7 Mechanical Property Measurement 7.1 Introduction Mechanical testing of materials and structural details is conducted to satisfy one or more of the following objectives:1)characterization of materials or processes,2)development of design allowables,3)qualification of materials for certain applications,4)quality control,5)assessment of strength and durability under sustained or cyclic loads,or 6)assessment of the influence of damage or degradation on residual strength. Aerospace wrought metal alloys are available in standard pre-fabricated forms with well-characterized properties.By contrast,composites are usually formed at the same time as the component is manufactured and therefore can have a very wide range of properties depending on the fiber,resin,lay-up,volume fraction, etc.Some properties of composites are more sensitive to environmental conditions.Thus,testing requirements are generally more demanding than is the case for metals. The use of mechanical testing for developing design allowables for composites is described in Chapter 13,and its use in the testing of adhesively bonded or mechanically fastened joints is described in Chapter 9. 7.1.1 Types of Mechanical Tests Most tests are conducted under static tensile,compressive,or shear loading. They may also be conducted under flexural loading,which induce tensile, compressive,and shear stresses in the various zones of the specimen.The static loading may be of short duration,taking only a few minutes,as in a standard tensile test to measure strength or stiffness. Static tests,most generally performed under tensile loading,may be also be prolonged for weeks or months,as in a creep test,to measure the long-term strength and strain stability-often at elevated temperature.These tests are usually conducted at various percentages of the short-term ultimate tensile strength,typically 10-50%. Cyclic loading tests to measure resistance to degradation and cracking under varying loads are essentially repeated static loading.The frequency of application is generally low,in the case of composites around 5-10 Hz,to avoid heating.Loading may be tension/tension,tension/compression,or reversed 213
7 Mechanical Property Measurement 7.1 Introduction Mechanical testing of materials and structural details is conducted to satisfy one or more of the following objectives: 1) characterization of materials or processes, 2) development of design allowables, 3) qualification of materials for certain applications, 4) quality control, 5) assessment of strength and durability under sustained or cyclic loads, or 6) assessment of the influence of damage or degradation on residual strength. Aerospace wrought metal alloys are available in standard pre-fabricated forms with well-characterized properties. By contrast, composites are usually formed at the same time as the component is manufactured and therefore can have a very wide range of properties depending on the fiber, resin, lay-up, volume fraction, etc. Some properties of composites are more sensitive to environmental conditions. Thus, testing requirements are generally more demanding than is the case for metals. The use of mechanical testing for developing design allowables for composites is described in Chapter 13, and its use in the testing of adhesively bonded or mechanically fastened joints is described in Chapter 9. 7.1.1 Types of Mechanical Tests Most tests are conducted under static tensile, compressive, or shear loading. They may also be conducted under flexural loading, which induce tensile, compressive, and shear stresses in the various zones of the specimen. The static loading may be of short duration, taking only a few minutes, as in a standard tensile test to measure strength or stiffness. Static tests, most generally performed under tensile loading, may be also be prolonged for weeks or months, as in a creep test, to measure the long-term strength and strain stability-----often at elevated temperature. These tests are usually conducted at various percentages of the short-term ultimate tensile strength, typically 10-50%. Cyclic loading tests to measure resistance to degradation and cracking under varying loads are essentially repeated static loading. 1 The frequency of application is generally low, in the case of composites around 5-10 Hz, to avoid heating. Loading may be tension/tension, tension/compression, or reversed 213
214 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES shear at constant amplitude or under spectrum loading,and may be aimed at simulating the actual loading conditions in a particular application. Dynamic loads are used to measure the resistance of the materials to impact or ballistic conditions.These tests are also conducted under tension,compression, shear,or flexure,or they may be conducted using an impactor or penetrator of some type.In some tests,the impact event may occur while the specimen is under tensile or compressive loading.Typically,loading occurs over a 1-millisecond time interval. Testing may be conducted at different temperatures and levels of absorbed moisture.They may also include exposure to a range of other environmental conditions,such as UV and solvents. The specimens may be simple coupons or they may be structural details with representative stress-raisers such as holes,filled with a fastener or open.The coupons or details may include representative damage such as sharp notches or impact damage. Test machines consist of loading frames,one fixed and one moving crosshead, separated either by a simple electromechanical screw action or by a servo- hydraulic piston.For simple static testing,the screw-driven machines are simpler and less costly and there is less danger of overload caused by accidental rapid movement of the crosshead.However,for versatility in loading (e.g.spectrum loading in fatigue testing)and in load capacity,the servo-hydraulic machines are unmatched. 7.1.2 Special Requirements for Testing Composites During the early development of composites,many of the test techniques used for metals and other homogenous,isotropic materials were used to determine the properties of composite materials.It was soon recognized that anisotropic composite materials often required special consideration in terms of mechanical property determination.Much of the test method development was also undertaken within individual organizations,thus standardization was difficult and many methods developed were not adequate for the newer,emerging materials. Since those early days,there has been a great deal of effort devoted to the standardization of test methods,and there are a number of reference sources that can be used to identify the relevant techniques.Test standards have been published by the American Society for Testing and Materials(ASTM)2-and the Suppliers of Advanced Composite Materials Association (SACMA),10.11 together with other information sources such as the U.S.Department of Defense Military Handbook 17 (MIL-HDBK 17;Polymer Matrix Composites).MIL- HDBK 17 is specifically suited to composite materials for aerospace applications and is generally used for test method selection. The test techniques briefly described here are the ones most commonly used when measuring stress and strain in the tensile,compressive,flexural,and shear load states,but they are not the only techniques that can be used.The most critical
214 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES shear at constant amplitude or under spectrum loading, and may be aimed at simulating the actual loading conditions in a particular application. Dynamic loads are used to measure the resistance of the materials to impact or ballistic conditions. These tests are also conducted under tension, compression, shear, or flexure, or they may be conducted using an impactor or penetrator of some type. In some tests, the impact event may occur while the specimen is under tensile or compressive loading. Typically, loading occurs over a 1-millisecond time interval. Testing may be conducted at different temperatures and levels of absorbed moisture. They may also include exposure to a range of other environmental conditions, such as UV and solvents. The specimens may be simple coupons or they may be structural details with representative stress-raisers such as holes, filled with a fastener or open. The coupons or details may include representative damage such as sharp notches or impact damage. Test machines consist of loading frames, one fixed and one moving crosshead, separated either by a simple electromechanical screw action or by a servohydraulic piston. For simple static testing, the screw-driven machines are simpler and less costly and there is less danger of overload caused by accidental rapid movement of the crosshead. However, for versatility in loading (e.g. spectrum loading in fatigue testing) and in load capacity, the servo-hydraulic machines are unmatched. 7.1.2 Special Requirements for Testing Composites During the early development of composites, many of the test techniques used for metals and other homogenous, isotropic materials were used to determine the properties of composite materials. It was soon recognized that anisotropic composite materials often required special consideration in terms of mechanical property determination. Much of the test method development was also undertaken within individual organizations, thus standardization was difficult and many methods developed were not adequate for the newer, emerging materials. Since those early days, there has been a great deal of effort devoted to the standardization of test methods, and there are a number of reference sources that can be used to identify the relevant techniques. Test standards have been published by the American Society for Testing and Materials (ASTM) 2-9 and the Suppliers of Advanced Composite Materials Association (SACMA), 1°'ix together with other information sources such as the U.S. Department of Defense Military Handbook 17 (MIL-HDBK 17; Polymer Matrix Composites). MILHDBK 17 is specifically suited to composite materials for aerospace applications and is generally used for test method selection. The test techniques briefly described here are the ones most commonly used when measuring stress and strain in the tensile, compressive, flexural, and shear load states, but they are not the only techniques that can be used. The most critical
MECHANICAL PROPERTY MEASUREMENT 215 issues that must be satisfied are that the test method used accurately creates the required stress state in the material and that the specimen failure be consistent with this stress state and not be artificially influenced by the test method. Because of the variabilties encountered in coupon testing,airworthiness authorities require multiple tests across several batches.MIL-HDBK 17 recommends a minimum of six specimens per test point and five batches of material to be tested.These requirements mean that the exploration of even a minimum of material properties entails a very large number of test specimens. When conducting tests to determine the strength and stiffness of a composite material,the first question that must be answered is"What mode of its performance is to be measured?"Composites,as with other materials,can have significantly different mechanical properties when tested in different ways.The main loading modes that are generally of interest are tension, compression,flexure,and shear-each has its own particular test techniques and difficulties. To facilitate design computations,the elastic properties of the composite lamina discussed in Chapter 6 are usually obtained first through simple coupon tests.Recall the relationships for in-plane elastic properties,noting that in most cases,in-plane properties will be used to design the laminate: 1/E1 -v12/E2 0 6 82 -v12/E1 1/E2 0 02 Y12 0 0 1/G12 T12 And since v2/E=v21/E2(See Chapter 6),only three tests are required to establish the in-plane elastic properties,in other words,0 tension,90 tension, and in-plane shear. Because it is not possible to conduct tests on single plies,the coupons are laid up with multiple plies,all orientated in the same direction.The exception is the in-plane shear in which,if a 45 tension test is selected (see below),plies are alternated between+and-45 symmetric about the center line. Strength values should not,in general,be taken from these coupons although they are very often taken to failure.The reasons for this are explained in Chapter 12.Laminate strength should be obtained from tests on representative laminates in which the orientations of the fiber lay-up are similar to those anticipated in the design.These values that are used in initial design are generally substantiated by tests on structural elements and often finally on full-scale structures.This is often referred to as the Testing Pyramid, which is illustrated in Figure 7.1. It must be understood that issues such as scale effects2 and complex load conditions13 can become important when testing composite components,and the data obtained from simple coupon tests can often only be used for comparing materials and not as accurate predictions of component behavior
MECHANICAL PROPERTY MEASUREMENT 215 issues that must be satisfied are that the test method used accurately creates the required stress state in the material and that the specimen failure be consistent with this stress state and not be artificially influenced by the test method. Because of the variabilties encountered in coupon testing, airworthiness authorities require multiple tests across several batches. MIL-HDBK 17 recommends a minimum of six specimens per test point and five batches of material to be tested. These requirements mean that the exploration of even a minimum of material properties entails a very large number of test specimens. When conducting tests to determine the strength and stiffness of a composite material, the first question that must be answered is "What mode of its performance is to be measured?" Composites, as with other materials, can have significantly different mechanical properties when tested in different ways. The main loading modes that are generally of interest are tension, compression, flexure, and shear--each has its own particular test techniques and difficulties. To facilitate design computations, the elastic properties of the composite lamina discussed in Chapter 6 are usually obtained first through simple coupon tests. Recall the relationships for in-plane elastic properties, noting that in most cases, in-plane properties will be used to design the laminate: 81 ~2 T12 l/El = --Vl2/E1 0 -lJ12/E 2 0 1/E2 0 0 l/G12 o" 1 0"2 '7"12 And since 1)12/E 1 m_ 1,21/E 2 (See Chapter 6), only three tests are required to establish the in-plane elastic properties, in other words, 0 ° tension, 90 ° tension, and in-plane shear. Because it is not possible to conduct tests on single plies, the coupons are laid up with multiple plies, all orientated in the same direction. The exception is the in-plane shear in which, if a 45 ° tension test is selected (see below), plies are alternated between + and - 45 ° symmetric about the center line. Strength values should not, in general, be taken from these coupons although they are very often taken to failure. The reasons for this are explained in Chapter 12. Laminate strength should be obtained from tests on representative laminates in which the orientations of the fiber lay-up are similar to those anticipated in the design. These values that are used in initial design are generally substantiated by tests on structural elements and often finally on full-scale structures. This is often referred to as the Testing Pyramid, which is illustrated in Figure 7.1. It must be understood that issues such as scale effects 12 and complex load conditions 13 can become important when testing composite components, and the data obtained from simple coupon tests can often only be used for comparing materials and not as accurate predictions of component behavior
216 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES SUB-COMPONENTS DETAILS ELEMENTS Fig.7.1 Testing pyramid for composite structures. 7.2 Coupon Tests 7.2.1 Tension Valid axial tension testing,particularly of strong unidirectional composites, can be a challenge.The load must be transferred from the testing apparatus into the specimen via shear,and the shear strengths of composites are often significantly lower than their tensile strength.Thus shear failure within the gripping region is often a problem. The standard test technique (outlined in Refs.2 and 3 for open-hole tension) describes the use of a parallel-sided,rectangular specimen with bonded end tabs. However,these tabs,which are normally made from a glass fabric/epoxy composite,are not strictly required.The key factor is the successful introduction of load into the specimen.Therefore,if acceptable failures are being obtained with reasonable consistency,then it can be assumed that the gripping method is working.A wide variety of bonded tab or unbonded shim configurations have been used successfully.One unbonded shim material sometimes used is a coarse mesh made of carborundum-coated cloth. Load measurement is performed via the load cell in the test machine,and strain measurement is done by an extensometer secured to the specimen or by adhesively bonded strain gauges.To measure Poisson's ratio,both the axial and transverse strain must be measured concurrently.Extensometers are normally preferred because they are reusable,easier to mount,and often more reliable at elevated temperatures or in high-moisture-content environments.If strain gauges are used,then the active gauge length (length of specimen over which the strain is measured)is recommended to be at least 6 mm for tape composites and at least as large as the characteristic repeating unit of the weave for woven materials
216 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES i SUtB_CnMpnNE~ITS /", ~ \'\ I \ L__~ ~_ ........... ~ ........ / cnup_~ ~ ',, ./,,,.,.__.._.,,_,_ ...,,_,._,..-_ ~_ _~_~__ ~_ ~_'.~ Fig. 7.1 Testing pyramid for composite structures. 7.2 Coupon Tests 7.2.1 Tension Valid axial tension testing, particularly of strong unidirectional composites, can be a challenge. The load must be transferred from the testing apparatus into the specimen via shear, and the shear strengths of composites are often significantly lower than their tensile strength. Thus shear failure within the gripping region is often a problem. The standard test technique (outlined in Refs. 2 and 3 for open-hole tension) describes the use of a parallel-sided, rectangular specimen with bonded end tabs. However, these tabs, which are normally made from a glass fabric/epoxy composite, are not strictly required. The key factor is the successful introduction of load into the specimen. Therefore, if acceptable failures are being obtained with reasonable consistency, then it can be assumed that the gripping method is working. A wide variety of bonded tab or unbonded shim configurations have been used successfully. One unbonded shim material sometimes used is a coarse mesh made of carborundum-coated cloth. Load measurement is performed via the load cell in the test machine, and strain measurement is done by an extensometer secured to the specimen or by adhesively bonded strain gauges. To measure Poisson's ratio, both the axial and transverse strain must be measured concurrently. Extensometers are normally preferred because they are reusable, easier to mount, and often more reliable at elevated temperatures or in high-moisture-content environments. If strain gauges are used, then the active gauge length (length of specimen over which the strain is measured) is recommended to be at least 6 mm for tape composites and at least as large as the characteristic repeating unit of the weave for woven materials
MECHANICAL PROPERTY MEASUREMENT 217 A successful test must cause failure within the gauge region.Failure at the tab edge(or gripped edge)or within the tab is unacceptable.Failure due to early edge delamination,which is normally caused by poor machining,is also unacceptable. Figure 7.2 illustrates typically a)unacceptable and b)acceptable specimen failures.Poor load system alignment is often a major contributor to premature failure,and it is highly desirable to evaluate system alignment with a suitably strain-gauged,alignment coupon 7.2.2 Compression There is still a great amount of debate among researchers as to the most appropriate method for compression testing or indeed whether there is a true axial compression test for composites.4.15 Generally,compression failure occurs through buckling,ranging from classical column buckling of the entire specimen Fig.7.2 Failure modes in tensile testing:a)unacceptable;b)acceptable
MECHANICAL PROPERTY MEASUREMENT 217 A successful test must cause failure within the gauge region. Failure at the tab edge (or gripped edge) or within the tab is unacceptable. Failure due to early edge delamination, which is normally caused by poor machining, is also unacceptable. Figure 7.2 illustrates typically a) unacceptable and b) acceptable specimen failures. Poor load system alignment is often a major contributor to premature failure, and it is highly desirable to evaluate system alignment with a suitably strain-gauged, alignment coupon. 7.2.2 Compression There is still a great amount of debate among researchers as to the most appropriate method for compression testing or indeed whether there is a true axial compression test for composites. 14'15 Generally, compression failure occurs through buckling, ranging from classical column buckling of the entire specimen a) 7 Fig. 7.2 Failure modes in tensile testing: a) unacceptable; b) acceptable