| A study of fatigue crack closure in steel was conducted to assess the significance of plasticity induced closure. The impact of constraint, mean stress, and overload were assessed in terms of technical significance for the material studied. The ability of measuring crack closure was also assessed through a comparison of farfield and nearfield measurement techniques.; A combined finite element analysis and experimental study of fatigue crack closure behavior in 1070M steel was conducted. The experiments and FE models were conducted to study the dependence of crack closure and crack growth rate on constraint in terms of specimen thickness and crack length. Similar dependencies on mean stress were studied in terms of load ratio and maximum stress intensity level. The overload effect was also studied.; Material property tests were conducted to determine the monotonic and cyclic stress-strain properties. Fatigue crack growth tests were conducted using standard compact tension (CT) specimens. Crack opening and closure levels (R{dollar}sb{lcub}rm op{rcub}{dollar} and R{dollar}sb{lcub}rm cl{rcub}{dollar}) were measured using the crack mouth opening displacement (CMOD) method, the backface strain gage (BFSG) method, and a new surface strain gage (SSG) method. A new method of determining the closure level was also developed. A finite element (FE) modeling technique was developed to simulate the crack growth and plasticity induced crack closure behavior. A new crack plane spring model, a new load pin/hole contact stress model, and a new substructuring technique were developed.; The experimental results indicated that a thickness effect on crack closure and crack growth rate existed. A crack length dependency was indicated, which was more significant for the lower stress intensity tests. The closure level was shown to be dependent on stress intensity level and load ratio. The overload effect was found to be thickness dependent. The FE model results were found to agree with the experimental results, except for the overload test which is explained and recommendations are made for future studies. |
