Filler Material Effect

Table 4.7 and Fig. 4.15 compare the impact response between the ultra lightweight cement composite (ULCC) filled and the normal weight concrete filled pipe-in-pipe composite structures. In Table 4.7, “ULCC” refers to the simulation case for the ULCC filled pipe-in-pipe specimen CCFPIP-2-1. The other simulation case, the “Normal weight concrete”, utilizes the same material model (type MAT_72R3 in LS-DYNA) for the filler materials but different parameters (e.g. density c and elastic modulus E ) based on the material tests. The c

compressive strength of the filler material in the two simulation cases are the same, i.e., 60

c

f  MPa, for comparison purpose.

Table 4.7 Filler material effect on the impact response for pipe-in-pipe composite specimens. Case No. c E (GPa) c f (MPa) c  (kg/m3₎ p m (kg) max P (kN) max w (mm) max  (mm) m P (kN) EAC ULCC 16.9 60 1470 152.0 508.2 73.5 23.9 402.7 293.6 Normal weight concrete 37.0 60 2400 186.5 558.2 73.0 24.0 395.9 240.5

In general, the impact response between the pipe-in-pipe composite structures filled with the ULCC and the normal weight concrete are quite similar, as illustrated in Fig. 4.15, indicating that the slight effect of the filler material density on the impact performance of the pipe-in-pipe composite structure. At the initial phase of the impact, the impact force accelerates the composite pipe from the zero velocity to a speed approaching that of the drop weight. Under the same impact velocity, the normal weight concrete filled pipe-in-pipe model (m_p 186.5kg) demonstrates a higher peak force (P ) than that for the ULCC filled pipe-in-pipe model _max

113 composite pipe filled with the ULCC, the composite pipe filled with the normal weight concrete presents an 18.3% lower energy absorption capacity (EAC due to the 22.7% heavier ) pipe weight influences directly the value for EAC [see Eq. (3.6)].

(a) Impact force history (b) Global displacement history

(c) Local indentation profile (d) Average of S1-1 and S1-2 strain history Fig. 4.15 Comparison of the impact response between pipe-in-pipe composite specimens

filled with the ULCC and the normal weight concrete.

4.5 Summary

This chapter carries out a finite element analysis using the explicit code in LS-DYNA on the transverse impact behavior of the cement composite filled pipe-in-pipe structures. Comparison between the experimental and the numerical results has verified the capability of the FE method in analyzing the behavior of the pipe-in-pipe composite structure under the transverse impact by employing appropriate material models, contact algorithm and boundary conditions, etc.  kN P (ms) t 0 15 30 45 60 800 200 0 400 600 ULCC

Normal weight concrete

(ms) t mm g w 0 25 50 75 100 100 25 0 50 75 ULCC

Normal weight concrete

(mm) y -1000 -500 0 500 1000 /R_o  0 0.3 0.4 0.2 0.1 ULCC

Normal weight concrete

 (ms) t 0 20 40 60 80 0.003 0 -0.001 0.001 0.002 ULCC

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Meanwhile, this chapter conducts an FE parametric study to investigate the effect of various impact velocities, geometric and material properties on the transverse impact performance of the pipe-in-pipe composite structures. The maximum impact force (P ) enhances apparently _max

with the increase of the impact velocity (V ), the composite pipe mass (_o m ) and the yield _p

strength of the steel pipe (_y). The impact indenter shape demonstrates an influence on the contact area between the indenter and the composite pipe and thus influences the maximum impact force (P ) to some extent. The post-peak mean force ( )_max P provides a good reference _m

to the real structural capacity for the pipe-in-pipe composite member under the external impacts. The post-peak mean force ( )P relies significantly on the yield strength of the steel m

pipe. Both the global displacement (w ) and the local indentation (_g  ) increase with the growth of the external impact energy (E_i ) and the decrease of the steel pipe strength. Moreover, the long impact duration also enlarges the global and the local deformation. In addition, the local indentation ( ) escalates with the increase of the contact area and the strength of the cement composite. Similar to the post-peak mean force ( )P , the energy _m

absorption capacity (EAC depends significantly on the strength of the steel pipe but relies ) mildly on the strength of the cement composite. Furthermore, the total deflection (w ) and the _t

composite pipe mass (m ) also present an influence on _p EAC based on the definition , expression for EAC in Eq. (3.6). Without the inner steel pipe, the composite pipe system behaves like a hollow steel pipe under the impact load due to the serious loss of steel-cement composite action. The density of the filler material influences the total mass of the pipe-in-pipe composite structure while demonstrates limited effect on the structural impact performance except for P and _max EAC the two parameters related to the pipe mass. ,

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CHAPTER FIVE

LOAD-INDENTATION RELATIONSHIP

5.1 Introduction

The experimental (Chapter 3) and the numerical (Chapter 4) investigation demonstrate that the pipe structures, including the hollow pipe, the cement composite filled pipe and the cement composite filled pipe-in-pipe structure, experience both the global bending deformation and the local indentation under the transverse impact. During the past few decades, many researchers have studied the global bending behavior for hollow steel pipes (Thomas et al., 1976; Khedmati and Nazari, 2012) and concrete-filled pipes (Han, 2004; Moon et al., 2012). Some researchers (Uenaka and Kitoh, 2011; Liew and Xiong, 2012) investigated the flexural capacity for concrete-filled pipe-in-pipe structures through three-point or four-point bending tests. For the local indentation behavior, researchers have examined the load-indentation relationship for hollow pipes through extensive experimental, theoretical and numerical studies (Thomas et al., 1976; Wierzbicki and Suh, 1988; Brooker, 2003a). Recently, Hou et al. (2013) explored the local indentation behavior for concrete-filled pipes. Currently, little literature is about the local indentation behavior for cement composite filled pipe-in-pipe composite members. Engineering applications of such composite structures in a harsh offshore

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environment requires an improved understanding on the load-indentation (P-δ) relationship for these pipe-in-pipe composite structures.

This chapter starts with a description on the indentation test program including three hollow steel pipes, three cement composite filled pipes and seven cement composite filled pipe-in-pipe specimens. The following section presents a numerical simulation on the hollow steel pipes and the pipe-in-pipe composite members and develops an improved P-δ expression for continuously supported hollow steel pipes by fitting the FE results. The next section proposes a two-stage approach to predict the P-δ relationship for cement composite filled pipe-in-pipe specimens. The subsequent section develops a three-phase method to estimate the P-δ relationship for cement composite filled pipes. The last section summarizes the observations and the conclusions drawn from this indentation behavior study.