S. Hashemi

Plane-stress essential work of ductile fracture for polycarbonate

The effect of specimen geometry, specimen size and the specimen orientation on the essential work of fracture for polycarbonate is investigated. Two different test geometries, namely the single-edge notched tension and double-edge notched tension specimens, are used to evaluate the essential work of fracture for crack propagation. It is shown that the specific essential work of fracture for crack propagation,we′ is independent of the test piece geometries and the size of the test piece. It seems that for a given sheet thickness,we is a fundamental material property being independent of the specimen geometry and size. The value ofwe does change with the orientation of the initial notch with respect to the melt flow direction. The straight-line relationships between the total specific work of fracture,wf, and ligament length,L, breaks down when the ligament length to specimen thickness ratio is less than about three, because the fracture data fall in the plane stress-plane strain transition region. A plane strain specific essential work of fracture,wle|′, was obtained by extrapolating the best regression line of the data to a zero ligament. For the initial notch in the melt flow direction, values forwe andwle′, were approximately 28 and 3 kJ m−2, respectively. The specific essential work of initiation,wle′ was about 4.3 kJ m−2 ·JR curves (J-Δa curves) were also obtained and it is shown that the intercept and the slope of theJr curve, i.e.JC and dJ/da, are related towe and the slope of thewf versus ligament plot.

Fracture toughness evaluation of ductile polymeric films

The energy for complete fracture in double edge-notched tension test specimens has been measured for a wide range of polymer films. Results indicated that the variation of the total specific work of fracture, wT, with ligament length, L, can be described by two straight lines, both of the form wT = we + β wpL, thus giving upper and lower intercept values at zero ligament length (i.e. we) for each film. The first term, we, is the energy absorbed per unit area of fracture, whereas the second term, wp, is the energy absorbed per unit volume of plastic deformation remote from the fracture surface. The lower we value was obtained from the extrapolation of the data within the mixed mode region (plane-stress/plane-strain) where the maximum net-section stress exceeded 1.15 times that of the tensile yield stress, σy, of the material, and the upper value was ascertained by extrapolating the data within the plane stress region where the net-section stress was 1.15 σy. It appears that the transition from plane stress to plane strain mode of fracture in thin films occurs at a ligament length much greater than 5B, where B is the specimen thickness. Moreover, it was found that the linearity of the data within the plane-stress region was not affected when ligament length values exceeded the plastic zone size. Moreover, variation of the extension to break with ligament length, for both pure plane stress and the mixed mode regions, was also linear; and the extrapolation values at zero ligament length were identified as crack opening displacements. Essential work estimated from the crack opening displacement agreed reasonably well with the extrapolated values.

Factors affecting work of fracture of uPVC film

Essential work of fracture (EWF) approach was used to evaluate the fracture toughness of uPVC film. It was found that the specific essential work of fracture, (we) is independent of specimen width, specimen gauge length, loading rate and test temperature, but dependent on the geometry of the test specimens. Test temperature and geometry were the only testing parameters affecting the specific non-essential work of fracture (βwp) in a very significant way. The plastic zone shape factor (β) was found to be very sensitive to both the geometry and temperature. It was established that both we and βwp could be partitioned into components that are linked to yielding (i.e. we,y and βywp,y) and necking/tearing (i.e. we,nt and βntwp,nt) processes. The only testing parameter that affected we,y was test temperature, whereas we,nt was affected by test temperature as well as geometry. All testing parameters used in this study affected the values of βywp,y and βntwp,nt.

Plane-stress fracture of polycarbonate films

The use of the specific essential work of fracture, We, to characterize fracture of polycarbonate films is described. It is shown that the plane-stress specific essential work of fracture for polycarbonate film can be obtained from single-edge-notched-tension specimens, by extrapolating the straight-line relationship between the total work of fracture, Wf, and ligament length, L, to zero ligament length. From the data, it seems that, for a given film thickness, We is almost independent of the specimen width but increases with increasing thickness. The non-essential work of fracture as obtained from the slope of a Wf versus L plot showed no significant width dependence, and for the majority of thicknesses it was almost invariant with thickness, indicating that the shape of the outer plastic zone surrounding the fracture process zone is almost invariant with the dimensions of the test specimen.

Effect of temperature on fracture properties of an amorphous poly(ethylene terephthalate) (PET) film

Fracture behaviour of an amorphous polyethylene terephthalate (PET) film with a glass transition temperature (Tg) of 72°C and a thickness of 0.21 mm was studied between 23 and 70°C using Double Edge Notched Tension (DENT) specimens. Within this temperature range, DENT specimens fractured by ductile tearing of the ligament region after ligament region had been fully yielded. The load-displacement curves obtained for different ligament lengths were geometrically similar to one another. On the basis of these, Essential Work of Fracture (EWF) methodology was used to determine fracture toughness of the PET film as a function of temperature. A linear relationship was obtained between the total specific work of fracture, wf, and ligament length, L, at temperatures under consideration. Results showed that specific essential work of fracture, we, is independent of temperature but the specific non-essential work of fracture (β wp) increases with increasing temperature and drops in value near the glass-transition temperature. A linear relationship was also found for yielding (wy) and necking/tearing (wnt) components of wf as a function of ligament length. The specific essential work components were found to be temperature dependent and whilst component wey decreased component went increased with increasing temperature. The contribution of went to we was substantially greater than that of wey at all temperatures.

Mechanical property relationships in glass-filled polyoxymethylene

The dependence of the mechanical properties such as strength, modulus and fracture toughness on the volume fraction of the reinforcing glass fibres and glass beads in polyoxymethylene (POM) matrix was studied. The majority of the measured quantities in tension or flexure tests, seemed to be linear functions of either the volume fraction of the glass fibres or the glass beads. The relationship between some individual mechanical properties seemed to be linear as well. Consequently, many of the mechanical properties of these POM composites (POM/GF and POM/GB) could be estimated from one measured property using the relationships presented. Also, the same property measured for the two composite systems was found to be linearly related. Consequently, the mechanical properties of one composite system, (i.e. POM/GB) could be used to determine that of the other system (i.e. POM/GF) at the same filler concentration.

The essential work of plane-stress ductile fracture of poly(ether-ether ketone) thermoplastic

The total work of fracture in a ductile material is not a material constant and the linear elastic fracture mechanics is not appropriate. Only the work performed per unit area in the fracture process zone, called the specific essential work of fracture, is a material constant for a given specimen thickness. The results of an experimental investigation on the essential work of fracture of a crystalline and a non-crystalline poly(ether-ether ketone) (PEEK) films are reported. Single-edge notched specimens were used to determine the specific essential work by extrapolating the straight-line relationship between the specific work of fracture and ligament length to zero ligament length. In this way, the specific essential work of fracture for the crystalline PEEK film of thickness 0.1 mm was measured to be 65.02 kJ m−2 and for the non-crystalline film of thickness 0.25 mm was measured as 62.71 kJ m−2. Advancing crack opening displacements (COD) have also been analysed and the specific essential work of fracture calculated from the COD values compared extremely well with those results obtained from the intercept of specific work of fracture versus ligament length.

Influence of short glass fibres and weldlines on the mechanical properties of injection-moulded acrylonitrile–styrene–acrylate copolymer

The present study investigated the dependence of various mechanical and fracture properties on the volume fraction, Φf, of reinforcing glass fibres in acrylonitrile–styrene–acrylate (ASA) copolymer. The addition of glass fibres enhanced the ultimate strength and modulus as measured in both tension and flexure but reduced the total work of fracture. The elastic modulus was not affected by the loading mode. The ultimate strength in flexure was found to be always greater than in tension by a factor of about 1.3. Both properties were found to be a linear function of Φf following the rule of mixtures:Pc=λPfΦf+Pm(1−Φf)where Pc is the measured property for the composite, Pf and Pm are the corresponding values for the fibre and the matrix, respectively, and λ is the overall efficiency of the reinforcing fibres. Addition of glass fibres to ASA polymer reduced both the notched and the unnotched impact strengths. Linear elastic fracture mechanics were used to determine values of the fracture toughness and the strain energy release rate. The fracture toughness did not change significantly with Φf, whereas the strain energy release rate decreased with increasing Φf. The presence of weldlines in the specimens had an adverse effect on all tensile properties except for the elastic modulus. The weldline integrity parameter for the modulus was between 1 and 0.95, and for strength it was between 0.87 and 0.20, decreasing linearly with increasing Φf.

Influence of short-fibre reinforcement on the mechanical and fracture behaviour of polycarbonate/Acylonitrile Butadiene Styrene polymer blend

The present study investigated the dependence of various mechanical and fracture properties on the volume fraction, φf, of the reinforcing glass fibres in Polycarbonate/Acylonitrile Butadiene Styrene (ABS) blends. The addition of glass fibres enhanced the ultimate tensile strength and modulus and reduced elongation (both to yield and to break) and total work of fracture. The elastic modulus was not significantly affected by the loading mode although the ultimate strength was significantly affected, giving flexural strength values of 1.5–1.6 times greater than tensile strengths. The elastic modulus and strength were linear functions of φf and thus followed the principle of rule of mixtures. The presence of weldlines in specimens had an adverse effect on most tensile properties except for the elastic modulus. Linear elastic fracture mechanics could not be used to assess the resistance to crack propagation of the present range of materials because their behaviour violated the principle assumptions upon which the theory is based. An alternative method was employed where the total work of fracture and the work of fracture corresponding to the maximum load were plotted as a function of initial crack length. These plots were reasonably linear for the polymer and its composites giving values of the resistance to steady state crack propagation JT and the J integral of maximum load Jm respectively. Values of JT and Jm decreased with increasing φf.

Mechanical properties of injection moulded styrene maleic anhydride (SMA) Part II Influence of short glass fibres and weldlines

The dependence of the various mechanical and fracture properties on the volume fraction ofshort glass fibres in the styrene maleic anhydride (SMA) polymer was investigated. Special attention has been given to describing the dependence of various mechanical properties on the volume fraction of the glass fibres, ϕf by way of the rule of mixtures. It was found that, strength, elastic modulus and fracture toughness, all follow a simple rule-of-mixtures of the form Qc=λQfϕf+Qm(1−ϕf), where Qc is the measured quantity for the composite, Qm and Qf are the corresponding values for the matrix and the fibre, respectively, and λ is the overall efficiency of the fibres, taking into account the orientation and the length of the fibres in the composite. It was also found that, while the presence of the weldline had no significant effect upon elastic modulus, its presence significantly reduced tensile strength and the fracture toughness of SMA and its composites.