Residual toughness of poly(acrylonitrile-butadiene-styrene) (ABS) after fatigue loading—effect of uniaxial fatigue loading

Toughness variation of non-notched poly(acrylonitrile-butadiene-styrene) (ABS) subjected to uniaxial fatigue loading was investigated. The experiments were conducted by applying fatigue loading to strip specimens first, from which dog-bone specimens were machined. The dog-bone specimens were tested to measure the strain for the on-set of fracture, named cracking strain here, thus to monitor the toughness change due to the fatigue loading.The test results showed that the fatigue loading caused the toughness drop in ABS, even before any visible crack was developed. Damage development and fracture behavior were then analyzed using scanning electron microscopy (SEM). The SEM analysis revealed that damage zones, not cracks, were initiated during the fatigue loading, and were the main cause of the toughness drop. Mechanisms for the damage initiation include matrix crazing and debonding of small rubber particles; however, large rubber particles remained intact. Based on the results, a deformation model is proposed for the damage zone initiation, which provides an explanation for the toughness change under the fatigue loading.     

Application of essential work of fracture concept to toughness characterization of high‐density polyethylene

Deformation and fracture toughness of high‐density polyethylene (HDPE) in plane‐stress tension was studied using the concept of essential work of fracture (EWF). Strain range for necking was determined from uniaxial tensile test, and was used to explain the deformation transition for 2‐staged crack growth in double‐edge‐notched tensile test. Through work‐partitioning, EWF values for HDPE were determined for each stage of the crack growth. Appropriateness of these EWF values to represent the material toughness is discussed. The study concludes that the EWF values for ductile polymers like HDPE may not be constant, but vary with the deformation behaviour involved in the crack growth process. POLYM. ENG. SCI., 47:1327–1337, 2007. © 2007 Society of Plastics Engineers     

Modeling and control for a Gough‐Stewart platform CNC machine

In this paper, a complete dynamic model on task space for a 6 degrees of freedom (DOF) Gough‐Stewart platform‐type computer numerical control (CNC) machine is derived. The rotation terms of the legs are included for those inertia effects cannot be negligible in the machine tool applications. The formulation derived by means of the Euler‐Lagrange method is convenient for designing the adaptive control law. Also, the average‐type force model for end milling process is derived and included in the dynamic model and control. A composite adaptive control scheme is developed by use of filtering dynamics technique. An appropriate estimator gain is designed in the parameter adaptation law that is useful for estimating the selected important cutting parameters. Experimental results verify the proposed adaptive control scheme can achieve good tracking performance. © 2004 Wiley Periodicals, Inc.     

Effect of Sintering Temperature on Thermoelectric Properties of p‐Bi2Te3 Alloys Produced by Gas Atomization

In this research, p‐type Bi₂Te₃–75% Sb₂Te₃ thermoelectric alloy powders were produced by gas atomization and subsequently sintered by hot pressing at different temperatures. The grain growth of the hot‐pressed samples was observed with increasing sintering temperature from 380°C to 460°C. The compressive strength increased with increasing hot‐pressing temperature due to the high relative density of bulk samples obtained at high temperatures. The effect of sintering temperature on thermoelectric (TE) properties was studied. The maximum power factor 3.48 mW/mK² was obtained for the sample hot pressed at 420°C due to the resulting high electrical conductivity and enhanced Seebeck coefficient values.     

Imaging blend morphology of unstained styrene-N-phenyl-male- imide-styrene-co-acrylonitrile using electron microscopy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of aspect ratio on large deformation and necking of polyethylene

Objective of the study is to examine influence of aspect ratio of rectangular cross-section on the tri-axial stress state developed by necking in tensile specimens of polyethylene. The first part of the paper presents an experimental study that used two types of high-density polyethylene (HDPE) as sample material to identify the thickness-dependent relationship between engineering stress and elongation from tensile tests. The experimental study also shows that thinner specimens, i.e. higher aspect ratio, have lower neck propagation speed and higher flow stress, thus higher rate of energy consumption for the neck propagation. The second part of the paper presents finite element simulation of large deformation and necking in HDPE when subjected to uni-axial tension. True stress–strain relationship and governing equation for visco-plastic deformation are determined from the finite element simulation based on experimental data for the two HDPEs, which reveals influence of aspect ratio of cross-section on the stress state during the necking process. Results from the study indicate that plane-stress condition prevails when the aspect ratio increases, i.e. by decreasing the specimen thickness. The finite element simulation also supports the observation that necking in specimens with higher aspect ratio, i.e. thinner specimens, generates higher percentage of reduction in the thickness direction but lower in the width direction. The overall capability for the deformation endurance was found to improve by reducing the specimen thickness. The paper concludes that finite element simulation has successfully demonstrated the influence of aspect ratio of the cross-section on the stress state in the necking process. The paper also concludes that by combining experimental testing and finite element simulation, time-dependent deformation behaviour can be separated from the time-independent deformation behaviour, which is almost impossible to achieve based on the experimental techniques that rely purely on measurement only.     

Cohesive zone with continuum damage properties for simulation of delamination development in fibre composites and failure of adhesive joints

A new approach is developed to implement the cohesive zone concept for the simulation of delamination in fibre composites or crack growth in adhesive joints in tension or shear mode of fracture. The model adopts a bilinear damage evolution law, and uses critical energy release rate as the energy required for generating fully damaged unit area. Multi-axial-stress criterion is used to govern the damage initiation so that the model is able to show the hydrostatic stress effect on the damage development. The damage material model is implemented in a finite element model consisting of continuum solid elements to mimic the damage development. The validity of the model was firstly examined by simulating delamination growth in pre-cracked coupon specimens of fibre composites: the double-cantilever beam test, the end-notched flexure test and the end-loaded split test, with either stable or unstable crack growth. The model was then used to simulate damage initiation in a composite specimen for delamination without a starting defect (or a pre-crack). The results were compared with those from the same finite element model (FEM) but based on a traditional damage initiation criterion and those from the experimental studies, for the physical locations of the delamination initiation and the final crack size developed. The paper also presents a parametric study that investigates the influence of material strength on the damage initiation for delamination.