Garth Pearce

Experimental Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

This paper presents quasi-static and dynamic modelling of bolted composite structures using the explicit finite element code PAM-CRASH. User controlled point link (PLINK) elements were investigated for modelling the bolted composite joints used in the structures. Simulation results were compared with quasi-static and dynamic structural testing reported previously. Two loading configurations were considered. It was shown that the PLINK element modelling approach agreed well with the experimental results for both loading configurations and for one case offered significant improvements over other simplified bolt modelling methods. A stacked shell modelling approach was used to model the interlaminar delamination damage present in the ball-loaded impact mode. The overall response of the structure was significantly improved by the addition of these energy absorbing interfaces.

A Stacked-Shell Finite Element Approach for Modelling a Dynamically Loaded Composite Bolted Joint Under in-Plane Bearing Loads

This paper presents the results of a study into a novel application of the “stacked-shell” laminate modelling approach to dynamically loaded bolted composite joints using the explicit finite element code PAM-CRASH. The stacked-shell approach provides medium-high fidelity resolution of the key joint failure modes, but is computationally much more efficient than full 3D modelling. For this work, a countersunk bolt in a composite laminate under in-plane bearing loading was considered. The models were able to predict the onset of damage, failure modes and the ultimate load of the joint. It was determined that improved debris models are required in order to accurately capture the progressive bearing damage after the onset of joint failure.

Contrast Enhancement of MicroCT Scans to Aid 3D Modelling of Carbon Fibre Fabric Composites

This paper presents a methodology for volume capture and rendering of plain weave and multi-layer fabric meso-architectures within a consolidated, cured laminate. Micro X-ray Computed Tomography (MicroCT) is an excellent tool for the non-destructive visualisation of material microstructures however the contrast between tows and resin is poor for carbon fibre composites. Firstly, this paper demonstrates techniques to improve the contrast of the microCT images by introducing higher density materials such as gold, iodine and glass into the fabric. Two approaches were demonstrated to be effective for enhancing the differentiation between the tows in the reconstructed microCT visualisations. Secondly, a method of generating three-dimensional volume models of woven composites using microCT scan data is discussed. The process of generating a model is explained from initial manufacture with the aid of an example plain weave fabric. These methods are to be used in the finite element modelling of three-dimensional fabric preforms in future work.

Study of magnetorheological fluids towards smart energy absorption of composite structures for crashworthiness.

ABSTRACT A study of magnetorheological fluids energy absorption capability towards development of composite retrofit technologies for aged helicopters is presented in this article. Design parameters that influence the performance of magnetorheological dampers were identified and evaluated based on the damping force as a measure of compatibility to energy absorption. Tensile, compression, and cyclic loading tests were also conducted experimentally to validate the numerical investigation. An extensive parametric study conveyed that the piston head radius plays a major role in achieving higher damping force. The fundamental behaviors and results of the damping force in the numerical studies were also in a good agreement with the experimental results. The novel parametric numerical analysis framework validated in this article will allow for efficient design and optimization of future dampers. Received 2 August 2013 Accepted 9 January 2015

Methodologies of stochastic simulation for helicopter accidents and nonparametric evaluation of the stochastic responses

ABSTRACT The paper aims to develop stochastic simulation and nonparametric analysis methods for investigation into and harm minimisation of helicopter accidents resulting from a total engine failure. An innovative, stochastic simulation scheme is established for investigating the variability in a full crash event by incorporating the methodologies of hierarchical stochastic simulation and nonparametric analysis. By employing Monte Carlo algorithms in flight emergency and structure crash simulations, a stochastic simulation method is developed for establishing robust correlations between the critical parameters prior to helicopter accidents and possible occupant injury metrics. Based on the nonparametric analysis performed on the stochastic injury responses, a solution is proposed to predict and evaluate the potential severity of occupants injuries resulting from this class of helicopter accidents. A comparison of using the nonparametric strategy into three different thicknesses of subfloor tubes develops the potential for evaluating the stochastic injury results of the helicopter accidents.

Effect of Ultrasonic Peening on Residual Stresses at a T-Butt Weld Toe

The current paper presents the results of neutron diffraction measurements of the throughthickness residual stress field at the toe of a T-butt weld, made from 10mm thick A350 grade black mild steel plates, after successful ultrasonic peening. A single ultrasonic peening treatment was carried out at the weld toe. Residual stresses were measured using the KOWARI instrument at ANSTO. The neutron diffraction technique was chosen for this study because of its ability to measure three-dimensional residual stress deep within the component at high resolutions. Although the nominal yield stress of the A350 grade plate is 350 MPa the actual yield stress is generally higher, in this case averaging out to about 400 MPa. Ultrasonic peening was highly effective, leading to a residual stress redistribution with a maximum compressive stress of about 250 MPa at the weld toe surface and a maximum tensile stress of 220 MPa, at a depth of almost 3mm into the base plate. The resulting compressive residual stresses at the weld toe surface will almost certainly increase substantially both the fatigue initiation and propagation lives, or may prevent fatigue completely. Since A350 steel is widely used in buildings, bridges and offshore structures, ultrasonic peening shows great promise as an in-situ peening method in order to improve weld fatigue performance. Introduction A350 grade black mild steel is one of the most widely used structural materials in the world, being commonly found in buildings, bridges and offshore structures. Welding is typically used to join two plates of structural steel and this often takes the form of a T-butt weld. In addition, other more complex geometrical joints are often simplified for stress analysis purposes by approximating them as two-dimensional T-butt plate models (e.g. skewed T-joints, tubular welded joints, pipe–plate joints, etc.). However, all such welds are potentially susceptible to fatigue crack initiation and slow but accelerating growth arising as a result of fluctuating service loads, often eventually resulting in fast fracture. Ultrasonic peening, more properly known as ultrasonic impact treatment (UIT), is a recent development of the well-established shot peening process. It was originally invented in 1972 in the former USSR to improve the fatigue and corrosion performance of ship and submarine hulls. UIT is similar to conventional needle or hammer peening in many respects. An important difference is that rather than using a pneumatic tool, which causes the needles or a single hammer-like rod to impact Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 19-24 doi: http://dx.doi.org/10.21741/9781945291173-4 20 the weld surface at a frequency of 25-100 Hz, with UIT, the weld is impacted by a small number of rods vibrating at a much higher frequency on the order of 18,000-27,000 Hz. This makes it a much quieter device, which vibrates at a lower intensity, so that the operator can use it for longer periods of time before tiring [1]. Ultrasonic peening is relatively cheap, can be applied in-situ and offers significant improvements in the lifespan of welded components when applied correctly. This occurs for three different reasons: removal of weld defects; reduction of stress concentration; and redistribution of tensile stresses and/or introduction of compressive stresses [2]. Experimental Methods Welding. Dimensions were 800×160×10mm for the base plate and 100×160×10mm for the attachment plate of each T-butt. During fabrication two base plates were tack welded back-to-back prior to welding of the double-beveled attachment plate to each, in order to minimize distortion. Balanced full-penetration GMAW fillet welding was employed, with six passes on each side. All welds passed ultrasonic testing (UT) for internal flaws and magnetic particle inspection (MPI) for surface flaws. Fig. 1 shows the weld detail of a typical T-butt joint before buffing. Fig. 1. Weld detail of typical T-butt joint before buffing. Ultrasonic Peening. Ultrasonic peening treatments were carried out at the base plate and attachment plate weld toes on both sides of one of the T-butts using an Applied Ultrasonics Esonix UIT apparatus. The impact pins were oriented 45° from the treated weld area (perpendicular to the toe line) and the tool was continuously moved in an oscillating motion in a path parallel to the direction of the weld. In order to develop the desired groove, the tool was worked back and forth between a 30° and a 60° angle while maintain the oscillating motion described above. Groove formation was continuously observed during the application process. A properly formed, shiny groove at the weld toe was obtained; this is the main quality assurance inspection point for treatment verification [3]. Neutron Diffraction. It was attempted to measure through-thickness residual stresses from the weld toe into the base plate for both as-welded and ultrasonically peened specimens using the KOWARI instrument at the ANSTO Bragg Institute. The (non-destructive) neutron diffraction technique was chosen for this study because of its ability to measure three-dimensional residual stress deep within the component at high resolutions. For the neutron measurements a 0.5×0.5×1mm gauge volume was used for the longitudinal, transverse and normal components. The experimental measurements on the ultrasonically peened sample were successful but those on the as-welded sample were unfortunately shown to be incorrect, as the scan was made slightly inside the weld rather than exactly at the weld toe. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 19-24 doi: http://dx.doi.org/10.21741/9781945291173-4 21 Parametric Equations. Brennan-Dover-Karé-Hellier (BDKH) parametric equations [4] are available for the stress intensity factor (SIF) geometric Y-factor at the deepest point of a semi-elliptical flaw at the toe of a T-butt weld, accurate for a wide range of geometric parameters under both tension (membrane) and pure bending loadings. These were derived from the results of eighty 8-noded shell finite element analyses in conjunction with the Niu-Glinka weight function [5]. From the models studied, the geometry validity limits for the equations developed are: Weld angle: 30°<α<60° Crack aspect ratio: 0<a/c<1.0 Crack depth: 0.01<a/T<1.0 (1) Weld toe radius: 0.01<ρ/T<0.066 Attachment width: 0.3<L/T<4.0 Fig. 2 shows the geometry of the T-butt weldments studied including the crack geometry, with all the geometric parameters above defined. Fig. 2. (a) Local weld geometry studied (b) Geometry and loading used to derive stress intensity factors (c) Crack geometry (semi-elliptical crack). Recently developed and unique Hellier-Brennan-Carr (HBC) T-butt weld toe surface stress concentration factor (SCF) [6] and stress distribution parametric equations [7] through the base plate thickness (i.e. the potential Mode I crack plane) are also available for similar ranges of geometric parameters and tensile (membrane) loading. Crack Growth Equations. The Paris Law [8] is commonly used to predict the (Stage 2) fatigue propagation life for a component or structure containing a sharp initial crack. It does not take into Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 19-24 doi: http://dx.doi.org/10.21741/9781945291173-4 22 account the influence of mean stress. Another equation which does incorporate the (second order) influence of mean stress on the propagation rate is the Forman Equation [9]. Computer Programs to Predict Fatigue Life. Two FORTRAN computer programs were written to predict the remaining fatigue life for a T-butt welded joint containing a semi-elliptical crack at the weld toe under tension (membrane) loading. The first of these programs uses the BDKH tension parametric equation in conjunction with the Paris Law, and is applicable to a stress-relieved joint. The second program uses the BDKH tension parametric equation together with the HBC tension parametric equation and the Forman Equation, and is applicable to both the as-welded and ultrasonically peened conditions where residual stresses are present. Experimental Results Residual Stresses. Although the nominal yield stress of the A350 grade plate is 350 MPa the actual yield stress is generally higher, in this case averaging out to about 400 MPa. Ultrasonic peening was highly effective, leading to a residual stress redistribution with a maximum compressive stress of about 250 MPa at the weld toe surface and a maximum tensile stress of 220 MPa, at a depth of almost 3mm into the base plate (refer to Fig. 3). Fig. 3. T-butt weld toe residual stresses through the base plate after ultrasonic peening. Since there are no measurements in the as-welded state, a typical as-welded residual stress distribution from the literature [10] was used instead, as shown in Fig. 4. The sample investigated using neutron diffraction with a 2×2×2mm gauge volume was a T-plate fillet weld, joining two 25mm thick plates. The plate material was BS 7191 grade 355 EMZ structural steel, which is very similar to A350 grade steel, and represents a large group of steels commonly used in the nuclear and offshore industries. SMAW welding with standard filler metal was used. Both welds consisted of four weld passes in an alternating sequence between the two sides. For the purpose of this work, the residual stress distribution at the T-butt weld toe centre line was scaled horizontally to fit a 10mm thick base plate. Geometric, Material and Loading Parameters. Table 1 contains values of the geometric parameters selected for the present analyses. The fatigue threshold, ∆KIth, was taken as 3.2 MPa√m. The Paris Law crack growth coefficient and exponent were C = 8.57 × 10 (SI units) and m = 3, respectively. Applied membrane stress varied from 0 to 100 MPa. The number of crack increments emplo

Fibre placement processes for composites manufacture

Abstract Fibre placement refers to composite fabrication processes that involve the laying down of reinforcing fibres along predefined trajectories in the component. It is also referred to as directed fibre placement and fibre steering. The goal of fibre placement is to maximise the performance of a particular part by utilising the highly directional strength of fibre reinforcement. Fibre placement can be used to improve stiffness and strength of components, to locally reinforce holes and cutouts, or even to produce a part with a complex load–deflection response (e.g. bend–twist coupling). Fibre placement technology tightly integrates design (the definition of the fibre trajectories) with manufacturing technology (the fabrication of those trajectories). Several technologies are addressed in this chapter that can be used with fibre placement techniques, including automated fibre placement, tailored fibre placement, and fibre patch preforming. Analytical and optimisation techniques for defining the fibre trajectories are also considered here. The principal stress and load path trajectory methods are discussed in this chapter, as well as the use of a genetic algorithm.

Atoms to Assemblies: A Physics-Based Hierarchical Modelling Approach for Polymer Composite Components

The development of new composite materials requires analysis and experimentation spanning scales from nanometres to metres, from “atoms to assemblies”. In this paper, concerned primarily with fibre reinforced epoxy composites, a methodology is presented which allows continuum level structural simulation to account for nanoand micro-scale size effects in composites. The novelty of this approach is the modular hierarchical nature of the simulation which ensures computational tractability, regardless of the length scales considered. Linking the nanoscale to the macroscopic scale in a single simulation allows for holistic materials development, including the addition of nanoadditives to polymer resin systems.