The development of a novel technique for small ring specimen tensile testing

The wide scale use of small specimens in routine testing programs could significantly reduce material resource requirements (factors of 10 are easily achievable). This is a major benefit to situations where there is not enough material to manufacture conventional, full-size specimens, such as first-stage gas turbine blade roots. However, limitations exist due to concerns over size effects, manufacturing difficulties, uncertainties related to the application of representative loading conditions and complex interpretation procedures of non-standard data. Due to these limitations, small specimen testing techniques have been mostly applied in ranking exercises and to determine approximate or simple material parameters such as Young’s modulus, minimum creep strain rate and fracture toughness. The small ring method is a novel, high sensitivity small specimen technique for creep testing that has been extended in the present work to the determination of tensile material properties. The main advantages of the small ring specimen are that it is self-aligning and has a large equivalent gauge length in comparison to other small specimens, resulting in much higher testing sensitivity. In the present work, this specimen type mimics conventional, full-size, monotonic testing, allowing for observations of elastic and plastic material response to be made. Wrought aluminium alloy 7175-T7153 small rings were tested at room temperature at 5 different loading (displacement) rates and the results compared to conventional, full-size, monotonic specimen equivalents. Finite element analysis was conducted in order to evaluate the equivalent gauge section and equivalent gauge length in the small ring specimen (which varied between circa 0.35–1.4 mm2 and 25–45 mm, respectively) to facilitate these comparisons. An analytical solution has also been derived in order to validate the finite element analysis

Analysis of contact area in a continuous application-and-peel test method for prepreg tack

The relationship between prepreg tack and the degree of intimate contact (DoIC) between prepreg and a rigid substrate was explored in the context of a continuous application-and-peel test method. Tack for a unidirectional prepreg tape was characterised for different surface combinations and varying test parameters (material feed rate, temperature) at a constant compaction pressure. Application of the prepreg to a transparent rigid substrate (glass), was carried out at matching test conditions to the prepreg tack measurements. Optical microscopy was utilised to acquire images of the contact area at the prepreg-glass interface. Image analysis of the micrographs enabled quantification of the contact area. The time- and temperature-dependent viscoelastic behaviour of the resin was explored directly on the prepreg using oscillatory parallel plate rheometry, and time-temperature superposition was applied to construct both tack and DoIC master curves. The shifted DoIC data showed that true contact area increases with decreasing shifted feed rates, until maximum contact area is achieved. Similarly, tack increases with decreasing shifted feed rates. However, at a critical feed rate, the bond failure mechanism switches from adhesive to cohesive failure. In cohesive failure, tack decreases with decreasing feed rate despite the high levels of DoIC

A Novel Method of Extraction of Blend Component Structure from SANS Measurements of Homopolymer Bimodal Blends

A new method is presented for the extraction of single-chain form factors and interchain interference functions from a range of small-angle neutron scattering (SANS) experiments on bimodal homopolymer blends. The method requires a minimum of three blends, made up of hydrogenated and deuterated components with matched degree of polymerization at two different chain lengths, but with carefully varying deuteration levels. The method is validated through an experimental study on polystyrene homopolymer bimodal blends with inline image. By fitting Debye functions to the structure factors, it is shown that there is good agreement between the molar mass of the components obtained from SANS and from chromatography. The extraction method also enables, for the first time, interchain scattering functions to be produced for scattering between chains of different lengths

Sustainable terpene triblock copolymers with tuneable properties for pressure sensitive adhesive applications

A series of triblock copolymers in a hard-soft-hard block configuration with varying hard block α-pinene methacrylate content and molecular weight and butyl acrylate soft segment have been synthesised and investigated for viability in pressure sensitive adhesive (PSA) applications. The morphologies vary from pockets of hard phase distributed within a continuous soft matrix, through to lamellar with co-continuous phases, and finally continuous hard phase with pockets of soft phase dispersed. Uniaxial tensile properties, probe adhesion performance and cyclic adhesive behaviour are presented for seven compositions including four short chain and three long chain copolymers, alongside a commercial benchmark PSA. Structure-property relationships for the novel elastomers are evaluated, establishing that short chain materials with 20-25 wt% αPMA offer similar tensile and adhesion performance to the commercial elastomer. Raising the hard phase concentration has been observed to provide a considerable increase in ultimate tensile strength, stiffness and peak tack force, but at the expense of significant reductions in ultimate tensile strain, adhesive bond displacement and vibrational dissipation. The results suggest that the performance of these sustainable materials can be tuned to produce viable PSAs with a range of useful properties

Adaptation of material deposition parameters to account for out-time effects on prepreg tack

A single-stage peel method was employed to determine the relationship between key processing parameters and tack for a standard aerospace carbon/epoxy prepreg subjected to various levels of room-temperature out-time. The temperature-dependent viscoelasticity of the resin was studied using parallel plate rheometry and modelled using a simple Arrhenius equation. Differential scanning calorimetry and gel permeation chromatography results showed that, over a period of 35 days under ambient conditions, resin Tg increased, while no significant change in polymer chain length was observed. Time-temperature superposition was applied to construct tack master curves for each out-time interval, which were shown to approximately coincide when considering shift factors attributed to changes in Tg. Process maps considering prepreg out-time were generated using tack master curves to inform process parameters and achieve desirable tack levels. This type of tailored process control is anticipated to improve resource utilization when manufacturing large preforms which take several days to complete

Double diaphragm forming simulation for complex composite structures

A finite element (FE) model has been developed to simulate the double diaphragm forming (DDF) process, to identify potential defects when forming complex 3D preforms from 2D biaxial non-crimp fabric plies. Three different metrics have been introduced to predict and characterise defects, which include local shear angles to determine ply wrinkling induced by over-shear, compressive strains in the primary fibre directions to determine bundle wrinkling, and tensile stresses in the primary fibre directions to determine fabric bridging. The FE simulation is in good agreement with experiments performed on a demonstrator component. Results indicate that fabric bridging occurs in large-curvature regions, which is the dominant defect in DDF, as wrinkling is generally lower than in matched-tool forming due to relatively low forming pressures (up to 1 bar). The axial tensile stress in fibres has been used as a measure to identify suitable positions and orientations for darts, to alleviate fabric bridging and improve surface conformity, whilst minimising the effect on the mechanical performance of the component

Compression moulding of composites with hybrid fibre architectures

Advanced Sheet Moulding Compounds (ASMC) and unidirectional (UD) prepregs have been co-compression moulded to form a hybrid composite material. In-mould flow influences the UD fibre architecture in two ways. When UD fibres are aligned transversely to the ASMC flow direction, shearing occurs which causes local changes in fibre volume fraction and fibre waviness. When the UD fibres are aligned with the ASMC flow direction, ply migration takes place. In general, the composite stiffness follows a rule of mixtures relationship, with the stiffness proportional to the UD fibre content. A grid analysis method has been developed to quantify distortion in the UD plies. Staging the resin to 50% cure was shown to reduce ply distortion during moulding, whilst maintaining suitable inter-laminar shear strength. Adding an interfacial prepreg ply between the reinforcing UD fibres and the ASMC charge successfully prevented distortion in the UD fibres, avoiding shear thinning and fibre migration

Time–temperature equivalence in the tack and dynamic stiffness of polymer prepreg and its application to automated composites manufacturing

A recently developed peel test designed to simulate the automated tape lay-up (ATL) process was used to measure tack and dynamic stiffness of newly developed ATL prepregs. Resin was extracted from the prepreg process before impregnation of the fibres. Isothermal small amplitude frequency sweeps were carried out in shear rheology to determine time–temperature superposition parameters in the form of Williams–Landel–Ferry equation. Gel permeation chromatography and differential scanning calorimetry demonstrated that the resin was not significantly changed during the prepregging process. The WLF parameters were used to transpose isothermal tack and dynamic stiffness results with excellent agreement found. This relationship offers manufacturers using composite prepreg a method to maximise and maintain tack levels at different feed rates by appropriate changes in temperature. This is of significant importance in improving the reliability of automated composite lay-up processes such as AFP and ATL, whose feed rate must vary to accommodate lay-up operations

High-pressure rheological analysis of CO2-induced melting point depression and viscosity reduction of poly(ε-caprolactone)

High-pressure rheology has been used to assess the effects of supercritical carbon dioxide (scCO2) on the melting point (Tm) and viscosity of poly (ε-caprolactone) (PCL) over a range of temperatures and pressures up to 300 bar over a wide range of shear rates. Plots of the storage and loss moduli against temperature show a significant shift of Tm to lower temperatures in the presence of CO2, indicating that the polymer crystals melt at temperatures much lower than the ambient pressure Tm. Furthermore, a significant decrease in the viscosity of two PCL grades with different molecular weight (Mn ~ 10 kDa and 80 kDa) was also detected upon increasing the CO2 pressure to 300 bar. Experimental viscosity data were fitted to the Carreau model to quantify the extent of the plasticising effects on the zero-shear viscosity and relaxation time under different conditions. Similar analyses were conducted under high-pressure nitrogen, to compare the effects obtained in the presence of a non-plasticising gas

Roles of chain length, chain architecture, and time in the initiation of visible crazes in polystyrene

Visible craze initiation stress has been measured for a wide range of linear and branched monodisperse polystyrenes (PS) soaked in diethylene glycol. Results show that, for a given time under stress, craze initiation in linear PS is disentanglement-dominated below a critical molar mass and chain scission-dominated above it. Branched monodisperse PS behaves similarly, with the relevant molar mass in this case being the span molar mass. Disentanglement craze initiation stress is found to vary linearly with log molar mass and log time. These observations can be explained in terms of Eyring-type stress acceleration of the process of chain retraction, required to achieve the entanglement loss necessary for creation of craze fibril surfaces. A single effective activation volume of 1.8 nm3 accounts for the dependence of crazing stress on molar mass, time, and temperature under uniaxial tensile stress, both as observed in the present data and in a previous study of rate/temperature dependence