Issue 78

V. Pisarev et alii, Fracture and Structural Integrity, 78 (2026) 96-117; DOI: 10.3221/IGF-ESIS.78.06

A NALYSIS OF THE RESULTS

C

omposite impact damage often exhibits considerable scatter with respect to estimations of residual strength or CAI performance. This might be explained by the fact that impact with the same energy might lead to different residual stress values for the same composite plate. To provide statistical evidence of this suggestion repeat experiments should be provided. Unfortunately, this question is not considered in present paper. The point is that problems are being observed at experimental stage, especially for 65 and 75 J impact energy. Difficulties are related to residual stress determination by combining the hole drilling method and deformation response measurements by electronic speckle-pattern interferometry. Main of them resides in that obtaining high-quality interference fringe patterns in contact interaction area, which are suitable for reliable quantitative interpretation, is very complex technical problem for composite coupons. There are two reasons of this. The first of them is that before performing optical interference experiments the initial (black) surface of the composite samples must be coated with a thin layer of mat white enamel to ensure laser light scattering. The second reason consists in discrete character of fiber distributions over the external layer of composite material. These facts mean that there are no guaranties of obtaining identical contact dimples with the same location with respect to fibers and the same surface quality for each specific impact. Moreover, impact with 65 and, especially, 75 J energy leads to considerable surface damage inside and outside the contact dimple. This, in turn, might negatively influence on a quality of interference fringe patterns generated by hole drilling in residual stress field. Initial plate (320×320×6.4 mm) was cut by eight coupons of dimensions 180×30×6.4 mm. High-quality interferograms were obtained for one from three samples impacted by 75 J (Sp_75); one from three impacted by 65 J (Sp_65) and one impacted by 55 J (Sp_55). The last coupon is called as Sp_75 Back. Thus, we have demonstrated for the first time that reliable quantitative results can be obtained for 65 and 75 J energy impact. This is the main essential feature of performed investigation. Anticipating the analysis of the influence of impact energy on the values of residual stresses, it is necessary to note a number of features related to the technical aspects of experimental research. First, the coordinates of points P1, P2, P3, and P4 are the same for specimens Sp_55, Sp_65, and Sp_75. This fact means that the distances from the contour of the contact dimple to the centers of the holes P2, P3 and P4 decrease with increasing impact energy. Secondly, the contours of the probe holes P5 and P6 coincide with the contour of the contact dimple, and the distances between the centers of the holes P5, P7 and P6, P8 are equal and amount to 14 mm. This means that the distance from the center of the dimple to the centers of the probe holes P5 and P6 increases with increasing impact energy. At the same time, the distance from the center of the dimple to the centers of the holes P7 and P8 decreases. The data shown in Fig. 8 indicate that, as expected, the impact energy influence on the values of the principal residual stress components is most noticeable for the circular area, which includes the contact dimple along with its boundary. This influence has a diverse character. Firstly, at point P1, an increase in the impact energy from 55 to 65 J leads to a noticeable increase in the absolute value of the compressive residual stresses. The next energy increment by 10 J has lower effect on the values of both components of the residual stresses (Fig.9a). Secondly, the tangential tensile residual stresses at point P5 reveal a different trend. Namely, an increase in the impact energy from 55 to 65 J leads to a smaller increase in the tensile component than the subsequent increase in energy from 65 to 75 J (Fig. 9b). Thirdly, the increase in tangential tensile residual stresses at point P6 with an increase in impact energy has a linear character (Fig.9c). The facts established in this work indicate the ambiguity of the response of the composite plate material to the impact energy in terms of residual stresses. This means that it is necessary to conduct a wide range of experimental studies that will ensure reliable use of residual stresses as a design parameter. We should briefly summarize the essential parameters inherent in experimental procedure. In all specimens a set of through holes of 2.0 mm diameter is made by hard coated drill bit by means of Bosch drilling machine. Low rotational speed is fixed at 60 rpm. Extremally low feed rate is equal to 0.25 mm/min. Both of these features ensure that there are no residual stresses induced during drilling. The validity of this statement is reliably confirmed by a good match between the real interferograms and corresponding reference fringe patterns numerically constructed using experimental values of residual stresses [22, 24]. Uncertainty analysis of residual stress determination should be involved for more deep understanding of the results obtained. Required procedure can be based on the methodology developed in works [22, 24]. It is well-known that measured parameters inherent in any destructive method, which are caused by relieved stresses, appear as an integral equation. This equation must be inverted to obtain residual stresses. In practice, usually stress distributions are discretized by a finite dimensional basis, to transform the integral equations into a linear algebraic equations system, which is often ill-conditioned. The inversion of the integral equations arising from relaxation methods is an ill-posed problem, which is a very complex and non-reliable mathematical procedure. Ill-posed problems always require additional assumptions to obtain a physically meaningful solution. This process falls under the name of regularization.

112