Modeling of facesheet crack growth in titanium–graphite hybrid laminates. Part II: Experimental results

doi:10.1016/S0013-7944(02)00087-5

Engineering Fracture Mechanics

Volume 70, Issue 6, April 2003, Pages 799-812

https://doi.org/10.1016/S0013-7944(02)00087-5 Get rights and content

Abstract

Titanium–graphite hybrid composite laminates exhibit a coupled damage growth mode of facesheet cracking and delamination. Part I of this work modeled the growth of the coupled damage mode. Fatigue experiments were conducted on single edge notch tension specimens to measure the crack growth rate. This paper compares the model predictions with experimental data. The three-dimensional finite element model successfully captured the damage growth behavior for two of the lay-ups ([Ti/0/90/0₂]_s and [Ti/90/0/90₂]_s) in the experimental program. However, in a third lay-up, [Ti/0/90/±30]_s, the underlying damage modes were found to be sufficiently different than the other two lay-ups and the model did not capture the steady-state growth behavior. The effects of temperature and specimen size were also investigated for TiGr laminates. Except for the effects of the load ratio, elevated temperatures did not affect the crack growth rate significantly. For wider specimens, the steady-state fatigue crack growth behavior was similar to the narrow specimens, indicating that the steady-state facesheet crack growth behavior is independent of specimen size.

Introduction

In Part I of this work [1], a bridged-crack model and two- and three-dimensional finite element models (2D and 3D FEMs) were constructed for the combined damage growth of a facesheet crack and delamination in titanium–graphite hybrid laminates (TiGr). The 3D FEM, using the virtual crack closure technique (VCCT), indicated that for long facesheet cracks the growth rate would be constant and primarily dependent on the applied stress and the delamination slope near the crack tip. The bridged-crack model produced significantly different predictions as compared to the 3D model and it was concluded that it did not capture the key parameters that influence the stress intensity factor. In Part II of this work, experiments were conducted to measure the facesheet crack growth behavior of [Ti/0/90/0₂]_s TiGr laminates. The experiments were compared to the results from the models developed in Part I of this paper. Experiments were conducted for both room temperature and elevated temperature conditions for a [Ti/0/90/0₂]_s laminate. In addition, two other laminates, [Ti/90/0/90₂]_s and [Ti/0/90/±30]_s, were tested to determine if the constant crack growth behavior depended on the laminate stacking sequence.

Section snippets

Material

The test specimens were 152 mm×38 mm rectangular coupons for the room temperature experiments and 305 mm×38 mm specimens for the elevated temperature experiments. The difference in specimen length for the elevated temperature experiments is due to the size requirements for the thermal chamber. The specimen lay-up for the base laminate was [Ti/0/90/0₂]_s. This lay-up is also known as TiGr 2-6-2, signifying two titanium plies, six 0° plies, and two 90° plies. The titanium layers were made from the

Test equipment and procedures

The fatigue tests were carried out using an Instron™ servo-hydraulic load frame with a sinusoidal waveform under load control. Damage observation was performed using a Questar™ long distance microscope system. The stage for the microscope was equipped with digital position encoders along all space dimensions, which allowed for measurement of the crack length without removing the specimen from the load frame. The fatigue cycling was interrupted and the load was held at the mid-point of the

Single edge notch crack growth behavior

The fatigue crack growth rate as a function of the applied stress intensity factor in the facesheet for the base laminate, [Ti/0/90/0₂]_s, is shown in Fig. 1 for three applied stress levels, σ_max=314, 419 and 524 MPa. The far-field stress level in the facesheet, determined using CLPT, was 97% of the total applied far-field stress level. The applied stress intensity factor is calculated using solutions found in Tada [6]. The results show that as the crack extends, i.e. for increasing values of

Conclusions

The experimental results for facesheet fatigue crack growth in TiGr laminates exhibit a consistent behavior for three different lay-ups. The crack growth rate initially decreased and after a short crack advance, the growth rate leveled out to a constant value. If the constant fatigue crack growth behavior exhibited by TiGr is a characteristic of the material, the crack growth rate, obtained either from experiments or models, can be utilized to characterize damage growth off a crack discovered

Acknowledgements

The authors are very grateful for the help and support of the HSCT group at The Boeing Company. In particular, Ron Zabora, Bill Westre, Antonio Rufin, Elaine Worden, Matthew Miller, Eric Sager, and Edward Li. In addition, experimental assistance was provided by John Kane, David Pinson, and Michelle Park.

This work was enabled by the support of NSF CAREER Grant CMS-9702399.

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