含裂纹结构剩余强度的一种估算方法

剩余强度分析对结构断裂安全性评价十分重要。现有的剩余强度分析方法中,Feddersen工程分析法具有代表性,它对简单含裂纹结构比较有效。但这类方法一般需要预先得到弹性断裂应力的连续曲线,才有可能进行短裂纹阶段的弹塑性修正和长裂纹时的净截面屈服修正,使用不方便,有时甚至不适用。本文认为含裂纹结构的破坏是静力学模式破坏和断裂力学模式破坏共同作用的结果,从而建议一种结构剩余强度的实用估算方法。通过表观断裂韧度准则确定的弹性断裂强度和净截面全面屈服后的静力学破坏强度的结合,估算剩余强度许用值。该方法可以方便地估算任意时刻的剩余强度。分别进行含中心裂纹的有限宽板和含裂纹耳片结构的剩余强度试验,并与估算结果进行比较,表明本文方法预测精度良好,有望推广到其他结构形式。文中强调,表观断裂韧度Kapp值的合理与否对估算结果有明显影响。

METHODOLOGY FOR RESIDUAL STRENGTH EVALUATION OF CRACKED STRUCTURES

Residual strength evaluation is playing a significant role in damage tolerance analysis of structures. Available methods to determine residual strength of engineering structures are those similar to Feddersen's, in which the strength evaluated on the basis of linear elastic fracture criterion must be limited by flow stress at short crack length and by net section yielding stress as crack ligament is getting quite narrow. Major deficiencies of such methods are that smooth curve of elastic fracture strength vs. crack length must be available before any modifications going along with it, and they are not applicable, or inconvenient to use for some situation. A simple procedure for evaluating residual strength of structures with cracks was developed based on the consideration that the strength of a cracked body is controlled by the mechanism of both linear elastic fracture and net section static failure. The linear elastic fracture strength is determined in terms of apparent fracture toughness criterion and the static failure strength equals the ultimate load that would result in entire breaking of ligament beyond crack tips. According to present method, it is convenient to obtain residual strength value at any given time. Two kinds of specimens, center cracked LY12CZ aluminum alloy plates (CCT specimens) and pin loaded straight lugs separately made of LD5CS aluminum alloy forging and 30CrMnSiNi2A steel alloy forging materials, were tested to obtain their residual strength. Good agreement was achieved between the results of residual strength predictions according to derived formula and test results. Nowadays, lack of fracture toughness data has become the major impediment for the application of residual strength evaluation methodologies.