Fracture Mechanics Tests Research Articles

Comparison of different titanium alloys welded by Yb:YAG fibre laser for thin sheet applications used for T-ducts in bleed air systems

Research in aerospace applications includes the replacement of well-known materials by newly developed alloys or by new manufacturing methods for the existing materials. In the frame of TiB-Air project funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) the development of a process chain consisting of deep drawing at elevated temperatures, chemical milling, contour machining by laser cutting and laser beam welding to produce pneumatic T-ducts used in bleed air systems is in focus. This production process of sheet metal parts could lower costs in terms of the process itself and the used materials: low alloyed Ti-alloys. Commercially pure titanium alloy (cp-Ti) is commonly used for these structures because of its balanced mechanical properties regarding tensile strength, yield strength, plastic strain and fatigue strength as well as good resistance against corrosion and oxidation. The possibility to substitute cp-Ti by low-alloyed Ti-alloys is examined in this work, by the comparison of two different low alloyed Ti-alloys, namely KS1.2ASN and Ti XT, with a cp-Ti alloy (Grade 4). Mechanical properties of the base materials, their weldability and the mechanical assessment of the laser beam welded butt joints in terms of static, cyclic and fracture mechanical behaviour is compared for sheet materials, with a thickness of 0.9 mm. Defect-free welding according to EN13919 acceptance criteria B was possible for all three alloys, no porosity problems occurred. The low strength alloy KS1.2ASN exhibited mechanical anisotropy between longitudinal direction and transverse direction in the tensile test, welded specimens of this alloy broke in the base material. Due to the tensile properties, both weld and base material of KS1.2ASN showed the least values for the fatigue strength and endurance limit strength. Ti XT and Grade 4 showed similar mechanical anisotropy and fractured in the base material, too. Fatigue strength of Ti XT is below Grade 4, but for the 50%-percentile the endurance limit strength is equal. Fracture mechanical testing showed that KS1.2ASN is a very promising alloy in the welded condition.

Development of ecological strain-hardening cementitious composites incorporating high-volume ground-glass pozzolans

A novel strain-hardening cementitious composite (SHCC) incorporating high-volume ground-glass pozzolans (HVGP) has been developed by coupling particle packing optimization with micromechanical tailoring. For the former, the compressible packing density model was adopted, while for the latter, single-fiber pull-out and fracture mechanics tests were conducted to compile model input parameters. Ground-glass pozzolans also known as glass powder (GP) at up to 100% replacement of fly ash (FA) were attempted. The resulting HVGP-SHCC formulations have self-consolidating ability and exhibited up to 75 MPa compressive strength, 9–15 MPa flexural capacity, 3–6 MPa tensile strength, up to ≈3% tensile strain capacity, and a significantly improved durability (up to 80% enhancement in electrical bulk resistivity). Results reveal that the compactness-based formulation of HVGP-SHCC yielded composites exhibiting higher strength than conventional SHCC containing high-volume fly ash (HVFA) with similar water-to-binder ratio while demonstrating acceptable tensile strain capacity and far better durability aspects. Research outcomes shape a forward step in the development of greener high-performance construction materials necessary for sustainable and resilient concrete infrastructures.

Building the Kitagawa-Takahashi diagram of flawed materials and components using an optimized V-notched cylindrical specimen

The present technical note illustrates a novel procedure for building the Kitagawa-Takahashi diagram of a flawed material and of notched components made of the same material. The novelty of the method over the existing technical literature is the use of a rounded V-notched cylindrical specimen, whose geometry was optimized to maximize the intensity of the asymptotical notch stress term. In this way, the sensitivity to experimental uncertainties is reduced and no cumbersome fracture mechanics tests are requested to determine the crack threshold of the defect free of the defect-free material counterpart. The only experimental input are the high-cycle fatigue strength and the mean size of the critical defect at the notch tip of three specimen batches differing in the size of the critical defect and/or in the notch root radius. The potentiality of the method is checked for an additively manufactured Ti-6Al-4V alloy.

Application of the material inhomogeneity effect for the improvement of fracture toughness of a brittle polymer

In a multilayered structure with a crack, a spatial change in the mechanical properties of the material strongly influences the crack driving force. This material inhomogeneity effect can be utilized to improve the fracture toughness of a given structure by inserting thin, soft interlayers into the material. The effectiveness of this procedure has been demonstrated on high-strength materials, such as metallic alloys and ceramics. It is shown in this article that the material inhomogeneity effect can be also successfully applied to polymers and that it is possible to predict the improvement in fracture toughness by a numerical analysis. First, a numerical case study based on the configurational force concept is performed on a brittle polymer matrix with interlayers made of materials with different strength and Young’s modulus. After selecting the most appropriate interlayer material, a composite is fabricated, which contains a single interlayer. Fracture toughness experiments show approximately 7 times higher fracture toughness for the composite in comparison to the homogeneous matrix material. Numerical fracture mechanics tests are performed on homogeneous and composite material using the cohesive zone model for crack growth simulation. A procedure to calibrate the cohesive zone parameters is worked out, which is relatively easy for the homogeneous material, but more sophisticated for the composite material. The numerical analysis provides a tool for predicting the fracture toughness of multilayered polymer composites.

On the transition of fracture toughness in metallic materials with thickness: An atomistic viewpoint

From thick to very thin specimens, fracture toughness of metallic materials peaks out before hitting much lower value compared to the constant plane strain fracture toughness value obtained using standard fracture mechanics testing of thick specimens. Understanding this behavior is essential for improving reliability of small-scale metallic devices currently being used for various critical applications. To understand this behavior, it is essential to study the process of dislocations emission and interaction at the crack front and its variation with specimen thickness. To this end, atomistic fracture simulations of pre-cracked single crystal FCC metal (Nickel) specimens representing both thick and thin specimens are performed. At first, stress-state dependent single crystal yield function based on the generalized Schmid-law is associated with the dislocation emission process at the crack front for both thin and thick specimens. Fracture simulations are then performed on single crystal specimens representing different thickness cases. Due to low stress triaxiality prevailing throughout the thickness of thin specimens, dislocation interaction with each other inside the specimen and then with the specimen surface leading to wedge-shaped groove formation on opposite surfaces at crack front is found to be the responsible mechanism of crack propagation in thin specimens. This mechanism provides enhanced fracture toughness to thin specimen compared to thick specimen in which crack propagation is based on high stress triaxiality at the core of the crack front making formation of microvoids and their coalescence as a dominant mechanism of crack propagation. The dislocation configurations generated at the crack front for thick and thin specimens are also studied and the mechanisms for dislocation multiplication in thin specimens compared to thick specimens are highlighted.

Application of the Master Curve concept on the choice of duplex stainless steel to avoid brittle fracture

ABSTRACTDuplex stainless steel gets more and more common in structural applications even in steel bridges. For these cases, the choice of duplex stainless steel material to avoid brittle fracture is of great importance as e.g. steel bridges are exposed to low temperatures, generally down to −30 °C or even lower, and fatigue loads. EN 1993‐1‐4 also covers duplex stainless steel, but the given toughness requirements for duplex stainless steel are insufficient. Due to this fact, first investigations have been carried out on various duplex stainless steel base materials and welded specimens based on the fracture mechanic based concept of EN 1993‐1‐10 including the Wallin Master Curve concept. The objective was to achieve a first insight in the applicability of EN 1993‐1‐10 for structural applications with different types of duplex stainless steels in principle. In this frame, Charpy and fracture mechanics tests have been evaluated for 1.4462, 1.4362 and 1.4162 duplex stainless steels of 30 mm and 50 mm thick base materials and weldments. The results have been transformed into the general approach used in EN 1993‐1‐10 originally developed for ferritic structural steel considering the structural detail of a welded longitudinal attachment with a semi elliptical surface crack at the weld toe. The results demonstrate that the investigated duplex stainless steels and their weldments show a very good toughness behaviour and that the EN 1993‐1‐10 model can be transformed for application of duplex stainless steel. The paper presents these first results.

The experimental and numerical study of fracture behavior of 58s bioactive glass/polysulfone composite using the extended finite elements method

The composites derived from the bioactive glasses, such as BG/polysulfone, have better mechanical properties and their characteristics are closer to human bone. In this paper, the fracture behavior of 58s BG/PSF composite has been investigated. The extended finite elements method (XFEM) was used as an alternative numerical method to the FEM, in order to model the fracture behavior of 58s BG/PSF composite with greater accuracy. The XFEM doesn’t require remeshing at each step and achieves the precise approximation of singularities by incorporating discontinuity behavior into the elements using enrichment functions. The aim of using the XFEM was to obtain stress intensity factors, displacements, stress and strain around the crack tip, fracture toughness as well as strain energy release rate of BG/polymer composite. Moreover, the 58s BG/PSF composite with 30% volume ratio of bioactive glass particles was synthesized using solvent casting method. Meanwhile, the SEM micrographs were prepared, and the fracture mechanic tests were accomplished based on ASTM D5045 standard test method. The obtained fracture toughness was in the range of 1.4 to 1.6 and the strain energy release rate is in the range of 1600 to 1900 J.m−2, which is comparable to the same properties of natural human bone. Also, the stress intensity factors and strain energy release rates were calculated by coding in MATLAB and modeling in ABAQUS, and the numerical results were validated with the analytical and experimental results.

Experimental and numerical investigation of residual stress effects on fatigue crack growth behaviour of S355 steel weldments

Fatigue crack growth tests have been conducted on S355 G10+M structural steel which is widely used in the fabrication of offshore structures. Fracture mechanics tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. All tests were performed at room temperature in air and the obtained results are compared with the literature data available on a range of offshore structural steels and also the recommended BS7910 trends using the 2-stage law and simplified law. The specimen orientation, with respect to the location of the extraction within the welded plate, has also been examined and discussed in this work. Residual stress measurements have been performed prior to testing by using the neutron diffraction technique. Finally, a numerical model has been developed in order to calculate the effective stress intensity factor range in the presence of residual stresses. The results have shown that the residual stresses play a key role in the fatigue life of the welded structures, especially in the near threshold region.

Fracture of laminated bamboo and the influence of preservative treatments

Treated bamboo can be made into large, durable structural elements which have the potential to become a transformative large-scale building material, but the fracture behaviour which determines their ultimate strength in various loading scenarios has not been studied. Laminated bamboo is a promising structural engineered bamboo material, and is generally made from bamboo treated to improve its durability. Studies into the structural behaviour of laminated bamboo indicate that different preservative treatments affect the structural properties of the composite differently, with conflicting evidence from tests in different load orientations. This study uses fracture mechanical testing and microscopy to develop an understanding of the fracture mechanics of engineered bamboo, and explains why the properties of the composite under tension, compression and bending may be affected differently by the treatment processes. Two types of treated Moso bamboo are studied alongside the same material with minimal processing. The treated material had gone through one of two commercial processes: bathing in a hydrogen peroxide bleach solution, or treatment by pressurised steam (described as caramelised). The results show that the critical strain energy release rate in the caramelised material is much lower than that in the bleached, and the fracture behaviour of the bleached material is closer to that of the raw bamboo. Fracture experiments included Mode I and Mode II fracture with cracks progressing parallel to the grain, and Mode I fracture with a crack progressing perpendicular to the grain. The results shed new light on the strength of structural-size elements.

Determination of fracture toughness from the small punch test using circular notched specimens

The small punch (SP) test method is an innovative technique which shows potential for residual life assessment of components in service. The main tasks of the technique are to derive standard material properties with a limited amount of material, including strength/toughness, creep and fatigue behaviour. In order to achieve the objective, many methods have been put forward since early 1980’s but even in the new EN standard in preparation no common acceptable methods are available yet for fracture mechanics testing of notched SP specimens.An investigation is carried out to exploit new approaches to determine fracture toughness from small punch tests, including a new type of notched specimen and a new analytic approach. The new type of specimen is a circular notched disc with thickness of 1 mm, and the diameter of the circular notch is 2.5 mm with depth of 0.5 mm which has been proposed and tested by JRC Petten, European Commission. The advantage of the circular notched specimen is that a plane strain condition is built around the crack tip which makes determination of the elastic fracture toughness KIC possible if the condition of small scale yielding is met. In comparing the volume of the new type specimen with the standard fracture specimens, it is only 1/5000 of that of the three point bending specimen (TPB) and 1/2000 of that of the compact tension specimen (CT).In this paper, a method based on fracture energy is proposed to estimate the J-integral for the circular notched specimen, and the procedure to determine the fracture toughness JIC is also addressed. In order to verify the proposed approach, a so-called “Local approach” analytical simulation has been performed to determine the J-R resistant curve and the fracture toughness JIC by using standard small punch tests. In comparison of the J-integral predicted in this paper with the literatures data and the master curve, a good agreement is also found.

Environmental degradation of fracture resistance in high-temperature water environments of low-alloy reactor pressure vessel steels with high sulphur or phosphorus contents

The fracture behaviour of low-alloy reactor pressure vessel steels with various sulphur and phosphorus contents, different environmentally-assisted cracking (EAC) and temper embrittlement (TE) susceptibilities was evaluated by elastic plastic fracture mechanics tests in air and various simulated light water reactors environments. A moderate but clear reduction of fracture initiation resistance occurred in a) high-sulphur steel with high EAC susceptibility in oxygenated high-temperature water with aggressive occluded crevice environment and preceding EAC growth, and in b) high-phosphorus steel with high TE susceptibility, where the reduction of fracture resistance was most pronounced in hydrogenated high-temperature water.

A Theory of Critical Distances based methodology for the analysis of environmentally assisted cracking in steels

This paper provides a methodology, based on the Theory of Critical Distances (TCD), for the hydrogen embrittlement analysis of steels in cracked and notched conditions. The TCD has been successfully employed in fracture and fatigue analysis, but it has not yet been applied under environmentally assisted cracking conditions. The point method and the line method, both belonging to the TCD, have been applied to two steels (X80 and S420) following an experimental programme composed of C(T) specimens containing defects with notch radii varying from 0 mm up to 2 mm. Fracture mechanics tests were carried out at 6 · 10−8 m/s of constant displacement rate under cathodic polarization conditions (5 mA/cm2). The aqueous environment has been prepared following the Pressouyre’s method. The study, which has been completed with finite element simulation analysis, reveals that the TCD provides accurate predictions of the hydrogen embrittlement behaviour of these steels in notched conditions.

Effect of dynamic strain ageing on environmental degradation of fracture resistance of low-alloy RPV steels in high-temperature water environments

Elastic plastic fracture mechanics tests on different low-alloy reactor pressure vessel (RPV) steels in air and various simulated light water reactor environments revealed a clear, but moderate reduction of the fracture initiation resistance of RPV steels with high susceptibility to dynamic strain ageing (DSA) in high-temperature water (HTW). In HTW, the reduction of fracture initiation resistance increased with decreasing loading rate and was most pronounced in hydrogenated HTW. The stronger reduction of fracture resistance in hydrogenated HTW was mainly due to the localization of plastic deformation by interaction between DSA and hydrogen.

Computational Modeling of Fracture Toughness of Al-Si and Al-Zn-Mg-Cu Alloys with Detected Porosity

This article presents the results of an experimental and numerical analysis of the fracture resistance of cast aluminum alloys subjected to variable loads over time based on the fracture mechanics method. The subjects of the analysis were the AlSi9 and AlZn6MgCu alloys (usually used for plastic processing) in which the presence of porosities in the form of separate or dispersed voids was revealed experimentally. The material for the experimental procedures was obtained directly from a casting or from casting samples. Depending on the type of the defect detected, different numerical models were proposed. The results of the numerical calculations were verified experimentally. Static tensile tests, fatigue low-cycle tests, fracture mechanics tests, fatigue crack propagation tests, a microscopic examination of the sample fracture area, and an x-ray tomographic analysis were all performed. A correlation of the results between the numerical analysis and the experimental work has been found. On the basis of the proposed calculation models, the fatigue life of castings of the analyzed alloys can be predicted in the case of concentrated and dispersed defects.

Failure investigation of liquefied petroleum gas cylinder using FAD and XFEM

To assess the reliability and performance of household liquefied petroleum gas (LPG) cylinder is important for safety standard. Such crack-like flaws in material of cylinder might lead to many serious catastrophes like fluid leakage and gas explosion. In this work, the failure assessment diagram (FAD) was determined according to the standard API 579 for inland produced LPG cylinders. A brand-new and used LPG cylinder of the same steel grade were investigated. Metallographic analyses, tensile tests and fracture mechanics tests using a single-edge tension SE(T) specimen were carried out. The direct current potential drop (DCPD) method was applied by the SE(T) test and the critical contour J-Integral for crack initiation was identified. Then, FE simulations of the cylinders afflicted with different crack configurations were performed under varying internal pressures. The corresponding critical pressure loads were determined and the level 3 FAD for a crack onset could be generated. It was found that the mechanical properties of used cylinder were obviously deteriorated that could therefore lead to an earlier failure when an unexpected certain pressure is given. In addition, FE simulations coupled with extended finite element method (XFEM) were conducted in order to describe the crack propagation in both cylinders. The results were firstly verified by comparing the load vs. crack mouth opening displacement (CMOD) curves. It was shown that the crack propagation in the aged LPG cylinder would become considerably stronger when unintended crack is present.

Effect of anisotropic microstructure of ODS steels on small punch test results

Hot rolling and hot extrusion of oxide-dispersion strengthened (ODS) ferritic steels give rise to anisotropic microstructures and mechanical properties and may provoke related phenomena such as secondary cracking. In this study, we consider the small punch (SP) test – a method, applicable in the case of small amounts of available material and well established for isotropic materials. The SP test was applied to investigate the effect of sample orientation on deformation and cracking for one hot-rolled and two hot-extruded ODS ferritic steels. Existing microstructural evidence is used to rationalize the observed anisotropic fracture behaviour. The SP test results are compared with those from existing fracture mechanics tests based on sub-sized C(T) samples. The applicability of the empirical conversion of SP-based into Charpy-based transition temperatures is evaluated. The fractographic manifestation of load drops in SP load-displacement curves is identified and the analogy to secondary cracking in fracture mechanics tests is shown.

Mechanical Behavior of Polymeric Materials for Mounting Systems of Solar-thermal Collectors

Three unreinforced and two glass fiber reinforced polymeric materials were tested with regard to their mechanical and fatigue crack growth (FCG) behavior. In the group of the unreinforced plastics, one elastomer modified copolymer (acrylic ester-styrene-acrylonitrile; ASA), one polymer blend of ASA and polycarbonate (PC) as well as one polymer blend of polybutylene terephthalate (PBT) and PC were investigated. As to the reinforced plastics, two glass fiber reinforced polyamide (PA) grades, differing in glass fiber length and glass fiber content, were characterized. Monotonic tensile tests and cyclic fracture mechanics tests were conducted at service relevant temperatures (0°C, 23°C, 60°C). For the three unreinforced plastics, significantly lower values for Young’s modulus and strength were obtained. Conversely, these materials revealed much higher strain-at-break values. Furthermore, glass fiber content dependent mechanical properties were determined for the reinforced plastics. With increasing temperatures, the Young’s moduli and strengths of all polymeric materials decreased distinctly. In terms of fatigue crack growth (FCG), the unreinforced plastics revealed an inferior FCG resistance at all temperatures. The FCG behavior of the reinforced plastics was influenced by the fiber orientation. Tests with the initial crack in mold direction exhibited a superior FCG resistance for the PA grade with the higher fiber content. In contrast, with the initial crack in cross direction, both glass fiber reinforced PA grades showed a similar FCG behavior. These effects are related to differences in the layer thicknesses of equally oriented fibers, which were obtained via microscopic investigation of the fatigue fracture surfaces.

Thermal and mechanical cyclic tests and fracture mechanics parameters as indicators of thermal shock resistance – case study on silica refractories

The paper discusses alternative methods to assess thermal shock in refractories. Thermal shock performance of two conventional silica bricks and two novice fused silica materials has been studied. Methods involving fracture mechanical tests of monotonic loading, repetitive thermal shock tests and cyclic strain controlled fatigue tests have been utilised. The amorphous fused silica is rather brittle. However low thermal expansion guarantees its superior thermal shock resistance. In-service crystallisation is to happen rather quickly. For crystallised fused silica of the studied morphology the tests coherently predict superior thermal shock resistance due to less brittle failure. Strain controlled fatigue test allows assessment of strain limits in the conditions of cyclic loading and thus combines the benefits of the thermal shock and monotonic fracture mechanics tests. Strain tolerance seems to be the property to correlate the results of the alternative test methods and those with the service loads.

A probabilistic Kitagawa-Takahashi diagram for fatigue strength assessment of cast aluminium alloys

Aluminium cast components containing intrinsic flaws were investigated in terms of fatigue strength. Therefore, quasi-static, fracture mechanical and fatigue tests were conducted at specimens exhibiting varying microstructural characteristics. Subsequently, the specimens were metallographically and fractographically analysed to evaluate spatial extent of the fracture initiating defects as well as the statistical distribution of defect sizes. Furthermore, the near defect-free fatigue strength was investigated to evaluate the upper boundary in fatigue strength for the Kitagawa-Takahashi diagram. In addition, the fracture mechanical test results were statistically evaluated as probabilistic values. Further on, a probabilistic evaluation of the crack resistance curve-concept was set up, taking the statistically distributed long crack threshold into account. The fatigue tests were finally utilized as probabilistic, defect correlated, fatigue design approach. The experimental results revealed that the fracture elongation of HIPped material increases by an average of 146% and the fatigue strength elevates of about 35%. The evaluated parameters from extensive crack propagation tests propose a long crack threshold of approximately 4 MPa √ m and an intrinsic threshold of slightly beneath 1 MPa √ m. The probabilistic Chapetti and El Haddad approaches lead to a sound correspondence with the experimental test data. Therefore, the presented fatigue assessment scientifically supports the lightweight design process especially for utmost probabilities of survival.

Modification of a Defect-Based Fatigue Assessment Model for Al-Si-Cu Cast Alloys.

Cast parts usually inherit internal defects such as micro shrinkage pores due to the manufacturing process. In order to assess the fatigue behaviour in both finite-life and long-life fatigue regions, this paper scientifically contributes towards a defect-based fatigue design model. Extensive fatigue and fracture mechanical tests were conducted whereby the crack initiating defect size population was fractographically evaluated. Complementary in situ X-ray computed tomography scans before and during fatigue testing enabled an experimental estimation of the lifetime until crack initiation, acting as a significant input for the fatigue model. A commonly applied fatigue assessment approach introduced by Tiryakioglu was modified by incorporating the long crack threshold value, which additionally enabled the assessment of the fatigue strength in the long-life fatigue regime. The presented design concept was validated utilising the fatigue test results, which revealed a sound agreement between the experiments and the model. Only a minor deviation of up to about five percent in case of long-life fatigue strength and up to about 9% in case of finite-lifetime were determined. Thus, the provided extension of Tiryakioglu’s approach supports a unified fatigue strength assessment of cast aluminium alloys in both the finite- and long-life regimes.