Key-hole Notches Research Articles

Fracture assessment of inclined double keyhole notches in polylactic acid specimens fabricated by fused deposition modeling and traditional method

AbstractThe fracture behavior of specimens made of polylactic acid and weakened by inclined double keyhole notches is investigated by both experimental and numerical methods. The test specimens are fabricated by two different methods; fused deposition modeling and traditional methods. The mechanical behavior of specimens was investigated under mixed‐mode loading with different notch inclination angles, various notch tip radius, and multiple raster angles. An experimental program was performed, and 72 new experimental data were provided. A fracture criterion based on the strain energy density is used to assess the critical fracture loads of the tested specimens. Good agreement is found between experimental and numerical data. The results showed that increasing the notch inclination angle and notch tip radius led to a decrease in the fracture load of the specimens. Also, fracture loads of the classical specimens are higher than the 3D‐printed specimens. In the 3D‐printed specimens, fracture loads for the samples with raster angles of 0°, 45°, and 90° have the highest value, respectively.Highlights Fracture assessment of polylactic acid specimens Specimens were weakened by using an eccentric inclined double keyhole notch Specimens manufactured by fused deposition modeling and traditional method Investigation of different notch inclination angles and notch tip radius Evaluation of specimens using experimental and numerical analysis methods

Brewing sustainability and strength: Biocomposite development through tea waste incorporation in polylactic acid

This study aims to enhance the mechanical properties of polylactic acid by incorporating black tea waste as an economical and sustainable additive capable of being recycled. The black tea waste, after the hot water color removal process, is milled to a fine and uniform powder size. The combination of these particles with polylactic acid is carried out using a twin-screw extruder. Specimens of pure polylactic acid and biocomposites containing 3 and 5 wt% black tea waste are manufactured using a hot press machine at a temperature of 200 °C. The filaments are also successfully extruded for three-dimensional printing purposes. Scanning electron microscopy images reveal that in the polylactic acid–3% tea biocomposite, the tea particles are appropriately dispersed within the polylactic acid matrix. The tensile test results indicate that biocomposite polylactic acid-3% tea has the highest mechanical properties, with a tensile strength of 67 MPa, demonstrating a 34% increase compared to polylactic acid. Furthermore, the biocomposite of polylactic acid–3% tea waste exhibits the best performance with a fracture energy of 2.5 kJ/m2 in the impact test. Hence, polylactic acid–3% tea biocomposite can be recommended as a suitable sustainable substitute. Finally, numerical simulations are performed on biocomposites containing double keyhole notches. This analysis is conducted to observe the behavior of biocomposite models with double keyhole notch under mixed-mode loading. The critical fracture load of the models is calculated using the strain energy density criterion. It is observed that an increase in the notch inclination angle and notch radius leads to a decrease in the fracture load in the models.

Prediction of last ply failure load of keyhole notched laminated composites using the virtual isotropic material concept

Composite laminates have become increasingly popular in industries like aerospace and shipbuilding. In practical applications, the creation of notches in composite laminates is often necessary to enable assembly and modifications. This rise in the use of notched composite laminates highlights the need for reliable methods to predict potential failures in these components. With these predictions, the safety and reliability of composite structures can be ensured by preventing catastrophic failures. The present paper investigates the use of the Virtual Isotropic Material Concept (VIMC) combined with the Finite Fracture Mechanics (FFM) criterion to predict the Last Ply Failure (LPF) load in keyhole notched composite laminates and under mode I loading conditions. Experimental tests were conducted to measure the LPF load of E-glass/Epoxy keyhole notched specimens, and the results were compared with the predicted results using the VIMC-FFM criterion. The paper also derives an expression for the Stress Intensity Factor (SIF) function for a crack that emanates from the keyhole notch tip under mode I loading conditions. The derived equation matches very well with numerical data and have a mean absolute error about 2% in the studied domain. The study shows that the VIMC-FFM criterion predicts the LPF load of keyhole notched composite laminates with acceptable accuracy and in a simple and low-cost manner.

Sensitivity analysis of notch shape on brittle failure by using uni-bond dual-parameter peridynamics

The brittle failure of components weakened by sharp or blunt notches is a topic of active and continuous research. Notches created during the design and manufacturing of structural components give rise to localized stress concentrations, which can generate a crack leading to catastrophic failure or to a shortening of the assessed structural life. To investigate how notches influence the fracture and bearing capacity of brittle materials by using peridynamics (PD), an extended uni-bond dual-parameter peridynamic (UDPD) model that can overcome the fixed Poisson’s ratio in bond-based PD and address the attenuation effect of the nonlocal PD force is developed. The expressions of normal stiffness and tangential stiffness with elastic modulus and Poisson’s ratio are derived and rewritten. Additionally, the critical microstrain energy density for the extended UDPD model is given to judge the fracture initiation, and the validity and accuracy of the proposed model are verified by four examples. Ultimately, six typical types of notches, i.e., rectangular notches, sharp V notches, U notches, blunt V notches, keyhole notches, and VO notches, were selected and investigated in brittle plates with four different tip radii to study the notch sensitivity of brittle failure to notch shape by using the extended UDPD model. The results show that the crack path and bearing capacity of the structure are sensitive to the notch shape and the tip radius of the notch, and a blunt notch with a larger tip radius (U notch, blunt V notch, keyhole notch, and VO notch, tip radius ρ⩾2mm) offers better performance in terms of structural safety than a sharp notch or a notch with a smaller tip radius.

Extension of the Equivalent Material Concept to Compressive Loading: Combination with LEFM Criteria for Fracture Prediction of Keyhole Notched Polymeric Samples

This work analyzes, both theoretically and experimentally, the fracture process of square specimens weakened by keyhole notches and subjected to compressive stresses. Two materials are covered: general-purpose polystyrene (GPPS) and poly(methyl methacrylate) (PMMA). Firstly, the load-carrying capacity (LCC) of the specimens is determined experimentally. Then, by using the equivalent material concept (EMC) for compressive conditions coupled with the maximum tangential stress (MTS) and the mean stress (MS) criteria, the LCC of the notched specimens is predicted. The results show that by using the approach proposed in the present investigation, not only can the critical loads in the keyhole notched polymeric specimens be precisely predicted, but also the corresponding compressive critical stress of the two mentioned polymers can be successfully estimated.

On the use of the Fictitious Material Concept in estimating the ultimate load of keyhole notched AA6061‐T6 specimens under large tension‐torsion deformations

AbstractThis paper examines the fracture of AA6061‐T6 specimens with ductile behavior having notches with keyhole geometry under combined loading of tension/torsion. The investigation is performed by theoretical and experimental methods. New fracture tests are performed using a loading fixture on the samples made of AA6061‐T6 weakened by keyhole notches with different notch tip radii. For theoretically estimating the onset of fracture in the tested notched specimens, a concept called “Fictitious Material Concept (FMC)” is used to equate the ductile material with a fictitious brittle material. Therefore, use of this concept allows use of the linear elastic brittle fracture criteria for ductile materials. The brittle fracture criteria utilized are the “mean stress (MS)” and “maximum tangential stress (MTS)” criteria. The small difference between the theoretical and experimental results confirms that use of the combination of FMC with the two criteria is successful in predicting failure of the keyhole notched AA6061‐T6 samples.

Limit curves for brittle fracture in key-hole notches under mixed mode I/III loading based on stress-based criteria

In this research, with the aim to estimate the onset of fracture in key-hole notched components, made of brittle materials and subjected to combined tension-tear loading, the fracture limit curves are developed in terms of the notch stress intensity factors (NSIFs) based on the maximum tangential stress (MTS) and mean stress (MS) criteria. In order to verify the curves developed, a series of fracture tests are carried out on key-hole notched specimens made of PMMA using an improved loading configuration. The samples are prepared with three various notch tip radii for the sake of evaluating the influence of the notch tip radius on the fracture toughness of the key-hole notched samples. It is shown that for all notch tip radii, the fracture limit curves for both MTS and MS criteria are in very good consistency with the experimental data. No meaningful difference is found between the curves of MTS and MS criteria, especially under plane-stress conditions. It is also observed that the notch fracture toughness of the PMMA specimens could be well estimated under both plane-stress and plan-strain conditions.

Mixed mode notch fracture toughness assessment of quasi-brittle polymeric specimens at different scales

In this study, the mixed mode I/II notch fracture toughness (NFT) of polymeric (PMMA and GPPS) specimens weakened by U- and key-hole notches is assessed experimentally and theoretically. For this aim, first, by performing experiments, the apparent fracture toughness values of cracked polymeric semi-circular bending (SCB) and Brazilian disk (BD) specimens with four different radii are specified. Next, multiple fracture experiments are conducted on the notched polymeric SCB and BD specimens at four different scales, from which the critical loads are measured and converted to the NFT values. Then, the critical radius of the fracture process zone (FPZ) for the notched polymeric specimens is acquired using two different equations already suggested in literature for cracks. A well-established stress-based failure model, namely the point stress (PS) criterion, is employed for anticipating the NFT values of the specimens tested at various scales, by taking advantages of two different critical distance formulas. The results show that at all of the four scales, the fracture toughness values of U- and key-hole notches achieved from PS criterion are in good agreement with the experimental results. Also, shown in this research is that the mixed mode NFT is considerably dependent on the nominal radius of the specimen, such that, as the specimen radius increases, the mixed mode NFT increases significantly. Moreover, it is exhibited that for the present specimens tested, both the critical distance formulas give almost the same predictions, proposing that each of them can be arbitrarily chosen for the NFT prediction.

Brittle fracture prediction of key-hole notched specimens by means of J-integral expression

The main goal of the present research is to check if J-integral criterion is capable of predicting the onset of brittle fracture in key-hole notched isostatic poly-granular graphite plates. In this way, this article provides the prediction of fracture loads in several key-hole notched rectangular graphite specimens with five different notch tip radii subjected to pure mode I loading condition, which has been previously reported in the literature by Lazzarin et al. (2013). The fracture load predictions for five dimensional cases were obtained through the J-integral calculations. It is revealed that the J-integral criterion is capable of predicting well the fracture loads of graphite key-hole notched specimens, independent of the size of key-hole notch.

Predicting the fracture trajectory in U, VO, and key-hole notched specimens using an incremental approach

In this paper, an incremental approach is proposed for predicting the fracture trajectory in specimens containing U, V with end hole (VO), and key-hole shaped notches. In this approach, the first initiation angle of crack is calculated from the maximum tangential stress (MTS) criterion corresponding to stress field around the notch border. Then, a short crack is generated from notch border along the calculated fracture angle. The next step is to obtain the fracture initiation angle for the new cracked sample using the MTS criterion formulated according to the stress field at the vicinity of the crack tip. This procedure is repeated until the crack reaches the back boundary of the specimen. The proposed method is evaluated by means of various experimental results reported in the previous studies for three sets of experiments conducted on the Brazilian disks containing the U-shaped, VO, and key-hole notches under mixed mode I/II loading. It is shown that the fracture paths predicted by the proposed approach are in good agreements with those obtained from the broken samples.

Fracture Assessment of Inclined Double Keyhole Notches in Isostatic Graphite

In the present contribution, the static strength of isostatic graphite using keyhole notch specimens under mixed mode loading is investigated. An experimental program was performed and in total, 18 new experimental data are provided. In addition, different loading mode ratios are considered by varying the inclination angle of the notch with respect to the direction of the applied load. The criterion based on the averaged value of the strain energy density over a control volume at the notch edge is applied to assess the static strength of specimens. A sound agreement is found between experimental data and the results obtained from strain energy density criterion.

Mixed mode I/II fracture investigation of Perspex based on the averaged strain energy density criterion

In this work, some recent mixed mode I/II fracture toughness results obtained from Perspex (or polymethylmethacrylate (PMMA)) with four simple cracked specimens subjected to the conventional three-point bend loading are reanalysed based on local energy concept. Although all the mentioned samples have been tested under the same and similar mode mixities, different fracture toughness envelopes were obtained for mixed mode I/II fracture of PMMA. The averaged strain energy density (SED) criterion has been applied in the past for different types of notched specimens (including U, V, O and keyhole notches). It is shown that the mixed mode tensile-in plane shear fracture toughness data obtained from the semicircular and triangular crack type specimens are successfully predicted for sharp cracked PMMA samples using the SED criterion.

Brittle failure of key-hole notches under mixed mode I/II loading with negative mode I contributions

Brittle failure of key-hole notches is investigated experimentally and theoretically under mixed mode I/II loading with negative mode I contributions. The key-hole notched Brazilian disk specimen made of the general-purpose Polystyrene is utilized for conducting the fracture experiments. To predict the experimental results, the key-hole notch maximum tangential stress and key-hole notch mean stress criteria are utilized. It is shown that both criteria could provide good predictions to the experimental results of the fracture load. Moreover, the key-hole notch mean stress criterion with the experimentally determined critical distance is shown to be the most accurate failure criterion.

Mode II Brittle Fracture Assessment of Key-Hole Notches by Means of the Local Energy

Abstract Brittle fracture of key-hole notches under pure mode II loading is investigated both experimentally and theoretically. In the experimental part, a new version of the well-known Brazilian disk specimen containing a central dumbbell-shaped slit with two key-ends, called the Key-BD specimen, made of polymethyl-methacrylate (PMMA), is utilized to perform fracture experiments at room temperature for different notch lengths and various notch radii. In the theoretical part, the experimentally recorded fracture loads are predicted by means of the strain energy density (SED) averaged over a specified control volume, which embraces the notch edge. It is found that the SED criterion can successfully predict the test results.

Key-Hole Notches in Isostatic Graphite: A Review of Some Recent Data

Fracture of the breakable isostatic graphite is concerned using the experimental and theoretically, the plates containing nicks holes key subject to varying degrees of mixed task. The main purpose of this work is a double one. In the first place, in order to offer a new set of experimental results on fracture of samples of graphite scored, with different values of the load mixed and radii notch and which may be helpful for researchers, as it enlarges the very limited data available; and secondly, to provide a criterion fracture of the polycrystalline graphite under the conditions mentioned above. The main purpose of this work is to offer a new set of experimental results (70 new data) on the fracture of the samples of the loose graphite of key holes, the different values of mixed loading, the tilt angle and radii notch and which may be of help as widens the very limited data available. By using the value of the average density of the stem of energy in a well defined, a criterion of fracture of the polycrystalline graphite under the conditions referred to above, it is proposed to predict the static resistance of the samples taken into account. The third part of the work deals with the analysis of the direction of initiation of fracture and spread of the crack in the early. The average value of the stem density of energy in a well defined is used to predict the static resistance of the samples taken into account. Good agreement is found between the experimental data for the tasks critical failure and the theoretical predictions based on average constant strain density of energy on the volume of the material.

Tensile Fracture Analysis of Key-Hole Notches by Means of the Strain Energy Density

The aim of the present research is twofold. Firstly, to provide a new set of experimental results regarding tensile brittle fracture in key-hole notches and secondly, to check the suitability of the local strain energy density (SED) criterion in predicting the test results. For this purpose, 21 fracture tests were conducted at room temperature on a new version of the well-known Brazilian disk specimen containing a central dumbbell-shaped slit with two key-ends (Key-BD specimen) made of PMMA. The experimentally obtained fracture loads were theoretically predicted for different notch radii by means of the local SED averaged over a specified control volume which embraces the notch edge. It was shown that the experimental results could be well predicted by means of the SED criterion.

Mode II notch fracture toughness measurement for key-hole notches by the disk test

A new version of the well-known Brazilian disk specimen weakened by a dumbbell-shaped slit with two key-shaped ends, called key-hole notched Brazilian disk specimen, made of polymethylmethacrylate, was utilized to measure experimentally the notch fracture toughness for key-hole notches of various radii and different lengths under pure mode I and pure mode II loading conditions. The experimental values of the mode II notch fracture toughness/mode I notch fracture toughness ratio were theoretically estimated by means of two brittle fracture criteria, namely, the maximum tangential stress and the mean stress criteria. Two groups of critical distances were used in the criteria. The first one was equal to that for sharp crack and the second one was directly determined from the mode I fracture tests. It was found that while both the criteria could estimate the experimental results satisfactorily, the agreement between the theoretical and the experimental results is excellent when one deals with larger notch lengths. Also found in the research was that choosing any of the critical distance groups did not affect the theoretical predictions significantly, meaning that the critical distances of sharp crack could be used in fracture prediction of key-hole notches.

Experimental and theoretical investigation of brittle fracture in key-hole notches under mixed mode I/II loading

The Brazilian disk specimen weakened by a central dumbbell-shaped slit with two key-shaped ends and made of PMMA is employed to perform mixed mode I/II brittle fracture experiments on key-hole notches for various notch lengths and radii and, also, different mode mixity ratios. The load-carrying capacity of the specimen resulting from the test machine at the onset of sudden fracture is theoretically predicted by means of a well-known brittle fracture criterion, namely the local strain energy density criterion. A good agreement is found to exist between the theoretical and the experimental results.

Brittle fracture in key-hole notches under mixed mode loading: Experimental study and theoretical predictions

Brittle fracture was studied experimentally and theoretically for key-hole notches under mixed mode loading. Brazilian disk specimen containing a dumbbell-shaped slit and made of polymethyl-methacrylate was proposed to measure the notch fracture toughness and the fracture initiation angle at room temperature. The experimental results were predicted by the maximum tangential stress and the mean stress criteria. Two groups of critical distances were utilized in the criteria. The first one was equal to that for sharp cracks and the second one was directly obtained from the fracture tests. It was found that both the criteria could predict the experimental results successfully.

Fracture Study on Key-Hole Notches Under Tension: Two Brittle Fracture Criteria and Notch Fracture Toughness Measurement by the Disk Test

Notch fracture toughness expressions were developed for predicting tensile fracture in members containing key-hole notches based on the point stress and the mean stress criteria. The criteria were verified by using the experimental results obtained from new fracture tests on the key-hole notched Brazilian disk specimens, made of PMMA. It was found that the mean stress criterion predicts the experimental results very well. Also, found in this research was that while the point stress model provides generally satisfactory estimations, except for a special case, the accuracy of this criterion enhances as the notch radius increases.