Opening Load Research Articles

Fatigue crack characterisation in 2024-T351 aluminium alloy through SEM observation combined with the CJP model

This work characterises crack growth in AA2024-T315 by combining different methods to further increase the reliability of the results. The Christopher-James-Patterson (CJP) model was fitted to experimental data obtained by digital image correlation (DIC). The effective value of the CJP stress intensity factors were successfully correlated with the ΔK-da/dN curve as obtained with Scanning Electron Microscopy measurements of the depth of striations on the fracture surface. This approach based on fitting the CJP model with DIC data and SEM observations allowed estimation of opening and closure loads and allowed the propagation rate and fracture mode to be effectively characterised.

An immersed interface methodology for simulating supersonic spacecraft parachutes with fluid–structure interaction

The capability of a computational fluid–structure interaction method for simulating thin shell structures with an immersed boundary method and a nonlinear structural dynamics solver is extended in order to simulate supersonic spacecraft parachutes. Methodologies for the representation and motion tracking of thin, sub-grid resolution, thickness Lagrangian geometries in a static Eulerian background mesh are presented in detail. Logical functions for constructing reliable near-wall finite difference operators in the presence of thin geometries and trapped/concave volumes of space are presented. The Darcy–Forchheimer momentum equation is solved as a jump condition at the immersed interface in order to model the porosity of parachute broadcloth. A parallel contact identification and enforcement strategy based on first principles is introduced and validated. The coupled method is then used to simulate wind tunnel experiments conducted to support the Mars Science Laboratory (MSL) mission. In these experiments, a sub-scale MSL disk-gap-band parachute is inflated in a range of Mach numbers and dynamic pressures. The computational method shows good agreement with the experimental data, and where available, other simulation and empirical data, in terms of the opening load magnitude at various Mach numbers and the sustained aerodynamic performance measured by the coefficient of drag. Following quantitative comparison, a qualitative analysis is performed to investigate the effect of freestream Mach number, material porosity, and the bluff upstream payload on the main flow features.

Mixed-mode fracture behavior of epoxy-based impact adhesives

The focus of our study was to determine the mixed-mode fracture behavior and the governing fracture (failure) criteria for different impact-resistant/toughened structural adhesives. Four of the five model adhesives used contained organic tougheners in the form of carboxyl terminated butadiene acrylonitrile (CTBN) copolymer, as well as polyurethane adducts. The main crystalline inorganic phases were calcite (CaCO 3 ), wollastonite (CaSiO 3 ) or calcium silicate (CaSiO 3 ), talc (Mg 3 Si 4 O 10 (OH) 2 ), zeolite which is an alumina silicate based mineral and has many different elements in its composition (M 2 /nO.Al 2 O 3 .xSiO 2 .yH 2 O., M can be Mg, Na, Ca, K, Li). The total amount of inorganic fillers was different in each adhesive. Independently Loaded Mixed-Mode Specimen (ILMMS) was used for independent measurement of P I (opening) and P II (in-plane shear) loads which were necessary to determine the strain energy release rate values, G I and G II without using a finite element analysis. Two versions of the ILMMS specimen geometry (a, b) were used: monotonic in opening and static in shear loads, and monotonic in shear and static in opening loads, respectively. Elliptical energy balance failure criterion and parabolic principal stress criterion were used for curve fitting. The experimental data revealed that all but one of the adhesives were more accurately represented by the energy balance criterion except for one that was more accurately represented by the principal stress criterion indicating more brittle behavior. Results obtained from ILLM specimen a showed that ranking varied as related with the G Ic and G IIc values and, also, the applied static load. For example, while Adhesive 2 was in the first rank based on G Ic value, it became third in the ranking based on G II c value. Thus, it can be suggested that ranking as well as the fracture behavior (i.e., cohesive vs. adhesive failure; interfacial separations) change with the extent of the mode mixity present in loading of toughened epoxy adhesives.

The effect of the distance between the end point of the osteotomy and the lateral cortex on the lateral cortical hinge fracture in medial opening-wedge high tibial osteotomy

Background The purpose of this study was to compare the effects of different distances between the end point of the osteotomy and the lateral cortex on the risk of lateral cortical fracture in the medial opening-wedge high tibial osteotomy (MOWHTO) procedure. Methods Eighteen synthetic tibia models were used. Saw cuts were performed on the test models (n=6 for all groups). Wedge gap distance and wedge opening load were evaluated using compression tests. Findings The mean maximum gap distance without a lateral cortical fracture was 19.90 mm in Group 5, 15.49 mm in Group 10, and 11.23 mm in Group 15. The differences between Group 5 and Group 10, Group 5 and Group 15, and Group 10 and Group 15 were statistically significant. The mean load just before the fracture was 13.24 N in Group 5, 18.31 N in Group 10, and 26.16 N in Group 15. The difference between Group 5 and Group 15 was statistically significant. No significant difference was observed between Group 10 and both Group 5 and Group 15. Interpretation As the end point of the osteotomy is brought gradually closer to the lateral cortex, wider gaps can be opened without a lateral cortical fracture. Thus, higher angle corrections can be achieved more safely by bringing the end point of the osteotomy closer to the lateral cortex, which should be preferred to reduce the risk of a lateral cortical hinge fracture during the MOWHTO procedure, from a clinical viewpoint.

The influence of concrete compressive strength on the punching shear capacity of reinforced concrete flat slabs under different opening configurations and loading conditions

In this paper, theoretical and numerical approaches were used to investigate the punching shear capacity of RC flat slabs without shear reinforcement under the effect of loading eccentricity and the presence of an opening adjacent to the column face. Fourty-eight flat slabs of 1800 × 1800 × 120 mm3 were analyzed, having different compressive strengths (30, 35, 40, and 45) MPa, loading eccentricities (0, 75, 150, and 225) mm, and opening configurations, including the opening existence and its location (with or opposite to the eccentricity side). No additional reinforcement was introduced after the opening was made to simulate the effect of an opening in an existing structure. The slabs were supported at 1500 mm in both directions using eight high-strength bolts and were analyzed up to failure under square column loading of 150 mm side. It was concluded that a linear relationship exists between the flat slab cracking, ultimate load capacity, and the concrete compressive strength under different loading eccentricity values. Furthermore, All simulated slabs failed in punching shear. Simulation results for all slabs were calculated using three different codes and compared to those obtained from the numerical simulation. It was found that all the codes underestimated the punching shear capacity of the investigated slabs. In addition, it was proved that the opening location with respect to the eccentricity side significantly affects the behavior of the flat slab, while all the international codes neglect this position.

Toughness arising from inherent strength of polymers

This study carries out spatial-resolved optical birefringence observations to quantify, for mode I (tensile opening) loading, the stress intensification at crack tip of brittle and ductile glassy polymers (polymethyl methacrylate—PMMA, polyethylene terephthalate—PET) as well as one type of elastomer: (ethylene propylene diene monomer—EPDM). We measure the stress buildup in a precut specimen by correlating retardation with the corresponding tensile stress. Given the adequate spatial resolution under 10μm and natural bluntness of intentional through-cuts, we are able to show that (a) during drawing at different stages up to the onset of fracture in precut PMMA and EPDM, the local stress saturates, namely, ceases to increase as r−1/2 upon approaching the cut tip (with r reaching rss in a range of 0.05–0.15 mm), (b) tip stress σtip, i.e., the tensile stress in the stress saturation zone (r≤rss), linearly grows with (operationally defined) stress intensity factor KI until fracture, reaching a level below the breaking stress σb observed of uncut specimen. Thus, the inherent strength σF(inh) under plane strain, taken to be the tip stress at fracture is only comparable to σb. Moreover, a characteristic length P, involved in the observed linearity between KI and σtip, i.e., in KI=σtipP1/2, is found to be comparable to 2πrss. Here rss appears to depend on the tip sharpness, which may be characterized by a radius of curvature ρtip. Thus, toughness given by the critical stress intensity factor KIc is determined by the product of σF(inh) and ρtip1/2, and the critical energy release rate GIc is given by the product of specific work of fracture wF = [σF(inh)]2/2E and ρtip.

Numerical and analytical studies of flexural behaviour of prestressed UHPC joints

This study numerically and analytically investigates flexural behaviour of precast ultrahigh performance concrete (UHPC) segments connected by post-tensioned prestressing strands with or without epoxy on keyed joint surfaces. The analytical formulas are proposed for calculation of joint opening load and ultimate flexural capacity, and a numerical model is established to investigate flexural behaviour of the jointed beam. The joint surfaces without epoxy are simulated by a pair of contact elements with a friction coefficient of 0.6. For the joint surfaces with epoxy, besides the contact elements, a spring element is defined for consideration of the epoxy and an optimized force–displacement relationship is first proposed and input for the spring property. The previous test data and data from one intact beam with low reinforcement ratio tested in the present study are used for validation of the analytical and numerical calculations. The numerical results agree well with the test results in terms of load-midspan deflection, load-joint opening displacement as well as failure modes. Based on the validated numerical model, the effect of number of prestressing strands and beam depth on the flexural behaviour of the prestressed jointed beam is investigated. This study will promote prestressed UHPC joints’ application in practical engineering.

Distribution of the through-thickness effective stress intensity factor range and its influence on fatigue crack growth rate curves

This paper focuses on an analysis of the through-thickness distribution of the effective stress intensity factor range of compact tension specimens, aiming to improve the characterisation of fatigue crack growth rate propagation curves. The maximum stress intensity factor and opening load are evaluated by standard analytical methods and three-dimensional numerical models for various crack lengths and crack front shapes to assess the relevance of considering real crack shapes. The findings reveal that standard procedures attributes only one value of velocity and one value of the fracture parameter for the entire front, whereas numerical methods report different values for both parameters along the thickness. Three-dimensional response surfaces are modelled and validated to estimate the fracture parameters, showing an incongruity in the use of fatigue crack growth rate curves obtained by standard two-dimensional procedures to assess the 3D crack growth.

Experimental research on the fatigue crack growth behaviour of Q420C

The lack of test data on the fatigue crack growth behaviour of Q420C steel affects its application in the field of wind power engineering. To fill this gap, this paper carried out an experimental study and theoretical analysis of the fatigue crack propagation behaviour of Q420C steel under constant amplitude. Six compact tension specimens were separately tested under six loading conditions comprising two maximum loads (Fmax = 8 kN or 10 kN) and three load ratios (R = 0.1, 0.3, or 0.5). The test results show that for the two crack growth rate calculation methods given by the ASTM E647–15, the secant method provides large discrete results, while the incremental polynomial method can obtain a relatively smooth rate curve. In addition, for the incremental polynomial method, the normalisation of the number of cycles in the ASTM E647–15 should not be used to achieve acceptable results. The rate curves of Q420C steel show a clear load ratio effect. Quantitative analysis of the effect of load ratios has shown that two-parameter models based on ‘unified theory’, such as Walker's model, can accurately describe the effect of load ratios. However, the theory of plasticity-induced crack closure cannot account for the effect of load ratios, as the crack opening load Fop is less than the minimum load Fmin for load ratios greater than 0.1. Finally, a comparison is carried out against the predictions by BS7910; results verify the applicability of BS7910 to Q420C steel material and demonstrate the different degrees of conservatism of all recommended design curves in BS7910.

Effective course of pumping of a well rod pump

Determining the length of the plunger’s pumping is an integral part of the calculation of the well pump. However, it should be borne in mind that the pumping course of the plunger is different for each well. Therefore, the article proposes to define it from the dynamogram, on the coordinate parameter “pumping course” which is length, starting from the point of the minimum load on the TPS before the start of the suction. The opening of the supercharged valve shows a minimum load level, which has reached the lowest value due to the maximum high pressure of liquid compression in the working cylinder, after opening load begins to increase. This is the main sign of the opening of the supercharged valve. Mathematical modeling of the interaction of the column of rods and liquid showed: the column of rods, having a higher mass than the mass of liquid, is not subjected to a large forced oscillation, which is reflected on the dynamogram. The opening moment of the discharge valve indicates the beginning of liquid injection or the effective stroke of the plunger, therefore, we need to find exactly this point on the dynamogram circuit.Towards the end of the turn down the column comes to its original state, freed from longitudinal bends and vibrations. Then it can be argued: on the distance of the plunger’s run his intermediate state cannot have an impact. However, this allegation requires proof.

Convergence Investigation of XFEM Enrichment Schemes for Modeling Cohesive Cracks

When simulating cohesive cracks in the XFEM framework, specific enrichment schemes are designed for the non-singular near-tip field and an iteration procedure is used to solve the nonlinearity problem. This paper focuses on convergence and accuracy analysis of XFEM enrichment schemes for cohesive cracks. Four different kinds of enrichment schemes were manufactured based on the development of XFEM. A double-cantilever beam specimen under an opening load was simulated by Matlab programming, assuming both linear and exponential constitutive models. The displacement and load factors were solved simultaneously by the Newton–Raphson iterative procedure. Finally, based on a linear or an exponential constitutive law, the influences of variations in these enrichment schemes, including (i) specialized tip branch functions and (ii) corrected approximations for blending elements, were determined and some conclusions were drawn.

Delamination and compression-after-impact properties of z-pinned composite laminates reinforced with circumferentially notched z-pins

The investigations performed here show a positive influence on the properties of z-pin reinforced laminates using circumferentially notches as a defined z-pin surface micro-structuring. In addition to significant improvements of the fracture mechanical properties under Mode-I crack opening loading, effective reductions of the resulting delamination areas that arise during an impact can be achieved. Furthermore, the residual strength properties of pinned laminates are more favourable than those of unpinned ones and can be further improved by the use of circumferentially notched z-pins. The determined values of the Mode-I critical energy release rate GIC can be increased by approx. 13% if micro-structured z-pins are used. Laminates reinforced with these z-pins also show reductions in the dimensions of the delamination area depending on the impact energy level. Also the compression-after-impact strength can be increased by 6% for low impact energy of 17 J in z-pinned laminates reinforced with circumferentially notched z-pins, compared to those reinforced with unnotched z-pins. For increasing impact energies the values of the pinned laminates with unnotched as well circumferentially notched z-pins converge. Therefore, it can be concluded that for higher impact energies the effect of a defined micro-structuring of the z-pins surfaces on the delamination area after impact and compression-after-impact properties is negligible.

Measurement of Elastic Stresses and Dissipated Energies at Cracks Is the Lock-In-Evaluation an Appropriate Tool?

Lock-In-Thermography is a common technique for determining local stresses in cyclic-loaded components. This so-called Thermographic Stress Analysis (TSA) uses the cyclic change of the temperature, which results from the elastic stress by the thermoelastic effect. To reduce the noise in the measured temperature signal, the evaluation is performed using a Discrete Fourier Transformation. Brémond [1] and Sakagami [2] have shown that in case of plastic deformation a second mode, coupled with the double loading frequency appears in the DFT-evaluation. This part is known as the so-called D-Mode and is assigned to dissipative energies. Investigations of Bär et al. [3] have shown that there are discrepancies in the elastic and dissipative temperature changes between the Lock-In evaluation and an analytical evaluation, showing that the Lock-In evaluation is just qualitatively but not quantitatively correct. In this study a detailed study of the temperature changes in the vicinity of a crack in AISI 316L was undertaken. A determination of temperature-force hysteresis was carried out on a precracked specimen. The comparison of temperature-force hysteresis based on a frequency synthesis of the different modes of the Lock-In evaluation with an analytical evaluation showed only an approximate match. The compensation of the thermoelastic effect with force proportional local stress changes uncovers the dissipative temperature behavior at the crack. A simple isolation of the stress change or rather the dissipative temperature changes out of the E Mode and D-Mode is not generally possible. Crack opening and compressive loads cause thermoelastic temperature changes, which affect the E-Mode as well as the D-Mode and higher harmonic Dk-Modes.

A Study on the Effects of the Interior Architecture on the Fracture Toughness of 3D Printed PLA Samples

Additive manufacturing (AM) using 3D printing techniques is widely used not only in prototyping, but also in production of structural elements in many applications such as medical science and biomechanical engineering. Therefore, it is highly important to investigate the fracture mechanics of components and engineering materials made with 3D printing techniques with the aim of application in biomechanical components. In this study, to investigate the effects of interior architecture on the mixed mode fracture behavior of 3D printed polylactic acid (PLA) components, special Arcan samples were produced at 70% filling ratio and four different filling types using fused filament fabrication technique. A special fixture has been designed that allowed the mixed-mode fracture experiments of the Arcan samples to be conducted on a unidirectional tensile test machine. The fracture tests were performed under 3 different loading angles of 0°, 45° and 90° as opening mode, mixed mode I / II and shear mode, respectively. In addition, the finite element analyses were also conducted to determine the geometric functions of the Arcan samples required for calculation of fracture toughness at different loading angles. Overall, the results of fracture toughness tests revealed that for the sections of the samples that are mainly exposed to opening and mixed-mode loading conditions, printing with the triangular filling pattern provides higher fracture toughness to the final products. In contrast, for the sections exposed to pure shear loadings, hexagonal printing pattern provides a better resistance against fracture.

Finite element contact stress analysis of the temporomandibular joints of patients with temporomandibular disorders under mastication

Background and Objectives: Temporomandibular disorders (TMD) represent a group of diseases occurred in the temporomandibular joint (TMJ) and its surrounding tissues. In epidemiological studies, up to 75% of adults have shown at least one sign of temporomandibular disorders during their examinations. The incongruous biomechanical environment in the TMJ is the main pathogenic factor of TMD. This study attempts to determine the mechanical differences in different groups of TMD patients through biomechanics and to explain the mechanical pathogenesis of TMD according to various cases.Methods: Eleven control subjects and eleven TMD patients were selected and divided into three groups: the control group, bilateral TMD group, and unilateral TMD group. The contact stresses of the articular discs, condyles and temporal bones were analyzed using finite element method and statistical analysis of variance.Results: The results indicated that the contact stresses in the joints with TMD were significantly greater in the Bi-Group (Bilateral TMD patients) compared to the Control-Group. The TMD side always exhibited greater stresses in the Uni-Group [Unilateral TMD patients) under various conditions (clenching on the TMD side or asymptomatic side). The greatest stress of all the groups occurred at the contralateral side with TMD when clenching on the asymptomatic side.Conclusions: Excessive protection would lead to greater stress on the affected side and increased TMD risk on the asymptomatic side. Clinically, the abnormal stress distributions of the disc represented poor buffering and articular clicking. The asymmetric distributions of the articular fossa manifested the deviation of mouth opening or inconsistent TMJ loading.

Study on fracture behavior of nickel-based single crystal superalloy subjected to high temperature fatigue using digital image correlation

Anisotropic mechanical behavior of nickel-based single crystal superalloy (NBSCS) is studied based on the displacement field near the crack tip during the fatigue crack growth at 980°C. A new distortion calibration technique for bi-prism-based single-lens 3D digital image correlation (BSL3DDIC) is proposed to in-situ and on-line characterize 3D full-field deformation with high precision. Virtual displacement extensometers (VDEs) are arranged from the displacement field for the crack opening displacement (COD) determination. To the best of our knowledge, the effect of VDEs locations on the precision of COD measurement is first theoretically analyzed, and the arrangement of VDEs has been optimized. Noticeably, similar quantitative analyses are conducted to determine crack opening ratio (COR) for crack closure evaluation. Moreover, via fitting full-field displacement data to a theoretical model for anisotropic material, not only mode-Ⅰ effective stress intensity factor (SIF) expressed as KeffI and mode-Ⅱ effective SIF (KeffΠ), but also theoretical SIF (KtheoI), crack opening SIF (KopenI) and crack opening load ratio (COLR) are obtained. Based on these important fatigue parameters, modified Paris’ law is used to describe the high-temperature anisotropic crack growth behavior.

Study of low-cycle fatigue crack growth behavior of central-cracked stiffened plates

This work is aims to study the low-cycle fatigue (LCF) crack growth behavior of central-cracked stiffened plates through experimental and numerical analysis methods. A central-cracked stiffened plate with two stiffeners is manufactured to test the influence of the stiffeners on the crack propagation in the base plate. The effects of loading amplitude, the stiffeners’ size and location on the crack growth life are investigated in the test. In numerical analysis, an elastic-plastic numerical model is developed to simulate the crack propagation of stiffened plates considering the influence of the plasticity-induced crack closure (PICC). The results show that PICC and the crack opening load can clearly reflect the crack arrest performance of the stiffener on the crack propagation in the plate. Increasing the stiffener stiffness or decreasing the space between the two stiffeners is beneficial to reduce the driving force acting on the crack tip, prolonging the crack growth life. This work is intended to contribute to a better understanding of the crack arrest performance of stiffened plates through the crack closure mechanism.

Numerical visualization of drop and opening process for parachute-payload system adopting fluid\u2013solid coupling simulation

In order to study the fluid–solid coupling dynamic characteristics of parachute-payload system during drop process and analyze the unsteady aerodynamic characteristics under finite mass opening conditions, an adaptive moving fluid mesh method is developed on the basis of the existing arbitrary Lagrangian–Eulerian (ALE) fluid–solid coupling method. The calculation results of open force and drop velocity on the C-9 parachute demonstrate the effectiveness of this method. On this basis, the effect of canopies with three different permeability on parachute-payload system motion characteristic including opening property, steady descent property and stability is studied. Comparative analysis is conducted for structures and characteristics of vortex with different canopy materials, and interference mechanism of unsteady flow for parachute-payload system in unsteady oscillation is revealed. The results show that the adaptive moving fluid mesh method can effectively eliminate restrictions of existing simulation methods for parachute-payload system and significantly reduce calculation time. For the lightweight parachute, permeability has significant effect on kinetic characteristic of parachute-payload system. Canopy with large permeability has small opening load and structural stress in opening stage. After opening, there are mainly small vortexes distributed evenly behind the canopy with good stability. However, canopy with small permeability has obvious breath behavior and oscillation in opening stage. The main vortexes periodically shed off after opening. With the change of permeability from small to large, Parachute-payload system eventually presents three steady descent modes: conical descent, gliding descent and stable vertical descent.Graphical abstract

Orthogonal sampling moiré method and its application in microscale deformation field measurement

Sampling moiré (SM) method is one of the modern deformation measurement methods in photomechanics. This method realizes the visualization measurement of deformation field by a sparse sampling of grating images, showing the advantages of high efficiency and strong robustness. However, in the traditional sampling moiré method, the unidirectional sampling mode is used with the result that the two-dimensional deformation cannot be acquired at the same time. In this paper, by combining the orthogonal sampling and Fourier phase analysis techniques, an orthogonal sampling moiré (OSM) method is proposed with which the two-directional moiré phase of specimen grating can be separated synchronously. The accuracy of the OSM method for the deformation measurement is verified by numerical experiments and PMMA uniaxial tensile tests. As an application, in a scanning electron microscope (SEM), the deformation field near the crack tip of small fatigue cracks is measured, based on which the crack opening displacement (COD) and the stress intensity factors (K) are obtained. Furthermore, the crack closure effects at different temperatures are characterized. The experimental results show that the crack closure significantly affects the process of small crack growth at both room temperature and high temperature, and the crack opening load at high temperature is greater than that at room temperature, which indicates that small cracks are easier to close at high temperature.

Effects of specimen type and thickness on elastic and elastic–plastic fracture parameters for I-II and I-III mixed mode cracks

This paper focuses on the effects of specimen type and thickness on the fracture parameters for I-II and I-III mixed mode cracks. Results show that mode III and mode II components emerge simultaneously near free surfaces of specimens under I-II and I-III mixed mode loadings, which is called coupling effect. For specimen under I-II mixed mode loading, the level of coupling effect is independent of specimen geometry. While for specimen under I-III mixed mode loading, the coupling effect is much affected by specimen geometry. Thicker middle cracked specimen has smaller coupling effect than thinner single edge notched specimen. Due to the non-negligible coupling effect on crack tip field and fracture behavior, a new mixity parameter taking the coupling effect into consideration is proposed to describe the mixed mode crack. Furthermore, mode I component is the main reason that constraint of I-II or I-III mixed mode crack exists, which is related with loading mode like tensile opening loading and bending opening loading. Specimen under tensile opening loading like middle cracked specimen is in a lower constraint than that under bending opening loading like single edge cracked specimen. Results also shows that the effect of specimen type on constraint is greater than that of thickness.