Intangible Assets

The gross carrying amount and accumulated amortization for intangible assets consisted of the following ($ in millions):



	March 31, 2019
	Gross Carrying Amount		Accumulated Amortization		Accumulated Impairments		Accumulated Foreign Exchange		Net Carrying Amount
Amortized Intangible Assets
Advanced Materials & Structures
Customer relationships	$	276.3	$	54.7	$	—	$	1.6	$	220.0
Developed technology	34.0		9.1		—		0.3		24.6
Trade names	21.8		0.8		20.7		0.3		—
Non-compete agreements	2.9		1.9		—		—		1.0
Patents	1.5		0.4		—		—		1.1
Total	$	336.5	$	66.9	$	20.7	$	2.2	$	246.7

Unamortized Intangible Assets (Advanced Materials & Structures)
Trade names	$	20.0	$	—	$	0.1	$	—	$	19.9



	December 31, 2018
	Gross Carrying Amount		Accumulated Amortization		Accumulated Impairments		Accumulated Foreign Exchange			Net Carrying Amount
Amortized Intangible Assets
Advanced Materials & Structures
Customer relationships	$	276.3	$	50.4	$	—	$	0.7		$	225.2
Developed technology	34.0		8.5		—		0.2			25.3
Trade names	21.8		0.8		20.7		0.3			—
Non-compete agreements	2.9		1.7		—		—			1.2
Patents	1.5		0.4		—		—			1.1
Total	$	336.5	$	61.8	$	20.7	$	1.2		$	252.8

Unamortized Intangible Assets (Advanced Materials & Structures)
Trade names	$	20.0	$	—	$	0.1	$	(0.1	)	$	20.0

Amortization expense of intangible assets was $5.1 million and $5.2 million for the three months ended March 31, 2019 and 2018, respectively. Finite-lived intangibles in the AMS segment are expensed using the straight-line amortization method. The estimated average aggregate amortization expense is $20.1 million in each of the next five years.