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The main objective of this research investigation was the effect of compressive loading on the fatigue response of graphite epoxy laminates as influenced by a circular hole and an aggressive environment. The investigation was undertaken in order to further characterize the behavior of graphite/epoxy laminates under fatigue loading. The primary emphasis of the program was on the accumulation of a statistically significant data base. Unnotched coupon properties of a quasi-isotropic and a 67% 0 degrees fiber laminate were investigated in a 82.2 degrees C (180 degrees F), 90% R.H. environment after prior conditioning in that environment. The interactional effects of absorbed moisture and fatigue loading on the fatigue lifetime of coupons containing a circular hole was investigated at room temperature and at 82.2 degrees C (180 degrees F), 90% R.H. using the quasi-isotropic laminate. The test program consisted of static tension and compression tests; tension-tension and tension- compression fatigue tests; and static tension and compression residual strength tests of coupons prior tested either under tension-tension or tension- compression fatigue loading.

The primary objectives of this program are to: 1) study in detail how mechanical loading parameters affect the fatigue life of graphite/epoxy laminates; 2) determine the effects of environmental and geometrical perturbations on the effects studied in objective 1; 3) analyze the results in a manner such that an expanded foundation is laid for formulating fatigue life models based on knowledge of failure mechanisms. The research investigation is divided into four tasks: Task I, Diagnostic Experimentation: Task II, Effects of Loading Parameters; Task III, Effects of Environment and Geometry; Task IV, Data Analysis and Reporting. Tasks I, II and III are conducted sequentially while Task IV is conducted simultaneously with all three tasks. This report describes the analytical and experimental work undertaken in Task I.

The main objective of this research investigation was the experimental determination of the effect of compressive loading on the fatigue response of graphite/epoxy lamiantes. The primary emphasis of the program was on the accumulation of a statistically significant data base. The data base was to be used for developing an analytical model with predictive capability to account for the effect of compressive loading. The test program included only unnotched coupons in a room temperature, laboratory air environment. The test program consisted of static tensile and compressive tests; tension-tension and tension-compression fatigue tests; and tensile and compressive residual strength tests of coupons tested either under tension-tension or tension-compression fatigue loading. The development of a large, statistically meaningful data base was accomplished. The general effect of compression on the fatigue response of a graphite/epoxy composite was determined. However, a general model with capability to account for compressive loading was not attempted. As an alternate, the concept of a fatigue threshold was postulated along with consideration of some substantiating data. Analysis of the data showed that due to the extent of data scatter, a model of the 'wear-out' or similar type would predict the need for unacceptably low stress levels to achieve the high probabilities of survival required for aircraft structures.

The purpose of this research investigation was to explore, in both a qualitative and quantitative fashion, the relationship among stiffness, strength, fatigue life, residual strength, and extent of damage of unnotched, graphite/epoxy laminates subjected to tension loading. The study consisted of experimentation on a wide range of laminates involving detailed observations of damage development and progression and the correlation of this damage to stiffness, strength, and fatigue life. Damage states were analyzed and extensively mathematically modeled in order to delineate and understand the relationship among the measured variables. Enough understanding of the tension loaded case was developed to allow development of a proposed, simple procedure for calculating strain to failure, stiffness, strength, data scatter, and shape of the stress-life curve for unnotched laminates subjected to tension load.

Contents: Overview of Tasks II and III; Material and Specimen Characterization -- Tasks II and III; Experimental Procedures; Static Tension and Compression Results; Task II Fatigue Results; Task III Fatigue Results; Damage Characterization; and Analysis of Results., See also Volume 3, AD-A115 185.

The primary objectives of this program were to: (1) investigate the effects of mechanical load history on the response of laminated graphite/epoxy composites; and (2) to analyze the results in a manner such that an expanded foundation is laid for formulating cumulative damage concepts. The research investigation was divided into three tasks: Task I, Diagnostic Experimentation; Task II, Effect of Loading Parameters; and Task III, Data Analysis and Reporting. In Task I, the laminate of interest was selected as a quasi-isotropic layup of T300/5208 graphite/epoxy material. The unidirectional properties of the material were characterized at room temperature and at 82.2 C (180 F) in both an as-received and a high moisture content condition. Moisture absorption and thermal expansion properties of the laminate were obtained. Static tension and compression properties of the quasi-isotropic laminate as well as those for unidirectional laminates were determined. During Task II, different types of mechanical load histories were applied, at room temperature, to coupons of the selected quasi-isotropic laminate. Distributions of either monotonic strength or cyclic life, which ever was appropriate, were obtained for the various applied load histories., See also report dated Jun 80, AD-A088 439.

Contents: Appendixes-Quality Control Plan; Specimen Weight Measurements; Static Test Data; Damage Growth Characteristics Under Fatigue Loading; Damage Characteristics of Specimens Tested for Residual Strength; Damage Measurements of Specimens Tested for Residual Strength; Damage as Determined by Metallographic Sectioning: Comparison of Damage as Determined by Holscan Ultrasonic C-Scan and Dib Enhanced X-Ray; Damage on Individual Layers of Specimens Deplied After Fatigue Cycling; Statistical Analysis of Panel Variability; and Discussion of Weibull Function and Parameter Estimation Procedures., See also Volume 1, AD-A115 183.

The report summarizes work directed at the verification of currently used compliance relationships for the wedge-open-loading (WOL) specimen geometry frequently used in fatigue crack propagation testing. An overview is given of compliance curve fitting procedures. A new procedure is proposed which appears to offer significant advantages. Compliance calibration results are presented for Ti-6Al-4V(RA) and Ti-6Al-4V(Beta) alloys. Analytical procedures for obtaining the stress intensity calibration function (C sub 3) from experimental compliance results were compared and the proposed three parameter fit was found to be superior. The advantages of this proposed data analysis procedure are: small numbers of constants, relationship between normalized compliance (compliance x modulus x thickness) and normalized crack length (crack length/specimen width) is differentiable and invertible, consistency with physical reasoning, and discussion of possible differences in compliance due to material differences can be restricted to a single parameter.