-field strain maps using digital image correlation. This local strain analysis approach revealed that the shear zones of the test geometry are not solely under pure shear conditions, but also experience superimposed bending. The local strain analysis approach increases the precision of measured elastic properties to ±15% of the literature value compared to deviations of 75-80% using the traditional global strain analysis approach. This study highlights the limitations of the global strain analysis approach in complex specimen geometries and demonstrates the effectiveness of digital image correlation in accurately determining elastic strain at the small length scale. Furthermore, both the low defect density in the samples as well as the small length scales allow for the exploration of orientation dependent strength levels close to the theoretical limit.

Embedding strain-rate sensitivities of multiple deformation mechanisms to predict the behavior of a precipitate-hardened WE43 alloy under a wide range of strain rates

A rare earth Mg alloy, WE43, exhibits high strength, good ductility, low anisotropy, and moderately high strain rate sensitivity. As such, the alloy is a viable candidate for high strain rate applications. In this work, a comprehensive set of mechanical and microstructure data recorded during quasi-static, high strain rate split Hopkinson bar (SHB), and impact tests on specimens of WE43 Mg alloy reported in (Savage et al., 2020b) is simulated and interpreted using an advanced Taylor-type crystal plasticity finite element (T-CPFE) model. The T-CPFE model is formulated physically to embed two sources of strain-rate sensitivities inherent to each slip and twinning mode in WE43, one that occurs under constant structure and another that affects structure evolution. The model parameters are established for the alloy by achieving agreement in the stress-strain response and microstructure evolution under quasi-static and SHB tests. Density functional theory calculations of anti-phase boundary (APB) energy are carried out to explain origins of the unusually large initial slip resistance for basal dislocations, which shear precipitates in the alloy. The initial slip resistances of the prismatic and pyramidal dislocations are, instead, rationalized by Orowan looping around precipitates. After calibration and validation, the model is shown to successfully predict WE43 response at much larger strain rates than those used for model calibration. Specifically, mechanical response, specimen geometry changes, twin volume fractions, and texture evolution are predicted for different orientations of the Taylor cylinders. Details of the modeling framework, comparison between simulation and experimental results, and insights from the results are presented and discussed.

High strain rate constitutive and fracture characterization of AA7075-T6 sheet under various stress states

The stress state and strain rate dependent fracture behavior of high strength aluminum sheet, AA7075-T6, is characterized experimentally and numerically. Experiments were performed using shear, hole tension, notch tension, and groove tension specimen geometries at strain rates in the regime 10−2 – 500 s−1. The temperature change on the specimen surface was measured utilizing high speed thermal imaging, while strains were acquired using DIC-based optical measurements. Mild positive rate sensitivity was observed in the flow response for equivalent strains up to approximately 20%, while the flow stress decreased at higher (500 s−1) strain rates as the strain levels increased. An increase in temperature was observed within the gauge region of the specimens during elevated strain rate (500 s−1) tests leading to thermal softening. Positive strain rate sensitivity was observed in the measured axial force response for the hole tension and notch tension specimens. The fracture limits of the AA7075-T6 in the shear, hole tension, and notch specimens decreased between strain rates of 0.01 and 10 s−1, then increased at strain rates between 10 and 500 s−1. A strain rate and temperature dependent Hockett–Sherby (HS-SRT) constitutive model was proposed to capture the dependency of the flow stress on the equivalent plastic strain, strain rate, and temperature. The generalized Drucker-Prager (GDP) fracture model was extended to capture the effect of strain rate on fracture initiation. Numerical simulations of the coupon experiments were then performed to extract the local stress and strain states to calibrate the GDP fracture model across the range of strains considered.

Impact of Notches on Additively Manufactured Inconel 718 Tensile Performance.

This study was completed in effort to characterize the notch sensitivity of additively manufactured (AM) Inconel 718 produced by laser powder bed fusion (L-PBF). Three different root radii on V-notched test specimens and smooth specimens were evaluated under tensile conditions for specimens built in vertical and horizontal orientations. Both the total axial strain and localized notch diametral strain were measured. Finite element analysis (FEA) was completed on each specimen geometry to confirm the actual strain measurements near the notch. Test results showed the tensile strength of the notched specimens were larger than the tensile strength values of the smooth specimens. These tensile results equate to a notch-sensitivity ratio (NSR) greater than one, indicating that the L-PBF Inconel 718 material is a notch-strengthened material. It is suspected that the notch strengthening is a result of increased triaxial stress produced near the notch tip causing added material constraints, resulting in higher strength values for the notched specimens. Fractography analysis was completed on the various fracture surfaces and identified a dominate ductile failure mode within all of the specimens; however, the amount of ductility reduced with smaller notch root radii. While this study provides the initial notch responses of the L-PBF Inconel 718, further research must be completed in regard to the impact of notches on more complex loading behaviors, such as fatigue and stress-rupture conditions.

A re-assessment of Ti-685 as a dwell sensitive titanium alloy and a definition for engineering relevant dwell behaviour

It is believed that the near-α titanium alloy Ti-685 was the first to demonstrate dwell sensitive fatigue behaviour whilst under engineering service as a gas turbine fan disc material. New fatigue data, micro-textural characterisation and fractography from a recent study which utilised remnant Ti-685 specimens originally manufactured for previous laboratory campaigns based at Swansea dating back to the late 1980s are now reported. In isolation, the present results failed to demonstrate a significant dwell effect in either aligned or basketweave microstructural variants. On this occasion, the limited sensitivity to dwell loading was explained by the combination of heat treatment artifacts and selection of a relatively small test specimen geometry. However, the present data were also reviewed in the context of the wider fatigue database generated by multiple studies over the past fifty years. In combination, stress-life (SN) curves defined the magnitude in applied peak stress necessary to demonstrate a dwell effect in this alloy. Ultimately, this facilitated a fundamental definition of dwell sensitivity that is relevant to the engineering community.