Compact Tension Fracture Tests Research Articles

High-temperature fracture behavior of carbon fiber reinforced PEEK composites fabricated via fused filament fabrication

This study investigates the influence of temperature on the mechanical and fracture properties of carbon fiber reinforced (CF) polyetheretherketone (PEEK) composites, with a specific focus on different layer heights used in the additive manufacturing process. Tensile and compact tension fracture tests were conducted at varying temperatures to evaluate the orthotropic mechanical properties and crack growth resistance curves, respectively. The key findings underscore the pronounced orthotropic mechanical behavior in additively manufactured CF-PEEK composites, showing substantial variations of up to 50% in Young's modulus and yield strength across orientations. Additionally, as test temperatures increase, crack initiation becomes easier, indicating reduced crack resistance at elevated temperatures. Notably, the effect of layer height on initiation toughness is relatively minor compared to temperature. Moreover, the study reveals that fracture energy for crack growth is less temperature-sensitive but depends on layer height, with larger defects in taller layers resulting in decreased damage resistance. These findings stress the importance of considering the distinct sensitivities of initiation and steady-state fracture energies to temperature, emphasizing the critical role of effective fracture toughness in evaluating material resistance to high-temperature fractures. These observations contribute to our understanding of the mechanical and fracture behavior of CF-PEEK composites under high operating temperatures. This understanding facilitates the optimization of their manufacturing processes and their application in various industries such as aerospace.

Coupling effect of thermal aging and pre-strain on fracture behavior of SMAW welded joints

A repairing process is commonly needed to correct installation slope and geometrical deviation of nuclear power piping, but the resulting pre-strain has obvious degradation impacts on the long-term operation of surge pipeline, especially for the shielded metal arc welding (SMAW) joints. In this paper, to investigate the embrittlement induced by thermal aging and pre-strain, the SMAW welded joints with the base metal Z2CND18-12N2 and welded metal ER316L were subjected to different pre-strain (0%, 8%, 12% and 15%) and the accelerated thermal aging from 0 h to 13000 h at 400 ℃. Then, the Charpy impact test and compact tension fracture test were conducted to determine fracture properties of the SMAW welded joints. Results showed that both thermal aging and pre-strain can significantly reduce the fracture properties of SMAW welded joints. The impact absorbed energy reached stable after thermal aging 500 h, but it has been decreasing with the pre-strain varying from 0% to 15%. Moreover, it is shown that for pre-strained SMAW welded joints, with the increase of thermal aging duration, the absorbed energy first recovers, and then decreases until stable. Compared with pre-strain, the influence of thermal aging is dominant. Finally, combined with microscopic analysis, the comprehensive effects of thermal aging and pre-strain on the embrittlement mechanism of SMAW welded joints were discussed.

Fracture behavior of 3D printed carbon fiber-reinforced polymer composites

This study examines the inter-layer and cross-layer fracture behavior of 3D printed short CFRP composites using both experimental and numerical methods. First, the orthotropic mechanical response of the 3D printed CFRP samples was characterized by tensile tests. The out-of-plane Young's modulus and yield strength were measured to be about 25% and 32% of the in-plane counterparts, respectively. Second, compact tension fracture tests were performed to characterize the fracture behavior of the CFRP composite samples. The crack resistance curves of inter-layer and cross-layer fracture were independently derived using the elastic-plastic fracture theory. Both cross-layer and inter-layer fracture exhibited stable crack growth with the rising crack resistance curves. Finite element simulations incorporating a cohesive zone method was used to numerically determine the critical cohesive strength and fracture toughness from the experimental results. The experimentally measured and numerically calibrated cross-layer fracture toughness was found to be three-fold of the inter-layer counterparts. In addition, fracture surface morphology and the crack-tip strain field evaluated by the digital image correlation technique revealed three important observations: (1) the cross-layer fracture resulted in a larger fracture process zone ahead of the crack tip compared to the inter-layer fracture, (2) the cross-layer fracture exhibited a fibril-bridging toughening mechanism along with fiber pull-out failure at the micro-scale, and (3) the inter-layer fracture showed a uncracked-ligament toughening mechanism arising from the locally enhanced inter-layer adhesion due to short fiber-bridging. The fracture mechanisms reported in this study provide practical strategies for designing and 3D printing CFRP composites with superior fracture-resistance.

Increasing Precision and Confidence Level in Fracture Energy Measurement by Optimizing the Number of Test Replicates for Disk-Shaped Compact Tension Fracture Test (ASTM D7313)

Abstract Different fracture-based tests and parameters have been developed in recent years to assess thermal cracking performance of asphalt mixtures. Agencies have adopted some of these tests, established threshold values, and incorporated them into their specifications to evaluate mixes during design and production. Because of the heterogeneous nature of asphalt mixtures, there can be a substantial amount of variability associated with mechanical test results obtained from fracture-based tests. To obtain unbiased properties with variabilities only inherent to the material itself and not caused by mixture production, specimen fabrication, or testing related factors, an appropriate number of replicate specimens need to be tested. This article provides the results of a study to establish a practical number of replicate tests that are required to obtain accurate and representative asphalt mixture fracture energy from the disk-shaped compact tension (DCT) fracture test for routine use by producers and agencies. Statistical analyses were performed to determine the precision and accuracy associated with the use of 4, 8, and 12 replicates. A total of 368 fracture energy measurements (16 replicates for 23 mixtures) were used for the analysis. The statistical analysis consistently indicated that 12 replicates results in higher confidence in the estimate of the mean. The maximum difference between measurements when 4 replicates are used ranges from 3 to 25 % but reduces to 1 to 9 % for 12 replicates. For 4 replicates, the data set used in this study indicated that 41 % of the time, it was not possible to confidently distinguish whether mixtures met a minimum threshold value or not. However, this error margin drops to 12 % when using 12 replicates. Based on these results, 12-replicate specimens are recommended to ensure necessary accuracy and repeatability for performance-based specifications using DCT fracture energy.

Relations between intralaminar micromechanisms and translaminar fracture behavior of unidirectional FRP supported by experimental micromechanics

The translaminar fracture behaviors of partially different unidirectional composite systems, constituted by the same carbon fibers but different (thermoset vs. thermoplastic) matrices, were characterized by means of compact tension fracture tests. The resulting crack resistance curves (R-curves) and fracture surfaces, were studied in detail and found to be rather different between those material systems, in spite of the same reinforcing fibers at similar volume fractions. In the attempt to justify this difference, the effects of the underlying micromechanisms were evaluated by employing experimental micromechanical measurements of the fracture characteristics of fibers, matrices and fiber/matrix interfaces. By means of a thorough analysis and quantification of the micromechanisms that contribute to the work of fracture, it was possible to decompose the translaminar fracture toughness of the composites into different contributions. Independently of the material considered, fibre bundle pull-out was found to be the mechanism that dissipates the highest amount of energy. Different patterns of bundle pull-out in different material systems were found to be the result of different outcomes from the competition of fracture micromechanims, and to be responsible for the differences between the translaminar fracture energies of both material systems. Moreover, it was realized that the energy dissipated in bundle pull-out, hence also the overall measured translaminar fracture toughness are strongly governed by in-situ effects, i.e. effects of specimen lamination features.

Comprehensive evaluation of low-temperature fracture indices for asphalt mixtures

For performance-based specifications and design processes, a number of cracking-related index parameters have been proposed for asphalt mixtures in recent years. A number of these parameters have been developed to utilise results from fracture tests. This study conducted a comprehensive evaluation of various fracture index parameters including fracture energy, Illinois flexibility index, stress intensity factor and toughness index. Over 200 tests from 61 distinct test data sets representing 21 asphalt mixtures are included. The focus of this study is on low-temperature cracking and all indices were evaluated using test results from the disk-shaped compact tension fracture tests conducted at low temperatures. The objective of this study was to determine if there is a relationship between various fracture index parameters as well as to determine typical measurement variability associated with each parameter. Comparisons were made between different indices and correlations were determined for the mix rankings provided by individual indices. The results indicate that fracture energy successfully captured the mix rankings and showed a strong correlation with other indices. In order to better capture post-peak softening behaviour of asphalt mixtures, the flexibility and toughness indices have been utilised; however, these parameters were found to have high variability. A new index called fracture strain tolerance has been proposed that was shown to provide the same level of distinction between mixtures as the flexibility and toughness indices while having considerably lower variability. Finally, several areas were identified for future extension of this research.

Prediction of ductile failure using a local strain-to-failure criterion

In this article, we provide the details of the predictive simulations performed by the University of Texas team in response to the 2012 Sandia Fracture Challenge (Boyce et al. in The Sandia Fracture challenge: blind predictions of ductile tearing. Int J Fract. doi: 10.1007/10704-013-9904-6, 2013). The material constitutive model was calibrated using the tensile test data through an optimization scheme. A modified Johnson–Cook failure criterion was also partially calibrated using the material characterization data obtained from a tension test and a compact-tension fracture test. These models are then embedded in a highly refined finite element simulation to perform a blind prediction of the failure behavior of the Sandia Fracture Challenge geometry. These results are compared with experiments performed by Sandia National Laboratories and additional experiments that were performed at the University of Texas at Austin with full-field three-dimensional digital image correlation in order to explore the different failure modes. It is demonstrated that a well-calibrated model that captures the essential elastic–plastic constitutive behavior is necessary to confidently capture the elasto-plastic response of challenging structural geometries; it is also shown that a simple ductile failure model can be used to predict ductile failure correctly, when proper calibration of the material model is established.

Exploring the reduction of laboratory testing for the cohesive zone model for asphalt concrete

The cohesive zone model is a popular tool used to predict cracking in asphalt concrete. However, two of the three inputs required for the cohesive zone model, tensile stress and fracture energy, come from destructive laboratory tests. If the peak load from a fracture test (which supplies fracture energy) could be substituted for the tensile strength, the amount of laboratory testing could be halved. This research compared the peak loads from the single-edge notch beam, semi-circular bend and disk-shaped compact tension fracture tests as well as the tensile strength from the standard indirect tension (IDT) test. Even accounting for specimen geometry, the data did not indicate consistent correlation of the three fracture tests with the IDT test across various testing temperatures, loading rates or sample type (laboratory vs field samples). Therefore, no fracture test provided a sufficiently robust correlation to recommend replacing the tensile strength with the peak load of the fracture test for the cohesive zone model.

Effects of in-plane electric fields on the toughening behavior of ferroelectric ceramics

Mode-I steady state crack growth in poled ferroelectric ceramics subjected to simultaneous electrical and mechanical loading is analyzed to investigate the effect of in-plane electric fields and polarization on the toughening behavior. A multiaxial, incremental constitutive law for domain switching is implemented within the finite element method to obtain the electromechanical crack tip fields. Simulation results are presented for the cases of initial remanent polarization states and applied electric fields perpendicular to the crack plane and parallel to the crack growth direction. Specific results from the calculations include the shapes and sizes of switching zones, and the toughening effects due to domain switching near the crack tip. Ferroelectric ceramics have been widely used in smart structure applications due to their large electromechanical coupling effects. Since ferroelectricdevices oftenoperate understrong mechanicaland electrical loading conditions, the brittle ferroelectric ceramic material is susceptible to fracture. Therefore, an understanding of ferroelectric fracture is a key issue for the efficient and reliable design of these devices. This paper is concerned with the study of the Mode-I fracture behavior of ferroelectric ceramics under combined in-plane electrical and mechanical loading. The scenario investigated here is illustrated in Figure 1. A ferroelectric material is initially poled by an electric field either perpendicular or parallel to the direction of crack growth. After the initial poling, an electric field is applied along (positive electric field) or opposite (negative electric field) to the initial poling direction. Finally, mechanical loading is applied and crack growth occurs. Experimental observations of the fracture properties of ferroelectrics under such conditions have been obtained on several materials from indentation tests and compact tension specimens. Using indentation tests on a PZT-8 material composition, Tobin and Pak [1993] showed that for cracks perpendicular to the poling direction, the apparent fracture toughness decreases with positive electric field and increases with a negative field. For cracks parallel to the poling direction, their results indicated that both positive and negative electric fields have little influence on the toughening. Tobin and Pak [1993] also observed that with no applied electric field the fracture toughness was greater for cracks parallel to the poling direction than for cracks perpendicular to the poling direction. Park and Sun [1995] investigated electric field effects on crack growth in a PZT-4 ceramic by using conventional compact tension fracture tests. Their results agree with those of Tobin and Pak [1993] for the case of electric field applied perpendicular to the cracksurface. However, theVickersindentationtestsofWangandSingh[1997]showedthatiftheapplied electricfields are perpendicular to the crack in a PZT EC-65 ceramic, then a positive electricfield impedes

Applicability of crack energy density to fracture strength evaluation of piezoelectric ceramics

A lot of studies have been made to grasp and describe fracture behavior of piezoelectric material under electromechanical loadings. However, even the criterion to describe the dependence of static fracture strength on electric field has not been established yet. In this paper, a fracture criterion based on mechanical contribution of crack energy density is proposed and its applicability is studied by analyzing not only the three-point bending fracture test results by the authors on piezoelectric ceramics called C-2 but also the compact tension fracture tests by Park and Sun on piezoelectric ceramics called PZT-4, based on the criterion.

Re-evaluating the toughness of human cortical bone

Data for fracture in human humeral cortical bone are re-analyzed to assess the validity for this material of linear-elastic fracture mechanics (LEFM), which is the standard method of analyzing toughness and one basis for analyzing clinical data relating to bone quality. A nonlinear fracture model, which is based on representing the damage zone in the bone by a cohesive model, is calibrated against a number of sets of test data for normal (not diseased or aged) human cortical bone taken from cadavers. The data consist of load vs. load-point displacement measurements from standard compact–tension fracture tests. Conventional LEFM is unable to account for the shape of the load–displacement curves, but the nonlinear model overcomes this deficiency. Calibration of the nonlinear model against one data curve leads to predictions of the peak load and the displacement to peak load for two other data curves that are, for this limited test set, more accurate than those made using LEFM. Furthermore, prior observations of damage mechanisms in bone are incompatible with the modeling assumption of LEFM that all nonlinearity is confined to a zone much smaller than the specimen and the crack length. The predictions of the cohesive model and the prior observations concur that the length of the nonlinear zone in human cortical bone varies in the range 3–10 mm, which is comparable to or larger than naturally-occurring bones and the specimens used to test them. We infer that LEFM is not an accurate model for cortical bone. The fracture toughness of bone deduced via LEFM from test data will not generally be a material constant, but will take different values for different crack lengths and test configurations. The accuracy of using LEFM or single-parameter fracture toughness for analyzing the significance of data from clinical studies is called into question. The nonlinear cohesive zone model is proposed to be a more accurate model of bone and the traction-displacement or cohesive law is hypothesized to be a material property. The cohesive law contains a more complete representation of the mechanics of material failure than the single-parameter fracture toughness and may therefore provide a superior measure of bone quality, e.g., for assessing the efficacy of therapy for osteoporosis.

Effect of Water Absorption Rates on Fracture Toughness of Sisal Textile Fiber Reinforced Composites

Using natural fibers that are inexpensive, lightweight and biodegradable, as the reinforcement for composites is difficult due to their poor interfacial properties between hydrophilic fiber and hydrophobic polymer matrices. It is necessary to evaluate fracture toughness of natural fiber reinforced composites according to water absorption rates to improve mechanical performance of those. In this study, compact tension fracture test was conducted to evaluate fracture toughness with the various specimens. The value of fracture toughness has the tendency to decrease as water absorption rate increases. And different surface treatment methods and different polymer matrices have influence on the value of fracture toughness.

Prediction of cleavage failure probabilities using the Weibull stress

In this work, the use of the Weibull stress as a measure of the failure probability of a cracked body is examined. Closed form expressions for the Weibull stress are presented for linear elastic and power law materials. These expressions allow Weibull stress values and failure probabilities to be estimated without the need for finite element analyses and provide insight into the use of the Weibull stress as a parameter for the prediction of cleavage failure of cracked bodies. Application of the Weibull stress to the prediction of transition region and upper shelf cleavage failure probabilities of ferritic steel welds is also discussed. The effect of ductile tearing, described by a Gurson-type material model, is examined. Numerical analyses of compact tension fracture tests are carried out and used to interpret experimental fracture toughness data for ferritic steel welds. A comparison between the finite element results and the experimental data indicates that the scatter in the cleavage fracture toughness values in the transition region are quite well captured by the Weibull stress. However, cleavage failure probabilities close to the upper shelf using the two parameter Weibull model are considerably higher than those observed in practice. A number of reasons for this discrepancy and suggestions to improve the predictions are proposed.

Design complexity and fracture control in the equine hoof wall.

Morphological and mechanical studies were conducted on samples of equine hoof wall to help elucidate the relationship between form and function of this complex, hierarchically organized structure. Morphological findings indicated a dependence of tubule size, shape and helical alignment of intermediate filaments (IFs) within the lamellae on the position through the wall thickness. The plane of the intertubular IFs changed from perpendicular to the tubule axis in the inner wall to almost parallel to the tubule axis in the outer wall. Morphological data predicted the existence of three crack diversion mechanisms which might prevent cracks from reaching the sensitive, living tissues of the hoof: a mid-wall diversion mechanism of intertubular material to inhibit inward and upward crack propagation, and inner- and outer-wall diversion mechanisms that prevent inward crack propagation. Tensile and compact tension fracture tests were conducted on samples of fully hydrated equine hoof wall. Longitudinal stiffness decreased from 0.56 to 0.30 GPa proceeding inwardly, whereas ultimate (maximum) properties were constant. Fracture toughness parameters indicated that no compromise results from the declining stiffness, with J-integral values ranging from 5.5 to 7.8 kJ m-2 through the wall thickness; however, highest toughness was found in specimens with cracks initiated tangential to the wall surface (10.7 kJ m-2). Fracture paths agreed with morphological predictions and further suggested that the wall has evolved into a structure capable of both resisting and redirecting cracks initiated in numerous orientations.

Mechanical Failure in PEEK and Its Short-Fiber Composites

Four commercial PEEK systems—two grades of neat PEEK and short GF and CF-fiber composites—were compression-molded. Compact tension fracture tests were performed, with temperature and specimen thickness as variables. The dependences of the critical stress intensity factor KQ and of mechanical nonlinearity on these variables are re ported. The filled systems show both nearly constant KQ and low ductility from —90 to +125°C. For all materials, KQ varies more slowly with temperature than does the tensile strength. Specimen thickness, varied from 0.29 to 1.25 cm, has a profound effect on neat PEEK, as it changes from plane stress to plain strain behavior Thickness has essentially no effect on the composites, which show plane strain behavior over the entire thickness range.

Evaluation of Fracture Toughness (JIC) Using Single Specimen Fracture Test Augmented by Finite Element Analysis

Fracture toughness is evaluated using the combination of results from a single compact tension fracture test and computationally developed key-curves. The key-curves—a family of load-deflection curves for the test specimen with different length cracks—were developed using the tensile flow properties of the material. By overlaying the experimental test results on these curves the crack length and crack growth during the test can be deduced. The value of J for each crack length is then evaluated from the experimental load-deflection data. The J versus crack extension curve is plotted and JIC evaluated. The method is applied to the evaluation of the toughness of depleted uranium and is shown to substantially reduce the scatter often observed in the J versus Δa data.

Modeling of Elastoplastic Fracture Behavior Using Acoustic Emission Methods

Using multiple-specimen JIC (R-curve) methods to determine elastoplastic fracture properties is time consuming and costly, and gives limited data about fracture behavior. This paper investigates quantitative acoustic emission analysis methods in modeling elastoplastic fracture in typical pressure vessel steels, and presents the results of a series of compact tension fracture tests, heat treated to exhibit a range of fracture behavior and monitored with acoustic emission. The paper also gives a theoretical formulation relating acoustic-emission parameters with crack length and GIC, and discusses several schemes for acoustic-emission source classification.

Optimization of the Electrical Potential Technique for Crack Growth Monitoring in Compact Test Pieces Using Finite Element Analysis

AbstractFinite element procedures are used to optimize the efficiency of the electrical potential technique for monitoring the initiation and slow growth of cracks, as applied to the compact tension fracture test piece. An analysis of various configurations of current input and potential measurement lead placement is performed to optimize the accuracy, sensitivity, and reproducibility of measurement and to maximize output voltages. Numerical calibration curves are computed for selected configurations and are confirmed by experimental measurements.

An application of the finite element method to elastic-plastic analysis of the compact tension fracture test specimen

The results of a finite element analysis of the compact tension fracture test specimen (CT-specimen) in the elastic-plastic range are presented with special attention to the fracture-characterizing parameters. The geometry considered is that of the usual CT-specimen of width w, thickness B, height H = 0.6 w, crack length c = 0.5 w and applied forces P. The problem is approximated as plane strain. The finite element program developed by the author employs simple constant strain triangles and works according to the incremental initial strain method. As yield criterion the Mises condition was used and in the plastic state the associated flow rule of Prandtl-Reuss was assumed. As material laws a linear hardening with constant slope E. = E/50 or the power law hardening relation of Ramberg-Osgood with h~rdening exponent n were supposed. (E = elastic modulus, ~F = uniaxial yield strength). The finite element network for one-half of the symmetric specimen consists of about 700 elements, focused essentially around the crack tip with smallest elements of the dimension c/100. About 100 load steps were carried out with each step having 10% of the initial load where yield first occurred.