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Forceful measures
Forceful measures Extensive testing of the mechanical properties of composite materials is required to qualify them for various applications. George Marsh reviews some of the methods available to determine properties such as strength, impact resistance and fatigue behaviour.
A
test laboratory for composites is a torture chamber. Its various frames, columns, crossheads and other structures are for ‘torturing’ materials, sometimes to the point of failure, in a controlled and measured way so that their mechanical properties and failure modes can be established. Using force to bend, pull, push, squash, hit, tear and shear material specimens, while accurately recording the reactions to the forces applied, is a familiar test discipline. But composites, as complex dual-phase entities, pose particular test challenges. Different forms of strength depend on which material phase is dominant. Tensile strength is fibre-dominated while compressive strength depends more on the resin. Failure modes range from ductile, where properties are resin-dominated, to brittle, where fibre volume fractions are high. In theory, an enormous amount of testing would be needed to cover a spectrum of fibre/resin combination possibilities (especially when processing and application variables are also taken into account), but in practice computer modelling can provide interpolation. Because composites are non-homogenous and often have variation between specimens, test results may exhibit high scatter, making statistical techniques important in designing test regimes and interpreting data. More complexity arises from the possibility that the material phases can become separated. Shear strength is a function of resin properties and its adhesion to the reinforcing fibres. Inplane shear loads are used to test the
36
REINFORCEDplastics
April 2004
fibre-matrix bond, while interlaminar shear loading tests the bond strength between layers. Resistance to impact (from falling objects, stones and debris, tools, bird strikes etc) can be crucial, requiring tests to establish failure modes and the energy needed to fracture a material. Particularly complex are sandwich structures, for which bend testing applies tensile, compression and shear loads to different portions of the laminate, while peel testing is used to quantify core-to-face bond strength.
Composites are non-homogenous and often have variation between specimens, making statistical techniques important. Extra complication arises where holes are present, typically for fasteners, raising a need for open and closed hole testing. Prolonged test programmes are needed to establish time-dependent properties such as fatigue behaviour and tolerance to heat, moisture and other environmental factors.
Static tests A common denominator in composites evaluation is static testing of material coupon specimens. A typical static coupon test machine, familiar in any test lab, comprises a vertical metal frame within which a crosshead moves up and down under electromechanical or servo-
controlled hydraulic pressure. Alternatively, a single column can accommodate the vertical movement of a loadapplying carriage. The crosshead or carriage applies tensile or compressive load to a sample held between itself and the machine base. Load is transferred to the test specimen via grips or, given an appropriate fixture, as a bending force in a three or four-point bend test. A load cell monitors the force applied while contact or non-contact (optical, laser, video) extensometers measure the resulting sample elongation. In the case of compression testing, axial extensometers can be used to measure sample shortening. Specimens are prepared for testing according to specifications from such bodies as ASTM, CEN, ISO, national standards organisations and original equipment manufacturers (OEMs) such as Boeing and Airbus. Designing grips to successfully hold plastics under stress is an art in itself. Basic machine configuration has changed little over the years, but precision engineering allied with computer control has made today’s machines more accurate, easier to use, and faster and better at recording and managing data than their predecessors. Customer requirements include safety (in particular guarding against possible damage caused by sample disintegration), a wide range of accessories and, especially in production quality control applications, rapid, automated operation. For cost-effectiveness, many users favour the multi-mode, multiple test capability offered by so-called ‘universal’ testing machines. Machine capability is specified in terms of the force that can be 0034-3617/04 ©2004 Elsevier Ltd. All rights reserved.
Forceful measures
applied, frame/column stiffness, the positional movement of which the crosshead is capable, movement speed, test stroke and working area, plus data sampling rate and accuracy. Under software control, today’s machines can automatically apply specified loads at given rates, run fatigue and other prolonged testing, and carry out complex test programmes such as cycling to load and extension limits, and relaxation testing. Rapidly analysing their recorded data statistically, advanced digital controllers can revise their own programs (for instance implementing further application of particular tests where necessary) or contribute directly to statistical process control for quality purposes.
What’s on offer One manufacturer offering a comprehensive range of universal testing machines able to carry out tensile, compression, bending and other tests, is the Zwick Roell Group. This German-headquartered company has produced load frame machines capable of exerting up to 6000 kN of force, though according to Alan Thomas, sales manager of the Zwick UK subsidiary, most customers wanting to test composites require force in the 50-100 kN range. Zwick has supplied electromechanical and servo-hydraulic machines to such leading names as Hexcel, Cytec, Advanced Composites Group, BAE Systems and the Composites Testing Laboratory in Ireland. Standard Zwick/Roell testers range from light bench-top models to large, floor-standing units supplied with various frames and drives along with a wide selection of accessories and versatile control units. Machines can exert up to 100 kN force (bench-top) and 250 kN (floor-standing) and may have two or four support/guide columns depending on the machine's force range. Precision ball-screw drives for the crosshead are operated by a DC motor for the 100 kN models, or an AC motor for 250 kN models. A ring-torsion strain gauge load cell measures the force applied, a digital distance transducer records the
Lloyd Instruments’ EZ50 50kN materials testing machine performing a tensile test on a composite sample. (Picture courtesy of Lloyd Instruments.)
machine’s stroke and analogue or digital extensometry is used to measure material elongation under load. A width reduction monitor and other measurement systems can be added. Provision can be included for subjecting the sample to high and low temperatures, for measuring torque and for multi-channel force measurement. Servo-hydraulic testers are favoured for more dynamic test purposes,
including cyclic tensile, compression and flexural tests with complex dynamic stressing. They feature stiff, resonancefree frames, with a choice of hydraulic power packs depending on the amplitude and frequency of force application required. Alan Thomas says that Zwick has developed several solutions to the problem of gripping composite specimens (whether coupons or small April 2004
REINFORCEDplastics
37
Forceful measures
Impact resistance Impact resistance of a composite material is one of the most important properties for a design engineer to consider. In real life, materials often absorb applied forces very quickly. Depending on the application, these could be falling objects, blows, collisions, drops, etc. A material is also more likely to fail when it is subjected to an impact blow in comparison to the same force being applied more slowly. The designer must determine: • the impact energies that the composite material or structure is expected to see in its lifetime; • the type of impact that will deliver that energy; and then • create a material that will resist such assaults over the projected life span. Impact testing is intended to simulate the impact conditions that a material or structure is expected to withstand in real life, whether it be a bird strike on an airplane or a collision between two ships. In measuring the energy required to break a test specimen, the test engineer must first determine appropriate parameters (such as impact velocity, energy and geometry, as well as temperature variations). (Source: Instron.)
components) effectively throughout a test programme. Hydraulic and pneumatic devices maintain a constant gripping force, which screw wedge-type grips cannot always achieve. This gives the best chance of avoiding physical damage to the specimen. Insert surfaces are spark-eroded to provide an optimum gripping interface. Test machines from competitor Instron, an essentially global company with its European headquarters in Britain, range from ‘simple’ mechanical systems through electromechanical universal test systems to upper-end servohydraulic machines. The 3300 Series, for example, encompasses affordable benchtop models with single columns providing load capacities of 0.5-5 kN and twin
38
REINFORCEDplastics
April 2004
columns for capacities of 5-50 kN. A high-torque closed loop-controlled DC motor drives the carriage/crosshead via pre-loaded ball screws. The system is compatible with existing Instron grips, fittings and extensometers. Measurement accuracies for load and strain are typically ±0.5% of scale reading, and crosshead speed accuracy is ±0.2% of the set speed. Also electromechanically driven are the 5000 Series, giving a 5 kHz data sampling rate, an open IEEE interface and a LabVIEW™ driver option for user programmability, and the 5500 Series for high-force applications. Most sophisticated of Instron’s testers are the latest SATEC™ universal hydraulic testing systems intended for high-capacity static tensile, compression, bend and shear testing with forces available up to 150 kN, 300 kN or 600 kN depending on the model. Designers have minimised demands on lab floor space by combining the load frame, hydraulic power supply, electronics and control unit into a single relatively compact package. In addition, Instron has launched a family of portable mechanical testers. The In-Spec 2200 Series is suitable for carrying out tension, compression and peel testing in the field, away from the laboratory and AC power. One model is hand-held, and the other is bench-top mounted.
Software savvy Modern systems for testing rigid plastics owe much of their growing capability to software and there has been a swing away from bespoke digital controllers to PC/Windows-based systems. A good example is the digital controller with a Windows 95/98/NT operating system that Zwick offers with its servo-hydraulic testers. Configurable as a multi-channel installation, the system samples up to eight channels of data synchronously at up to 10 kHz. Signals entering the system are converted to a standard digital format, using analogueto-digital converters where necessary. Powerful 32-bit processors incorporate
digital signal processing algorithms to facilitate direct digital control under which a wide range of accurate ramps speeds can be applied in mixed-mode options, cyclic generation or mean and amplitude formats. Adaptive gain caters for non-linear specimen transfer characteristics. Statistical analysis options provide static, flexural and compression test possibilities, and fracture mechanics. Measurement electronics used with a range of force transducers designed for load cell use provide automatic reading of setting and calibration parameters, zero point and sensitivity compensation, temperature stabilisation, plus high measurement accuracy and resolution. Clip-on extensometers can measure elongation at specific force levels to an accuracy of 1% and resolution of 0.01 m and can establish yield points and modulus. Capabilities like these can be added to older machines under a Zwick upgrade scheme. Modernisation packages can include the latest PC-based software, as well as an exchange drive system, control and measurement electronics, and sensor calibration. In line with EC guideline 89/655/EEC, modernisation includes a review of safety protection and emergency-stop provision, which are then upgraded as necessary. Other manufacturers offer similar upgrades for their machine ranges. Another example of contemporary computer data acquisition and management possibilities is provided by the Benzwin 2000 software offered by US company Benz Material Testing Instruments with its TT2100-2300 tensile testers. This PC/Windows 2000based package facilitates the display in ‘real time’ of force and elongation on a computer monitor, so that users can watch the values change as load is progressively applied. Test curve displays and statistical analysis summaries are presented. The system saves time by storing test data and holding it for comparison and model building so that stress and strain analysis requires minimal
Forceful measures
repeat testing and calculation. Used with this software, Benz machines cater for a wide range of tensile, elongation, compression, shear, peel, flexural and proof of load testing. A useful way to gain insight into the behaviour of reinforced plastics under strain is to apply acoustic monitoring, an established technique requiring digital signal processing and analysis. According to Chris Keeping, a product specialist with UK’s Lloyd Instruments, it is difficult to determine safe working loads for composites from normal load/extension graphs, but acoustic emission testing can accomplish this most effectively, by ‘hearing’ when fibres start to break under increasing load. The load at which acoustic activity rises rapidly is that at which physical damage has started to occur, so safe working load can then be taken as a percentage of this level. The technique can also be used to establish the maximum bending or flexure a material can accept before the tensioned outer fibres start to fracture. The system relies on ‘ultrasonic microphone’ transducers attached to the test specimen or to the grips holding the specimen. Lloyd Instruments has supplied AE100 acoustic emission monitors to users ranging from universities and test houses to Formula 1 race car teams. The company’s latest NEXYGEN™ MT instrument control software offers a video capture facility for recording live tests for analysis, training and other purposes. Video windows can be displayed with graphs and numerical results. During playback, a synchronised marker moves along the test trace and the video can be paused at any time for further analysis. Users can view the video image at any part of the trace by moving the marker with a mouse. Recording can be synchronised for all test types including multi-stage, multispeed and cycling. Normal PC limits on the amount of video content that can be stored may be extended by using an image compression facility. Imagery,
compressed or otherwise, can be exported by e-mail. NEXYGEN MT is the company’s primary software package offered with its latest Chatillon® and Davenport™ test machines. Like other manufacturers’ test software, Instron’s Series IX/s package, supplied with 3300 Series test systems, provides easy set-up and use, with automatic recognition and calibration of load and strain transducers. Users can set up test routines and allocate parameters, save measurements and retrieve analyses, histories and test data. High-end software, such as the same company’s Merlin system, can take account of standardised test methods such as the ASTM and EN ISO tests that major measurement organisations such as the UK’s NPL encourage as a means of standardising testing practice.
Impact Resistance to impact can be tested by the direct means of dropping weights onto a sample or swinging a pendulum against it. Test equipment specialists like Zwick, Instron and Lloyds Instruments cater for both these techniques. The pendulum method offers fine control of impact velocity and energy, both through the selection of appropriate pendulum length and by setting the swing displacement. Zwick, for instance, says that instruments within its 5100 Series are highly suitable for evaluating the impact strength of a wide range of thermosets and thermoplastics reinforced with unidirectional, random or multiaxial single or hybrid fibre forms. Tests can be run according to industry-standard Izod and Charpy methods, the latter being considered especially suitable for materials likely to exhibit interlaminar shear fracture. Linking machines with a PC and testXpert® software provides the flexibility to create comprehensive test protocols, statistics and graphics as well as to store data in a range of formats. Alternatively, drop weight testers (also falling dart testers) may be used to
determine failure characteristics, damaging energy levels, buckling depth and dynamic cushioning performance to established test standards. Impact energy and velocity are set by appropriate selection of the drop weight and fall distance. Machines in the Zwick/Amsler range, to take a representative example, utilise a drop weight which is raised to the required height on a crosshead riding inside a frame. Included with each machine are an electrically interlocked safety cage and a remote crosshead release system. Testers can be supplied without instrumentation, or complete with force, acceleration and displacement sensors along with a PC-based data acquisition system able to run ImpactWin® software. Establishing residual strength after impact, important since damage in composites is often internal and not obvious, requires the combined use of impact testers and universal test machines.
Qualification Qualification of composites, particularly for high-integrity aerospace, military and other applications, will continue to require extensive testing to establish basic mechanical properties. The growing use of composites in high-specification applications suggests bright prospects for the mechanical test equipment market of the future. Particular winners will be those manufacturers offering products that can speed up testing, improve accuracy and tracability, simplify operation and, above all, contribute to the cost-effectiveness that is so vital to the composites sector. ■
Benz Materials Testing Instruments; website: www.benztesters.com; Instron; website: www.instron.com; Lloyd Instruments; website: www.lloydinstruments.co.uk; Zwick Roell; website: www.zwick.com.
April 2004
REINFORCEDplastics
39
A
test laboratory for composites is a torture chamber. Its various frames, columns, crossheads and other structures are for ‘torturing’ materials, sometimes to the point of failure, in a controlled and measured way so that their mechanical properties and failure modes can be established. Using force to bend, pull, push, squash, hit, tear and shear material specimens, while accurately recording the reactions to the forces applied, is a familiar test discipline. But composites, as complex dual-phase entities, pose particular test challenges. Different forms of strength depend on which material phase is dominant. Tensile strength is fibre-dominated while compressive strength depends more on the resin. Failure modes range from ductile, where properties are resin-dominated, to brittle, where fibre volume fractions are high. In theory, an enormous amount of testing would be needed to cover a spectrum of fibre/resin combination possibilities (especially when processing and application variables are also taken into account), but in practice computer modelling can provide interpolation. Because composites are non-homogenous and often have variation between specimens, test results may exhibit high scatter, making statistical techniques important in designing test regimes and interpreting data. More complexity arises from the possibility that the material phases can become separated. Shear strength is a function of resin properties and its adhesion to the reinforcing fibres. Inplane shear loads are used to test the
36
REINFORCEDplastics
April 2004
fibre-matrix bond, while interlaminar shear loading tests the bond strength between layers. Resistance to impact (from falling objects, stones and debris, tools, bird strikes etc) can be crucial, requiring tests to establish failure modes and the energy needed to fracture a material. Particularly complex are sandwich structures, for which bend testing applies tensile, compression and shear loads to different portions of the laminate, while peel testing is used to quantify core-to-face bond strength.
Composites are non-homogenous and often have variation between specimens, making statistical techniques important. Extra complication arises where holes are present, typically for fasteners, raising a need for open and closed hole testing. Prolonged test programmes are needed to establish time-dependent properties such as fatigue behaviour and tolerance to heat, moisture and other environmental factors.
Static tests A common denominator in composites evaluation is static testing of material coupon specimens. A typical static coupon test machine, familiar in any test lab, comprises a vertical metal frame within which a crosshead moves up and down under electromechanical or servo-
controlled hydraulic pressure. Alternatively, a single column can accommodate the vertical movement of a loadapplying carriage. The crosshead or carriage applies tensile or compressive load to a sample held between itself and the machine base. Load is transferred to the test specimen via grips or, given an appropriate fixture, as a bending force in a three or four-point bend test. A load cell monitors the force applied while contact or non-contact (optical, laser, video) extensometers measure the resulting sample elongation. In the case of compression testing, axial extensometers can be used to measure sample shortening. Specimens are prepared for testing according to specifications from such bodies as ASTM, CEN, ISO, national standards organisations and original equipment manufacturers (OEMs) such as Boeing and Airbus. Designing grips to successfully hold plastics under stress is an art in itself. Basic machine configuration has changed little over the years, but precision engineering allied with computer control has made today’s machines more accurate, easier to use, and faster and better at recording and managing data than their predecessors. Customer requirements include safety (in particular guarding against possible damage caused by sample disintegration), a wide range of accessories and, especially in production quality control applications, rapid, automated operation. For cost-effectiveness, many users favour the multi-mode, multiple test capability offered by so-called ‘universal’ testing machines. Machine capability is specified in terms of the force that can be 0034-3617/04 ©2004 Elsevier Ltd. All rights reserved.
Forceful measures
applied, frame/column stiffness, the positional movement of which the crosshead is capable, movement speed, test stroke and working area, plus data sampling rate and accuracy. Under software control, today’s machines can automatically apply specified loads at given rates, run fatigue and other prolonged testing, and carry out complex test programmes such as cycling to load and extension limits, and relaxation testing. Rapidly analysing their recorded data statistically, advanced digital controllers can revise their own programs (for instance implementing further application of particular tests where necessary) or contribute directly to statistical process control for quality purposes.
What’s on offer One manufacturer offering a comprehensive range of universal testing machines able to carry out tensile, compression, bending and other tests, is the Zwick Roell Group. This German-headquartered company has produced load frame machines capable of exerting up to 6000 kN of force, though according to Alan Thomas, sales manager of the Zwick UK subsidiary, most customers wanting to test composites require force in the 50-100 kN range. Zwick has supplied electromechanical and servo-hydraulic machines to such leading names as Hexcel, Cytec, Advanced Composites Group, BAE Systems and the Composites Testing Laboratory in Ireland. Standard Zwick/Roell testers range from light bench-top models to large, floor-standing units supplied with various frames and drives along with a wide selection of accessories and versatile control units. Machines can exert up to 100 kN force (bench-top) and 250 kN (floor-standing) and may have two or four support/guide columns depending on the machine's force range. Precision ball-screw drives for the crosshead are operated by a DC motor for the 100 kN models, or an AC motor for 250 kN models. A ring-torsion strain gauge load cell measures the force applied, a digital distance transducer records the
Lloyd Instruments’ EZ50 50kN materials testing machine performing a tensile test on a composite sample. (Picture courtesy of Lloyd Instruments.)
machine’s stroke and analogue or digital extensometry is used to measure material elongation under load. A width reduction monitor and other measurement systems can be added. Provision can be included for subjecting the sample to high and low temperatures, for measuring torque and for multi-channel force measurement. Servo-hydraulic testers are favoured for more dynamic test purposes,
including cyclic tensile, compression and flexural tests with complex dynamic stressing. They feature stiff, resonancefree frames, with a choice of hydraulic power packs depending on the amplitude and frequency of force application required. Alan Thomas says that Zwick has developed several solutions to the problem of gripping composite specimens (whether coupons or small April 2004
REINFORCEDplastics
37
Forceful measures
Impact resistance Impact resistance of a composite material is one of the most important properties for a design engineer to consider. In real life, materials often absorb applied forces very quickly. Depending on the application, these could be falling objects, blows, collisions, drops, etc. A material is also more likely to fail when it is subjected to an impact blow in comparison to the same force being applied more slowly. The designer must determine: • the impact energies that the composite material or structure is expected to see in its lifetime; • the type of impact that will deliver that energy; and then • create a material that will resist such assaults over the projected life span. Impact testing is intended to simulate the impact conditions that a material or structure is expected to withstand in real life, whether it be a bird strike on an airplane or a collision between two ships. In measuring the energy required to break a test specimen, the test engineer must first determine appropriate parameters (such as impact velocity, energy and geometry, as well as temperature variations). (Source: Instron.)
components) effectively throughout a test programme. Hydraulic and pneumatic devices maintain a constant gripping force, which screw wedge-type grips cannot always achieve. This gives the best chance of avoiding physical damage to the specimen. Insert surfaces are spark-eroded to provide an optimum gripping interface. Test machines from competitor Instron, an essentially global company with its European headquarters in Britain, range from ‘simple’ mechanical systems through electromechanical universal test systems to upper-end servohydraulic machines. The 3300 Series, for example, encompasses affordable benchtop models with single columns providing load capacities of 0.5-5 kN and twin
38
REINFORCEDplastics
April 2004
columns for capacities of 5-50 kN. A high-torque closed loop-controlled DC motor drives the carriage/crosshead via pre-loaded ball screws. The system is compatible with existing Instron grips, fittings and extensometers. Measurement accuracies for load and strain are typically ±0.5% of scale reading, and crosshead speed accuracy is ±0.2% of the set speed. Also electromechanically driven are the 5000 Series, giving a 5 kHz data sampling rate, an open IEEE interface and a LabVIEW™ driver option for user programmability, and the 5500 Series for high-force applications. Most sophisticated of Instron’s testers are the latest SATEC™ universal hydraulic testing systems intended for high-capacity static tensile, compression, bend and shear testing with forces available up to 150 kN, 300 kN or 600 kN depending on the model. Designers have minimised demands on lab floor space by combining the load frame, hydraulic power supply, electronics and control unit into a single relatively compact package. In addition, Instron has launched a family of portable mechanical testers. The In-Spec 2200 Series is suitable for carrying out tension, compression and peel testing in the field, away from the laboratory and AC power. One model is hand-held, and the other is bench-top mounted.
Software savvy Modern systems for testing rigid plastics owe much of their growing capability to software and there has been a swing away from bespoke digital controllers to PC/Windows-based systems. A good example is the digital controller with a Windows 95/98/NT operating system that Zwick offers with its servo-hydraulic testers. Configurable as a multi-channel installation, the system samples up to eight channels of data synchronously at up to 10 kHz. Signals entering the system are converted to a standard digital format, using analogueto-digital converters where necessary. Powerful 32-bit processors incorporate
digital signal processing algorithms to facilitate direct digital control under which a wide range of accurate ramps speeds can be applied in mixed-mode options, cyclic generation or mean and amplitude formats. Adaptive gain caters for non-linear specimen transfer characteristics. Statistical analysis options provide static, flexural and compression test possibilities, and fracture mechanics. Measurement electronics used with a range of force transducers designed for load cell use provide automatic reading of setting and calibration parameters, zero point and sensitivity compensation, temperature stabilisation, plus high measurement accuracy and resolution. Clip-on extensometers can measure elongation at specific force levels to an accuracy of 1% and resolution of 0.01 m and can establish yield points and modulus. Capabilities like these can be added to older machines under a Zwick upgrade scheme. Modernisation packages can include the latest PC-based software, as well as an exchange drive system, control and measurement electronics, and sensor calibration. In line with EC guideline 89/655/EEC, modernisation includes a review of safety protection and emergency-stop provision, which are then upgraded as necessary. Other manufacturers offer similar upgrades for their machine ranges. Another example of contemporary computer data acquisition and management possibilities is provided by the Benzwin 2000 software offered by US company Benz Material Testing Instruments with its TT2100-2300 tensile testers. This PC/Windows 2000based package facilitates the display in ‘real time’ of force and elongation on a computer monitor, so that users can watch the values change as load is progressively applied. Test curve displays and statistical analysis summaries are presented. The system saves time by storing test data and holding it for comparison and model building so that stress and strain analysis requires minimal
Forceful measures
repeat testing and calculation. Used with this software, Benz machines cater for a wide range of tensile, elongation, compression, shear, peel, flexural and proof of load testing. A useful way to gain insight into the behaviour of reinforced plastics under strain is to apply acoustic monitoring, an established technique requiring digital signal processing and analysis. According to Chris Keeping, a product specialist with UK’s Lloyd Instruments, it is difficult to determine safe working loads for composites from normal load/extension graphs, but acoustic emission testing can accomplish this most effectively, by ‘hearing’ when fibres start to break under increasing load. The load at which acoustic activity rises rapidly is that at which physical damage has started to occur, so safe working load can then be taken as a percentage of this level. The technique can also be used to establish the maximum bending or flexure a material can accept before the tensioned outer fibres start to fracture. The system relies on ‘ultrasonic microphone’ transducers attached to the test specimen or to the grips holding the specimen. Lloyd Instruments has supplied AE100 acoustic emission monitors to users ranging from universities and test houses to Formula 1 race car teams. The company’s latest NEXYGEN™ MT instrument control software offers a video capture facility for recording live tests for analysis, training and other purposes. Video windows can be displayed with graphs and numerical results. During playback, a synchronised marker moves along the test trace and the video can be paused at any time for further analysis. Users can view the video image at any part of the trace by moving the marker with a mouse. Recording can be synchronised for all test types including multi-stage, multispeed and cycling. Normal PC limits on the amount of video content that can be stored may be extended by using an image compression facility. Imagery,
compressed or otherwise, can be exported by e-mail. NEXYGEN MT is the company’s primary software package offered with its latest Chatillon® and Davenport™ test machines. Like other manufacturers’ test software, Instron’s Series IX/s package, supplied with 3300 Series test systems, provides easy set-up and use, with automatic recognition and calibration of load and strain transducers. Users can set up test routines and allocate parameters, save measurements and retrieve analyses, histories and test data. High-end software, such as the same company’s Merlin system, can take account of standardised test methods such as the ASTM and EN ISO tests that major measurement organisations such as the UK’s NPL encourage as a means of standardising testing practice.
Impact Resistance to impact can be tested by the direct means of dropping weights onto a sample or swinging a pendulum against it. Test equipment specialists like Zwick, Instron and Lloyds Instruments cater for both these techniques. The pendulum method offers fine control of impact velocity and energy, both through the selection of appropriate pendulum length and by setting the swing displacement. Zwick, for instance, says that instruments within its 5100 Series are highly suitable for evaluating the impact strength of a wide range of thermosets and thermoplastics reinforced with unidirectional, random or multiaxial single or hybrid fibre forms. Tests can be run according to industry-standard Izod and Charpy methods, the latter being considered especially suitable for materials likely to exhibit interlaminar shear fracture. Linking machines with a PC and testXpert® software provides the flexibility to create comprehensive test protocols, statistics and graphics as well as to store data in a range of formats. Alternatively, drop weight testers (also falling dart testers) may be used to
determine failure characteristics, damaging energy levels, buckling depth and dynamic cushioning performance to established test standards. Impact energy and velocity are set by appropriate selection of the drop weight and fall distance. Machines in the Zwick/Amsler range, to take a representative example, utilise a drop weight which is raised to the required height on a crosshead riding inside a frame. Included with each machine are an electrically interlocked safety cage and a remote crosshead release system. Testers can be supplied without instrumentation, or complete with force, acceleration and displacement sensors along with a PC-based data acquisition system able to run ImpactWin® software. Establishing residual strength after impact, important since damage in composites is often internal and not obvious, requires the combined use of impact testers and universal test machines.
Qualification Qualification of composites, particularly for high-integrity aerospace, military and other applications, will continue to require extensive testing to establish basic mechanical properties. The growing use of composites in high-specification applications suggests bright prospects for the mechanical test equipment market of the future. Particular winners will be those manufacturers offering products that can speed up testing, improve accuracy and tracability, simplify operation and, above all, contribute to the cost-effectiveness that is so vital to the composites sector. ■
Benz Materials Testing Instruments; website: www.benztesters.com; Instron; website: www.instron.com; Lloyd Instruments; website: www.lloydinstruments.co.uk; Zwick Roell; website: www.zwick.com.
April 2004
REINFORCEDplastics
39