Three-point Bending Loading Conditions Research Articles

Effect of peening treatment on fatigue strength of welded joints of aged steel

This study experimentally investigates the effect of peening treatment on the fatigue strength of welded joints of aged steel. Fatigue tests were conducted on T-shaped fillet welded joints made from three different types of aged steel under three-point bending loading conditions. To reveal fatigue strength improvement mechanism, residual stress measurement, hardness measurement, and metallographic analysis were performed at the weld toe for both as-weld and peened specimens. Fractured surfaces were examined to identify crack initiation sites and propagation. The findings reveal that peening treatment introduces compressive residual stress on the peened surface of welded joints of aged steel equal to or higher than conventional steel welded joints. Higher improvement of hardness was observed in the near surface of the peened welded joints of aged steel, and the improvement continued up to around the zero crossing points of the residual stress distribution. Additionally, peening treatment might refine near-surface grains of aged steel welded toe similar to conventional steel. The fatigue strength improvement effect of the peening treatment was the same for aged steel as conventional one. Cracks were observed on the peened surface of the aged steel due to over-peening treatment. However, it doesn't affect the fatigue strength improvement of the welded joints of aged steel. In summary, it can be said that peening treatment can improve fatigue strength of welded joints of aged steel in the same manner of conventional steel.

Damage assessment during ultrasonic fatigue testing of a CF-PEKK composite using self-heating phenomenon

The knowledge and experience with the very high cycle fatigue (VHCF) behavior are limited compared to the low cycle and high cycle fatigue behavior of fiber-reinforced composite materials. Accelerated cyclic loading using an ultrasonic fatigue testing system is one potential solution to evaluate the longevity of composite materials within a reasonable time. Insufficient sampling rates and the inherent inaccuracies of non-contact measurement devices pose critical challenges in the adaptation of accelerated fatigue experiments. This study aims to overcome these challenges by combining the temperature response of the composite specimen and the input parameters for the dynamic response of the in-house developed ultrasonic fatigue testing system to detect damage before failure. Increasing- and constant-amplitude fatigue experiments were conducted on a carbon fiber satin fabric-reinforced poly-ether-ketone-ketone (CF-PEKK) composite material. These experiments were carried out under three-point bending loading conditions at a cyclic frequency of about 20 kHz. Various parameters, such as the surface temperature, the cyclic displacement, and the input resonance parameters of the ultrasonic generator, were recorded during these experiments. The VHCF behavior of the CF-PEKK material system was characterized based on these results and through digital light optical microscopy.

Stress and strain calculation method for orthotropic polymer composites under axial and bending ultrasonic fatigue loads

Accelerated fatigue testing is one potential solution to evaluate the very high cycle fatigue behavior of composite materials within a reasonable amount of time. The ultrasonic fatigue testing methodology can be adopted to realize fatigue experiments up to 109 cycles at 20 kHz, compared to conventional fatigue experiments usually carried out between 5–50 Hz. The determination of cyclic stresses during ultrasonic loading remains to be one of the major challenges. The cyclic stresses during ultrasonic fatigue loading were investigated for a carbon fiber 5H satin fabric reinforced in Polyetherketoneketone (CF-PEKK) composite material. Two experimental setups were developed to perform ultrasonic testing under uni-axial and three-point bending loading conditions. A 3D-Scanning Laser Doppler Vibrometer (3D-SLDV) and a single-point Laser Doppler Vibrometer (LDV) were integrated into the test systems to measure the oscillation displacement of the CF-PEKK specimens during ultrasonic cyclic loading. These displacement measurements were used to calculate the resulting strains and stresses under elastic loading conditions. The experimental results were found to be in good agreement with those obtained from finite element models, providing evidence for applying the proposed method.

Bending behavior of triply periodic minimal surface foam-filled tubes

Foam-filled tubes represent one of the best energy-absorbing components for future crashworthiness applications. Triply Periodic Minimal Surface (TPMS)-filled tubes were investigated under quasi-static and dynamic three-point bending loading conditions. The TPMS-filled tubes enhanced specific energy absorption up to 46% compared to the sum of empty tubes and the core response separately. The validated computational models were used to extend the computational study to investigate graded core of TPMS-filled tubes under bending loading. The study results show that the mechanical response of TPMS-filled tubes can be tuned using different relative densities or/and distributions of relative density of the core.

Mechanical behavior of resin pin-reinforced composite sandwich panels under quasi-static indentation and three-point bending loading conditions

This paper reports the mechanical behavior of resin pin-reinforced composite sandwich panels made from polyvinyl chloride core and glass/epoxy face sheets under indentation of a hemispherical indenter and three-point bending loading conditions. The goal was to study the effects of reinforcing parameters such as the number, arrangement, and diameter of the through-the-thickness resin pins on the indentation maximum load, the bending strength, and energy absorption characteristics of the tested samples under these loading conditions. The results revealed that using the resin pins to reinforce the polyvinyl chloride foam core led to increase the indentation maximum load up to 47%, and the maximum bending load up to 34%, compared to nonreinforced foam–core sandwich structures. Also, the presence of resin pins led to the change in the failure modes of specimens (i.e. from local to global deformation and failure) and consequently increased the energy absorption capability of sandwich structures by 31% and 68% respectively under indentation and bending loads.

Collapse modes of aluminium honeycomb sandwich structures under fatigue bending loading

Abstract Aluminium honeycomb sandwich panels are an interesting lightweight structural solution for several applications such as marine structures, aerospace, automotive and aeronautics. In many of these applications, in-service conditions produce fatigue loadings: as a result, safer use of aluminium honeycomb sandwich structures requires a deep knowledge of their fatigue response, which was seldom studied in previous literature. The aim of the current study is to evaluate the fatigue response of aluminium honeycomb sandwich panels subjected to three-point bending loading conditions. The experimental investigation was performed on a commercial aluminium honeycomb sandwich structure with an overall thickness of 11 mm. A preliminary static analysis was performed both under three and four point bending conditions. The static tests allowed the identification of the static bending strength and the absence of a significant strain rate influence. Crashworthiness parameters were evaluated and a slight better performance was found under four point bending. The combination of static tests with Computed Tomography analysis resulted in the observation of the phenomena involved in static bending response of aluminium honeycomb sandwich structures, which are mainly dependent on cell walls buckling. Fatigue tests were conducted under three-point bending conditions. The influence of boundary conditions on fatigue life and on collapse modes were investigated by considering different supports spans. For one condition the S-N curve was obtained and its equation was compared to literature results. Two different collapse mechanisms were observed depending on the supports span: for larger supports span a fracture of the tensioned skin was observed, whereas lower supports span produced core shear. The former mode differed significantly from static failure with the same boundary conditions. In both cases, failure occurred suddenly and this should be taken into consideration in industrial applications. An analytical model was applied to predict fatigue collapse modes and limit loads. A fatigue failure map describing the relationship between supports span, collapse modes and fatigue limit loads was obtained, in order to provide a quantitative tool for aluminium honeycomb sandwich structures design. The fatigue failure map was able to accurately predict the experimental results.

A peridynamic model for brittle damage and fracture in porous materials

We introduce a peridynamic (PD) model for simulating brittle damage and fracture in elastic porous materials based on an Intermediate Homogenization (IH) approach. In this approach, instead of explicitly representing the detailed pore geometry, we use homogenization but maintain some information about the microstructure (porosity) in the model. Porosity is introduced in the model as initial peridynamic damage, implemented by stochastically pre-breaking peridynamic bonds to match the desired porosity value. We validate the model for elastodynamics using wave propagation speed and apparent elastic moduli in porous glasses of various porosities. We then use the model to study the fracture behavior of Berea sandstone under three-point bending loading conditions and an off-center pre-notch. Results agree very well with experiments: we obtain different failure modes depending on the length of the off-center pre-notch. With a short pre-notch, most damage (and subsequent failure) happens in the center of the beam. In this case, material porosity makes the fracture behavior “insensitive” to the presence of the off-center pre-notch. With a long pre-notch, damage starts from near its tip, and progresses in mixed mode towards the loading point. The model captures the location of crack initiation to be the right-corner of the notch, exactly as shown by the experimental acoustic emission results. In contrast, a fully-homogenized model for the porous sandstone fails to reproduce the failure mode sensitivity on the pre-notch length. The results show the importance of using information from the microscale when trying to capture the important effects local heterogeneities (like pores) have on brittle damage initiation and growth in porous materials.

Fiber Bridging Induced Toughening Effects on the Delamination Behavior of Composite Stiffened Panels under Bending Loading: A Numerical/Experimental Study.

In this paper, a research activity, focused on the investigation of new reinforcements able to improve the toughness of composite materials systems, is introduced. The overall aim is to delay the delamination propagation and, consequently, to increase the carrying load capability of composite structures by exploiting the fiber bridging effects. Indeed, the influence of fiber bridging related Mode I fracture toughness (GIc) values on the onset and propagation of delaminations in stiffened composite panels, under three-point bending loading conditions, have been experimentally and numerically studied. The investigated stiffened panels have been manufactured by using epoxy resin/carbon fibers material systems, characterized by different GIc values, which can be associated with the material fiber bridging sensitivity. Experimental data, in terms of load and delaminated area as a function of the out-of-plane displacements, have been obtained for each tested sample. Non-Destructive Inspection (NDI) has been performed to identify the debonding extension and position. To completely understand the evolution of the delamination and its dependence on the material characteristics, experiments have been numerically simulated using a newly developed robust numerical procedure for the delamination growth simulation, able to take into account the influence of the fracture toughness changes, associated with the materials’ fiber bridging sensitivity. The combined use of numerical results and experimental data has allowed introducing interesting considerations of the capability of the fiber bridging to substantially slow down the evolution of the debonding between skin and reinforcements in composite stiffened panels.

Temperature-dependent mechanical behaviour of PMMA: Experimental analysis and modelling

An experimental study of temperature-dependent mechanical behaviour of Poly-methyl methacrylate (PMMA) was performed at a range of temperatures (20 °C, 40 °C, 60 °C and 80 °C) below its glass transition point (108 °C) under uniaxial tension and three-point bending loading conditions. This study was accompanied by simulations aimed at identification of material parameters for two different constitutive material models. Experimental flow curves obtained for PMMA were used in elasto-plastic analysis, while a sim-flow optimization tool was employed for a two-layer viscoplasticity model. The temperature increase significantly affected mechanical behaviour of PMMA, with quasi-brittle fracture at room temperature and super-plastic behaviour (ε>110%) at 80 °C. The two-layer viscoplasticity material model was found to agree better with the experimental data obtained for uniaxial tension than the elasto-plastic description.

A Comparative Experimental Investigation of 3D Angle-Interlock Woven Composite between Quasi-Static Tension and Three-Point Bending Loading Conditions

As one new kind of 3D textile structural composite which has a strong potential in the application field of structural engineering, the mechanical behavior of 3D angle-interlock woven composite (3DAWC) needs to be extensively investigated. In this paper, a comparative experimental analysis of 3DAWC between quasi-static tension and three-point bending loading conditions were described, which has not been well studied so far. To evaluate the mechanical properties of the 3DAWC, the stress-strain/deflection curves were obtained under quasi-static tension and three-point bending loading, respectively. Furthermore, the Young’s modulus and peak values in stress were also compared. All of these results show that the 3DAWC has good integral performance.