Automated monitoring of the crack propagation in mode I testing of thermoplastic composites by means of digital image correlation

One of the main challenges of the mode I double cantilever beam (DCB) test is the simultaneous determination of the applied load and displacement with the developing delamination length. The present work addresses this issue by side-view tracking the crack propagation by means of digital image correlation (DIC). Two different reduction methods were developed to determine the crack length from the DIC data. On the one hand, the crack tip position was defined by the high strain concentration in the immediate vicinity of the crack tip, and on the other hand, by crack tip opening displacement (CTOD). The data obtained enabled the calculation of the energy release rate of carbon fibre reinforced thermoplastic specimens with either run-arrest or stable crack extension. For reasons of comparability, top surface analysis (TSA), as recently reported, was also carried out. Following this approach, the crack propagation was tracked applying DIC to the top specimen surface. The methods developed showed a good correlation with both the standardised procedure and TSA. It was shown that DIC can be used as an alternative to the conventional optical measuring tools to follow the crack propagation in the mode I DCB test.     

Effect of defect size on fracture strength of dental low fusion porcelain

Various grinding defects were produced on the surface of specimen dental low fusion porcelain in an attempt to establish the relationship between defect size and fracture strength. In addition, the applicability of the process zone size-fracture criterion in assessing the material properties of dental low fusion porcelain was examined. Super porcelain AAA E3 (Noritake Co., Japan) was used as dental low fusion porcelain. The bending strength and fracture toughness value were estimated by the three-point bending test. After glazing, grinding flaws were introduced by grinding the specimen with abrasive papers of various mesh sizes. In order to calculate the fracture toughness value of dental low fusion porcelain, we introduced a surface crack using a Vickers indenter. The results were discussed based on the process zone size-fracture criterion. The size of cracks caused by grinding was estimated with the process zone size-fracture criterion and Newman-Raju formula. As the defect size decreased, the fracture stress approached the strength for smooth specimen without defect. The K(c) value showed a tendency to approach the K(lc) value when the defect size increased. The relationship between the fracture stress, sigma(F), and the equivalent crack length, a(e), was in good agreement with the theoretical relations deduced from the criterion in dental low fusion porcelain.     

Relevance of the femtolaser notch sharpening to the fracture of ethylene-propylene block copolymers

The effect of the notch sharpening on the fracture toughness measured under Linear Elastic Fracture Mechanics (LEFM), Elastic-Plastic Fracture Mechanics (EPFM) and Post-Yielding Fracture Mechanics (PYFM) approaches has been evaluated. Bulk and film specimens of an ethylene-propylene block copolymer have been analyzed. The samples for fracture characterization were sharpened using a steel razor blade and the femtosecond laser ablation technique. Both notching techniques give rise to crack tip radii of the very same size. The fracture toughness of the specimens sharpened via femtolaser were ∼10%, ∼75% and ∼90% lower than that of the specimens sharpened via razor blade when determined with the help of LEFM, the EPFM approach as the multiple specimen method, and by the Essential Work of Fracture, respectively. Both in the bulk samples as in the films, the presence of plastic deformation, either large or small, occurring ahead of the crack tip during the sharpening seems to be the reason for the difference in the fracture values.     

On the determination of the point of fracture initiation by the load separation criterion in J-testing of ductile polymers

The application of an experimental approach based on the load separation criterion for the determination of the point of fracture initiation in a fracture test on a ductile polymer was critically examined. To this aim, the fracture process outlined by the application of this method was related to that described by the visual analysis of the fracture surfaces obtained in fracture tests on nominally identical specimens, in which different levels of crack extension were produced. The material examined was an acrylonitrile-butadiene-styrene (ABS) resin, and the fracture tests were performed at low loading rate on single-edge notched in bending specimens.The results demonstrated that this load separation criterion based methodology is a promising approach for the determination of the point of fracture initiation, and for material fracture resistance, J_Ic, evaluation. The method also has experimental simplicity and a high degree of repeatability.     

Fracture toughness of gamma irradiated polycarbonate sheet using the essential work of fracture

Polycarbonate (PC), a ductile polymer, has been found by both linear elastic fracture mechanics and impact tests to present a ductile-brittle transition, which depends on notched specimen thickness, test speed and gamma irradiation. Owing to large amounts of plastic deformation, fracture toughness measurements by these test methods are not precise. In the present communication, a better method, the Essential Work of Fracture (EWF), to assess the fracture characteristics in plane state of stress was for the first time used to evaluate the fracture toughness of PC sheets subjected to gamma irradiation dose. Three-points bend tests of sharp pre-cracked specimens with different ligament lengths were 340 kGy gamma irradiated. EWF results showed that the total fracture work increased linearly with length for both non-irradiated and gamma irradiated conditions. A significant decrease in EWF fracture toughness was associated with brittleness promoted by gamma irradiation. This brittleness was also confirmed by macro and microscopy (SEM) evidence.     

Refined fixture design for effective essential work of fracture toughness characterization of m-LLDPE thin films

Characterization of Mode-I fracture toughness of ductile polymeric thin films is nontrivial. Proper specimen preparation and experimental procedures are required to ensure in-plane tensile loading. In this study, a custom-built double-edge notched tensile test fixture was employed to characterize the Mode-I essential work of fracture (EWF) toughness of metallocene linear low-density polyethylene (m-LLDPE) films. Effects of specimen geometry, strain rate and film orientation on the specific essential work of fracture, w_e, and the specific non-essential work of fracture, w_p, were investigated. Results indicate both EWF parameters are independent of the crosshead speed, gauge length (distance between upper and lower clamps) and specimen width within the ranges tested. w_e is significantly higher for thinner films and for crack propagation perpendicular to the blown film machine direction (MD). The usefulness of EWF for evaluating m-LLDPE fracture toughness is discussed.     

“On the use of the essential work of fracture procedure in the determination of the fracture energy of tough polymeric materials”

The determination of the energy of fracture of tough polymeric materials in plane strain conditions by means of the essential work of fracture procedure has nowadays two open questions. The first is related to the ligament length that has to be used when the impact specimens break partially upon testing, and the second is related to the way to assure that the crack propagation occurs in an already yielded zone. The use of i) a “true ligament length” associated with only the broken area and ii) impact specimens yielded in the notch zone in a previous tensile test, are evaluated and proposed.     

Measurement of rapid crack propagation in pressure pipes: A static S4 approach

A method for creating rapid crack propagation in pressurized pipes under slow static loading using modified S4 apparatus is described. In the development of the method a complexity involved with dynamic loading in the S4 test (ISO 13477) is eliminated by the use of a displacement controlled static loading machine. The experimental system consisted of an universal testing machine, a low compliance wedge loading device, notch tip quenching apparatus and a pipe specimen where a through thickness hole is drilled to accommodate the wedge loading device. The pipe specimen is made in such a way that a section containing a hole is free from the internal pressure, while the rest of the specimen is made to carry the internal pressure which would eventually drive the unstable crack along the pipe axis. The idea of such rapid crack initiation under static loading was derived from the concept of time-temperature equivalence, where impact loading may in part be simulated by lowering the temperature at the site of rapid crack initiation. The details of the method for rapid crack propagation under static loading are described and the correlation of the results to rapid crack propagation obtained by ISO 13477 is illustrated. The two methods were shown to compare quite well in terms of critical pressure determination and the details regarding normalized rapid crack length versus the internal pressure curve as well as the crack propagation pattern.     

A finite element method based comparative fracture assessment of carbon black and silica filled elastomers: Reinforcing efficacy of carbonaceous fillers in flexible composites

This study is focused on numerical investigation on fracture behaviors of carbon black (CB) and silica filled elastomeric composites. Finite element analysis (FEA) in compliance with multi-specimen method is used to calculate J-integral and geometry factor of the rubber composites up to a displacement of 20 mm for single edge notch in tension (SENT) and double edge notch in tension (DENT) specimens. An empirical relationship between crack tip opening displacement (CTOD) and crack advancement is established depending on notch to width ratio (NWR). The stress contours across the notches for SENT and DENT specimens is discussed briefly. It is found that fracture propagation resistance of CB filled elastomer is 125% more than that of silica filled elastomer. Although, Silica filled elastomer have good tensile strength and crosslink density but it fails to replace carbon black in terms of fracture properties. The critical J-integral for CB filled elastomer is 18.7% and 32.2% more than silica filled elastomer for SENT and DENT specimens respectively. The effect of specimen type on various fracture properties is also explored. The factor of safety is found to be significantly more in case of CB filled elastomers making them less vulnerable to crack propagation and catastrophic failure.     

Experimental approach for microscale mechanical characterization of polymeric structured materials obtained by additive manufacturing

In this paper, an original experimental method is developed for local strain characterization at the surface of additively manufactured polymeric materials. The process used herein is material extrusion. This experimental method is based on the use of microscopic speckle pattern deposited at the surface of micro single edge notched specimen (μ_SENT) made of acrylonitrile butadiene styrene (ABS). Two configurations of filament orientation were used for the specimen manufacturing. Images of the μ_SENT specimen surface were recorded during in-situ tensile test. The quantitative analysis of images was made by digital image correlation (DIC). The evolutions of the local strain heterogeneities and the crack tip are evidenced on the kinematic fields. It is shown that the crack propagates in the low resistance path which is the interface area between filaments. It is also evidenced that the intersection of perpendicular filaments in two adjacent layers blocks crack growth. The local strain evolutions at the surface of the specimen are compared to the macroscopic response of the material. The method developed herein allows the determination of the materials mechanical properties. The identification of the crack tip location using digital image correlation (DIC) and J-integral calculation lead to plot the J-R curve. The J-R curves comparison of the two specimen configurations shows that the fracture toughness is directly related to the material structure.     

Characterization and modeling of slow crack growth behaviors of defective high-density polyethylene pipes using stiff-constant K specimen

In this study, slow crack growth (SCG) resistances of defective and normal high density polyethylene (HDPE) pipes were measured using the stiff-constant K (SCK) specimen, where the stress intensity factor (SIF) was maintained at a constant value within a certain crack length range. A significantly reduced SCG resistance was observed in the defective pipe; a detailed procedure for evaluating SCG kinetics using the SCK specimen has been provided herein. The results of a fracture surface analysis indicate that the white window patterns, resulting from poor carbon black dispersion, are the main reason for poor SCG performance. In addition, a crack layer (CL) model was derived for the SCK specimen geometry and was compared with experimental results. It was observed that the crack and process zone growth resistance parameters were significantly lower in the case of the defected pipe than those in the case of the normal pipe.     

Experimental studies on dynamic fracture behavior of thin plates with parallel single edge cracks

Dynamic fracture behavior of polymer PMMA thin plates with three- and four-parallel edge cracks was studied by means of the method of caustics in combination with a high-speed Schardin camera. A series of dynamic caustic patterns surrounding the crack tip and fracture path of the specimen were recorded simultaneously by two types of focused images. Some dynamic fracture parameters such as the dynamic stress intensity factor, crack velocity and acceleration were determined. The evolution of dynamic stress intensity factors on the parallel edge cracks, due to the dynamic unloading effect, was analyzed from the viewpoint of the release of elastic strain energy.     

Direct fracture toughness determination of a ductile epoxy polymer from digital image correlation measurements on a single edge notched bending sample

This work presents a combined experimental and numerical study on the fracture toughness behaviour of a ductile epoxy resin system. Quasi-static fracture tests using single edge notched bending (SENB) specimens were conducted under room temperature conditions. In addition, the digital image correlation technique was employed to experimentally map the full-field displacements and strains around the notch and crack tip, allowing direct calculation of the J-integral fracture toughness. The magnitude of fracture toughness was found to be 1.52 ± 0.03 kJ/m², showing good consistency with the results measured according to the standard analytical formulations. A numerical model of the single edge notch bending specimen was built to compute the local strain field around the crack tip, together with the fracture toughness parameter. Good agreement was confirmed for both the experimental J-integral fracture toughness and the local surface strains around the crack-tip from the digital image correlation based optical technique, compared to the results obtained by numerical simulation. The fracture surfaces of the samples were examined using an optical microscope to analyze the failed surface morphology and the corresponding failure mechanisms.     

J-testing of polymers via the load separation criterion based ESIS TC4 procedure: Effect of the specimen size

The Technical Committee 4, “Polymers, Polymer Composites and Adhesives”, of the European Structural Integrity Society (ESIS TC4) developed a draft protocol based on the load separation criterion to determine two fracture parameters (an initiation parameter, J_I,lim, and a crack growth parameter, m_s) without the need to measure the crack growth (Δa). This is especially beneficial, since the measurement of Δa is prone to errors. The developed testing scheme displays promising results, as shown in a round-robin testing exercise. To further push this testing scheme, it is necessary to verify the specimen size scaling possibility. Hence, in this work, single edge notched in bending (SE(B)) specimens with different sizes, but geometrically similar, were manufactured. ESIS TC4 testing scheme was successfully applied to specimens with the different sizes, and data of J_I,lim and m_s were obtained. The observed effect of the specimen size on the aforementioned fracture parameters is presented and discussed.     

Investigation of the Slow Crack Growth Behavior of Static and Cyclic Loaded Specimens of Polyethylene by 2D and 3D Optical Fracture Surface Analysis

Summary: A fracture surface investigation was conducted to study the applicability of cracked round bar (CRB) specimens for an accelerated extrapolation concept for a lifetime assessment of polyethylene (PE) pipes. Scanning electron microscopy and topography metrology with InfiniteFocus were used to study the slow crack growth behavior in CRB specimens at different loading conditions. The results confirm the compliance of the CRB test with the requirements of linear elastic fracture mechanics.     

Griffith fracture of a phenol–formaldehyde polymer

Cast samples of a phenol–formaldehyde polymer with a crack of length defined by a metallic foil inclusion were fractured in tension. The stress at fracture was inversely proportional to the square root of the crack length, in agreement with the Griffith equation for brittle fracture. The behavior did not conform to the Griffith equation with respect to the experimental value of surface free energy, which was several orders of magnitude higher than a theoretically calculated value. However, as the temperature of tensile testing was raised, the experimental value did approach the calculated value. Consistently the appearance of the fracture surface was observed to change from one showing evidence of plastic deformation at room temperature to a featureless appearance, characteristic of brittle fracture, at higher temperatures.     

Crack growth in medical-grade silicone and polyurethane ether elastomers

One major problem with ball and socket artificial discs is the migration of wear particles to the surrounding tissues. This debris can cause inflammation that can lead to implant loosening. Encapsulating the artificial disc with an elastomer sheath could prevent this problem by retaining the wear particles within the disc. The encapsulation sheath will face millions of tensile cycles during the implant life and, therefore, it must have the ability to withstand large strains without fracture. Using cyclic displacement, crack nucleation was applied on dumbbell specimens and crack growth was applied on rectangular specimens with an initial crack. Both tests were performed on Silex silicone and polyurethane ether elastomer specimens, both with a Shore durometer hardness of 40 shore A. No samples completely failed during the crack nucleation tests after five million cycles. The polyurethane ether elastomer showed a slower rate of crack growth life (421 k cycles to reach 70 mm crack length) than silicone elastomer (221 k cycles to reach the same crack length) in the control group. Accelerated ageing decreased the hardness and the crack growth rate of the polyurethane elastomer but had the opposite effect for the silicone elastomer. Gamma sterilization increased the crack growth rate and did not affect the hardness of the polyurethane elastomer. The hardness and the crack growth rate of the silicone elastomer were increased after gamma sterilization.     

Impact tensile fracture testing of a brittle polymer

The fracture behavior of a brittle polymer, methylmethacrylate–butadiene–styrene resin, under impact tensile loading was studied using single-edge-cracked specimens. The dynamic load and displacement were measured with a Piezo sensor and a high-speed extensometer, respectively. The load and displacement diagram, i.e., the external work, U_ex, applied to the specimen was used to determine the elastic energy, E_e, and non-elastic energy, E_n, due to viscoelastic and plastic deformation, and the fracture energy, E_f, for creating new fracture surface, A_s. The energy-release rates were then estimated using G_t=U_ex/A_s and G_f=E_f/A_s. The values of G_t and G_f were correlated with the fracture loads and the mean crack velocities determined from the load and time relationships.     

连续碳纤维增强的聚芳醚酮复合材料的层间破坏

用双悬臂梁和端开口弯曲试件分别研究了连续碳纤维增强的聚芳醚酮复合材料(CF/PEK-C)的Ⅰ型和Ⅱ型的层间破坏。CF/PEK-C的Ⅰ型层破坏的线弹性断裂判据G_(Ⅰc)和弹塑性断裂判据J_(Ⅰc)分别为0.69KJ/m~2且与裂纹长度无关。CF/PEK-C的Ⅱ型层间破坏的稳定性,与裂纹和半距之比α/L有关。当α/L小于0.7时,表现为不稳定的Ⅱ型层间破坏的断裂韧性G_(Ⅱc)为1.62KJ/m~2。当α/L大于0.7时,则为稳定的Ⅱ型层间破坏。此时的G_(Ⅱc)与临界点的选择有关。由亚临界点和0.95点法得出的G_(Ⅱc)值分别为1.73和2.74KJ.M~2。     

Crack-craze opening profiles near a crack tip in a polytetrafluoroethylene

The crack opening and craze profiles near a crack tip in a polytetrafluoroethylene have been experimentally investigated. A double-edge-crack plate specimen under uniaxial tensile load was used in the experiment and the experimental procedure was performed using the Digital Image Correlation method, which is a well-established optical-numerical method for estimating full-field displacement. A theoretical model of the stress intensity factor based on linear elastic fracture mechanics combined with a classical saturated expression was proposed. The proposed model is in good agreement with experimental data and predictions of the model may be used to verify the non-linear behavior from crack and craze (cohesive) zones.