Composite services and defect assessment

Citation

(2000), "Composite services and defect assessment", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 5. https://doi.org/10.1108/aeat.2000.12772eac.001

Composite services and defect assessment

Composite services and defect assessment

Keywords Exhibitions, Composites, Defects

The study of damage in composites and the detection of cracks, delaminations, etc., and consequent degradation of properties has been the subject of research and development for some time. A recent exhibition at NEC Birmingham included indications of current developments in the design of composites and detection of defects.

Systems involving laser strain mapping and laser shearography have been produced by laser testing instruments, which are applicable to a wide range of engineering products. The integrity of a part can now be assured so that the measurement of strain does not involve, as previously, time-consuming and error prone, single point surface contact methods. These constraints are overcome with a wholefield optical technique which has been developed at Loughborough University.

The Laser Strain Mapper is a PC-based instrument designed to view an object and produce a complete surface strain map in the time it takes to capture a TV image. It allows the whole surface viewed to be strain measured at one time, without touching the surface and to produce accurate strain maps quickly and economically. Results from the unit appear as full colour strain maps. Object interrogation is in three stages; whole object, selected areas, and zoom optics to highlight target areas.

The system is based on electronic speckle pattern shearing interferometry (ESPI), which uses a laser to illuminate an object. The laser/object relationship generates "fringe patterns" within the Laser Strain Mapper optics, which are processed to yield both qualitative and quantitative data that describe how an object moves or vibrates. Hardware and software processing translate the data to produce a 2D strain map which shows the strain experienced by the object surface during the measurement period.

An application shows the strain in a boron patch bonded over a 2RAM wide slot in an aircraft skin, in which, under tensile load, the strain over the slot is clearly defined. Here, numerical values of the in-plane strain are visualised to present a pseudo 3D "C scan" of the inspected area (Figure 1). The 3D box can be rotated at will to obtain the optimum response. Results can be presented in spreadsheet form if required.

Figure 1 A pseudo 3D "C scan" of the inspected area

Laser shearography is a form of video holography, which provides more than a quarter of a million adjacent, real-time strain gauges on the surface of the structure being imaged. This is performed with little or no surface preparation. Shearography is very sensitive to changes in surface strain due to subsurface flaws and, in order to highlight these flaws, the surface is put under a uniform stress. For large composite bonded constructions such as in aircraft, the stress is generated by the creation of a two-stage partial vacuum at the surface. The equipment is optimised to measure the out-of-plane movement caused by the vacuum, which coincides with the bonded structure's weakest vector. The system captures a shearogram of the surface while a small holding vacuum is applied which locks the vacuum hood to the surface. The image is used as a baseline for subsequent real time images to be compared against, whilst the vacuum pressure differential is increased by 1psi (approx 20 inches of water).

Each placing of the vacuum hood covers about 800cm², with an examination of that area carried out by two separate, consecutive inspections within five seconds. With overlapped placings, a coverage rate of 5 to 10m² per hour can be achieved on a uniform surface. A well-bonded area will exhibit a regular, uniform, wide spaced fringe pattern, as the surface reacts to the applied stress. An unbonded or disbonded core will create a weaker area of the surface. These weak areas affect the strain pattern at the surface and are therefore detected as a characteristic interference fringe set, often referred to as a double bull's eye or butterfly pattern.

Range of ultrasonic testing

From Parametrics NDT comes a wide variety of pulser-receivers, providing a suitable base for a number of ultrasonic test systems. Applications include thickness gauging, flaw detection, and materials characterization. A recent product is the Model 5072PR which is a versatile instrument designed to provide high-energy broadband performance over a range of uses. The pulser-receiver, which is used in conjunction with transducers and an oscilloscope, is the most basic unit in any NDT or research ultrasonic system. The pulser section produces an electrical pulse to excite a piezoelectric transducer, which emits an ultrasonic pulse. In pulse-echo applications, this pulse travels through the test material until it is reflected from an interface back to the transducer. In through-transmission applications, this pulse travels through the test material to a second transducer acting as a receiver. In either case, the transducer converts the pulse into an electrical signal which is then amplified and conditioned by the receiver section and made available for further analysis. Front panel controls permit quick and easy selection of instrument parameters for optimising signal responses for necessary inspection tasks.

The Model 5072PR is designed to provide high energy, high gain performance necessary for low frequency investigation of attenuating materials. It also shows high signal recovery and low noise performance over an extended bandwidth suited for thin materials analysis and high resolution flaw detection. Graphite epoxy composites and fibreglass are among the range of materials suited. This model may be used with a gated peak detector for high speed flaw detection, C-scan imaging or attenuation measurement applications.

Other types of units made by Parametrics include computer controlled ultrasonic pulser-receivers, typical of which is the Model 5800 (Plate 1). This is designed for general purpose ultrasonic testing, with emphasis on computer-based systems for automatic testing, or repetitive manual tasks where operators can work with stored setups. Remote control is accomplished using typically, a PC linked to the Model 5800 either by a serial RS-232 connection or by the GPIB bus (IEEE-488). The-user interface at the controlling computer is the same, whichever link is used. Local control is accomplished by means of a front panel touch keypad. All instruments settings may be viewed locally through an eight line by 40 character LCD display which features adjustable contrast and back lighting. Functions most frequently used have direct access keys; others are menu driven. As with other models, it is well suited for general puspose ultrasonic testing on many types of materials.

Plate 1 The Model 5800 ultrasonic pulser-receiver from Parametrics NDT

The ICAM scanning acoustic microscope produced by Ultrasonic Sciences Ltd is a high speed, fully digital and integrated system designed for rapid NDT inspection of integrated circuit (IC) packages and similar parts. It can be used in development, production and failure analysis situations to give repeatable and detailed information on internal features, showing the presence of manufacturing, processing and in-service defects. ICAM is the product of the company's years of expertise in mechanical, electronic and software design and requires a minimum of operator skill. At the same time, it has the flexibility to offer detailed analysis when the situation demands.

The electronic system incorporates the company's range of ultrasonic, data acquisition and I/O boards, with all parameters controlled digitally on the PC bus. High frequency polymer transducers are supplied with the system and the software is designed for repetitive inspection tasks, with the capability for more advanced and detailed scanning and analysis when required.

A range of products

Notable from NPL was the extensive range of testing services which are available to industry. These include mechanical testing, characterisation of advanced materials, design and modelling, materials degradation in aggressive environments, and adhesives processing and performance. Also featured was the miniaturised thermo-mechanical fatigue testing system, which can determine physical properties of materials, including electrical resistivity, thermal conductivity, and thermal expansion.

Many aspects of composites' performance and design are being studied, a particularly useful service being Composites Design Analysis (CoDA) which is a Windows-based software package which assists in the preliminary stage of engineering design of polymer matrix composites. The CoDA software package consists of four modules which are available individually.

The synthesiser module enables the full three-dimensional mechanical properties of composite materials to be generated taking into account the volume fraction and properties of the reinforcement, its orientation, and the properties of the matrix polymer. Both continuous and discontinuous reinforcement can be analysed, and the materials which can be evaluated vary from reinforced thermoplastics, CSMs, and GMTs to aerospace prepregs. The software also computes the thermal and moisture expansion properties in the two principal directions. The synthesised properties can be used in the CoDA design modules or in other design packages, including finite element analysis (FEA). It is possible to input directly into this materials database measured properties, manufacturer's and textbook values. As these data inputs are frequently incomplete, the synthesiser facility can generate the missing properties.

The lay-up module allows these synthesised materials to be layered in the required manner, with material type, layer thickness, layer orientation and stacking sequence as free choices, including sandwich structures. This module calculates the thermoelastic and strength properties using a laminate analysis incorporating a Tsai-wu failure criterion.

The panel module assists in the design of panel structures, enabling the user to determine the stiffness and strength of circular and rectangular panels, sandwich panels and ribbed plate elements. Loading conditions considered include point, line and pressure. The software can also be used to predict stress and deflection, and to determine critical material properties or geometric parameters. Like the panel design module, the beam module predicts strength, stress levels, stiffness and deflections for generic beam shapes, such as hollow, solid, box and channel designs as well as T and I sections. Loading conditions include flexural, torsion and mixed loading. Strength predictions include assessment of the relative likelihood of failure occurring by buckling or material fracture under banding and shear stresses.

Recent composite performance and design activities at NPL have also included several aspects of sandwich structures. These include improved data analysis procedures for the climbing drum test, the validation of a multiple beam test for flexure and shear stiffness properties, and the development of sandwich test methods for thick skinned sandwich laminates (Figure 2). The latter work is being undertaken in close collaboration with Liverpool University with a spin-off programme at Liverpool John Moores University on creep performance of the same sandwich panels.

Figure 2 Modified three-point beam specimen for thick skinned sandwich laminates

In another area, and based on the pin bearing test previously developed, further work has studied the effect of multiple bolt arrays in composite bolted joint design. In particular, the calculation of bearing and by-pass stresses in order to predict the failure mode and load, has been studied. The work was validated for four types of composites to illustrate the generality of the approach. These were carbon-fibre epoxy, pultrusion, woven glass-fibre epoxy, and glass mat thermoplastic. Good agreement was obtained between predicted and measured results, with the procedures developed being prepared as an additional module within CoDA (preliminary design software). The original pin-bearing test has been submitted to ISO for standardisation and precision data obtained in a UK round-robin organised by NPL.

Also studied by NPL is the surface roughness of a range of representative fibres used as reinforcements in composite material. This illustrates the capabilities and limitations of atomic force microscopy. In a composite, the development of a stable bond between the resin matrix and the reinforcing fibre depends on the efficiency of adhesion at the interface. Mechanical interaction associated with the surface roughness is one of the mechanisms potentially involved, although the effect has not been quantified at this stage. The study has used atomic force microscopy to analyse three-dimensional surface features in several fields with very high spatial resolution in a sub micron size range. The images and data can be used for textural quality assessment. Standard roughness values were obtained indicating the difference between untreated and treated carbon fibres.