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Success of a hydraulic fracturing operation depends on the height and width of the induced fractures. One of the critical components controlling fracture size is fracture toughness of the formation. In this work, mode I fracture toughness of Berea Sandstone and Mancos Shale is measured by combining semi-circular bend test (SCB) and digital image correlation (DIC). Experiments were carried out in different notch orientations with respect to bedding. DIC is used to measure full-field displacements and to visualize and quantify fracture process zone (FPZ). Full-field displacements from DIC are utilized in Williams’ series solution to extract critical stress intensity factor, or fracture toughness. Accuracy of measuring fracture toughness using DIC displacements depends on area of interest (AOI), field of view (FOV), and the number of terms of solution (N). A parametric study is conducted, allowing to choose an optimal set of these parameters for evaluation of fracture toughness in rock specimens. It is known that fracture toughness values obtained directly from the SCB test, using conventional maximum load method, are underestimated due to the effect of nonlinear behavior caused by the fracture process zone. FPZ length is considered as an increase in the effective crack length. Irwin’s correction for effective crack length is utilized to measure fracture toughness values obtained directly from the SCB test that account for the fracture process zone. Fracture toughness values measured using DIC method and Irwin’s correction method are in a good match for both Berea Sandstone and Mancos Shale. Both methods show higher fracture toughness for samples in arrester orientation. In addition, the results show that FPZ length in Berea Sandstone is much larger than in Mancos Shale.

The current work investigates the effect of the size of the consumable sheet during friction stir spot welding of AA6063-T6 and CRCA/IS-513 thin sheets through experiments and finite element (FE) simulations. Size ratio, which is defined by shoulder diameter (SD) and consumable diameter (CD), was optimized through bend tests, material flow, and microstructure analysis. Temperature, axial force, torque, effective strain, and strain rate are measured through experiments and simulations. Bendability, joint grain size, intermetallic compounds, and microstructures are revealed following the joining process. FE simulations are performed in DEFORM 3D with calibrated Hollomon and Voce flow stress models. The peak temperature rose 52.7% when the SD was increased from 9 to 15 mm, but only 36% when the SD was increased from 18 to 21 mm. Stir zone with recrystallized grains of 4.3 μm size was observed in Case 3 (15 mm SD and 20 mm CD). Case 5 (21 mm SD and 28 mm CD) had higher hardness (298 HV) because of IMCs at the joint interface (1.8 µm thick AlFe and FeAl 3 ). In case 3, the axial force during FSSW-C peaked at 2835 N, increasing 147% from case 1 before decreasing 38%. In case 1, the torque was 216 N-mm; in case 5, it was 864 N-mm. Root bend shear tests showed higher fracture loads than face bend tests. FE simulated temperatures agreed with experimental values within 6.5%. At the upper surface of the consumable sheet, Case 5 exhibited the highest effective strain of 121.08 mm/mm and strain rate of 115 s −1 . On the other hand, case 1 recorded the lowest values of 47.12 mm/mm and 21.20 s −1 , which is consistent with the literature on the impact of SD. Using the results, a size ratio of 0.75 is recommended for optimal joining of dissimilar sheets.

Asphalt composite concrete pavement is one of the common pavement forms in China. However, due to the influence of design, materials, construction quality, and other aspects, asphalt composite concrete pavement develops various degrees of cracks after being put into use, which affects the service performance and life of asphalt pavement. The Burgers model is used to examine the effects of fiber-volume-fraction and length–diameter-ratio on the viscoelastic mechanical behavior model parameters and viscoelastic properties of asphalt composite concrete through the bend test for creep of polyester fiber asphalt composite concrete beam. The findings indicate that the fiber’s ability to control asphalt composite concrete bending creep deformation increases initially and subsequently diminishes as fiber-volume-fraction and length–diameter-ratio increase. Fiber-volume-fraction and length–diameter-ratio effects can be fully reflected by fiber amount characteristics. A viscoelastic mechanical behavior model of fiber-reinforced asphalt composite concrete is developed on this foundation while taking into account the influence of fiber amount characteristic factors. Theoretical study and practical research indicate that the ideal fiber volume ratio of polyester fiber asphalt composite concrete is 0.35 percent, the ideal length-to-diameter ratio is 324, and the ideal fiber amount characteristic parameter is 1.13. The test results can provide a certain reference value for the improvement of the long-term durability of fiber asphalt composite concrete pavement of road engineering.

The prevalence of adolescent idiopathic scoliosis (AIS) is increasing, partly due to a lack of physical activity. In a cross-sectional study with 18,216 pupils (5th, 6th, and 8th grades) from four Croatian counties using the forward bend test (FBT; presumed AIS), the prevalence of AIS and its correlation with physical activity were evaluated. Pupils with presumed AIS were less physically active than their peers without scoliosis (p < 0.001). Abnormal FBT was more prevalent among girls than boys (8.3% vs. 3.2%). Boys were more physically active than girls (p < 0.001). Pupils with presumed AIS were less physically active than their peers without scoliosis (p < 0.001). A higher prevalence of presumed AIS was found among inactive or just recreationally active schoolchildren than among those engaged in organized sports (p = 0.001), girls especially. Pupils with presumed AIS were less active and had fewer weekly sports sessions than their peers without scoliosis (p < 0.001). Notably low prevalence of AIS was detected among pupils engaged in soccer (2.8%, p < 0.001), handball (3.4%, p = 0.002), and martial arts (3.9%, p = 0.006), while it was higher than expected in swimming (8.6%, p = 0.012), dancing (7.7%, p = 0.024), and volleyball (8.2%, p = 0.001) participants. No difference was detected for other sports. A positive correlation was found between time spent using handheld electronic devices and the prevalence of scoliosis (rs = 0.06, p < 0.01). This study confirms the increasing prevalence of AIS, particularly among less athletic girls. Further, prospective studies in this field are required to explain whether the higher prevalence of AIS in these sports is due to referral or other aspects.

Understanding the microstructure–property relationship in aluminum extrusions is crucial to leverage their potential in automotive lightweighting. The sensitivity of the processing history to the microstructure and through-thickness variations poses a major challenge since it leads to strong directionality in plasticity and fracture. Reliable characterization of the mechanical response under relevant stress states is crucial for the development of modeling strategies and performance ranking in alloy design. To this end, tensile and 3-point bend tests were performed for an aluminum extrusion produced on a laboratory-scale extrusion press at Rio Tinto Aluminium. Direct measurements of surface strains during bending using stereoscopic digital image correlation revealed that a larger bend angle in the VDA238-100 test does not necessarily imply a higher fracture strain. The T4 sample tested in the extrusion direction sustained a bend angle of 104° compared to 68° in T6 for the same nominal bend severity (ratio of sheet thickness to punch radius), despite comparable major fracture strains of 0.60 and 0.58, respectively. It is proposed that the work-hardening behavior governs the strain distribution on the outer bend surface. The higher hardening rate in the T4 condition helped distribute deformation in the bend zone more uniformly. This delayed fracture to larger bend angles since strain is accumulated at a lower rate. To assess whether the effect of the hardening behavior is manifest at a microstructural lengthscale, microcomputed tomography (μ-CT) scans were conducted on interrupted bend samples. The distribution and severity of damage in the form of cracks on the outer bend surface were distinct to the temper and thus the hardening rate.

BackgroundThe VDA 238–100 tight radius V-bend test can be used to efficiently characterize the bendability and fracture limits of sheet metals in severe plane strain bending. Material performance in plane strain bending is critical for the selection of advanced high strength steels for energy absorbing structural components.ObjectiveThe detection of failure based upon a reduction in the punch force can lead to erroneous predictions of failure for ductile or thin gage alloys in the VDA 238–100 test. New failure criteria were proposed and evaluated across a range of automotive steels.MethodsFour detection methods in the V-bend test were evaluated based upon the load drop, bending moment, novel stress metric and the strain rate for seven steels with strength levels from 270 to 1500 MPa. The appropriate failure threshold was identified from visual inspection of the surface during bending.ResultsThe vertical punch force will decrease as a consequence of the mechanics in the V-bend test at intermediate bend angles even without fracture. The novel stress-based metric accounts for sheet thinning and could successfully identify “false positives” and punch lift-off when considering the strain-rate evolution.ConclusionsFailure detection using the VDA load threshold method may significantly under-report the bend performance of alloys with intermediate-to-high bendability or thin gauges. The proposed stress-based metric can reliably detect fracture for bend angles in excess of 160° and be readily calculated using the existing data. The VDA load threshold for failure can work well for materials that exhibit significant cracking.

Composite laminated structural members in segmented cylindrical composite pressure vessels are integrated with metallic counterparts using mechanical fasteners. Load transfer in such structures will be through the laminate bearing surface via the mechanical fasteners. Earlier studies indicate a close association of bearing strength with compressive strength of the laminate and mode-2 shear failure. Specimen tests are conducted to examine the variation of strength with respect to laminate sequence, preload on fasteners, the presence of low modulus separators, and laminate thickness. Compressive stress predictions close to bearing locations are found to be higher when compared to the specified macrocompressive strength of carbon fiber-reinforced polymer (CFRP). Stress measurements close to the hole are hampered by the presence of bolt head and washers. Therefore, 3-point bend tests are conducted to examine the stability of CFRP layers in the compressive stress region, and the measured strains confirm sustenance at compressive strain levels. Additional tests are conducted with fiber-metal-laminates (FMLs) to study the effects of cyclic loading on metals and the strength of bonding between FRP and metal. Effect of yielding and post-yielding residual stresses on metal, CFRP as well as the bonded interface between FRP and metal is examined. Bond failure is noticed at 1.4% strain in aluminum layer, and the CFRP layer under compressive strain sustained this loading. Finite element (FE) analysis of bearing test and 3-point bend test is performed to predict the maximum stress near the bearing surface and compressive strength response of laminates, respectively.

Quick, low-cost, and high-quality manufacturing is considered a key factor in today’s industry. Therefore, researchers have turned to inventing new methods and technologies for meeting such industrial requirements. Liquid impact forming is one such method which is being increasingly developed in different industries, such as automotive and aerospace. Considered to be a tube hydroforming process, this forming method utilizes liquid pressure to produce the desired shape. In this study, the liquid impact forming process, which was applied to a thin-walled tube made of 6063 aluminum alloy, was experimentally and numerically investigated. In the experimental section, a new die was designed and manufactured for deforming the cross section of the aluminum tube into a hexagonal profile. To investigate the characteristics of the hexagonal profile obtained from the forming process, tensile and three-point bend tests were performed. According to the results obtained from the tensile test, the tensile yield strength in the workpiece increased by 21 MPa due to work hardening. The results obtained from the three-point bend test indicated that the flexural strength of the circular tube was greater than that of the hexagonal profile due to its greater moment of inertia. The numerical results included plastic equivalent strain distribution, variations in the profile thickness, and the force required for the forming process. Upon comparing the workpiece thicknesses obtained from numerical simulation and measurements, a good agreement was observed.

Fused filament fabrication (FFF) is the most widely used additive manufacturing (AM) technology for printing thermoplastic materials, among them the ABS. A significant problem of 3D-printed parts manufactured by AM-FFF is the anisotropy of their mechanical properties. Thus, it is of great importance to understand the impact of the build strategy of the mechanical properties and failure mechanisms of AM-FFF ABS components. This research aims, at least partly, to fill this gap by studying the structure and mechanical behavior by performing fracture surface analysis of AM-FFF ABS specimens under the three-point bend test. For this purpose, three build orientations (flat, on-edge and upright), each built at 0°/90° and -45°/+45° raster angles and oblique printed samples (0°, 15°, 30°, 45°, 60°, and 75°) built at -45°/+45° raster angles were prepared. The results revealed that the build direction with the lowest density, the flexural modulus of elasticity, flexural strength, and deflection was in the upright direction for both 0°/90° and -45°/+45° raster orientations. Overall, two main failure modes were observed for the tested specimens: (1) inter-layer/inter-raster bond failure, which is the main contributor to failure of all upright samples and (2) intra-layer/trans-raster failure, which is the main contributor to failure of flat and on-edge specimens printed at -45°/+45° raster orientation. The results of the oblique printed samples demonstrate that a single crack initiation can transform into a few inter-laminar and intra-laminar fracture surfaces due to competing stress fields and structural gradients

Fiberglass composites have been extensively studied in recent decades. However, studies on damage in randomly oriented chopped glass fiber composites are limited. In this work, experiment and numerical analyses were performed to investigate damage in randomly oriented chopped glass fiber-reinforced plastics (GFRP) for flexural behavior. The 3.3-mm-thick composite laminate was fabricated by hand lay-up method and cured under atmospheric conditions for at least 24 h. Then, the fabricated laminate specimens were experimentally tested using the 3-point bend test (3-PBT) according to ASTM D790 with a universal testing machine (WDW-20) to evaluate the flexural strength and various damages under the applied load. The experimental analysis showed a breaking load and flexural strength of 230.75 N and 159.5 MPa, respectively. In addition, an optical microscope is used to study the different failure modes and their interactions with each ply. Cracks in the matrix and delaminations are observed between the successive plies of the specimens. In addition, integrated Hashin failure criteria are implemented to identify the various damages in laminated composites in the commercially available finite element software (FE) ANSYS 19.0. The simulated results show good agreement with the experimental results.