Ran Y. Kim

Experimental and Analytical Studies On the Onset of Delamination in Laminated Composites

A large number of laminates prone to delamination under uniaxial loading have been tested. The onset of delamination is determined on the basis of acoustic emission data. A two parameter failure criterion, i.e. average interlaminar normal stress failure criterion on σ=, has been used in conjunction with the stress analysis done by applying the global local model [5]. A comparison between the experimental and the predicted edge delamination threshold strengths has been made. For the class of laminates studied, the onset of edge delamination load can be accurately predicted.

Fatigue Behavior of Composite Laminate

The paper discusses a characterization of fatigue behavior of [0/±45/90] s glass/epoxy laminate in terms of the following parameters: static properties; S-N relationship; reliability; effect of thickness variation; damage initiation and growth; temperature increase; secant modulus change; effect of preloading on residual modulus and strength; and effect of ply failure on compression buckling strength. Some of the findings are the following. The primary failure process responsible for up to about 106 cycles of fatigue life is the wear-out followed by the chance failure. Change of secant modulus can be used as a measure of damage extent. Preloading to a high level has negligible effect on the residual tensile strength when the fatigue stress is low. However, ply failure and partial delamination result in a moderate loss of compression buckling strength.

Proof Testing of Composite Materials

The paper presents a concept of proof testing for composite materials. A unique relationship between the static strength and time to rupture is demonstrated for a unidirectional glass/epoxy composite subjected to static fatigue. The maximum significance level at which a Weibull distribution is applicable to represent scatter is 35% for the static strength and 61% for the fatigue life. Limitations and variations of the strength degradation model for the life prediction are discussed.

Steady-state cracking and edge effects in thermo-mechanical transverse cracking of cross-ply laminates

Abstract Composites for space applications such as advanced satellites will require the use of new and/or improved materials so that more stringent dimensional stability requirements can be met. In this work, we study the effect of ply-level initiation of damage under loading and processing stresses and imposed thermal loading. In particular, included in the study is the prediction and demonstration of a condition of steady-state cracking and the temperature change and loading level necessary to create this damage state, thus leading to the possibility of a predetermined loading range over which layer cracking will not propagate. We also consider the influence of the free-edges on the initiation of layer transverse cracking and potential microcracking that may develop at the fiber/matrix interface. The condition of constrained edge cracking is demonstrated where cracks develop but do not propagate across the laminate. To accomplish these studies unidirectional composites and various cross-ply laminates of IM7/5250-4 (graphite/bismaleimide) were produced and characterized.

Effect of Temperature and Moisture on Pin Bearing Strength of Composite Laminates

A series of pin bearing tests has been conducted to examine the effect of temperature and moisture on the strength of graphite/epoxy laminates. Three laminate orientations [OZ/±45] 1

Effect of Curing Stresses on the First Ply-failure in Composite Laminates

’ [902 /:t45] 2

Dimensional stability of composite in a space thermal environment

and [0/90/:t45] 2

Effect of Stacking Sequence on the Notched Strength of Laminated Composites.

were studied under room temperature and 260°F. Wet specimens with 1.5% of moisture content by weight were also tested under the above temperatures. Based on the experimental results under the conditions tested, a number of conclusions can be drawn. The pin bearing strength at the 260°F-wet condition was 40% less than the strength at room temperature-dry in all three orientations. It is apparent from the test results that no interaction between temperature and moisture on the strength degradation can be

A Longitudinal Compression Test for Composites Using a Sandwich Specimen

This paper describes an experimental technique of detecting the first ply-failure in laminated composites using a combination of long strain gages and acoustic emission measurement. The material system chosen for this study is T300/5208 graphite/epoxy laminates of [0/903] S ,[±45/902] S and [0/±45/90] S orientations. Each specimen was instrumented with a strain gage and an acoustic emission transducer. In general, the strain gage technique was good for only those cracks that formed under the gage; the cracks in other areas were barely detected by this technique. The acoustic emission technique, however, was very effec tive regardless of crack locations; matrix cracking always manifested itself in a sudden acoustic emission. The reliability of the above techniques was established by visually observing cracks on photomicrographs. Further more, the experimental data for all three types of laminates agreed fairly well with the analytical predictions with and without residual stresses pres ent. Thus, the effect of residual stresses on the first ply-failure has been delineated.

On the off-axis and angle-ply strength of composites

Abstract In this study, strain gages have been used to investigate variations in thermal expansion coefficient (CTE) with damage of two representative aerospace structural materials, AS4/3501-6 and XN-70/RS3. Refinements in the measurement technique allowed the accurate determination of very low CTEs. The longitudinal and transverse CTEs of unidirectional composites were determined over the temperature range −101° to 121°C. From these data, the CTEs of cross-ply laminates were predicted by using classical laminated plate theory and verified experimentally. Specimens of these laminates were then subjected to a series of uniaxial tensile loadings or thermal cycles −101°C to 121°C (−150° to 250°F) to produce cracks within the transverse plies of the laminate. The laminate CTE was measured after increments in the mechanical loading or thermal cycling and the corresponding crack density quantified by microscopic examination of a polished specimen edge. The CTE variation with crack density in the cross-ply laminate was predicted by using a 2-D variational model and an exact 3-D laminate theory. The results indicate that the variation in CTE due to ply cracking in a cross-ply laminate can be quantitatively predicted under mechanical loading as well as thermal cycling.