218 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES cross-section to local microbuckling of fibers that often leads to failure through the process of kink band formation."Therefore,the greater resistance to buckling the test fixture provides to the specimen,the higher the value of compressive strength that is obtained. Many different test methods and specimen configurations have been developed over the decades in an attempt to limit specimen buckling,and there are a number of tests that have become the most widely used in current practice.The Illinois Institute of Technology Research Institute (IITRD method,4 which has become an ASTM standard,and the modified ASTM D695 method10 (currently a SACMA standard),are two methods used for un-notched specimens (Figs.7.3 and 7.4,respectively).The SACMA Recommended Test Method 3R-9411 is commonly used for open-hole compression testing Fig.7.5. As with the tension test,tabs are not absolutely required for the specimen, although they are strongly recommended for specimens made with unidirectional reinforcement.The main criteria is that correct failure occurs within the gauge LINEAR BEARINGS TABBED KNURLED SPECIMEN SURFACE CLAMPING CLAMPING WEDGES SCREWS LOAD- ALIGNMENT ALIGNMENT BLOCK RODS Fig.7.3 The IITRI compression test rig
218 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES cross-section to local microbuckling of fibers that often leads to failure through the process of kink band formation. 4 Therefore, the greater resistance to buckling the test fixture provides to the specimen, the higher the value of compressive strength that is obtained. Many different test methods and specimen configurations have been developed over the decades in an attempt to limit specimen buckling, and there are a number of tests that have become the most widely used in current practice. The Illinois Institute of Technology Research Institute (IITRI) method, 4 which has become an ASTM standard, and the modified ASTM D695 method ]° (currently a SACMA standard), are two methods used for un-notched specimens (Figs. 7.3 and 7.4, respectively). The SACMA Recommended Test Method 3R-9411 is commonly used for open-hole compression testing Fig. 7.5. As with the tension test, tabs are not absolutely required for the specimen, although they are strongly recommended for specimens made with unidirectional reinforcement. The main criteria is that correct failure occurs within the gauge I ~ ~ .~"~. KNURLED WEDGES LINEAR TAOBED • ~i CLAMPING --SCREWS Lo, o ¸ ALIGNMENT ALIGNMENT Fig. 7.3 The IITRI compression test rig
MECHANICAL PROPERTY MEASUREMENT 219 COVER PLATE SPECIMEN PLACED BETWEEN COVER LOADING PLATE AND APPLIED SUPPORT PLATE THROUGH END LOADING OF TABBED SPECIMEN SUPPORT PLATE HARDENED INSERT BASE SUPPORT Fig.7.4 Modified ASTM D695 compression test rig. FRONT SUPPORT PLATE BACK SUPPORT PLATE TEST WINDOW FRONT SUPPORT ALIGNMENT PLATE PLATE SPECIMEN PLACED BACK SUPPORT BETWEEN FRONT AND PLATE BACK SUPPORT PLATES AND END LOADED Fig.7.5 Rig for the SACMA recommended test method 3R-94 for open-hole compression testing
MECHANICAL PROPERTY MEASUREMENT 219 SOVER PLATE LOADING APPLIED THROUGH END LOADING OF TABBED SPECIMEN SPECIMEN PLACED BETWEEN COVER PLATE AND SUPPORT PLATE SUPPORT PLATE HARDENED INSERT BASE SUPPORT Fig. 7.4 Modified ASTM D695 compression test rig. FRONT SUPPORT PLATE ~ SPECIMEN PLACED BETWEEN FRONT AND BACK SUPPORT PLATES AND END LOADED TEST WINDOW PLATE Fig. 7.5 Rig for the SACMA recommended test method 3R-94 for open-hole compression testing
220 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES area;if this does not occur correctly,then the data point should not be used. Figure 7.6 illustrates examples of acceptable and unacceptable failures,with any failure residing solely in the tabbed or gripped region being considered unacceptable.Due to the very short gauge length,it is likely that the failure location could be near the grip/tab termination region;this is still an acceptable failure. Compression tests are very sensitive to the flatness and parallelism of the specimens and/or tabs,and within the test specifications,the required tolerances are outlined.Gripping of the specimens and system misalignment are generally the biggest cause of data scatter,and of particular relevance to the test methods that use stabilizing lateral supports(SACMA SRM 1R-94 and 3R-94)is the issue of bolt torque.An over-torque of the bolts allows more of the applied load to be carried by the lateral supports through friction,thereby increasing the apparent compressive strength.Generally,the bolts are tightened up to a"finger-tight" level,a fairly arbitrary measurement;however studies'5 have shown that the torque should not exceed approximately 1 Nm. 7.2.3 Flexure A flexure test is,without doubt,one of the simplest types of tests to perform and thus has long been popular(Fig.7.7).The main difficulty is that it does not a) b) Fig.7.6 Failure modes in compression testing:a)unacceptable;b)acceptable
220 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES area; if this does not occur correctly, then the data point should not be used. Figure 7.6 illustrates examples of acceptable and unacceptable failures, with any failure residing solely in the tabbed or gripped region being considered unacceptable. Due to the very short gauge length, it is likely that the failure location could be near the grip/tab termination region; this is still an acceptable failure. Compression tests are very sensitive to the flatness and parallelism of the specimens and/or tabs, and within the test specifications, the required tolerances are outlined. Gripping of the specimens and system misalignment are generally the biggest cause of data scatter, and of particular relevance to the test methods that use stabilizing lateral supports (SACMA SRM 1R-94 and 3R-94) is the issue of bolt torque. An over-torque of the bolts allows more of the applied load to be carried by the lateral supports through friction, thereby increasing the apparent compressive strength. Generally, the bolts are tightened up to a "finger-tight" level, a fairly arbitrary measurement; however studies 15 have shown that the torque should not exceed approximately 1 Nm. 7.2.3 Flexure A flexure test is, without doubt, one of the simplest types of tests to perform and thus has long been popular (Fig. 7.7). The main difficulty is that it does not a) b Fig. 7.6 Failure modes in compression testing: a) unacceptable; b) acceptable
MECHANICAL PROPERTY MEASUREMENT 221 FLEXURE THREE POINT BENDING 15 PLIES 8.36cm 42.50N】 1143cm 1.27Gm 14.6IN.】 0.60N.I 44 1.27cm 0.01N. 1.27cm FLEXURE:FOUR POINT BENDING 0.601N.) ↑.27cm (0.50N.J Fig.7.7 ASTM test methods for the flexure test. provide basic material property information because of the variation in stress- state within the specimen.The stress-state on the loading side is compression and on the supported side is tension;the mid-plane of the specimen is in pure shear. Therefore,depending on the relative values of the tension,compression,and shear strengths of the material,any one of these properties may be measured. The ratio of the support span length to specimen thickness is normally set long enough so that shear failure does not occur(32:1 is common)but whether failure initiates on the tensile or compressive face will be dependent on the material. Although the flexure test does not provide basic design data,its use is normally justified if the actual components are subjected to flexure.This is a valid argument if the span length to thickness ratio is similar to the laboratory test specimens.If not,the failure mode of the component in service may be different, and thus any comparison of the laboratory testing is not valid. The details of the standard flexure test are contained within the ASTM specification D790-84a'and this provides the recommended dimensions and cross-head speeds for various materials.There are two possible test configurations that can be used:three-point bending and four-point bending (Fig.7.7).Although the three-point bending test requires less material,the four- point bending has the advantage that uniform tensile or compressive stresses (with zero shear)are produced over the area between the loading points,not just under the loading point as in three-point bending test.In the three-point test,the high local stresses at the loading point affects the failure mode and load.It should be noted that excessive bending of the specimen before failure can render the test
MECHANICAL PROPERTY MEASUREMENT 221 FLEXURE" THREE POINt 8ENDING 15 PLIES ,27 em ~O,EO INJ Fig. 7.7 ASTM test methods for the flexure test. provide basic material property information because of the variation in stressstate within the specimen. The stress-state on the loading side is compression and on the supported side is tension; the mid-plane of the specimen is in pure shear. Therefore, depending on the relative values of the tension, compression, and shear strengths of the material, any one of these properties may be measured. The ratio of the support span length to specimen thickness is normally set long enough so that shear failure does not occur (32 : 1 is common) but whether failure initiates on the tensile or compressive face will be dependent on the material. Although the flexure test does not provide basic design data, its use is normally justified if the actual components are subjected to flexure. This is a valid argument if the span length to thickness ratio is similar to the laboratory test specimens. If not, the failure mode of the component in service may be different, and thus any comparison of the laboratory testing is not valid. The details of the standard flexure test are contained within the ASTM specification D790-84a 5 and this provides the recommended dimensions and cross-head speeds for various materials. There are two possible test configurations that can be used: three-point bending and four-point bending (Fig. 7.7). Although the three-point bending test requires less material, the fourpoint bending has the advantage that uniform tensile or compressive stresses (with zero shear) are produced over the area between the loading points, not just under the loading point as in three-point bending test. In the three-point test, the high local stresses at the loading point affects the failure mode and load. It should be noted that excessive bending of the specimen before failure can render the test