Brittleness and toughness of polymers and other materials

Extreme Mechanics Letters

Theoretical analyses and experiments have been carried out to investigate fracture and failure behavior of glassy polymers, aiming to obtain new insights into the extreme mechanics of plastics. Our birefringence measurements quantify the local stress buildup at cut tip during different stages of drawing of a precut specimen. Based on brittle polymethyl methacrylate (PMMA), ductile bisphenol A polycarbonate (PC) and polyethylene terephthalate (PET), we find several key results beyond the existing knowledge base. (1) The inherent fracture and yield strengths F(inh) and Y(inh) differ little in magnitude from the breaking and yield stress (b and y) respectively measured from uncut specimens. (2) Stress intensification (SI) near a pre-through-cut build up ceases because of finite tip sharpness. (3) The stress tip at cut tip shows a trend of approximate linear increase with the stress intensity factor KI = 0(a) 1/2 or far-field load 0 for all three polymers and different cut size a. (4) A characteristic length scale P emerges from the linear relation between tip and KI. For these glassy polymers, P is on the order of 0.1 mm, apparently determined by the tip bluntness that occurs during the precut making. (5) Fracture toughness of brittle polymers is characterized by critical stress intensity factor KIc = F(inh)(2P) 1/2 , revealing relevance of the two crucial quantities. (6) The critical energy release rate GIc for brittle glass polymers such as PMMA is determined by the product of its work of fracture wF (of uncut specimen) and P. (7) The elusive fractocohesive length Lfc defined in the literature as GIc/wF naturally arises from the new expression for GIc as stated in (6), i.e., it is essentially proportional to P. These results suggest that a great deal of future work is required to acquire additional understanding with regards to fracture and failure behaviors of plastics.

Polymer Bulletin, 2011

Brittleness is a significant property considered in product design and the research and development of materials. However, for a long time the methods to determine brittleness have been largely ''hand-waving'' arguments or else circumferential properties-in other words describing numerous properties related to brittleness but not actually quantifying brittleness itself. We have defined brittleness of polymeric materials quantitatively with applications to multiple areas. Relationships between brittleness and both tribology and mechanics have been discovered and are described. Moreover, the definition has been applied in the development of multilayer composite materials; structural integrity of the composites decreases with increasing brittleness. Other applications and the fact that toughness is not an inverse of brittleness are also discussed.

Chemistry & Chemical Technology

The property of brittleness for polymers and polymer-based materials (PBMs) is an important factor in determining the potential uses of a material. Brittleness of polymers may also impact the ease and modes of polymer processing, thereby affecting economy of production. Brittleness of PBMs can be correlated with certain other properties and features of polymers; to name a few, connections to free volume, impact strength, and scratch recovery have been explored. A common thread among all such properties is their relationship to chemical composition and morphology. Through a survey of existing literature on polymer brittleness specifically combined with relevant reports that connect additional materials and properties to that of brittleness, it is possible to identify chemical features of PBMs that are connected with observable brittle behavior. Relations so identified between chemical composition and structure of PBMs and brittleness are described herein, advancing knowledge and improving the capacity to design new and to choose among existing polymers in order to obtain materials with particular property profiles.

Materials Letters: X, 2019

The isobaric thermal expansivity is an underestimated property. The entire composite structures disintegrate into pieces on heating-without the use of any mechanical force-when the constituents have significantly different expansivities. While one typically tries to relate mechanical properties among themselves and thermodynamic ones among themselves too, we provide an equation relating the linear isobaric expansivity to the brittleness B for a number of polymers with a large variety of chemical structures and consequently with a large variety of properties.

Journal of Materials Science, 2010

Brittleness of materials-whether it occurs naturally or with aging-affects significantly performance and manifests itself in various properties. In the past, brittleness was defined qualitatively, but now a definition of brittleness for viscoelastic materials exists, enabling analysis of all types of polymer-based materials. The quantity brittleness, B, has been evaluated for neat thermoplastics, but here composites and metal alloys are also assessed. The physical significance of brittleness is connected to the dimensional stability of materials. The connections of brittleness to tensile elongation and to fatigue are explored while its relationship to surface propertiesspecifically wear by repetitive scratching-is examined more closely. The economic impact of wear results in monetary loss associated with failure and reduced service life of plastic parts-thus its connection to brittleness finds use across a broad spectrum of industrial applications which utilize plastics for manufacturing, processing, etc. We also demonstrate a correspondence between impact strength (Charpy or Izod) and brittleness of polymers. It is shown that the assumption hardness is equivalent to brittleness is inaccurate; this fact has important implications for interpreting the results of mechanical testing of viscoelastic materials.

The fracture behaviour of materials in the ductile-to-brittle region is neither completely brittle nor entirely ductile. Besides, scatter in toughness results has been reported in polypropylene and nylon. At the moment there is no general agreement on the methodology to determine the fracture toughness in the transition region. In this work an assessment of different proposed methods based on LEFM, EPFM and statistical approach was carried out over two materials: polypropylene homopolymer (PPH) and a blend of PPH containing 20 wt.% of elastomeric polyolefin (PPH/POes). The methods analysed were Fernando-Williams method, plastic zone corrected LEFM proposed by Gerin et al., G ST /G INST method by V-Khanh and De Charentay, JR curve method by Santarelli et al., and a statistical approach proposed by the authors in a previous work. The results of this analysis indicate that the Fernando-Williams and Plastic zone corrected LEFM methods, based on LEFM, tended to underestimate the fracture toughness, being very conservative. On the other side, JR method may overestimate the toughness, as in PPH/POes blend case. The G ST /G INST and Statistical methods appear to be the most adequate to characterise the fracture toughness of PPH and PPH/POes blend. The values of the characteristic fracture toughness found by both methods were slightly smaller than the minimum determined experimentally and proved very close between themselves.

2002

Toughness is an important mechanical proprerty and often the deciding factor in materials selection. The continuing growth in the use of plastics for engineering and other applications is due in no small measure to the development, during the past five decades, of new and tougher plastics materials. The problem facing the raw materials manufacturer is not simply to increase toughness. For many applications, the requirement is for a moderately priced polymer which can be moulded easily, and which exhibits adequate stiffness and toughness over a wide range of temperatures. Most of the major plastics manufacturers have devoted a significant part of their research and development effort to the search for materials with these characteristics. There are two basic solutions to this problem. One is to produce completely new polymers, based upon novel monomers, as in the case of polycarbonates and polysulphones. The second approach consists in modifying existing polymers through the addition of a second rubbery component. Rubber-toughened plastics constitute a commercially important class of polymers, which are characterised by a combination of fracture resistance and stiffness. The best known members of the class are toughened polystyrene, or HIPS, and ABS, but there are also toughened grades of polypropylene, PVC, epoxy resin, and a number of other polymers. The paper reports on the work carrried out by the Department of Polymer Engineering, UTM, in these areas. It will discuss the results of the recent investigations which have been conducted to enhance the toughness of three commercial polymers that is PP, PVC and PS.

Procedia Structural Integrity, 2017

Polymer, 2005

This paper presents a new test method that measures fracture toughness of polymeric materials when subjected to in-plane shear loading (mode II), and compares the toughness with that in tension mode (mode I). The new test method uses an Iosipescu device to apply the shear load, and determines the toughness based on the concept of essential work of fracture (EWF). Three physical-based criteria were used to verify the occurrence of mode II fracture. The new test method was then used to evaluate toughness of poly(acrylonitrile-butadiene-styrene) (ABS). The results suggest that for the ABS, the ratio of toughness in mode II to mode I is about 2.5 which leads to the dominance of mode I fracture in most loading conditions. The results also showed that for ABS in mode I fracture, the specific work of fracture (defined as the absorbed energy for fracture divided by the cross sectional area of the ligament between the notch tips) depends on ligament length; while in mode II fracture, it depends on ligament thickness. The study concludes that the new test method has a good potential for evaluation of mode II fracture toughness of polymers, though further study using polymers of different characteristics will be needed to confirm universality of the test method in the measurement of mode II fracture toughness.

Makromolekulare Chemie. Macromolecular Symposia, 1993

The deformation and toughness of amorphous glassy polymers is discussed in terms of both the molecular network structure and the microscopic structure at length scales of 50-300 nm. Two model systems were used: polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) blends (PS-PPE; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non-adhering core-shell-rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50-80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non-adhering rubbery particles ('holes'). The value of the critical matrix ligament thickness (ID,) uniquely depends on the molecular structure: at an increasing network density, ID, increases independent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain-rate and temperature dependency.