Plane Strain Fracture Toughness Testing Research Articles

Fatigue crack growth properties of PMMA material used in underwater transparent structures

ABSTRACT Polymethyl methacrylate (PMMA) is an ideal material used in underwater transparent structures such as submersible windows and the fully-transparent pressure chamber of the sightseeing submersibles. The failure of PMMA structures usually come from small crack-like defects. In this paper, the fatigue crack growth experiments were carried out on PMMA materials with different thicknesses (B = 3, 6, 12 mm) and stress ratios (R = 0.1, 0.3) as well as plane strain fracture toughness test for unstable crack propagation conditions. Thickness effect and dwell time effect are experimentally investigated in addition to load ratio effect. The applicability of an improved crack growth rate model on PMMA material is validated by test results. The application of this model for prediction of crack growth rates can greatly save experimental time under different working conditions and can be applied in fatigue assessment of full-transparent structures under random loading.

Effect of Fe-Rich Phases on Fracture Toughness of High Strength Al-Zn-Mg-Cu Alloy

In order to analyze the effect of insoluble Fe-rich phases on the fracture toughness of high-strength Al-Zn-Mg-Cu alloys, plane-strain fracture toughness test, scanning electron microscopy, finite element analysis and quantitative modeling were used to investigate the fracture toughness of three commercial high-strength Al-Zn-Mg-Cu alloys. The results show that the fracture mode of the three experimental alloys is strongly related to the Fe-rich phases, which are prone to initiate intergranular fracture of the experimental alloys. Finite element analysis proved that the larger the shape factor of the Fe-rich phases, the more sensitive it is to external stress and more prone to secondary cracking, which is consistent with the fracture morphology observation. Quantitative analysis shows that the size, volume fraction and morphology of the Fe-rich phases have a greater influence on the fracture toughness of the high-strength Al-Zn-Mg-Cu alloy. The fracture toughness of the alloy will be improved when the shape factor of the Fe-rich phases is small and close to circular and the volume fraction of the Fe-rich phases is small. The fracture toughness value of the Al-Zn-Mg-Cu alloys can be better predicted when the deviation of the shape and distribution of the Fe-rich phases is considered.

Crack path direction in plane-strain fracture toughness assessment tests of quasi-brittle PLA polymer and ductile PLA-X composite

Plane-strain fracture toughness is one of the main parameters in linear elastic fracture mechanics and its purpose is to show the material capability to withstand load while having a defect. Main validity aspect for such an assessment is to provide a wide enough crack front to enable plane-strain condition. Nonetheless, in FDM (Fused Deposition Modeling), due to structural anisotropy caused by polymer material properties and Additive Manufacturing (AM) process parameters, more validity aspects must be met. During the plane-strain fracture toughness test a crack must follow a straight line from initiation up to the point of structural failure. Plane-strain fracture toughness assessment is conducted according to the ASTM D5045-14 standard for testing of polymer materials. Tests are performed on SENB (Single Edge Notched Bend) specimens, made from two similar polymer materials: quasi-brittle PLA and ductile PLA-X composite. Specimens are manufactured with four different AM process parameters, i.e., layer height, infill density, printing orientation and one specimen batch was dried before testing.

Applicability of compact tension specimens for evaluation of the plane-strain fracture toughness of steel

AbstractThe plane-strain fracture toughness test method enables the determination of the plane-strain fracture toughness (KIC) of metallic materials by tests using a variety of precracked specimens, such as bend, compact tension (CT), arc-shaped, disk-shaped specimens. In this study, to determine the effect of the heat treatment process on the specimen thickness required for a valid plane-strain test in accordance with the ASTM standard, fracture tests are performed by using CT specimens made of AISI 1040 carbon steel. Furthermore, by taking into consideration the effect of residual stress caused by heat treatment in the vicinity of the notch, additional fracture tests are also performed. The specimens are machined from rolled plates in the L-T rolling direction (crack plane is perpendicular to the rolling direction). Having performed experimental analyses, the applicability of the plane-strain fracture toughness tests using CT specimen made of AISI 1040 steel is investigated.

Experimental analysis of mechanical properties of stir casted aluminium-graphene nanocomposites

Aluminium (Al) based materials are widely used in aerospace and automobile industries due to its higher strength to weight ratio. Under fatigue loading the components are prone to failure, which is one of the key factors in designing the components. In the present study, Graphene (Gr) is added to Al at different weight percentages (1%, 2% and 3%) using stir casting. Tensile strength and fracture toughness of the composites were determined using tensile testing of Dogbone specimens and plane strain fracture toughness testing of C (T) speci-mens in mode-I, respectively. The result showed that for Al-Gr nanocomposites, with the increase in Graphene percentages, tensile strength was found to be in-creased however the fracture toughness was decreased. It is also found that the manufacturing method followed in the present work to be cost effective method for Al-Gr nano composite. The analysis of different tests suggests that the addition of 2wt% Graphene found to be the saturation point in the nanocomposites and any further addition of Graphene can lead to the degradation in the quality of composite.

Perspective on the mechanical response of pineapple leaf filler/toughened epoxy composites under diverse constraints

In view of fiber orientation problem and seminal ecological economics, the present research work is concentrated on the development of ‘commercial adhesive grade’ epoxy composite reinforced with surface-modified low-cost ‘agronomic waste’- based pineapple leaf (PALF) fillers. To overcome the potential adhesion and moisture absorption issues, surface modification of fillers is accomplished with alkaline treatment. The change occurred in the crystal structure and thermal stability of fillers before and after the treatment is quantified using X-ray diffraction and thermogravimetric analysis. The composite samples are subjected to the tensile test with varying strain rate to investigate its effect on the tensile properties and failure behavior of materials. In addition to this, the plane-strain fracture toughness and flexural test have also been carried out to evaluate the composite properties in bending mode with and without the preexisted crack in materials. The dynamic mechanical analysis has also been conducted from room temperature to 150 °C to evaluate the temperature dependence of mechanical properties. The crystallinity of treated filler has been improved by 8 % due to the loss of non-cellulosic components from filler molecules. The highest value of ultimate tensile strength and Young’s modulus are 22 MPa and 570 MPa, respectively. The maximum values of storage modulus of 10 GPa and glass transition temperature of 91 °C were observed for 2.5 % and 12.5 % of filler content, respectively. From these findings, it is envisaged that the balanced tensile and dynamic mechanical properties of PALF–epoxy composites will elucidate their potential applications.

Influence of probability distribution in measurement uncertainty of plane-strain fracture toughness test

The evaluation of measurement uncertainty by testing laboratories has a direct impact on the interpretation of the results. In several cases, it is recommended to determine the measurement uncertainty by the Monte Carlo method (MCM), which considers the propagation of distributions rather than the propagation of uncertainty. Measurement uncertainty of plane-strain fracture toughness KIC test was evaluated through the MCM and an examination of the influence of the probability distribution on the uncertainty values was performed through analysis of variance (ANOVA). In addition, results were compared to those obtained by the Guide to the Expression of Uncertainty in Measurement (GUM). Results demonstrate the importance of using the Monte Carlo method for the evaluation of measurement uncertainty and confirm that the probability distribution of input data has a significant influence on the expanded uncertainty values obtained for the plane-strain fracture toughness test.

Measurement uncertainty of plane-strain fracture toughness KIC testing by the Monte Carlo Method

Abstract The reliable determination of materials' mechanical properties is a fundamental factor for their application in engineering, and the estimation of the measurement uncertainty in testing laboratories has a direct impact on the interpretation of the results. Recent literature demonstrates that one of the most widely used methodologies for uncertainty estimation, the Guide to the Expression of Uncertainty in Measurement (GUM), has limitations, especially in cases where the mathematical model has a high degree of non-linearity. Furthermore, it makes approximations for the final probability distribution. In these cases, it is recommended that the measurement uncertainty is determined by the Monte Carlo Method (MCM), which considers the propagation of the distribution rather than the propagation of uncertainties. Thus, given the limitations of the GUM method and the importance of estimating the measurement uncertainty of mechanical tests, this work aims to implement the measurement uncertainty estimation for the plane-strain fracture toughness (KIC) test of metallic materials through the Monte Carlo Method. The results of the work confirm the importance of estimating the measurement uncertainty of fracture toughness tests.

Effect of nano/micro B4C and SiC particles on fracture properties of aluminum 7075 particulate composites under chevron-notch plane strain fracture toughness test

Reinforcing aluminum with SiC and B4C nano/micro particles can lead to a more efficient material in terms of strength and light weight. The influence of adding these particles to an aluminum 7075 matrix is investigated using chevron-notch fracture toughness test method. The reinforcing factors are type, size (micro/nano), and weight percent of the particles. The fracture parameters are maximum load, notch opening displacement, the work up to fracture and chevron notch plane strain fracture toughness. The findings demonstrate that addition of micro and nano size particles improves the fracture properties; however, increasing the weight percent of the particles leads to increase of fracture properties up to a certain level and after that due to agglomeration of the particles, the improvement does not happen for both particle types and size categories. Agglomeration of particles at higher amounts of reinforcing particles results in improper distribution of particles and reduction in mechanical properties.

Thermal Mechanical Processing Effects on Microstructure Evolution and Mechanical Properties of the Sintered Ti-22Al-25Nb Alloy.

This work illustrates the effect of thermal mechanical processing parameters on the microstructure and mechanical properties of the Ti-22Al-25Nb alloy prepared by reactive sintering with element powders, consisting of O, B2 and Ti3Al phases. Tensile and plane strain fracture toughness tests were carried out at room temperature to understand the mechanical behavior of the alloys and its correlation with the microstructural features characterized by scanning and transmission electron microscopy. The results show that the increased tensile strength (from 340 to 500 MPa) and elongation (from 3.6% to 4.2%) is due to the presence of lamellar O/B2 colony and needle-like O phase in B2 matrix in the as-processed Ti-22Al-25Nb alloys, as compared to the coarse lath O adjacent to B2 in the sintered alloys. Changes in morphologies of O phase improve the fracture toughness (KIC) of the sintered alloys from 7 to 15 MPa·m−1/2. Additionally, the fracture mechanism shifts from cleavage fracture in the as-sintered alloys to quasi-cleavage fracture in the as-processed alloys.

Tensile and fracture behavior of boron and carbon modified Ti-15-3 alloys in aged conditions

This work illustrates the effect of boron and carbon addition on the mechanical behavior of a beta Ti alloy, Ti–15V–3Cr–3Al–3Sn (Ti-15-3), in differently aged conditions. The alloys were prepared by consumable vacuum arc melting followed by forging and hot rolling. These were subsequently solution treated and aged at different temperatures above 500°C for 8h. Standard tensile and plane strain fracture toughness tests were carried out to understand the mechanical behavior of the alloys and its correlation with the microstructural features characterized by scanning and transmission electron microscopy. Both the boron- and the carbon-containing alloys exhibit improved strength with comparable elongation to failure values as compared to the base Ti-15-3 alloy. The presence of TiB and TiC precipitates in a matrix of fine α with β results in lower fracture toughness (KIC) in the boron- and carbon-containing alloys as compared to the base alloy. However, at higher aging temperatures KIC improves due to more tortuous crack path because of the presence of coarse α-phase. An empirical relationship has been proposed correlating KIC with the volume fraction, size and interspacing of α in these alloys.

The linear elastic analysis of cracked bodies and crack paths

The linear elastic analysis of cracked bodies is a Twentieth Century development, with the first papers appearing in 1907, but it was not until the introduction of the stress intensity factor concept in 1957 that widespread application to practical engineering problems became possible. Linear elastic fracture mechanics (LEFM) developed rapidly in the 1960s, with application to brittle fracture and fatigue crack growth. The first application of finite elements to the calculation of stress intensity factors for two dimensional cases was in 1969. Finite element analysis had a significant influence on the development of LEFM. Corner point singularities were investigated in the late 1970s. It was soon found that the existence of corner point effects made interpretation of calculated stress intensity factors difficult and their validity questionable. In 1998 it was shown that the assumption that crack growth is in mode I leads to geometric constraints on permissible fatigue crack paths. Current open questions are. The need for a new field parameter, probably a singularity, to describe the stresses at surfaces. The use of strain energy density is promising. Adequate description of fatigue crack path stability. How best to allow for the influence of corner point singularities in three dimensional numerical predictions of fatigue crack paths. Development of an adequate plane strain fracture toughness testing standard. Some background information is given in appendices.

Study on strength and fracture toughness of Al-Zn-Mg-Cu-Ti(-Sn) alloys

The strength and fracture toughness of Al-Zn-Mg-Cu-Ti(-Sn) alloys were investigated by performing tensile and plane strain fracture toughness (KIC) tests. Detailed observations with optical, scanning electron and transmission electron microscopy were conducted to analyze microstructure and fracture surfaces of the alloys. The results revealed that addition of Sn refined the solution-aging grain size of matrix and reduced coarsening rate of precipitate during aging. Narrower precipitation free zones and more discontinuous distribution of grain boundary precipitates were observed to be displayed in the Sn-containing alloy. Small size second phase particles Mg2Sn were observed to form in the Sn-containing alloy and distribute in the fine dimples of fracture surface. These features of microstructure were believed to impart higher strength and fracture toughness of the Sn-containing alloy on overaging.

Microstructure and Mechanical Properties of Al-Li Alloy 2397-T87 Rolled Plate

The microstructure, tensile property and fracture toughness of Al-Li alloy 2397-T87 rolled plate were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, tensile and plane-strain fracture toughness tests. The results show that a pronounced texture variation through the plate thickness was found. Near the surface, Goss texture dominated. While in the center of the plate, typical β fiber texture and a scattering of cube texture were observed. And the subsurface layer exhibited a very weak texture. From the center to the subsurface, the fraction of β fiber texture and cube texture decreased. In contrast, the fraction of shear type texture reaching the maximum in subsurface layer increased. The tensile properties in different layers along the thickness direction were inhomogeneous. The strengths near the surface were lower than those in the center. And the through-thickness strength properties variation in the rolling direction was more remarkable than that in the long transverse direction. In the same thickness layer, the fracture toughness and the strengths were anisotropic. The strengths in the rolling direction were higher than those in the long transverse direction and the short transverse direction, and the strengths in the short transverse direction were the lowest. The fracture toughness in L-T orientation was the highest, followed by that in T-L orientation, and the fracture toughness in S-L orientation was the lowest.

Fatigue behavior and plane-strain fracture toughness of sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy

In this study, the tensile properties, high cycle fatigue behavior and plane-strain fracture toughness of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy were investigated, comparison to that of sand-cast plus T6 heat treated magnesium alloy which named after sand-cast-T6. The results showed that the tensile properties of the sand-cast alloy are greatly improved after T6 heat treatment, and the fatigue strength (at 107cycles) of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy increases from 95 to 120MPa after T6 heat treatment, i.e. the improvement of 26% in fatigue strength has been achieved. The plane-strain fracture toughnesses KIC of the sand-cast and sand-cast-T6 alloys are about 12.1 and 16.3MPam1/2, respectively. In addition, crack initiation, crack propagation and fracture behavior of the studied alloys after tensile test, high cycle fatigue test and plane-strain fracture toughness test were also investigated systematically.

Experimental investigation of mixed-mode-I/II fracture in polymer mortars using digital image correlation method

The purpose of this work is to investigate experimentally the effects of mixed-mode I/II loading on the fracture behavior of polymer mortars. Three point bend beam specimens with offcenter notch were used in plane-strain fracture toughness tests. Two approaches have been simultaneously used to determine the crack opening displacements. The crack mouth opening displacement was measured by means of a clip gauge, while the crack tip opening displacement was estimated through the Digital Image Correlation method. Fracture parameters, such as critical values of crack opening displacement, energy release rate and mixed-mode stress intensity factor were estimated. In addition, the values of crack mouth opening displacement and crack tip opening displacement were used to evaluate the rotational factor. Finally, the effect of sliding mode-II in fracture was not easily observed using clip gage, however this effect was noticed employing Digital Image Correlation method.

A 50‐year retrospective review of three‐dimensional effects at cracks and sharp notches

ABSTRACTThis review is a brief survey of three‐dimensional effects at cracks and sharp notches. The overall aim is to review developments over the past 50 years leading up to the current state of the art. The review is restricted to linear elastic, homogeneous, isotropic materials, with any yielding confined to a small region at a crack or notch tip. It is also restricted to static loading and to constant amplitude fatigue loading. An enormous amount of theoretical and experimental information relevant to three‐dimensional effects has been published in the past five decades, so the review is, of necessity, highly selective. Theoretical topics covered are linear elastic fracture mechanics, including Volterra distorsioni, stress intensity factors, corner point singularities, crack front line tension, displacement analysis of cracks and notches, and through thickness distributions of stresses and stress intensity factors. Crack path topics covered are fatigue crack path constraints, determination of fatigue crack paths, oscillating crack fronts in thin sheets and the transition to slant crack propagation in thin sheets. Plane strain fracture toughness testing, including standards, is covered. Overall, it can be concluded that the existence of three‐dimensional effects at cracks and sharp notches has been known for many years, but understanding has been limited, and for some situations still is. Understanding improved when the existence of corner point singularities and their implications became known. Increasingly powerful computers made it possible to investigate three‐dimensional effects numerically in detail. Despite increased understanding, three‐dimensional effects are sometimes ignored in situations where they may be important.

Studies on fracture toughness of cold wire addition in narrow groove submerged arc welding process

Due to the faster joint completion rates coupled with good mechanical properties, narrow gap submerged arc welding (NGSAW) is widely used for fabrication of thick-walled pressure vessels. Several researchers are working on further increase in productivity in NGSAW. In this paper, we propose to increase the quality and productivity in NGSAW through cold wire addition without addition of heat input. Further toughness at sub-zero temperature is also enhanced. The improvement in toughness in cold wire NGSAW is demonstrated through different tests such as impact energy test, fracture toughness tests, plane strain fracture toughness test K Ic, and crack tip opening displacement test.

Effect of sample pre-cracking method and notch geometry in plane strain fracture toughness tests as applied to a PMMA resin

The influence of test method factors (notch shape, square or angular, and pre-cracking method, by tapping onto or pressing a razor blade) on the results obtained in plane strain fracture toughness test according to standard ASTM D5045 using SENB specimens made of a commercial PMMA resin were investigated. Results were analyzed quantitatively by comparing the obtained KIc values and qualitatively by observing their effect on the Moiré fringes observed using photoelasticity, showing that, at 95% significance level, the KIc values are affected by the pre-cracking method, with the most conservative value being obtained when natural pre-cracks were introduced by tapping onto a razor blade (KIc = 1.15 ± 0.11 MPa·m0.5). This correlates with a perturbation in the stress field close to the pre-crack tip observed in the photoelasticity test sample when it was introduced by pressing the razor blade. Surprisingly, notch geometry only slightly affects the results.

Prestressed double network thermoset: preparation and characterization

An innovative scheme to prepare Prestressed double network (PDN) epoxies is presented using a judicious combination of tetrafunctional curatives that have similar molecular weights but different reaction kinetics. A diglycidyl ether of bisphenol A epoxy monomer was reacted stoichiometrically and sequentially with various molar ratios of an aliphatic polyetheramine curing agent and an aromatic curing agent. Deformation was imposed on the partially cured resins after the formation of the first network, and postcuring was conducted at 50% compressive strain. Physical properties of the resulting PDN epoxies were examined using thermomechanical analysis, dynamic mechanical analysis, uniaxial tensile test, and plane-strain fracture toughness test. The application of prestress resulted in no changes in glass transition temperature, coefficient of linear thermal expansion, and Young’s modulus. However, a marked increase in fracture toughness is observed, accompanied by strong birefringence and visible roughness on the fracture surface.