Papers by Keyword: R-Curve

Abstract: Fracture resistance curves (R-curves) have served as a robust tool in characterizing the entire fracture process of engineering materials. However, obtaining such curves for asphalt concrete (AC) mixtures is cumbersome due to the non-linear inelastic behavior of the mixtures. In this research, a single-specimen technique is developed based on the unloading compliance method which is used for metals. AC mixtures with limestone aggregate and PG58-22 binder were prepared. Beam specimens were fabricated and single-edge notched beam (SE(B)) fracture testing was conducted at low temperatures. A loading-partial unloading regime was used in the experiments and crack growth increments were captured by digital images throughout the tests. Using a multi-variable regression analysis, modified compliance equations were obtained for AC and R-curves of the mixtures could be constructed. It was revealed that the R-curve developed by ASTM E1820 compliance method could potentially overestimate the resistance of the mixtures against low-temperature fracture. The constructed R-curve exhibits a lower semi-vertical region addressing lower resistance of the mixture in the crack blunting phase. Also, the post-peak phase of the fracture shows a significantly lower slope in the constructed R-curve which denotes lower resistance of the mixture against unstable crack propagation.

Abstract: In a previous research by authors, a methodology was developed to derive J-R curves for Hot Mix Asphalt (HMA) mixtures using an elastic-plastic approach where a comprehensive understanding of crack propagation regime could be achieved. In this research, the effect of crumb rubber modification of HMA binder is studied in terms of R-curves and crack propagation at low temperatures. Mode I Single edge notched beam (SE(B)) fracture tests were conducted in temperature levels of 0 °C, -10 °C, and -20 °C. PG58-22 and PG64-22 binders were used in the fabrication of HMA samples. Modified specimens consist of 20% crumb rubber along with the incorporation of 3% warm mix admixture. Crack growth resistance curves were obtained in SE(B) tests by means of image processing and recording of the progressive crack length. Elastic-plastic J-R curves revealed that crumb rubber modified mixtures exhibit higher crack growth resistance for each bitumen performance grade. As well, increased ductility and cohesive energy can be observed according to the R-curves as the mixtures are modified by crumb rubber.

Abstract: Numerical models based on cohesive zones are usually used to model and simulate the mechanical behavior of laminated carbon fiber reinforced polymers (CFRP) in automotive and aerospace applications and require different interlaminar properties. The current work focuses on determining the interlaminar fracture toughness (G_IC) under Mode I loading of a double cantilever beam (DCB) specimen of unidirectional CFRP, serving as prototypical material. The novelty of this investigation is the improvement of the testing methodology by introducing digital image correlation (DIC) as an extensometer and this tool allows for crack growth measurement, phenomenological visualization and quantification of various material responses to Mode I loading. Multiple methodologies from different international standards and other common techniques are compared for the determination of the evolution of G_IC as crack resistance curves (R-curves). The primarily metrological sources of uncertainty, in contrast to material specific related uncertainties, are discussed through a simple sensitivity analysis. Additionally, the current work offers a detailed insight into the constraints and assumptions to allow exploration of different methods for the determination of material properties using the DIC measured data. The main aim is an improvement of the measurement technique and an increase in the reliability of measured data during static testing, in advance of future rate dependent testing for crashworthiness simulations.

Abstract: This article examines the fracture toughness of end notched flexure (ENF) composite specimens of three different lay-ups. Experiments were conducted on these glass/epoxy specimens and the critical fracture energy,, was evaluated based on compliance based beam method (CBBM). Classical methods require crack length measurements, which are not easy to obtain as propagation occurs rapidly without a clear opening. The CBBM is based on crack equivalent concept, which does not require crack length monitoring during propagation and hence the crack growth resistance curve (R-curve) can be generated in a much easier way. Moreover, the CBBM accounts the non-negligible energy dissipation in the fracture process zone (FPZ) in addition to stress concentrations near crack tip, contact between specimen arms at the pre-crack region and root rotation effects. Hence, the complete R-curves of ENF specimens of different lay-ups were obtained using the CBBM with higher degree of accuracy. It was observed that the unidirectional specimen did show higher propagation toughness value than the angle-ply and cross-ply specimens.

Abstract: Ceramic particle reinforced metal matrix composites (PRMMCs) combine the strength and brittleness of ceramics with the toughness of a metallic matrix. In order to use these materials in construction and operational design their fracture mechanical behavior must be evaluated. In this study, a 30 vol.-% Al₂O₃ reinforced austenitic TRIP steel processed by powder metallurgical technique was investigated using precracked miniature SENB-specimens in 3-point-bending. An elastic-plastic analysis by means of the J-integral method in combination with optical crack observation showed the materials ability of stable crack growth, i. e. R-curve behavior. In addition to the mechanical tests microstructural studies were performed, whereby particle debonding and fracture as well as martensitic phase transformation and crack bridging within the matrix were identified as fracture energy dissipating mechanisms.

Abstract: The life expectancy or failure of aerospace pressure vessels is evaluated by the critical stress intensity determined by the crack growth resistance curve of a material. Load versus crack mouth opening displacement data is generated from the Compact Tension specimens made from the weld joints of maraging steel rocket motor segments. The steps involved to generate critical stress intensity factor is explained. A power law is adopted to model the crack extension in terms of stress intensity factor and determine the maximum failure load of weld specimens. Maximum failure loads of CT specimens obtained by test and analysis are presented.

Abstract: Weight reduction is the main driving force in automotive and aircraft structural design. As a result, magnesium alloys, with their high potential for lightweight construction, have attracted a considerable amount of industrial attention. The determining criterion for the structural applications of magnesium alloys is the availability of efficient joining technologies for the construction of lightweight structures and the availability of reliable data for the assessment of their damage tolerance behaviour. Laser beam welding (LBW), as a high-speed and easily controllable process, allows the welding of complex geometric forms that are optimised in terms of mechanical stiffness, strength, production velocity and visual quality. The work accomplished in this study addresses the challenges of the LBW process for typical joint configurations using the magnesium alloy AZ31HP: butt joints, T joints and overlap joints. LBW processes were developed for use with a 3.3-kW Nd:YAG laser to optimise the mechanical performance of such joints with respect to tensile strength, fatigue, fatigue crack propagation and mechanical fracture behaviour. The relationships between the LBW process and the microstructural and mechanical properties of welds were established. Compared to state-of-the-art aerospace alloys, AZ31HP demonstrates that magnesium alloys have potential for use in structural applications, with AZ31HP being comparable to AA2024T351 and AA6061T6. Welded AZ31HP exhibits better crack resistance than the base material, so fully welded integral structures made from magnesium alloys can be used in lightweight construction.

Abstract: The prediction of the strength of adhesively bonded joints has been investigated using a variety of failure criteria such as maximum stress or strain, and fracture mechanics approaches. Fracture mechanics approaches based on the critical strain energy release rate, for crack propagation are applicable to highly cross-linked structural adhesives and have the advantage of avoiding the explicit consideration of the bi-material singularities inherent in adhesive joints. In the present work, the finite-element simulation of such adhesive joint has been performed and the R-curves of two different rubber-toughened epoxy adhesives were measured using double cantilever beam (DCB) specimens. The FE results are applied to be compared with the experimental results which were reported in the literature.

Abstract: The recent rewriting of the Bazant's Size Effect Law which has suggested the existence of an additional asymptotic regime for intermediate structure sizes is compared to numerical simulations of fracture of geometrically similar notched structures of different sizes. The quasibrittle failure is simulated through Cohesive Zone Model (bilinear softening) using a constant set of cohesive parameters whatever the specimen size. The different asymptotic regimes expected for the size effect on the nominal strength are shown in fair agreement with the size effect observed on the results obtained from numerical simulations. The existence of the new asymptotic regime expected for intermediate structure sizes is, in particular, clearly revealed by this comparison.

Abstract: In fracture tests involving the Single-Edge-Notched beam loaded in Three or Four-Point-Bending as well as in other specimen shapes, the specimen weight contributes to the overall loading of the system. Unless special methods are put into practice, the contribution of the specimen weight is not compensated, leading to misevaluations of fracture parameters obtained in the experiments. In this work a method taking into account the exact contribution of the specimen self-weight is proposed to evaluate the Resistance-curve of mortar. Cohesive crack modeling accounting for the structure self-weight is used to generate the necessary load-displacement response to perform the validation of the proposed method.