The effect of polymerization conditions and crystallinity on the mechanical properties and fracture of spherulitic nylon 6

The molecular and structural parameters controlling the mechanical properties, deformation and fracture of spherulitic nylon 6 have been investigated. The nylon was prepared by the anionic polymerization of ε-caprolactam and the polymerization conditions were varied to give samples having a range of spherulite diameter, molecular weight and degree of crystallinity. The tensile properties and fracture mode of the nylon varied considerably with degree of crystallinity and polymerization temperature. High crystallinity and low polymerization temperatures below 423 K gave a brittle material. Polymerization above 423 K resulted in a ductile material which showed a yield drop. In this material final fracture was preceded by the formation of inter and trans spherulitic cracks which coalesced to form a large cavity that led to final failure. In nylon having a low degree of crystallinity, fracture was fibrillar in nature and occurred by the ductile drawing of the material to strains greater than 250%.     

Effects of Processing Variables on Fatigue in Molded PEEK and Its Short Fiber Composites

The effects of various material and processing parameters on the fatigue behavior of PEEK and its short glass and carbon fiber reinforced composites have been investigated. In particular, the significance of molecular weight, fiber type and increased matrix crystallinity, induced by post-forming annealing, are considered in this paper. Standard S-N (peak stress against log (cycles to failure(( fatigue data curves have been generated and crack propagation rates have been monitored as a function of the alternating stress intensity factor. Scanning electron microscopy (SEM), optical microscopy, X-ray diffraction and differential scanning calorimetry (DSC) have all been used to characterize the microstructural parameters and failure mechanisms. Results show that the increased crystallinity, induced by annealing, significantly improves fatigue crack resistance for both neat PEEK and its short fiber composites. Generally, superior fatigue behavior is associated with the higher molecular weight matrix and with carbon rather than glass fibers. However, it proved difficult to unravel the complex interdependence of various matrix factors such as molecular weight and crystallinity, and fiber type, length, aspect ratio and preferred orientation. Nevertheless, a combination of the tie molecule density concept and a reinforcing efficiency model, developed to take into account these various parameters, has helped to explain the complexity of the results.     

The tensile behaviour of polyethylene terephthalate

Measurements of the load/extension curves of polyethylene terephthalate (PET) over a wide range of temperatures showed four regions of behaviour. These were brittle fracture, ductile, cold-drawing, and uniform extension. A particular study was made of the transitions between brittle fracture and ductile failure, and between ductile and cold-drawing, since these define the limits of the three regimes of failure observed at temperatures below the glass transition and softening range.The effects of molecular weight and crystallinity were examined. The brittle strengths measured in tension at low temperatures showed a very large scatter. There was evidence, in spite of this scatter, that the brittle strength falls with decreasing molecular weight. The yield behaviour was not affected, so that the brittle/ductile transition moves to higher temperatures.Crystallinity affects both brittle strength and yield behaviour. The brittle strength falls with increasing crystallinity, whereas the yield stress rises. Both effects combine to raise the temperature of the brittle/ductile and the ductile/cold-drawing transitions.Stress/temperature curves were also constructed for notched specimens. Notching raises the effective yield stress and reduces the brittle strength so that the brittle/ductile transition is moved to higher temperatures. The observed effects are in qualitative agreement with theoretical predictions of the plastic constraint at the tip of a notch, and thus the latter gives a satisfactory qualitative explanation of notch sensitivity. Notching leads to brittle failure at room temperature, and in notched specimens the brittle strength rises as the temperature is decreased. The brittle strength of the lowest molecular weight sample was again significantly less than that of the higher molecular weight samples.Joint appointment between ICI Ltd and Bristol University.     

Effects of vacuum,mold temperature and cooling rate on mechanical properties of press consolidated glass fiber/PET composite

Glass fiber reinforced polyethylene–terephthalate (PET) matrix composites manufactured by a rapid press consolidation technique were investigated as functions of vacuum, mold temperature, and cooling rate among the many possible processing parameters. Tensile, impact, fracture toughness and short beam shear tests were carried out and these mechanical properties were compared with respect to crystallinity. It was found that the mechanical properties strongly depend on vacuum, mold temperature, and cooling rate. The degree of crystallinity (X_C) in composites affects tensile properties to some degree, but impact properties were affected much more. It also affects the degree of ductility depending mold temperature in consolidation and cooling rate, which determines the impact energy of this material.     

The effect of processing temperature and time on the structure and fracture characteristics of self-reinforced composite poly(methyl methacrylate)

A novel material, self-reinforced composite poly(methyl methacrylate) (SRC-PMMA) has been previously developed in this laboratory. It consists of high-strength PMMA fibers embedded in a matrix of PMMA derived from the fibers. As a composite material, uniaxial SRC-PMMA has been shown to have greatly improved flexural, tensile, fracture toughness and fatigue properties when compared to unreinforced PMMA. Previous work examined one empirically defined processing condition. This work systematically examines the effect of processing time and temperature on the thermal properties, fracture toughness and fracture morphology of SRC-PMMA produced by a hot compaction method. Differential scanning calorimetry (DSC) shows that composites containing high amounts of retained molecular orientation exhibit both endothermic and exothermic peaks which depend on processing times and temperatures. An exothermic release of energy just above T_g is related to the release of retained molecular orientation in the composites. This release of energy decreases linearly with increasing processing temperature or time for the range investigated. Fracture toughness results show a maximum fracture toughness of 3.18 MPa m1/2 for samples processed for 65 min at 128°C. Optimal structure and fracture toughness are obtained in composites which have maximum interfiber bonding and minimal loss of molecular orientation. Composite fracture mechanisms are highly dependent on processing. Low processing times and temperatures result in more interfiber/matrix fracture, while higher processing times and temperatures result in higher ductility and more transfiber fracture. Excessive processing times result in brittle failure.     

On the essential work of fracture in polymer–metal multilayers

Polyethylene terephtalate (PET) metallized with aluminium by physical vapour deposition was investigated through classical physical chemistry techniques and mechanical characterization. The amount of aluminium altered the amount of crystallinity of the PET substrate, but appeared unrelated to the mechanical properties obtained with regular tensile test. In contrast, the essential work of fracture (EWF), as obtained with Cotterell tests, permitted to better discriminate the perforation resistance. It is shown that increasing the amount of crystallinity within the PET linearly reduced the EWF.     

Impact behavior of hydroxyapatite reinforced polyethylene composites

Hydroxyapatite particulate reinforced high density polyethylene composite (HA-HDPE) has been developed as a bone replacement material. The impact behavior of the composites at 37 °C has been investigated using an instrumented falling weight impact testing machine. The fracture surfaces were examined using SEM and the fracture mechanisms are discussed. It was found that the fracture toughness of HA-HDPE composites increased with HDPE molecular weight, but decreased with increasing HA volume fraction. Examination of fracture surfaces revealed weak filler/matrix interfaces which can debond easily to enable crack initiation and propagation. Increasing HA volume fraction increases the interface area, and more cracks can form and develop, thus decreasing the impact resistance of the composites. Another important factor for the impact behavior of the composites is the matrix. At higher molecular weight, HDPE is able to sustain more plastic deformation and dissipates more impact energy, hence improving the impact property.     

Experimental and numerical studies of the notch strengthening behaviour of semi-crystalline ultra-high molecular weight polyethylene

In the present work, both experimental work and Finite Element Modeling (FEM) have been used to study the effects of notch geometry on the stress/strain behaviour and fracture of the ultra-high molecular weight polyethylene (UHMWPE) at quasi static conditions. The effects of notch profile radii on UHMWPE fracture behaviour, predicted stress/strain distribution, triaxial state of stress and hydrostatic pressure across the neck have been studied. Different techniques have been used to study the effects of notch geometry on the UHMWPE properties. Differential Scanning Calorimetry (DSC) has been used to study the changes in the tested material crystallinity. The universal testing machine has been used to measure the changes in the tested material mechanical properties. The SEM has been used to examine the change in the tested material fracture surface. Finally, the Finite Element Code (ANSYS10) has been used to investigate the effects of notch geometry on the predicted stress/strain distribution across the neck. The results show a notch strengthening behaviour for the tested material, where the axial yield properties increased significantly with the reduction of notch radii, while the axial ultimate properties decreased significantly for the notched specimens compared with plain ones. Also the predicted stress/strain distribution and stress triaxiality show a strong dependence on the notch geometry. The remarkable effects of notch geometry on the predicted stress/strains distributions across the neck show the importance of careful design of UHMWPE artificial joint components with the aim to eliminate the presence of stress risers such as undercuts and fillets.     

Failure mechanisms and bending strength of Fuchsia magellanica var. gracilis stems

In the course of biological evolution, plant stems have evolved mechanical properties and an internal structure that makes them resistant to various types of failure. The mechanisms involved during damage development and failure in bending are complex and incompletely understood. The work presented builds on a theoretical framework outlined by Ennos and van Casteren, who applied engineering mechanics theory to explain why different woody stems fail in different ways. Our work has extended this approach, applying it to a detailed analysis of one particular species: Fuchsia magellanica var. gracilis. When subjected to three-point bending, stems of this species exhibited one of two failure mechanisms: a plastic hinge or a greenstick fracture. We developed a predictive model using a computer simulation and a mathematical analysis using the theory of plastic bending. Required material properties were obtained from tests, the literature and imaging techniques. We found that greenstick fractures are more likely to occur in more lignified stems with a higher density. We discovered a new failure mode: an internal crack caused by tensile transverse stress. This work helps in understanding how plants have evolved their bending resistance and may assist in the creation of novel engineering structures inspired by these principles.     

Static fatigue behaviour of linear low-density polyethylenes

Various thermal histories were utilized to generate samples with the same crystalline microstructure (i.e. degree of crystallinity, supermolecular structure, tie molecule density and lamellar thickness) for linear low-density polyethylenes (LLDPEs) with the same molecular weight, molecular weight distribution and branch frequency but different branch length. The static fatigue properties were found to improve with decreasing applied load for samples with the same type of short-chain branches. The failure time of static fatigue (t _f) was found to increase dramatically as the branch length increased. An equation was used to predict t _f from the stress, the branch length and other material parameters. In addition, the initial growth rate of the crack opening displacement and the time required to reach the critical opening displacement at the notch roots of the specimens were observed to decrease and increase, respectively, with increasing branch length. This dramatic improvement in static fatigue properties is attributed to the increasing sliding resistance of the polymer chains through the crystal and through entanglements in the amorphous region as the branch length of LLDPEs increases.     

聚乙烯管材专用料的结构与性能

利用凝胶渗透色谱(GPC)、差示扫描量热法(DSC)、J积分法和动态力学分析(DMA)等研究了聚乙烯管材专用料的分子量分布和共聚单体对其结晶行为、断裂韧性和耐蠕变性能的影响。结果表明,相对于单峰分子量分布的共聚聚乙烯而言,双峰分子量分布的均聚聚乙烯结晶度从54.5%增加到60.9%,断裂韧性从5.85 kJ/m2提高到6.67 kJ/m2。此外,拉伸性能和耐蠕变性能也得到提高。     

A load–deformation formulation with fracture representation based on the J–R curve for tubular joints

This study presents a new fracture formulation to describe the ductile tearing and unstable fracture failure for circular hollow section (CHS) joints under monotonically increasing brace tension. The initiation of the ductile tearing occurs when the crack driving force in an assumed initial shallow crack reaches the material fracture toughness determined from a standard fracture toughness test. The joint behavior prior to the ductile crack initiation follows a previously proposed nonlinear formulation based on the latest strength equations recommended by the International Institute of Welding. The load–deformation characteristics beyond the crack initiation assume that the energy release rate and the amount of crack extension adhere to the experimentally measured J–R curve, prior to the unstable fracture failure. Unstable fracture, which leads to the total loss of the joint capacity, occurs when the crack driving force reaches the maximum fracture resistance determined from the J–R curve test. The proposed load–deformation representation for tubular joints, when implemented in the large-scale K-frame pushover analysis with a material fracture toughness test, predicts successfully the global frame response governed by the joint fracture failure, as observed in the frame test.     

Observed and simulated fracture pattern in diametrically loaded discs of rock material

To investigate the influence of a tensile stress gradient on fracture initiation and fracture growth in rock material, a configuration, consisting of a diametrically loaded disc with a hole on the diameter perpendicular to the loaded diameter, is used. The maximum local tensile stresses the material is able to withstand increase as the stress gradient increases. Depending on the diameter and the eccentricity of the hole, the disc splits along the loaded diameter or macro-fracturing starts at the hole. However, the tensile stresses at the top and the bottom of the hole are for nearly all cases considerably higher than the stress along the loaded diameter and than the macroscopic tensile strength of the material, determined by conventional Brazilian tests. To better understand this particular fracturing behaviour, numerical simulations of the experiments are conducted using the boundary element code DIGS, which allows the incorporation of weak elements (flaws), representing defects and weaknesses in the rock material. It is shown that the influence of the stress gradient on the stress concentration at the tip of mobilised defects lies at the origin of the particular fracturing behaviour in the diametrically loaded disc with a hole. The study of the new configuration leads also to a number of conclusions with regard to the failure in diametrically loaded discs in general. Based on the flaw model, fracture initiation in the Brazilian test has to be attributed to fracture growth of a mobilised defect, situated in the area close to one of the platens.     

Simulation of fracture in heterogeneous elastic materials with cohesive zone models

In brittle composite materials, failure mechanisms like debonding of the matrix-fiber interface or fiber breakage can result in crack deflection and hence in the improvement of the damage tolerance. More generally it is known that high values of fracture energy dissipation lead to toughening of the material. Our aim is to investigate the influence of material parameters and geometrical aspects of fibers on the fracture energy as well as the crack growth for given load scenarios. Concerning simulations of crack growth the cohesive element method in combination with the Discontinuous Galerkin method provides a framework to model the fracture considering strength, stiffness and failure energy in an integrated manner. Cohesive parameters are directly determined by DFT supercell calculations. We perform studies with prescribed crack paths as well as free crack path simulations. In both cases computational results reveal that fracture energy depends on both the material parameters but also the geometry of the fibers. In particular it is shown that the dissipated energy can be increased by appropriate choices of cohesive parameters of the interface and geometrical aspects of the fiber. In conclusion, our results can help to guide the manufacturing process of materials with a high fracture toughness.     

Stress and failure analysis of the crankshaft of diesel engine

In this work the failure analysis of the crankshaft of diesel engine was performed. Visual examination of the crankshaft fracture showed that beach marks, typical for fatigue failure were observed. Additional observations of the crack initiation zone indicated that crack origin was not covered by material defects or corrosion products. Performed hardness test of the fractured crank pin showed that large HRC values were observed in central part of the pin only. On the corner of cylindrical pin surface where the crack origin was located the hardness of material was much smaller. In order to explain the reason of premature crankshaft damage, the finite element method was utilized. The results of nonlinear static analysis showed that during work of the engine with maximum power the high stress area was located in crack initiation zone. Based on results of performed investigations it was concluded that the main reason of premature fatigue failure was high-cycle fatigue of the material in external zone of the crank pin where the small structural radius was designed. In final part of the work the recommendations for increase of the fatigue life of analyzed crankshaft were formulated.     

Room temperature fracture processes of a near-α titanium alloy following elevated temperature exposure

Near-α titanium alloys are used at higher temperatures than any other class of titanium alloys. As a consequence of thermal exposure, these components may develop locally elevated oxygen concentrations at the exposed surface which can negatively impact ductility and resistance to fatigue crack initiation. In this work, monotonic and fatigue fracture mechanisms of Ti–6Al–2Sn–4Zr–2Mo–0.1Si samples exposed to laboratory air at 650 °C for 420 h were identified by means of a combination of quantitative tilt fractography, metallographic sectioning, and electron backscatter diffraction. These mechanisms were compared and contrasted with those operative during similar tests performed on material is the as-received condition with uniform oxygen content. While faceted fracture was not observed during quasi-static loading of virgin material, locally elevated concentrations of oxygen near the surfaces of exposed samples were shown to change the fracture mode from ductile, microvoid coalescence to brittle facet formation and grain boundary separation at stresses below the macroscopic yield point. Similar features and an increased propensity for facet formation were observed during cyclic loading of exposed samples. The effects of this time-dependent degradation on monotonic and cyclic properties were discussed in the context of the effect of oxygen on crack initiation and propagation mechanisms.     

Impact of some environmental conditions on the tensile, creep-recovery, relaxation, melting and crystallinity behaviour of UHMWPE-GUR 410-medical grade

The present work was undertaken to examine the effect of some environmental media (sodium hydroxide NaOH solution, water, ice, UV irradiation dose and pre-heat treatment) on the mechanical (quasi-static tensile creep-recovery and relaxation) and physical/thermal (melting and crystallinity) behaviour of the ultra high molecular weight polyethylene (UHMWPE-GUR 410-medical grade), that has several biomedical and engineering applications. The results show changes in the mechanical properties due to these environmental effects. The pre-heat treatment has significantly enhanced the tensile properties compared to virgin specimens’ properties. Improvement due to pre-heat treatment at 100 °C is more than that at 50 °C. Specimens’ storing in ice, NaOH and water has not affected significantly the tensile properties. All properties except fracture strain have enhanced due to specimens exposure to UV irradiation. The differential scanning calorimetry results indicate that environmental media have not any noticeable effects on the melting temperature. However, a significant increase in the degree of crystallinity was observed for all specimens versus that for virgin specimens. The creep and permanent strains of the tested virgin material increase with temperature and lineally increase with applied load. The specimens’ exposure to environmental media has improved the creep resistance and the permanent creep strain when compared with that for virgin ones. Remarkable increase was observed in the initial relaxation and residual stress of the exposed specimens against that for virgin specimens.     

Prediction of crack initiation at blunt notches and cavities - size effects

Crack initiation at corners, V-notches and other situations such as interfaces breaking a free surface (delamination initiation) cannot be correctly predicted by the usual brittle fracture criteria (either Griffith or maximum stress). They give contradictory results and neither one nor the other agrees with the experiments. An additional characteristic length is required to define a satisfying criterion giving rise to the so-called “Finite fracture mechanics”. The crack is supposed to jump this length which depends both on the material properties and the local geometry of the structure; it is not a material parameter. In most cases this crack increment is small. The size effect arises with the interaction between the crack increment and another length characterising a microstructure such as a pore diameter, a notch root radius or an interface layer thickness. The remote load at failure depends on the actual value of this microstructure parameter whereas it was not expected in all cases. Assuming that the two interacting lengths remain small compared to the size of the global structure, an asymptotic procedure allows bringing into evidence the change in the apparent resistance of the structure due to this phenomenon. Results are compared with experiments in various domains: polymers, ceramics and rocks.     

Fracture Criteria for Automobile Crashworthiness Simulation of Wrought Aluminium Alloy Components

In automobile crashworthiness simulation, the prediction of plastic deformation and fracture of each significant, single component is critical to correctly represent the transient energy absorption through the car structure. There is currently a need, in the commercial FEM community, for validated material fracture models which adequately represent this phenomenon. The aim of this paper is to compare and to validate existing numerical approaches to predict failure with test data. All studies presented in this paper were carried out on aluminium wrought alloys: AlMgSi1.F31 and AlMgSiCu‐T6. A viscoplastic material law, whose parameters are derived from uniaxial tensile and compression tests at various strain rates, is developed and presented herein. Fundamental ductile fracture mechanisms such as void nucleation, void growth, and void coalescence as well as shear band fracture are present in the tested samples and taken into consideration in the development of the fracture model. Two approaches to the prediction of fracture initiation are compared. The first is based on failure curves expressed by instantaneous macroscopic stresses and strains (i. e. maximum equivalent plastic strain vs. stress triaxiality). The second approach is based on the modified Gurson model and uses state variables at the mesoscopic scale (i. e. critical void volume fraction). Notched tensile specimens with varying notch radii and axisymmetric shear specimens were used to produce ductile fractures and shear band fractures at different stress states. The critical macroscopic and mesoscopic damage values at the fracture initiation locations were evaluated using FEM simulations of the different specimens. The derived fracture criteria (macroscopic and mesoscopic) were applied to crashworthiness experiments with real components. The quality of the prediction on component level is discussed for both types of criteria.     

Structure–fracture properties relationship for Polypropylene reinforced with fly ash with and without maleic anhydride functionalized isotactic Polypropylene as coupling agent

The deformation and fracture behavior of PP/ash composites with and without maleic anhydride functionalized iPP (MAPP) as coupling agent was investigated, focusing on the effect of ash content and loading conditions. A decreasing trend of tensile strength and strain at break values with filler content was observed for unmodified composites, whereas these properties were roughly independent of ash content for the composites with MAPP. In quasi-static fracture tests, all materials displayed ductile behavior. Most composites exhibited improved fracture properties with respect to the matrix as a result of the toughening mechanisms induced by the ash particles. Under impact loading conditions, in contrast, all materials displayed fully brittle behavior. Impact critical fracture energy values of the composites were higher than those of PP and they also presented a maximum which was explained in terms of the comprehensive analysis of the crystallinity development in PP. The incorporation of MAPP led to better dispersion of ash particles in the matrix but was detrimental to the material fracture behavior independently of loading conditions. Increased interfacial adhesion promoted by MAPP hindered particle-induced toughening mechanisms.