Single Edge Notch Tensile Research Articles

Constraint correction of high strength steel: Selection of test specimens and application of direct calculations

This study presents a practical procedure for application of constraint correction of high strength steels and its weldments. Tests have been carried out on single edge notch bend (SENB) specimens with shallow and deep notch and SENT (single edge notch tensile) specimen with shallow notch, to cover the constraint effect. The yield strength of the material was 700 MPa. In order to apply the results on real structures it is crucial that the constraint correction procedures are made available to designers. The procedures must be easy to perform, have high accuracy and low costs. In order to meet these requirements the concept of linespring has been applied for performing direct calculations of components.

Polycarbonate and co-continuous polycarbonate/ABS blends: influence of notch radius

The influence of notch tip radius in the range of 1–0.002 mm was studied on polycarbonate (PC) and co-continuous PC/acrylonitrile–butadiene–styrene (ABS). Co-continuous PC/ABS blend was obtained by mixing PC and ABS containing 15% polybutadiene (PB) in a twin screw extruder. PC and PC/ABS specimens were injection moulded into test bars. A notch was milled-in, with notch tip radius of 1, 0.5, 0.25 and 0.1 mm. Very sharp notches with a radius of 0.015–0.002 mm were obtained with an Excimer LAZER. The specimens were tested by single edge notch tensile tests at 1 m/s (apparent strain rate 28.5 s −1) and at different temperatures (−60 to 130 °C). Initiation and propagation phases of the fracture process were monitored and the brittle–ductile transition temperature ( T bd) determined. It appeared that the amount of deformation in the initiation phase of fracture was extremely sensitive to notch tip radius. Temperature measurements of the deformation zone showed that the size of the deformation zone decreased with decreasing notch radius. The T bd of PC increased rapidly with decreasing notch radius, until the glass transition temperature was approached. Remarkably, for PC the notch sensitivity was strongest around the standard notch tip radius of 0.25 mm. This means that a small deviation of this standard notch leads to large deviations in the results. The PC/ABS blend was much less sensitive to notch tip radius and the T bd was almost constant. Thus the sensitivity of PC to sharp defects can be neutralised by adding ABS.

Polycarbonate and co-continuous polycarbonate/ABS blends: influence of specimen thickness

The influence of specimen thickness on the fracture behaviour of polycarbonate (PC) and co-continuous PC/ABS (50/50) blends was studied in single edge notch tensile tests at 1m/s and different temperatures (−80 to 130°C). Specimen thickness ranged from 0.1 to 8mm. In the co-continuous PC/ABS blends the rubber concentration in the ABS was 0, 15 and 30wt%. The change in fracture toughness was typified by the change in brittle-to-ductile transition temperature (Tbd).Tbd of pure PC depended strongly on specimen thickness, leading to very low transition temperatures for thin PC specimens. PC/ABS 0%, a 50/50 blend of PC and SAN (i.e. ABS without polybutadiene (PB)), was a brittle blend and showed a very high Tbd close to the Tg of SAN. Tbd did not seem to be influenced by specimen thickness. PC/ABS blends with 15 and 30% PB in ABS showed improved Tbd compared to PC/SAN and PC, indicating effective rubber toughening. Tbd decreased with decreasing thickness for PC/ABS specimens thicker than 1.5mm. However, Tbd increased with decreasing thickness for specimens below 1.5mm thickness. In thin specimens, the rubber-filled blend is less effective rubber toughening. The plane strain stress condition needed for rubber cavitation is apparently not present in thin specimens.

Strain rate effects in polycarbonate and polycarbonate/ABS blends

Acrylonitrile–butadiene–styrene was blended in polycarbonate using a twin-screw extruder, producing dispersed and co-continuous blends. The blends were injection moulded into test bars. These materials were tested in single edge notch tensile tests as a function of test speed over a test speed range of 10 −4–10 m s −1 (2.85×10 −3–2.85×10 2 s −1). Infrared measurements were made to monitor the temperature development in the material during fracture. The fracture energies were higher at high test speeds than could be expected based on the low speeds results. This effect was due to an increased fracture stress, initiation strain, and propagation strain. With infrared measurements a temperature increase of 30–40 °C was observed. Also the size of the deformation zone increased with test speed. A possible explanation is a thermal blunting resulting from adiabatic heating at high speeds. The brittle–ductile transition temperature increased linearly with the logarithm of the strain rate by a factor of 18.6. At high test speeds the brittle–ductile transition developed with the formation of shear lips.

INVESTIGATION OF STATIC AND DYNAMIC FRACTURE TOUGHNESS OF SHORT CERAMIC FIBER REINFORCED POLYPROPYLENE COMPOSITES

Short ceramic fiber reinforced polypropylene composites have been investigated to determine their static and dynamic fracture toughness for different reinforcing fiber contents. The composites were reinforced with fibers produced by a carding technique combined with needle-punching. Static fracture toughness (K c) was measured on single-edge notched tensile (SEN-T) specimens, while dynamic fracture toughness (K d) was tested by impact strength Charpy specimens. Specimens in both cases were cut transverse (T) and in longitudinal (L) directions. Test results show that dynamic fracture toughness is larger than the static one. During loading of SEN-T specimens the burst-type acoustic emission (AE) signals were monitored. From AE signals it can be concluded that the main damage form is the pull-out in the T specimens, and debonding in L ones. These results were supported by scanning electron microscopy micrographs taken from fracture surfaces.

Co-continuous polycarbonate/ABS blends

Co-continuous PC/ABS (50/50) blends were studied with a variable polybutadiene (PB) content (0–40%) in ABS. Polycarbonate (PC), styrene-acrylonitrile (SAN) and PB were blended in two steps using a twin screw extruder. Rectangular bars were injection moulded and notched Izod impact tested at different temperatures and in single edge notch tensile tests at 1m/s and different temperatures. Co-continuous PC/ABS gave a brittle-to-ductile transition temperature lower than expected based on notched Izod results for dispersed ABS in PC. The brittle-to-ductile transition temperature, in the co-continuous PC/ABS blends, decreased with increasing rubber content in SAN. The fracture energies showed an optimum at 15% PB in SAN while at the same time a delamination was seen on the ductile fracture surface, due to failure of the PC/SAN interface. Delamination disappeared when the rubber content in SAN or the temperature was increased. Specimens containing a welding were injection moulded to study the influence of rubber and AN content in the SAN on the interface. Weldline strength of the blends was very poor compared to PC, but improved with increasing rubber content in SAN.

Finite volume analysis of dynamic fracture phenomena. I. A node release methodology

A novel node release numerical methodology based on the Finite Volume discretisation (Ivankovic et al., 1994) has been developed and applied for the simulation of (non-linear) elastic fractures where crack propagation velocities are not a priori known. The analysis is performed in two-dimensions by means of an unconditionally stable implicit time-marching scheme and a second order accurate conservative spatial differencing scheme. Application to single crack propagation problems has demonstrated the validity and accuracy of the proposed formulation for dynamic crack propagation. The results obtained by two examples of crack propagation in single edge notch tensile (SENT) and double cantilever beam (DCB) specimens show good agreement with selected numerical, experimental and analytical results. Based on these results, the formulation appears to be well suited to the study of rapid crack propagation (RCP) phenomena in brittle materials. This, combined with the simplicity and accuracy of the formulation, may provide a solid foundation for more physically realistic RCP computational methodologies other than that of the single linear elastic crack representation considered here.

An acoustic emission study of the temperature-dependent fracture behavior of polypropylene composites reinforced by continuous and discontinuous fiber mats

The temperature-dependent fracture toughness, K Q, and acoustic emission, AE, behavior of glass-mat-reinforced thermoplastic polypropylene (GMT-PP) composites with continuous (GM-C) and discontinuous (GM-D) fiber mats were studied at three different temperatures: −40°C, room temperature (RT) and +90°C. Apart from the mat type, the matrix viscosity and the fiber/matrix adhesion were also varied in the GMT-PP samples. K Q was determined on statically loaded single-edge notched tensile (SEN-T) specimens. During loading of the SEN-T specimens, the burst-type AE signals were monitored. It was established that the mat type and the testing temperature strongly affect the fracture-toughness response. The K Q values of the GMT-PPs with GM-C reinforcements were superior to those with GM-D. Matrix and interfacial variables marginally affected the K Q values, however. Making use of mathematical statistics, correlations were found between K Q data and selected AE signal parameters.

Effects of ?-? transformation on the static and dynamic tensile behavior of isotactic polypropylene

It was demonstrated that the mechanical stress-induced βα-transformation in isotactic polypropylene (iPP) is associated with considerable toughness enhancement. This toughness improvement depends on the test conditions (loading frequency). The toughness of β-iPP was superior to the α-iPP by 13% under static (characterized by a load frequency of ca. 5 × 10−4 Hz) and 70% under dynamic (tensile impact with a loading frequency in the range of ca. 3 × 102…103 Hz) conditions, respectively. By applying the essential work of fracture (EWF) concept to single-edge notched tensile (SEN-T) specimens it was shown that for the toughness upgrading observed, energy dissipation in the enlarged plastic zone is responsible. The occurrence of the βα-transformation was evidenced by differential scanning calorimetry (DSC). Based on DSC measurements it was found that the degree of βα-trans-formation depends on the local strain. At high strain values the βα-conversion is complete (at elogation at break in uniaxial static tensile test), while this transformation is only partial at lower strains (at tensile impact). In addition, in the plastic (or deformation) zone the βα-conversion changed locally, and can be used for mapping of this region. © 1996 John Wiley & Sons, Inc.