Deformation and Fracture Behaviour of Polymers

Non-destructive inspection of a component means that one has to look for deviations from structural integrity [1]. So there is a need to know what the structure should look like if it were good — and what the real situation is. This comparison can be difficult for complex structures because small deviations may not be obvious. So a method is required which responds only to the characteristic properties of a defect, while it ignores the potentially complicated ‘good’ structure. In this way the non-relevant information is suppressed, while all attention can be focused on the defect.

On the one hand, the polymer industry is characterized by an increasing specialization and variety of products. On the other hand, there is also an increasing range of applications for plastics. Chemical and physical modification of the basic polymer materials, often as a result of customers’ demands, has led to a confusing variety of materials on offer. The choice of a specific polymer is often influenced by an insignificant change in the material properties. In addition to such small changes, the cost is decisive in practice.

The damage behaviour of composites is of interest for the purpose of development and application of materials. The strength and lifetime of composite materials are determined by the stress stages of the initiation of micro-failure processes and by the transition of the damage from a subcritical stage to a critical stage. Inter-fibre fracture, delamination and fibre failure are processes of special importance. The damage behaviour of composite materials depends considerably on the crack resistance of the matrix, on the debonding energy of the fibre—matrix interface and, finally, on the delamination resistance of the layers of the composite.

Composite materials, one of the most rapidly growing classes of materials, are being used increasingly for technological applications, especially when the tribological behaviour of the material is of interest. The properties of polymer-based materials can be varied and optimized over a wide range with regard to the requirements of the particular application. Some tribological applications, e.g. sliding elements and sliding bearings, require low friction and wear, whereas for other applications, e.g. in clutches and brakes, high friction combined with low wear is necessary (Table Dl). Tribological systems of polymer-based composites against steel are of high interest for technological purposes. Yet much of the present knowledge on the tribological behaviour of polymer-based composite materials, especially high-temperature-resistant (>150 ºC) thermoplastics, is empirical, and very limited predictive capability currently exists. In particular, experimental results of friction and wear tests under conditions close to those occurring practice and basic knowledge of the friction and wear mechanisms in the tribological interphase of polymer materials are needed.

Heterophasic polymer composites are often used for brake linings. The nature and quantities of the components of the composite have a significant influence on its properties, including, in particular, the tribological properties. Asbestos, which has traditionally been used as an additive to frictional materials, had to be removed from these composites because of its negative influence on human health. As a result, an urgent need arose to develop a new material with properties similar to or even better than those of asbestos. In particular, the new material must ensure an appropriately high level of the friction coefficient and its stability as the temperature rises during brake operation. At the same time, it is necessary to maintain the mechanical characteristics of the brake lining material, particularly its crack resistance. The resistance of the material to microcrack formation and propagation has a decisive influence on the life of facings and thus also on the safety and economy of use of a device.

Some components of rotating machines, for example axial-compressor blades, are manufactured from composite materials instead of steel to reduce the weight, with the aim of increasing the throughput (larger blades or a higher revolution rate). Consideration of complex aspects of the transfer of forces into the composite and the mechanical vibration behaviour is necessary before substituting steel by a high-performance composite [1].

The artificial replacement of joints opened a new era in the treatment of degenerative diseases. It became possible to relieve patients from pain and to give them back nearly normal mobility. Today more than 500 000 prostheses are implanted in the hip joint alone, all over the world every year. With the development of joint replacement during the past 40 years, much research has been done to prolong the survival of joint prostheses and to make them reliable for the rest of the patient’s life. Polymer materials have continued this development; they have become an important constituent part of most prosthesis systems and have prolonged their survival to a range of 12–15 years today. On the other hand, polymers seem to contribute to an aseptic loosening of prostheses after this period, so that they need to be improved further.

Voice prostheses and tracheal prostheses are functional prostheses that are implanted to copy a natural function. Voice prostheses are implanted in patients who have had their larynx removed for medical reasons. With a voice prosthesis, fast voice rehabilitation is possible. The prosthesis generates the voice only indirectly. It makes possible an airflow that can cause vibrations of the entrance musculature and mucous membrane of the oesophagus. It is a pharyngo-tracheal shunt valve. Because of the overpressure in the trachea, air streams into the oesophagus or the low pharynx area. On the other hand, the passage of saliva and undigested food into the trachea from the oesophagus is prevented because of the valve effect. Perfect valve function is necessary for correct functioning: dysfunction necessitates a prosthesis change. There is a range of products on the market that differ mainly as far as the valve is concerned. The material of choice is silicone rubber. Polyurethane has also been tested as a material. Details of clinical use are given in [1–3].

Singer and Blom published in 1980 their technique of the tracheo-oesophageal puncture (T-E puncture), including the application of the first commercially manufactured valve prosthesis. With this method, they started an ever-broadening interest in the voice rehabilitation of patients without a larynx. In the last 20 years different variations of this prosthesis have been developed, which have been tested successfully with several thousand patients [1,2].

Polymer materials are of importance as substitute materials in medicine because their individual properties are specifically adjustable. Their compatibility with the biological system (biocompatibility) and the functionality and stability of the materials used are decisive factors for medical practice, especially for the purposes of implants. The materials commonly used in medicine include PET (textile vascular prostheses), PTFE, PUR (vascular replacements, blood pumps, cardiac valves), polyolefines such as PP or PE (threads, joint replacement), silicone elastomers (prostheses in the ear, nose and throat field, and special orthopaedic items) and polyamide. Pharyngo-tracheal valve prostheses, also referred to as voice prostheses, are used for the rehabilitation of patients who have had their larynx completely removed.

In their numerous areas of application, elastomer materials are subject to a number of complex wear factors. A typical example of this phenomenon is the abrasion of the tread of a tyre or the pattern of chipping and chunking damage observed under certain circumstances on truck tyre treads [1].

Elastomer materials are used in tyres, conveyor belts, engine bearings and isolation materials, for example. Definite requirements for the material properties depending on application field exist. For tyres, in addition to safety, economic and ecological aspects, wet-skid resistance, rolling resistance and wear resistance are important properties. A predictable lifetime of products is necessary; this is, however, restricted by an inadequate control of wear phenomena.

Polymer concrete is a relatively young material that has been used in the building industry since the 1950s (known then as reactive resin concrete). Since the beginning of the 1980s, polymer concrete has been applied in mechanical engineering. Here, machine bases and components of fast-running machines have been made increasingly of polymer concrete because of its high damping properties for sound and mechanical vibrations and because it is simple to make complicated shapes with it.

Epoxy resins are used in a wide range of applications. At room temperature crosslinked epoxies typically exhibit a high modulus and a nearly elastic stress—strain behaviour, but they have poor resistance to fracture. Complex material behaviour can often be observed owing to different mechanical and thermal properties (for example, in a sandwich construction), which can impair the overall mechanical—thermal reliability. In these systems, local defects such as cracks can appear as a result of inhomogeneities of the composite material, internal stresses and high thermal and mechanical loading gradients [1–3]. Consequently, the methods of fracture mechanics have an increasing significance regarding the assessment of the failure behaviour. The aim is an optimization of the toughness of epoxies under extreme conditions. Concerning the dimensioning, geometry-independent parameters have to be used in order to guarantee optimum reliability and operational safety in industrial applications (pp. 3–26). The usual compact tension specimens require a large amount of material and are often unsuitable for experimental investigations. In the future, the application of miniature specimens for the determination of fracture mechanics parameters should be made possible so that an efficient toughness evaluation of newly developed materials can be performed [4].

The mechanical behaviour of non-linear viscoelastic materials depends on the time, the temperature, the loading rate and the magnitude of the load. The aim of the work described in this chapter was an investigation of the effect of a biaxial state of stress on the characteristic functions, such as the secant modulus versus strain. A model is presented that allows the simulation of non-linear viscoelastic materials under a multi-dimensional state of stress.Therefore, a suitable test specimen and testing machine were developed. Because the specimen is produced by injection moulding, both isotropic and anisotropic specimens can be produced. The testing machine enables strain- and force controlled experiments. Uniaxial quasi-static strain-controlled experiments on test specimens made from polyamide 6 (PA 6) and from a styrene-butadiene-styrene (SBS) block copolymer demonstrate the effect of the process parameters on the mechanical anisotropy. As far as the isotropic PA 6 is concerned, approaches based on energy absorption and equivalent stresses show the possible ways to transform the secant modulus from a uniaxial to a biaxial state of stress in the case of quasi-static loads. Dynamic experiments demonstrate that the load history must be taken into account when considering multi-dimensional states of stress. The approach mentioned above cannot achieve this. Therefore, a three-dimensional model has been developed to simulate multi-dimensional load histories. This three-dimensional deformation model is based upon the uniaxial deformation model and is constructed from a parallel arrangement of a certain number of basic elements. Each basic element consists of an elastic Hookean element and a damper system (3D damper) to describe the viscous properties. The arrangement is calibrated by means of isothermal strain-controlled tensile tests at different temperatures. Along with suitable calculation algorithms (relaxation algorithm and retardation algorithm), this model offers the ability to simulate any multi-dimensional load history caused by direct stresses. The model allows the simulation of both quasi-static and dynamic loading at different temperatures. By comparing the experimentally determined static and dynamic stress-strain curves with simulated curves, the suitability of the model developed here is demonstrated. Relaxation effects are overpredicted by the mode L in comparison with experimental results. Better calibration should lead to an improved correspondence.The aim of further investigations should be the expansion of this model to thermal stresses and, later, implementation in finite-element programs.

The increasing requirements for lightweight construction are leading to a wider acceptance of alternative materials for purposes where metals have traditionally been used. As a result of new technologies, these materials are being used for the substitution of conventional materials in components and structures. Polymer materials are fording increasing application in components subject to thermal and detergent loading. At present, thermoplastics based on mass-produced polymers are favoured because of their well-known advantages of properties, processing and price. The reservations about the use of these materials are due to a lack of knowl-edge about the parameters relevant to structural use and the lack of reliable morphology—property correlations.

The adaptation of material characteristics to specific applications by using different fillers and reinforcements and/or using composite materials is an international trend in the development of the properties of polymers. A necessary condition for specific requirements on the strength and toughness level of polymers to be satisfied is a knowledge of the deformation and fracture mechanisms [1,2].

Glass-fibre-reinforced composites based on thermosets show good mechanical properties, such as high specific strength and stiffness, combined with an excellent chemical resistance. An increasing use of such materials in a wide range of engineering applications and structures, especially as lining materials for containers and pipelines, can be observed. In recent years, many research programmes have been studied the mechanical properties of composites. However, independently of these investigations, reservations about the application of composite materials in the chemical industry exist because exact criteria for lifetime, chemical resistance and moisture ageing are not or are rarely available. Additionally, the evaluation of the damage state of a composite, as well as of the functionality, reliability and impact behaviour, is complicated. A successful failure analysis of structures made from composites depends strongly on suitable non-destructive test and inspection methods.

Among thermoplastic polymers, polypropylene (PP) is one of the most rapidly developing product families owing to its property profile, the range of possible modifications and its excellent environmental record [1]. Associated with this development are a quantitative growth well above the average and a continuous expansion of its application areas. Technological applications demand tailor-made PP systems with an optimized adjustment of polymer design, processability and properties in the final application. Here, it is often assumed that the properties of a moulded component remain constant for its whole lifetime — even if the producers and processors of PP are well aware that the properties of polymers change continuously over a very long period of time and even if the producers and processors take account of this fact, e.g. by fixing the time between processing and standardized testing.