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It is possible to consider a solid polymer as either a uniform mechanical continuum which responds in a predictable way to stress and strain or.
Table of contents
- Fracture in polymers
- Fracture Behaviour of Polymers
- Deformation and Fracture Behaviour of Polymer Materials | cothemportnaru.tk
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- 1. Introduction
The determination of J-crack resistance R -curves is necessary for a complete toughness evaluation. The fluorescence dyeing technique showed to be an efficient, accurate and reliable method to enhance the measurement of stable crack growth portions on roughly structured fracture surfaces. Within the study a fractional factorial statistical design of experiments was used to prove the reliability and accuracy of the method. Fluorescence microscopy has its definite advantages compared to light microscopy or scanning electron microscopy SEM especially when it is applied by use of a 3D digital microscope with depth from defocus which was used for the study.
Finally additional information about the toughness behaviour of highly reinforced PA6 compounds with different amounts of elastomeric modifier could be revealed by means of the newly developed fluorescence adsorption-contrast method 3D-FAC method. The acoustic emission AE analysis is a structure-sensitive test method of plastic diagnostics, which enables the characterisation of the damage kinetics as well as damage mechanisms under specific conditions.
The AE analysis is linked to release of stored elastic energy, which propagates as spherical volume wave in the material. In this work, short-glass fibre-reinforced thermoplastic materials were examined in the quasi-static tensile test in the environmental scanning electron microscope ESEM with simultaneously recording of the AE as well as under impact-loading conditions in the instrumented Charpy impact test ICIT. Therefore, it was possible to couple the mechanical, the acoustic and the micromechanical results to describe the damage kinetic as well as the damage mechanisms.
Fracture in polymers
The coupling of the AE analysis with the ICIT allows the assessment of the damage kinetic and therefore, the determination of the damage initiation under impact-loading conditions. However, in dependence on various bonding conditions, different results could be found. For good bonding conditions, the damage initiation takes placed before the material behaviour changes from elastic to elastic—plastic behaviour.
This could be found for the short-glass fibre-reinforced high-density polyethylene materials. For the fibre-reinforced polybutene materials, the first AE takes place at the point of elastic—plastic material behaviour. An energetic approach of the damage initiation by the parameter J Si shows an independent behaviour from the polymer matrix as well as from the glass fibre content. For all investigated thermoplastics, a J Si -value of 0. Damage to plastic products often results in a fracture.
Therefore, the fracture surface examination fractography is particularly important when performing failure analysis. By application of standardized fracture surface structures, the location of crack initiation and the direction of crack growth can be determined. Furthermore, the influence of temperature, type of loading, loading speed or stress due to media on the plastic product can be detected.
Fracture Behaviour of Polymers
It is very important that fractography is applied in combination with other methods of polymer diagnostics and analytics as a tool for damage assessment. In particular at filled and reinforced polymeric materials, the applicability of fractography is limited because of the jagged fracture surface induced by fillers and reinforcement materials. Regarding lifetime of plastic products made of polyethylene PE , resistance against slow crack growth failure is crucial. Therefore, in recent years material suppliers took a lot of effort into the improvement of PE materials leading to highly stress cracking resistant grades e.
PE RC. This advantage in application unfortunately causes problems in material testing: As testing times become longer, highly resistant grades cannot be characterised quantitatively with standard test methods, like the Full-Notch Creep Test, anymore.
Therefore, improved respectively new testing methods for the characterisation of slow crack growth resistance of PE have been developed. The effect of crystallisation conditions, polymorphism and processing on the impact resistance of some nucleated pipe grades was studied.
A dependency between polymorphism, process conditions and final pipe properties is reported. A short-term method for measuring the slow crack growth was successfully developed and applied, and a crack has been observed via microscopy. It was observed that a large plastic zone is formed in the specimen before the crack initiation and also ahead of the crack tip.
Pipe failure in long-term applications is characterised by creep crack initiation and creep crack growth. Both can be determined by fracture mechanics-based tests of the pipe materials. The overall stable crack initiation and propagation behaviour of fracture mechanics specimens cut from plastic pipes that were composed of different polyolefin materials were investigated using concepts of elastic—plastic fracture mechanics including the crack propagation kinetics.
The effect of specimen shape, orientation, welding and lading rate on the crack resistance R behaviour of these materials has been thereby assessed. It was found in principle that specimen shape, orientation and welding have an influence indeed but only an unexpected small one on crack initiation behaviour and, particularly, on crack propagation behaviour.
Deformation and Fracture Behaviour of Polymer Materials | cothemportnaru.tk
The crack initiation toughness is not sensitive to the orientation in most cases. In contrast, the crack propagation toughness is significantly affected by the orientation where the values for crack propagation in extrusion direction are larger than ones for crack propagation crosswise to that.
This confirms that the morphology affects the stable crack propagation behaviour more than the stable crack initiation behaviour. In agreement with results of the microindentation test, fracture mechanics investigations also show that a lower welding pressure and a larger welding temperature, respectively, have no or a positive effect on the mechanical and fracture mechanics properties, whereas a larger pressure and a lower temperature, respectively, result in deterioration of the performance of the welded joint.
Furthermore, the R-curve behaviour was investigated using specimens cut from bilayer pipe segments. It has been shown that an additional layer has a clear impact on the R-curve behaviour compared to the crack propagation in single-layer pipes, which can be explained thereby that the plastic constraint was affected by this additional layer. For clarification of the toughness-in- or -decreasing effect of an additional layer with differing mechanical characteristics on the layer where the crack was growing, R-curve ratios were introduced, that showed that the asymmetric mechanical properties of different layers were directly reflected in an asymmetric impact on the stable crack initiation and propagation behaviour.
The main aim of the paper is to study the influence of both material inhomogeneity and weld bead geometry on crack propagation in welded polyolefin pipes. Lifetime of three pipes welded by different welding procedures is numerically estimated. Circumferential crack is of interest during the crack propagation through the pipe wall and the stress intensity factor is evaluated.
The change of weld bead notch radius is not proved to be important for slow crack growth. It is shown that non-optimal welds can significantly decrease lifetime of pipe systems. The results of this research can be used for lifetime estimation and prediction of creep crack growth and further optimisation of welding conditions and butt weld technology. The main reason for the flexibilisation of this epoxy sealing material is the necessity of the materials ability to compensate relative movements between the pipe segments.
Epoxy EP modified with ethylene—propylene—diene rubber EPDM powder, reactive liquid polymer ATBN—amine-terminated butadiene—acrylonitrile , and epoxidised modifiers as well as two customised epoxies were analysed. The formulations with epoxidised modifier and the customised proprietary epoxy showed a slope in this temperature range. Generally the glass transition temperature decreased and thus a change of the materials operating temperature range occurred.
Thermogravimetric analysis showed that all epoxy modifications absorb water after immersion. Due to the water absorption a plasticisation effect was observed. Further changes in the materials operating temperature ranges have to be considered.
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A comparison with neat epoxy shows for all formulations a reduction of the mechanical properties like tensile strength and modulus of elasticity. However, no enhancement of strain at break could be achieved by modifying with EPDM powder. The measured major strain was negligible. Results of the pressure and inclination test at the fibre concrete test rig show, that the tightness as well as the flexibility inclination of the repaired socket can be guaranteed up to a pressure of 1.
Finally, a pressure test in a first field application confirmed the suitability of the epoxy based material C-EP 1 for use as a robot processed sealing material. Modern elastomer and film testing involves the interaction of methods of mechanical and fracture-mechanical testing on the one hand and methods for the investigation of material composition analytics , the structural composition and the physical and chemical properties on the other hand.
A comprehensive description of the deformation and fracture behaviour of elastomers and films is only possible if a number of parameters are used. New test methods based on scientific findings are better suited for the evaluation and optimisation of the properties of these materials than technological methods that are based on empirical findings. One example shown here is supplementing the tear test with the conventional and instrumented notched tensile-impact impact tests, which provide further insights into the deformation and fracture behaviour.
The influence of the content of the peel component and the influence of the peel angle on the peel behaviour was investigated. In case of the adhesive peel system, an independence of the adhesive energy release rate on the peel angle was found. In case of the cohesive peel system, two zones of G aIc could be identified, 1 G aIc is independent of the peel angle and 2 G aIc depends linearly on the peel angle. This difference in G aIc could be strongly addressed to the type of crack propagation. In general the results revealed that fracture mechanics values like G Ic and G aIc can be used to get further and deeper structure-sensitive information of the investigated material system.
Polymeric stretch films are wrapped around a packaging unit to keep it bundled and safe for transportation and storage. To maintain the generated retention forces and prevent a peel-off of the film, adhesive interactions between the film layers are required. Resistance against initiation and stable peel front propagation across the interface of the cold-welded peel films are supposed to indicate detailed adhesive interactions.
Next to the energy release rate G IC that considers all kind of deformations during the peel process, the adhesive energy release rate G aIC was determined using the standard peel cling test for polymeric stretch films according to ASTM D The crack resistance curve concept was applied and crack initiation values could be determined next to stable crack propagation.
The results might indicate multiple adhesive interactions due to the surface morphology and viscoelastic behaviour of the investigated films. In accordance to the calculated crack initiation, a physical crack initiation could be observed.
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Epoxide systems, with diglycidyl ether of bisphenol A as the most popular representative, are known for their outstanding thermal stability. Nevertheless, lifetime predictions are indispensable for many applications. Therefore, the change of mechanical properties of an epoxide adhesive system was analysed.
The observed embrittlement of the system could be explained and shown by chemical degradation reactions. To determine the long-term stability accelerated testing methods had to be applied: Steel panels were bonded with a thin layer of the adhesive system and aged at elevated temperatures. Lifetime predictions were based on the adhesive strength, which can be evaluated by different methods like peeling or shearing tests.
The lifetime was strongly depending on the chosen parameter for the adhesive strength. However, if only the time at the high temperature was calculated, cyclic thermal loads came to the same results as constant high temperature regimes. The fatigue bahaviour of short-glass fibre-reinforced polypropylene was investigated in dependence on the fibre orientation. For that, a special plate geometry was developed which allows to exclude the effect of machining and leads to original injection surfaces at least in the region of the main slightly concentrated load. Furthermore, the plates consist of a notch region and a knit line.
Morphology analysis can be done by sectioning and stereological principles or by X-ray computed tomography. The fatigue strength shows a strong correlation to fibre orientation, but the samples taken from the knit line and the notch region do not follow this correlation.
In these regions the situations are more complex. The application of different methods for the characterization of the evolution of damage development of dissipated energy, strain, normalized modulus, temperature does not give a clear indication of damage; there are no clear signs of damage until shortly before the fracture. High-speed camera in combination with digital image correlation can give information on local strain and therefore on the localized occurrence of damage at a relatively early stage of fatigue life.
A new biaxial testing machine from COESFELD is presented, which enables essential new and more comprehensive opportunities for non-destructive and fracture mechanical testing of elastomers, mainly under dynamical load. Some possible applications are described in detail.