《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 3）Chapter 10 Thick-Section Composites.pdf_大学文库

MIL-HDBK-17-3F Volume 3,Chapter 10 Thick-Section Composites CHAPTER 10 THICK-SECTION COMPOSITES 10.1 INTRODUCTION AND DEFINITION OF THICK-SECTION Thick-section composites are ones where the effect of geometry (thickness-to-span ratio),material constituents(matrix and fiber stiffness/strength properties),lamination scheme,processing.and service loading exhibit three-dimensional states of stress.For instance,all loadings induce multiaxial stresses into individual plies of composite materials that are made of multi-directional ply laminates (either woven or nonwoven),even though the overall loadings may only be uniaxial.When transverse (through- thickness)stresses and strains occur to a significant degree,they must be accounted for in analysis,de- sign and testing.A significant degree is achieved when these effects contribute to failure (e.g.,delamina- tion),excessive deflection or vibration.Frequently,these stresses and strains induce failures that cannot be accurately predicted by conventional two-dimensional analyses for thin laminates.These two- dimensional analyses are usually based on material response data obtained from traditional shear and uniaxial tensile/compressive testing techniques.In thick section composites,where any one of six stress components may significantly contribute to failure,a failure criteria must distinguish between different types of failure modes by associating the contribution of each three-dimensional stress component to a unique mode of failure,be it fiber,matrix or interface dominated.An appropriate failure criteria for thick section composites must consider the following laminate failure modes: Fiber Dominated Matrix Dominated Interface Dominated Fiber pull-out Transverse cracking Interface disbonding Fiber tensile failure Interlaminar cracking Interface delamination Fiber micro-buckling Intralaminar cracking Compressive delamination Fiber shear failure Edge delamination For example,thick-section composites made of high stiffness and strength fiber-reinforced plies often exhibit significant transverse shear and transverse normal deformations (the type of three-dimensional stress contributions that are negligibly small in thin laminates).The thickness effect can also be influ- enced by short wavelength loadings and,in dynamics,high frequency vibrations.These three- dimensional effects are considerably more pronounced in composites than in homogeneous isotropic ma- terials due to their inherently high material compliances in the transverse direction relative to the axial fiber direction.Moreover,composite laminates exhibit much lower strength in the transverse direction, and at ply interfaces,making them particularly susceptible to matrix cracking and delamination. Thick section composites can also be defined from the standpoint of fabrication effects associated with a large number of plies.Process induced stresses can be significant and,therefore,warrant special attention.Fabrication effects of special concern include residual stresses,wrinkling,micro-cracking,exo- therm,volatile removal,compaction,machining,and mechanical joining and/or adhesive bonding.To minimize these effects,special resins,processing,tooling,and cure cycles may be necessary. In thick laminates,typically two competing objectives are desired,namely,minimization of process induced residual stresses and maximization of production rates(i.e.,minimization of the processing time required to achieve complete cure).Fast cure cycle times,involving steep heating and cooling rates,will generally lead to high process induced residual stresses.On the other hand,slowly bringing all part thicknesses up to complete cure simultaneously will minimize,if not eliminate,all process induced resid- ual stresses.This,however,is accomplished at the expense of extended cure cycle times.It is also im- portant to note that process induced residual stresses may in fact be intentionally introduced to cancel,or otherwise mitigate,large superimposed in-service stresses. 10-1

MIL-HDBK-17-3F Volume 3, Chapter 10 Thick-Section Composites 10-1 CHAPTER 10 THICK-SECTION COMPOSITES 10.1 INTRODUCTION AND DEFINITION OF THICK-SECTION Thick-section composites are ones where the effect of geometry (thickness-to-span ratio), material constituents (matrix and fiber stiffness/strength properties), lamination scheme, processing, and service loading exhibit three-dimensional states of stress. For instance, all loadings induce multiaxial stresses into individual plies of composite materials that are made of multi-directional ply laminates (either woven or nonwoven), even though the overall loadings may only be uniaxial. When transverse (throughthickness) stresses and strains occur to a significant degree, they must be accounted for in analysis, design and testing. A significant degree is achieved when these effects contribute to failure (e.g., delamination), excessive deflection or vibration. Frequently, these stresses and strains induce failures that cannot be accurately predicted by conventional two-dimensional analyses for thin laminates. These twodimensional analyses are usually based on material response data obtained from traditional shear and uniaxial tensile/compressive testing techniques. In thick section composites, where any one of six stress components may significantly contribute to failure, a failure criteria must distinguish between different types of failure modes by associating the contribution of each three-dimensional stress component to a unique mode of failure, be it fiber, matrix or interface dominated. An appropriate failure criteria for thick section composites must consider the following laminate failure modes: Fiber Dominated Matrix Dominated Interface Dominated . Fiber pull-out . Transverse cracking . Interface disbonding . Fiber tensile failure . Interlaminar cracking . Interface delamination . Fiber micro-buckling . Intralaminar cracking . Compressive delamination . Fiber shear failure . Edge delamination For example, thick-section composites made of high stiffness and strength fiber-reinforced plies often exhibit significant transverse shear and transverse normal deformations (the type of three-dimensional stress contributions that are negligibly small in thin laminates). The thickness effect can also be influenced by short wavelength loadings and, in dynamics, high frequency vibrations. These threedimensional effects are considerably more pronounced in composites than in homogeneous isotropic materials due to their inherently high material compliances in the transverse direction relative to the axial fiber direction. Moreover, composite laminates exhibit much lower strength in the transverse direction, and at ply interfaces, making them particularly susceptible to matrix cracking and delamination. Thick section composites can also be defined from the standpoint of fabrication effects associated with a large number of plies. Process induced stresses can be significant and, therefore, warrant special attention. Fabrication effects of special concern include residual stresses, wrinkling, micro-cracking, exotherm, volatile removal, compaction, machining, and mechanical joining and/or adhesive bonding. To minimize these effects, special resins, processing, tooling, and cure cycles may be necessary. In thick laminates, typically two competing objectives are desired, namely, minimization of process induced residual stresses and maximization of production rates (i.e., minimization of the processing time required to achieve complete cure). Fast cure cycle times, involving steep heating and cooling rates, will generally lead to high process induced residual stresses. On the other hand, slowly bringing all part thicknesses up to complete cure simultaneously will minimize, if not eliminate, all process induced residual stresses. This, however, is accomplished at the expense of extended cure cycle times. It is also important to note that process induced residual stresses may in fact be intentionally introduced to cancel, or otherwise mitigate, large superimposed in-service stresses

MIL-HDBK-17-3F Volume 3,Chapter 10 Thick-Section Composites In thick laminate design,cure simulation plays a very important role in developing a deeper under- standing of the cure kinetics and the degree of cure at any point in the time domain.Such simulation is also able to predict processing stresses even during the cure cycle.This can be an important tool for pre- diction and preventing in-process part fabrication failures where both stresses and associated strengths are low. The structural analyst needs to know the multiaxial strength and deformation characteristics for effi- cient thick composite material design.The full potential of thick composites cannot be realized until the material response under multiaxial service loadings can be established.Technical progress in the design, analysis and associated material testing of thick composites remain much less developed than the gener- ally accepted methodology associated with thin composite material characterizations and applications. The step-by-step method for analysis of thick section composites is illustrated by the flow chart in Fig- ure10.1. THICK-SECTION COMPOSITES PROCESSING STRUCTURAL MATERIAL TESTING MATERIAL TESTING PHYSICAL PROPERTIES UNI-AXIAL STRESS MATERIAL PROPERTIES THERMAL/CHEMICAL ALLOWABLES E.G,etc. PROCESS ANALYSIS MU儿TI-AXAL METHODS COMBINED STRESSES STRUCTURAL ANALYSIS METHODS PRocESs INDUCED FAILURE CRITERIA sErvICE LOAD LAMINATE STRESSES LAMINATE STRESSES MARGIN OF SAFETY FIGURE 10.1 Flowchart illustrating thick-section composites analysis method. 10.2 MECHANICAL PROPERTIES REQUIRED FOR THICK-SECTION COMPOSITE THREE-DIMENSIONAL ANALYSIS The purpose of this section is to define the three-dimensional(3-D)orthotropic stiffness properties necessary to conduct a 3-D point stress analysis,and the failure strength and strain allowables required to calculate a margin of safety.This section will: a)Define the stiffness properties currently used to conduct a conventional two-dimensional(2-D) analysis (Volume 1,Section 6.7). 10-2

MIL-HDBK-17-3F Volume 3, Chapter 10 Thick-Section Composites 10-2 In thick laminate design, cure simulation plays a very important role in developing a deeper understanding of the cure kinetics and the degree of cure at any point in the time domain. Such simulation is also able to predict processing stresses even during the cure cycle. This can be an important tool for prediction and preventing in-process part fabrication failures where both stresses and associated strengths are low. The structural analyst needs to know the multiaxial strength and deformation characteristics for efficient thick composite material design. The full potential of thick composites cannot be realized until the material response under multiaxial service loadings can be established. Technical progress in the design, analysis and associated material testing of thick composites remain much less developed than the generally accepted methodology associated with thin composite material characterizations and applications. The step-by-step method for analysis of thick section composites is illustrated by the flow chart in Figure 10.1. THICK-SECTION COMPOSITES PROCESSING MATERIAL TESTING PHYSICAL PROPERTIES THERMAL/CHEMICAL PROCESS ANALYSIS METHODS PROCESS INDUCED LAMINATE STRESSES UNI-AXIAL STRESS ALLOWABLES MULTI-AXIAL COMBINED STRESSES FAILURE CRITERIA MARGIN OF SAFETY STRUCTURAL MATERIAL TESTING MATERIAL PROPERTIES E, G, etc. STRUCTURAL ANALYSIS METHODS SERVICE LOAD LAMINATE STRESSES FIGURE 10.1 Flowchart illustrating thick-section composites analysis method. 10.2 MECHANICAL PROPERTIES REQUIRED FOR THICK-SECTION COMPOSITE THREE-DIMENSIONAL ANALYSIS The purpose of this section is to define the three-dimensional (3-D) orthotropic stiffness properties necessary to conduct a 3-D point stress analysis, and the failure strength and strain allowables required to calculate a margin of safety. This section will: a) Define the stiffness properties currently used to conduct a conventional two-dimensional (2-D) analysis (Volume 1, Section 6.7)

MIL-HDBK-17-3F Volume 3,Chapter 10 Thick-Section Composites b)Define the additional stiffness properties needed to conduct a three-dimensional(3-D)stress analysis. c)Define the testing required to experimentally determine the 3-D stiffness properties and the failure strengths and strains for uniaxial loading (Section 10.2.3.1)and multiaxial loading (Section 10.2.3.2) d)Discuss the methodology for predicting laminate stiffness properties through the thickness using the 3-D lamina properties(Section 10.2.4). The symbols and nomenclature used in the handbook(Volume 3,Section 1.3.1)apply to 2-D and 3-D composites and utilize 1,2,3 for lamina axes and x,y,z for an oriented laminate axis directions. 10.2.1 2-D composite analysis The two-dimensional composite analysis procedures (Volume 3,Section 5.3.1)apply when the through the thickness stresses are not significant.For unidirectional laminates that have low stresses in the thickness or 3-direction (o3=723=713),plane stress),the stress-strain relationship (Reference 10.2.1)is, {e可=1{o 10.2.1(a S11 S12 0r 1 E2 = S12 S22 02 10.2.1(b) Y12 0 0 S66712 In terms of the engineering elastic constants obtained by simple tests 1 V12 0 E 01 V21 1 0 E2 E2 02 10.2.1(c) Y12 T12 0 0 G12 The reciprocity relationships for stiffness is 2=21 10.2.1(d) E1 E2 For the plane stress two-dimensional analysis,the four independent elastic material properties are: E1,E2,G12,V12 In-plane failure stress and strain values can be obtained from the same test used for determining the stiff- ness as discussed in Section 10.2.3.1. 10.2.2 3-D composite analysis When the stresses and strains in the thickness direction are significant,(applied values are approach- ing their allowables)the problem requires a three-dimensional orthotropic stress analysis.A 3-D analysis is frequently necessary as the section thickness of a composite increases or when thin sections have out- of-plane loading (bending moment,lateral pressures,etc.)which results in,for example,interlaminar ten- sile stresses in a corner radius or interlaminar shear stresses in a beam or plate. 10-3

MIL-HDBK-17-3F Volume 3, Chapter 10 Thick-Section Composites 10-3 b) Define the additional stiffness properties needed to conduct a three-dimensional (3-D) stress analysis. c) Define the testing required to experimentally determine the 3-D stiffness properties and the failure strengths and strains for uniaxial loading (Section 10.2.3.1) and multiaxial loading (Section 10.2.3.2) d) Discuss the methodology for predicting laminate stiffness properties through the thickness using the 3-D lamina properties (Section 10.2.4). The symbols and nomenclature used in the handbook (Volume 3, Section 1.3.1) apply to 2-D and 3-D composites and utilize 1, 2, 3 for lamina axes and x, y, z for an oriented laminate axis directions. 10.2.1 2-D composite analysis The two-dimensional composite analysis procedures (Volume 3, Section 5.3.1) apply when the through the thickness stresses are not significant. For unidirectional laminates that have low stresses in the thickness or 3-direction στ τ b g 3 23 13 = = , plane stress), the stress-strain relationship (Reference 10.2.1) is, ij ij ns ns ε σ = 10.2.1(a) 1 2 12 11 12 12 22 66 1 2 12 = S S 0 S S 0 0 0 S ε ε γ σ σ τ R S | T | U V | W | L N M M M O Q P P P R S | T | U V | W | 10.2.1(b) In terms of the engineering elastic constants obtained by simple tests 1 2 12 1 2 12 = ε ε γ ν ν σ σ τ R S | T | U V | W | − − L N M M M M M M M O Q P P P P P P P R S | T | U V | W | 1 0 1 0 0 0 1 1 12 1 21 2 2 12 E E E E G 10.2.1(c) The reciprocity relationships for stiffness is ν 12 ν 1 21 E E2 = 10.2.1(d) For the plane stress two-dimensional analysis, the four independent elastic material properties are: E1 2 , E , G12 , ν 12 In-plane failure stress and strain values can be obtained from the same test used for determining the stiffness as discussed in Section 10.2.3.1. 10.2.2 3-D composite analysis When the stresses and strains in the thickness direction are significant, (applied values are approaching their allowables) the problem requires a three-dimensional orthotropic stress analysis. A 3-D analysis is frequently necessary as the section thickness of a composite increases or when thin sections have outof-plane loading (bending moment, lateral pressures, etc.) which results in, for example, interlaminar tensile stresses in a corner radius or interlaminar shear stresses in a beam or plate

MIL-HDBK-17-3F Volume 3, Chapter 10 Thick-Section Composites 10-4 10.2.2.1 Unidirectional lamina 3-D properties For the orthotropic unidirectional lamina there are nine independent constants as shown by the following stress-strain relationship (Reference 10.2.1): ε ε ε γ γ γ σ σ σ τ τ τ 1 2 3 23 31 12 1 2 3 23 31 12 R S | | | T | | | U V | | | W | | | L N M M M M M M M M M O Q P P P P P P P P P R S | | | T | | | U V | | | W | | | = SSS 000 SSS 000 SSS 000 000 S 0 0 0000 S 0 00000 S 11 12 13 12 22 23 13 23 33 44 55 66 10.2.2.1(a) or in terms of the engineering constants, ε ε ε γ γ γ ν ν ν ν ν ν σ σ σ τ τ τ 1 2 3 23 31 12 1 21 2 31 3 12 1 2 32 3 13 1 23 2 3 23 31 12 1 2 3 23 31 12 1 000 1 000 1 000 000 1 0 0 0 0 00 1 0 0 0 0 00 1 R S | | | T | | | U V | | | W | | | − − − − − − L N M M M M M M M M M M M M M M O Q P P P P P P P P P P P P P P R S | | | T | | | U V | | | W | | | = E EE EE E E EE G G G 10.2.2.1(b) There are three reciprocal relationships that must be satisfied for an orthotropic material. They are ν 12 ν ν ν ν ν 1 21 2 13 1 31 3 23 2 32 EE EE E E3 === , , 10.2.2.1(c) There are nine independent elastic material properties required for an orthotropic lamina E1 E2 E3 G12 G13 G23 12 13 23 , , , , , , ν , ν , ν When materials have a different stiffness in tension from in compression, it is common practice to use an average value when the difference is small. If the stiffness difference is significant, use the stiffness (tensile or compressive) that is representative of the application loading. 10.2.2.2 Oriented orthotropic laminate 3-D properties The compliance matrix and associated nine elastic constants required to conduct a 3-D analysis are defined in this section and are for a oriented balanced and symmetric laminate loaded in the x, y, or z direction. Most practical composite laminate lay-ups generally are balanced and symmetric to prevent thermal warpage during processing. If the laminate is unbalanced and unsymmetric, or loaded "off-axis" to the principal orthogonal directions, then the matrix is fully populated with the Chentsov's coefficients d i µ ij kl , and coefficients of mutual influence η η ij i i ij , , d i , (see References 10.2.1, 10.2.2.2). The compliance matrix for the balanced and symmetric laminate loaded in the x, y, or z direction is

MIL-HDBK-17-3F Volume 3, Chapter 10 Thick-Section Composites 10-5 ε ε ε γ γ γ σ σ σ τ τ τ x y z yz zx xy x y z yz zx xy SSS SSS SSS S S S R S | | || T | | | | U V | | || W | | | | L N M M M M M M M M M M O Q P P P P P P P P P P R S | | || T | | | | U V | | || W | | | | = 000 000 000 000 00 0000 0 00000 11 12 13 12 22 23 13 23 33 44 55 66 10.2.2.2(a) In terms of the effective engineering elastic constants this relationship is, ε ε ε γ γ γ ν ν ν ν ν ν σ σ σ τ τ τ x y z yz zx xy x yz y zx z xy x y zy z xz x yz y z yz zx xy x y z yz zx xy E EE EE E E EE G G G R S | | || T | | | | U V | | || W | | | | − − − − − − L N M M M M M M M M M M M M M M M M O Q P P P P P P P P P P P P P P P P R S | | || T | | | | U V | | || W | | | | = 1 000 1 000 1 000 000 1 0 0 0 0 00 1 0 0 0 0 00 1 10.2.2.2(b) There are three reciprocal relationships that must be satisfied by the effective laminate stiffnesses. They are, ν xy ν ν ν ν ν x yx y xz x zx z yz y zy EE EE EEz === , , 10.2.2.2(c) There are nine independent effective elastic material constants required for analysis of the oriented laminate, Ex y z xy xz yz xy xz yz , E , E , G , G , G , ν , ν , ν 10.2.3 Experimental property determination The current and most commonly used approach for failure analysis of 2-D composites is to experimentally determine the strength and stiffness values for the unidirectional lamina from simple uniaxial tests and use a failure criterion to account for the various load direction interactions to calculate the margin of safety. These uniaxial tests are defined in Section 10.2.3.1 for 2-D and 3-D composites. Another approach is to conduct multiaxial tests that provide loading in the proper proportions to simulate the actual load applications. The multiaxial testing and methodology are discussed in Section 10.2.3.2. There are considerable challenges associated with both uniaxial and multiaxial, mechanical testing of thick section composite materials. A partial list of experimental testing considerations is presented below: − Test system and load introduction − Gripping system and fixturing − Computer control and interface − Adequate displacement control over specimen centroid location − Specimen design and optimization − Unknown states of stress within thick composites − Multiaxial extensometry and other measurement devices and techniques

MIL-HDBK-17-3F Volume 3.Chapter 10 Thick-Section Composites Inclusion and treatment of environmental effects Data acquisition and analysis Multiaxial yield and failure criteria Size effect and scaling law Edge effects treatment Static and dynamic testing,including fatigue and impact loadings Sensitivity to stress concentrations -NDE of damage 10.2.3.1 Uniaxial tests The type of common tests conducted on the unidirectional laminate to obtain the conventional 2-D in-plane tensile,compressive,and shear stiffness,as well as failure strength and strains are summarized in Figures 10.2.3.1(a)through 10.2.3.1(c).These tests are also discussed in detail in Volume 1,Section 6.8.The additional unidirectional laminate design property tests needed when a 3-D (thick-section) analysis is required are summarized in Figure 10.2.3.1(d)and described in detail in Figures 10.2.3.1(e) and 10.2.3.1(f).Test methods available to obtain these properties are summarized in Table 10.2.3.1(a). Further test method development is needed for tension and compression testing in the 3 or through-thickness direction. For oriented laminates,the additional design properties tests needed in addition to the 2-D tests for a 3-D analysis are summarized in Figure 10.2.3.1(g).The 3-D through the thickness stiffnesses can also be predicted from the unidirectional lamina stiffnesses by the methods discussed in Section 10.2.4(Theoreti- cal Property Determination).Table 10.2.3.1(b)summarizes the test methods available for determining 3-D properties for an oriented laminate.Furthermore,test method development is also needed for tension and compression testing in the z-thickness direction similar to the need for unidirectional laminate testing. An example of representative thick-section composite properties for an intermediate modulus car- bon/epoxy material system are presented in Tables 10.2.3.1(c)and(d)for the unidirectional lamina and [0/90]oriented laminate.The lamina properties were taken from Reference 10.2.3.1(a)and the [0/90]data were obtained by a Hercules test program from an 80-ply (t=0.59 in.,15mm)fiber-placed,auto- clave-cured laminate(Reference 10.2.3.1(b)). Tables 10.2.3.1(a)and (b)identify three uniaxial compressive test methods for testing composites greater than 0.250 inches(6.35 mm)in thickness.Both the David Taylor Research Center(DTRC)and the Alliant Techsystems testing fixtures,which are shown in Figures 10.2.3.1(h)and 10.2.3.1(i),respec- tively (see References 10.2.3.1(a)and 10.2.3.1(c),respectively),were developed for uniaxial compres- sion testing of thick prismatic columnar shaped composite material specimens.The US Army Research Laboratory-Materials Directorate (ARL)(Reference 10.2.3.1(d))test method utilizes a cubic specimen loaded directly between two steel platens with no associated fixturing.The development of compression data relative to the different material orientations identified in Tables 10.2.3.1(a)and(b)is accomplished through independent,successive uniaxial load applications.Successive uniaxial compression tests,that consist of one-directional load applications per material orientation,can be undertaken with conventional, medium-to-high capacity load frames.With proper care and specimen fixturing,these tests may also be used for determining unidirectional compressive material strengths and failure characteristics. The primary feature that both the DTRC and the Alliant Techsystems test fixtures provide is that they have been developed for maintaining proper gripping and alignment of the test specimens as well as pro- viding constraints to minimize any potential specimen end brooming (specimen splitting)under compres- sive load applications.Any potential onset of apparent,specimen end splitting and fixture-induced test specimen material cracking,may cause significant material strength reductions.Special tabbing as well as associated specimen-tabbing connection detail may be required for some uniaxial compression testing of thick composites. 10-6

MIL-HDBK-17-3F Volume 3, Chapter 10 Thick-Section Composites 10-6 − Inclusion and treatment of environmental effects − Data acquisition and analysis − Multiaxial yield and failure criteria − Size effect and scaling law − Edge effects treatment − Static and dynamic testing, including fatigue and impact loadings − Sensitivity to stress concentrations − NDE of damage 10.2.3.1 Uniaxial tests The type of common tests conducted on the unidirectional laminate to obtain the conventional 2-D in-plane tensile, compressive, and shear stiffness, as well as failure strength and strains are summarized in Figures 10.2.3.1(a) through 10.2.3.1(c). These tests are also discussed in detail in Volume 1, Section 6.8. The additional unidirectional laminate design property tests needed when a 3-D (thick-section) analysis is required are summarized in Figure 10.2.3.1(d) and described in detail in Figures 10.2.3.1(e) and 10.2.3.1(f). Test methods available to obtain these properties are summarized in Table 10.2.3.1(a). Further test method development is needed for tension and compression testing in the 3 or through-thickness direction. For oriented laminates, the additional design properties tests needed in addition to the 2-D tests for a 3-D analysis are summarized in Figure 10.2.3.1(g). The 3-D through the thickness stiffnesses can also be predicted from the unidirectional lamina stiffnesses by the methods discussed in Section 10.2.4 (Theoretical Property Determination). Table 10.2.3.1(b) summarizes the test methods available for determining 3-D properties for an oriented laminate. Furthermore, test method development is also needed for tension and compression testing in the z-thickness direction similar to the need for unidirectional laminate testing. An example of representative thick-section composite properties for an intermediate modulus carbon/epoxy material system are presented in Tables 10.2.3.1(c) and (d) for the unidirectional lamina and [0/90] oriented laminate. The lamina properties were taken from Reference 10.2.3.1(a) and the [0/90] data were obtained by a Hercules test program from an 80-ply (t=0.59 in., 15mm) fiber-placed, autoclave-cured laminate (Reference 10.2.3.1(b)). Tables 10.2.3.1(a) and (b) identify three uniaxial compressive test methods for testing composites greater than 0.250 inches (6.35 mm) in thickness. Both the David Taylor Research Center (DTRC) and the Alliant Techsystems testing fixtures, which are shown in Figures 10.2.3.1(h) and 10.2.3.1(i), respectively (see References 10.2.3.1(a) and 10.2.3.1(c), respectively), were developed for uniaxial compression testing of thick prismatic columnar shaped composite material specimens. The US Army Research Laboratory - Materials Directorate (ARL) (Reference 10.2.3.1(d)) test method utilizes a cubic specimen loaded directly between two steel platens with no associated fixturing. The development of compression data relative to the different material orientations identified in Tables 10.2.3.1(a) and (b) is accomplished through independent, successive uniaxial load applications. Successive uniaxial compression tests, that consist of one-directional load applications per material orientation, can be undertaken with conventional, medium-to-high capacity load frames. With proper care and specimen fixturing, these tests may also be used for determining unidirectional compressive material strengths and failure characteristics. The primary feature that both the DTRC and the Alliant Techsystems test fixtures provide is that they have been developed for maintaining proper gripping and alignment of the test specimens as well as providing constraints to minimize any potential specimen end brooming (specimen splitting) under compressive load applications. Any potential onset of apparent, specimen end splitting and fixture-induced test specimen material cracking, may cause significant material strength reductions. Special tabbing as well as associated specimen-tabbing connection detail may be required for some uniaxial compression testing of thick composites